ISSN 0412-1961
CN 21-1139/TG
Started in 1956

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Hydrogen-Induced Cracking Resistance of Novel Cu-Bearing Pipeline Steels
Xianbo SHI, Wei YAN, Wei WANG, Yiyin SHAN, Ke YANG
Acta Metall Sin    2018, 54 (10): 1343-1349.   DOI: 10.11900/0412.1961.2017.00522
Abstract   HTML PDF (5109KB)  

Hydrogen-induced cracking (HIC) resistance of pipeline steels is one of the most important properties for sour gas service pipelines. For the conventional pipeline steels, the strength level mainly depends on the Mn content. However, as the Mn content increases, the unfavorable microstructures such as large size martensite/austenite (M/A) islands, bainite or martensite will be generated, which will deteriorate the HIC resistance of the steels. Therefore, it is hard to simultaneously improve strength level and HIC resistance for pipeline steel. The nature of HIC in pipeline steel is hydrogen embrittlement, which is essentially the redistribution of H atoms into the matrix of steel. So, how to make the distribution of H atoms in the steel as evenly as possible without causing local enrichment is a key factor to improve the HIC of pipeline steels. In this work, the susceptibilities of HIC of traditional X80 and novel Cu-bearing pipeline steels were studied with comparison. The results showed that the X80 pipeline steel behaved bad HIC resistance. The hydrogen-induced cracks mainly expanded along the interface between M/A islands and the matrix. However, the novel Cu-bearing pipeline steels with different Cu contents exhibited excellent HIC resistance, showing no cracks were produced after HIC test. It was analyzed that nano-sized Cu-rich precipitates in the Cu-bearing pipeline steels are speculated to act as the beneficial hydrogen traps, and these uniformly dispersed fine Cu-rich phases in matrix provide many sites for the distribution of H atoms, which helps to avoid the localized high concentration H atoms enrichment leading to hydrogen embrittlement. Taking nano-sized Cu-rich phases as a type of beneficial hydrogen traps provides a new way for the development of new pipeline steels with high strength and excellent HIC resistance.

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Numerical Simulation of Anomalous Eutectic Growth of Ni-Sn Alloy Under Laser Remelting of Powder Bed
Lei WEI, Yongqing CAO, Haiou YANG, Xin LIN, Meng WANG, Weidong HUANG
Acta Metall Sin    2018, 54 (12): 1801-1808.   DOI: 10.11900/0412.1961.2018.00139
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Eutectic is one of the most commonly observed solidification patterns, the growth morphology of which is important to materials properties. Anomalous eutectic is typically coarser and globular than lamellar eutectic, which is commonly observed during solidification of binary eutectic alloy, including deep undercooled melt and laser remelting process. The morphological evolution mechanism of anomalous growth is still unknown due to the lack of simulation evidence. During laser remelting process, the anomalous eutectic is sandwiched between lamellar eutectic at the bottom of melt pool. Comparing to deep undercooled melt, laser remelting has simpler temperature field distribution, which can be simplified into directional solidification. Thus, simulations of anomalous eutectic growth in laser remelting process are feasible. In the present work, the anomalous eutectic growth mechanism under laser remelting conditions was simulated using a low mesh induced anisotropy cellular automaton (CA) model. Firstly, a two-dimensional lamellar eutectic CA model of CBr4-C2Cl6 alloy was established, and the morphological transition from 1λO to 2λO was simulated. The calculated results are in good agreement with experiments and phase field simulations. By setting the interface cells containing three phases (α, β and liquid phases), the model can continuously change the α and β phase volume fractions in the CA model, making it easier for the model to capture the instability of lamellar eutectic. Compared with the results of the phase field model, the intermediate 1λO-2λO state of oscillation instability of 1λO and 2λO which is consistent with the experimental results was calculated. Based on the above-mentioned binary eutectic CA model, the lamellar eutectic to anomalous eutectic transition at the bottom of the molten pool was simulated. Under the condition of initial low cooling rate, the fine lamellar eutectic is decoupled, it leads to the overgrowth of β-Ni3Sn phase. During the subsequent accelerated cooling process, α-Ni nucleated in the liquid phase at the front of the solid/liquid interface, and the β-Ni3Sn phase wrapped around the α-Ni phase forming anomalous eutectic morphology. During the laser remelting process, there is indeed a rapid change of solidification rate from zero to scanning speed rate from the bottom to the top of the melt pool, and therefore coincides with the solidification conditions of the variable pulling velocity used in the CA simulations.

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Distribution Characteristics of Twin-Boundaries in Three-Dimensional Grain Boundary Network of 316L Stainless Steel
Tingguang LIU, Shuang XIA, Qin BAI, Bangxin ZHOU, Yonghao LU
Acta Metall Sin    2018, 54 (10): 1377-1386.   DOI: 10.11900/0412.1961.2018.00062
Abstract   HTML PDF (5151KB)  

Grain boundaries are sources of failure and weakness due to their relatively excess free volume compared to the lattice of polycrystalline materials exposed to aggressive environment. The control of grain boundary degradation has become one of the key issues of materials science and engineering. It has been found that the coincidence site lattice (CSL) boundaries, especially Σ3 (the twin boundaries), have stronger resistance to intergranular degradation than random boundaries. Materials with a high proportion of CSL boundaries that could disrupt the connectivity of random boundaries have better performance to resist intergranular failures. However, the distribution characteristics of twin boundaries in grain boundary network are still unclear. In this work, three-dimensional electron backscatter diffraction (3D-EBSD) was used to map the 3D grain boundary network of a 316L stainless steel. The topological characteristics of triple junction and quadruple junction in the presence of twin boundaries were investigated. The distribution of twin boundaries around grains and grain boundaries was analyzed. The results show that the twin boundary number fraction in the 3D grain boundary network is lower than the measured twin boundary area fraction, indicating that the average area per twin boundary is larger than random boundary. Most of triple junctions in the 316L stainless steel have one twin boundary. The proportion of triple junctions with two twin boundaries is about 9.4%. A quadruple junction has three twin boundaries at most. Most of quadruple junctions have one or two twin boundaries. About 7.9% of quadruple junctions have three twin boundaries. The 3D-EBSD data of 316L includes 1840 grains, 7353 random boundaries and 1824 twin boundaries. On average, a 3D grain in the 3D microstructure has 11 faces (39.85 neighboring faces that includes all boundaries of the grain and all boundaries that connected with the grain by lines or points), in which the number of twin boundaries is 2.03 (8.02) on average. A 3D grain boundary has 9.35 neighboring boundaries, in which the number of twin boundaries is 1.99 on average.

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Effects of Tool Rotation Rates on Superplastic Deformation Behavior of Friction Stir Processed Mg-Zn-Y-Zr Alloy
Guangming XIE, Zongyi MA, Peng XUE, Zongan LUO, Guodong WANG
Acta Metall Sin    2018, 54 (12): 1745-1755.   DOI: 10.11900/0412.1961.2018.00174
Abstract   HTML PDF (10684KB)  

Compared to conventional Mg-Al and Mg-Zn system magnesium alloys, the Mg-Zn-Y-Zr heat-resistant alloy exhibits high thermal stability due to the addition of Y earth element, which is an ideal candidate for producing high strain rate superplasticity (HSRS, strain rate≥1×10-2 s-1). Recently, the HSRS of Mg-Zn-Y-Zr alloy was achieved by friction stir processing (FSP), because the FSP resulted in the generation of fine and equiaxed recrystallized grains and fine and homogeneous second phase particles. However, the study on superplastic deformation mechanism of FSP Mg-Zn-Y-Zr alloy at various parameters is limited relatively. Therefore, at the present work, six millimeters thick as-extruded Mg-Zn-Y-Zr plates were subjected to FSP at relatively wide heat input range of rotation rates of 800 r/min to 1600 r/min with a constant traverse speed of 100 mm/min, obtaining FSP samples consisting of homogeneous, fine and equiaxed dynamically recrystallized grains and fine and uniform Mg-Zn-Y ternary phase (W-phase) particles. With increasing rotation rate, within the FSP samples the W-phase particles were broken up and dispersed significantly and the recrystallized grains were refined slightly, while the fraction ratio of the high angle grain boundaries (grain boundaries misorientation angle≥15°) was increased obviously. Increasing rotation rate resulted in an increase in both optimum strain rate and superplastic elongation. For the FSP sample obtained at 1600 r/min, a maximum elongation of 1200% was achieved at a high-strain rate of 1×10-2 s-1 and 450 ℃. Grain boundary sliding was identified to be the primary deformation mechanism in the FSP samples at various rotation rates by superplastic data analyses and surfacial morphology observations. Furthermore, the increase in rotation rate accelerated superplastic deformation kinetics remarkably. For the FSP sample at 1600 r/min, superplastic deformation kinetics is in good agreement with the prediction by the superplastic constitutive equation for fine-grained magnesium alloys governed by grain boundary sliding mechanism.

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Effect of H2O(g) on Decarburization of 55SiCr Spring Steel During the Heating Process
Kai ZHANG, Yinli CHEN, Yanhui SUN, Zhijun XU
Acta Metall Sin    2018, 54 (10): 1350-1358.   DOI: 10.11900/0412.1961.2017.00558
Abstract   HTML PDF (8719KB)  

Spring steel is an important steel widely used in the national economic construction. Its application environment is quite harsh, so it strongly demands for a high quality of the surface. However, the decarburization behavior on the surface seriously affects the surface quality and reduces the fatigue performance of materials. Decarburization is caused by chemical reaction between oxidizing atmosphere and steel surface. Hydrosphere, as a component of the oxidizing atmosphere, could lead to a significant influence on decarburization process. Our experimental materials, taken from continuous casting billet, were polished carefully by abrasive paper in order to remove stains, original scale and decarburization layer. Components of furnace gas were controlled by mixing units designed independently. Spring steel samples were heated by a tube vacuum furnace in 600~950 ℃. The mixed atmosphere contains (15%~20%)CO2, (2%~4%)O2, hydrosphere of different contents and N2 in balance. With the condition of the mixed atmosphere, influence of hydrosphere on surface decarburization of spring steel 55SiCr was investigated by 3D measuring laser microscope. The results show that, with the influence of the mixed atmosphere, decarburization is able to happen during low temperature interval, 650 ℃~Ac1 (Ac1: starting temperature of austenitization during slow heating). The thickness of total decarburized layer increases correspondingly with the temperature. In the mixed atmosphere, slight decarburization occurs at 650 ℃ and obvious ferrite decarburization layer can be detected within a temperature range of 700~950 ℃, which is more serious than that in the air. The peak temperature of ferrite decarburization is 850 ℃ under both mixed atmosphere and air. The surface decarburization in low temperature region is related to the dissolution of cementite in pearlite lamellae driven by carbon concentration gradient. When the decarburization degree deepens as time goes, partial pearlite colony begins to shrink, much line form cementite dissolves gradually, and the line cementite becomes short and punctate. The grain morphology of ferrite decarburization layer is different from that who generates within α+γ phase region, which is small in size and without a strong orientation. Hydrosphere in mixed atmosphere could increase porosity of oxidation layer and destroy the important protection mechanism of preventing decarburization by compact scale. It is deduced that existing of hydrosphere offers a chance for low temperature decarburization process occurring and hydrosphere plays an important role in deepening the decarburization degree of samples.

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Effect of Electric-Magnetic Compound Field on the Microstructure and Crack in Solidified Ni60 Alloy
Yinghua LIN, Ying YUAN, Liang WANG, Yong HU, Qunli ZHANG, Jianhua YAO
Acta Metall Sin    2018, 54 (10): 1442-1450.   DOI: 10.11900/0412.1961.2018.00134
Abstract   HTML PDF (5527KB)  

Ni60 alloy has been widely used in many application fields due to its excellent wear resistance, corrosion resistance and high temperature oxidation resistance. However, uneven microstructure was easily formed due to the effect of heat shock and heat accumulation during laser multi-track overlap process. Moreover, Ni60 alloy powder was composed of a variety of elements. The composition segregation and high content CrB, (Cr, Fe)23C6 were easily present in the coating during the laser cladding process, which can easily lead to the cracking of Ni60 alloy coating. In this work, multi-layer Ni60 alloy coating was prepared by electric-magnetic compound field assisted laser cladding. Synthesis of Ni60 alloy coating was analyzed by coloring agent, OM, SEM, EDS, XRD and microhardness tester. The results showed that cracks and large pores were to appear at the coating when the electric-magnetic compound field was not applied, and the molding quality was also poor. When the electric-magnetic compound field was applied, the surface cracks of Ni60 alloy coating were suppressed, the pores were eliminated, and the molding quality of the coating was also improved. Meanwhile, the particle size of the brittle phase (CrB, (Cr, Fe)23C6) was decreased from 4~6 μm to 2~4 μm by the aid of the electric-magnetic compound field, and the degree of particle cluster was also reduced, which was beneficial to the elimination of the internal crack. XRD, microstructure and microhardness analysis results showed that the brittle phase content, particle segregation, lattice distortion and hardness were reduced under the condition of electric-magnetic compound field, leading to the decrease of crack initiation probability, so the crack of Ni60 alloy coating was remarkably reduced.

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Effect of Cold Rotary-Swaging Deformation on Microstructure and Tensile Properties of TB9 Titanium Alloy
Dechun REN, Huhu SU, Huibo ZHANG, Jian WANG, Wei JIN, Rui YANG
Acta Metall Sin    2019, 55 (4): 480-488.   DOI: 10.11900/0412.1961.2018.00241
Abstract   HTML PDF (19419KB)  

TB9 titanium alloy has been widely used for aerospace due to it's superior low stiffness, corrosion resistance and workability. It has been reported that cold deformation can improve the comprehensive mechanical properties of titanium alloys. At the same time, the cold rotary-swaging deformation facilitates the production of small batches and the acquisition of special shape and size bars. However, current studies on the microstructure and properties of cold rotary-swaged titanium alloys are not systematic. So, the effects of cold deformation rate on the microstructure, texture evolution and mechanical property of TB9 alloy during cold rotary-swaging were investigated using OM, EBSD, XRD, TEM and tensile test. The results showed that the grain size of TB9 titanium was refined with the increase in diameter reduction. Meanwhile, with the deformation increases, the grains rotation along the swaging axis occurs, forming a preferred orientation, the textures change from initial {001}<110> and {001}<100> to α-fiber and γ-fiber {001}<110>, {112}<110> and {111}<110>. All of grains refinement, texture components and substructures contributed to the enhancement of strength after cold rotary-swaging. And the ductile kept on a high level after 70% cold working, which means the TB9 titanium has a great cold deformation ability.

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Effect of Manganese Content on Tensile Deformation Behavior of Fe-Mn-C TWIP Steels
Dongdong LI, Lihe QIAN, Shuai LIU, Jiangying MENG, Fucheng ZHANG
Acta Metall Sin    2018, 54 (12): 1777-1784.   DOI: 10.11900/0412.1961.2018.00129
Abstract   HTML PDF (6127KB)  

Twinning-induced plasticity (TWIP) steels exhibit excellent mechanical properties including high tensile strength and good plasticity owing to their high strain-hardening rate. The high strain-hardening rate results mainly from deformation twinning; in addition, plane slip and dynamic strain ageing also have some contribution to strain-hardening rate. Until now, the influences of some alloy elements such as C, Al and Si on tensile properties of Fe-Mn-C based TWIP steels have received much attention. However, the effect of Mn content on the microstructure and tensile properties of twinning-dominated Fe-Mn-C TWIP steels is still not clear. In this work, the microstructure, tensile properties and strain hardening behavior of two Fe-Mn-C TWIP steels (Fe-13Mn-1.0C and Fe-22Mn-1.0C, mass fraction, %) were studied by using OM, TEM, SEM-EBSD and monotonic tensile tests. The results show that the yield and tensile strengths of the steel decrease while the elongation to fracture increases with the increase of Mn content. At low tensile strains, the increase of Mn content delays the formation of deformation twins. However, at higher strain level, the deformation twinning rate becomes higher and hence more deformation twins are produced in the steel with higher Mn content than that in the steel with lower Mn content. Furthermore, the thickness of deformation twins increases with increasing the Mn content. The twinning and tensile deformation behavior in the two steels are also discussed.

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Effect of Pulsed Magnetic Field on the Microstructure of TC4 Titanium Alloy and Its Mechanism
Qingdong XU, Kejian LI, Zhipeng CAI, Yao WU
Acta Metall Sin    2019, 55 (4): 489-495.   DOI: 10.11900/0412.1961.2018.00257
Abstract   HTML PDF (5361KB)  

In this work, the effect of pulsed magnetic treatment (PMT) on the microstructure of TC4 titanium alloy was investigated. TC4 titanium alloy is widely used in the manufacture of the blade of aviation engine. The microstructure of TC4 titanium alloy determines its property. PMT is a novel method used to modify the microstructures of alloys and has been explored in several papers recently. PMT has many advantages in the aspect of efficiency, energy-saving, non-deformation, etc. Therefore, the effect of PMT on the microstructures of TC4 titanium alloy was explored in this work. The variation of the dislocation density and the grain boundary angle of TC4 titanium alloy was observed after PMT. In the experiment, the magnetic induction density is 2 T, the pulse frequency is 5 Hz and the pulse number is 100. According to XRD tests, the dislocation density in TC4 alloy after PMT increased about 10.9%. KAM maps in EBSD test were used for evaluating the same area's dislocation density of the TC4 alloy before and after PMT. The dislocation distribution of TC4 titanium alloy changes notably: the in-grain dislocation density became more homogeneous and some local high-density areas disappeared, the distribution of dislocation near grain boundaries caused the angles of the grain boundaries altered and the fraction of low-angle grain boundaries decreased while the fraction of Σ11 grain boundaries (CSL grain boundary) increased. The motivation mechanism of the dislocation in TC4 titanium alloy under PMT was speculated based on the experimental results and some previous researches. The PMT may change the energy state of the electrons in pinning area of dislocations, which accelerates the electrons transformation from singlet state to triplet state and then increases the mobility of the vacancy or impurity atoms so that the dislocation de-pinning could occur under the original stress field and thus leads to dislocation movement and transformation of microstructure.

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Study on Vaporizing Foil Actuator Welding Process of 5A06/0Cr18Ni10Ti with Interlayer
Shan SU
Acta Metall Sin    2019, 55 (8): 1041-1048.   DOI: 10.11900/0412.1961.2018.00432
Abstract   HTML PDF (8480KB)  

Aluminum alloy and stainless steel composite structure have been widely used in the chemical industry. Aluminum alloy and stainless steel are difficult to weld by fusion weld method because of differences in physical and chemical properties. Joints of aluminum alloy 5A06 and 0Cr18Ni10Ti stainless steel with good mechanical properties were created using vaporizing foil actuator welding with an interlayer. The interlayer was welded to both the target and the flyer on a ring-shaped welded area. The influences of the input energy on the time of the occurrence of vaporization and mechanical properties of the joints were analyzed. Single collection system and photonic Doppler velocimetry system were used to analyze the burst time, discharge current and voltage changes with energy input increased, and microstructure and element distribution were analyzed by OM and SEM with EDS. The results show that as the input energy increases, the vaporization of the foil occurred earlier and achieved higher impact velocity, resulting in a larger diameter of the welded area. The peak tensile load and shear load were increased with energy input increased, the peak tensile load is 44.0 kN and peak shear load is 2.1 kN with 9 kJ energy input. The Al3003 was joint to 5A06 in symmetric wavy pattern and joint with 0Cr18Ni10Ti stainless steel by intermetallic compounds. The joining areas were not aligned.

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Stress Relaxation Mechanism for Typical Nickel-Based Superalloys Under Service Condition
JIANG He,DONG Jianxin,ZHANG Maicang,YAO Zhihao,YANG Jing
Acta Metall Sin    2019, 55 (9): 1211-1220.   DOI: 10.11900/0412.1961.2019.00121
Abstract   HTML PDF (38878KB)  

Since nickel-based superalloys are more and more used as fasteners, it is necessary to investigate the stress relaxation behavior and mechanism of nickel-based superalloy. In present work, the stress relaxation mechanism for four typical nickel-based superalloys (GH4169, GH4169D, GH4738, GH350) for fasteners under service condition was investigated. The stress relaxation tests were carried out according to GB/T 10120-2013 in the temperature range of 600~780 ℃ and initial stress range of 260~510 MPa, and the stress relaxation curves were recorded. The microstructure was studied by FESEM and TEM. The results show that the stress decreases fast in the initial stage of stress relaxation test and then trends to be steady. The stress relaxation stability decreases with increasing temperature. There is no apparent change in the microstructure after stress relaxation test. TEM observation shows that the major mechanism of stress relaxation is the movement of dislocations, and the stress relaxation properties of different alloys depend on the inhibition of dislocation movement. The species, size, shape and distribution of phases determine the ability to hinder dislocation movement and the stress relaxation property of different alloys. GH4169 alloy gets the stress relaxation property mainly by γ' phase, γ'' phase and δ phase hindering the movement of dislocations. In GH4169D alloy, both γ' phase and η phase participate in the stress relaxation process. γ' phase in GH4738 alloy can effectively impede the movement of dislocations and provide good stress relaxation property. The combined effect of γ' phase and η phase guarantees the stress relaxation stability of GH350 alloy.

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Deformation Behavior and Strengthening-Toughening Mechanism of GH4169 Alloy with Multi-Field Coupling
WANG Lei,AN Jinlan,LIU Yang,SONG Xiu
Acta Metall Sin    2019, 55 (9): 1185-1194.   DOI: 10.11900/0412.1961.2019.00085
Abstract   HTML PDF (28386KB)  

The superalloy is one of key metal materials, representing the level of scientific and technological development. Nickel-based superalloy is the most important which has been widely used for rotating component of aerospace. Nickel-based GH4169 alloy shows excellent combination properties including good fatigue property, excellent oxidation and corrosion resistance, as well as the microstructure stability during long-term ageing. The using amount of GH4169 alloy is about 45% of total wrought superalloys. For satisfying high performance of aero-engine, both strength and ductility of GH4169 alloy at high temperature are required to be simultaneously improved for safety servicing. It is an effective method to strengthen alloys by adding alloying elements. The alloying element addition ratio of GH4169 alloy is more than 40%, which unavoidably leads to hard deforming and plasticity declining, so that it restricts the further application of the alloy. Therefore, it is key to find methods realizing strengthening-toughening and without any losing of hot-deforming ability. In this work, the plastic deformation behavior and strengthening-toughening mechanisms of GH4169 alloy with multi-field coupling (electric-pulse current (EPC)/temperature/stress) were investigated. The results show that the deformation resistance of GH4169 alloy decreases and plastic deformation ability increases with multi-field coupling. The thermal vibration of atoms enhances and thus leads to decreasing of Peierls force with multi-field coupling, which is the essential factor on decreasing of deformation resistance and increasing of plastic deformation coordinate ability. When the alloy aged with electric-pulse treatment (EPT)/temperature coupling, the ultimate strength, yield strength and fracture elongation increase simultaneously at elevated temperatures. The vacancy concentration increases of the alloy aged with EPT/temperature coupling. Vacancy induces ultrafine nm-sized γ" phase to precipitate during tensile deformation at high temperature, which is the key factor on strength and ductility improvement. At the same time, because of the EPT/temperature coupling ageing, part of γ" phases precipitate around dislocation, while, due to the increasing of γ" phase size, the ductility of the alloy will be improved. With the multi-field coupling treatment, the strengthening-toughening of GH4169 alloy can be realized depended on an appropriate distribution of two kind sizes of γ" phase.

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Topological Modelling of the B2-B19' Martensite Transformation Crystallography in NiTi Alloy
Zhaozhao WEI, Xiao MA, Xinping ZHANG
Acta Metall Sin    2018, 54 (10): 1461-1470.   DOI: 10.11900/0412.1961.2018.00078
Abstract   HTML PDF (2265KB)  

Some NiTi alloys, generally known as shape memory alloys or smart materials, exhibit larger negative thermal expansion (NTE) strain than that of traditional nonmetallic NTE materials. Furthermore, high strength and better ductility of NiTi alloy make it more advantageous compared to nonmetallic materials. The NTE response of NiTi alloy may be attributed to the transformation strain that originates from the volume change accompanying the B2-B19' martensitic transformation in the alloy. Therefore, it is of great importance and interests to study the martensitic transformation crystallography in NiTi alloy. In this work, the martensitic transformation crystallography in an equiatomic NiTi alloy was investigated by using the topological model, as well as the optimum twist criterion developed lately. The optimum twist angle, ωo, in NiTi alloy was determined to be -0.969°, and the so-obtained transformation crystallography results, including the habit plane index and parent-martensite orientation relationship, agree well with the corresponding experimental data and theoretical calculations in the literature. Furthermore, the martensitic transformation strain of NiTi alloy was calculated based on the analysis of interfacial dislocation movement, and the total transformation strain can be resolved into an in-habit-plane shear strain and an axial strain perpendicular to the habit plane. In particular, the value of the axial strain that represents the volume change due to the B19' to B2 transformation was found to be negative, indicating that the NiTi alloy shrinks during the reverse martensitic phase transformation when heated, which might shed some light on the relationship between the NTE mechanism and martensitic transformation in NiTi alloy. The measured NTE strain is much smaller than the theoretical calculated phase transformation strain in NiTi alloy, due to the self-accommodation effect of martensite variants and the compensation of transformation strains in polycrystalline materials.

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Concurrence of Phase Transition and Grain Growth in Nanocrystalline Metallic Materials
Feng LIU, Linke HUANG, Yuzeng CHEN
Acta Metall Sin    2018, 54 (11): 1525-1536.   DOI: 10.11900/0412.1961.2018.00318
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The concurrence of solid-state phase transition and grain growth is ubiquitous in thermal processing of metallic materials; understanding the concurrence is significant for manipulation of microstructure and design of structured materials with high strength and good ductility. This work will briefly review the recent progresses on the concurrence in nanocrystalline metallic materials, with particular attention to the physical origin, typical examples, underlying mechanisms, as well as microstructures, of the concurrence. On this basis, perspectives on scientific understanding of the occurrence in nanocrystalline materials are addressed.

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Exploration on the Unified Model for Fatigue Properties Prediction of Metallic Materials
Zhefeng ZHANG, Rui LIU, Zhenjun ZHANG, Yanzhong TIAN, Peng ZHANG
Acta Metall Sin    2018, 54 (11): 1693-1704.   DOI: 10.11900/0412.1961.2018.00331
Abstract   HTML PDF (5438KB)  

The fatigue of metallic materials can be divided into high-cycle fatigue (HCF) and low-cycle fatigue (LCF); the damage of these two types of fatigue is commonly evaluated through stress amplitude and strain amplitude of cyclic loading, respectively. The mismatch of the evaluation standards between HCF and LCF leads to difficulties in the design and selection of anti-fatigue materials. Under this condition, systematic researches on fatigue properties and microscopic damage mechanisms of HCF, LCF and extra-low-cycle fatigue (ELCF) for pure Cu and Cu-Al alloys were summarized in this work. On the bases of the experimental results, a three-dimensional fatigue model is proposed, which is simultaneously applicable to both the HCF and LCF properties. The model is built up in a three-dimensional coordinate system of stress amplitude-strain amplitude-fatigue life; it could be associated with the cyclic stress-strain (CSS) curve, S-N curve and E-N curve through the projection method, or be transformed into the Basquin equation, Coffin-Manson equation and hysteretic energy model under specific conditions. In this way, this generally applicable fatigue model helps provide a new viewpoint for the evaluation and optimization of fatigue properties based on the classical fatigue theories.

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Preparation and Corrosion Resistance of the ZnAl-LDHs Film on 6061 Al Alloy Surface
Yusheng ZHANG, Youbin WANG, Chunmin LI, Bingtao ZHOU, Keke CHENG, Yuezhou WEI
Acta Metall Sin    2018, 54 (10): 1417-1427.   DOI: 10.11900/0412.1961.2018.00020
Abstract   HTML PDF (10413KB)  

6061 Al alloy is widely used for structural components in the structural, building, aerospace and automobile industry because of their good extrude-ability, high strength and low density. 6061 Al alloy is easily corroded during the corrosive environments containing Cl-, which lead to the decreasing of the alloy service life. Therefore, the Al alloy need to adopt surface treatment to improve the corrosion resistance. The chromate passivation was the most effective surface treatment technology of Al alloys in the past. However, Cr(VI) could pollute the environment and harmful to human body. Thus, it is necessary to develop chromium-free films to protect Al alloy. Layered double hydroxides (LDHs) are an environment-friendly and smart material, which could be used for anticorrosion film. The inhibitor can be inserted into the film layer due to its unique anion exchanging capacity. Therefore, the LDHs is acquired self-healing performance and applied to the anticorrosion field of Al alloy. Meanwhile, vanadate is a good inhibitor and it has many forms under different pH conditions. Moreover, different forms of vanadate have different influence on LDHs film. The corrosion resistance of LDHs and its modified form films on Al alloys need deep study. In this work, ZnAl-LDHs films with NO3- were prepared on the suface of 6061 Al alloy via a facile in-situ growth method, and then ZnAl-LDHs-NO3 films were intercalated with the corrosion inhibitor VO43- and VO3- to obtain the ZnAl-LDHs-VO4 and ZnAl-LDHs-VO3 films, respectively. The structure, morphology and composition of as-prepared ZnAl-LDHs films were investigated by XRD, fourier infrared spectrometer (FT-IR) and SEM; the corrosion behavior of ZnAl-LDHs films in the 3.5%NaCl (mass fraction) solution were studied by the electrochemical workstation and the 3D microscope. The results show that the plate-like ZnAl-LDHs microcrystals are perpendicular to the substrate and cover almost the entire Al alloy substrate surface. Compared with the 6061 Al substrate, ZnAl-LDHs films can not only decrease the corrosion current (Icorr), but also increase the corrosion potential (Ecorr) and charge transfer resistance (Rct) of the 6061 Al alloy. It is suggested that ZnAl-LDHs films could significantly enhance the corrosion resistance of 6061 Al substrate, indicating an effective protection for 6061 Al alloy by the ZnAl-LDHs film. The ZnAl-LDH-VO3 film is of the highest corrosion resistance in the studied ZnAl-LDHs films.

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Influence of Magnetic Shielding on the Power Loss of Induction Heating Power Supply in the Electro-magnetic Induction Controlled Automated Steel Teeming System
Ming HE, Xianliang LI, Qingwei WANG, Lianyu WANG, Qiang WANG
Acta Metall Sin    2019, 55 (2): 249-257.   DOI: 10.11900/0412.1961.2018.00083
Abstract   HTML PDF (2298KB)  

In order to reduce the influence of ladle structure on the power loss of power supply in the electromagnetic induction controlled automated steel teeming (EICAST) system, a method of setting magnetic shielding material on the bottom and sides of induction coil is firstly proposed. The influence of the magnetic shielding on the magnetic flux density and the optimal heating position of induction coil are analyzed by numerical simulation, and the correctness of simulation results is verified by laboratory experiments. In addition, the best magnetic shielding sizes and structure for this new technology are determined respectively. The results show that the magnetic shielding method can effectively reduce the power loss of induction coil and improve the optimum heating area of induction coil. When using copper as a magnetic shielding material, the best sizes of magnetic shielding are height of 200 mm, length of 290 mm, width of 290 mm and thickness of 1 mm. At this time, the best heating position of induction coil will move upward, and the moving distance is 20.2 mm, which is beneficial to the installation of induction coil and the improvement of its service life. To improve the strength of nozzle brick and ensure the service life of nozzle brick, a new structure is applied, and its magnetic shielding effect is almost the same as the former. These research works are very important for the wide application of the EICAST technology.

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Hydrogenated Vacancy: Basic Properties and Its Influence on Mechanical Behaviors of Metals
Jun SUN, Suzhi LI, Xiangdong DING, Ju LI
Acta Metall Sin    2018, 54 (11): 1683-1692.   DOI: 10.11900/0412.1961.2018.00341
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Hydrogen embrittlement, the degradation of mechanical behaviors due to the existence of hydrogen, is an industrially and environmentally critical problem in metals and alloys. Yet the fundamental mechanism(s) of embrittlement are still controversial, and the molecular-level damage events shrouded in mystery. In hydrogen embrittlement phenomena, the molecular-level agents of damage are hypothesized to be hydrogen-vacancy complex (Va+nH→VaHn), hereupon called hydrogenated vacancy. Contrary to vacancy, hydrogen-vacancy complex has good thermal stability and low diffusivity. When metals undergo plastic deformation at low homologous temperature in the presence of hydrogen, the mechanically driven out-of-equilibrium dislocation processes produce extremely high concentrations of hydrogen-vacancy complexes. Under such high concentrations, these complexes prefer to grow by absorbing additional vacancies and act as the embryos for the formation of proto nano-voids. Our work provides the insight on the microscopic mechanism of hydrogen embrittlement. Moreover, this work also helps understanding some unique mechanical behaviors induced by hydrogen.

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Research on Single SiC Fiber Reinforced TC17 CompositesUnder Transverse Tension
Jialin LIU, Yumin WANG, Guoxing ZHANG, Xu ZHANG, Lina YANG, Qing YANG, Rui YANG
Acta Metall Sin    2018, 54 (12): 1809-1817.   DOI: 10.11900/0412.1961.2018.00124
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Transverse mechanical properties of titanium matrix composites (TMCs) play an important role during its engineering service. Although SiCf / TC17 composite is one of the most promising TMC candidates for aeroengine, as we know its transverse properties have not been reported yet until now. In this work, the transverse strength of single SiC fiber reinforced TC17 composite was evaluated using cruciform specimen. The surface and cross-section of fractured specimen were investigated by SEM to determine the failure position during tensile test. Finite element simulation method was also used to analyze the mechanism of interfacial failure and crack propagation. During the transverse tensile test of single fiber specimen, the initial non-linearity in the stress-strain curve occurred at the stress of (271±12) MPa, which indicated the beginning of fiber-matrix interface failure. SEM observation showed that the crack in the center of sample appeared at the interface of reaction layer and carbon coating with a 24°~68° angle to the applied loading direction and its length extended with the increase of the applied stress. The finite element simulation results based on bilinear cohesive element model showed that transverse fracture of composite interface was shear failure mode, which agreed well with the test results. Before the occurrence of non-linearity in the stress-strain curve, the crack initiated at the circular interface between reaction layer and carbon coating with a 40°~50° angle to the applied loading direction. Crack initiation locations in test samples were different with those in simulation samples, because the actual composite interface was rough and some micro-flaws formed in the interface, whereas it was assumed to be an ideal rigid interface for simulation. Then the crack propagated along both circumferential and axial directions because of the shear stress. With the crack growing, the interface close to 0°angle to the applied loading direction failed first caused by the radial tensile stress, whereas the interface near 90° failed later as a result of circumferential shear stress. After complete failure of the interface, stress redistribution occurred around the SiC fiber and the interface separation increased with the increasing of the applied load, which gave rise to the yielding and deforming of the matrix near fiber until the final fracture of the composite.

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High-Resolution Transmission Electron Microscopic Study of Various Borides Precipitated in Superalloys
Xiuliang MA, Xiaobing HU
Acta Metall Sin    2018, 54 (11): 1503-1524.   DOI: 10.11900/0412.1961.2018.00342
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Microelement B is widely added into almost all commercial superalloys because B contributes to strengthening grain boundaries at high temperature during service. Generally, B is present in two different forms. Besides the solute state in matrix, B tends to react with transition elements at high temperatures, giving rise to various borides including M2B, M3B2 and M5B3 phases. An accurate knowledge of the microstructural characterizations of these borides is of great importance for a better understanding of the structure-property relationship and designing materials with improved properties. By means of various advanced techniques based on the aberration-corrected transmission electron microscopy (TEM), microstructural features of above borides have been systematically investigated. Various defect features which were controversial in the past have been clarified. In this paper, after a brief review on the studies of borides, the atomic-scale information on the microstructural features has been presented. Finally, some prospects for future studies have been proposed.

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Progress in Interface Modification and Nanoscale Study of Diamond/Cu Composites
Di ZHANG, Mengying YUAN, Zhanqiu TAN, Ding-Bang XIONG, Zhiqiang LI
Acta Metall Sin    2018, 54 (11): 1586-1596.   DOI: 10.11900/0412.1961.2018.00355
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Due to the superiority in high thermal conductivity, low thermal expansion and good resistance from heat and corrosion, diamond/Cu composites show great prospect in thermal management applications. However, the thermal properties of diamond/Cu composites are impeded by their interface incompatibility. Interface modification is an effective method to enhance interfacial bonding and reduce interfacial thermal resistance. Based on the principles and factors related with interface design, this paper briefly reviewed some hot topics in diamond/Cu composites, including the main research progress, issues remained to be solved and nanoscale interface design with layer thickness lower than 200 nm, and its prospect of the future development.

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Microstructures and Growth Orientation of Directionally Solidification Mg-14.61Gd Alloy
Yan YANG, Guangyu YANG, Shifeng LUO, Lei XIAO, Wanqi JIE
Acta Metall Sin    2019, 55 (2): 202-212.   DOI: 10.11900/0412.1961.2018.00053
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As one of the most promising heat-resistant magnesium alloys, Mg-Gd series alloy has a wide application prospect in the industrial fields of aerospace, cars, and rail transit. There have been extensive researches on the performance improvement of Mg-Gd series alloys. As known, dendrites are the common solidification microstructures of castings of magnesium alloys, and solidification conditions have a significant effect on dendrite morphologies and growth orientation, which could strongly affect the mechanical properties of castings, thus it is critical to study the grain growth regularity for predicting the performance of magnesium castings. However, there are few studies on numerical simulation of dendrite growth process and growth orientation of magnesium alloys. Solidification behavior of magnesium alloys can be scientifically studied via directional solidification technology, and cellular automaton finite element (CAFE) method should be effective to simulate the dendrite growth process of magnesium alloys. In present work, microstructures and growth orientation of directionally solidified Mg-14.61Gd alloy under the temperature gradient G=30 K/mm and the withdrawal rate v=10~200 μm/s were investigated by EBSD measurement method and CAFE numerical simulation method. It was found that α-Mg primary phase presented unidirectional dendritic morphologies on longitudinal cross-section. The growth interface appearance of α-Mg changed from the protruding forward growth to the flat growth gradually and the dendritic arm spacing decreased gradually with the increasing v. when v increased from 10 μm/s to 100 μm/s, the main growth orientation of α-Mg changed from <1120> and <1010> to <1120>, and the deviation angle (θ) from solidification heat flow direction reduced from 11.0° to 5.7°, the reason for this lied mainly in the change of the heat flux. Further increasing v up to 200 μm/s, the main growth direction of α-Mg was still in <1120>, but the value of θ increased to 10.6°, and the anisotropy of the crystal was the dominant factor then. It was proved that the CAFE numerical simulation model could predict the grain structure and growth orientation reasonably for Mg alloy.

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TEM Analysis of Microstructure Evolution Process of Pure Tungsten Under High Pressure Torsion
Ping LI, Quan LIN, Yufeng ZHOU, Kemin XUE, Yucheng WU
Acta Metall Sin    2019, 55 (4): 521-528.   DOI: 10.11900/0412.1961.2018.00165
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Refractory metal tungsten has wide applications in many fields such as aerospace, national defense, military and nuclear industry due to its excellent comprehensive mechanical properties. As the demand for high-performance materials in the new era is increasing, existing materials cannot meet the performance requirements under extreme conditions. The high pressure torsion (HPT) process can produce severe shear deformation and densify the material effectively, leading to ultrafine-grain structure with non-equilibrium grain boundaries and having a significant effect on improving the overall performance of pure tungsten materials. HPT process is used to prepare an ultrafine-grain material with excellent comprehensive performance, which can broaden the application field of refractory metal tungsten and promote the engineering application of high-performance materials. The HPT experiment was carried out on commercial pure tungsten at a relatively low temperature, and the microstructure evolution during HPT processing at various turning numbers has been investigated by means of EBSD, TEM and HRTEM. It was found that with the strain increasing, the grains were refined significantly, dislocation density and the ratio of non-equilibrium grain boundary increased obviously. Moreover, it was transparent that the low angle grain boundary transform into high angle grain boundary during HPT processing. At the same time, the dislocation structure moved to grain boundary gradually so that there was no obvious defect in fined grains. When the equivalent strain increased to 5.5, the deformation mode of grains transformed from intracrystalline sliding to grain boundary sliding, because the size of some grains was close to the mean free path of dislocation.

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Stress Relaxation and Elastic Recovery of Monocrystalline Cu Under Water Environment
Junqin SHI,Kun SUN,Liang FANG,Shaofeng XU
Acta Metall Sin    2019, 55 (8): 1034-1040.   DOI: 10.11900/0412.1961.2019.00041
Abstract   HTML PDF (8286KB)  

The stress relaxation and elastic recovery have an important effect on the mechanical and electrical properties of metallic crystal materials, which restricts the range of application and working life of materials. However, during plastic deformation of materials, the relaxation and elastic recovery behaviors are still not very clear at the nanoscale. In this work, the stress relaxation and elastic recovery of monocrystalline Cu under water environment is studied by molecular dynamics simulation. The results indicate the stress acting on Cu surface decreases at constant strain, meaning the occurrence of stress relaxation phenomenon. The stress relaxation increases with water film thickening compared with no-water environment. The separation between Cu atoms dramatically decreases with the increasing indentation depth at indenting stage, and there is no clear change in the nearest interatomic separation at stress relaxation stage, but the separation increases rapidly due to the release of elastic energy and dislocation energy at the unloading stage. The nucleated dislocations within Cu coated by water film are obviously more than that without water, which suggests the water film increases the unrecovered deformation in the total nanoindentation process. During unloading, partial dislocations disappear because of the deformation energy release, while the water film impedes the elastic recovery and plastic release.

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Influence of Size Factor on Calculation Accuracy of Welding Residual Stress of Stainless Steel Pipe by 2D Axisymmetric Model
Peiyuan DAI,Xing HU,Shijie LU,Yifeng WANG,Dean DENG
Acta Metall Sin    2019, 55 (8): 1058-1066.   DOI: 10.11900/0412.1961.2018.00567
Abstract   HTML PDF (7704KB)  

Austenitic stainless steel, owing to its good mechanical properties and excellent corrosion resistance, is widely used in petroleum, chemical, nuclear power and other fields. Welding is an extremely important manufacturing method in industrial production. When the thermal elastic-plastic finite element method (TEP-FEM) is used to simulate welding residual stress, especially in thick welded joints, a long calculation time is generally needed. Therefore, it has become an urgent problem to develop an efficient and high-precision computational approach to simulate welding residual stress. In this work, numerical simulation and experimental methods were combined to explore the effect of size on the calculation precision of welding residual stress of SUS316 stainless steel by the 2D axisymmetric model, in order to clarify the applicability of 2D axisymmetric model in the prediction of welding residual stress in pipe butt joints. This research can provide theoretical support for the development of computational methods suitable for engineering applications. Based on the finite element software MSC. Marc, the temperature field and welding residual stress distribution of three different sizes of pipes were calculated by 2D axisymmetric model and 3D model. The calculated residual stress distributions in the thin pipe model are compared with the experimental measurements. The results show that calculated residual stress by the 2D axisymmetric model agrees well with the 3D model. However, in the weld seam near the inner surface and the areas near the weld seam, a deviation on the residual stress distribution between in the 2D axisymmetric model and in the 3D model was observed, which is significant as the pipe size increases. For practical engineering applications, with the regardless of the stress problems at the beginning and end positions, the 2D axisymmetric model can be used instead of the 3D model to calculate the residual stress of the girth weld, which is very beneficial to calculation time saving.

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Microstructure and Texture Evolution of AZ31 Mg Alloy Processed by Multi-Pass Compressing Under Room Temperature
Liping DENG,Kaixuan CUI,Bingshu WANG,Hongliang XIANG,Qiang LI
Acta Metall Sin    2019, 55 (8): 976-986.   DOI: 10.11900/0412.1961.2019.00050
Abstract   HTML PDF (15871KB)  

Mg alloy has hexagonal structure and exhibits poor workability at room temperature, which is attributed to the difficulty in activating a sufficient number of independent slips to accommodate the deformation. Twinning plays an important role in plastic deformation of Mg alloys during low and medium temperature to accommodate the imposed strain, especially the strain along the c-axis. Therefore, the microstructure and texture evolutions of AZ31 Mg alloy during multi-pass compressions at room temperature were investigated by EBSD technology. The results show that the microstructure and texture evolutions are mainly controlled by tension twinning during multi-pass compression. And the more the strain passes, the severer the texture transformation. The c-axes of the grains are almost rotated to the compression direction by tension twins. The twins generated during multi-directional compression can separate grains and then refine them. However, the de-twinning can rotate the grains back to the initial orientations, which is against the texture weakening. The Schmid law governs the characteristics of {101ˉ2} twinning, and thus controls the texture evolution. Both the residual matrix and the pre-deformation induced twins intersect with the twins generated during subsequent deformation. And this can separate the grains and weaken the texture strength. The number and morphology of the activated twin behavior during multi-pass compression would be influenced by the pass reductions, consequently affecting the grain refinement.

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