Effect Of Grain Refinement Research Articles

The {332} Twinning Behavior of a Ti-15Mo Medical Alloy during Cyclic Deformation and Its Effect on Microstructure and Performance.

In this study, the microstructural evolution of a Ti-15Mo medical alloy was investigated, when the in situ cyclic tensile strain had 2% amplitude and the tension-compression cyclic deformation had 1%, 2%, and 3% amplitude. The Vickers hardness and wear resistance of the alloy were also optimized due to the grain-refining effect after cyclic deformation and annealing. The twinning-induced plasticity (TWIP) was considered the main deformation mechanism of the Ti-15Mo alloy during the tensile-compressive cycle deformation with suitable strain amplitude. The {332}<113> twins and boundaries were the main contributors to the grain refinement. The optimal microstructure, hardness, and wear resistance were obtained in the alloy deformed by tension-compression cyclic strain with a 3% strain amplitude. The wear resistance of the annealed alloy in Hank's solution was excellent in contrast to the original Ti-15Mo alloy due to its reasonable microstructure and hardness. It is clear that abundant twins were formed and retained in the coarse grains of the original alloy after cyclic deformation and annealing, which provided the expected refined grains and performance.

Effects of Li addition on the properties of biodegradable Zn–Fe–Li alloy: Microstructure, mechanical properties, corrosion behavior, and cytocompatibility

Biodegradable Zn-based alloys have excellent mechanical properties, a good corrosion rate, and favorable biocompatibility, which have broad application prospects. In this study, new biodegradable Zn-0.5Fe-xLi (x = 0, 0.2, 0.5, 0.8 wt%) alloys were successively prepared by melting and hot extrusion, and the effects of Li content on the microstructure, mechanical properties, corrosion behavior, and biocompatibility of the alloys were investigated. The formation of FeZn13 and eutectic LiZn4 phases in the Zn-0.5Fe-xLi alloys and the grain size of the Zn-0.5Fe-0.5Li alloys significantly reduced with an increase in Li content. The synergistic effect of solid solution and grain refinement of Li significantly improved the strength of the Zn-0.5Fe-0.5Li alloy and ensured the ductility of the alloy, which resulted in its optimum overall performance. The yield strength was 353.6 ± 6.8 MPa, the ultimate tensile strength was 441.9 ± 2.6 MPa, and the elongation was 26.8 ± 0.8%. In addition, the Zn-0.5Fe-0.5Li alloy showed excellent corrosion properties. The electrochemical corrosion rate was 0.735 mm/year, and the surface corrosion degradation morphology was uniform after immersion. The Zn-0.5Fe-0.5Li alloy also demonstrated the most favorable in vitro cytocompatibility in cellular experiments. Overall, the extruded Zn-0.5Fe-0.5Li alloy is a promising biodegradable implant material with excellent mechanical properties, a suitable corrosion rate, and good cytocompatibility.

Investigating the Dynamic Mechanical Properties and Strengthening Mechanisms of Ti-6Al-4V Alloy by Using the Ultrasonic Surface Rolling Process.

The demand for titanium alloy has been increasing in various industries, including aerospace, marine, and biomedical fields, as they fulfilled the need for lightweight, high-strength, and corrosion-resistant material for modern manufacturing. However, titanium alloy has relatively low hardness, poor wear performance, and fatigue properties, which limits its popularization and application. These disadvantages could be efficiently overcome by surface strengthening technology, such as the ultrasonic surface rolling process (USRP). In this study, the true thermo-mechanical deformation behavior of Ti-6Al-4V was obtained by dynamic mechanical experiment using a Hopkinson pressure bar. Moreover, USRP was applied on the Ti-6Al-4V workpiece with different parameters of static forces to investigate the evolution in surface morphology, surface roughness, microstructure, hardness, residual stress, and fatigue performance. The strain rate and temperature during the USRP of Ti-6Al-4V under the corresponding conditions were about 3000 s-1 and 200 °C, respectively, which were derived from the numerical simulation. The correlation between the true thermo-mechanical behavior of Ti-6Al-4V alloy and the USRP parameters of the Ti-6Al-4V workpiece was established, which could provide a theoretical contribution to the optimization of the USRP parameters. After USRP, the cross-sectional hardness distribution of the workpiece was shown to initially rise, followed by a subsequent decrease, ultimately to matrix hardness. The cross-sectional residual compressive stress distribution of the workpiece showed a tendency to initially reduce, then increase, and finally decrease to zero. The fatigue performance of the workpiece was greatly enhanced after USRP due to the effect of grain refinement, work hardening, and beneficial residual compressive stress, thereby inhibiting the propagation of the fatigue crack. However, it could be noted that the excessive static force parameter of USRP could induce the decline in surface finish and compressive residual stress of the workpiece, which eliminated the beneficial effect of the USRP treatment. This indicated that the choice of the optimal USRP parameters was highly crucial. This work would be conducive to achieving high-efficiency and low-damage USRP machining, which could be used to effectively guide the development of high-end equipment manufacturing.

Achieving strength-ductility synergy in zirconium via ultra-dense twin-twin networks

Hierarchical nano-twinned structures have been proven to be an effective way in breaking the strength-ductility dilemma in face-centered cubic metals. However, it is challenging to introduce high-density of twinned structures in hexagonal-close packed (HCP) metals because deformation twinning are limited in fine-grained structures. Here, we propose an approach to introduce ultra-dense twin-twin networks in high purity HCP Zr with ultra-large grain size (>200 µm) via cryogenic biaxial-rolling. Tensile tests show that the yield strength and the elongation of the densely twinned Zr are increased by 76.5 % and 28.6 % compared with its coarse-grained counterparts, and reaches 360 MPa and 13.5 %, respectively. The strength of twinned Zr is enhanced by the synergy effect of grain refinement (from 241 µm to 1.2 µm) and the obstruction of dislocations by high-density intersecting twin boundaries (TBs). The ductility and strain-hardening ability are improved by the activation of massive prismatic 〈a〉 and pyramidal 〈c + a〉 dislocations. The three-dimensional TB networks are effective 〈c + a〉 dislocation sources which enhance the plasticity along 〈c〉 direction. Our work provides a new strategy to design ultra-dense twinned structures and sheds light on the twinning-induced plasticity in HCP metals.

Studies on the Mechanical, Strengthening Mechanisms and Tribological Characteristics of AA7150-Al2O3 Nano-Metal Matrix Composites

Stir-casting with ultrasonic cavitation produced nano-Al2O3-filled AA7150 matrix composites in this study. The SEM microstructure study shows that all composites include nano-Al2O3 particles with consistent particle sizes and homogenous distribution. EDS and XRD showed no secondary phases or impurities in the composite. Optical microscopy showed intense ultrasonic cavitation effects, and nano-Al2O3 particles caused grain refinement in the AA7150 matrix. The composite’s mechanical characteristics improved when the Al2O3 nanoparticle weight percentage (wt.%) increased. With only 2.0 wt.% nano-Al2O3 particles, the composites yielded 232 MPa, 97.52% higher than the sonicated AA7150 matrix alloy. Multiple models were used to characterize the strength of the AA7150 nano-Al2O3 composite. The findings showed that thermal incongruity, Orowan strengthening, the Hall–Petch mechanism, and load transfer effects contributed the most towards the increased strength of the composite. Increasing the nano-Al2O3 wt.% in the AA7150 matrix improved hardness by 95.08%, yield strength by 90.34%, and sliding wear resistance by 46.52%. This enhancement may be attributed to the combined effects of better grain refinement, enhanced dispersion with dislocation strengthening, and better load transfer between the matrix and reinforcement, which are assisted by the inclusion of reinforcements. This result was confirmed by optical studies.

Preparation of Al-Ti-Sc master alloys and refining effects on the 6016 aluminum alloy

The 6016 aluminum alloy has good plasticity but low strength. In this work, Al-Ti-Sc master alloy was prepared by thermite reduction method in order to obtain an effective grain refiner with a simplified preparation process and a reduced cost. The refining effects of the ternary master alloy on the 6016 aluminum alloy were checked to improve the strength and plasticity simultaneously. The chemical reactions during the preparation process of the Al-Ti-Sc master alloy was studied by thermodynamic software of HSC 6.0. The content of Sc and Ti in the master alloy reaches 2.48 wt% and 1.4 wt%, respectively, on proper experimental conditions, and the main phases of the master alloys are α-Al and Al3(Sc,Ti) phase. In the refined alloys, the average grain size can reach 27 μm, and the ultimate tensile strength and the elongation after fracture reaches 280 MPa and 35% in cast state, respectively. Compared to the pristine 6016 aluminium alloy, the significantly enhanced strength and good plasticity of the refined alloys contribute to the combined effects of grain refinement and precipitated phase strengthening. For the fracture of low doped alloys, originates from cleavage fracture mechanisms, and for the ones with high dosages, ductile fracture becomes dominant.

Effects of extrusion die cavity on the texture components and the associated mechanical properties for ZK60 magnesium alloy

This work managed the extrusion strain path by designing various extrusion die cavities, successfully realizing the texture modification for the ZK60 magnesium alloy. The mechanisms involving the texture dependence on the extrusion die cavity as well as their effects on the mechanical properties were emphatically investigated. Results showed that dynamic recrystallization refined the grain size and improved the microstructure homogeneity in the three extrusion specimens, but did not produce too large microstructure differences. By comparison, significant texture differences developed owing to the various extrusion die cavities, which here were mainly reflected in the strong or weak texture components for the c-axes//TD and the c-axes//ND. Such texture differences started from the deformation texture instead of the recrystallization texture whose roles only consisted in dispersing the texture component and reducing the texture intensity. The results from the finite element analysis and the visco-plastic self-consistent model indicated that, in order to accommodate the different strain components induced by the extrusion die cavities, slip systems or tension twinning were activated differently, and this was the critical reason causing the above texture differences. One modified Hall-Petch relationship was adopted to analyze the conjoint effects of grain refinement and texture variation on the yield stress. Additionally, the quantitative results about deformation mechanism activation fractions demonstrated that the texture variations influenced the competition relationships between the twinning induced deformation and the slip dominant deformation, and the former generally produced the lower yield stress and the increasing stage of strain hardening rate, while the latter produced the higher yield stress and the continuous decline of strain hardening rate.

Synergistic effect of TiN ceramic particle and grain refinement in enhancing the higher performance Ti–V–Al based shape memory alloy

In the present study, in-situ TiN ceramic particles were introduced into Ti–V–Al based shape memory alloys to optimize mechanical properties and friction resistance. The results revealed that increasing the TiN content resulted in the transition from α" to β phase, and led to a reduction in the martensitic transformation temperature. Additionally, the TiN ceramic particles were found to significantly reduced the grain size of the Ti–V–Al based shape memory alloys. Compared to other Ti–V–Al-X shape memory alloys, the introduction of TiN ceramic particles enhanced the mechanical and wear properties of the Ti–V–Al shape memory alloy. These improvements can be attributed to a combination of solid solution strengthening, grain refinement and precipitation strengthening. Notably, the (Ti–V–Al)100-xNx shape memory alloy exhibited a maximum yield strength of 568 MPa and a fracture strength of 788 MPa. Furthermore, the micro-hardness of the Ti–V–Al based shape memory alloy increased monotonously from 355HV to 393HV, while the wear coefficient steadily decreased from 0.53 to 0.48. In conclusion, by adjusting content of TiN ceramic particles, it is possible to obtain a lightweight Ti–V–Al shape memory alloy with tailored martensitic transformation temperatures, enhanced strength and hardness, as well as excellent friction properties.

Performance optimization of ternary Ti−Nb−Cu shape memory alloys based on d electron theory

The phase constituents of Ti−Nb−Cu shape memory alloys were designed based on d electron theory, in order to optimize their performances. The XRD analysis and TEM observation revealed that Cu addition led to the reduction of the bond order () and metal d-orbital energy level (), which caused the evolution of phase constituents. With increasing Cu content, the variation of phase constituents can be concluded as follows: β+α″→β+ω→β+α″+ω→β. The yield strength, ultimate tensile strength and elongation of Ti−Nb−Cu shape memory alloys firstly increased and then decreased with increasing Cu contents. By optimizing the content of Cu alloying element, Ti−Nb−Cu shape memory alloys possess superior mechanical properties with yield strength of 528 MPa and ultimate tensile strength of 742 MPa, which can mainly be attributed to the comprehensive effect of solution strengthening and grain refinement as well as precipitation strengthening.

Biodegradation performance of ball burnished magnesium alloy in stimulated body fluids for bioresorbable implant applications

Magnesium alloys are found to be potential bioresorbable materials. However, the application range of these alloys as bioresorbable materials has been limited due to rapid degradation in physiological fluids. The current study deals with the enhancement of degradation behavior of Mg Ze41A alloy in SBF solution by ball burnishing. Further, the influence of burnishing parameters on biodegradation rate has been investigated by Tafel plots, polarization resistance, electrochemical impedance spectroscopy and surface morphology analysis of degraded samples. Additionally, the biodegradation rate has been estimated by ANFIS modeling. The biodegradation rate of Mg Ze41A has been reduced from 1.106 mm/yr to 0.367 mm/yr with an improvement of 68.82% at optimum burnishing condition of burnishing force 50 N, burnishing speed 1400 RPM, burnishing feed 125 mm/min and 2 passes. ANOVA analysis has revealed that burnishing force and feed have contributed the most for the enhancement. The shift in degradation potential towards noble direction and reduction in degradation current density in potentiodynamic polarization curves and the highest charge transfer resistance in electrochemical impedance spectroscopy of the burnished samples are observed for burnished samples. The results reveal that ball burnishing process is a potential surface modification method in reducing the biodegradation rate of Mg Ze41A alloy. The ball burnishing has resulted in lowest biodegradation rate at a degradation current density of 5.743 μA/cm2 and degradation potential of −1.513 V. The ANFIS model developed with 81 fuzzy rules and triangular membership function estimated the biodegradation rate with an accuracy of 95.03%. The decrease in biodegradation rate of ball burnished magnesium substrate in SBF is essentially due to cumulative effect of compressive residual stress, microhardness, surface finish, work hardening and grain refinement.

Low-cycle fatigue behaviour of extruded 7075 aluminium alloy bar: Competition of grain sizes and textures

The low-cycle fatigue (LCF) behaviour of a large 7075 aluminium alloy bar was investigated, and the results indicated that the microstructure at the center (0R) and surface section of the bar (R) were different in terms of grain sizes and textures. The microstructure of the 0R specimen was characterised by coarser elongated grain and higher <111> texture intensity compared to that of the R specimen. The LCF life of the 0R specimen was higher than that of the R specimen at a strain amplitude range of 0.6%–0.9%, thereby indicating the dominant effect of higher texture intensity. However, the 0R specimen sustained short LCF life at low strain amplitudes of 0.4% and 0.5%, indicating the predominant effect of grain refinement. The fracture analysis results indicated that there were wider striations for the 0R specimen at a strain amplitude of 0.4%. Interestingly, the yield strength, ultimate tensile strength and elongation of the 0R specimen were higher than that of the R specimen. The higher mechanical properties of the 0R specimen was consistent with its longer low cycle fatigue life at strain amplitude range of 0.6%–0.9%. Moreover, the features of the fracture, including fracture initiation and fast fracture regions, were also elaborated. The competition of grain sizes and texture evolutions determined the LCF life of the alloy depending on the strain amplitude range: the grain size effect gradually overshadowed that of the texture with decreasing strain in terms of fatigue life.

A Rolled Mg–8Al–1Sm–1Zn Alloy with High‐Strength Ductility via Single‐Pass Liner Rolling

In this article, the microstructure and mechanical properties of Mg–8Al–1Sm–1Zn alloy prepared by multi‐pass conventional rolling and single‐pass liner rolling are investigated. The alloy is subjected to multi‐pass conventional rolling with a cumulative deformation of 60% and single‐pass liner rolling with a deformation of 60% after solution‐treated (T4 temper) at 400 °C, respectively. In the results, it is shown that the best mechanical properties are obtained by single‐pass liner rolling at 400 °C with a deformation of 60% deformation under the combined effect of grain refinement and precipitation strengthening. The average grain size of the alloy is 7.2 μm and the microhardness is 80 HV. The tensile yield strength, ultimate tensile strength, and elongation at room temperature are 255 MPa, 285 MPa, and 12.7%, which are 14.9%, 15.3%, and 10.4% higher than those of the conventional multi‐pass rolled alloy, respectively.

Enhanced Mechanical Properties, Corrosion Resistance, Cytocompatibility, Osteogenesis, and Antibacterial Performance of Biodegradable Mg-2Zn-0.5Ca-0.5Sr/Zr Alloys for Bone-Implant Application.

Magnesium (Mg) alloys are widely used in bone fixation and bone repair as biodegradable bone-implant materials. However, their clinical application is limited due to their fast corrosion rate and poor mechanical stability. Here, the development of Mg-2Zn-0.5Ca-0.5Sr (MZCS) and Mg-2Zn-0.5Ca-0.5Zr (MZCZ) alloys with improved mechanical properties, corrosion resistance, cytocompatibility, osteogenesis performance, and antibacterial capability is reported. The hot-extruded (HE) MZCZ sample exhibits the highest ultimate tensile strength of 255.8 ± 2.4MPa and the highest yield strength of 208.4 ± 2.8MPa and an elongation of 15.7 ± 0.5%. The HE MZCS sample shows the highest corrosion resistance, with the lowest corrosion current density of 0.2 ± 0.1µA cm-2 and the lowest corrosion rate of 4 ± 2µm per year obtained from electrochemical testing, and a degradation rate of 368µm per year and hydrogen evolution rate of 0.83 ± 0.03mL cm-2 per day obtained from immersion testing. The MZCZ sample shows the highest cell viability in relation to MC3T3-E1 cells among all alloy extracts, indicating good cytocompatibility except at 25% concentration. Furthermore, the MZCZ alloy shows good antibacterial capability against Staphylococcus aureus.

Tensile properties and microstructural evolution of 17–4 PH stainless steel fabricated by laser hybrid additive manufacturing technology

Wire-based laser directed energy deposition (LDED) technology is an attractive and efficient additive manufacturing technology, which shows great potential in the process of manufacturing high-value engineering components. However, the thermal effect will lead to a large amount of residual tensile stress in the components, and the large crystal structures after solidification seriously affects the tensile performance. The pursuit of high mechanical properties in additive manufacturing components have prompted researchers to seek new strengthening process to produce high-strength components. Therefore, in this paper, laser shock peening (LSP) composite wire-based LDED were used for the additive manufacturing of 17–4 PH stainless steel under the specified experimental conditions, and the effects of LSP on microstructure evolution, microhardness, and tensile properties of wire-based LDED components were systematically studied. Results showed that uniform grains with an average size of 2.31 μm were generated on the top surface of specimen due to the ultra-high plastic strain induced by LSP shock wave. The size of hierarchical martensitic units shows varying degrees of reduction after LSP treatment, which is shown by the width decrease of packet, block and lath. Dynamic recrystallization caused by rearrangement and annihilation of high-density dislocations promoted the grain refinement. Meanwhile, the residual stress was completely converted from tensile state into compressive state with a maximum value of 425 MPa, and the microhardness was also enhanced to a peak value of 432 HV. In addition, we observed that the high-density dislocation environment created by LSP treatment promoted the further precipitation, and the aggregation phenomenon of precipitates was observed in the dense dislocation area. Furthermore, the tensile strength and elongation of LSP treated specimen were improved significantly, which was attributed to the combined effects of grain refinement, introduction of dislocation structures, precipitation strengthening and gradient residual compressive stress. In summary, laser hybrid additive manufacturing technology provides a new idea and method for the production of high-performance precipitation hardening stainless steel components.

Micron-sized SiC particles reinforced TC4 composites: Mechanical properties and strengthening mechanisms

SiC fiber, W fiber, C fiber and other continuous fiber-reinforced titanium matrix composites (TMCs) have limited their application prospects due to their anisotropy, interface reaction, and high costs. In contrast, the SiC particles (SiCp) reinforced TMCs are of particular interests due to their ease of fabrication, lower costs, and isotropic properties. Micron-sized SiCp reinforced TC4 (Ti-6Al-4V) composites (SiCp/TC4) were synthesized via spark plasma sintering (SPS). The microstructure and mechanical properties of SiCp/TC4 composites have been investigated. The results showed that SiCp/TC4 composites exhibite a typical Widmanstatten structure composed of plate α, grain boundary α phase, and intercrystalline β phase. TiC particles formed via the reaction of SiCp with the TC4 matrix distributed at grain boundaries. In the meanwhile, a small amount of Ti5Si3 have been observed around the β phase. The strength of the composites increased continuously with the increase of SiCp contents. Due to effects of grain refinement and load transfer strengthening and solution strengthening from C, O and Si elements, the composites of 0.5 wt% SiCp/TC4 has achieved a high yield strength and an ultimate tensile strength of 1053 MPa and 1144 Mpa, which are higher than TC4 matrix for 33.5% and 27.1%. SiCp/TC4 composites have widespread application prospects in aerospace, military and automotive industries.

Improving surface integrity and wear resistance of selective laser melted 316L stainless steel using ultrasonic nanocrystal surface modification

In this study, ultrasonic nanocrystal surface modification (UNSM) was employed to process 316L stainless steel fabricated using selective laser melting (SLM). The surface integrity, microstructure evolution, and mechanical behavior of this alloy before and after UNSM treatment were systematically characterized. The results show that the surface integrity and wear resistance of the alloy has been significantly improved due to the synergistic effect of grain refinement, work hardening, and beneficial compressive residual stresses that resulted from UNSM processing. The effect of the gradient structure and residual stress introduced by UNSM processing on the strengthening mechanism and its effect on friction and wear behavior have been discussed. The improved surface finish, the grain refinement layer, and the presence of compressive residual stress of the UNSM SLM 316L stainless steel alloy successfully demonstrated UNSM to be a promising post-processing surface treatment technology to enhance the fatigue performance of additively manufactured metallic materials.

Investigating the role of processing temperature on the microstructure evolution, mechanical properties, and corrosion behaviour of equal channel angular pressed AZ31 Mg alloy

AZ31 magnesium (Mg) alloy was subjected to equal channel angular pressing (ECAP) through Bc route at three different temperatures (200, 250 and 300 °C) to investigate the role of processing temperature on the microstructure evolution. All the sample were processed for one complete cycle which includes 4 consecutive pressings. Workpiece processed at 200 °C has shown relatively higher grain refinement (2.1 ± 1.4 μm) compared with the samples processed at 250 °C (5.6 ± 2.7 μm) and 300 °C (7 ± 5.5 μm) from a starting size of 36 ± 4.1 μm. Increased hardness was measured for the ECAPed samples at 200 °C due to the higher level of grain refinement. Workpiece produced at 200 °C exhibited higher tensile strength (270.9 ± 6.3 MPa) compared with the other processed samples and base alloy. Surface energy measurements from the contact angles show the positive effect of grain refinement on increasing the hydrophilic nature of AZ31 alloy. Corrosion experiments from the potentiodynamic polarization studies demonstrate lower corrosion current for the base alloy compared with all of the processed alloys. The deteriorated corrosion performance in the ECAPed alloys can be claimed to the increased crystal imperfections due to ECAP. From the results, it is understood that the lower processing temperatures are preferable to achieve higher level of grain refinement in ECAP of AZ31 Mg alloy. Excellent mechanical properties can be achieved in AZ31 alloy by ECAP and the decreased corrosion resistance is a concern to pay more attention.

Insights into the evolution of texture and mechanical properties during warm rolling and annealing of Mg–2Zn-0.5Ca alloy

In this work, we systematically investigated the microstructure, texture evolution and mechanical properties of Mg–2Zn-0.5Ca alloy during warm rolling and annealing. The results showed that the net-like Mg6Zn3Ca2 secondary phases are evolved into short wire-like and fragmented into smaller pieces with the increasing rolling reduction. Besides, some recrystallization grains can be generated with a higher rolling reduction, resulting in a significant grain refinement, which is prone to suppress the activation of twinning. The formation of a double-peak texture with basal poles splitting from the normal direction (ND) towards the transverse direction (TD) can be mainly ascribed to the prismatic <a> slip during warm rolling. The formation of TD-oriented recrystallized grain leads to a transformation from a broadening of angular distribution towards the rolling direction (RD) to a broadening of angular distribution towards the TD. Moreover, the yield strength is dramatically enhanced by the synergetic strengthening effect of grain refinement, work hardening, and texture evolution. After annealing, the sample with high rolling reduction can achieve a combination of high yield strength and high formability due to the fine grains structure containing massive precipitates, which is also featured by an elliptical shape texture with basal poles inclining towards RD and TD.

Achieving 2.2 GPa Ultra-High Strength in Low-Alloy Steel Using a Direct Quenching and Partitioning Process.

Advanced high-strength steels (AHSS) have a wide range of applications in equipment safety and lightweight design, and enhancing the strength of AHSS to the ultra-high level of 2 GPa is currently a key focus. In this study, a new process of thermo-mechanical control process followed by direct quenching and partitioning (TMCP-DQP) was developed based on Fe-0.4C-1Mn-0.6Si (wt.%) low-alloy steel, and the effects of microstructure evolution on mechanical properties under TMCP-DQP process and conventional hot rolled quenched and tempered process (HR-QT) were comparatively studied. The results show that the TMCP-DQP process not only shortened the processing steps but also achieved outstanding comprehensive mechanical properties. The TMCP-DQP steel exhibited a tensile strength of 2.23 GPa, accompanied by 11.9% elongation and a Brinell hardness of 624 HBW, with an impact toughness of 28.5 J at -20 °C. In contrast, the HR-QT steel exhibited tensile strengths ranging from 2.16 GPa to 1.7 GPa and elongations between 5.2% and 12.2%. The microstructure of TMCP-DQP steel primarily consisted of lath martensite, containing thin-film retained austenite (RA), nanoscale rod-shaped carbides, and a minor number of nanoscale twins. The volume fraction of RA reached 7.7%, with an average carbon content of 7.1 at.% measured by three-dimensional atom probe tomography (3DAP). Compared with the HR-QT process, the TMCP-DQP process resulted in a finer microstructure, with a prior austenite grain (PAG) size of 11.91 μm, forming packets and blocks with widths of 5.12 μm and 1.63 μm. The TMCP-DQP process achieved the ultra-high strength of low-alloy steel through the synergistic effects of grain refinement, dislocation strengthening, and precipitation strengthening. The dynamic partitioning stage stabilized the RA through carbon enrichment, while the relaxation stage reduced a small portion of the dislocations generated by thermal deformation, and the self-tempering stage eliminated internal stresses, all guaranteeing considerable ductility and toughness. The TMCP-DQP process may offer a means for industries to streamline their manufacturing processes and provide a technological reference for producing 2.2 GPa grade AHSS.

Effect of isothermal multidirectional compression on microstructure and mechanical properties of Al–Zn–Mg–Cu–Zr alloy with minor Sc addition

Sc microalloying and isothermal multidirectional compression (IMC) are important methods to refine grains and improve the mechanical properties of Al–Zn–Mg–Cu–Zr alloys. Herein, EBSD, SAXS and TEM have been carried out to investigate the effect of IMC and minor Sc on the microstructure, especially for grain boundary wetting, and mechanical properties of aged Al–Zn–Mg–Cu–Zr alloys by melting and hot compression process parameter optimization. The results show that the strength and plasticity can be obviously improved simultaneously by minor Sc addition and IMC. Especially, stripy GBs (one kind of typical GB complexion) have been widely observed for alloy samples after microalloying Sc and IMC, which indicates the occurrence of GB wetting for Al–Zn–Mg–Cu–Zr alloy. And the GB wetting degree gradually increases for alloys with only microalloying Sc, only IMC and (microalloying Sc + IMC). Combined with GB wetting, the formation of Al3(Sc, Zr) due to Sc addition could inhibit the motion of dislocation and grain boundary, which promotes formation and inhibit growth of recrystallized grains, leading to further grain refinement. Besides, GB wetting degree would affect the precipitate distribution, especially for grain boundary precipitates. Also, GB wetting probably promote grain bridging, which is beneficial to improve the yield strength and ductility of Al–Zn–Mg–Cu–Zr alloys. The concurrent effect of grain refinement, the precipitate including finer Al3(Sc, Zr) particles, dislocation strengthening and grain bridging contribute to the simultaneous improvement of the strength and ductility of Al–Zn–Mg–Cu–Zr alloys under IMC and Sc. This study provides a new understanding on microalloying Sc combined with IMC on the microstructure and properties of Al–Zn–Mg–Cu–Zr alloys.