Predominant Deformation Mechanism Research Articles

Influences of ion bombardment, grain size, and gamma irradiation on surface topography, microstructural, and mechanical characteristics of Cu-2 wt% Sb alloy

The Cu-2 wt% Sb alloy was prepared and exposed to various heat treatments at temperatures ranging from 573 to 973 K to get distinct grain diameters. Different samples of various grain diameters were individually sputtered in an argon glow discharge for various times (0.5, 1, 2 h) using a DC magnetron sputtering device. The surface topography and microstructural features of a Cu-2 wt% Sb alloy were examined utilizing an optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The Cu3Sb\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Cu_{3} Sb$$\\end{document} IMC phase was found to segregate at the grain boundaries. Results showed that the topographical features of sputtered samples (grain boundaries, tiny cones, cratered cones, and etch pits) were mainly affected by grain size and sputtering time. Moreover, the mechanical properties of a Cu-2 wt% Sb alloy were examined. The parameters of the stress–strain curves (Young modulus Y, 0.2% offset stress σy0.2, fracture stress σf, parabolic work-hardening coefficient χp\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\upchi }_{{\ ext{p}}}$$\\end{document}, and ductility εf\\%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\upvarepsilon }_{{\ ext{f}}} {\ ext{\\% }}$$\\end{document}) were measured under different testing conditions. Generally, these stress parameters decrease as the grain diameter increases. These parameters, with the exception of εf\\%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\upvarepsilon }_{{\ ext{f}}} {\ ext{\\% }}$$\\end{document}, increase as the strain rate increases, while εf\\%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\upvarepsilon }_{{\ ext{f}}} {\ ext{ \\% }}$$\\end{document} decreases as the strain rate increases. Additionally, selective samples of the present alloy were irradiated using γ-radiation with different doses 0.5, 1, 1.5, and 2 MGy. The Vickers microhardness of annealed and irradiated samples lowered as the grain size augmented, while it increased as the irradiation dose increased. Based on the obtained activation energy Q value of 20.65 kJ/mol., it is indicated that the predominant deformation mechanism in the Cu-2 wt% Sb alloy is the motion of dislocation through the Cu-matrix.

Temperature-dependent deformation behavior of Hastelloy X Ni-based superalloy fabricated via laser powder bed fusion

Understanding the deformation behavior of additive manufacturing components across a wide spectrum of operating temperatures is crucial from an application standpoint. This study investigated the tensile deformation behavior of Hastelloy X (HX), fabricated via laser powder bed fusion (LPBF), over a temperature range of -50 °C to 900 °C through uniaxial tensile tests. The focus was on the temperature-dependent tensile properties and deformation mechanisms of HX in a fully heat-treated condition, involving initial hot isostatic pressing followed by solution treatment. The results indicate that as the temperature increases, the ultimate tensile strength decreases sharply from approximately 790 MPa at -50 °C to 610 MPa at 200 °C, then gradually declines to 530 MPa at 600 °C, and finally drops to 230 MPa at 900 °C. Meanwhile, the yield strength falls rapidly from 345 MPa at -50 °C to 230 MPa at 200 °C, then slowly reduces to 170 MPa at 600 °C, stabilizing up to 900 °C. Electron back scattered diffraction analysis revealed that with rising temperature, the predominant plastic deformation mechanism transitions from deformation twinning to slip bands and eventually to dynamic recrystallization. Remarkably, the strain hardening capacity increases with rising temperature below 600 °C, attributed to the beneficial effects of deformation twins and subsequent slip bands. Furthermore, dynamic strain aging takes place between 300 °C and 600 °C, slightly limiting the improvement of plasticity within this intermediate temperature range. Understanding these temperature-dependent deformation mechanisms in heat-treated LPBF HX would provide valuable insights into its performance over a wide range of service temperatures.

Superplastic behavior of a metastable β-type Ti alloy governed by grain size: Microstructure evolution and underlying deformation mechanism

The superplastic forming (SPF) technology has gained widespread adoption in the manufacturing of intricately shaped components made from Ti alloys with high dimensional accuracy due to its near net forming capabilities. It is crucial to systematically investigate the superplastic behavior in metastable β-type Ti alloys. However, the microstructure evolution and underlying superplastic deformation mechanism governed by grain size in these alloys remain unclear. In this study, three types of fine-grained (<10 μm) Ti-15V-3Cr-3Sn-3Al (Ti-15-3) alloys were obtained using friction stir processing (FSP), and tensile tests were conducted at temperature of 650 °C with strain rates ranging from 1 × 10−4 s−1 to 3 × 10−3 s−1. The deformation behavior of the coarse-grained (>3 μm) Ti-15-3 alloy can be described as quasi-superplastic rather than strictly superplastic. In this microstructure, plasticity is synergistically contributed by grain boundary sliding (GBS), continuous dynamic recrystallization (CDRX), and dynamic phase transformation (DPT). For the fine-grained (<3 μm) Ti-15-3 alloy, deformation enters into the superplastic region with GBS becoming the predominant deformation mechanism accompanied by DPT. Additionally, strain/stress promotes α phase precipitation from the β matrix, which inhibits grain growth and serves as a supplementary stress accommodation mechanism facilitating continuous operation of GBS and ultimately achieving exceptional superplasticity.

Weak strain-rate sensitivity of hardness in the VCoNi equi-atomic medium entropy alloy

In this study, high strain rate nanoindentation was utilized to elucidate the dynamic behavior and the underlying deformation mechanisms of the equi-atomic vanadium-cobalt-nickel (VCoNi) medium entropy alloy. The hardness at high strain rate (average indentation strain rate: ∼11,000 s−1) exceeded that at low strain rate (constant indentation strain rate: 0.01 s−1) by only ∼3.5 %. Planar slip was identified as the predominant deformation mechanism at both strain rates, with no evidence of deformation twinning or phase transformation. The low strain rate sensitivity of VCoNi can be attributed to its lower lattice friction relative to that of conventional BCC metals and limited dislocation-dislocation interaction compared to that in typical FCC metals. The low strain rate sensitivity observed in VCoNi may also extend to other intermediate stacking fault energy alloy systems in which planar slip is prevalent.

Collapse Pattern in Gypseous Soil using Particle Image Velocimetry

Abstract Gypseous soil is prevalent in arid and semi-arid areas, is from collapsible soil, which contains the mineral gypsum, and has variable properties, including moisture-induced volume changes and solubility. Construction on these soils necessitates meticulous assessment and unique designs due to the possibility of foundation damage from soil collapse. The stability and durability of structures situated on gypseous soils necessitate close collaboration with specialists and careful, methodical preparation. It had not been done to find the pattern of failure in the micromechanical behavior of gypseous sandy soil through particle image velocity (PIV) analysis. This adopted recently in geotechnical engineering to track the motion of soil grains and using tracer particles by applying digital particle image analysis. It has also been used to study the displacement distribution in some cases of granular materials. Therefore, the goal of this study is to find out how gypseous sand medium moves when in contact with a rigid strip foundation that is under static stress and plane strain conditions. The experimental model would focus on two common types of wetting, namely water table rise and dry conditions. The PIV showed that the collapse pattern under the footing is of the type of punching shear failure. The predominant mechanism of soil deformation was the vertical compression of the gypseous granular soil. The results showed that understanding gypseous sandy grain displacement and failure patterns at the local scale is crucial for enhancing the design of foundations under static stress conditions.

Shock responses of nanocrystalline molybdenum via molecular dynamics simulation: Grain size and shock intensity effects

Molybdenum (Mo) is a strategic metal for the manufacture of aerospace equipment, satellite components, and vehicle armor. Thus, understanding its behaviors under harsh conditions like shock compression is crucial for its practical utilization. Through molecular dynamic simulations, we have explored the mechanical responses and microstructural evolutions of nano-polycrystalline (NPC) Mo under different shock intensities, with grain sizes ranging from 5 to 33 nm. Our study reveals that grain size considerably influences the Hugoniot data and waveform of NPC Mo. NPC Mo with a smaller grain size exhibits higher compressibility and lower Hugoniot shear stress. As the grain size increases, the presence of a double-wave structure becomes more pronounced. Additionally, with the increase in shock intensity, there is a reduction in the shock front width. Significantly, when the shock stress ranges from approximately 60 to 100 GPa, twinning structures are detected in samples with grain sizes ranging from 10 to 33 nm. Moreover, the elevated temperature behind the shock wave further promotes detwinning reactions. When the shock stresses exceed 100 GPa, twinning–detwinning as well as amorphization-recrystallization become the predominant deformation mechanisms, almost unaffected by grain size. As the shock stress exceeds 250 GPa, the atoms in the samples become completely disordered. These findings provide new insights into the mechanical responses as well as the microstructural evolutions of NPC Mo under shock compression.

Deformation mechanism and microstructure evolution during superplastic deformation of friction stir processed CoCrFeNiMn high entropy alloy

Since superplastic formation offers a significant advantage in preparing the complex structure parts of HEAs, the study of their superplastic behavior is extremely important. Using friction stir processing (FSP) with an improved convex hemispherical shape tool, this study reported for the first time an equiaxed ultrafine-grained CoCrFeNiMn HEA (489 nm) with a maximum elongation of 870% at 675 °C and 3 × 10−4 s−1. This superplastic property is much larger than ever reported in the CoCrFeNiMn HEA, which is even larger than that of the nano-sized CoCrFeNiMn HEA (10 nm) prepared by the high-pressure torsion. A high proportion of high angle grain boundaries and twin boundaries, a hard Cr-rich phase, and the sluggish diffusion effect of the CoCrFeNiMn HEA were primarily responsible for the exceptional superplasticity. Additionally, this study provides the first elucidation of the mechanism underlying Cr precipitation and Cr-rich phase growth, as well as the function of low-energy annealed twins during superplastic deformation. The diffusion of Cr along dislocations and grain boundaries facilitated grain boundary sliding as the predominant deformation mechanism. This study elucidates the superplastic deformation mechanism and microstructure evolution, thereby furnishing theoretical guidance for the practical implementation of superplastic forming of complex components of HEAs. Additionally, it presents an efficient method for fabricating such components.

Temperature study of deformation twinning behaviour in nickel-base Superalloy 625

Deformation behaviour in the Nickel-base superalloy 625 has been studied by tensile testing at four temperatures: 295, 223, 173 and 77 K. The microstructure has been investigated using TEM, FIB-SEM, EBSD and ECCI techniques. Deformation in the alloy turns out to be a competitive course of events between at least two deformation mechanisms, namely dislocation slip and deformation twinning. Slip is the predominant deformation mechanism at higher temperatures. While at 77 K, deformation induced twinning gives an extra degree of freedom as one of the main deformation mechanisms, i.e., the material shows a twin induced plasticity, TWIP, behaviour. Ab initio calculations indicate that the influence of cryogenic/sub-zero temperatures on the stacking fault energy of this alloy can be limited and therefore the formation of deformation twins cannot be determined solely by the stacking fault energy. The results implies that critical strain and strain hardening rate influences the deformation twinning onset and twinning rate.

Deformation mechanisms and mechanical properties of rolled AZ31 alloy subjected to precompression and subsequent annealing: Effect of annealing temperature

This study investigates the effect of annealing temperature on the microstructure, deformation behavior, and mechanical properties of a rolled AZ31 alloy having {10–12} twins. To this end, rolled AZ31 samples are precompressed to 6.0% along the rolling direction (RD) to introduce {10–12} twins and then are annealed at 200, 250, and 300 °C for 1 h. The results reveal that as the annealing temperature increases, the grain size of the annealed samples increases and their RD-oriented basal texture is strengthened. In addition, the number of {10–12} twin boundaries gradually decreases and they almost disappear after annealing at 300 °C. Under tension along the RD, the predominant deformation mechanism of the annealed samples changes from detwinning of preexisting twins to {10–12} twinning as the annealing temperature increases. The annealed samples exhibit different tensile strengths and elongations according to the annealing temperature. Particularly, the tensile elongation increases by 56%, from 13.2% to 20.6%, as the annealing temperature increases from 200 to 300 °C. The variations in the mechanical properties of the annealed samples with the annealing temperature are discussed in detail on the basis of their microstructural characteristics and deformation behaviors.

Low-cycle fatigue behaviour of magnesium alloy thick plate joints fabricated via differential double-shoulder friction stir welding

Magnesium (Mg) alloy thick plates welded by friction stir welding (FSW) often exhibit poor formability and uneven microstructures, leading to a significant deterioration in the mechanical properties. In this work, AZ31 Mg alloy plates with a thickness of 20 mm were successfully welded by differential double-shoulder friction stir welding (DDS-FSW). The low-cycle fatigue response behaviour was characterised, and the damage mechanism of the joints were studied. The results indicated that the fatigue life of the DDS-FSW joints in both parameters exceeded that of the FSW joint at 0.3 % strain amplitude, with increases of 200 % and 82 %, respectively. The predominant deformation mechanisms are basal slip and {10 1¯ 2} twinning. Basal slip is more likely to be activated in the FSW joint, whereas {10 1¯ 2} twinning is more easily activated in the DDS-FSW joints. Furthermore, the fatigue toughness (W0) and damage transition exponent (β) could be increased by improving the asymmetric distribution of the stir zone in the joint, the strength of “orientated micro-interface”, and the proportion and distribution uniformity of twins.

Molecular dynamics simulations of tensile and creep-ratcheting behaviour of CNT reinforced columnar nanocrystalline Al nanocomposites

High-temperature cyclic loading is a major factor affecting the life of a structural component. In this study, the tensile and creep-ratcheting behaviour of three volume fractions of CNT-reinforced columnar nanocrystalline aluminium has been studied using hybrid potentials (EAM, AIREBO and LJ potentials) at two different stress ratios (R = 0.4, 0.6) and three different temperatures (T = 300 K, 467 K and 653 K). The aim is to determine the stress-strain curve, strain-time curve, overall dislocation density, dislocation types, and mainly the underlying deformation mechanisms of CNT-based NC Al matrix nanocomposites. Increasing the CNT volume fraction in the matrix has shown improved UTS values, enhanced dislocation annihilation rate, dissipation of the hysteresis loop energy, lower strain accumulation and higher time to fracture. The specimen fails quickly at higher temperatures which is depicted by the widening of the hysteresis loop. The predominant deformation mechanisms like grain boundary diffusion, widening, merging, sliding and rotations have been observed. Grain boundary diffusion drives the creep-ratcheting deformation behaviour under three temperatures and the misalignment has been noticed at the interaction of CNT-NC Al nanocomposite during the creep-ratcheting deformation at high temperature. The Shockley-partial and perfect dislocations have been determined to control the underlying deformation mechanism. The current work can serve as a keystone in the creep-ratcheting analysis of structural materials employed in high-temperature loading environments.

Temperature effect on the transition between grain boundary migration and sliding

Grain boundary (GB) dynamics plays an important role in the mechanical and physical properties of nanocrystalline metals. In this study, we investigate the temperature effect on GB deformation transition using molecular dynamics simulations of [100] symmetric tilt GBs in Au bicrystals. Different deformation behaviours were revealed in GBs with the same structures at varied temperatures. GB sliding occurs as the predominant deformation mechanism at elevated temperatures, while GB migration dominates at low temperatures. The temperature-dependent critical stresses for GB migration and GB sliding were compared, revealing that temperature alters the critical GB misorientation at which GB migration transitions to GB sliding. Our study provides new insight into GB-mediated plastic deformation in nanocrystalline materials.

Room temperature creep mechanisms of Ti–6Al–4V ELI alloy with equiaxed microstructure under different applied stresses

Titanium alloys are often used to make deep-sea pressure hulls and the creep strain produced during service is nonnegligible. The variation of dislocation structures with room temperature creep stresses in equiaxed α grains of Ti–6Al–4V ELI alloy was examined using transmission electron microscope. The plastic deformation mechanism varied with the applied stress, mainly manifested by the change of dislocation patterns and the sequential activation of slip systems. At the stress below the creep threshold, even though almost no macroscopic creep was generated, a small number of immobile dislocations and dislocation networks occurred in the sample. The networks were composed of dislocations with different Burgers vectors, and part of them would evolve into low angle grain boundaries (LAGBs). At the stresses above the threshold, the density of dislocations in α grains increased with the stress level, and the basal <a>, pyramidal <a> and <c+a> slips were activated successively which also contributed to creep and participated in the formation of LAGBs. Prismatic <a> slip was always the predominant creep deformation mechanism for the studied stress range. The activation of difficult slip systems was attributed to the stress concentration and stress redistribution between soft and hard phases/grains. In general, the amount of LAGBs increased with the applied creep stress, but the LAGBs formed by dislocation networks decreased.

Microstructural evolution and yield strength of a novel precipitate-strengthened Fe-based superalloy during thermal aging at 700 °C

Microstructures and compressive yield strength of a novel γ′-strengthened Fe-based superalloy with about 55 wt% iron are investigated after aging at 700 °C for various durations. We found that the γ′ particle always displays a spherical shape and its size increases only from 9 ± 1.6 to 51 ± 14.0 nm with increasing the aging time up to 10,000 h. Meanwhile, the distribution of carbides at grain boundaries is always discontinuously, and no topologically close-packed phases are visible in the alloy during aging. Compressive deformation tests disclose that the yield strength of the experimental alloy at 700 °C increases firstly and then decreases with particle size. Microstructural observations uncover that the predominant deformation mechanism changes from shearing of γ′ particles by weekly-coupled dislocations to shearing by strongly-coupled dislocations and then to Orowan looping with enlarging particle size at the beginning of plastic deformation. Based on these results, it is deemed that the evolution of microstructure and deformation mechanism with aging time leads to the changes in the yield strength during thermal aging.

Microstructures and mechanical properties of a L12-structured precipitation strengthened Co-based superalloy

A L12-structured precipitation (γˊ) strengthened Co-based superalloy is identified based on thermodynamic calculation and experimental validation. An alloy with composition of Co9Al5Mo30Ni2Ta10Cr is fabricated to explore the relationships among processing, microstructures and mechanical properties. Typical γ/γˊ two-phase microstructures have been obtained after heat treatments, where the spherical γˊ phase with diameter of ∼100 nm is uniformly distributed in the face centered cubic (FCC) matrix. The L12 structured γʹ-(Ni, Co)3(Al, Ta) phase can be stabilized, which coherently precipitates in the FCC matrix (γ) during aging treatment. The alloy exhibits promising strength with comparison to traditional Co-based superalloys at a temperature range from room temperature (RT) to 1000 °C. The predominant deformation mechanism transits from shearing of dislocation pairs to dislocations bypassing γʹ phase when deformation temperature increases from RT to 1000 °C.

Investigation on the indentation performance of 3D printed re-entrant diamond auxetic metamaterial: printability and tailorability for futuristic applications

PurposeAuxetics are the class of cellular materials with a negative Poisson’s ratio. This paper aims to study the low-cost 3D printing capabilities and printing variations and improve the indentation performance of the re-entrant diamond auxetic metamaterial by tuning the structural parameters that have not been reported.Design/methodology/approachThe design of experiment strategy was adopted to study the influence of re-entrant angle, diamond angle and thickness-to-length ratio on relative density, load, stiffness and specific energy absorption (SEA) during indentation experimentally. Grey relational analysis was chosen as a multi-objective optimisation technique to optimise structural performance. Surrogate models were proposed to uphold the metamaterial’s tailorability for desired application needs. The fit and efficacy of the proposed models were tested using specific statistical techniques. The predominant deformation mechanisms observed with the alteration in structural parameters were discussed.FindingsThe improvements noticed are 48 times hike in load, 112 times improvement in stiffness and 10 times increase in SEA for optimised structures. The surrogate models are proven to predict the outputs accurately for new input parameters. In-situ displacement fields are visualised with an image processing technique.Originality/valueTo the best of the authors’ knowledge, the indentation performance of the re-entrant diamond auxetic metamaterials has not been reported and reported for the first time. The influence of geometrical parameters on the newly developed structure under concentrated loading was evaluated. The geometry-dependent printing variations associated with 3D printing have been discussed to help the user to fabricate re-entrant diamond auxetic metamaterial.

Thermomechanical properties and the deformation mechanism of nickel monoaluminide-based alloys produced by L-PBF in combination with gasostatic treatment and aging

The synthesized NiAl-based material was manufactured by laser powder bed fusion (L-PBF) followed by gasostatic treatment and aging using spheroidized CompoNiAl-M6 (NiAl–8Cr–6Co–1Nb-0.9Hf) alloy powder (the 20–50 μm fraction). The structure was examined by high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED); nanoparticles of the refractory phases were identified. The compressive mechanical properties of the alloy were measured at 20, 800, 900, and 1000 °C. Doping with niobium and hafnium was shown to strengthen the alloy with fine-grained Laves and Heusler phases, as well as the (HfxNby)C carbide phase, which reduce the size of NiAl grains and prevent grain boundary diffusion. Due to this fact, the material can be used at temperatures above 800 °C. Ultimate compressive strength (σb) at 900 °C was 575 MPa, while being 260 MPa at 1000 °C. The dislocation substructure of the deformed sample was examined, and the predominant deformation mechanisms (dislocation glide, dislocation climb, and shear of the crystal lattice of the matrix phase) were identified. In situ TEM nanotensile testing revealed that fine-grained refractory phases increase σb from 1360 MPa to 1870 MPa. Clusters and agglomerates of strengthening phases act as stress concentrators and cause premature alloy failure because of stress localization at the particle interface.

Characterization of Prismatic Slip in SiC Crystals by Chemical Etching Method

In 4H-silicon carbide crystals, basal plane slip is the predominant deformation mechanism. However, prismatic slip is often observed in single crystals grown by the physical vapor transport method as the diameter expands to 6 inches or larger. Thermal modeling has shown that occurrence of prismatic slip is attributed to increased radial thermal gradients. While X-ray topography can be used to characterize the presence and extent of prismatic slip, the feasibility of using the chemical etching method to assess the extent of prismatic slip in an industrial setting is investigated. The distribution of scallop shaped etch pits oriented along the directions that correspond to prismatic dislocations, correlate well with the results of the thermal model that predicts the occurrence of prismatic slip dislocations. This capability of the etch pit method to characterize prismatic slip can be used to manage radial thermal gradients during PVT growth.

Effect of manganese on mechanical properties and deformation mechanism of CoCrFeNi high entropy alloys

In this study, the predominant mechanical properties and deformation mechanisms of CoCrFeNiMnx high entropy alloys (HEAs) were explored by using the molecular dynamics (MD) simulation. A uniaxial tensile process with different temperatures (100–700 K) and compositions of manganese (x = 0, 5, 10 and 20) were conducted. Besides, a circular defect was created on the CoCrFeNiMnx HEAs to investigate the damage tolerance of these HEAs. The equiatomic CoCrFeNiMn HEA performed a strongly temperature dependence. The ultimate tensile strength (UTS) was 4.83 GPa at 100 K, and was decreased with the temperature increased. Based on the dislocation distribution and von Mises strain results, the early stage deformation was governed by the 1/2 < 110 > -type dislocation slip. On the contrary, the grain boundaries glide was dominated the later stage deformation. In addition, the manganese content in this work was higher than the limit to form the face-centered cubic (FCC) single phase solid solution. Instead, hexagonal close-packed (HCP) and amorphous phases appeared, and thus decreased the strength of CoCrFeNiMnx HEAs. The Mn0 HEA showed the highest yield stress, UTS and Young’s modulus, which were 4.9, 4.8 and 218.4 GPa, respectively. In addition, the damage tolerance of these HEAs was enhanced by the amorphous phase formation, which inhibited the propagation of cracks and improved the ductility. Based on the results, the Mn5 HEA presented the best damage tolerance than others.

Extraordinary combination of strength and ductility in an additively manufactured Fe-based medium entropy alloy through in situ formed η-nanoprecipitate and heterogeneous microstructure

The current study investigated the mechanical properties of Fe65Ni15Co8Mn8Ti3Si (at%) medium entropy alloy (MEA), printed by laser-based direct energy deposition (DED) in the room and liquid nitrogen environments. The as-printed microstructure contains various levels of heterogeneities, including different grain sizes, dual-phase microstructure, in situ formed η nanoprecipitate, cellular structure, and elemental segregation generated during the DED process. The impact of each observed heterogeneity on the mechanical properties and the predominant deformation mechanisms were examined. The corresponding MEA revealed a high ultimate tensile strength (UTS) of ∼1.02 GPa, total elongation (TE) of ∼46%, and exceptional cryogenic mechanical properties with a UTS and TE of 1.83 GPa and 40%, respectively. According to microstructural observations, the deformation-induced phase transformation from face-centered cubic (FCC) to body-centered cubic (BCC) is the predominant strengthening mechanism at both room and liquid nitrogen temperatures in addition to solid-solution strengthening and dislocation-mediated plasticity. The non-shearable η nanoprecipitates also enhanced the mechanical properties through precipitation/stacking fault strengthening. Hetero-deformation-induced (HDI) strengthening also occurred in the presence of dual-phase microstructure, cellular microstructure, and elemental segregation in the FCC phase. This upgraded synergy of tensile strength and ductility at liquid nitrogen temperature can be explained through the phase transformation and additional activation of twinned martensite formation, which results in two-step strain-hardening behavior. The presented results expand possibilities for developing DED-processed ferrous HEAs/MEAs to overcome the strength and ductility trade-off at room and liquid nitrogen temperatures.