Substructure Evolution Research Articles

In-situ EBSD study of the active slip systems and substructure evolution in a medium-entropy alloy during tensile deformation

Electron backscatter diffraction (EBSD) coupled with in-situ tensile loading is powerful for investigating the microstructural evolution of alloys. Thermo-mechanically treated f.c.c. medium-entropy alloys (MEAs) typically have high densities of annealing twin boundaries (ATBs), which can not only strengthen but also toughen the MEA via interacting with dislocations. However, the evolution of ATBs and other substructures in plastically-deformed MEA has not yet been revealed. Plastic deformation involving dislocation evolution, active slip systems, lattice rotation, boundary transformation, and grain subdivision in a polycrystalline MEA Ni41.4Co23.3Cr23.3Al3Ti3V6 was studied using in-situ EBSD. The slip was accompanied by heterogeneous lattice rotation among grains and within grains, where inhomogeneous plasticity was accommodated by geometrically-necessary dislocations (GNDs). Both GND and low-angled boundaries (LABs) densities substantially increased with progressive strain, which was mainly concentrated in sites approaching ATBs or grain boundaries (GBs). Located stress, lattice rotation, or curvature caused a loss in the coherence of ATBs, which resulted in integrity loss with increasing strain and promoted a decrease in density by 60 %. Further, lattice rotation incompatibility due to constraints from neighboring grains leads to grain fragmentation into various misorientated volumes, which were separated by LABs or high-angle boundaries (HABs). The grain orientation angle increased with progressive strain and crystallographic 〈111〉 orientation gradually spread toward a tensile direction. Slip systems with maximum Schmid factor were activated first at ε ≥ 3.9 %, which is almost the same with experimental slip traces. Both single slip and double slip occurred during plasticity, where straight slip traces tend to curve due to lattice curvature. Slip transfers are not only controlled by geometric compatibility factor, which can occur between some neighboring grains with low geometric compatibility factor but high Schmid factor.

Influence of phase stability on postshock mechanical properties and microstructure evolution in Ti-17 alloy

There has been a growing surge of interest in examining the shock response of titanium alloys, owing to their considerable potential for military applications. The present study aims to reveal the influence of phase stability on the shock-induced mechanical response and substructure evolution of a metastable β titanium alloy, namely, Ti-17. This investigation included extensive work, such as plate impact tests, quasi-static reloading compression tests, and electron microscope analyses. The microstructural evaluations following the shock-wave loading unveil planar slip as the prevailing deformation mechanism in Ti-17 with a bimodal microstructure with stable α and β phases. However, when the shock stress exceeds 10 GPa, the activation of {101¯1}α nano-sized twins was observed, leading to improved reloading ductility. This implies a novel strategy to achieve excellent strength-plasticity compatibility in titanium alloys through appropriate shock-wave loading. Conversely, in Ti-17 with an equiaxed β microstructure, the metastability of the β phase leads to the activation of shock-induced α″ martensite, shock-induced ω, and planar slip. Two distinct forms of interaction involving the α″ laths, i.e., shear and truncation, were also observed. Phase stability greatly influences substructure evolution, which ultimately controls the reloading mechanical properties of the postshock Ti-17 alloy.

Plastic deformation and strengthening mechanisms in CoNiCrFe high entropy alloys: The role of lattice site occupancy

The work herein presents the designing of two γʹ strengthened high entropy alloys guided by density function theory (DFT) and thermodynamics calculations with compositions Co34Ni34Cr12Al8Nb3Ti4Fe5 and Co31.5Ni31.5Cr12Al8Nb3Ti4Fe10 (referred as 5Fe and 10Fe). These alloys in the peak aged condition (900 °C for 20 h) exhibit similar precipitates sizes, shapes, volume fractions and γ/γʹ lattice misfit (∼ 0.56). Intriguingly, despite their microstructural similarities, these alloys show different trends in yield strength (YS) evolution over a temperature range. The 5Fe alloy shows a better combination of strength and ductility at room temperature (RT), with YS and elongation of 970 ± 25 MPa, ∼ 18 (%), respectively, in comparison to 850 ± 20 MPa, and ∼ 15(%) in the 10Fe alloy. The precipitate chemistry analyses carried out by 3D atom probe tomography suggest that Fe atoms occupy B-sites in the 5Fe alloy, while it occupies both A and B-sites in the 10Fe alloy. The site occupancy behaviour rendered a higher stacking fault energy (SFE) of the 5Fe alloy, making the γʹ shearing more difficult compared to the 10Fe alloy. The synchrotron X-ray measurements further confirm higher stacking fault (SF) probability in the γ matrix compared to γʹ precipitates in the 5Fe alloy. The role of deformation substructure evolution is also carefully discussed to explain the differences in the high temperature behavior. These results on the effects of alloying chemistry in high entropy alloys enable tuning the mechanical properties of alloys and widening the alloy spectrum with improved high-temperature properties.

Subgrain Size Modeling and Substructure Evolution in an AA1050 Aluminum Alloy during High-Temperature Compression.

For materials with high stacking fault energy (SFE), such as aluminum alloys, dynamic recovery (DRV) and dynamic recrystallization (DRX) are essential softening mechanisms during plastic deformation, which lead to the continuous generation and refinement of newborn subgrains (2° ˂ misorientation angle ˂ 15°). The present work investigates the influence of compression parameters on the evolution of the substructures for a 1050 aluminum alloy at elevated temperatures. The alloy microstructure was investigated under deformation temperatures ranging from 300 °C to 500 °C and strain rates from 0.01 to 0.1 s-1, respectively. A well-defined substructure and subsequent subgrain refinement provided indication of the evolution laws of the substructure under high-temperature compression. Corresponding experimental data on the average subgrain size under various compression conditions were obtained. Two different independent average subgrain size evolution models (empirical and substructure-based) were used and applied with several internal state variables. The substructure model employed physical variables to simulate subgrain refinement and thermal coarsening during deformation, incorporating a corresponding dislocation density evolution model. The correlation coefficient (R) and root mean square error (RMSE) of the substructure-based model were calculated to be 0.98 and 5.7%, respectively. These models can provide good estimates of the average subgrain size, with both predictions and experiments reproducing the expected subgrain size evolution using physically meaningful variables during continuous deformation.

Stress sensitivity and its influence on high-temperature creep behavior of a novel 2nd-generation low-cost nickel based single crystal superalloy

To meet the application requirements under complex service conditions for aero engine blades, the influence of stress-state on creep behavior of a self-designed Re-low Ni-based single crystal superalloy at 1038 °C over a wide stress range of 137–207 MPa was investigated. With applied stress decreasing from 172 to 137 MPa, the creep life rapidly extends by a factor of 2.41, showing strong stress dependence. Further, the creep mechanisms under different stresses are discussed based on microstructural characterization and threshold stress calculation results. With the increasing applied stress, interfacial dislocation networks exhibit sparse and irregular shapes. Particularly, in necking zone of the fractured specimen at 1038 °C/137 MPa, due to the long-duration accumulation of deformation storage energy, dislocation arrays within γ′ rafts gradually evolve into dislocation walls through recombination and annihilation of dislocations, then develop into subgrain boundaries. Finally, some recrystallization grains with large misorientations (>10°) form by subgrain-rotation. Moreover, based on the calculated threshold stress, dislocations climbing can play a major role in creep deformation throughout the steady-state stage under a stress range of 137–207 MPa. Nevertheless, with the arrival of tertiary stage, the increasing actual stress resulted from necking deformation can also promote dislocations to cut or bypass γ′ rafts under lower stresses (137 and 172 MPa), which is consistent with the final evolution of dislocation substructures. Overall, the findings from this work are helpful to understand the high-temperature creep mechanism of low-cost single crystal superalloys, so as to improve the creep resistance of materials.

Excellent strength-ductility synergy assisted by dislocation dipole-induced plasticity in Co-free precipitate-strengthened medium-entropy alloy

Precipitation strengthening is one of the most effective approaches for developing advanced structural materials with outstanding strength-ductility combinations. However, most compositional designs of precipitate-strengthened HEAs/MEAs compromise the cost-property tradeoff owing to the addition of expensive Co element. In this study, a Co-free FeCrNi-based precipitate-strengthened medium entropy alloy (denoted as Al0.2Cr0.9FeNi2.2Ti0.2) with a near-equiatomic FeCrNi matrix and a high content (∼ 35 %) L12 nanoprecipitates was designed using a mixing strategy. The microstructural features, mechanical performance, deformation substructure evolution, and strengthening mechanisms were systematically investigated using EBSD, TEM, and APT. Tensile tests indicated that the current alloy aged within a moderate temperature range achieved an exceptional strength-ductility combination compared to existing Co-containing and Co-free HEAs/MEAs. Particularly, the alloy aged at 700 ℃ (denoted as 700A) demonstrated a high ultimate tensile strength of 1606 MPa and a large ductility of 25 %, benefiting from both precipitation hardening and an unusual strain-hardening sustainability. Such anomalous strain-hardening sustainability can be attributed to the dislocation dipole-induced plasticity. High-density dislocation dipoles can simultaneously provide additional strain hardening by reducing the dislocation mean free path and enhance plastic deformation compatibility by acting as stress delocalization origins, thereby contributing to excellent strength-ductility synergy. These findings will not only open a new door for the future development of high-performance Co-free precipitate-strengthened HEAs/MEAs, but also deepen the understanding of the work-hardening mechanisms in these alloys.

Studying the effect of Cr content on microstructure and deformation behavior of (NiCo)88-xCrxAl10Ta2 medium-entropy alloy through L-DED gradient material

Optimization of Cr content is crucial for γ'-strengthened NiCoCr-based medium entropy alloys, influencing the stacking fault energy, deformation behavior and the precipitation of brittle σ phase. In this study, (NiCo)88-xCrxAl10Ta2 gradient materials were prepared by L-DED, and the influence of Cr content on the phase precipitation and tensile deformation behavior was investigated. As the Cr content decreases from 24.1 at% to 13.9 at%, the aging time for σ phase precipitation at the grain boundaries at 750°C increases from 100 hours to 215 hours. Meanwhile, the coarsening rate of γ' phase at 750°C are found to be 7.84×10−28 m3/s and 13.6×10−28 m3/s for the Cr content of 24.1 at% and 18.7 at%, respectively. It suggests that the reduction in Cr content notably suppresses the precipitation of σ phase, and accelerates the coarsening rate of γ' phase. Tensile testing of the gradient material reveals that the lower Cr content region exhibits fewer and shorter stacking faults during tensile deformation due to its higher stacking fault energy, leading to the rapid development of local strain, subsequent necking, and eventual fracture. Taking into account the evolution of microstructure, hardness, and deformation substructure with varying Cr content, the appropriate Cr content in (NiCo)88-xCrxAl10Ta2 medium entropy alloy is 17–24 at%.

Substructure Evolution and Mechanical Properties in Martensitic Stainless Steel During a High Cycle Number of Repetitive High Stress Loading–Unloading Process

Substructure Evolution and Mechanical Properties in Martensitic Stainless Steel During a High Cycle Number of Repetitive High Stress Loading–Unloading Process

Continuous Dynamic Recrystallization and Deformation Behavior of an AA1050 Aluminum Alloy during High-Temperature Compression

Continuous dynamic recrystallization (CDRX) forms a new recrystallized microstructure through the progressive increase in low-angle boundary misorientations (LAGBs) during the hot forming of metallic materials with high stacking fault energy (SFE), such as aluminum alloys. The present work investigates the effect of deformation parameters on the evolution of the dynamic recrystallization microstructures of an AA1050 aluminum alloy during compression at elevated temperatures. The alloy microstructure is investigated at deformation temperatures and strain rates in the range of 300 °C to 500 °C and 0.001 to 0.8 s−1. A well-defined substructure and subsequent DRX grains provide indication that recrystallization can proceed with continued strain under high-temperature compression. At a strain rate of 0.1 s−1, the DRX fraction is observed to be 0.25 at a temperature of 300 °C. This fraction increases to 0.32 as the temperature rises to 400 °C. The recrystallization mechanism is identified by analyzing the flow stress, the evolution of the subgrain misorientation angle, and the distribution of recrystallized grains. The observations of discontinuous dynamic recrystallization (DDRX) and CDRX under various deformation parameters are discussed. Moreover, the main substructure evolution laws observed from the high-temperature compression of an AA1050 Al alloy are summarized.

Microstructural origins of enhanced work hardening and ductility in laser powder-bed fusion 3D-printed AlCoCrFeNi2.1 eutectic high-entropy alloys

Limited tensile ductility usually restricts the practical applications of new classes of high-strength materials in many industrial fields. Therefore, in-depth understanding of the work hardening behavior and its underlying plastic deformation mechanism are critical for the newly developed high-entropy alloys (HEAs). In this work, a geometric atomistic model of face-centered cubic (FCC)/ordered body-centered cubic (BCC (B2)) interfaces and the evolution of dislocation substructures have been investigated to explore the microstructural origins of work hardening responses for two additively manufactured AlCoCrFeNi2.1 eutectic high-entropy alloys (EHEAs) with the respective lamellar and cellular microstructures. Unlike the lamellar interphase interfaces with the most classical Kurdjumov-Sachs (KS) FCC-BCC relationship of {111}FCC∥{110}B2〈011〉FCC∥〈111〉B2, the Nishiyama-Wassermann (NW) relationship, namely {111}FCC∥{110}B2〈011〉FCC∥〈001〉B2, is observed to be dominant on the cellular interphase interfaces. Furthermore, our intermittent high-resolution transmission electron microscopy (HR-TEM) results directly show that the deformation of lamellar AlCoCrFeNi2.1 alloy first proceeds with massive stacking faults (SFs) and then dislocation walls developed across phases interfaces, due to the effective dislocation transfer capability of lamellar boundaries. The large uniform elongation of the cellular AlCoCrFeNi2.1 alloy is attributed to the stable and progressive strain-hardening mechanism that is stemmed from the activated multiple slip systems, deformation-induced SF networks, and the associated Lomer-Cottrell locks in the middle and later stages of plastic deformation. Moreover, the nano-bridging of FCC cells in the 3D-printed microstructure provides unique channels for dislocation movement, which offsets the “blocking effect” of cellular boundaries and thus suppresses the pre-mature fracture.

Substructure and texture evolution of a novel near-α titanium alloy with bimodal microstructure during hot compression in α+β phase region

Ti65 alloy has extensive potential applications in manufacturing high-temperature components in the aerospace industry. During thermomechanical processing in the α+β phase region, complicated microstructure and texture evolution occurs. In this work, isothermal hot compression experiments were conducted to comprehensively investigate the substructure and texture evolution of Ti65 alloy with bimodal microstructure via the electron backscatter diffraction (EBSD) technique, and the influence of processing parameters on the evolution of substructure and texture was systematically analyzed. The experimental results showed that the distribution of grain boundaries and the transformation of low-angle boundaries (LABs) to high-angle boundaries (HABs) exhibited sensitivity to temperature and strain rate. The fraction of sub-grain boundaries decreased, while the fraction of HABs increased with the increasing of strain rate or temperature due to the spheroidization of the lamellar α phase accompanied by the wedging of the β phase during deformation and the relative misorientation angle distribution between different α variants precipitated from β phase. Temperature rising at a high strain rate resulting from adiabatic heat promoted atomic activity, caused dislocations to be absorbed by sub-grain boundaries or HABs, and finally reduced the dislocation density in both αp and αs phases. Dynamic spheroidization was the main mechanism with increasing temperature, and coarsening of the lamellar α phase was the main mechanism with increasing strain rate. With the increase of temperature or decrease of strain rate, the texture of the αp phase became stronger and the basal and pyramidal slip could be activated. For the αs phase, spheroidization and variant selection together influence the orientation distribution. As a result, the texture evolution showed a complicated tendency.

Microstructural evolution and spheroidization mechanism of powder metallurgy Ti-6Al-4 V alloys after high-temperature forging

After high-temperature forging and rapidly cooling, the microstructure evolution and spheroidization mechanism of powder metallurgy Ti-6Al-4 V alloy have been clarified. The spheroidization process is divided into two asynchronous stages. The initial stage is primarily characterized by the initial shearing and twisting of GBα grain boundaries, as well as the deformation of lamellar structure. Subsequently, the fragmentation of original GBα grain boundaries and the fracture of intra-granular α lamellae are observed. This process is accompanied by grain twisting and substructure formation, especially the division of lamellae through interface splitting and the segregation of local chemical elements. The differential accumulation of elements around the fractured and twisted grains suggests that element segregation and interfacial energy play a significant role in microstructure evolution. Therefore, the static spheroidization process largely depends on the formation and evolution of dislocation substructures, and the reduction of interfacial energy may be a key factor driving this process. These findings provide a theoretical basis for manufacturing metals with specific anisotropic properties by controlling microstructure evolution.

Controlling of cellular substructure and its effect on mechanical properties of FeCoCrNiMo0.2 high entropy alloy fabricated by selective laser melting

Fine cellular substructures are typical microstructure feature of high entropy alloys (HEAs) produced via selective laser melting (SLM), playing a pivotal role in improving the mechanical properties. Nonetheless, the controlling of cellular substructure and its impact on the mechanical properties remains ambiguous. This study investigates the effect of energy densities on the cellular substructure evolution and mechanical properties of the FeCoCrNiMo0.2 HEA. It is found the increase in energy density causes a decrease in temperature gradient (G) and solidification rate (R) in molten pool, consequently leading to the increase of cellular substructure size and the intensification of Mo segregation at cellular substructure boundaries. The cellular substructure size (d) can be described by formula d=80(GR)−1/3, in which G and R can be obtained from discrete element method - computational fluid dynamics (DEM-CFD) simulation. The strength of the FeCoCrNiMo0.2 HEA is significantly affected by dislocation strengthening (σd) and segregation strengthening (σs), which are further determined by the cellular substructure size and the Mo segregation, respectively. The increase of cellular substructure size leads to the decrease of σd, while the intensification of Mo segregation results in the increase of σs. The competition between these two strengthening effects leads to an optimized and excellent tensile property of 707 MPa in yield strength, 947 MPa in ultimate tensile strength and 34 % in fracture elongation at a moderate energy density of 47 J/mm3. The findings provide guidance towards the advancement of high-performance high entropy alloys fabricated by SLM in terms of cellular substructure controlling.

Understanding the sigmoidal primary creep behaviour in a multi-component solid-solution Co-base alloy between 700 – 850 °C

This investigation was motivated by the lack of understanding of the role of solid solution strengthening and substructure evolution during the creep of multi-component alloys. Thus, we studied the tensile creep behaviour of a multi-component solid-solution Co-base alloy (Co41-Ni23-Cr26-W4.7-Fe2-Mn1-C0.5) at 650 – 900 °C and stress (σApplied) range: 80 – 400 MPa. Uncommon sigmoidal primary creep (SPC) was witnessed at 700–850 °C and in the intermediate stress range. The stress exponent (∼5 – 7) and activation energy (∼270 kJ/mol ≈ Qlattice) suggest that dislocation creep controls the creep rate. Transmission electron microscopy (TEM) studies revealed substructural evolution with creep strain. At 750 °C and 160 MPa, at creep strain (ε) = 0.25 %, multiple pinning points along the dislocation length were seen, and a thorough analysis confirmed them as jogs. Friction stress estimation from dislocation curvature from multiple TEM micrographs revealed that effective stress on dislocations is high (>0.6σApplied) while solid solution strengthening is nominal during creep. Overall analysis suggests that the creep rate in the multi-component alloy is recovery controlled while jogs' motion determines the SPC creep rate.

Mesoscale modeling of continuous dynamic recrystallization in Ti-10V-2Fe-3Al alloy

Continuous dynamic recrystallization (CDRX) is the dominant restoration mechanism of near-β titanium alloys during hot deformation. But till now, there is seldom visual modeling of the process which limits its industrial application. Based on CDRX mechanisms characterized by low-angle grain boundaries (LAGBs) formed and partially transformed into high-angle grain boundaries (HAGBs), the present study established a cellular automata (CA) model to investigate the substructure evolution and mechanical response of Ti-10V-2Fe-3Al alloy. Besides, a new set of equations for subgrain rotation considering the effect of the α-phase was integrated into the CA model to improve its applicability in titanium alloy. The model was validated by experimental results and then applied to predict the substructure evolution during hot working. Simulation analysis revealed that, except for high strain rates and temperatures, a high fraction of α/β phase boundaries could also promote the development of CDRX. The proposed CA model provided an effective method for microstructure optimization in hot forging of the Ti-10V-2Fe-3Al alloy.

Condition Monitoring and Quantitative Evaluation of Railway Bridge Substructures Using Vehicle-Induced Vibration Responses by Sparse Measurement

Bridge substructure failure has been responsible for numerous recorded bridge collapses, particularly for small- and medium-span bridges, so it is crucial to effectively monitor the performance of the bridge substructures for efficient maintenance and management. The current vibration-based approaches for quantitatively evaluating bridge substructures rely on in-situ experiments with a multitude of sensors or impact vibration test, making it challenging to implement long-term online monitoring. This paper proposes an accurate, low cost, and practicable method to achieve online quantitative monitoring of railway bridge substructures using only one vibration sensor and operational train-induced vibration responses. The newly derived flexible-base Timoshenko beam models, along with the random decrement technique and Levenberg–Marquardt–Fletcher algorithm, are employed to identify the modal parameters and quantitatively assess the condition of bridge substructures. The proposed method is numerically verified through an established 3D train-bridge-foundation coupling system considering different damage scenarios. In addition, a real-world application is also conducted on the 2nd Songhua River bridge in the Harbin–Dalian high-speed railway, aiming at examining the effectiveness and robustness of the method in condition monitoring of bridge substructure under a complete freeze-thaw cycle. The results indicate that the proposed methodology is effective in extracting the modal parameters and monitoring the state evolution of the bridge substructures, which offers an efficient and accurate strategy for condition monitoring and quantitative evaluation of railway bridge substructures.

Deformation substructure and texture evolution of (VCoNi)99.9C0.1 medium-entropy alloy under different rolling reductions

The deformation substructures and texture characteristics of (VCoNi)99.9C0.1 medium-entropy alloy (MEA) upon the different rolling reductions were systematically investigated in this work. The rolling deformation caused the deviation of annealing twin orientation and thus led to the gradually reduced annealing twin variants. The dislocation induced-orientation analysis was carried out within the deformed grains, confirming the presence of three kinds of orientation spreads (type 1, type 2 and type 3). The dominant orientation spreads of type 1, type 2 and type 3 were emerged in a sequential order of low, medium and heavy strains, respectively. As strains progressed, the intensities of 111<12̅1> R1 and {111}<011̅> R2 components were significantly weakened, while {112}<111̅> Copper and {123}<634̅> S components were consistently persisted with the strengthening orientations. For the heavily deformed sample, the strong {110}<001> Goss and {110}<11̅2>Brass components were revealed and the maximum intensity was obtained at a location between Goss and Brass components, which were ascribed to the chemical short-range order-assisted planar slip. It was worth noting that the carbide particles show the weak effect on the texture components and intensity distribution. As increasing rolling reductions, rolling deformation led to the significant improvement of strength, due to the work hardening.

Multi-scale investigation of microstructure and texture evolution during equal channel angular pressing of silver

A multi-scale investigation was carried out on pure silver subjected to equal channel angular pressing up to 3 passes. Microstructure and texture were examined using scanning and transmission electron microscopy as well as diffraction of neutron and high-energy synchrotron radiation. The evolution of the deformation substructure in the material is in accordance with its low stacking fault energy followed by restoration mechanisms leading to overall microstructural refinement. The restoration mechanism sets in as early as after 3 passes. Twinning involved in the deformation process supports grain refinement through mechanisms of dynamic recrystallization involving strain-induced boundary migration contributing to a steady-state grain size. The global texture, as measured by neutron diffraction, forms through a twin-assisted deformation mechanism. The texture is found heterogeneous through thickness.Graphical abstract

Corrosion behavior of selective laser melting-manufactured bio-applicable 316L stainless steel in ionized simulated body fluid

Additive manufacturing (AM) is gaining increasing popularity in various fields, including biomedical engineering. Although AM enables fabrication of tailored components with complex geometries, the manufactured parts typically feature several internal issues, such as unpredictable distribution of residual stress and printing defects. However, these issues can be reduced or eliminated by post-processing via thermomechanical treatment. The study investigated the effects of combinations of AM and post-processing by the intensive plastic deformation method of rotary swaging (variable swaging ratios) on microstructures, residual stress, and corrosion behaviors of AISI 316L stainless steel workpieces; the corrosion tests were performed in an ionized simulated body fluid. The results showed that the gradual swaging process favorably refined the grains and homogenized the grain size. The imposed swaging ratio also directly influenced the development of substructure and dislocations density. A high density of dislocations positively affected the corrosion resistance, whereas annihilation of dislocations and formation of subgrains had a negative effect on the corrosion behavior. The first few swaging passes homogenized the distribution of residual stress within the workpiece and acted toward imparting a predominantly compressive stress state, which also favorably influenced the corrosion behavior. Lastly, the presence of the {111}||swaging direction texture fiber (of a high intensity) increased the resistance to pitting corrosion. Overall, the most favorable corrosion behavior was acquired for the AM sample subjected to the swaging ratio of 0.8, exhibiting a strong fiber texture and a high density of dislocations.

Multi-stage strain-hardening and nano-twinning strengthening Co40Cr22Ni15Fe14Mo4Si3Mn2 multi-principal element alloy

CoCrNiFe multi-principal element alloy (MPEA) plates were prepared by melting and rolling. Thermomechanical treatment (cold rolling and aging) was developed to modify the formation of lattice defects and precipitates. The results show that the Co40Cr22Ni15Fe14Mo4Si3Mn2 MPEA have excellent room temperature ultimate tensile strength and hardness of 1946 MPa and 684 HV0.1. Electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) techniques were used to reveal the microstructure and provide insight into the mechanism. The strengthening mechanism comes from the multiple factors, and most notably, the interaction between nano-twins and Lomer-Cottrell locks. The presence of nano-twins and stacking faults has a significant impact on the strain hardening behavior of the alloy. At the same time, the occurrence and development of deformation substructure dominated by dislocation configuration is the reasons for the material to achieve multi-stage strain hardening (MSSH). This method of realizing MSSH behavior by adjusting microstructure is of great significance in the design and application of FCC MPEA and can be extended to other FCC alloys.