Pre-deformation with low strain can effectively control the thermal stability of metastable austenite. Till now, research has mainly focused on the effect of pre-deformation on martensitic transformation at one or more temperatures. However, research is still lacking on the effect of pre-deformation on the temperature at which martensite is formed (M_s), the final martensite content, and the transformational kinetics during continuous cooling. Furthermore, the mechanism underlying how pre-deformation affects martensitic transformation has not been reported. In this work, the influence rule and the corresponding effect of pre-deformation with low strain on martensitic transformation induced by temperature under continuous cooling from 300 K to 4 K was studied with 321 stainless steel samples by using the quasi-in-situ observation technique. The results show that M_s and the final amount of martensite increased under pre-deformation with low strain, and the martensitic transformation during continuous cooling was simultaneously accelerated. The quasi-in-situ observation demonstrated that the slip bands introduced by pre-deformation effectively provided nucleation sites for ε-martensite transformation. Accordingly, the formed ε-martensite increased the number of α'-martensite nucleation sites during continuous cooling, and finally promoted α'-martensite transformation. This builds on the theory proposed by other researchers that the dislocation defects introduced by pre-deformation directly provide the nucleation sites for α'-martensite transformation, and thus, promote martensitic transformation. In addition, by analyzing the nucleation behavior and nucleation priority at slip band defects, it is shown that the nucleation behavior of slip bands introduced by the pre-deformation was similar to that of faulted austenite induced by temperature. However, it is worth noting that the slip bands introduced by pre-deformation had a relatively higher nucleation priority. The crystallography of α'-martensite in the pre-deformed samples was analyzed, and it was found that the slip bands effectively changed the variant selection of α'-martensite so that the texture of α'-martensite was modified. This study advances the existing theory of martensitic transformation and provides theoretical guidance for the proactive control of temperature-induced martensitic transformation.

As a typical bearing steel, GCr15 steel tends to solidify over a wide temperature range during casting because of its high carbon content. The size of the mushy zone is relatively large, causing macrosegregation and porosity defects in bearing steel. The morphology of the solidification structure plays an important role in governing macrosegregation severity. Solidification structures have conventionally been characterized by measuring the primary or secondary dendrite arm spacing in a dendritic network, but these measures do not adequately describe the branched appearance of secondary and tertiary arms. In this work, fractal dimension and specific surface area have been introduced, and the solidification structure integral morphology characteristics of different locations in the continuous casting billet of GCr15 bearing steel have been quantitatively investigated. Then, the permeability of the interdendritic channels were calculated based on fractal dimension and specific surface area. The size of the billets was 220 mm × 220 mm, and the sampling location was in the cross section of the billet. Two superheats (20 and 35^oC) were considered for studying the integral characteristics of the solidification structure. First, fractal dimension can describe the self-similar complexity of the solidification structure, and specific surface area can describe dendritic coarsening. Second, it was determined that the fractal dimension was larger and the specific surface area was smaller at 35^oC superheat compared with 20^oC superheat. This indicates that the self-similar complexity of dendrites is larger, and the dendrite coarsening is more significant at high superheating. Finally, the permeability in the equiaxed grain zone calculated using fractal dimension and specific surface area is lower at 20^oC superheat. The smaller the permeability, the greater the flow resistance of the liquid, which is more conducive to the control of the macrosegregation defects. In addition, to effectively restrain the formation of macrosegregation defects at high superheat, the cooling rate in the equiaxed grains zone should increase by adjusting the process parameters under isothermal solidification conditions.

Lath martensitic steels are widely used in high strength structural materials. Coherency strains in quenched lath martensite induce huge dislocation densities, which are the sources of the alloys' strength, whereas the way its microstructure functions is still unclear. The plastic deformation behavior of lath martensite in ultrahigh strength steel was investigated using in situ neutron diffraction technology. Diffraction data were analyzed using the Z-Rietveld and convolutional multiple whole profile (CMWP) fitting procedures. Transformation dislocations in the as-quenched martensite were mixed with edge and screw components and showed characteristics of random distribution. Significant work hardening of lath martensite can be better understood by considering the increase in dislocation density along with changes in dislocation arrangement. With increased tensile strain, the total dislocation density increased with the increasing amount of edge-type components and the decreasing amount of screw-type components. The hard orientation packets showed characteristics of work hardening with an increased dislocation density, whereas the soft orientation packets showed characteristics of work softening with a decreased dislocation density. The partitioning of the applied load was carried out within two types of packets, which further promoted the formation of long-range internal stresses after deformation.

Based on the limitations of severe plastic deformation (SPD) technology, such as a small effective deformation area and huge forming load, in the preparation of ultrafine grained/nanocrystalline materials, preparing industrial grade bulk ultrafine grained materials is difficult. In this work, a new SPD method, titled 3D-SPD, for preparing bulk ultrafine grained bars is proposed based on the cross-rolling principle. A severe torsion and compression deformation region was constructed using specially-curved conical rolls and guide plates, super large feed angles, and diameter reduction ratios. During the 3D-SPD process, a billet entered the deformation region from the large diameter side of the roll, where the deformation pressure was MPa grade and able to realize the SPD process with effective strains greater than 6.5. A crack control model based on Oyane criteria was established. Through the optimization analysis of damage factors under different deformation conditions, the crack induced by the Mannesmann effect was effectively restrained. Based on theoretical and experimental results, optimal parameters were determined as follows: cone angle 5°, feed angle 24°, diameter reduction ratio 50%, temperature 700^oC, ovality coefficient 1.02, and roll speed 40 r/min. A 25 mm-diameter ultrafine grained bar of 45 steel was obtained by the single pass deformation. The average grain size was refined from 46 μm to 1 μm, and the yield and tensile strengths were increased by 46% and 42%, respectively.

The urgent need for lightweight, high-accuracy, and personalized products has led to the rapid development of additive manufacturing. Selective laser melting (SLM), which is a very promising additive manufacturing technique, has attracted remarkable attention. The mechanical properties of SLM parts are highly related to the formation of pores and cracks. In this work, SLM parameters for AlSi10Mg alloy were optimized, and the SLM AlSi10Mg sample with a high relative density of 99.63% was obtained. The SLM sample exhibited good properties, including an ultimate tensile strength (UTS) of approximately 478 MPa, a total elongation of 8%, and an average hardness of 122 HV along the horizontal direction. However, due to a high cooling rate, an inhomogeneous microstructure with refined grains and a Si network was obtained. To achieve a homogeneous microstructure and further improve the elongation of the SLM samples, the effect of heat treatments on the microstructure and mechanical properties of the SLM samples along the horizontal direction was analyzed. After the heat treatments, the strength of the samples changed significantly and the elongation was significantly improved. Further, after a solid solution treatment at 540oC for 1 h, the UTS significantly decreased to approximately 246 MPa and the elongation increased to more than 22%. For the sample annealed at 236oC for 10 h, a UTS of approximately 368 MPa and elongation of approximately 17% were obtained. Moreover, the sample subjected to ageing at 130oC for 4 h exhibited a high strength similar to the level of the SLM sample, the elongation was increased to approximately 11.9%, and the hardness was approximately 133 HV which is 10% higher than that of the SLM sample. The improved performance of the aged samples can be attributed to the combination of solution strengthening, microstructural refinement, and precipitation strengthening. The results show that low-temperature ageing is the optimized heat-treatment method for SLM samples with fine microstructures.

Mg-air batteries have excellent applicability in the fields of electrochemical energy storage and conversion due to their high theoretical voltage (3.09 V) and high specific energy density (6.8 kW·h/kg). Nevertheless, the high polarization and low Coulombic efficiency reduce the inherently outstanding discharge performance of the Mg anode. Alloying and plastic deformation have been utilized to overcome these drawbacks by developing novel anode materials with relatively enhanced performance. In this work, the effect of microstructural characteristics on the discharge performance and electrochemical behaviors of the extruded Mg-2Bi-0.5Ca-0.5In (mass fraction, %) alloy as an anode for Mg-air batteries have been systematically discussed. Results indicate that the extruded alloy primarily consists of complete dynamically recrystallized grains with an average grain size of (10.92 ± 0.23) μm. The texture component is mainly composed of nonbasal texture consisting of texture components of basal poles from the normal direction to extrusion direction by around 45^o-60^o. The alloy contains α-Mg, nanoscale Mg₃Bi₂ phases, and microscale Mg₂Bi₂Ca phases. Furthermore, the extruded alloy exhibits a stable discharge process and negative discharge potential of -1.628 V at 10 mA/cm² in a half-cell test. Moreover, the Mg-air battery based on the extruded alloy as an anode exhibits a high cell voltage and power density. For instance, the cell voltage and peak power density reach up to 0.72 V and 86.4 mW/cm² at 120 mA/cm², respectively, which is significantly higher than commercially accepted AZ31 and AM50 anodes for Mg-air batteries. The outstanding discharge properties are primarily attributed to the re-deposition of metallic In at the electrode surface, the weakened texture intensity, the uniform microstructure and the loose and thin discharge products film.

Growing attention has been placed on high-entropy alloys (HEAs) owing to their promising mechanical properties. Particularly, HEAs in which the main crystal structure is fcc are attracting significant attention. Although such alloys exhibit a good combination of strength and ductility, they cannot meet the increasing demands of applications because of limited yield strengths. In recent years, researchers have tried to improve yield strengths of HEAs by refining grains and introducing interstitial atoms. However, the processing cost is high and is often accompanied by the significant loss of ductility. In this study, we propose a simple processing route incorporating cold rolling at medium thickness reductions and short-time annealing at medium temperatures to obtain a heterogeneous structure in Fe-Mn based HEAs consisting of deformed grains with an average diameter of several tens of microns and recrystallized ultrafine grains. By simultaneously introducing multiple strengthening mechanisms, including the strengthening contributed by the microstructural characteristics of dense dislocations, grain refinement, precipitates, ε-martensite, α-martensite, and recovery twins, as well as the strengthening induced by deformation twinning and ε-martensite phase transition that occurs continuously during deformation, the yield strength of the alloy significantly increases compared with that of the fully recrystallized material and reaches 825 MPa. Simultaneously, due to the activation of significant deformation twinning and deformation-induced martensitic transformation, the uniform elongation of the alloy is about 28.6%. The proposed material fabrication method is simple, cost-effective, and can effectively improve the mechanical properties of Fe-Mn based HEAs, providing new insight into optimizing the mechanical properties of low stacking fault energy alloys of the fcc structure.

GH3625 superalloy is a type of solid-solution strengthened nickel-based wrought superalloy having Mo and Nb as the main strengthening elements. Because of its excellent high-temperature mechanical properties and oxidation resistance below 650 ^oC, it can be used in harsh stress and atmosphere environments. It is mainly used as a pipe material for aeroengine fuel main pipe, nuclear power steam generator heat transfer pipe, and pressure pipe, etc. Owing to the high alloying degree of nickel-based superalloys, large deformation resistance, and narrow thermal processing temperature range, the pipe preparation process is complicated. In this study, the as-cast and homogenized pipe billets were used for a short-flow hot extrusion pipe preparation test using the same process. The homogenized pipe billet was extruded successfully, and the pipe burst occurred during the extrusion of the as-cast billet. The pipe burst behavior was studied by OM, SEM, and EBSD, with an EDS analysis. The results showed a considerable amount of Laves phases in the as-cast pipe billet, and the Laves phases and micro-segregation were essentially eliminated after homogenization. Adiabatic heating of the as-cast pipe billet leads to the Laves phase remelting during the hot extrusion process, which is the main reason for pipe bursting during a hot extrusion process. The cracking mode of the pipe burst is a quasi-cleavage fracture, combining brittle fracture and ductile fracture with the predominance of the brittle fracture.

316 stainless steel is the first choice for bipolar plate material in fuel cells; however, it suffers from passivation-induced corrosion and conductivity deficiencies. In this work, Fe-Cr-Ni alloy was refined using the cluster-plus-glue-atom model to obtain stainless steels with balanced corrosion and electrical performances. For austenite 316L stainless steel, the unit is described as a 16-atom cluster formula [Ni-Fe₁₁Ni₁]Cr₃. By fixing the three atoms of a glue, Cr₃ is required to achieve sufficient corrosion resistance, and new compositions with varying Ni contents are designed following [Ni-Fe_13-xNi_x-1]Cr₃ = Fe_13-xNi_xCr₃ (x = 1-5). The designed alloys were arc melted at least five times, copper-mold suction casted into 10-mm cylindrical rods under an argon atmosphere, homogenized at 1150^oC for 2 h, and water quenched. Under the simulated bipolar plate service environment (0.5 mol/L H₂SO₄ + 2 × 10^-6 HF aqueous solution), as the Ni content increases, the self-corrosion current density decreases to 1.10 and 0.29 μA/cm² after acid passivation and electrochemical nitridation, respectively. These values are well below compared to the commercial 316L stainless steel (7.51 and 0.47 μA/cm²) and close to the current industry target (0.5 μA/cm²) for bipolar plates. At the same time, the contact electrical resistance (under 0.064 MPa pressure) decreases to 0.98 and 1.03 Ω·cm² after acid passivation and electrochemical nitridation, respectively, which is superior to the 316L stainless steel (1.1 Ω·cm²). Thus, optimal alloy composition [Ni-Fe₁₀Ni₂]Cr₃ can be used as the right substrate material of the bipolar plate instead of the 316L stainless steel. The electrochemical nitridation method is the proper surface treatment method for stainless steel bipolar plates, and this method improves the alloy's corrosion resistance while maintaining the same level of contact resistance.

5356 aluminum alloy has been widely applied in transportation, aerospace and other fields owing to its low density, excellent fatigue property, and superior corrosion resistance. Aluminum alloy is widely manufactured by the arc additive technique that operates at a fast manufacturing speed with simple equipment and high material utilization. The property of 5356 aluminum alloy is closely related to its microstructure. To better control the property of this alloy for the additive manufacturing of forming parts, it is necessary to study the evolution of its microstructure. In this work, 5356 aluminum alloy forming parts were produced by tungsten inert gas welding (TIG) arc additive manufacturing, and their microstructures and mechanical properties were analyzed. The 5356 aluminum alloy formed by TIG additive manufacturing was composed of α-Al matrix and β(Al₃Mg₂) phase. As the deposition height increased, the layer microstructure transformed from equiaxed grains to columnar grains and tended to stabilize at thermal equilibrium. The top layer exhibited a dendritic microstructure with serious segregation of the Mg element. The middle and lower microstructures were varied and included equiaxed grains, columnar grains, and a mixture of these, with improved Mg-element segregation. As the deposition height increased, the microhardness in the layer first decreased and then stabilized. The microhardness was larger in the interlayers than in the deposition layers. The pores gathered in the interlayers might explain the lower yield strength of the thin-walled parts than the theoretically calculated value. The tensile strength, yield strength, and elongation were all anisotropic, and the tensile property was better in the transverse than in the longitudinal direction. This result was attributable to pore accumulation between the layers of the thin-walled parts and to the uneven microstructure.

Liquid-liquid phase separation was used to design phase-separated metallic glasses with special properties. In this work, Zr₆₀Cu_40-xFe_x phase-separated metallic glasses were designed by partial substitution of Cu by Fe in Zr₆₀Cu₄₀ metallic glass. The liquid-liquid phase separation behavior of Zr₆₀Cu_40-xFe_x alloy was investigated. The results show that the miscibility gap of the binary Cu-Fe system can be extended into the Zr₆₀Cu_40-xFe_x system and that liquid-liquid phase separation into Cu-rich and Fe-rich liquids occurred during rapid cooling. On the basis of the behavior of liquid-liquid phase separation of the Zr₆₀Cu_40-xFe_x system, the effect of partial substitution of Cu by Fe on the microstructure and phase formation of the Zr₆₀Cu_40-xFe_x alloys was investigated. The microstructure evolution and the competitive mechanism of phase formation in the as-quenched Zr₆₀Cu_40-xFe_x alloy were discussed. For the Zr₆₀Cu₂₀Fe₂₀ alloy, liquid-liquid phase separation into Cu-rich and Fe-rich liquids and then liquid-glass transition occurred during rapid cooling and resulted in a heterogeneous structure with glassy Fe-rich matrix embedded with glassy Cu-rich nanoparticles. Considering this structure, the electrical properties and nanoindentation behavior of the as-quenched Zr₆₀Cu₂₀Fe₂₀ alloy were examined. The abnormal change in electrical resistivity during crystallization and the effect of nanoscale phase separation on the shear transformation zone of the Zr₆₀Cu₂₀Fe₂₀ alloy were analyzed.

Thermal barrier coatings (TBCs), which mainly comprise top and bond coats, have been applied to the hot components of gas turbine engines owing to their low thermal conductivity, high-temperature oxidation resistance, gas corrosion resistance, and so on. However, a thermally grown oxide (TGO) layer, which germinates between top and bond coats, has a considerable effect on the service life of TBCs. Moreover, the microstructure optimization and growth inhibition of a TGO layer are crucial. The surface modification of bond coat in TBCs has been introduced to reduce and optimize the growth rate of a TGO layer. Among these methods, the surface shot peening of bond coat has yet to be extensively elucidated at high service temperatures. Furthermore, the influence of surface shot peening on isothermal oxidation behavior has rarely been reported in literature. In the present research, NiCrAlYSi coating was prepared using vacuum arc ion plating. The influence of the surface shot peening process on the isothermal oxidation behavior of NiCrAlYSi coating was investigated in detail, which indicated that the surface roughness of NiCrAlYSi coating reduced after the shot peeing. In this process, the compactness and smoothness of NiCrAlYSi coating improved, which could avoid the formation of an abnormal oxidation area in the coating due to the penetration and diffusion of oxygen atoms inside the coating. A uniform and low thickness deviation TGO layer could be generated at the surface. The TGO layer growth rate of NiCrAlYSi coating with 0.4 MPa and 5 min shot peening reduced by 60% compared to no shot peening, and the oxidation resistance of NiCrAlYSi coating improved.

Conduction plasma arc welding is widely used to weld thin stainless-steel plates in a liquefied natural gas carrier, in which high arc energy density can be achieved through the constraint effects of the constricting nozzle. However, the welding current is relatively low, such that a keyhole is not formed inside the molten pool in conduction plasma arc welding, causing significantly different arc physical characteristics and molten pool dynamic behaviors from those of keyhole plasma arc welding. In this study, a one-way coupled electrode-arc-molten pool model was developed, and spectral analysis, infrared thermography, and particle tracing methods were used to investigate the arc physical characteristics and molten pool dynamic behaviors in conduction plasma arc welding. In conduction plasma arc welding, numerical and experimental results show that plasma impinges on the surface and flows toward the edge of the molten pool. Two contrary convective eddies were found inside the molten pool. The counterclockwise eddy at the center of the molten pool is driven by arc pressure, Marangoni forces, and Lorentz forces, and the clockwise eddy at the rear part of the molten pool is driven by plasma shear stress, Marangoni forces, and buoyancy forces. Additionally, the maximum temperature of the molten pool in conduction plasma arc welding is higher than that in keyhole plasma arc welding due to higher arc energy density and weaker convection.