Dislocation-twin Interaction Research Articles

Molecular Dynamics Study on the Mechanical Behaviors of Nanotwinned Titanium

Titanium and titanium alloys have been widely applied in the manufacture of aircraft engines and aircraft skins, the mechanical properties of which have a crucial influence on the safety and lifespan of aircrafts. Based on nanotwinned titanium models with different twin boundary spacings, the impacts of different loadings and twin boundary spacings on the plastic deformation of titanium were studied in this paper. It was found that due to the different contained twin boundaries, the different types of nanotwinned titanium possessed different dislocation nucleation abilities on the twin boundaries, different types of dislocation–twin interactions occurred, and significant differences were observed in the mechanical properties and plastic deformation mechanisms. For the {101-2} twin, basal plane dislocations were likely to nucleate on the twin boundary. The plastic deformation mechanism of the material under tensile loading was dominated by partial dislocation slip on the basal plane and face-centered cubic phase transitions, and the yield strength of the titanium increased with decreasing twin boundary spacing. However, under compression loading, the plastic deformation mechanism of the material was dominated by a combination of partial dislocation slip on the basal plane and twin boundary migration. For the {101-1} twin under tensile loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane and crack nucleation and propagation, while under compression loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane and twin boundary migration. For the {1124} twin, the interaction of its twin boundary and dislocation could produce secondary twins. Under tensile loading, the plastic deformation mechanism of the material was dominated by dislocation–twin and twin–twin interactions, while under compression loading, the plastic deformation mechanism of the material was dominated by partial dislocation slip on the basal plane, and the product of the dislocation–twin interactions was basal dislocation. All these results are of guiding value for the optimal design of microstructures in titanium, which should be helpful for achieving strong and tough metallic materials for aircraft manufacturing.

The abnormal deformation behaviors in commercial pure Ti during high speed compression

The effect of high speed compression on the deformation behaviors of pure titanium (Ti) was investigated in this paper. The results indicated that deformation twinning dominated the entire deformation process. In addition to the commonly observed twinning modes 101̅2<101̅1>,112̅1<112̅6>and 112̅2<112̅3>twinning, the uncommon twinning modes 101̅1<101̅2>, 112̅4<224̅3> twinning, and a rare twinning mode 112̅3<112̅2> twinning which has not been reported in previous literature, were observed. The 112̅3<112̅2> twinning presented an orientation relationship of 87° <101̅0>. Moreover, deviations from the ideal twinning relationship were observed in some twins which might due to the deformation induced dislocation-twin interactions. Alongside twinning, high angle kink bands with a misorientation of 61°<71017̅3> were locally activated to accommodate the deformation. These abnormal deformation behaviors, including the formation of unusual twins and high angle kink bands, occurred in grains that were less favorable for triggering common dislocation and twinning modes, based on the Schmid factor calculations. The high strain rate during high speed compression limited heat dissipation time and generated additional energy, thereby enhancing the flow stress and raising the temperature, which facilitated the occurrence of these abnormal deformation behaviors.

Microstructure-Based Modeling of Deformation and Damage Behavior of Extruded and Additively Manufactured 316L Stainless Steels.

In this study, we investigated the micromechanical deformation and damage behavior of commercially extruded and additively manufactured 316L stainless steels (AMed SS316L) by combining experimental examinations and crystal plasticity modeling. The AMed alloy was fabricated using the laser powder bed fusion (LPBF) technique with an orthogonal scanning strategy to control the directionality of the as-fabricated material. Optical microscopy and electron backscatter diffraction measurements revealed distinct grain morphologies and crystallographic textures in the two alloys. Uniaxial tensile test results suggested that the LPBFed alloy exhibited an increased yield strength, reduced elongation, and comparable ultimate tensile strength in comparison to those of the extruded alloy. A microstructure-based crystal plasticity model was developed to simulate the micromechanical deformation behavior of the alloys using representative volume elements based on realistic microstructures. A ductile fracture criterion based on the microscopically dissipated plastic energy on a slip system was adopted to predict the microscopic damage accumulation of the alloys during plastic deformation. The developed model could accurately predict the stress-strain behavior and evolution of the crystallographic textures in both the alloys. We reveal that the increased yield strength in the LPBFed alloy, compared to that in the extruded alloy, is attributed to the higher as-manufactured dislocation density and the cellular subgrain structure, resulting in a reduced elongation. The presence of annealing twins and favorable texture in the extruded alloy contributed to its excellent elongation, along with a higher hardening rate owing to twin-dislocation interactions during plastic deformation. Moreover, the grain morphology and defect state (e.g., dislocations and twins) in the initial state can significantly affect strain localization and damage accumulation in alloys.

Anomalous ductility-strength synergy occurred after cold rolling in a high Gd content Mg alloy

During cold rolling in magnesium (Mg) alloys, there will always be serious ductility deterioration, which greatly restricts the application of this processing method. In this work, ductility-strength synergy occurred after cold rolling was carried out on a 〈0001〉 fiber textured Mg-17.5Gd-Zr alloy (wt.%). The ductility-strength synergy can be mainly attributed to texture randomization, microstructure refinement, and easier basal slip transfer condition in the twinned grains. Firstly, the activation of multiple twinning behaviors, i.e., {10−12}, {10−11}, and {11−21} twinning, and related twin-twin interaction restrict the formation of strong basal texture and facilitate the texture randomization. Secondly, profuse high-angle grain boundaries form through dislocation-twin interaction, twin-twin interaction, and dislocation interaction, which promote microstructure refinement. Thirdly, the basal slip transfer becomes easier in the twinned grains owing to the activation of multiple twinning behaviors and their impingements. Activation of {11−21} twinning and 〈c + a〉 dislocations and the 〈0001〉 fiber texture which is favorable for multiple twinning activating during cold rolling are considered to be the key roles for microstructure and texture optimization.

Effect of Cr and Ni on mechanical response and microstructural evolution of nanocrystalline ferrite: A molecular dynamics study

Microalloying plays a critical role in improving the mechanical properties of steel. To offer a better theoretical guide for experimental research at the atomic level, this paper investigated the synergistic mechanism of adding trace amounts of alloy Cr and Ni and the microstructure evolution of nanocrystalline ferrite during the mechanical response process. First-principles calculations were implemented to investigate electronic properties. Hybrid molecular dynamics and Monte Carlo simulations were employed to explore the deformation mechanism under uniaxial tension and scratching. Specifically, comprehensive differences between doped and pure nanocrystalline ferrites were explored regarding local stress-strain state, dislocation evolution, twin expansion, and grain boundary activity. The results show that Cr- and Ni-doped nanocrystalline ferrite has higher strength and better wear resistance. The potential mechanism is that the addition of Cr and Ni enhances the atomic bonding strength with Fe atoms, hinders the movement of dislocations caused by lattice distortion, and suppresses grain boundary slip and migration, thereby improving the resistance to plastic deformation and grain boundary stability. Theoretical calculations based on microstructure indicate that compared to solid solution strengthening, Ni-induced grain boundary strengthening plays a dominant role in improving yield strength. Under large deformation, the trend of mechanical response is reversed. The suppression of dislocation motion by Cr reduces the dislocation density and dislocation entanglement, resulting in flow stress and local scratch force being smaller than that of pure samples. However, the formation of more nanoscale twins and twin-dislocation interactions enhances strain-hardening ability during tensile. Finer nanostructured subgrains are formed under scratching. These results provide valuable insights into the understanding of the strengthening mechanism and plastic deformation mechanism of Cr-Ni system low alloy steel under dynamic loading.

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.

Strong compressive γ-TiAl alloy produced by dual-wire-fed electron beam directed energy deposition: Microstructural evolution and strengthening mechanisms

Dual-wire-fed electron beam directed energy deposition (EB-DED) of TiAl alloy is of increasing interest due to its potential to produce high performance components at low cost. However, the strengthening mechanisms involved, in particular the complex relationship between microstructure and mechanical properties, are still unclear. This study uses a combination of experimental fabrication and numerical simulation. The aim is to investigate the microstructural evolution and strengthening mechanisms of a fully columnar γ-TiAl alloy produced by the dual-wire-fed EB-DED technique. The results show that cyclic heating significantly increases the dislocation density in the TiAl alloy. In addition, dislocation networks, twinning intersections and dislocation-twin interactions were observed. Compression test results show that the dual-wire-fed EB-DED prepared γ-TiAl alloy exhibits 26% higher yield strength and fracture toughness, as well as 25% higher work-hardening stress compared to the traditionally processed TiAl alloy of the same composition. The nanoscale twinning and interaction structures are identified as the unique strengthening mechanisms, contributing 194 MPa to yield strength and 1196 MPa to work hardening. This study provides valuable insights into the microstructural characteristics and strengthening mechanisms of TiAl alloy produced using the dual-wire-fed EB-DED.

Strengthening mechanism based on dislocation-twin interaction under room temperature multi-directional forging of AZ80 Mg alloy

Twin and dislocation have critical influence on the mechanical properties of the widely used AZ80 magnesium alloy. In the present work, we designed room temperature multi-direction forging (MDF) at different strain amplitudes (3% and 6%) with a same strain, introduced twin and dislocation structures and improved the mechanical properties of the alloy. The results showed that the material reached the limit of deformation with a cumulative strain of 0.72. With the increase of strain, the grain size decreased and the dislocation density increased gradually. At low strain amplitude, the transformation of dislocations at twin boundaries (TBs) caused twinning expansion and detwinning. While at high strain amplitude, interactive twin structure hindered the over-expansion of twins. In addition, high-density dislocations accumulate at the twin boundaries, which stimulates the nucleation of twin variants and further refines the grains. In the process of plastic deformation, dislocation strengthening contributed the main yield strength, and fine twins provided a certain level of hardening ability. Through the interaction between the dislocations and twins, the strength and plasticity of the material were synergistically improved, and excellent mechanical properties with ultimate tensile strength of 486 MPa, yield strength of 385 MPa, and elongation of 8.1% were finally obtained.

Microstructure and compressive deformation behavior of W-Ni3Al-Co model alloys by two-step sintering

The regulation of microstructure can improve the mechanical properties of tungsten alloys. To regulate the microstructure, the W-Ni3Al-Co alloys with different Ni3Al/Co ratios were prepared using a two-step sintering method, and the microstructure and compressive deformation behavior were investigated. The results showed that the particle sizes of the W-rich phase in the network structure were refined with the increase of the Ni3Al/Co ratio, and a large fraction of twin boundaries were detected in the adhesive phase. The compressive deformation behavior of a W-7Ni3Al-3Co alloy with excellent compressive properties was investigated by the EBSD technique. The results demonstrated that twins in the FCC binder phase hindered the stress concentration in the binder phase and at the interface of the W/binder phases through twin-dislocation interactions. The W network structure underwent the uniform global deformation that restricted grain boundary sliding during deformation and prevented crack extension and consolidation. Twins in the binder phase and the W network structure were both important factors in improving the compressive properties of W-Ni3Al-Co alloys.

Dynamic deformation behaviors and mechanisms of CoCrFeNi high-entropy alloys

Due to their unique microstructures and chemical compositions, high-entropy alloys (HEAs) exhibit remarkable mechanical properties, such as high strength, excellent ductility and good thermal stability. However, to date, there have been limited studies on the dynamic behaviors of HEAs. Here, we systemically investigated the mechanical behaviors and deformation mechanisms of CoCrFeNi HEAs and their connections under dynamic loading by combining mechanical tests and molecular dynamics simulations. Compressive testing on polycrystalline CoCrFeNi HEA samples at different strain rates showed that both yield and flow stresses increase with strain rate and exhibit a significant strain rate sensitivity due to solid solution strengthening and lattice distortion of the HEA. The strain rate sensitivity of the CoCrFeNi HEA transitions from 0.010 at low strain rates (5.0×10−5–2.5×103 s−1) to 0.333 at high strain rates (2.5×103–6.5×103 s−1). Large-scale molecular dynamics simulations further revealed that this transition is related to the plastic deformation mechanism transition from dislocation nucleation and slip at low strain rates to massive dislocation nucleation and drag at high strain rates. Furthermore, the CoCrFeNi HEA exhibited a remarkable strain-hardening capability at high strain rates, which originated from the formation of primary and secondary nanoscale twins and twin-twin and twin-dislocation interactions. Our current study sheds light on the plastic deformation mechanisms of HEAs under dynamic loading, providing guidance for the design and fabrication of HEAs with excellent dynamic mechanical properties.

Texture tailoring and microstructure refinement induced by {11−21} and {10−12} twinning in an extruded Mg-Gd alloy

An extruded Mg-17Gd (wt%) alloy with < 0001 > fiber texture was compressed along different directions at room temperature. Electron backscatter diffraction characterization was used to investigate the texture tailoring, twinning and dynamic recrystallization (DRX) behaviors. The results indicate that {10−12} and {11−21} twinning activated. The [0001] pole tilted to compression direction with the strain increasing and twinning dominated the texture transition. Activation of {10−12} twinning can be governed by Schmid law, while it is more appropriate to explain the activation of {11−21} twinning by the basal slip transfer condition in the twinned grains according to the slip trace analysis. In addition, operation of {11−21} twinning can facilitate the nucleation of {10−12} twin in the same grain by changing Schmid factor (SF) in the vicinities of {11−21} twin. Grain refinement occurred in the specimens with large strain and the average grain size decrease from ∼27 µm to ∼12 µm. The mechanism of grain refinement can be summarized as follows: the mobility of twin boundaries will be restricted because of the twin-twin and dislocation-twin interaction and converted to random high angle grain boundaries (HAGBs). The random HAGBs offer the nucleation sites and the strain accumulation within twins offer the driving force which facilitate the formation of DRXed grains at random HAGBs. Additionally, the SF difference within parent grain may result in the formation of HAGBs, and these HAGBs can connected the random HAGBs that result from the dislocation-twin interaction which result in the formation of new grains.

Dislocation-twin interaction in medium entropy alloy containing a high density of oxygen interstitials

Dislocation-twin interactions play important roles in tuning the plastic deformation of materials. Here, we report the great influence of interstitial oxygen atoms on dislocation-twin interactions in NiCoCrO medium entropy alloy through in-situ transmission electron microscopy mechanical testing, atomic resolution energy-dispersive X-ray spectroscopy characterizations, and integrated differential phase-contrast scanning transmission electron microscopy imaging. The spreading lattice distortion in medium-entropy alloy was found to enable large oxygen interstitial solubility. Oxygen atoms occupied both the tetrahedral and octahedral interstitial sites, and tended to be enriched at defects such as twin boundaries and stacking faults. Oxygen-enriched regions near the twin boundary hindered dislocations from direct transmitting or cross-slip into another twin plane during plastic deformation, resulting in significant dislocation entanglement, partial dissociation of dislocations, and the deviation of slip directions. Our results give insight into the strengthening mechanisms associated with the synergetic effects of twin boundary and interstitial atoms.

In situ observation of distance dependence of the plasticity behavior of the crack tip in nanosized AuAg alloys

Understanding the deformation behavior of crack tips in metals is of great significance for improving fracture toughness. However, how crack tips in nanosized metallic alloys behave under loading is unclear, because most previous studies focused on pure metals. In this study, the atomic-scale deformation behavior of the crack tip in AuAg alloy nanocrystals was observed in situ. We revealed that the deformation mechanism near the crack tip depended on the distance from the tip. For the ‘near region’ close to the crack tip, plastic deformation was governed by partial dislocations, twinning, and their interactions. For the ‘far region’, further than ~15 nm from the crack tip, full dislocations dominated, and their interactions resulted in Lomer-dislocation (LD) lock formation and destruction. We uncovered that the combination of blunting dislocation-twin interactions, twin-twin intersections, and formation and destruction of LD locks, as a previously unrecognized fracture toughness improvement mechanism in metals. • The atomic-scale deformation behavior of crack tip in AuAg alloy nanocrystals was studied in situ. • The dislocation-twin interactions, twin-twin intersections, and formation and destruction of LD locks were observed. • The deformation mechanism near the crack tip depended on the distance from the tip.

Effect of cold rolling and subsequent heat treatment on microstructural evolution and mechanical properties of Fe-Mn-Al-C-(Ni) based austenitic low-density steels

Thermo-mechanical processing in a two-phase domain of low-density steel (LDS) has the potential of enhancing strength-ductility combination. In the present study, the effects of alloy composition, cold rolling and heat treatment temperature (973–1173 K) on concurrent precipitation of κ-carbide/B2 particles and recrystallization and subsequently, mechanical properties include hardness variation and tensile response are evaluated. In order to understand the synergistic effect of Mn/Ni ratio towards selecting the two-phase processing window, three LDS alloy compositions, namely Fe–28Mn–9Al-0.9C, Fe–28Mn–9Al-0.9C–5Ni and Fe–15Mn–9Al-0.9C–5Ni (in weight percent) are vacuum induction melted, hot rolled and then cold rolled to 50% reduction in thickness at room temperature. The LDS alloys containing Ni showed much delayed recrystallization as compared to Ni-free LDS. The Mn/Ni ratio played a vital role towards controlling precipitation and recrystallization and accordingly, the mechanical properties. All the three alloys exhibited similar hardness, strength and ductility levels after 50% cold reduction. Upon heat treatment, there was precipitation of intergranular and intragranular B2 phase particles along with κ-carbides, whose fraction decreased with increasing the temperature. The tensile response of LDS alloys was influenced primarily by the volume fraction of these precipitates as well as the recrystallized austenite grains. Planar slip activity, dynamic slip band refinement in conjunction with dislocation-twin interaction mechanisms were noticed in fully recrystallized austenite and thereby leading to enhanced strain hardening and ductility. Furthermore, lowering Mn in Ni containing LDS reduced the processing temperature window of the alloy in two-phase region.

Temperature-dependent dynamic compressive properties and failure mechanisms of the additively manufactured CoCrFeMnNi high entropy alloy

CoCrFeMnNi high entropy alloy (HEA) parts were fabricated by laser powder bed fusion (LPBF), and their dynamic compressive properties at different temperatures as well as the resulting microstructures were analyzed. The HEAs showed an unprecedented strength-ductility combination, especially at a cryogenic temperature of 77 K and a high strain rate of 3000 s−1. Under this testing condition, the yield strength (YS) of the HEAs amounted to 665 MPa. Regardless of the testing temperature, the deformation mechanism of all investigated HEAs was dominated by a synergistic effect consisting of deformation twinning and dislocation pile-up around twins. The fraction of twin boundaries and dislocation density within the deformed microstructure of the HEA correlated with the test temperature. At 77 K, the formation of nanotwins together with dislocation slip prevailed and contributed to pronounced twin-twin and twin-dislocation interactions which effectively restricted the dislocation movement and, hence, contributed to a higher YS as well as strain hardening rate in comparison to that of the HEAs at room temperature of 298 K. The LPBF-fabricated HEAs showed unpronounced thermal softening even at a high testing temperature of 1073 K. Continuous dynamic recrystallization was restricted in the HEA because of its inherent sluggish dislocation kinetics and low stacking fault energy.

Observing Dislocations Transported by Twin Boundaries in Al Thin Film: Unusual Pathways for Dislocation-Twin Boundary Interactions.

Twins are generally regarded as obstacles to dislocations in face-centered cubic metals and can modify individual dislocations by locking them in twin boundaries or obliging them to dissociate. Through in situ tensile experiments on Al thin film in a transmission electron microscope, we report a dynamic process of dislocations being transported by twin lamella via periodic twinning and detwinning at the atomic scale. Following this process, a 60° dislocation first transforms into a sessile step of the twin boundary, then migrates under stress as a step and finally reverts back into a 60° dislocation. Our results reveal a novel evolution route of dislocations by a dislocation-twin interaction where the twins act as transport vehicles rather than as obstacles. The potential implications of this mechanism on toughening are also discussed.

Distribution of dislocation-twin interaction in TWIP steel under different loading modes

Twinning induced plasticity (TWIP) steel with nanotwinned structure enables it to have adjustable strength and elongation. However, the dislocation reactions at the twin boundary (TB) in nanotwinned materials have obvious tension-compression asymmetry and orientation effects, and the corresponding mechanism of dislocation-TB interactions is not yet fully understood. In this study, the tension-compression asymmetry and orientation effect of dislocation-TB interactions in twinned TWIP steel is investigated using the discrete dislocation dynamics simulations and Schmid's law. The different types of dislocations dominate the TB migration in the twinned TWIP steel during tension and compression with the same loading orientation, and the asymmetric dislocation reactions at TB are attributed to the different leading partials produced after the dissociation of dislocation under different loading modes (tension or compression). Moreover, the complex dislocation-TB interactions, such as dislocations traversing TB, cross slip, dissociation, pile up and so on, are determined by considering the orientation effect of dislocation activation and the asymmetry of dislocation dissociation. The research advances the understanding of asymmetric behavior of twin and its influence on the macro hardening in plastic deformation of TWIP steels.

Atomic-level study of {[formula omitted]} deformation twinning in pure Ti and Ti-5at.% Al alloy

In this work, pure Ti and Ti-5at.%Al alloy are both cold rolled, and then characterized to observe deformation twins {101¯1}. Many two-layer steps are observed at the {101¯1} twin boundary (TB) in pure Ti by high-resolution transmission electron microscopy (HRTEM). However, in addition to two-layer steps, a large number of four-layer steps are observed at the {101¯1} twin boundary in Ti-5at.%Al alloy. To understand the experimental observations, molecular dynamics (MD) simulations are performed to analyze the {101¯1} deformation twinning at the atomic scale. MD simulations are also showing similar results to the experimental observations in pure Ti and Ti-5at.%Al alloy. The two-layer and four-layer steps are twinning dislocations (TDs) with the Burgers vectors of 12×4γ2−93+4γ2<101¯2¯> ±16<12¯10> (b⇀2) and 4γ2−93+4γ2<101¯2¯> (b⇀4) by topological analysis, respectively. For pure Ti, the twin growth is carried out only by the slip of TDs b⇀2. For Ti-5at.% Al alloy, twinning is carried out by the gliding of TDs b⇀2 and TDs b⇀4. Due to the poor mobility of TDs b⇀4, the nucleation of the twinning dislocations b⇀4 (4-layer steps) requires some special nucleation sites, such as intersections between the stacking fault (SF) and TB (SF-TB intersection) in Ti-5at.% Al alloy. The SF-TB intersection is a sessile imperfect disconnection, and this defect is immobile, which leads to high stress at the SF-TB intersections to facilitate the nucleation of TD b⇀4. The Al element enhances the high density of basal stacking faults (BSFs) and SF-TB intersections at the twin-tip, and indirectly promotes the nucleation of TD b⇀4 in Ti-5at.% Al alloy. In addition, Al element also facilitates the nucleation of matrix dislocations, resulting in twin-dislocation interactions that promote the generation of TDs.

An EBSD based investigation on the deformation mechanisms under HCF-creep interaction in a Ni-based superalloy (alloy 617M)

The study presents an insight into the deformation mechanisms under high cycle fatigue-creep interaction (HCF-creep interaction) in a Ni-based superalloy (Alloy 617 M), using electron backscatter diffraction (EBSD) as an important tool. HCF-creep interaction tests are carried out at 700 °C by imposing periodic holds at mean stress (σm) during HCF cycling. Detailed EBSD investigation using kernel average misorientation (KAM) map indicates a non-uniform distribution of local misorientation especially at the grain boundaries, with increase in σm, which is attributed to the increased creep damage therein. Creep cavitation through grain boundary sliding is found to increase with an increase in the hold temperature to 750 °C owing to a higher dislocation mobility triggered by precipitate coarsening and dislocation climb. Change in twin boundary character from Σ3 to other higher angle boundaries caused by dislocation-twin interaction is noticed under HCF-creep interaction, leading to partial detwinning. A decrease in the fraction of Σ3 annealing twin boundaries through partial detwinning is observed, which is found to be more pronounced when the creep hold is maintained at 750 °C. This caused a loss of strength at the above temperature, through coalescence of secondary intergranular cracks, in turn leading to intergranular failure. In contrast, the hold imposed at 700 °C with similar loading parameters resulted in run out.

Superior strength-ductility synergy and strain hardenability of Al/Ta co-doped NiCoCr twinned medium entropy alloy for cryogenic applications

Metallic materials with outstanding cryogenic mechanical properties are highly demanded for cryogenic engineering applications. Here we developed a novel (NiCoCr)92Al6Ta2 medium-entropy alloy (MEA) towards superior cryogenic mechanical properties via elaborating the chemical composition designing and the thermo-mechanical processing. It appears that the twinned (NiCoCr)92Al6Ta2 MEA shows a strongly temperature-dependent mechanical behavior, i.e., when the testing temperature from 298 down to 77 K, the yield strength, ultimate strength and tensile ductility are increased from ∼600 to ∼800 MPa, from ∼1.0 to ∼1.35 GPa and from ∼52% to ∼90%, respectively. An excellent strength-ductility synergy and extraordinary strain hardening capacity were realized in this alloy, in particular the product of tensile strength and elongation is more superior to their reported counterparts. There is a strongly temperature-dependent deformation mechanism transition from the ordinary planar-slip at 298 K to the cooperative plastic mechanisms at 77 K, including stacking faults (SFs) and deformation twins. The excellent cryogenic mechanical properties of this designed MEA stems from the synergic effects of nanotwins, hierarchical SFs and Lomer-Cottrell locks, as well as their extensive interactions, rarely observed in their siblings deformed at room temperature. Furthermore, the deformation twinning acting as an additional/significant mechanism for plasticity and favoring strain hardening, along with the effect of temperature-dependent dislocation-twin interactions, collaboratively delays the onset of necking for enhanced ductility at 77 K.