Low Stacking Fault Energy Materials Research Articles

Atomistic modeling of idealized equal channel angular pressing process

Severe plastic deformation (SPD) processes are forming techniques that impose large plastic strains to achieve ultra-fine grained and nanocrystalline microstructures. The refined microstructure results in a significant increase in strength and in many cases without a considerable loss in ductility. Despite their long existence, a comprehensive understanding of the deformation and grain refinement mechanisms during SPD is still lacking. Atomistic simulations that are regularly used to obtain insights into material behavior have rarely been used to study SPD processes. In this work, we use large scale atomistic simulations of the molecular statics/dynamics kind to model the equal channel angular pressing (ECAP) process, a specific SPD process. Three different materials—Al, Ni and Cu—are used to model high, medium and low stacking fault energy materials. Large simulation cells are used to facilitate grain refinement at the nanoscale. The simulations are carefully analyzed in terms of stress–strain behavior, dislocation activity and grain refinement. All samples show the formation of deformation twins at the nanoscale, which generally detwin at increased strains via dislocation twin interactions. A significant number of stair-rod dislocations form via dislocation–dislocation interactions and are present at low angle grain boundaries in the early stages of deformation. Such stair-rod dislocations also result in the formation of stacking fault tetrahedra which are present in significant numbers in all samples. Finally, difficulties in studying grain refinement at very large strains in atomistic simulations are discussed.

Additive friction stir deposition of SS316: Effect of process parameters on microstructure evolution

Solid state nature of additive friction stir deposition (AFSD) additive manufacturing process is very advantageous in terms of defect formation and microstructural refinement in the material. Current study presents the process optimization, microstructural evolution and kinetics of recrystallization for AFSD deposited low stacking fault energy material - SS316. As deposited microstructure shows equiaxed ultra fine grains with an average grain size of ~5.0 ± 0.5 μm. Shear deformation at high temperature during processing leads to the operation of restoration mechanisms. Observation of necklace type microstructure in the as deposited SS316 is attributed to discontinuous dynamic recrystallization during processing. Recrystallization kinetics of the AFSD SS316 is characterized using Johnson-Mehl-Avarami-Kolmogorov (JMAK) model. Deformation – thermal cycling during AFSD process resulted in inconsistent recrystallization kinetics. Variation in strain, strain rate and temperature during processing with processing parameters result in varying microstructure and tool wear. High strength-ductility combination and sustained work hardening in as deposited SS316 appear to arise from transformation and twinning during deformation, leading to the formation of hierarchical twins and martensitic phase after deformation. • A high strength and work hardening alloy – SS316 is additively manufactured using a novel additive friction stir deposition. • Porosity free builds with fine equiaxed grain structure is formed in as deposited material as a result of discontinues dynamic recrystallization during processing. • Microstructure of the deposited material and the tool wear during processing varied with variation in the process parameters. • Activation of twining and transformation during deformation resulted in excellent tensile properties with yield strength of 415 ± 10 MPa and ductility 75 ± 5% in as deposited material.

Microstructure, texture, and mechanical properties of asymmetrically cold-rolled Ni1.5FeCrCu0.5 high-entropy alloy

This study deals with the microstructural evolution, texture development, and mechanical properties of the cold-rolled and annealed Ni1.5FeCrCu0.5 high-entropy alloy (HEA). In addition to symmetric rolling (SR), asymmetric rolling (AR) was performed with different velocity ratios (VRs) between the working rolls. Electron backscatter diffraction (EBSD) measurements showed that the grain size of the annealed samples was drastically reduced by increasing the VR of the rolls and imposing shear strain. Moreover, annealing was associated with forming a high volume fraction of annealing twins. Recovery was accelerated by increased accumulated shear strain through the thickness of the ARed strips. The X-ray diffraction (XRD) method was employed to investigate the crystallographic texture in the processed high-entropy alloy. Macrotexture data revealed that the intensity of E {111} <110> and F {111} <112> shear texture components were remarkably higher in the ARed sheets. However, the intensities of S {123} <634> and Copper {112} <111> orientations were decreased in favor of the Brass {011} <211> component, indicating the pure-metal to alloy type texture transition in low stacking fault energy (SFE) materials. Compared to the homogenized sample, the Cube {001} <100> component was dramatically weakened by the annealing of the cold-rolled specimens. In terms of the mechanical properties, the pronounced grain refinement of the post-roll annealed strips enhanced the yield strength (YS) and ultimate tensile strength (UTS) from 332 to 503 in the homogenized condition to 625 and 709 MPa in the asymmetrically rolled (ARed) sample with a velocity ratio of 8. Finally, plastic anisotropy analysis demonstrated that the sheet drawability was gradually improved by increasing the velocity ratio due to the increased mean r-values and reduced planar anisotropy (Δr) values.

Verification of the Tanaka non-proportional isotropic cyclic hardening model under asynchronous loading

Non-proportional cyclic loading path induces intrinsic strain hardening to the low stacking fault energy materials. The hardening is manifested in increased stress response which underestimation leads to non-conservative life prediction. This study aims to verify the Tanaka non-proportional isotropic cyclic hardening model under a wide spectrum of multiaxial asynchronous loadings. The Tanaka model is coupled with the Chaboche kinematic hardening model for stress path estimation. Modeling results are compared with fatigue experiments for two non-alloyed steel grades, E235 and E355. The Tanaka model improves the performance of the Chaboche model and exhibits its applicability to asynchronous loading.

On the grain boundary character evolution in non equiatomic high entropy alloy during hot rolling induced dynamic recrystallization

Grain boundary character evolution during dynamic recrystallization (DRX) in a non-equiatomic Fe40Mn40Co10Cr10 high entropy alloy has been examined systematically through single pass hot rolling of 30, 50 and 70 pct reduction. It was observed that the extent of DRX is highly dependent upon the hot rolling reduction. The recrystallization area fraction was considerably higher at 50 pct as compared to 30 pct and 70 pct rolling reduction. In recrystallized regions, a significant proportion of coincidence site lattice (CSL) boundaries (Ʃ≤29) were observed regardless of the rolling reductions. Interestingly, a higher proportion of CSL boundaries (particularly Ʃ3 and Ʃ9) were observed amongst the grain boundaries between recrystallized grains of last and second last necklace layer (L-S); and grain boundaries between recrystallized grains of the last necklace layer (L-L). However, most of the grain boundaries between last necklace layer and the parent deformed matrix (L-D) have been observed to be random high angle grain boundaries (HAGBs). Generation of significant fraction of second order twin boundaries (Σ9) along with first order twin boundaries (Σ3) adjacent to the recrystallization front (i.e. behind the migratinggrain boundary) is a consequence of multiple twinning processes expected to occur during DRX. Here, role of Σ3 twin boundaries on the recrystallization process has been discussed. Grain boundary character evolution and its influence on DRX in the presently studied High-Entropy Alloy (HEA) has been inferred to be similar to previously reported observations on conventional low stacking fault energy materials.

Investigation of microstructure and mechanical properties evolution in 7050 aluminum alloy and 316L stainless steel treated by laser shock peening

Laser shock peening (LSP) has been widely applied to enhance the mechanical properties of metallic materials by modifying their sub-surface microstructures. However, controversies still exist on whether grain refinement can be obtained after the LSP process. To investigate the effect caused by LSP in metallic materials, a 7050 aluminum alloy and 316L stainless steel, which are typically high and low stacking fault energy (SFE) materials, respectively, were selected for this study. The microstructure modified by different LSP cycles and energy densities in both materials was illustrated by Electron Backscatter Diffraction (EBSD). The result shows that no grain refinement was observed regardless of the laser cycles and energy density. The most evident change was the increase of dislocation density, and higher dislocation density was observed with the increase of LSP cycles and energy density. The hardness and residual stress measurements around the LSPed areas show that LSP can effectively introduce a plastic deformation layer ranging from 600~1300 μm. It was revealed from the tensile tests that the yield strength of both materials was improved after the LSP process with the scarification of their elongations. Moreover, a potential method to calculate the dynamic yield stress of metallic materials was put forward with the help of the LSP process.

Deciphering the role of multiple generations of annealing twins on texture evolution in cold-rolled high entropy alloys during annealing

The occurrence of multiple generations of annealing twins (MGAT) in low stacking fault energy (SFE) materials leads to significant texture weakening with the evolution of some new prominent texture components. However, the exact correlation between MGAT and texture evolution is a long-standing enigma. In the current research, the presence of MGAT was confirmed by transmission Kikuchi diffraction (TKD) and high resolution electron backscattered diffraction (EBSD) in partially (700 °C/5 min) and fully (700 °C/1 h) recrystallized specimens of equiatomic CoCrFeMnNi high entropy alloy, respectively. Further, the twin clusters in the vicinity of the potential nucleation sites (Brass, Goss and S orientations) were investigated to gain insights on the contribution of MGAT on texture development. The newly developed (φ1 = 90°, Φ=30°, φ2 = 45°) orientation showed its roots originating from 2nd generation twin of the Brass orientation while Cube orientation was evolved from the 1st and 5th generation twins of the S orientation.

The impact of the stacking fault energy of nanostructured metals on phenomena during annealing at the high hydrostatic pressure

Abstract The present study investigates the impact of stacking fault energy on the microstructure evolution and mechanical properties of nanostructured metals that differ in stacking fault energy, annealed under high hydrostatic and atmospheric pressure. Ag and Ni were selected as materials of low and high stacking fault energy, respectively. To this end, nanostructured metals were obtained by high pressure torsion and subsequently annealed by high hydrostatic pressure annealing, performed under 2 GPa at 0.4 homologous temperature for 1h. For comparison, similar experiments at the same temperature and time were performed under atmospheric pressure. After deformation and annealing, the microstructures were examined using transmission and scanning electron microscopy, and further analysed in terms of grain size, coefficient of grain size variation, and twinning frequency. The stored energy and peak temperatures were measured by differential scanning calorimetry. The mechanical properties were evaluated from microhardness measurements and tensile tests. It is demonstrated that the pressure applied during annealing leads to a more profound retardation of microstructure evolution in the low stacking fault energy material, mainly due to a higher deformation nanotwin density. The twinning deformation mechanism generates a higher dislocation density and a lower grain size than those achieved by dislocation slip.

Investigation of microstructure evolution and martensite transformation developed in austenitic stainless steel subjected to a plastic strain gradient: A combination study of Mirco-XRD, EBSD, and ECCI techniques

The evolution of the microstructure and deformation mechanism at different levels of plastic strain are investigated for 304 L austenitic steel with a combination of micro X-ray diffraction (XRD), electron backscattered diffraction (EBSD) and electron channeling contrast imaging (ECCI). A plastic strain gradient is developed along the longitudinal rolling direction in a wedge-shaped 304 L austenitic steel sample. The graded deformed microstructure includes various amounts of deformation bands, ε- and α′-martensites, and their intersections form during the plastic deformation. ECCI observations reveal several deformation mechanisms of the formation of partial dislocations, dislocations, and interactions. This study suggests geometrically necessary deformation bands (GNDBs) are introduced and stored instead of geometrically necessary dislocations (GNDs) in low stacking fault energy (SFE) materials such as austenite steels. Consequently, increasing the strain gradient leads to an increase in the geometrically necessary martensitic transformation (GNMT); this is the result of the deformation-induced martensite in these materials. In addition to the statistically stored dislocations (SSDs), GNDs are generated at the grain boundaries of the fragmented grains to preserve the continuity of the grains. Accordingly, the strain hardening of the austenite steel includes multiple interactions of the deformation bands, SSDs, GNDs, GNDBs, and GNMT. From the viewpoint of microstructure design, our study provides quantitative information about the relationship between the amount of plastic deformation and the extent of microstructure evolution, in a continues design space.

Effect of rolling temperatures on mechanical and fracture behavior of AA 3003 alloy and pure Cu

Ultrafine-grained materials have an enormous scope of structural and functional applications owing to their higher mechanical properties compared to that of coarse-grained materials. The influence of room temperature and cryogenic temperature rolling of AA 3003 alloy and pure Cu on its mechanical properties and fracture behavior has been investigated in the present study. Both the materials are rolled to a thickness of 2 mm and 1 mm with a true plastic strain of 0.40 and 1.09, respectively. The deformed samples are characterized by tensile test, hardness test, and X-ray diffraction analysis. It has been observed that tensile strength and hardness of cryorolled (CYR) samples are higher than cold rolled (CR) samples and it is due to the fact that rolling at an extremely low temperature (− 196 °C) suppresses the dynamic recovery and hence mobility of dislocations becomes difficult as a result dislocation density increases. In CYR, low stacking fault energy material (Cu) has higher tendency to increase strength and work hardened compared to higher stacking fault energy materials (AA 3003) without much loss of ductility. SEM pictorial view of fractography revealed that CYR samples have mix mode of failure compared to CR samples. The dimple size is gradually decreased with increasing plastic strain and that is due to severe strain hardening and grain refinement.

On the Impact of Texture and Grain Size on the Pseudoelastic Properties of Polycrystalline Fe\u2013Ni\u2013Co\u2013Al\u2013Ti Alloy

The effects of thermomechanical treatments on crystallographic texture and grain size evolution and their impact on the pseudoelastic properties in Fe41–Ni28–Co17–Al11.5–Ti2.5 (at.%) were studied in the present paper. The results show that cold rolling leads to brass-type texture in this alloy, which is typical for low stacking fault energy materials. Thermal treatments up to 1300 °C were conducted and it is shown that the presence of β-phase helps to control grain growth. After the dissolution of the secondary phase induced by heat treatment at higher temperatures, a strong {230}〈001〉 recrystallization texture evolves in cold rolled samples already upon imposing medium reduction ratios. Finally, good pseudoelastic properties are found in conditions being characterized by adequate texture and grain sizes spanning over the entire thickness of the samples tested.

Evaluation of microstructure and texture formation during annealing of cold-rolled FeCrCuMnNi multiphase high-entropy alloy

Abstract Microstructure and texture variations of a cold-rolled multiphase FeCrCuMnNi high-entropy alloy were studied after different annealing treatments. Samples were heat-treated at different temperatures and for different time, and then, subjected to different tests including XRD and SEM-EBSD. The results reveal that the FCC1 phase, which goes through more strain, has lower melting temperature, and recrystallizes earlier (lower temperature and shorter time). In addition, it is seen that particle stimulated nucleation is more effective on the recrystallization of this phase compared to FCC2 phase. A significant number of FCC1 nuclei form around the BCC particles. Nucleation of FCC2 phase initiated at 800 °C mostly at the grain boundaries and the inhomogeneities. The FCC1 phase was almost fully recrystallized at this temperature. The annealing process led to the elimination of rolling textures, while the Brass component remained a major component in the recrystallized samples. Increasing annealing temperature as well as annealing time led to the formation of Cube texture component, which is a major component of recrystallized low stacking fault energy materials. Furthermore, D and Rt-Co components formed during recrystallization as a result of the formation of annealing twins.

Critical Strain for Dynamic Recrystallisation. The Particular Case of Steels

The knowledge of the flow behavior of metallic alloys subjected to hot forming operations has particular interest for metallurgists in the practice of industrial forming processes involving high temperatures (e.g., rolling, forging, and/or extrusion operations). Dynamic recrystallisation (DRX) occurs during high temperature forming over a wide range of metals and alloys, and it is known to be a powerful tool that can be used to control the microstructure and mechanical properties. Therefore, it is important to know, particularly in low stacking fault energy materials, the precise time at which DRX is available to act. Under a constant strain rate condition, and for a given temperature, such a time is defined as a critical strain (εc). Unfortunately, this critical value is not always directly measurable on the flow curve; as a result, different methods have been developed to derive it. Focused on carbon and microalloyed steels subjected to laboratory-scale testing, in the present work, the state of art on the critical strain for the initiation of DRX is reviewed and summarized. A review of the different methods and expressions for assessing the critical strain is also included. The collected data are well suited to feeding constitutive models and computational codes.

Investigation of Electroplastic Effect on Four Grades of Duplex Stainless Steels.

Since the late 1950s, an effect of electrical current in addition to joule heating on the deformation of metals called the Electroplastic Effect (EPE) has been known. It is used nowadays in the so-called Electrically Assisted Forming (EAF) processes, but the understanding of the phenomenon is not very clear yet. It has been found that EPE increases the formability of high stacking fault energy (SFE) materials, while low SFE materials reach fracture prematurely. Since Duplex Stainless Steels (DSSs) possess a microstructure consisting of two phases with very different SFE (low SFE austenite and high SFE ferrite) and they are widely used in industry, we investigated EPE on those alloys. Tensile tests at 5 A/mm2, 10 A/mm2 and 15 A/mm2 current densities along with thermal counterparts were conducted on UNS S32101, UNS S32205, UNS S32304 and UNS S32750. The DSS grades were characterized by means of optical microscopy, X-ray diffraction and their mechanical properties (ultimate tensile strength, total elongation, uniform elongation and yield stress). An increase in uniform elongation for the electrical tests compared to the thermal counterparts as well as an increase in total elongation was found. No differences were observed on the yield stress and on the ultimate tensile strength. Un uneven distribution of the current because of the different resistivity and work hardening of the two phases has been hypothesized as the explanation for the positive effect of EPE.

Significant Contribution to Strength Enhancement from Deformation Twins in Thermomechanically Processed Al0.1CoCrFeNi Microstructures

Strengthening mechanisms from thermomechanical processing treatments were explored in single-phase FCC high-entropy alloy Al0.1CoCrFeNi. Cold work offers substantial strengthening in this low stacking fault energy material owing to the resultant high work hardening rates. An enormous increase in yield strength of ~ 275% was obtained in 40% rolled material, but was accompanied by a steep drop in ductility. Recovery and recrystallization annealing treatments were investigated for improving elongation and obtaining better balance of strength–ductility combinations. Formation of novel microstructures from the different processing routes was examined. X-ray diffraction peak broadening and mechanical test results were coupled to estimate micro-strain in the different conditions and understand micro-strain’s correlation to strength. Retention of large-scale deformation twins formed during cold rolling is shown to play a key role in elevation of yield strength after heat treatments.

Softening Mechanisms and Microstructural Evolution in a Low Stacking Fault Energy Material Between Warm and Hot Working Temperatures

A low stacking fault energy material is subjected to warm working as well as hot working, i.e. in the temperature range 873 K (600 °C)–1173 K (900 °C). The evolution of microstructure is quantified using average grain size and grain size distributions. The microstructural transitions resulting from continuous dynamic recrystallization and discontinuous dynamic recrystallization are examined. The resultant softening is studied using macro- and micro-indentation experiments. The applicability of empirical relations between hardness and microstructural parameters is critically examined. The SFE at each of these temperatures is evaluated using a thermodynamic model and correlated with the softening mechanism.

Severe Plastic Deformation by Equal Channel Angular Pressing and Rolling: The Influence of the Deformation Path on Strain Distribution

The present work compares two deformation techniques, rolling and Equal Channel Angular pressing (ECAP), and the response offered by three different materials that differ in Stacking Fault Energy (SFE): AA1010 Al, commercially pure Cu, and an austenitic stainless steel. The objective of this investigation is to study the effect of each deformation mode on tensile behavior, deformation mechanism, texture, and microstructure and to establish the influence of the stacking fault energy on said effects. The results show that the different strain paths of ECAP and rolling do not affect the strength, but rolling leads to an accentuated texture and thus to elastic and plastic anisotropy. This finding has practical relevance for micro manufacturing techniques. Furthermore, it is observed that lower SFE results in smaller domain size and higher dislocation density, which are microstructural details related to strength and to the work hardening capacity. Finally, both techniques are able to produce a high amount of high angle grain boundaries, a feature that characterizes refined microstructures. These processes operate at different strain rates; thus, in low SFE materials, a more effective grain fragmentation by deformation‐induced twins is observed after the ECAP process.

Micro-mechanical aspects of texture evolution in nickel and nickel–cobalt alloys: role of stacking fault energy

The micro-mechanisms associated with the evolution of deformation texture in nickel and nickel–cobalt alloys, with stacking fault energy (SFE) ranging from high to low, has been investigated. Pure nickel and nickel-20 wt.% cobalt alloy, which are high SFE materials, develop a characteristic copper-type texture, which is attributed to dislocation slip. In the medium SFE nickel-40 wt.% cobalt alloy, the texture is copper-type up to 95% reduction; however, subsequent reduction to 98% causes the texture to shift towards brass-type. Microstructural examination suggests that the occurrence of Cu-type shear bands (SBs) preferentially in the Cu {1 1 2}-oriented grains has led to this texture transition. In nickel-60 wt.% cobalt alloy, which is a low SFE material, texture is brass-type from the early stages of rolling. Deformation mechanisms show a gradual transition from deformation twinning to Bs-type SBs, as a function of strain. The strength of the final texture is a synergistic effect of twinning and shear banding. The absence of Cu component during the process of brass-type texture evolution goes against Wassermann’s prediction of texture transition. A modified theory for the formation of brass-type texture is proposed.

Solute drag on perfect and extended dislocations

The drag force exerted on a moving dislocation by a field of mobile solutes is studied in the steady state. The drag force is numerically calculated as a function of the dislocation velocity for both perfect and extended dislocations. The sensitivity of the non-dimensionalized force–velocity curve to the various controlling parameters is assessed, and an approximate analytical force–velocity expression is given. A non-dimensional parameter S characterizing the strength of the solute-dislocation interaction, the background solute fraction , and the dislocation character angle , are found to have the strongest influence on the force–velocity curve. Within the model considered here, a perfect screw dislocation experiences no solute drag, but an extended screw dislocation experiences a non-zero drag force that is about 10 to 30% of the drag on an extended edge dislocation. The solutes can change the spacing between the Shockley partials in both stationary and moving extended dislocations, even when the stacking fault energy remains unaltered. Under certain conditions, the solutes destabilize an extended dislocation by either collapsing it into a perfect dislocation or causing the partials to separate unboundedly. It is proposed that the latter instability may lead to the formation of large faulted areas and deformation twins in low stacking fault energy materials containing solutes, consistent with experimental observations of copper and stainless steel containing hydrogen.

High strain-rate compressive deformation behavior of the Al0.1CrFeCoNi high entropy alloy

Abstract High entropy alloy is a new class of structural metallic materials. No work, so far, has been carried-out to understand high strain-rate plastic deformation behavior and resulting microstructure. This work focuses on understanding the deformation behavior of an Al0.1CrFeCoNi HEA at high strain-rate (HSR). HSR plastic deformation in compression mode was carried out using split-Hopkinson pressure bar. The pre- and post-deformation microstructures were studied using electron microscopes. A high strain-rate sensitivity of yield strength, significant work hardening, and profuse twinning are main characteristics observed during deformation of the alloy at HSR. Overall, the deformation behavior of the alloy was consistent with low stacking fault energy materials.