Deformation Twinning Research Articles

Creep Behaviors and Deformation Mechanisms at Intermediate Temperatures in a Novel γ‐γ′ Nickel‐Based Superalloy

The creep behaviors of a novel polycrystalline nickel‐based superalloy are investigated at a wide applied stress range of 500–950 MPa and temperature ranging from 650 to 750 °C. And the microstructure and deformation mechanisms are also analyzed. The results indicate that all creep curves of the novel nickel‐based superalloy are mainly consist of the primary and accelerated stages, whereas the steady‐state stage is not obviously observed. The minimum creep rate increases and the corresponding creep life decreases with the applied stress increasing at constant temperature. The relative longer creep life at 650 °C could be attributed to the coupling of stacking faults in the interior of γ′ and deformation twinning.

Grain boundary engineering using self-organized high-index twins produced by twin-twin reactions in β-Ti alloys: A phase field study

Introducing coherent twin boundaries is an effective way to develop submicron/nano-sized grains with improved thermal stability in polycrystals, which exhibit enhanced mechanical properties of metals, such as high strength, high hardness and high fracture toughness. However, crystalline defects, such as dislocations and disclinations, usually arise at twin boundary junctions, which lead to local stress concentration and property degradation. Here we show that a large number of high-index coherent twin boundaries (e.g., Σ11 and Σ19a) can be produced through a well-controlled deformation twinning process. By using metastable β-Ti alloys as an example and analyzing the broken symmetry associated with deformation twinning, we demonstrate that the abnormal high-index twins result from intercorrelated twinning pathways and twin-twin reactions. The formation mechanism of self-organized multi-twin structures has been analyzed systematically through phase field modeling and simulations. It has been found that, under well designed thermo-mechanical condition, regularly distributed high-index twin boundaries and geometrically-compatible twin boundary junctions (i.e., dislocation/disclination-free) can be produced. The stress/strain condition required to obtain self-organized twins has been quantitatively determined, which provides a theoretical guideline for the development of self-organized submicron/nano-twin metals for unprecedented properties.

Activation of deformation twinning in ultrafine-grained high-entropy alloys via tailoring stacking fault energy and critical twinning stress

In this study, we propose a new material design approach to activate deformation twinning in ultrafine-grained (UFG) AlCoCrCuFeNiTi-based high-entropy alloys (HEAs) by considering both thermodynamic and microstructural aspects. Stacking fault energy of the solid-solution phase is reduced by tailoring composition with increasing Co/Ni ratio and diminishing the concentration of Al and Ti through precipitation. Furthermore, grain size is adjusted to enable the flow stress to surpass the critical twinning stress. It results in the activation of nano-twinning in the UFG HEAs, making the alloys exhibit high strain-hardening after yielding from 1.6 to 2.3 GPa.

Shuffling pathway of anti-twinning in body-centered-cubic metals

In this study, anti-twinning and deformation twinning of body-centered-cubic (BCC) metals under nanoindentation were examined. Molecular dynamics (MD) simulations of BCC tantalum (Ta) showed both the existence of anti-twinning and a novel two-layer-by-two-layer anti-twinning pathway consisting of a staggered shuffle sliding of {112} atomic planes in the [111¯] and [1¯1¯1] directions, generating finite shear displacement along the anti-twin direction. The density functional theory (DFT) multi-dimensional interlayer slipping energy landscape of the newly observed shuffling anti-twinning process has a lower energy barrier for both twin nucleation and growth than the recently proposed layer-by-layer anti-twinning process.

Formation of intracrystalline deformation twins in austenitic Mn-steel at the early stage of plastic deformation

In austenitic Mn-steels with low stacking fault energy, the deformation twin is known to play an important role in plastic deformation. Usually, the position grain boundary is preferred to observe and investigate the formation process of the deformation twin, while other suitable formation positions may be neglected. Here, high-resolution transmission electron microscopy is used to observe and characterize the formation of intracrystalline twins in 120Mn13 steel at the early stage of tensile plastic deformation at the atomic scale. The result shows that intracrystalline twins nucleate through overlapping stacking faults generated by three different perfect dislocations dissociating on consecutive {111}-type planes, which is different from twin nucleation mechanisms that have been proposed. Subsequently, they become large-sized intracrystalline twins or twin bands throughout the whole grain by twin nuclei growing themselves and connecting to adjacent twin nuclei or existing twins in the same growth direction. Intracrystalline twins are mainly formed in three positions, namely near the grain boundary, at the end or side of existing twins. The stacking fault sources of intracrystalline twins at different positions are different.

Exceptional strength-ductility synergy at room and liquid nitrogen temperatures of Al7.5Co20.5Fe24Ni24Cr24 high-entropy alloy with hierarchical precipitate heterogeneous structure

The study of alloys exhibiting noteworthy strength-ductility synergy at ambient and cryogenic temperatures has been a persistent area of interest in materials engineering. This interest extends to the recent development of high-entropy alloys (HEAs). The current investigation delves into the impact of diverse thermo-mechanical treatments on the phase and microstructure evolution in a face-centered cubic (FCC) Al7.5Co20.5Fe24Ni24Cr24 HEA. The transition from solid-solution annealing to recrystallization annealing leads to the formation of the desired hierarchical B2+L12+σ precipitates, accompanied by a heterogeneous FCC matrix. The initiation of the B2 phase originates from nucleation on defect-rich sites, such as deformation bands. However, the coherent L12 phase homogeneously forms in the FCC matrix at intermediate temperature aging, as these sites are scarce or occupied. A heterogeneous structure emerges from the transition in annealing temperatures and the pinning effect of the B2 precipitates. The resulting heterogeneous structure exhibits an exceptional strength-ductility synergy at both room and liquid nitrogen (LN2) temperatures. This is evident in its mechanical properties with a yield strength of ∼717 MPa / ∼1109 MPa, an ultimate tensile strength of ∼1086 MPa / ∼1609 MPa, and an elongation of ∼34.3 % / ∼43.2 % at room / LN2 temperatures. The formation of deformation twins (DTs) is facilitated by localized stress buildup from hetero-deformation-induced (HDI) hardening stress at room temperature. The exceptional strength and ductility at LN2 temperature are attributed to a combination of factors. These include a high-density of stacking faults (SFs), DTs, and their interactions, including those with precipitates, SFs-based substructures, and Lomer-Cottrell locks. These multiple deformation mechanisms ensure consistent and sustained strain-hardening even under substantial strain. This paper sheds light on the complex interplay of microstructure, deformation mechanisms, and mechanical properties in the Al7.5Co20.5Fe24Ni24Cr24 HEA, potentially guiding the development of ultra-strong yet ductile alloys for cryogenic applications.

Cross-slip of extended dislocations and secondary deformation twinning in a high-Mn TWIP steel

Twinning-induced plasticity (TWIP) steels have become important materials in industry owing to the good combination of strength and ductility and high strain hardening rate. The excellent mechanical properties are highly related to the glide of dislocation and deformation twinning. However, the cross-slip behavior of the extended dislocation and the mechanism of deformation twinning are still controversial. Here, the partial dislocation motion and austenite twinning of a high-Mn steel at the early stage of deformation were investigated using in-situ tensile transmission electron microscope (TEM) technique. Results show that a large number of plane glide and cross-slip of extended dislocations can occur at the early stage of deformation. Extended dislocation nodes can be formed as a result of the reaction between adjacent extended dislocations on the same glide plane. In-situ tensile TEM experiments confirm two cross-slip models of partial dislocation: (1) the Friedel-Escaig model, cross-slip based on constriction of extend dislocation and re-dissociation; (2) the Fleischer model, cross-slip involving Lomer-Cottrell dislocation. Based on experimental results and energy calculations, it can be confirmed that the formation mechanism of austenite primary deformation twin induced by partial dislocations is different from that of secondary deformation twin. Grain boundary emits partial dislocation into the grain to form stable stacking fault which induces austenite primary deformation twin. The formation of secondary deformation twin is related to cross-slip of extended dislocation. Only the cross-slip of an extended dislocation containing a 90° partial dislocation can induce the formation of secondary deformation twin by introducing the Frank partial dislocation.

Achieving strength-ductility synergy in zirconium via ultra-dense twin-twin networks

Hierarchical nano-twinned structures have been proven to be an effective way in breaking the strength-ductility dilemma in face-centered cubic metals. However, it is challenging to introduce high-density of twinned structures in hexagonal-close packed (HCP) metals because deformation twinning are limited in fine-grained structures. Here, we propose an approach to introduce ultra-dense twin-twin networks in high purity HCP Zr with ultra-large grain size (>200 µm) via cryogenic biaxial-rolling. Tensile tests show that the yield strength and the elongation of the densely twinned Zr are increased by 76.5 % and 28.6 % compared with its coarse-grained counterparts, and reaches 360 MPa and 13.5 %, respectively. The strength of twinned Zr is enhanced by the synergy effect of grain refinement (from 241 µm to 1.2 µm) and the obstruction of dislocations by high-density intersecting twin boundaries (TBs). The ductility and strain-hardening ability are improved by the activation of massive prismatic 〈a〉 and pyramidal 〈c + a〉 dislocations. The three-dimensional TB networks are effective 〈c + a〉 dislocation sources which enhance the plasticity along 〈c〉 direction. Our work provides a new strategy to design ultra-dense twinned structures and sheds light on the twinning-induced plasticity in HCP metals.

Promoting nanoscale deformation twinning through FCC phase decomposition in AlCoCrFeMo0.05Ni2 high entropy alloy

The high temperature plasticity of AlCoCrFeMo0.05Ni2 high entropy alloy (HEA) is significantly enhanced after annealing. However, the specific mechanisms are not fully understood. In this study, a polycrystalline HEA model with dimensions of 18 nm×18 nm×26 nm was used to analyze stress mapping under uniaxial tension through Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). Complemented by experimental results, one can see that the decomposition of the FCC phase promotes the formation of nanoscale deformation twins. The formation of deformation twinning is most likely one of the primary reasons for the substantial improvement in high temperature plasticity. The findings provide valuable insights for controlling the high temperature performance of the alloy.

Plastic deformation and strengthening mechanism in CoNiV medium-entropy alloy fiber

High/medium-entropy alloys (H/MEAs) are regarded as a potentially viable alternative to conventional metallic fibers for the production of ductile, high-strength fibers, to resolve the inherent trade-off between strength and ductility. The present study involved the cold drawing technique to produce a CoNiV MEA fiber measuring 300 μm in diameter with a length of more than 3 m. The mechanical properties of the FCC matrix can be improved through the inclusion of an appropriate amount of the κ phase via the optimized thermal treatment process. In addition to a yield strength of 1681 MPa and a well-coordinated elongation of 13.4 %, the ideal CoNiV fiber demonstrated a substantial ultimate tensile strength of 1932 MPa. Further calculations revealed that the κ phase, which possesses a substantial Von Mises stress of approximately 2715 MPa and an area fraction of 18.2 ± 1.1 %, was observed to be a primary contributor to the strength. Deformation twins were generated in the FCC matrix as a result of the ultra-high flow stress, which provided adequate ductility. This study offers significant contributions to the understanding of the deformation mechanisms and strengthening effect of the κ phase, thereby facilitating the development of high-performance metallic fibers.

Mechanical response in tension of twin-induced Mg alloys in wide strain rate regimes

Effect of twin boundary with/without segregation of solute atoms on deformation behavior in tension is examined in Mg–3Al–1Zn (AZ31) alloys from low strain rates to high strain rates. Solute atoms (Al and/or Zn atoms) are segregated at {10 1‾ 2} twin boundaries by static annealing at a temperature of 423 K with a holding time of 150 min. The room-temperature tensile mechanical response of the specimens, which have pre-induced twin boundaries, shows detwinning behavior, regardless of the strain rate between 10−5/s and 100/s. The yield strength of these specimens is also unlikely to be affected by strain rate, because twin boundary mobility, i.e., the shrinkage, is an athermal process. On the other hand, the specimen with segregating of solute atoms at twin boundaries exhibits an increased strength and a decreased strain hardening as compared with those in the specimen having non-segregated twin boundaries. Newly formed deformation twins of several types, i.e., the {10 1‾ 2} twins, {101‾ 1} twins and {101‾ 2}-{101‾ 1} double twins, are the origin of fracture in these specimens.

Achieving well-balanced strength and ductility via dual nanoscale precipitate structures in Co-free high-entropy alloys

Recent studies confirmed that introducing nanoprecipitates as strengthening phase into high-entropy alloys (HEAs) is an extremely effective means to achieve the excellent strength-ductility synergy. In this work, we successfully created a dual nanoscale precipitate structure in a Co-free Ni2.5CrFeAl0.3Ti0.2 HEA by controlling thermo-mechanical process. Compared to the recrystallized alloy, homogeneous L12 precipitates in FCC matrix and heterogeneous L12/BCC precipitates near grain boundaries were formed in the aged alloys. The dual-nanoprecipitate structure induces a long-range back stress during deformation process, effectively contributing to a balance of high strength and good ductility at 298K. Especially, the mechanical properties of the recrystallized and aged alloys are dramatically improved without strength-ductility tradeoff, showing a multiple-stage strain-hardening behavior at 77 K. In the aged alloys, the nanoprecipitates shorten the mean free path of dislocation movement to effectively induce the precipitation strengthening appreciably depending on the precipitation size. Different from planar dislocations slip dominated by stacking faults at 298K, multiple stacking fault networks and deformation twins were activated sequentially with increasing the tensile strain, thus forming a dynamic Hall-Petch effect at 77K. The current work could help to understand the correlation between mechanical properties and deformation mechanisms in precipitation-strengthening HEAs, and the transformation of these deformation mechanisms provide an insight for developing outstanding-performance HEAs through trigging various hardening modes.

A Novel Hexagonal Close-Packed High-Entropy Alloy with Outstanding Strength-Ductility Synergy

Refractory high-entropy alloys (HEAs) are new potential candidates in high temperature applications. However, most present refractory HEAs are single-phase body-centered cubic (BCC) structure, which is brittle at room temperature. Then strategies to ductile the refractory HEAs and maintain their good high temperature strength at the same time should be under consideration. In the present study, a novel WReOsIr HEA with hexagonal close-packed (HCP) structure was developed. This alloy not only has excellent high-temperature strength (416.7 MPa at 1473 K), but also exhibits good ductility (30.7%) at room temperature. The better room temperature plasticity is found to originate from the deformation twins formed inside the grains.

Deformation behavior and mechanical anisotropy in ZrB2-based ceramic

The deformation behavior and mechanical properties of ZBHOx ceramic (ZrB2 based ceramics with different amounts of (HfO2 + 2 vol% CaO), x = 10 - 25 vol%) have been investigated. The results demonstrated that the flow stress of ZBHOx ceramic decreased with an increase in the (HfO2+2 vol% CaO) addition at the same strain rate. The flow stress decreased from ∼178 MPa for ZBHO10 to ∼13 MPa for ZBHO20 at 1700 °C and a strain rate of 2 × 10−4 s−1, representing a reduction of an order of magnitude. The intergranular deformation twinning and the amorphous phase contribute to the grain boundary sliding and grain rotation of the boride matrix grains, resulting in a significant reduction in flow stress. The matrix grains orientations on the P face (the face perpendicular to the extrusion direction) primarily tend to be along the {0001} plane, while that on the V face (the face parallel to the extrusion direction) tend to be along the {-12-10} plane or {01–10} plane. The textured ZBHOx ceramics exhibit distinct mechanical anisotropy because of the significant orientation difference in the microstructure. The hardness of textured ZBHOx ceramics on V face is higher than that on P face. And the fracture toughness of ZBHOx ceramics exhibits anisotropy on the V face, the fracture toughness in the V-o direction (the direction perpendicular to the P face) is higher than that in V-p direction (the direction parallel to the P face) of ZBHOx ceramics.

Mechanical properties and deformation mechanisms of C-doped interstitial high-entropy alloy CrMnFeCoNi: Effects of strain rate and C content

The effects of strain rate on mechanical properties, microstructural evolution and underlying deformation mechanisms of two kinds of interstitial CrMnFeCoNi high entropy alloys (HEAs), (CrMnFeCoNi)99.5C0.5 and (CrMnFeCoNi)99.0C1.0, are investigated under quasi-static and dynamic tension. Dynamic loading is carried out via a split Hopkinson tension bar system. The pre- and post-deformation microstructures are characterized with electron back scatter diffraction and transmission electron microscopy. Both yield strength and work hardening increase remarkably with increasing strain rate. Plastic deformation in both alloys under quasi-static and dynamic loading are dominated by dislocations, stacking faults, kink bands and deformation twins. Twinning is activated easier under dynamic loading, and twin density decreases with increasing carbon content due to the increased stacking fault energy. The Khan-Liu constitutive model can describe the experimental results for these two interstitial HEAs in a wide strain range.

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.

A combined EBSD and machine learning study of predicting deformation twinning in BCC Fe81Ga19 alloy

Compared to face-centered cubic (FCC) metals, body-centered cubic (BCC) metals demonstrate more intricate deformation behavior and microstructural characteristics because of the influence of intrinsic unstable stacking faults and defect structure. To investigate the unique phenomenon of deformation twinning in Fe81Ga19 alloy with BCC structure at moderate temperature and low strain rate, four twin nucleation models were constructed using a combination of electron backscatter diffraction (EBSD) technique and machine learning. These models are aimed to predict twin nucleation of grains and explore the relationship between twin nucleation and microstructural attributes. The study revealed that amongst the 235 grains analyzed during in-situ compressive deformation, 32 grains experienced twinning. Nine attributes influencing twin nucleation were selected and ranked according to their importance. Grain area, average image quality, neighboring grain count, and the Schmid factor of twinning systems exhibited significant influence on twin nucleation. Decision tree and tree ensemble (XGBoost) achieved 94% and 96% accuracy, respectively, on the test set, showcasing robust generalization capability. Due to the optimization of hyperparameters, support vector machine (SVM) and artificial neural network (ANN) exhibited outstanding performance. The ANN model achieved an accuracy of 97% and an F1 score of 0.91 on the test set, while the SVM model achieved an accuracy of 98% and an F1 score of 0.94, indicating superior performance. Furthermore, the study revealed that the twinning stress in Fe81Ga19 alloy exhibited weak responsiveness to temperature during deformation, and the intrinsic factors of grains were identified as crucial factors influencing twin nucleation.

Improvement of tensile properties through Nb addition and heat treatment in additively manufactured 316L stainless steel using directed energy deposition

Metal additive manufacturing (AM) has been applied in various fields because it allows the manufacture of parts with complex shapes. In this study, 316 L stainless steel specimens with 0 and 2 wt% Nb were fabricated using direct energy deposition, and heat treatment was performed at various temperatures. The addition of Nb changed the microstructure of the steel, including the grain size and Nb segregation. When heat treatment was performed, microstructural evolution of the cell boundary, dislocation density, and deformation twin density occurred. These microstructural changes improved the tensile properties of the heat-treated specimens containing Nb. The yield strength, tensile strength, and elongation of the specimens containing Nb with heat treatment at 800 °C were 530 MPa, 757 MPa, and 39%, respectively, which represent great improvements compared to specimens without Nb. These results show that the addition of Nb to 316 L stainless steel together with heat treatment at an appropriate temperature endow the additively manufactured stainless steel with excellent tensile properties.

Effect of Manganese on the Strength-Toughness Relationship of Low-Carbon Copper and Nickel-Containing Hull Steel.

The influence of varying the manganese (Mn) contents of high-strength copper-containing hull steel on its microstructural evolution and mechanical properties was investigated. With increasing Mn content from 2 to 5%, the tensile strength of the steel increased by ~100 MPa, while the elongation of steel remained at ~23.5%, indicating good plasticity. However, the 2Mn sample had 128 J higher low-temperature (-84 °C) impact work than the 5Mn sample. The microstructures of different Mn steels were composed of fresh martensite (FM), ferrite/tempered martensite (F/TM), and reversed austenite (RA). The increase in Mn content markedly increased the presence of RA and intensified the work hardening caused by the transformation-induced plasticity (TRIP) effect during the tensile process. However, as the phase transformation in different Mn steels occurred in the early stage of strain and did not extend throughout the entire plastic deformation process, increasing plasticity via phase transformation was difficult. In addition, although the volume fraction of RA increased significantly in 4Mn and 5Mn steels, the stability of RA significantly decreased. The presence of numerous metastable blocks and coarse lath-like RA contributed little to low-temperature impact work and was even detrimental to toughness. The substantial fresh martensite resulting from phase transformation facilitated microcrack generation, owing to rapid volume expansion and mutual impacts, thus reducing the work required for crack formation. Additionally, the abundance of deformation twins significantly reduced the work needed for crack propagation. These combined actions significantly reduced the low-temperature toughness of 4Mn and 5Mn steels.

Fracture behavior and constitutive relations of Sn-3.0Ag-0.5Cu solder alloy at cryogenic temperature

The mechanical behavior, underlying fracture mechanism and constitutive relations of Sn-3.0Ag-0.5Cu (SAC305) solder alloy at cryogenic temperature through in-situ uniaxial tensile experiments were investigated over a wide temperature range of 293 K–77 K. The activation of deformation twin in the solder alloy contributes to improve the tensile strength and quasi-static toughness, and maintains the fracture elongation at 35–45% with the decline of ambient temperature. SAC305 solder alloy with a mixed mode of ductile and brittle fracture achieves the optimal balance of strength, ductility and toughness at 123 K, and the large mismatch between twins thickening rate (∼13 μm/s) and dislocation slip velocity (∼3.6 μm/s) finally leads to the brittle fracture at 77 K. In addition, Anand model as a unified viscoplastic constitutive law has been employed to describe the constitutive relations of SAC305 solder alloy and fails in fitting the stress-strain curves at invalid temperature below 233 K. Hollomon equation is conducted to perfectly characterize the constitutive relations of the alloy at temperatures lower than 233 K. The method for controlling the formation of deformation twins in the solder alloy and solder joints is really beneficial to solve the reliability problems for cryogenic electronics.