Search

This study delves into effects of deep cryogenic treatment (DCT) on enhancing the hydrogen embrittlement resistance of CoCrFeMnNi high-entropy alloy fabricated via laser powder-bed-fusion (L-PBF). Comparatively assessing as-print, conventional heat treatment, and DCT, we uncover how nanotwin formation within matrix serves as a critical mechanism to combat adverse effects of hydrogen embrittlement. This work reveals that DCT not only mitigates inherent residual stresses from L-PBF, thereby fostering dislocation redistribution and microstructural stabilization, but also synergizes with high-density dislocation cells. Our findings articulate a nuanced understanding of microstructural evolution in response to post-treatments and consequential enhancement of hydrogen embrittlement resistance.Highlights Laser-based additively manufactured CoCrFeMnNi undergoes post heat treatment and deep cryogenic treatment to tailor dislocations and residual stress.The inherent residual stress acts as the driving force for the redistribution of dislocations, leading to the development of a stable microstructure after deep cryogenic treatment.High-density dislocation cells and hydrogen collaborate to lower the local stacking fault energy, triggering gradient nanotwins.Nanotwin-ability effects by deep cryogenic treatment (77 K × 36 h) on the hydrogen embrittlement resistance of CoCrFeMnNi with interior defects are verified.

High-strength materials are essential for applications in the structural industry. However, their limited formability restricts broader industrial use. This study proposes a novel methodology to achieve an excellent combination of formability and strength. Surface heterostructuring through laser treatment induces local austenitization on the as-rolled martensitic stainless steel. The resultant bendability was significantly improved, enabling its bending to ∼180° folding while maintaining high tensile strength. Consequently, the present surface heterostructuring suggests achieving the superior combination of strength and formability by tailoring the microstructure specifically to forming. This surface heterostructuring will explore the empty area of the formability-strength relationship.

This study investigated toughness properties of FCC-based Fe45Co30Cr10V10Ni5-xMnx high entropy alloys (HEAs). Ductile-dimpled rupture mode in all alloys and test temperatures complicated precise explanations on Charpy impact energy, but the instrumented Charpy test provided a breakdown of total energy (ET) into initiation energy (EI) and propagation energy (EP), offering critical insights into Mn-content- and temperature-related energy variations. Pmax and TP, reflecting maximum flow stress and time, respectively, until initiation of crack propagation from the notch tip, involved a transition of deformation mechanisms from slip to TWIP, then to BCC-TRIP, thereby inducing increased strain hardening and delaying plastic instability at the notch tip. At 25 °C, an increase in Mn content prompted a transition in deformation behavior from slip to TWIP and subsequently to BCC-TRIP, correlating well with increased Pmax and TP and consequently ET due to enhanced strain hardenability. At −196 °C, predominant BCC-TRIP activity in all alloys led to increased formation of BCC martensite, intensifying strain hardening and requiring higher Pmax for crack initiation than at 25 °C. Due to minimal slip line field formation associated with reduced plastic deformation, however, cracks initiated directly from the notch-tip center, representing fast initiation of crack propagation and consequently reduction in EI and ET. Thus, utilizing parameters from instrumented Charpy tests, including Pmax, TP, EI, EP, and ET, provided insights into fracture phenomena and their interrelations in the present HEAs.

This study aimed to develop a structural analysis method to replace and validate large-scale gas gun tests for evaluating the impact resistance of electronic equipment. By measuring projectile's deceleration through experiments and simulations, stress and strain during collisions were quantified. It was demonstrated that optimizing projectile and electronic equipment designs could enhance impact resistance. Simulation results indicated that changes in projectile velocity and head angle led to variations in maximum deceleration and penetration depth, with decreasing the head angle proving more effective in reducing maximum deceleration. Comparisons between experimental and simulation results showed good agreement, particularly in plots between maximum deceleration and target collision depth. As the head angle increased, there was a tendency for maximum deceleration to increase and target penetration depth to decrease. Comparing peak deceleration curves from gas gun tests and simulations revealed similar trends, indicating consistent results. This research provides important information for evaluating the impact resistance of electronic equipment and demonstrates that combining experiments with simulations can yield more reliable results.

Weldability plays a crucial role in the journey of high entropy alloys towards their engineering applications. In this study, gas metal arc welding was performed to join an as-rolled CoCrFeMnNi high entropy alloy using Monel 400 as the filler wire. The present research findings demonstrate a favorable metallurgical chemical reaction between the Monel 400 filler and the CoCrFeMnNi high entropy alloy, resulting in compositional mixing within the fusion zone that promotes a solid-solution strengthening effect, counteracting the typical low hardness associated to the fusion zone of these alloys. The weld thermal cycle induced multiple microstructure changes across the joint, including variations in the grain size, existing phases and local texture. The grain size was seen to increase from the base material toward the fusion zone. An FCC matrix and finely sparse Cr-Mn-based oxides existed in both base material and heat affected zone, while in the fusion zone new FCC phases and carbides were formed upon the mixing of the Monel 400 filler. The role of the filler material on the fusion zone microstructure evolution was rationalized using thermodynamic calculations. Texture shifted from a γ-fiber (in the base material) to a strong cubic texture in the fusion zone. Digital image correlation during tensile testing to fracture coupled with microhardness mapping revealed that, stemming from the process-induced microstructure changes, the micro and macromechanical response differed significantly from the original base material. This study successfully established a correlation between the impact of the process on the developed microstructural features and the resultant mechanical behavior, effectively assessing the processing-microstructure-properties relationships towards an improved understanding of the physical metallurgy associated to these advanced engineering alloys. In conclusion, this work provides an important theoretical framework and practical guidance for optimizing the engineering applications of high entropy alloys.

This study investigates microstructural changes and the presence of multiple stages in strain hardening ratio (SHR) resulting from the sequential occurrence of deformation-induced martensitic transformation (DIMT) during the tensile deformation of an additively manufactured ferrous medium entropy alloy (AMed-FeMEA). The inherent microstructural heterogeneity in AMed-alloys contributes to distinctive deformation mechanisms and instantaneous hardening behavior at varying strain levels. To comprehensively address this phenomenon, a semi-empirical constitutive model is presented that associates multiple stages in SHR with the occurrence rate of DIMT. The model demonstrates its applicability by characterizing tensile deformation of AMed-FeMEA at both room temperature and liquid nitrogen temperature.

The configurational design of heterostructures has received considerable attention to achieve a superior combination of strength and ductility. Although heterostructuring for enhanced tensile properties has been widely analyzed, studies on the effect of heterogeneity on sheet formability are still insufficient. Sheet metal formability is one of the most crucial factors for determining hetero-materials’ industrial utilization. To broaden the industrial applicability of heterostructured layered materials, investigating the effects of heterogeneity in the layered structure on formability is necessary. This study investigated the stretch formability of aluminum-steel-aluminum clad sheets using the Erichsen cupping test. The three-layered clad sheets showed additional hardening with a slight increase in ductility owing to hetero-deformation-induced hardening because the strain-hardening exponent has a linear relationship with stretchability or necking resistance. After necking, damage evolution owing to severe internal stress concentration led to delamination at the interface. Interestingly, delamination serves as a block to full cracking through the thickness during forming. This study contributes to the industrial applicability of heterogeneous structuring by elucidating the characteristics of heterogeneity and formability.

The current research work introduces a novel processing technique involving a combination of hot rolling and a direct quench and partitioning treatments to produce an ultra-high strength, low-carbon and lean-composition steel with superior mechanical properties and enhanced stretch flangeability. The methodology involves the introduction of a secondary partitioning step after a one-step direct quenching and partitioning (DQP) process. A detailed investigations on microstructures, tensile properties and stretch flangeability (using hole expansion testing) were carried out. The martensite-austenite two phase microstructure resulted in a remarkably improved product of strength and elongation (PSE, 24 GPa.%), the hole expansion ratio of 45% and a total elongation of 21.7%. It is shown that the stability of retained austenite, rather than its volume fraction, has a significant impact on the strain hardening rate, and therefore influences strength, ductility and stretch flangeability. The results indicate that tailoring retained austenite stability is essential for optimizing the mechanical performance and stretch flangeability of quenched and partitioned steels. Introducing secondary partitioning into the Q&P process provides a feasibility to achieve a large fraction of total retained austenite, (predominantly film-type, along with small-sized blocky retained austenite islands in the microstructure), which results in high-strength Q&P steels with excellent global and local formability. [ABSTRACT FROM AUTHOR]

This study delves into effects of deep cryogenic treatment (DCT) on enhancing the hydrogen embrittlement resistance of CoCrFeMnNi high-entropy alloy fabricated via laser powder-bed-fusion (L-PBF). Comparatively assessing as-print, conventional heat treatment, and DCT, we uncover how nanotwin formation within matrix serves as a critical mechanism to combat adverse effects of hydrogen embrittlement. This work reveals that DCT not only mitigates inherent residual stresses from L-PBF, thereby fostering dislocation redistribution and microstructural stabilization, but also synergizes with high-density dislocation cells. Our findings articulate a nuanced understanding of microstructural evolution in response to post-treatments and consequential enhancement of hydrogen embrittlement resistance. Laser-based additively manufactured CoCrFeMnNi undergoes post heat treatment and deep cryogenic treatment to tailor dislocations and residual stress. The inherent residual stress acts as the driving force for the redistribution of dislocations, leading to the development of a stable microstructure after deep cryogenic treatment. High-density dislocation cells and hydrogen collaborate to lower the local stacking fault energy, triggering gradient nanotwins. Nanotwin-ability effects by deep cryogenic treatment (77 K × 36 h) on the hydrogen embrittlement resistance of CoCrFeMnNi with interior defects are verified. [ABSTRACT FROM AUTHOR]

This study investigates microstructural changes and the presence of multiple stages in strain hardening ratio (SHR) resulting from the sequential occurrence of deformation-induced martensitic transformation (DIMT) during the tensile deformation of an additively manufactured ferrous medium entropy alloy (AMed-FeMEA). The inherent microstructural heterogeneity in AMed-alloys contributes to distinctive deformation mechanisms and instantaneous hardening behavior at varying strain levels. To comprehensively address this phenomenon, a semi-empirical constitutive model is presented that associates multiple stages in SHR with the occurrence rate of DIMT. The model demonstrates its applicability by characterizing tensile deformation of AMed-FeMEA at both room temperature and liquid nitrogen temperature. [ABSTRACT FROM AUTHOR]

Cu–Fe alloys have drawn extensive attraction due to excellent multi-functional properties, including electrical, magnetic, and mechanical properties. However, the immiscible nature of Cu–Fe alloys results in heterogeneous microstructure and unexpected mechanical properties. In this study, a small amount of Zr was added to the cast Cu60Fe40 (wt%) alloy to suppress the liquid–liquid phase separation during solidification of the alloy. The microstructure and mechanical properties of the Zr-free and Zr-containing Cu–Fe alloy were investigated. It is found that the addition of Zr successfully inhibits the heterogeneous microstructural evolution caused by liquid–liquid phase separation, while the FeZr-rich precipitates form in the dual-phase microstructure. The FeZr-rich precipitates affect grain growth of the Zr-containing alloy through the Zener pinning effect, leading to extra grain boundary strengthening and precipitation hardening. The resulting microstructure of the Zr-containing alloy substantially enhances the strength of the alloy with a loss of ductility. The fractured surface reveals that microcracks are formed near the precipitates, which is associated with the reduced elongation of the Zr-containing alloy. [ABSTRACT FROM AUTHOR]

Hole expansion ratio is widely used to estimate the stretch flangeability of sheet metals which is a critical property of formability and to evaluate the efficiency of a forming process. Although many experiments were conducted in the past to identify the key tensile properties affecting hole expansion ratio, their results failed due to the data scarcity. This work demonstrates a machine learning framework coupled with imputation methods to augment both the quantity and quality of collected experimental data. Especially, a generative adversarial imputation network (GAIN) is used to impute the missing tensile properties in the collected experimental data. With the imputed data, the hole expansion ratio is predicted through an extra tree regressor. In terms of the imputation performance, GAIN resulted in the lowest root mean square error of 0.09146 when 50 known tensile properties are randomly removed and imputed with GAIN. In terms of the hole expansion ratio prediction performance, the extra tree regressor showed the lowest root mean square error of 0.124 compared to other machine learning models. Finally, the influence of each tensile property on the hole expansion ratio is analyzed using Shapley additive explanations, an explainable artificial intelligence technique. In this study, the influences of various tensile properties on hole expansion ratio were quantitatively determined for the first time via machine learning and this analysis will accelerate the exploration of sheet metals with high formability performances. [ABSTRACT FROM AUTHOR]