Low Stacking Fault Energy Alloys Research Articles

Effect of cyclic loading on microstructure and plastic deformation in heat-treated Co–28Cr–6Mo alloy fabricated via laser powder bed fusion: An in situ synchrotron X-ray diffraction study

We present a systematic microstructure-oriented plasticity investigation of an additively manufactured cobalt-based alloy aiming to relate microstructural changes to the fatigue resistance. A load-controlled cyclic test in the low-cycle fatigue regime was utilized to study mechanical response and microstructural changes in the alloy after reverse phase transformation heat treatment. Deformation-induced FCC → HCP phase transformation was followed using in situ synchrotron X-ray diffraction combined with ex situ electron backscatter diffraction and scanning electron microscopy. Scanning transmission electron microscopy provided experimental evidence of the presence of precipitates in the solution annealed sample. It was found, that a stepwise increase in plastic deformation is attributed to nucleation and growth of different martensitic crystallographic variants. These insights into plasticity and microstructural changes suggest that the reverse phase transformation heat treatment is an effective approach for improving the fatigue resistance of low stacking fault energy alloys, that are susceptible to deformation-induced martensitic transformation.

Evolution of recrystallization texture in medium to low stacking fault energy alloys: Experiments and simulations

The present work aims to investigate the evolution of static recrystallization microstructure and texture in medium to low stacking fault energy (SFE) alloys. In these categories of materials, the process of recrystallization becomes complex because of the presence of extensive deformation heterogeneity. Ni-xCo (40 and 60wt.% Co) alloy has been chosen for this purpose, where Ni-40Co belongs to medium SFE, and Ni-60Co belongs to low SFE regimes. The effect of solid solution strengthening and deformed microstructural features on the mechanism of recrystallization are explored. Both the alloys were subjected to 50% cold-rolling reduction followed by isothermal annealing at 600°C. Recrystallization texture of Ni-40Co shows a non-uniform α-fiber having a peak intensity at the Goss component, whereas Ni-60Co shows uniform α-fiber texture. Both the alloys also show rotated cube (Rt C) and rotated Cu (Rt Cu) components after recrystallization. Apart from that Ni-60Co exhibits brass recrystallization (BR) texture component. The differences in the recrystallization texture in both the Ni-Co alloys are attributed to the role of different heterogeneous deformation features in the microstructure and transition from Cu-type to Bs-type as-deformed texture. The recrystallization microstructure is dominated by the formation of substantial annealing twin (Σ3) boundaries, which also produces many new orientations; thereby weaken the recrystallization texture. These experimental findings and the proposed mechanism were used as input to simulate the recrystallization microstructure and texture in the alloy using the parallelized cellular automata (CA) technique. Parallelized CA model has successfully predicted the evolution of recrystallization texture and microstructure through simulation except parallel annealing twin feature.

Microstructural Evolution and Distributions of Grain Boundary in SPD Processed Al–3 mass%Mg Alloy

SPD-processed Al alloys have an ultra-fine-grained microstructure and extremely high strength. The strength of SPD-processed Al alloy exceeds the estimated it by the grain size using Hall-Petch relationship. SPD-processed Al has high dislocation density because the excess introduced strain remains in dynamically recrystallized grains. The excess dislocation may form the low angle grain boundary during dynamic recrystallization. Al-Mg alloy represents especially large strengthening by dislocation hardening after SPD not only smaller grain size than pure Al. This is because the stacking fault energy (SFE) of the alloy is lowered by Mg solute atom. The dislocation movement become suppressed during DRX in low SFE alloys. As a result, SPD-processed Al-Mg alloys have extremely fine-grained (~0.25 µm) microstructure. SFE of the alloy may also affect the formation of low angle grain boundary and dislocation accumulation in the grain interior. The dislocation distributions could result the degree of extra-hardening. The distributions of dislocation and low angle grain boundary in ECAE processed Al-3 mass%Mg alloy were investigated by EBSD and XRD techniques. Annealing after ECAE for recovery the strain provides the relaxation of the strain hardening, in spite of the LAGB fraction remains static.

Cryo-rolling and annealing-mediated nano/ultrafine structure, texture, and properties of extremely low stacking-fault energy high entropy alloys: Comparative perspectives

The effect of heavy (∼90% thickness reduction) cryo-rolling (CryoR) on microstructure, texture, and tensile properties of an extremely low stacking fault energy (SFE) Co20Cr26Fe20Mn20Ni14 high entropy alloy (HEA) was investigated. Comparisons with other heavily cold- (CR)/cryo-rolled materials were also presented to highlight the effect of CryoR. CryoR resulted in the formation of heterogeneities, shear bands, deformation twins, stacking faults, and deformation-induced nano-lamellar structure with a spacing of 35 ± 8 nm. Remarkably, the nano-grain size after CryoR was smaller than other low SFE alloys and achieved at a much lower strain level. These outcomes indicated the combined effect of CryoR, massive solid-solution formation, and extremely low SFE towards extreme grain refinement. CryoR also resulted in the development of a strong brass texture ({110}<112 >), similar to other low SFE alloys. Annealing resulted in recrystallization, precipitation, and dissolution of Cr-rich tetragonal σ phase precipitates in the CryoR material at lower temperatures due to finer nanostructure and greater stored energy for transformation. Grain growth was inhibited up to 950 °C by the presence of the σ precipitates but was triggered beyond that by the dissolution of the precipitates. Annealing textures retained weak α-fiber (normal direction (ND)//<110>) components and a high fraction of random components, which resembled other low SFE HEAs. Meanwhile, ultrafine microstructures and multistage strain-hardening resulted in a remarkable ambient temperature strength-ductility (1025 MPa-20%) combination in the CryoR and annealed HEA.

Charge-density based evaluation and prediction of stacking fault energies in Ni alloys from DFT and machine learning

A combination of high strength and high ductility has been observed in multi-principal element alloys due to twin formation attributed to low stacking fault energy (SFE). In the pursuit of low SFE alloys, a key bottleneck is the lack of understanding of the composition–SFE correlations that would guide tailoring SFE via alloy composition. Using density functional theory (DFT), we show that dopant radius, which have been postulated as a key descriptor for SFE in dilute alloys, does not fully explain SFE trends across different host metals. Instead, charge density is a much more central descriptor. It allows us to (1) explain contrasting SFE trends in Ni and Cu host metals due to various dopants in dilute concentrations, (2) explain the large SFE variations observed in the literature even within a given alloy composition due to the nearest neighbor environments in “model” concentrated alloys, and (3) develop a machine learning model that can be used to predict SFEs in multi-elemental alloys. This model opens a possibility to use charge density as a descriptor for predicting SFE in alloys.

Simultaneous Twinning and Microband-Induced Plasticity of a Compositionally Complex Alloy with Interstitial Carbon at Cryogenic Temperatures

Intermediate to low stacking fault energy (SFE) high entropy alloys (HEA) have shown an excellent combination of strength and ductility as a result of deformation twinning and martensite transformation. However, even in the absence of these mechanisms HEA can show a good strength-ductility combination, as is the case with non-equiatomic (Fe40.4Ni11.3Mn34.8Al7.5Cr6)C1.1. The room temperature mechanical behavior of this alloy has been associated with Taylor lattice and microband formation. The current research focuses on tensile cryogenic deformation of this alloy and investigates if these features and/or alternate mechanisms like deformation twinning are obtained. Surprisingly, it is not one or the other but both deformation twinning and microband formation that are observed during cryogenic deformation. The activation of both deformation mechanisms is a combination that is not often reported as the former is generally associated with intermediate to low SFE alloys and the latter with intermediate to high SFE alloys. The activation of twinning in (Fe40.4Ni11.3Mn34.8Al7.5Cr6)C1.1 is attributed to the high yield stress-temperature variation, as a result of solid solution strengthening being far greater than in other commonly researched compositionally complex alloys. A ductility retention down to 4 K was observed, while simultaneously showing a significant increase in flow stress. Despite the intermediate to high SFE deformation behavior, (Fe40.4Ni11.3Mn34.8Al7.5Cr6)C1.1 exhibits excellent cryogenic strength-ductility combination.

Achieving an exceptional ductility at room temperature in a low SFE copper alloy fabricated by severe plastic deformation

Abstract Low stacking fault energy alloys often exhibit low ductility. However, sometimes these alloys show unusual mechanical behavior following specific processing parameters. In this study, a low stacking fault energy α+β brass was processed by equal channel angular pressing (ECAP) at 350 °C in ‘route C’ through three passes. Room temperature tensile testing showed that ductility, surprisingly, reached ~80%, whilst good tensile strength was maintained. Investigation of alloy microstructure revealed a combination of deformation mechanisms, including slip and twinning, accompanied by grain boundary serration and grain fragmentation. It is suggested that these deformation modes triggered these unexpected mechanical properties in this intrinsically brittle alloy. The required energy for discontinuous recrystallization was supplied after only one pass in the α phase, and three passes in the β phase, which then prompted good ductility. In order to study texture evolution, the macro-texture was measured. A high fraction of recrystallized grains and change in strain path fostered the development of Goss, Rotated Goss and Rotated Cube components in the α phase. While in the β phase {011} and Goss components were dominant. Despite the high Schmid factor, Hall-Petch effects and work hardening led to increase of strength and hardness after the final pass.

Correspondence between grain refinements and flow softening behaviors at Nimonic 80A superalloy under different strain rates, temperatures and strains

As for a low stacking fault energy alloy, its strain softening degree evaluation and relative mechanism description significantly contribute to deep understanding its hot flow behaviors. An evaluator, decline ratio of flow stress (DRFS) was introduced and computed in this work to quantitatively evaluate the softening degree and its variation of a Nimonic 80A superalloy, and then the dominant softening mechanism was determined as dynamic recrystallization (DRX). The basic stress-strain data for DRFS computation come from a series of isothermal compressions with height reduction of 60% over a wide range of temperature and strain rate. The onsets of DRX were identified from the work hardening rate curves (i.e. θ versus σ) and their derivative. The results show that the critical onset stress σc decreases with increasing temperature under a fixed strain rate, while increases with increasing strain rate under a constant temperature. In the softening process with DRX mechanism, grain size evolution and DRFS variation are the intrinsic and extrinsic indicators respectively, and their correspondence under different deformation conditions contributes to understand the relationships between microstructures and macro-mechanical properties. A superimposed map of grain size contour plot over DRFS map was constructed at the true strain 0.916. The results show that with increasing strain rate, the value of DRFS decreases to 0.05 until the strain rate reaching to 1s−1 and then increases to 0.18 at low temperature, while the grain size persistently decreases to 8.2µm. It indicates that the relationship between grain size evolution and DRFS variation is nonlinear.

Nucleation of deformation twins in nanocrystalline fcc alloys

An earlier dislocation model for predicting the grain size effect on deformation twinning in nanocrystalline (nc) face-centred-cubic (fcc) metals has been found valid for pure metals but problematic for alloys. The problem arises from the assumption that the stacking-fault energy (γSF) is twice the coherent twin-boundary energy (γfcc), which is approximately correct for pure fcc metals, but not for alloys. Here we developed a modified dislocation model to explain the deformation twinning nucleation in fcc alloy systems, where γSF ≠ 2γtwin. This model can explain the differences in the formations of deformation twins in pure metals and alloys, which is significant in low stacking-fault energy alloys. We also describe the procedure to calculate the optimum grain size for twinning in alloy systems and present a method to estimate γtwin.

Influence of precipitate size and morphology on grain refinement in nickel aluminium bronze

Nickel aluminium bronze (NAB) was subjected to equal channel angular pressing (ECAP) using routes BA and C at 400°C to investigate the effect of precipitate size and morphology on grain refinement in low stacking fault energy alloys. Both routes produced dynamically recrystallised grains of ∼550 nm in size although only route BA was able to create a uniform distribution of the refined grains. The large unrefined regions in NAB processed via route C was thought to arise from its inability to redistribute the various precipitate phases, as recrystallisation was enhanced around the coarse κII rosettes and refined κIII lamellae but reduced in the areas containing fine κIV precipitates.

Correlation of Grain Size, Stacking Fault Energy, and Texture in Cu-Al Alloys Deformed under Simulated Rolling Conditions

The effect of grain size and stacking fault energy (SFE) on the strain hardening rate behavior under plane strain compression (PSC) is investigated for pure Cu and binary Cu-Al alloys containing 1, 2, 4.7, and 7 wt. % Al. The alloys studied have a wide range of SFE from a low SFE of 4.5 mJm−2for Cu-7Al to a medium SFE of 78 mJm−2for pure Cu. A series of PSC tests have been conducted on these alloys for three average grain sizes of ~15, 70, and 250 μm. Strain hardening rate curves were obtained and a criterion relating twinning stress to grain size is established. It is concluded that the stress required for twinning initiation decreases with increasing grain size. Low values of SFE have an indirect influence on twinning stress by increasing the strain hardening rate which is reflected in building up the critical dislocation density needed to initiate mechanical twinning. A study on the effect of grain size on the intensity of the brass texture component for the low SFE alloys has revealed the reduction of the orientation density of that component with increasing grain size.

Effects of stacking fault energy on the creep behaviors of Ni-base superalloy

Cobalt in a 23wt.% Co containing Ni-base superalloys was systematically substituted by Ni in order to study the effects of stacking fault energy (SFE) on the creep mechanisms. The deformation microstructures of the alloys during different creep stages at 725°C and 630MPa were investigated by transmission electron microscopy (TEM). The results showed that the creep life increased as the SFE decreased corresponding to the increase of Co content in the alloys. At primary creep stage, the dislocation was difficult to dissociate independent of SFE. In contrast, at secondary and tertiary creep stages the dislocations dissociated at γ/γ′ interface and the partial dislocation started to shear γ′ precipitates, leaving isolated faults (IFs) in high SFE alloy, while the dislocations dissociated in the matrix and the partials swept out the matrix and γ′ precipitates creating extended stacking faults (ESFs) or deformation microtwins which were involved in diffusion-mediated reordering in low SFE alloy. It is suggested that the deformation microtwinning process should be favorable with the decrease of SFE, which could enhance the creep resistance and improve the creep properties of the alloys.

Microstructural investigation on strengthening mechanisms of AISI 304L austenitic stainless steel during cryogenic deformation

In the present paper, microscopy techniques and mechanical tests were used to investigate in detail the strengthening mechanisms of an AISI 304L austenitic stainless steel during cryogenic deformation. The strain hardening rate–strain response of the alloy indicated three distinct regimes of hardening, being very similar to that previously reported for other low stacking fault energy alloys. The hardening rate initially decreased up to a strain of ∼6% (stage I). Then, a second stage of increasing hardening rate began (stage II). At strains larger than ∼25%, stage III with decreasing hardening rate followed. It was suggested that the formation of ϵ-martensite and α′-martensite together is responsible for the appearance of stage II. The high strain hardening values of the alloy in stage II were related to the increased fraction of α′-martensite and dislocation pile-ups behind the Lomer–Cottrell locks. The appearance of stage III was attributed to the difficulty of α′-martensite nucleation and ease of dislocation cross-slip at higher strains.

Role of stacking fault energy on the deformation characteristics of copper alloys processed by plane strain compression

Abstract Samples of Cu–Al and Cu–Zn alloys with different compositions were subjected to large strains under plane strain compression (PSC), a process that simulates the rolling operation. Four compositions in the Cu–Al system, namely 1, 2, 4.7 and 7 wt.% Al and three compositions in the Cu–Zn system of 10, 20 and 30 wt.% Zn, were investigated. Adding Al or Zn to Cu effectively lowers the stacking fault energy (SFE) of the alloy and changes the deformation mechanism from dislocation slipping to dislocation slipping and deformation twinning. True stress–true strain responses in PSC were documented and the strain hardening rates were calculated and correlated to the evolved microstructure. The onset of twinning in low SFE alloys was not directly related to the low value of SFE, but rather to build up of a critical dislocation density during strain hardening in the early stage of deformation ( ɛ

Construction of Processing Map for Biomedical Co-29Cr-6Mo-0.23C-0.14N Alloy by Using Compression Tests

The high-temperature deformation behavior of Co-29Cr-6Mo-0.23C-0.14N alloy was investigated by carrying out compression tests; the tests were carried out in the temperature range of 1000–1200°C and strain rates ranging from 0.01 to 30 s−1 with a height reduction of approximately 65%. The optimum hot-working conditions were determined from the processing map based on the dynamic materials model. Dynamic recrystallization was observed to occur over the entire temperature and strain rate range. However, uniformly sized grains were formed for strain rates higher than 1 s−1, which is considered to be the optimum hot-working condition. In addition, authors suggested that in extremely low stacking fault energy alloy, explosive and homogeneous formation of mechanical twinning should be considered to be stable state, and new processing map with more detailed description of deformation mechanism seems necessary in future.

Influence of stacking fault energy and short-range ordering on dynamic recovery and work hardening behavior of copper alloys

True stress vs. true strain responses of Cu–6 wt.% Al and Cu–12 wt.% Mn alloys are presented. While Cu–6 wt.% Al alloy shows the typical mechanical response of low stacking fault energy alloys, the Cu–12 wt.% Mn alloy behaved similarly to medium to high stacking fault energy alloys. These findings clearly show that while short-range ordering triggers slip planarity, it has a minor effect on total dynamic recovery of these copper alloys.

Plastic Behavior of Metals in Reverse Straining after Large Pre-Strains

The present work examines the reversal response of a face-centered cubic (fcc) polycrystalline metal after large pre-strains. While reversal responses among different fcc metals are similar after small pre-strains, they can vary widely after large pre-strains depending on material and microstructure. In this article, these characteristics are considered to be governed by three distinct mechanisms: (1) reverse glide of dislocations previously held by backstresses, (2) reverse glide of dislocations previously held by barriers, and (3) ‘reverse hardening’ by reverse glide over stable dislocation barriers formed in pre-straining. These small-scale mechanisms are incorporated into a polycrystal code to investigate their influence on the macroscopic reversal response and to interpret large strain reversal tests in the literature. It is shown that mechanism (2) is responsible for worksoftening and reductions in hardening rate and mechanism (3) for the overshoot seen in α- brass and other low stacking fault energy alloys. Mechanism (1) is responsible for the Bauschinger effect and occurs in all metals. A large fraction of second phases leads to a strong Bauschinger effect that can either reduce or postpone the effects of mechanisms (2) and (3).

A criterion for shear banding localization in polycrystalline FCC metals and alloys and critical working conditions for different microstructural variables

This article investigates the effect of stress state on the shear banding localization and microstructure evolution in polycrystalline FCC metals and alloys. It is worth to indicate that this investigation was previously conducted on 70/30 brass and extended through the present work to include different stacking fault energy (SFE) materials ranging from low SFE alloys; such as 316L Stainless steel to medium SFE metals such as copper to examine the generality (if any) of the previous findings in 70/30 brass. Stress states investigated are plane strain compression (PSC) and simple compression (SC). It is known that shear bandings are more likely to develop vigorously in PSC. The tests were also conducted on specimens with different grain sizes. It was shown that the strain hardening rate curves of SC and PSC coincided up to a certain value of an effective strain beyond which the hardening rate curve of PSC started to deviate and drop rapidly, which indicated the presence of extensive shear banding. These observations were verified through detailed optical microscopy of deformed specimens at these strain values.

Evaluation of activation parameters by Reed-Hill model

Differential strain rate tests have been carried out at different temperatures and at various stages along the stress-strain curve at a given temperature for a series of Ni—Co alloys of varying stacking fault energy (SFE). The thermal stress component has been estimated and apparent activation enthalpies have been determined. The likely reasons for poor applicability of the Reed-Hill approach to modelling flow stress and activation parameters for a low SFE alloy, and its better applicability to the temperature range where thermal obstacles conform to a linear force-displacement relationship, are discussed. A mixed analysis including both the Reed-Hill approach and a conventional method is suggested for the determination of activation parameters using limited experimentation.

Shear banding and recrystallization nucleation in a Cu–2%Al alloy single crystal

High-purity Cu–2%Al single crystals with initial C{112}〈111〉 orientation have been deformed by channel-die compression at 77 and 293 K to develop fine shear bands in a twin-matrix (T-M) substructure. The microstructural and microtextural changes during deformation and the early stages of recrystallization have been studied in detail by TEM/CBED and SEM/EBSD. Local orientations measurements indicate that the scattering of the original T-M orientations within shear bands towards two, twin-related positions is due to the localised lattice rotation around the TD ‖ 〈011〉 axis. The starting points for the occurrence of new, recrystallized grains are the {110}〈100〉 and {114}〈221〉 components of the as-deformed microtexture inside the shear bands. As recovery and recrystallization proceed, the orientations of the primary nuclei systematically evolve to adopt simple boundary misorientation relations of ∼30°〈111〉 type with respect to one of the two components of the deformed matrix. The results of the present study clarify the respective roles of all three basic mechanisms, i.e. oriented nucleation, growth (defined here as ∼30°〈111〉) and recrystallization twinning which are involved in the initial stages of recrystallization texture development in this low stacking fault energy alloy.