Formation Of Deformation Twins Research Articles

Insight into the effect of vibration parameters on deformation mechanism during scratching of nanograin iron alloy by molecular dynamics

Molecular dynamics simulations were employed to study the deformation behavior and microstructure evolution of nanocrystalline iron alloy during vibration-assisted scratching. The mechanical properties of the plastic deformation layer after scratching were tested by uniaxial tension. The results show that dislocation multiplication and twinning occur sequentially during loading, while dislocation rebound and detwinning occur during unloading. High stress induces dislocation multiplication. High strain rate facilitates the formation of deformation twins. Under the inhibitory effect of deformation twins on dislocation activity and the elastic recovery of the surface, dislocation density decreases with increasing frequency but initially increases and decreases with increasing amplitude. The plastic deformation layer at 10GHz frequency and 1nm amplitude exhibits optimal yield strength due to dislocation and nanotwin strengthening.

Temperature Effect on Deformation Mechanisms and Mechanical Properties of Welded High-Mn Steels for Cryogenic Applications.

High-manganese steel (high-Mn) is valuable for its excellent mechanical properties in cryogenic environments, making it essential to understand its deformation behavior at extremely low temperatures. The deformation behavior of high-Mn steels at extremely low temperatures depends on the stacking fault energy (SFE) that can lead to the formation of deformation twins or transform to ε-martensite or α'-martensite as the temperature decreases. In this study, submerged arc welding (SAW) was applied to fabricate thick pipes for cryogenic industry applications, but it may cause problems such as an uneven distribution of manganese (Mn) and a large weldment. To address these issues, post-weld heat treatment (PWHT) is performed to achieve a homogeneous microstructure, enhance mechanical properties, and reduce residual stress. It was found that the difference in Mn content between the dendrite and interdendritic regions was reduced after PWHT, and the SFE was calculated. At cryogenic temperatures, the SFE decreased below 20 mJ/m2, indicating the martensitic transformation region. Furthermore, an examination of the deformation behavior of welded high-Mn steels was conducted. This study revealed that the tensile deformed, as-welded specimens exhibited ε and α'-martensite transformations at cryogenic temperatures. However, the heat-treated specimens did not undergo α'-martensite transformations. Moreover, regardless of whether the specimens were subjected to Charpy impact deformation before or after heat treatment, ε and α'-martensite transformations did not occur.

Fine grains, dispersed phases and strong crystal defect interactions inducing excellent strength-ductility synergy in powder sintered Cu–18Sn-0.3Ti alloy via hot extrusion and solution treatment

In order to prepare the Cu–18Sn-0.3Ti alloy with excellent mechanical properties to realize the favorable collaborative deformation of Cu–18Sn-0.3Ti alloy and Nb in the preparation process of Nb3Sn superconducting wires by bronze method, the hot extrusion process of the Cu–18Sn-0.3Ti alloy prepared by Direct Current assisted Hot Pressed sintering based on micron atomized spherical alloy powders was systematically investigated by the combination of experiments and finite element simulation in this work. The ultimate tensile strength, yield strength and elongation can reach 683.5 MPa, 414.3 MPa and 33.3 %, respectively, through the optimization of hot extrusion parameters and subsequent solution treatment. The results show that hot extrusion with high strain rate induced the formation of fine α grains and dispersed δ phases. Especially, strong dislocations-annealing twins (ATs) interaction occurred, which was manifested as the pile-up of dislocations at twin boundaries of ATs, slippage of dislocations in multiple directions inside ATs and storage of dislocations inside ATs. Meanwhile, the high strain rate induced the formation of deformation twins inside partial grains. The grain refinement of α matrix, formation of dispersed δ phases and strong interactions of crystal defects were the main reasons for the strength-ductility enhancement of hot extruded samples, and laid an excellent microstructure foundation for further improving the mechanical properties of alloy by subsequent solution treatment. The results not only describe a feasible method for simultaneously enhancing the strength and ductility of Cu–18Sn-0.3Ti alloy, but also provide a theoretical guidance for the strength-ductility enhancement of multicomponent alloys.

Microstructure evolution and work hardening behavior of medium/high carbon Fe–Mn–C TWIP steel during tensile deformation at 298 K and 223 K

In this paper, a comparative investigation was conducted on the tensile properties, microstructure evolution, and work hardening behavior of Fe18Mn0.6C and Fe18Mn1.0C TWIP steels at 298 K and 223 K by tensile tests and multi-scale microscopic characterization. With decreasing deformation temperature, two steels exhibit increased yield strength and ultimate tensile strength. However, the total elongation decreases for both, with Fe18Mn0.6C steel showing a more pronounced decrease. The work hardening behavior of two steels at 298 K and 223 K was analyzed. It is observed that the work hardening rate of both steels is higher at 223 K compared to 298 K, attributed to enhanced dislocation accumulation and the formation of deformation twins or martensite at lower temperatures. Particularly, Fe18Mn1.0C steel exhibits a continuous upward stage in work hardening at any temperature, driven by its higher dislocation density. On the other hand, Fe18Mn0.6C steel at 233 K shows a relatively stable trend in work hardening rate, attributed to the presence of martensite generated during deformation, which enhances its work hardening ability. Furthermore, the contribution of different strengthening mechanisms to the total flow stress was evaluated. The calculation results highlight dislocation strengthening as the primary source of flow stress in two steels at different temperatures. These findings provide theoretical support for understanding the mechanical properties of Fe–Mn–C TWIP steel at low temperature and optimizing the material design.

Hydrogen-enhanced densified deformation twins in 304L austenitic stainless steel fabricated by selective laser melting

Twin boundaries are the preferred site for crack nucleation and propagation in a hydrogen-rich environment. The relationship among hydrogen, cellular structure, and deformation twins is significant in further enhancing the resistance to hydrogen embrittlement of austenitic stainless steels produced by SLM. It has been demonstrated that cellular structures with high dislocation densities play a positive role in increasing stress above a critical level for deformation twin formation. The cellular structure affects the distribution of hydrogen, with hydrogen presence at cellular boundaries reducing its accumulation at grain boundaries. Hydrogen increases the density of deformation twins and reduces the thickness of individual deformation twins. This phenomenon can be attributed to an increase in nucleation sites for the deformation twin, which is closely related to the decrease in stacking fault energy and the increase in dislocation density caused by hydrogen. Both cellular structure and deformation twins play a positive role in improving the hydrogen embrittlement resistance of austenitic stainless steels.

Increase in interplanar distances and formation of amorphous shear bands in deformed diamond

Amorphous transformation-deformation bands have been observed in diamond under cyclic loading (at planetary ball milling) near the graphite-diamond equilibrium line at temperatures below the Debye temperature. The maximum stresses and temperature in the diamond do not exceed 6 GPa and 420 K, respectively. These bands add to the set of plastic deformation mechanisms in diamond. TEM methods and conventional X-rays were used to investigate the mechanisms of plastic deformation of diamond. The observed mechanisms include amorphous deformation bands, bond breaking, twin formation, and phase transitions of diamond-lonsdaleite and diamond-onions. The increased interplanar distances provide evidence of the plastic deformation mechanism due to bond breaking. The distances of 0.222 and 0.255 nm are significantly larger than the initial value of 0.206 nm, which is characteristic of d111 diamond. This increase in distance cannot be attributed to lattice expansion caused by point defects, but rather to the breakage of interatomic bonds.

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.

A bimodal microstructure to improve the strength-ductility properties of Ti-Cu alloys by using eutectoid reactive laser depositing

A novel bimodal structure was achieved in Ti-6wt%Cu by laser metal deposition. The microstructures and tensile behaviors of this heterostructure, comparing with typical colony structure, were examined to understand the mechanisms of strengthening and deformation. The bimodal structure consisting of nano-scale eutectoid lamellae and fine Widmanstätten-α exhibits the strength-ductility combination and excellent strain hardening ability. The shear deformation mechanism in ultrafine lamellae and hetero-deformation induced hardening in bimodal structure contribute to the high strain hardening. In contrast, the colony structure with coarse laths and Ti2Cu precipitates show inferior dislocation strengthening leading to a lower tensile strength, while the formation of deformation twins ensures a higher ductility.

On twinning and shear banding in copper single crystals with rolling texture orientations plane strain compressed at high strain rate

Crystal lattice rotations developed in copper single crystals with (110)[1–1–2](Br), (110)[001](G), (112)[11–1](C), and S (346)[63–5](S) orientations have been examined to assess the role of an impact loading on twinning, deformation banding and shear banding. The single-crystalline specimens were channel-die compressed to logarithmic strains of 0.9 with a strain rate of 4.7 × 105 s−1. The microstructures were characterised using scanning (SEM) and transmission (TEM) electron microscope, whereas electron backscattered diffraction (EBSD) in SEM and X-ray diffraction have been applied to evaluate the texture evolution. The results showed that a very high strain rate promoted the formation of deformation twins in all orientations. However, a strong variation in the twin density and number of twin families was observed depending on the orientation of the single crystal. In the Br- and G-oriented crystals, only single needle-shaped twins were observed, whereas in the C- and S-oriented crystals, the formation of compact clusters of deformation twins of two generations on different twinning planes was identified. SEM/EBSD measurements revealed that deformation bands were formed in the crystallites with G and Br orientations, whereas shear bands (SBs) were observed in the C- and S-oriented crystals. This study demonstrates that the rotation of twin and matrix platelets in narrow areas of SBs developed in C- and S-oriented crystallites, combined with deformation twinning in the re-oriented primary matrix platelets can result in the formation of texture components near the {110}<001> orientation. Finally, a crystallographic model of the formation of SBs in twinned structures of face-centred cubic metals deformed at high strain rates is proposed.

Effect of uniaxial compression on the microstructural evolution and magnetic properties of 20Mn23AlV non-magnetic structural steel

In this study, to evaluate the stability of the micro phase in the non-magnetic steel, the 20Mn23AlV high-manganese non-magnetic structural steel was compressed at room temperature. The research focused on analyzing the mechanical and magnetic properties of the compressed samples under conditions of substantial deformation. The results show that the Vickers hardness of the high-manganese non-magnetic structural steel exhibits a significant increase during deformation, primarily attributed to the formation of numerous deformation twins. X-ray diffraction (XRD) analysis confirms the exclusive presence of austenite phases, with no other phases detected. Additionally, when assessing the volume fractions of α′-martensite, calculations from hysteresis loops reveal values of approximately ∼0.03 % and ∼0.09 % for deformations at 56 % and 70 %, respectively. Remarkably, even with such minimal amounts of ferromagnetic α′-martensite generated, the steel still adheres to the criteria for non-magnetic steel usage. To further elucidate the formation of deformation twins and potential nucleation sites for α′-martensite, we conducted observations of microstructures under varying deformation conditions using transmission electron microscopy (TEM).

Effect of heating-assisted ultrasonic surface rolling on the microstructure evolution and wear behavior of martensitic steel

In this study, a plastic deformation layer and a nano-gradient structure were prepared on the surface of high-strength martensitic steel using the Heating-Assisted Ultrasonic Surface Rolling (HUSR) technique. The microstructure, texture evolution, and wear performance of the plastic deformation layer were characterized and analyzed using techniques such as Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM). The influence of HUSR treatment on the friction and wear behavior of martensitic steel was discussed. The results indicate significant changes in the texture components, dislocation structures, and grain distribution in the plastic deformation layer prepared by HUSR. The predominant textures in the plastic deformation layer are Rotated Cube {001}<110>, Cube {001}<100>, F {111}<112>, and E {111}<110>. HUSR treatment induces the formation of abundant deformation twins and stacking faults (SF) on the surface of sample. This ultimately results in a 28.3 % increase in the surface hardness of the HUSR sample and the introduction of residual compressive stress of −746.2 MPa. The friction coefficient and wear volume of the HUSR sample are significantly reduced. After HUSR treatment, the martensitic steel exhibits excellent wear resistance.

Enhancing strength and ductility in high Nb-containing TiAl alloy additively manufactured via directed energy deposition

Intermetallic TiAl alloys have found applications in the aerospace and automotive fields. Nevertheless, to satisfy the increasingly complex environmental requirements, these lightweight alloys must be improved particularly in strength and ductility. Additive manufacturing (AM) exhibits a rapid solidification rate and may strengthen intermetallic TiAl alloys. In this study, an additively manufactured (AMed) Ti-48Al-8 Nb alloy with fine formability was successfully fabricated by optimizing the power used during directed energy deposition. The preferential crystal plane for the obtained alloy is {111}γ due to its lowest surface energy, and the presence of the thermal gradient during the AM process results in the alloy growing along the building direction. The TiAl alloy AMed at 500 W delivers an excellent tensile strength of 880 MPa that is 1.71 times higher than that of its as-cast counterpart, as well as elongation of 0.7% at room temperature. The superior properties of this alloy are due to the formation of numerous deformation twins during tensile deformation at room temperature that enables twin intersections to produce high-density nano-twins while contributing to stress release. Moreover, the AMed TiAl alloy exhibits outstanding high-temperature strength retention performance.

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.

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.

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.

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.

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.

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.

Failure-mode dependence on the formation of deformation twinning in the fourth-generation single crystal Ni-based superalloy at high temperatures

In this work, the twinning-related deformation mechanisms under different failure modes of a fourth-generation single crystal (SX) superalloy at high temperatures were investigated. Notably, the twinnability of alloy increased in the sequence as follows: pure tension, in-phase (IP) thermal mechanical fatigue (TMF), isothermal low cycle fatigue (LCF), hot compression, out-of-phase (OP) TMF. High-temperature compressive loading which facilitated the successive operation of {111}〈1 1 2〉 viscous slipping, was indispensable for the nucleation of micro-twins. However, this stimulative effect could be markedly offset by the co-existence of high-temperature tensile loading. Moreover, the activation of different {111}〈1 1 2〉 slipping systems as well as potential interactions between stacking faults played a significant role in the formation and thickening processes of deformation twins. In the experimental alloy system, the presence of deformation twins was attested to impair the fatigue resistance of the specimen. These findings provided new insights for ensuring the safe service of the fourth-generation SX superalloys.

Enhancing tensile properties of laser additive manufactured FeCoNiCrMn high entropy alloy via adding Fe-based metallic glass

It has always been difficult to significantly improve the tensile strength of FeCoNiCrMn high entropy alloy (HEA) by traditional technical methods. Here, high-strength Fe-based metallic glass (MG) was precisely selected as the strengthening phase to reinforce the mechanical properties of FeCoNiCrMn HEA fabricated by laser additive manufacturing (LAM) technology. The results showed that although the addition of Fe-based MG with 5 wt% did not change the original phase composition of HEA, it facilitated grain refinement to a great extent. Of special significance was that the strength and ductility of HEA-MG exhibited a synergistic enhancement (σy, σUTS and εf were increased by 23.8 %, 28.4 % and 31.0 %, respectively) after Fe-based MG addition, deriving from the grain refinement, increased the dislocation density and formation of deformation twins.