Non-equiatomic FeMnCoCr high entropy alloy (HEA) exhibits exceptional strain hardening and fracture resistance due to the coupled activation of dislocation slip, deformation twinning, and martensitic transformation. As fracture ultimately governs the reliability and failure of structural materials, achieving high fracture toughness alongside strength is essential for damage-tolerant design. However, yield strength and fracture toughness are often mutually antagonistic, necessitating targeted microstructural design to achieve an optimal balance. To this end, rotary swaging was employed to achieve a sub-micron grain size in the FeMnCoCr alloy, followed by annealing at different temperatures to obtain recovered, partially recrystallized, and fully recrystallized FCC microstructures. A systematic variation in mechanical response is observed with annealing temperature: yield strength decreases from 770 ± 26 MPa (773 K) to 370 ± 23 MPa (973 K), while fracture toughness (KJIc) increases from 159 ± 7 MPa.m0.5 to 226 ± 8 MPa.m0.5. The sample annealed at 873 K exhibits the optimal combination of strength (yield strength ∼670 ± 16 MPa) and fracture toughness (∼191 ± 7.5 MPa.m0.5). This balance is attributed to a trimodal microstructure comprising recovered austenite, nano-twinned reversed austenite, and partially recrystallized austenite grains, enabling synergistic strengthening and plasticity. In contrast, the sample annealed at 973 K shows superior fracture toughness and ductility (strain to failure ∼0.70 ± 0.04), driven by enhanced FCC→HCP martensitic transformation. These results demonstrate that thermo-mechanical processing effectively tailors the interplay among microstructure, transformation-induced plasticity, and fracture resistance, providing a robust pathway to optimize the strength-fracture toughness trade-off in metastable HEAs.

Friction stir processing of peak-aged, age-hardenable Al alloys often produces a softened processed zone. The recovery of strength through post-aging is limited by solute loss to incoherent precipitates formed in the wake of the tool, and by suppressed natural aging due to reduced supersaturated vacancy concentrations. Since pre-stretching prior to aging accelerates precipitation kinetics of the primary strengthening T1 phase in Al-Cu-Li-Mg alloys, its potential for restoring joint strength is examined in this study. AA2198 sheets were friction stir processed, followed by pre-stretching to 2% and 4% strain and subsequent aging at 155 degrees C for 8, 16, and 24 h. Global and local mechanical responses were evaluated using microhardness mapping and transverse tensile tests coupled with digital image correlation, and correlated with microstructural evolution characterized by differential scanning calorimetry and transmission electron microscopy. The magnitude of pre-strain and aging duration played a critical role in tailoring the mechanical properties. A 4% pre-strain followed by 24 h aging resulted in significantly enhanced yield (similar to 403 MPa) and ultimate tensile strengths (similar to 434 MPa) with adequate uniform elongation, attributed to profuse T1 precipitation that selectively strengthened the initially weak friction stirred regions. In contrast, a 2% pre-strain aged for 16 h at 155 degrees C provided an improved strength-ductility synergy. These findings demonstrate an effective pathway to mitigate property degradation in T8-tempered Al alloys processed by friction stir-based techniques.

Friction stir processing of peak-aged, age-hardenable Al alloys often produces a softened processed zone. The recovery of strength through post-aging is limited by solute loss to incoherent precipitates formed in the wake of the tool, and by suppressed natural aging due to reduced supersaturated vacancy concentrations. Since pre-stretching prior to aging accelerates precipitation kinetics of the primary strengthening T1 phase in Al-Cu-Li-Mg alloys, its potential for restoring joint strength is examined in this study. AA2198 sheets were friction stir processed, followed by pre-stretching to 2% and 4% strain and subsequent aging at 155 °C for 8, 16, and 24 h. Global and local mechanical responses were evaluated using microhardness mapping and transverse tensile tests coupled with digital image correlation, and correlated with microstructural evolution characterized by differential scanning calorimetry and transmission electron microscopy. The magnitude of pre-strain and aging duration played a critical role in tailoring the mechanical properties. A 4% pre-strain followed by 24 h aging resulted in significantly enhanced yield (~403 MPa) and ultimate tensile strengths (~434 MPa) with adequate uniform elongation, attributed to profuse T1 precipitation that selectively strengthened the initially weak friction stirred regions. In contrast, a 2% pre-strain aged for 16 h at 155 °C provided an improved strength–ductility synergy. These findings demonstrate an effective pathway to mitigate property degradation in T8-tempered Al alloys processed by friction stir–based techniques.

The fracture toughness and underlying deformation mechanisms of a non-equiatomic FeMnCrNiSi high-entropy alloy were systematically investigated using single-edge notched bend (SENB) testing and uniaxial tensile deformation. Two microstructural states were examined: a hot-rolled (HR) condition processed at 1223 K (single-phase FCC) and an annealed state (AN1173) heat-treated at 1173 K for 4 h (FCC + tetragonal sigma). The HR sample state exhibits superior strength-ductility synergy, with a yield strength of 570 ± 20 MPa, and a strain to failure of 0.33 ± 0.03, compared to 316 ± 17 MPa, and 0.12 ± 0.02 for the annealed sample state. Correspondingly, the fracture toughness of the HR sample is approximately twice (JIc ∼ 80 ± 8 kJ/m2, KJIc ∼ 133 ± 7 MPa.m0.5) that of the annealed specimen (JIc ∼ 42 ± 6 kJ/m2, KJIc ∼ 95 ± 7 MPa.m0.5). Microstructural analysis reveals that deformation in the HR sample is accommodated by a combination of dislocation slip and mechanical twinning, promoting homogeneous plasticity and enhanced crack-tip blunting. In contrast, annealing at 1173 K promotes sigma-phase precipitation, resulting in a significant reduction in fracture toughness. The brittle, mechanically incompatible sigma-phase generates localized stress concentrations, dislocation pile-up, and early crack initiation. Moreover, sigma-phase precipitation suppresses the development of the flow stress required for deformation twinning, thereby limiting crack-tip plasticity and strain accommodation. These results demonstrate that controlling sigma-phase precipitation is essential for preserving twinning-assisted plasticity and achieving an optimal balance between strength and fracture toughness in high-entropy alloys.

This study investigates microstructural transition and the corresponding strengthening mechanisms in a CoCuFeMnNi High Entropy Alloy (HEA) subjected to two thermo-mechanical routes-one at room temperature (RT) and another at cryogenic temperature (CT). These approaches result in a unique combination of strength and ductility. Cryogenic rolling leads to an exceptionally high strength of similar to 1.3 GPa, due to pronounced band formation and orientation hardening. The bands in cryo-rolled samples appear finer compared to the thicker bands in room-temperature rolled samples. However, this ultra-high strength is accompanied by relatively lower ductility. To counteract this, annealing at 900 degrees C for 5 min was performed, leading to equiaxed grain formation in all samples, with finer grains observed in the cryo-deformed specimens. The combination of rolling, annealing, and deformation temperature enhances the strength-ductility trade-off. Tensile testing reveals a yield point phenomenon and dynamic strain aging in the annealed specimens. The generated atom probe tomography data shows that the Cu nano-cluster could be the region behind them.

The present study aims to investigate the impact of unidirectional and cross-rolling modes on the microstructure, texture evolution, and mechanical property anisotropy of cold-rolled Ti6Al4V sheets. Tensile tests conducted at ambient and cryogenic temperatures revealed that cross-rolled sheets exhibited superior mechanical isotropy and reduced in-plane anisotropy compared to unidirectionally rolled sheets. Microstructural analysis using electron backscatter diffraction (EBSD) revealed elongated primary alpha grains in unidirectionally rolled sheets and refined, equiaxed primary alpha grains in cross-rolled sheets, indicative of effective dynamic recrystallization. Kernel Average Misorientation (KAM) analysis revealed higher dislocation density and enhanced recrystallization in cross-rolled samples, thereby improving mechanical properties. Furthermore, bend tests performed in accordance with ASTM standards demonstrated excellent ductility and crack-free surfaces for both rolling modes. These findings demonstrate that cross-rolling significantly reduces in-plane anisotropy and enhances isotropy and mechanical performance, making it a preferred processing technique for producing high-reliability titanium alloy sheets for critical aerospace and engineering applications.

A mechanistic understanding of the tensile response and fracture behaviour of a non-equiatomic FeMnCoCr high-entropy alloy (HEA) is developed through a combined experimental and multiscale simulation framework. A crystal plasticity model is formulated that explicitly incorporates dislocation slip, deformation twinning, and face centered cubic (FCC) to hexagonal close packed (HCP) martensitic transformation, along with basal, pyramidal slip and extension twinning in the HCP phase. The constitutive framework is integrated within a finite element scheme to examine crack-tip deformation and fracture processes. Both FCC and HCP phases exhibit pronounced strain hardening, supported by correlative electron backscattered diffraction analyses. At low strains, plasticity is dominated by dislocation slip in the FCC phase, followed by progressive FCC→HCP transformation. With increasing strain, the HCP-martensite accommodates deformation via basal slip, pyramidal slip, and extension twinning, enabling sustained work hardening and delaying strain localization. The evolution of multivariant, lamellar HCP-martensite results in hierarchical microstructural refinement, which governs fracture resistance across multiple length scales. This includes microcrack formation along FCC/HCP interfaces, development of crack-tip shielding zones (10-100 μm), and macroscopic crack deflection and bifurcation. These mechanisms collectively enhance crack tortuosity and reduce the effective driving force for crack propagation. The study establishes a direct linkage between transformation-induced plasticity, HCP-martensite deformability, and improved fracture toughness in low-stacking-fault-energy HEAs.

In the present investigation, the temperature-dependent impact behavior of a dual-phase (FCC + sigma) Fe42Mn28Co10Cr15Si5 high entropy alloy was systematically investigated using Charpy V-notch impact tests conducted at 298, 223, and 77 K. The impact toughness decreases sharply with decreasing temperature, from 24.2 ± 2.6 J at 298 K to 10.2 ± 1.3 J at 77 K, indicating a progressive loss of plastic deformation capacity under cryogenic conditions. The degradation in impact toughness is accompanied by a reduction in strain-induced martensitic transformation, attributed to an increase in the critical resolved shear stress for slip and to preferential cracking of the partially ordered sigma phase, which is necessary for superior high-temperature strength. Fractographic and microstructural analyses reveal that cracks preferentially nucleate and propagate along grain boundaries decorated with sigma phase particles, which act as stress concentrators and promote dislocation pileups. These findings underscore the critical role of controlling the size, spatial distribution, and volume fraction of the sigma phase in tailoring the dynamic fracture resistance of sigma-phase-containing metastable high entropy alloys, particularly for cryogenic applications.

Abnormal grain growth (AGG) during subsequent temper condition is one of the challenges associated with the friction stir processed (FSP) Al alloys, as it leads to substantial microstructural inhomogeneity and thus inferior mechanical properties. The mechanisms governing AGG in FSP Al alloys and the reasons behind the suppression of AGG via pre-strain rolling still remain elusive. In the present study, AA2198 (Al–3.2Cu–1.0Li–0.3 Mg–0.4Ag–0.1Zr) alloy subjected to FSP was used for a systematic study by linking AGG with secondary particles, mechanisms, and morphological evolution. The as-FSP samples were cold rolled to a 20 pct reduction in thickness (pre-strained condition). The samples in both as-FSP and pre-strained conditions were solution treated for 1 hour at 510 °C and were subsequently characterized in detail to examine the microstructural evolution. As anticipated, the as-FSP samples showed substantial AGG; however, no AGG was observed in the pre-strained samples. The study reveals that the soluble particles are ineffective in pinning down the grain boundaries, and insoluble dispersoids effectively govern the AGG. The governing mechanism is found to be the grain boundary mobility advantage, and it leads to a unique morphological signature containing large irregular AGG grains and island matrix grains. It is noted that the growth advantage arises via anisotropy in grain boundary mobilities and insufficient volume fraction of dispersoids. The mechanism of grain boundary mobility advantage and the reasons behind its suppression in AA2198 alloy are discussed in detail.

The severe plastic strain-induced mechanochemical alterations in a AA2195-0.025Sc alloy is experimentally investigated. The alloy is subjected to high-pressure torsion (HPT) processing at room temperature. Microstructural and microchemistry evolution is systematically characterized by scanning electron microscopy, transmission electron microscopy, and atom probe tomography techniques. Precipitate dissolution (Al3(LiScZr) and Al2Cu precipitates) and grain fragmentation occurs at an onset strain of 1.0 and continues till strain 4 resulting in disappearance of precipitates and appearance of high segregation at grain boundaries. Thereafter, at higher strains, the segregated solutes form nanoclusters at grain boundaries, triple junctions and HPT-induced vacancy cluster sites. Diffusion calculations are performed to account for the experimentally observed changes in precipitate state by evaluating the increased atomic mobility of solutes (Li, Sc, and Zr) by HPT-induced vacancies, dislocations, and grain boundaries. Enhanced atomic mobility of Li can be attributed to the high density of vacancies, dislocations, and grain boundaries, whereas solute drag by moving grain boundaries result in enhanced diffusion of Sc and Zr. The increase in Sc and decrease in Zr in the reprecipitated phases could be attributed to the rapid diffusion of Sc along grain boundaries along with trapping of Zr at grain boundaries.

Friction stir welding/processing (FSW/P) is a versatile solid state joining/processing technique for a range of alloys and critical applications. However, accurate prediction of mechanical response of FSW/P structures is challenging due to the limited description of the constitutive behavior of the processed region with heterogeneous microstructure. To accurately model the FSW/P structures, a methodology utilizing digital image correlation and through thickness uniformity assumption is proposed, which facilitated determination of highly resolved constitutive behavior information from a friction stirred tensile specimen. This allowed the development of accurate finite element (FE) models of FSW/P specimen including regions of rapid variations in constitutive behavior across the processed zone. The related experiments were conducted on a third-generation aerospace grade Al-Cu-Li alloy. The predicted bulk constitutive behavior and the strain distribution were a good match to the experimental data in the friction stirred as well as in a subsequent heat-treated specimen. Further, the FE models were extended to evaluate the effect of through-thickness microstructure heterogeneity and surface concavity common to FSW/P. This study demonstrates that the local constitutive behavior determined using the through thickness uniformity assumption accurately predicts the bulk deformation behavior of FSW/P specimen. This approach allows to simulate the deformation behavior of FSW/P structures in complex loading conditions.

Metastable high entropy alloys (HEAs) provide an exceptional combination of strength and ductility by the synergistic operation of slip, twinning, and transformation; however, their fracture behaviour remains unexplored. In the present investigation, tensile and elastic-plastic fracture toughness tests with a 2D digital image correlation setup were carried out for different microstructural states of Fe42Mn28Co10Cr15Si5 HEA. Finite element analysis (FEA) coupled with combinatorial site-specific electron backscatter diffraction helps in developing a meso and micro scale mechanistic understanding of the extrinsic and intrinsic toughening processes. The calculated J-integral and plastic zone size using FEA simulations were corroborated with experimental results. The crack growth resistance (J-R) curve was evaluated across three distinct processing conditions: hot rolled (HR), 1 h annealed at 1173 K (AN1173), and 4 h annealed at 1373 K (AN1373). The HR material exhibited higher strength (yield strength = 630 f 8 MPa), while the AN1373 demonstrated highest ductility (0.74 f 0.04). The mode I plane strain fracture toughness was highest for the AN1373 (125.4 f 15.8 MPa.m0.5) and lowest for the AN1173 (46.3 f 7.4 MPa.m0.5). The Cr-rich sigma phase at grain boundaries in the HR and AN1173 led to pronounced intergranular fracture, resulting in lower fracture toughness and plasticity. The multiple variants of martensite in the AN1373 microstructural state, results in refined microstructure by interactions of transformation variants and dislocations that enhance the strength, ductility, and crack tip plasticity. The findings underscore the significant impact of intrinsic toughening on the fracture and deformation behaviour of the Fe42Mn28Co10Cr15Si5 HEA.

Unnotched and notched tensile properties of Ti-6Al-4V (Ti64) alloy, additively manufactured using the laser powder bed fusion (LPBF) technique, at cryogenic temperatures of 90, 77 and 20 K were investigated. The LPBF process parameter combination was chosen such that the investigated alloy has not only a minimum in porosity, but also an equiaxed prior beta microstructure, which predominantly contains acicular alpha/alpha' lath structure in basket-weave morphology, that results in a high strength and ductility combination as well as insignificant mechanical anisotropy at room temperature (300 K). Tensile tests revealed that the yield (sigma(y)) and tensile (sigma(u)) strengths of LPBF Ti64 are superior while elongation to failure (e(f)) is comparable to those of the conventionally manufactured (CM) Ti64 down to 77 K, owing to the fine alpha/alpha' lath microstructure in the former. While the progressive reduction of -basal and prismatic dislocation slip activity with decreasing deformation temperature enables a steep rise in sigma(y), the unnotched specimens fractured catastrophically without macroscopic yielding at 20 K. Detailed postmortem analyses revealed the absence of significant twinning-based deformation in LPBF Ti64, in stark contrast to the CM ones at cryogenic temperatures. Instead, deformation kink bands with varying sizes and misorientation were found to be increasingly active within the prior beta grains. A large misorientation at the kink band boundary within the beta grain leads to void nucleation followed by crack growth, which eventually results in brittle failure at 20 K. While relatively notch insensitive from 300 to 77 K, LPBF Ti64 was notch brittle at 20 K. These results highlight the importance of the unique hierarchical micro- and meso-structural features of LPBF Ti64 on the tensile mechanical behavior, especially at cryogenic temperatures.

The hot deformation behavior of an alloy is significantly influenced by its constituent alloying elements. Nickel base superalloys, such as the XH62 alloy, have been modified with multiple alloying elements to impart high-temperature strength and creep resistance. However, modifications to the chemistry of the alloy have led to increased microstructural complexity, posing several challenges during thermo-mechanical processing. In this paper, we aim to determine the optimal combination of strain rate and temperature for thermo-mechanical processing of XH62 alloy. To achieve this, isothermal hot compression test was conducted on as-cast samples of the XH62 alloy in a wide range of temperatures (1050-1200 °C) and different strain rates (10-3-10 s-1). The deformation behavior was analyzed by examining flow curves, establishing empirical relationships, and generating a processing map that distinguishes stable and unstable domains. The microstructural changes in the samples following hot compression tests have been correlated to the stable and unstable regions of the XH62 processing map. This paper not only addresses the challenges associated with secondary processing of complex superalloys but also presents practical guidelines for optimizing the forging parameters, thereby facilitating the production of high-quality components.

This paper presents a comprehensive metallurgical analysis conducted on representative samples obtained from two batches of 17-4PH steel rods in order to investigate the underlying factors contributing to the observed differences in their mechanical properties. The samples underwent a solution annealing process followed by an aging treatment to achieve H1025 temper condition. Surprisingly, the mechanical properties of one batch namely 91/5 were found to be inferior compared to the other batch 91/7, with only the latter meeting the specified requirements. To gain a deeper understanding of these differences, a range of analytical techniques were employed. Local variation in austenite stabilizers and its corresponding effect on microstructure of batch 91/5 was found to be the root cause of lower mechanical property.

Abstract This article aims to summarize the work on cryogenic strength and toughness and to present the fractography of aluminum alloys. It presents case studies on the importance of understanding the fractography of aluminum alloys and the role of microstructure in the appearance of fractographic features, with variables comprised of in-plane/through-thickness anisotropy, test temperature, heat treatment condition, and the effect of welding.

Aluminum/carbon nanotubes composites have been processed by ingot metallurgy route. The cast ingots are subjected to 94% thickness reduction by groove rolling. In the composite with the lower concentration of aluminum/carbon nanotubes, the deformation of the matrix is homogeneous due to sparse distribution of aluminum/carbon nanotubes. However, on higher aluminum/carbon nanotube addition, sub-grain formation is confined closer to the prior aluminum/carbon nanotube regions indicating heterogeneity in deformation. Deformation texture of the rolled composites weakened with increasing aluminum/carbon nanotube content, due to the random distribution of sub-grain orientation near aluminum/carbon nanotube particles. Raman spectroscopy of the composite indicated two peaks first at 1336 cm−1 and the second at 1566 cm−1 corresponding to the D and G bands, respectively, and also the generation of surface defects in aluminum/carbon nanotubes during the rolling process.

Friction stirred materials have unavoidable heterogeneity in the processed microstructure, which is more severe in the transverse direction of the run and deteriorates structural strength. Effect of post friction stir pre-stretching and aging on this microstructural heterogeneity in an Al-Cu-Li alloy sheet is investigated using highly resolved local tensile behaviour. Local stress-strain curves, at 1mm spacing were obtained using transverse tensile tests accompanied by a continuous strain measurement using digital image correlation and the hybrid model was used for strain hardening analysis. Local mechanical and strain hardening behaviour is correlated with microstructural information obtained from transmission electron microscopy and differential scanning calorimetry. Aging with and without pre-stretch, both effectively increased the global strength, but the aging response was heterogeneous in all the cases due to variation in solute content and the strain hardening ability within the processed region. Direct aging enhanced the microstructural heterogeneity and led to strain localization, while strain hardening during pre-stretch contributed more strength to the weaker regions, enhanced T1 precipitation and thus effectively reduced the strength heterogeneity. This study provides insight into detailed mechanical behaviour information within a friction stir processed material which can be applied to strengthen structures produced with friction stir technology.