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

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    Orginal Article
    Double Extension Twin and Its Related CompoundTwin Structures in Mg
    Zhangzhi SHI, Xuefeng LIU
    Acta Metall Sin, 2018, 54 (12): 1715-1724.  DOI: 10.11900/0412.1961.2018.00291
    Abstract   HTML   PDF (3832KB) ( 481 )

    This paper summarizes recent research progresses on {10$\bar{1}$2}-{10$\bar{1}$2} double extension twin and its related compound twin structures in Mg. Tension-compression asymmetry of Mg with strong texture can be greatly alleviated through sequential multi-directional deformations, which consist of several sequential bi-axial deformations. There exist 36 possible double extension twin variants, which can be classified into four misorientation groups according to their misorientations with respect to the grain matrix. One of the groups appears with a much higher frequency than the others, which cannot be perfectly explained by Schmid factor (SF) rule. Primary and secondary extension twins form intergranular and intragranular compound twin structures without any one-for-all mechanism. SF rule and m' factor, which evaluates how much twinning shear can pass through an interface, partly or even totally fail to explain the formation of the compound twin structures, presenting challenge to make clear mechanism of twin formation under complex loading conditions. It is suggested that modelling on the formation of intragranular compound twin structure and experimental characterization of interfacial structures of primary twin-twin boundary and secondary twin boundary should be paid much attention in the future.

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    Microstructure and Mechanical Properties of Additive Manufactured 2319 Alloy by Electron BeamFreeform Fabrication
    Jing YU, Jijie WANG, Dingrui NI, Bolv XIAO, Zongyi MA, Xinglong PAN
    Acta Metall Sin, 2018, 54 (12): 1725-1734.  DOI: 10.11900/0412.1961.2018.00052
    Abstract   HTML   PDF (10373KB) ( 624 )

    Aluminum alloys have the advantages of light weight and high strength, and they are important structural materials in aerospace field. The additive manufacturing technology of aluminum alloys has a potential application prospect in the field of on-orbit manufacturing in the future, and the technology of electron beam fuse deposition is the best process selection due to its unique technical advantages. In the present study, 2319 aluminum alloy wires with diameter of 2 mm were used for additive manufacturing (AM) by electron beam freeform fabrication (EBF3), with a sample of 150 mm×35 mm×52 mm being printed. The microstructure and mechanical properties of the printed sample in three directions were investigated. The results showed that bulk materials of the 2319 alloy can be printed without macroscopic defects under selective EBF3 parameters, with a relative density of 99.3% compared to the initial wires. The average grain size of the printed sample was less than 10 μm, containing primary Al2Cu phases, fine particles, and coarse impurity phases. There are some tiny voids in the printed sample, and the sizes of the voids are 5~15 μm. The ultimate tensile strengths of the printed sample were 161, 174 and 167 MPa in the length, width and height directions. After a T6 treatment, the coarse phase were basically dissolved and some finer phases were re-precipitated. Due to the dominant effect of dispersion strengthening, the mechanical properties of the sample were significantly improved, and the ultimate tensile strengths of the sample in three directions were increased to 423, 495, and 421 MPa, respectively.

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    Research on Prediction Method of Liquation Cracking Susceptibility to Magnesium Alloy Welds
    Shujun CHEN, Xuan WANG, Tao YUAN, Xiaoxu LI
    Acta Metall Sin, 2018, 54 (12): 1735-1744.  DOI: 10.11900/0412.1961.2018.00151
    Abstract   HTML   PDF (10112KB) ( 362 )

    Magnesium alloys has a wide application prospect due to their good properties, such as high specific strength and specific stiffness, but the susceptibility of liquation cracking is also pretty high. The liquation in partially melted zone of AZ-series magnesium alloys were investigated with circular-patch welding test. The AZ91, AZ31 base alloys were welded with AZ61 and AZ92 filler wires by using the cold metal transter metal inert-gas (CMT-MIG) welding. The results show that, the liquation occurred along the weld edge of AZ91 with the eutectic reaction occurring between γ (Mg17Al12) phase and Mg-rich phase. The liquation susceptibility of AZ31 was pretty low as γ (Mg17Al12) was not present in base metal of AZ31. Meanwhile, a new method for predicting liquation cracking based on binary phase diagram was proposed. When the initial solidification temperature of weld is higher and the solidification temperature range of weld is shorter than those of base metal, the liquation crack susceptibility of weld is mostly higher. When the initial solidification temperature of weld is close to or below that of base metal, and the solidification temperature range of weld is close to or longer than that of base metal, the liquation cracking susceptibility of weld is lower. This method worked well on predicting the effect of composition of base metal and filler wires on liquation cracking, and the predicting results are consistent with the experimental results. That is, the liquation cracking susceptibility is higher with AZ91 base metal used than that with AZ31 base metal. And, the liquation cracking susceptibility is lower with AZ92 filler wire than that with AZ61 filler wire.

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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) ( 398 )

    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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    Phase Transformation and Texture Evolution During Cold Rolling and α'-M Reversion in High Manganese TRIP Steel
    Lina WANG, Ping YANG, Kai LI, Feng'e CUI, Weimin MAO
    Acta Metall Sin, 2018, 54 (12): 1756-1766.  DOI: 10.11900/0412.1961.2018.00222
    Abstract   HTML   PDF (11916KB) ( 489 )

    To meet the requirement of environment, economy and safety, advanced high strength steels including dual phased (DP), complex phased (CP), transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) steels are widely used for automotive steel. Among them, high manganese TWIP and TRIP steels are particularly appealing due to their outstanding tensile strength and elongation. In contrast to high manganese TWIP steel, high manganese TRIP steel exhibits higher strength and work hardening rate due to strain induced martensitic transformation. The enhanced mechanical properties of high manganese TRIP steel are determined by both the stability of the retained austenite (γ ) and the initial microstructure. Strain induced martensitic transformation and subsequent reversion from deformed martensite to γ during annealing is often applied as one of the most effective methods for microstructure improvement. Microstructure and texture characteristics of high manganese TRIP steel during cold rolling together with the reversion of deformed bcc martensite (α'-M) at high temperature were investigated. It is shown that the γ was almost completely transformed into α'-M at medium cold rolling reduction. And a higher reduction after α'-M saturation resulted in dominantly the deformation of α'-M, hence thin laths paralleled to the rolling direction (RD) were obtained. The main components in α'-M were {113}<110>, {554}<225> and rotated cube ({001}<110>) textures at medium cold rolling reduction, which are the typical phase transformation textures. The {113}<110> texture rotated toward a more stable orientation {223}<110> and led to a strong cold rolling texture (<110>//RD) with increasing reduction. The reversion of martensite and recrystallization of γ proceeded at temperature ranging from 650 ℃ to 850 ℃. The reversion of α'-M proceeded in a diffusional mechanism, accompanying with the redistribution of Mn and Al between γ and α'-M. Deformed α'-M was merged by the adjacent γ , and columnar γ grains with a large amount of subgrains were obtained. The texture of reverted γ was approximately the same as that of the deformed γ , this phenomenon called texture inheritance was formed by the direct growth of γ . Subsequently, recrystallization of γ grains occurred by sub-grain coalescence and the columnar γ grains were instead by equiaxed γ grains.

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    Influence of Inter-Pass Temperature on Residual Stress in Multi-Layer and Multi-Pass Butt-Welded 9%Cr Heat-Resistant Steel Pipes
    Lei HU, Xue WANG, Xiaohui YIN, Hong LIU, Qunshuang MA
    Acta Metall Sin, 2018, 54 (12): 1767-1776.  DOI: 10.11900/0412.1961.2018.00051
    Abstract   HTML   PDF (10027KB) ( 338 )

    9%Cr heat-resistant steels have been abundantly used in boilers of modern thermal plants. The 9%Cr steel components in thermal plant boilers are usually assembled by fusion welding. Many of the degradation mechanisms of welded joints can be aggravated by welding residual stress. Tensile residual stress in particular can exacerbate cold cracking tendency, fatigue crack development and the onset of creep damage in heat-resistant steels. It has been recognized that welding residual stress can be mitigated by low temperature martensitic transformation in 9%Cr heat-resistant steel. Nevertheless, the stress mitigation effect seems to be confined around the final weld pass in multi-layer and multi-pass 9%Cr steel welded pipes. The purpose of this work is to investigate the method to break through this confine. Influence of martensitic transformation on welding stress evolution in multi-layer and multi-pass butt-welded 9%Cr heat-resistant steel pipes for different inter-pass temperatures (IPT) was investigated through finite element method, and the influential mechanism of IPT on welding residual stress was revealed. The results showed that tensile residual stress in weld metal (WM) and heat affected zone (HAZ), especially the noteworthy tensile stress in WM at pipe central, was effectively mitigated with the increasing of IPT. The reasons lie in two aspects, firstly, there is more residual austenite in the case of higher IPT, as a result, lower tensile stress is accumulated during cooling due to the lower yield strength of austenite; secondly, the higher IPT suppresses the martensitic transformation during cooling of each weld pass, thus the tensile stress mitigation due to martensitic transformation was avoided to be eliminated by welding thermal cycles of subsequent weld passes and reaccumulating tensile residual stress. The influence of IPT on welding residual stress relies on the combined contribution of thermal contraction and martensitic transformation. When the IPT is lower than martensite transformation finishing temperature (Mf), thermal contraction plays the dominant role in the formation of welding residual stress, and tensile stress was formed in the majority of weld zone except the final weld pass. While, compressive stress was formed in almost whole weld zone due to martensitic transformation when the IPT is higher than martensite transformation starting temperature (Ms).

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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) ( 583 )

    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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    Out-of-Plane Constraint Effect on the Fracture Toughness of Single Edge Notch Tension Specimens
    Yizhe LI, Baoming GONG, Xiuguo LIU, Dongpo WANG, Caiyan DENG
    Acta Metall Sin, 2018, 54 (12): 1785-1791.  DOI: 10.11900/0412.1961.2018.00122
    Abstract   HTML   PDF (6674KB) ( 534 )

    The crack-tip stress and strain fields of single edge notch tension (SENT) specimen are similar to those of the full-scale pipe containing surface cracks under longitudinal tension and/or internal pressure. It is well known that material's fracture toughness is not constant, and the specimen size has a significant influence on fracture toughness. It is thus essential to consider the transferability from fracture specimens in laboratory testing to practical structures, i.e., size effects or constraint effects. However, the specimen dimensions for SENT specimens recommended by current design procedures have not validated the out-of-plane constraint effect on the fracture toughness. In this work, the effect of specimen thickness on the crack tip opening displacement (CTOD) of SENT specimen was investigated using an API X90 grade steel. Full-field deformation measurement by digital image correlation (DIC) technique and stretching zone width (SZW) examination were performed to analyze the size effects on fracture toughness. The results show that the critical crack initiation toughness is highly sensitive to specimen thickness, and decreases significantly as specimen thickness increases until the thickness-to-width ratio (B/W) equals to 4, beyond which the effect of specimen thickness becomes relatively weak. As the specimen thickness increases, the maximum longitudinal strain and stretching zone width decrease sharply, and the location of high-strain zones changes significantly; when B/W≥3, strain is initiated from the area opposite the cracked side rather than from the crack tip, indicating a strong loss of plasticity for thicker specimens. A dimension size is recommended for the fracture toughness testing to take the out-of-plane constraint into account for SENT specimen.

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    High-Temperature Oxidation Resistance of Mo-Si-B Alloys with Different B Contents
    Bin LI, Xiaohui LIN, Rui LI, Guojun ZHANG, Laiping LI, Pingxiang ZHANG
    Acta Metall Sin, 2018, 54 (12): 1792-1800.  DOI: 10.11900/0412.1961.2018.00015
    Abstract   HTML   PDF (7802KB) ( 327 )

    The oxidation behaviors of the Mo-Si-B alloy with B content in the range of 5% to 17% (atomic fraction) were experimentally investigated at temperatures ranging from 1000 ℃ to 1300 ℃. The microstructures and antioxidant mechanisms were also analyzed. Results showed that the oxidation behaviors were affected by both B content and oxidation temperature. The formation and growth process of oxidation film were mainly influenced by the B element which could improve the fluidity of surface glass phase and adjust the volume fraction and microstructure of α-Mo, Mo3Si and Mo5SiB2. The Mo-Si-B alloy with the B content increasing was favourable for quick forming and uniform covering by improving the mobility of the glass, but which decreased the oxidation resistance due to the sufficient liquidity of the oxidation film at high temperature. The oxidation resistance of the Mo-Si-B alloy is controlled by B content at low temperature and α-Mo content at high temperature, respectively. A large quantity of Mo5SiB2 phase and a small quantity of α-Mo phase existed in the high B content of Mo-12Si-17B alloy, which could promote the oxide layer to form rapidly but also cover uniformly under the temperature range of 1000~1300 ℃. The discussion illustrates that the fine-grained microstructure combining with the distributed intermetallics is a specific role to ensure the excellent oxidation resistance of Mo-Si-B alloy.

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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
    Abstract   HTML   PDF (3436KB) ( 252 )

    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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    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
    Abstract   HTML   PDF (5177KB) ( 336 )

    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 Temperature Deformation Behavior of High Strength and Toughness Ti-Ni Base Bulk Metallic Glass Composites
    Yanchun ZHAO, Hao SUN, Chunling LI, Jianlong JIANG, Ruipeng MAO, Shengzhong KOU, Chunyan LI
    Acta Metall Sin, 2018, 54 (12): 1818-1824.  DOI: 10.11900/0412.1961.2018.00256
    Abstract   HTML   PDF (3456KB) ( 362 )

    Room-temperature brittleness and strain-softening during deformation of bulk metallic glasses, and limited processability of shape memory alloys have been stumbling blocks for their advanced functional structural applications. To solve the key scientific problems, a new shape memory bulk metallic glass based composite, through the approach using transformation-induced plasticity (TRIP) effect of shape memory alloys to enhance both ductility and work-hardening capability of metallic glasses, and superplasticity of bulk metallic glass in supercooled liquid region to realize near net forming, was developed in this work. And the Ti-Ni base bulk metallic glass composites (BMGCs) rods were prepared by the levitation suspend melting-water cooled Cu mold process. Microstructure, thermal behavior, mechanical properties and high temperature deformation behavior of the alloy were investigated. The results show that the as-cast alloy microstructure consists of amorphous matrix, undercooled austenite and thermally-induced martensite. Besides, the size of the crystal phase precipitated on the amorphous matrix increases from the surface to the inside. The alloy exhibits excellent comprehensive mechanical properties at room temperature. The yield strength, fracture strength and the plastic strain of alloy are up to 1286 MPa, 2256 MPa and 12.2%, respectively. Under compressive loading in the supercooled liquid region, the composite exhibits approximate Newtonian behavior at lower strain rate in higher deformation temperature, and the optimum deformation temperature is T>480 ℃ and the intersection part with supercooled liquid region (SLR). When the temperature is 560 ℃ and the strain rate is 5×10-4 s-1, the stress sensitivity index m and the energy dissipation rate ψ are 0.81 and 0.895, respectively. Furthermore, the volume of activation is quantified to characterize the rheological behavior.

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    Protection Mechanism Study of Enamel-Based Composite Coatings Under the Simulated Combusting Gas Shock
    Cean GUO, Minghui CHEN, Yimin LIAO, Bei SU, Dongbai XIE, Shenglong ZHU, Fuhui WANG
    Acta Metall Sin, 2018, 54 (12): 1825-1832.  DOI: 10.11900/0412.1961.2018.00230
    Abstract   HTML   PDF (4520KB) ( 326 )

    High-temperature-resistant enamel coatings have been reported to be applied in non-critical hot end components of aero-engine and gas turbine recently. Although the enamel with a series of excellent properties can be as high-temperature-resistant coating material under appropriate condition, its lower soft point and inherent brittleness limit their use in broader application under severe service condition. Enamel-based composite coatings (an enamel matrix with the addition of ceramic particles and/or metal platelets) can remarkably increase the properties of the enamel coating and their protection mechanism under dynamic thermal shock needs further investigation. In this work, two kinds of enamel-based composite coatings, 70%enamel+25%Al2O3 and 70%enamel+20%Al2O3+10%NiCrAlY (mass fraction, %) abbreviated to E25A and E20A10M respectively, were designed and fired on K38G superalloy substrate, and their protection mechanism was comparatively studied at 900 ℃ under the simulated combusting gas shock. The thermal shock fire was produced by the mixture gas of C3H8+O2 and its ejecting pressure on the coating surface was 0.4 MPa. After the temperature has been stable at 900 ℃, samples were hold for 15 s and then cooling down in air for 120 s, constituting a thermal shock cycle. Results indicated that, after 150 cyc of thermal shock, both the coatings bond well with the alloy substrate, thus shows high resistance to spallation along interface. For the E25A coating, its microstructure had no obvious change after thermal shock and the surface is still intact. The addition of secondary phase Al2O3 increases the stability of enamel at high temperature. With regard to the E20A10M coating, holes and cracks form consecutively, and peeling off occurs at surface after thermal shock. Interfacial reaction between the NiCrAlY particles and enamel following Cr(NiCrAlY)+ZnO(enamel)→CrO(interface)+Zn↑ results in the formation of enamel swelling, which then, under the synergistic effect of combusting gas shear stress and interface thermal stress, leads to the peeling off of enamel and metal inclusions at surface.

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    Preparation and Performance of Spherical Ni Powder for SLM Processing
    Yajuan ZHANG, Haibin WANG, Xiaoyan SONG, Zuoren NIE
    Acta Metall Sin, 2018, 54 (12): 1833-1842.  DOI: 10.11900/0412.1961.2018.00153
    Abstract   HTML   PDF (7956KB) ( 432 )

    3D printing has attracted increasing interests in the field of metallic materials as it can effectively shorten the production cycle and create parts with complex shapes, which can hardly be produced by traditional methods. However, the gas atomization, as the mainstream method of preparing metal and alloy powders to meet the requirements of the processing of selective laser melting (SLM) at present, still has some limitations, such as hollow and/or satellite balls in the powder. This influences directly the density and performance of the printing parts. Moreover, the laser absorption in the smooth surface of powder particle is generally less than 10% in the laser processing, which hinders rapid heating of the powder. It has been found that the material can obtain multiple absorption of laser energy by increasing the surface roughness of powder particles, which can effectively improve the laser absorption rate and is beneficial to get the dense printing parts. Based on this, a novel method combining low temperature spray-drying with heat treatment was developed to prepare Ni powder with high purity, good sphericity, high flowability and narrow particle size distribution. The microstructure and laser absorptivity of the prepared Ni powder were compared with those of the commercial Ni powder prepared by gas atomization, and their influences on the microstructure and properties of the 3D printed bulk materials were investigated. It is found that the laser absorptivity of the Ni powder prepared by spray-drying is more than 2 times as high as that of the commercial Ni powder. This leads to a wider melting channel, smaller surface tension and liquid-bridging force between particles in the printing process. As a result, the spheroidization phenomenon occurred on the surface of the printed bulk material can be avoided by the use of the spray-dried powder, and the relative density is achieved as 99.2% at the as-printed state. In the microstructure of the printed bulk material, in addition to the cellular crystals, there are a number of fine columnar crystals, grown across the interlaminar boundaries, which is favorable for a high bonding strength between the interlayers.

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