Ce microalloyed H13 hot die steel is widely used in manufacturing hot extrusion and die casting mold of magnesium-aluminium alloy because of its excellent combination of hot strength and impact toughness. The C content in Ce microalloyed H13 steel is approximately 4% (mass fraction), and the alloy elements content such as Cr, Mo, V et al are about 8%. Therefore, it is easy for primary carbide to precipitate during the solidification of the molten steel due to the segregation of alloy elements. Most researchers study the precipitation mechanism of primary carbide in the two-dimensional perspective. A few people are involved in the three-dimensional morphology of the primary carbide, especially the thermal stability of the primary carbide in the three-dimensional perspective in the Ce microalloyed H13 steel. Therefore, the precipitation mechanism and thermal stability of the primary carbide were systematically studied in this work. First, the SEM and inclusion automatic analysis system were used to analyze the morphology, number density and size of the inclusions in Ce microalloyed H13 steel. The three-dimensional morphology of the primary carbide in sample was observed after electrolyzed in a non-aqueous solution. The voltage was 20 V, the electrolysis time was about 3 min and the electrolyte was composed of 1% tetramethylammonium chloride, 10% acetylacetone, and 89% methanol (volume fraction). Three samples were heated to 1150, 1200 and 1250 ℃ for 1 h to investigate the thermal stability of the primary carbide. Finally, Factsage 7.2 software was used to calculate the precipitation mechanism and thermal stability of the primary carbide. Elemental Ce can effectively react with O, S, P and As elements to form the corresponding Ce-O, Ce-S and Ce-P-As inclusions. There is a huge difference between the two-dimensional and three-dimensional morphologies of the primary carbide, the two-dimensional morphology is strip and the three-dimensional morphology is irregular flake. Ti-V-rich carbide precipitates first, and then acts as the nucleation core of V-rich carbide. When the heating temperature reaches 1250 ℃, the V-rich carbide has completely dissolved, and the Ti-V-rich carbide begins to dissolve. The three-dimensional morphology of the wrapped Ti-V-rich carbide is completely exposed after the V-rich carbide disappears completely. The Ce-O inclusion is formed before solidification, and the primary carbide precipitates at the end of the solidification of molten steel. As the Ce content in molten steel increases, the stability diagram of Ce2O2S and Ce-S increases gradually. The precipitation temperature of Ti-V-rich carbide is approximately 1360 ℃, and the V-rich carbide starts to precipitate at about 1200 ℃. The calculated results are keeping well with the experimental observations. The damage of primary carbide in Ce microalloyed H13 steel can be partly reduced by higher heating temperature, but cannot be completely removed.
Duplex stainless steel (DSS) is a type of steel with ferritic-austenitic duplex structure. It has been widely used in the engineering field such as petrochemicals and oceans. Recently, a series of economical DSSs with TRIP effect have been developed by replacing Ni-Mo with Mn-N. Generally, most structural components are subjected to periodic alternating loads during service, and then cyclic deformation which causes different mechanical responses with monotonous loading condition occurs. In this work, the mechanical properties of a Mn-N bearing DSS Fe-19.6Cr-2Ni-2.9Mn-1.6Si during cyclic deformation condition were studied and the microstructural mechanism was characterized by TEM. The results show that the Fe-19.6Cr-2Ni-2.9Mn-1.6Si steel exhibits enhanced mechanical properties and a typical "three-stage" hardening characteristic due to TRIP effect under monotonic loading condition. Cyclic hardening/softening characteristics of the Fe-19.6Cr-2Ni-2.9Mn-1.6Si steel are sensitive to strain amplitude and the number of cycle (N). At a small strain amplitude, cyclic hardening occurs firstly when N<5 cyc, then cyclic softening starts and cyclic deformation gradually trends to a stabilization. At a large strain amplitude, after a rapidly cyclic hardening (N<5 cyc), the Fe-19.6Cr-2Ni-2.9Mn-1.6Si steel is continuously softened until failure and no stabilization occurs. The dislocation walls form in ferrite during cyclic deformation which responsible for the overall cyclic softening of the Fe-19.6Cr-2Ni-2.9Mn-1.6Si steel; While austenite undergoes cyclic induced ε martensite transformation at large strain amplitude whereby the softening is suppressed, so that the cyclic softening rate of the Fe-19.6Cr-2Ni-2.9Mn-1.6Si steel increases rapidly with the increase of the (plastic) strain amplitude, followed by a slow increase and a final decrease. Compared with the monotonous loading condition, the Fe-19.6Cr-2Ni-2.9Mn-1.6Si steel shows a law of "hardening→softening→ re-hardening" with the increase of strain amplitude. In particular, there is a three-stage linear relationship between logarithmic cyclic stress amplitude and logarithmic plastic strain amplitude (lgσa-lgεa), and the corresponding cyclic hardening index (n') are: 0.16 (stage I), 0.09 (stage II) and 0.17 (stage III), respectively. The change of n' in each stage is related to the coordinated deformation between two phases (I→II) and the cyclic induced ε martensitic transformation (II→III).
Micro/nano-structured bainitic steel provides a unique combination of ultra-high strength and high ductility due to their structure consisting of micro/nano-scale bainitic-ferrite and retained austenite, but the toughness is a little bit low. The retained austenite plays a leading role for the toughness, and it can significantly increase the toughness of micro/nano-structured bainitic steel by refining the size of blocky retained austenite and improving the content of film retained austenite. Simultaneously, the structure of retained austenite affects the stability of retained austenite, and even can change the micro-deformation and determine the toughness. This work has been refined retained austenite of medium-carbon bainitic steel by using two-step bainitic transformation to study phase transformation of retained austenite through heat treatment. The effect of retained austenite on the impact toughness in medium-carbon micro/nano-structured steels was analyzed by one (300 ℃ for 6 h) and two-step (300 ℃ for 2 h, then 250 ℃ for 24 h) bainitic transformation processes. The microstructure, phase fraction, misorientation, crystallographic grain size and impact energy of different heat treatment steels were observed, detected and analyzed. The results showed that the impact property of two-step bainitic transformation was significantly higher than that of one-step bainitic transformation in medium-carbon steel, which the impact energy in -40 ℃ increased from 31 J to 42 J. The main reason is the new bainitic ferrite was formed in two-step bainitic transformation, the untransformed retained austenite was divided and refined by new bainitic-ferrite, reducing the formation of massive martensite during water quenching after isothermal bainite process. It significantly improve the toughness of the steel because the fracture energy was increased, owing to making crack bifurcation and even preventing the propagation of cracks in the impact process. Through the above-mentioned studies, this research not only precisely refines the retained austenite structure, reveals the effect of retained austenite stability on deformation mechanism and resolves toughness mechanism, but also provides the theoretical guidance for the production of micro/nano-structured bainitic steels in combination with good toughness.
The deformation structures, such as deformation twins, dislocations and kink bands, play an important role in the plasticity of magnesium alloys during the deformation process. However, due to the complexity of hcp structure, the deformation structures of the magnesium alloys, especially the interactions between deformation structures are still not well understood. Thus, it is of great scientific significance to study the microstructure of magnesium alloys, especially to characterize their structural characteristics of the interaction areas, which plays a significant role in understanding the structure and performance relationships of magnesium alloys. In this work, a combination of TEM and SAED pattern was applied to study the interaction mechanism associated with different kinds of deformation structures in Mg-Al-Zn (AZ31) alloy. When the applied external force is not beneficial for deformation twins and dislocations, kink bands act as a supplementary deformation mode to coordinate the asymmetry of hcp structure. According to crystallographic analysis, it is found that under the action of tensile stress nearly lie on basal plane in hcp structures, the basal dislocation pairs form and move to the opposite directions, forming tension kink band with the interface of {101ˉ2} plane. The angle between the tension kink band interface and the basal plane is about 43°. The tension kink bands can further contribute to the strength and toughness of the material. These results will open a new insight into the understanding of interaction mechanism of deformation structures and greatly promote the development of Mg alloys.
Button ingots were prepared by arc melting and feed bars for directional solidification were prepared by optimized drop casting technique. The Ti-43Al-3Si directionally solidified bars with lamellar boundaries perpendicular to the growth direction were prepared at the growth rate of 180 mm/h in an optical floating zone furnace. Cylindrical sections were cut from these perpendicular lamellae with appropriate direction and then fixed on polycrystalline TiAl bars by mechanical setting method to serve as the initial seeds. The ultimate Ti-43Al-3Si seeds with parallel lamellar microstructure were successfully prepared from these initial seeds at the growth rate of 5 mm/h. To avoid the nucleation of stray grains, drop-cast bars with the shape of frustum of a cone were used for the preparation of ultimate seeds. At the growth rates of 5 and 180 mm/h, the primary phase of Ti-43Al-3Si alloy was always the α phase. Polysynthetically twinned (PST) crystals of Ti-47Al alloy were obtained from the Ti-43Al-3Si ultimate seeds and the seeding process was studied by microscopic analysis. Lamellar microstructure of the seed kept stable and recrystallization of the seed was not found. Lamellar orientation of Ti-47Al PST crystals was successfully controlled by the ultimate Ti-43Al-3Si seed.
Ni-based single crystal (SX) superalloys are widely used for production of blades in gas turbines and aircraft engines for their superior mechanical performance at high temperatures. In this work, the formation and evolution of low angle grain boundary (LAGB) of a SX blade during directionally solified (DS) process were investigated by EBSD and XCT. It indicated that the alignment of dendrites was deteriorated with the increasing of the height along the growth direction during the DS process of SX blade. LAGBs were found in the SX blade. The misorientation angle and the frequency of LAGB were obviously enhanced with the increasing of the distance away from the initiated location of LAGB. Crystal orientation measurements showed that the orientation distribution of dendrites in the extended zone was concentrated, while the dispersion of dendrite orientation in the blade body increased, but it was still around that of the extended zone. The reason for the formation of LAGB may be related to the shell hindering the melt shrinkage which resulted in the production of the force and then led to the rotation of secondary dendrites. Larger size voids on the surface would be beneficial to the formation of LAGB. In addition, it was found that dendrites with the orientation approching [001] eliminated the stray grains between them and impinged to form LAGB.
Magnesium alloys are well known for their low density, high specific strength. However, they are often limited by unsatisfactory mechanical properties. To meet the challenge of growing demand for light structural applications, metal matrix composites (MMCs) have attracted more attention. Carbon nanotubes (CNTs) have attracted much attention as the ideal reinforcements for MMCs due to their excellent mechanical strength and Young's modulus. In this work, 0.1%CNTs/AZ91 (mass fraction) magnesium matrix composites were prepared by low temperature powder metallurgy and hot extrusion. The magnesium alloy and composites were observed and analyzed by SEM, XRD and TEM. The room temperature mechanical properties of the composites were tested by Instron 5982 machine. The results showed that the CNTs distributed uniformly in the composites. The CNTs have an effect on reducing grain size, promoting precipitation of β-Mg17Al12 and weakening basal texture. The compressive strength and yield strength of the composites reached 617 and 445 MPa, which increased by 8.8% and 7.2%, respectively. The tensile strength and yield strength were 393 and 352 MPa, which 4.5% and 6.0% MPa higher than the matrix, respectively. It can be found that fine grain strengthening and load transfer play a leading role in improving the strength in the 0.1%CNTs/AZ91 magnesium matrix composites.
Laser additive manufacturing technology is a feasible technology for the fabrication of bulk metallic glass with complex geometry. It has the characteristics of small molten pool and high cooling rate. However, crystallization often occurs in heat affected zone (HAZ). In this work, laser multiple remelting of Zr55Cu30Al10Ni5 metallic glass by pulsed laser was carried out and the morphological evolution of the HAZ crystalline phase in the multiple remelting process was studied. The results show that with the increase of the remelting times, the crystalline grains number and size are both improved. With the growth of the grains, the crystallization caused by the growth of the crystalline grains in the molten pool also becomes more and more remarkable. Both the size and number of the grains in the HAZ increase linearly with the increase of the remelting times. The nucleation rate and growth rate of different metallic glass plates are close, whereas the initial crystalline grains number and size are different, which are attributed to the different cooling process in the copper casting of the metallic glass plates.
Zirconium alloys are widely used as fuel cladding and core structure materials for water-cooled nuclear reactors due to its low thermal neutron absorption cross section, good corrosion resistance, moderate mechanical properties and good compatibility with UO2. Corrosion is one of the main factors affecting the service life of zirconium alloy cladding. The influence of initial corrosion behavior of zirconium alloys and the crystal structure of the oxide film formed at the early stage on the microstructural evolution of the oxide film at the later stage has gradually attracted people's attention. Sn is an important alloying element in zirconium alloys. In order to study the effect of Sn on the initial corrosion behavior of zirconium alloys, coarse-grain TEM thin samples of Zr-0.75Sn-0.35Fe-0.15Cr and Zr-1.5Sn-0.35Fe-0.15Cr (mass fraction, %) zirconium alloys were corroded in 280 ℃, 6.3 MPa and 0.01 mol/L LiOH aqueous solution for short period. In order to ensure the observation of the crystal structure evolution process under the same thickness and grain orientation, the cross-section thin section samples were cut by focused ion beam (FIB), and then the surface and cross-section microstructures of the corroded samples were observed by TEM. Based on the difference in oxygen content caused by the different thickness of the sample around the hole in TEM sample, the effect of Sn on the initial corrosion behavior of zirconium alloy was investigated, as well as the nucleation and growth process of early oxide film. Results showed that the lattice of α-Zr evolved with the increase of oxygen content in the sample from the beginning of oxidation to the formation of ZrO2. The evolution of the oxide layer on the grain oriented [0001] underwent sub-oxide layer, lattice distortion layer and m-ZrO2 layer. Compared with Zr-0.75Sn-0.35Fe-0.15Cr alloy, Zr-1.5Sn-0.35Fe-0.15Cr alloy had a thicker oxide layer in the thin section, a lower proportion of lattice distortion layer and a higher proportion of the m-ZrO2 layer. This illustrates that increasing the Sn content promotes the initial corrosion process of zirconium alloys.
Zr-based amorphous alloys are characterized by high glass forming ability, high thermal stability and excellent mechanical properties. The amorphous alloys in thermodynamic metastable state have the tendency to change to metastable state with lower energy or even crystal structure in equilibrium state under certain temperature or pressure conditions. At present, few researches have been conducted on the mechanical behavior of partially crystallized Zr-Cu-Ni-Al-Nb amorphous alloys, especially the fracture behavior under dynamic loading. In this work, as-cast Zr-Cu-Ni-Al-Nb amorphous alloy was annealed to accomplish different levels of nano-crystallization by controlling holding time. DSC, XRD, HRTEM, SEM, quasi-static and dynamic compression tests were utilized to research the effect of nano-crystallization on compressive strength and fracture mechanism of Zr-based amorphous alloy under different strain rates. The results indicated that the volume fraction and size of nanoscale crystalline phase inside Zr-based amorphous alloy increased with the increasing of annealing holding time. The compressive strength of annealed Zr-based amorphous alloy increased first and then decreased with the increase of holding time. The variation of strain rates also affected the compressive strength, which decreased when the strain rate increased from 1×10-3 s-1 to 1×103 s-1, and increased when the strain rate continually increased to 3×103 s-1. Different degrees of nano-crystallization had an impact on the fracture characteristics of Zr-based amorphous alloy. As the degree of crystallization increased, the fracture morphology of compression samples changed from vein-like patterns to quasi-cleavage features and then to river patterns.
Quenching and partitioning (Q&P) steel is a kind of high strength and toughness steels which has a majority of martensite at room temperature and a certain amount of retained austenite through the quenching and carbon distribution heat treatment process of cold-rolled carbon-silicon-manganese steel. In this work, the typical Q&P high-strength steel, QP980 steel, is taken as an example to carry out the physical simulation study of the whole process of heat treatment. A creep-like strain equation coupled with temperature and time is proposed to describe the volume change of materials during Q&P heat treatment. The phase transformation kinetics equation, phase transformation strain and phase transformation plasticity equation of Q&P heat treatment with the influence of quenching temperature were established, and the thermal expansion coefficient of each phase of Q&P steel was obtained. According to the coupling principles of temperature, microstructure and stress-strain field, a numerical simulation model for the whole process of Q&P heat treatment was developed. In this model, the physical simulation of QP980 steel thermal-elastoplastic incremental constitutive equations are implemented to commercial finite element software ABAQUS as the user subroutines. The model was validated by Q&P heat treatment experiment on Gleeble thermal-mechanical simulator. The calculated values of the models are both in good agreement with the experimental values.
Low alloy high strength steel, owing to its good mechanical properties and low cost, is widely used in bridge, building, pressure vessel and other engineering structures. Steel structures will inevitably produce residual stress and deformation after welding with the characteristics of concentrated heat source and local heating. Heat treatment is recognized as an effective method to eliminate the residual stress after welding. However, there is no quantitative and systematic study on the mechanism of heat treatment to eliminate residual stress when numerical simulation method is used to study post weld heat treatment (PWHT). Meanwhile, creep is an important factor in the process of PWHT on low alloy high strength steel. It is necessary to study the influence of creep on residual stress prediction so as to develop a simplified creep model used in practice more efficiently. Based on MSC. Marc software platform, a thermal-elastic-plastic finite element method (T-E-P FEM) considering creep effect is developed. The stress field during welding and PWHT of Q345 remelting joint was simulated by the integrated calculation method. The effect of creep on welding residual stress during PWHT was emphatically studied. Based on the results of numerical simulation, the mechanism of eliminating residual stress by PWHT was explored. At the same time, the residual stresses of welded and heat treated joints were measured by blind-hole method, and the results were compared with those of numerical simulation. In addition, the effect of two different creep models on the calculation accuracy of residual stress in PWHT is also discussed. A simple and efficient creep model suitable for engineering application is proposed for Q345 low alloy high strength steel. The results show that the residual stresses obtained by numerical simulation agree well with the experimental values, which verifies the validity of the integrated calculation method developed. It is necessary to consider creep effect in the process of PWHT, otherwise the residual stress after heat treatment will be seriously overestimated. By using the simple creep model proposed, the calculation efficiency could be increased by about 10 times with less loss of calculation accuracy.
Hydrostatic pressure was one of the critical factors affecting deep-sea corrosion. Theoretical research showed that increasing hydrostatic pressure could improve the activity of metal materials, increase the difference in activity between coupled metal materials, and aggravate the galvanic corrosion. At present, there were many researches on the corrosion behavior of metallic materials under hydrostatic pressure, but there were few researches on the influence of hydrostatic pressure on the corrosion behavior of metal materials. Due to the requirements of structure and performance in the marine environment, equipment components with different electrochemical properties must be connected. In such a harsh environment, galvanic corrosion would obviously accelerate. Therefore, it was very necessary to study the galvanic corrosion behavior of metallic materials under the condition of the deep sea. Fe-based alloys and Al-based alloys have been widely used in the marine environment, and there have been many studies on corrosion of Fe-based alloys and Al-based alloys in the deep-sea environment. As a result of single composition and structure, taking ultra-pure Al and ultra-pure Fe as the research object, the influence of phase, inclusion and other factors on corrosion behavior under hydrostatic pressure could be avoided, which was helpful to clarify the influence of hydrostatic pressure on corrosion behavior of ultrapure Al coupled with ultra-pure Fe. The influence of hydrostatic pressure on the corrosion behavior of ultrapure Al coupled with ultrapure Fe was studied in 3.5%NaCl using electrodynamic polarization and electrochemical noise. The discrete wavelet transform was utilized to remove the direct current drift of noise signal, and then the stochastic analysis based on the shot noise theory was carried out. The Hilbert-Huang transform was utilized to analyze the time-frequency characteristics of the noise signal. The surface morphology of corrosion samples was observed by SEM. The pressure distribution was simulated by finite element method. The results showed that the ultrapure Al was self-passivation in 3.5%NaCl solution under different hydrostatic pressures, pitting corrosion occurred after coupling with ultrapure Fe. With the increase of hydrostatic pressure, the galvanic potential of coupled ultrapure Al and ultrapure Fe decreased gradually, and the galvanic current increased gradually. The increase of hydrostatic pressure accelerated the pitting generation rate of ultrapure Al in galvanic corrosion, but inhibited the growth probability of pitting corrosion and reduced the tendency of local corrosion. When hydrostatic pressure was atmospheric, pitting corrosion could expand along the horizontal and vertical directions. In the presence of hydrostatic pressure, pitting corrosion was easier to expand along the horizontal direction.
The eutectic Sn-Ag-Cu (SAC) alloy is the most widely used solder alloy in consuming electronic devices owing to its good wettability and mechanical properties. However, the growth of automotive electronics working at a higher temperature than consuming electronics, requires more reliable solder alloy during microelectronic packaging, which can be achieved by elemental alloying. In this work, the modified Sn-Ag-Cu alloy with Ni, Sb and Bi addition was soldered on the surface of NiSn, NiAu and NiPdAu coating layer respectively, and the effects of elemental addition on the interfacial microstructure and shear strength were investigated systematically. It was found that compared to the commercial SAC305 solder joint, the modified Sn-Ag-Cu solder joint has a thinner interfacial IMC layer while a higher shear strength under the same reflowing conditions, although the formed IMC species are all (Cu, Ni)6Sn5 at both the chip side and the printed circuit board (PCB) sides. The addition of Ni, Sb and Au elements can reduce the IMC growth rate, while the addition of Pd increases the IMC growth. For the same solder alloy and reflowing process, the thickness of IMC layer on NiPdAu is the largest, while that on NiAu coating layer is the smallest. Au or Pd addition in the coating layer affects the distribution of Ag3Sn from dispersive particles to net-like morphology, resulting in an improvement of solder shear strength. The addition of Ag and Cu elements can increase the volume proportion of (Cu, Ni)6Sn5 and Ag3Sn in the solder alloy, hence to increase the shear strength of the solder joints. The solution and precipitation of Bi in the solder alloy can also contribute to the higher shear strength of the modified Sn-Ag-Cu solder joint, although its melting point is decreased to about 213 ℃. Therefore, the shear strength of modified solder alloy with Ni, Sb and Bi elements is higher than that of commercial SAC305.
