Strain Accommodation Mechanisms Research Articles

Deformation anisotropy and lamellar α re-orientation of TC17 alloy under the different slip modes of HCP structure

Deformation anisotropy and re-orientation for α phase during isothermal compression of TC17 alloy with basketweave microstructure were investigated systematically by quantitative analysis the influence of tilt angle between compression axis and c-axis on LABs density, rotate factor and ratio of activated slip system. The test results revealed that the tilt angle between compression axis and c-axis affects the deformability of α grain by affecting the rotation tendency and the number of activated slip systems. It results in the different prone of lamellar α with different orientation to globularization. Therein, the variation of the rotation factor with tilt angle conforms to the quadratic function, reaching a maximum value around 45°. When the rotation factor is small, the ratio of (01–11) [1–210] slip system with small Burgers vector at the phase boundary is large, the rotation tendency is poor, and the intragranular dislocation slip replaces the grain rotation as the strain accommodation mechanism, the lamella is more prone to globularization. And when the tilt angles are between 0°–13°, 13°–35°, 35°–78° and 78°–90°, the most easily activated slip systems are pyramidal <a+c> slip, basal <a> slip, pyramidal <a> slip and prismatic <a> slip, respectively. The more slip systems may be activated, the lamella is more prone to globularization. In addition, isothermal compression destroys the initial preferred orientations of the α grains and causes the lamellar α to rotate towards the direction that forms (0 0 0 1) plane texture, which is formed due to the activation of the prismatic <a> slip. Then, the (0 0 0 1) plane texture will be weakened due to a large number of randomly oriented grains generated by recrystallization under larger height reduction.

Dislocation structures and residual stresses in duplex stainless steel fabricated by laser powder bed fusion with 430 and 316L powders

The duplex stainless steel is fabricated using laser powder bed fusion (LPBF), with raw materials prepared by mechanically mixing 430 and 316 L powders in a 1:1 mass ratio. The as-built alloys exhibit good overall mechanical properties, and theoretical strength calculations are performed based on the flow strength model. Uniquely, the distribution of dislocations, residual stresses, and strain accommodation mechanisms are investigated using electron backscattered diffraction and transmission electron microscopy. The results show that the microstructure of the as-built alloys is predominantly ferritic phase, with an austenitic phase content of approximately 4 %. The ultimate tensile strength is 938 ± 4 MPa and the elongation is 19 ± 0.58 %. The as-built alloys possess high dislocation densities, with the dislocation density in the ferritic phases being higher than in the austenitic phases. In particular, austenitic phases present barely any local misorientation and minimal rotations within grains, stacking faults are introduced to accommodate the solidification strains. However, ferritic phases exhibit a high dislocations density, with dislocations present on the (101), (011‾) and (211) planes, accommodating solidification strains and cooling stresses through elastoplastic deformation, and building up geometrically necessary dislocations.

In-situ observation of variant pair formation in bainite of low carbon steel by digital holographic microscopy

Digital holographic microscopy (DHM) was employed to investigate the formation process and shape evolution of the most frequently observed bainite variant pairs in low-carbon alloy steel. Through in-situ observation coupled with theoretical predictions, the continuous changes in surface relief corresponding to bainite and adjacent austenite were measured. Additionally, the crystal orientations of the observed variant pairs were determined using electron backscattered diffraction (EBSD). The results indicated that at the onset of the transformation, the bainite variant shape agreed with theoretical predictions, while the adjacent austenite experienced plastic deformation. However, as the transformation proceeded towards completion, the shape of some bainite variants deviated from the theoretical predictions, while their adjacent austenite remained undeformed. This shift of deformation strain between austenite and bainite suggests a change in strain accommodation mechanism. Furthermore, the observed ‘tent-shaped’ surface relief was attributed to back-to-back mutually accommodating variant pairs, resulting in reduction of the total shape strain, compared to the theoretical prediction.

Intergranular deformation mechanism stimulated by {10-12} extension twins in AZ81-La alloy

In polycrystalline Mg-RE magnesium alloys, the lattice parameter, dislocation dynamics and critical resolved shear stress (CRSS) for various deformation modes are altered with the addition of RE atom, finally affecting deformation mode and strain accommodation mechanism among neighboring grains. Uniaxial compression tests were performed on as-extruded AZ81-La Mg alloy samples with the c-axis of the majority of crystals vertical to the compression direction. Twin variants and dominant slip systems were examined by electron backscattered diffraction (EBSD) technique. It is found by in-grain misoriention axis (IGMA) analysis that the plastic deformation is mainly accommodated by a combination of the pyramidal <c+a> slip and the basal <a> slip. The dominant twin variant is {01¯12} <011¯1> and {11¯02} <1¯101>. Here, we applied a modified displacement gradient accommodation (m-DGA) method to evaluate the selection mechanism of twin/slip in neighbour grain stimulated by {101¯2} extension twin. It is found that the activated slip system and/or twin variant in the neighbouring grain is determined by the accommodation of the major shear strain induced by {101¯2} extension twin in such a way that strain concentration along grain boundaries can be relaxed.

Pressure-dependent large-scale seismic anisotropy induced by non-Newtonian mantle flow

SUMMARY Observations of large-scale seismic anisotropy can be used as a marker for past and current deformation in the Earth’s mantle. Nonetheless, global features such as the decrease of the strength of anisotropy between ∼150 and 410 km in the upper mantle and weaker anisotropy observations in the transition zone remain ill-understood. Here, we report a proof of concept method that can help understand anisotropy observations by integrating pressure-dependent microscopic flow properties in mantle minerals particularly olivine and wadsleyite into geodynamic simulations. The model is built against a plate-driven semi-analytical corner flow solution underneath the oceanic plate in a subduction setting spanning down to 660 km depth with a non-Newtonian n = 3 rheology. We then compute the crystallographic preferred orientation (CPO) of olivine aggregates in the upper mantle (UM), and wadsleyite aggregates in the upper transition zone (UTZ) using a viscoplastic self-consistent (VPSC) method, with the lower transition zone (LTZ, below 520 km) assumed isotropic. Finally, we apply a tomographic filter that accounts for finite-frequency seismic data using a fast-Fourier homogenization algorithm, with the aim of providing mantle models comparable with seismic tomography observations. Our results show that anisotropy observations in the UM can be well understood by introducing gradual shifts in strain accommodation mechanism with increasing depths induced by a pressure-dependent plasticity model in olivine, in contrast with simple A-type olivine fabric that fails to reproduce the decrease in anisotropy strength observed in the UM. Across the UTZ, recent mineral physics studies highlight the strong effect of water content on both wadsleyite plastic and elastic properties. Both dry and hydrous wadsleyite models predict reasonably low anisotropy in the UTZ, in agreement with observations, with a slightly better match for the dry wadsleyite models. Our calculations show that, despite the relatively primitive geodynamic setup, models of plate-driven corner flows can be sufficient in explaining first-order observations of mantle seismic anisotropy. This requires, however, incorporating the effect of pressure on mineralogy and mineral plasticity models.

Remarkable cryogenic properties by deformation twinning in a coherent precipitation-containing high entropy alloy

Developing structural alloys for critical cryogenic applications is a challenging yet essential task, primarily due to the ductile-brittle transition phenomenon. To address this issue, we developed a twinning-aided coherent precipitation-containing Co45Cr15Ni30Al5Ti5 high entropy alloy (HEA) with remarkable cryogenic properties. The activation threshold for twinning was successfully reduced via the disordering behavior of the L12 phase. In the present work, distinct temperature-dependent deformation behavior was observed. The dominant room-temperature strain accommodation mechanism was found to be the dislocation slip coupled with architectured stacking faults (SFs) and immobile Lomer-Cottrell (L-C) locks. Whereas, deformation twins were readily formed under cryogenic tensile loading, leading to significantly increased yield strength and ultimate tensile strength, approaching 1.5 GPa and 1.9 GPa, respectively. The remarkable cryogenic work hardening behavior contributes to persistent elongation of up to 31 % and the super-high ultimate tensile strength. The presence of SFs, deformation twins, and their interactions provide significant strain-hardening capability.

Deformation mechanisms in a novel multiscale hetero-structured Mg alloy with high strength-ductility synergy

Most of the high-strength Mg alloys exhibit poor ductility at room temperature. Architecturing appropriate heterostructure in metallic materials gives rise to the breakthrough of the inherent strength-ductility trade-off dilemma. Here, we prepared a novel multiscale hetero-structure in Mg-9Al-1 Zn (AZ91) alloy, consisting of bimodal-structured center-region and fine-grained surface-layer, providing high strength and high ductility, i.e. yield strength of ∼303 MPa, tensile strength of ∼400 MPa, and elongation of ∼14.5 %. The multiscale microstructural heterogeneities were induced via a simple rolling process that combines the advantages of both accumulative rolling bonding (ARB) and hard plate rolling (HPR). By utilizing quasi-in-situ electron backscattered diffraction (EBSD)-digital image correlation (DIC) characterizations combined with the determination of dislocation/disclination distributions, it has been found that diversified strain accommodation mechanisms are stimulated by the strain incompatibilities near hetero-interfaces in the multiscale hetero-structure. In particular, non-basal slips have been activated largely in both the bimodal-structured center-region and fine-grained surface-layer. Moreover, a unique slip trace distribution has been observed in the fine grains adjacent to the basal-oriented coarse grain. Such a strain accommodation can better coordinate local strain incompatibility among the hetero-interface. The uniaxial loading–unloading–reloading tensile loop tests demonstrate that the multiscale heterogeneities impart a pronounced hetero-deformation induced hardening during tensile deformation. Multiple deformation modes, including the non-Schmid and non-basal slips, have been activated substantially in the multiscale hetero-structured Mg alloy, effectively relaxing local stress concentration and achieving stable plastic flow at a high-stress state, which is beneficial to improving strain-hardening ability and strength-ductility synergy. Our work could shed light on designing and processing strong and ductile Mg alloys, by tailoring proper microstructural heterogeneity to activate cooperated accommodating deformation modes.

Microstructural mechanisms of hysteresis and transformation width in NiTi alloy from molecular dynamics simulations

Martensitic transformations in shape memory alloys are often accompanied by thermal hysteresis, and engineering this property is of prime scientific interest. The martensitic transformation can be characterized as thermoelastic, where the extent of the transformation is determined by a balance between thermodynamic driving force and stored elastic energy. Here we used molecular dynamics simulations of the NiTi alloy to explore hysteresis-inducing mechanisms and thermoelastic behavior by progressively increasing microstructural constraints from single crystals to bi-crystals to polycrystals. In defect-free single crystals, the austenite-martensite interface moves unimpeded with a high velocity. In bi-crystals, grain boundaries act as significant obstacles to the transformation and produce hysteresis by requiring additional nucleation events. In polycrystals, the transformation is further limited by the thermoelastic balance. The stored elastic energy can be converted to mechanisms of non-elastic strain accommodation, which also produce hysteresis. We further demonstrated that the thermoelastic behavior can be controlled by adjusting microstructural constraints.

Stable strain hardening behavior of Ti-4Al-5Mo-5V-5Cr-1Nb alloy during plastic deformation

Strain hardening is crucial to the formability and reliability of structural materials, however, such capacity will be exhausted as strain increases. In current work, a stable strain hardening character is found in the compression of Ti-4Al-5Mo-5V-5Cr-1Nb alloy up to 0.4 strain. Microstructural characterization on deformed samples reveals the development of multiscale α′′ lath frames. The stable strain hardening behavior is understood via unveiling the roles of α′′ lath during deformation. It is noted that these α′′ laths impede dislocations and reduce their mean free path as strain increases. Meanwhile, a high density of dislocations can be observed within these nanoscale α′′ laths, indicating they are also involved into deformation. Such a strain accommodation mechanism greatly relieves the stress concentrated at β/α′′ interfaces enabling the resistance to crack initialization.

Graphene enables equiatomic FeNiCrCoCu high-entropy alloy with improved TWIP and TRIP effects under shock compression

Graphene (Gr) reinforced high-entropy alloy (HEA) matrix composites are expected as potential candidates for next-generation structural applications in light of outstanding mechanical properties. A deep comprehension of the underlying deformation mechanisms under extreme shock loading is of paramount importance, however, remains lacking due to experimentally technical limitations in existence. In the present study, by means of nonequilibrium molecular dynamics simulations, dynamic deformation behaviors and corresponding mechanisms in equiatomic FeNiCrCoCu HEA/Gr composite systems were investigated in terms of various shock velocities. The resistance to dislocation propagation imparted by Gr was corroborated to encourage the elevated local stress level by increasing the likelihood of dislocation interplays, which facilitated the onset of twins and hexagonal close-packed (HCP) martensite laths. Meanwhile, the advent of Gr was demonstrated to endow the HEA with an additional twinning pathway that induced a structural conversion from HCP to parent face-centered cubic (FCC) inside HCP martensite laths, different from the classical one that necessitated undergoing the intermediate procedure of extrinsic stacking fault (ESF) evolution. More than that, by virtue of an increase in flow stress, the transformation-induced plasticity (TRIP) effect was validated to be additionally evoked as the predominant strain accommodation mechanism at higher strains on the one hand, but which only assisted plasticity in pure systems, and on the other hand, can also act as an auxiliary regulation mode together with the twinning-induced plasticity (TWIP) effect under intermediate strains, but with enhanced contributions relative to pure systems. One may expect that TRIP and TWIP effects promoted by introducing Gr would considerably inspire a synergistic effect between strength and ductility, contributing to the exceptional shock-resistant performance of FeNiCrCoCu HEAs under extreme regimes.

Evidence for austenite to non-modulated martensite transformation crystallography and variant organization in Ni-Mn-Ga-Co ferromagnetic shape memory alloys

Knowledge of transformation crystallography and variant organization of product phase was a prerequisite for tuning microstructure and enhancing functionalities in materials with displacive structure-transformation. However, the transformation orientation relationship (OR) between parent austenite and non-modulated (NM) martensite in traditional NiMnGa ferromagnetic shape memory alloy was not yet well-determined via direct experimental results, thus leading to ambiguity in understanding the self-accommodated configuration. In this work, by generating a microstructure with coexisting austenite and NM martensite through minor Co substitution and proper heat treatment, the transformation OR was unambiguously determined to be the N-W relation with {111}A//{101}NM and <2¯11>A//<101¯>NM on the basis of accurate EBSD orientation measurements. Analysis on deformation gradient matrix constructed from the N-W OR revealed that, the internal nano-twined structure allowed a maximum profit for eliminating the overall latticed deformation caused by transformation, overweighing the “sandwich” micro-variant pair structure. Such strain accommodation mechanisms consequently contributed to the resultant self-accommodated hierarchically twinned structure of martensite. Moreover, it was found that the prior austenite grain boundaries (PAGBs) provided dominated nucleation sites for NM martensite variants, where the preferential variant selection at PAGBs mainly depended on the inclination angle between the PAGB and matching close-packed direction (<2¯11>A//<101¯>NM) of variants. The present study provided comprehensive information on transformation crystallography and displacive characters of austenite to NM martensite transformation, which was useful for the efforts in property optimization and theoretical simulation.

Grain size effect on the mechanical properties of nanocrystalline magnesium aluminate spinel

To develop transparent materials with superior mechanical properties, nanocrystalline magnesium aluminate (MgAl2O4) spinel with grain sizes ranging from 3.7 to 80 nm has been synthesized by environmentally controlled pressure assisted sintering. In this study, we investigated the microstructure and grain size dependence of the mechanical properties of nanocrystalline MgAl2O4 by performing transmission electron microscopy, nanoindentation, uniaxial micropillar compression, and micro-cantilever bending. Electron microscopy confirmed that the environmentally controlled pressure assisted sintering technique produces a nearly fully dense grain structure with a porosity of less than 1% in larger grain-sized ceramics and observably pore-free grain structures in the smaller grain-sized ceramics. Mechanical characterization revealed that nanoindentation hardness, compressive fracture strength, and fracture toughness each exhibit distinct grain size dependence. Our experimental results and numerical analyses point to a change in dominant strain accommodating mechanisms from dislocation-based plasticity to shear banding as the grain size is reduced, as previously suggested by the literature. Practical implications of the change in strain accommodation mechanisms manifest as the emergence of indentation size effect, weak grain size dependence of hardness and strength, and a ∼2-fold increase in apparent fracture toughness for the smaller grain-sized ceramics.

Unusual martensitic strain accommodation mechanism in an electron beam processed metastable β-Ti alloy

This study reports an unusual martensitic strain accommodation mechanism, in an electron beam processed metastable β-Ti alloy, exhibiting a fine-grained microstructure with good yield strength and elastic modulus matching. Microstructural analysis reveals that ultra-fine laths of martensites were embedded within the fine-grained β structure. Detailed TEM and orientation relationship analyses further unravel that the observed microstructure consists of α’’ martensites associated with different habit plane and transformation twins, one with {7100}α’’//{755}β type habit plane and forming {111}α’’ type I twin while another with {471}α’’//{443}β type habit plane, forming a {351}<2¯11>α’’ type II twin. This structure can be interpreted as the product of rapid solidification and thermal stress relaxation which manipulated the formation of distinct martensites that originates from the fluctuated lattice instability, lattice distortion, and transformation strain. This finding provides an alternative route for utilizing the martensitic strain accommodation mechanism, benefiting additive manufacturing β-Ti alloy components with desired mechanical performance.

On the warm temperature strain accommodation mechanisms of Ti–6Al–4V alloy holding different starting microstructures

The strain accommodation mechanism of Ti–6Al–4V alloy with different initial microstructure; equiaxed α+β, lamellar α+β, dual phase α+ά, and fully ά martensite was studied in warm temperature deformation regime. Toward this end, a series of tensile tests were performed at room temperature, 400, 500 and 600 °C. The progressive substructure development during tensile deformation of the equiaxed α+β microstructure at 400 °C increased the strain-hardenability (n-exponent) of the material. However, activation of continuous dynamic recrystallization at higher temperature of 600 °C as main strain accommodation mechanisms increases the portion of non-uniform elongation. This was attributed to the resistance against localized deformation and the occurrence of diffused necking. In the case of initial lamellar microstructure, mechanical fragmentation and thermal disintegration of β phase, and globularization of α-phase via boundary splitting mechanism were dominant restoration mechanisms. Martensite reverse transformation and dynamic recrystallization also served as the primary source of softening and strain compensation in dual-phase α+ά initial microstructure. This was led to the development of bimodal/trimodal microstructure representing acceptable balance between strength and ductility. The microstructural evolution in fully ά martensite was relatively analogous to that of α+ά microstructure apart from activation of load transition mechanism. The correlation of strain accommodation capability of each microstructure and tensile formability was discussed in details.

The enhanced warm temperature ductility of Ti-6Al-4V alloy through strain induced martensite reversion and recrystallization

Two different initial microstructures of Ti-6Al-4V alloy, α+ά dual phase and fully ά-martensite, were deformed in tensile mode at warm temperatures of 300 °C and 600 °C. The simultaneous occurrence of martensite reverse transformation and dynamic recrystallization were identified as the main responsible to enhance the ductility values. A bimodal/trimodal microstructure arose during the thermomechanical processing of the dual phase α+ά at 600 °C which resulted in an acceptable balance between strength and ductility. The lower ductility of full ά martensite than dual phase α+ά microstructure was related to the lower extent of involved strain accommodation mechanisms and absence of load transition mechanism.

Precise measurement of strain accommodation in a Mg-Gd-Y-Zn alloy using cross-correlation-based high resolution EBSD

Mg-RE alloys show improved ductility compared to other classical magnesium alloys at room temperature. The present study focused on the strain accommodation mechanism in a wrought Mg-5.4Gd-1.8Y-1.5Zn alloy at the microstructural scale. For this purpose, a tensile specimen was uniaxially strained in-situ in tension up to 6%, and the lattice rotation tensor and geometrically necessary dislocation (GND) density were measured using cross-correlation-based high angular resolution electron backscatter diffraction (HR-EBSD) technique. The strain incompatibility within the Mg-Gd-Y-Zn alloy was accommodated by basal slip together with pyramidal dislocation. The GND measurements via HR-EBSD showed a high occurrence of pyramidal <c + a> dislocation even in well-aligned grains. The orientation dependence of basal <a> and pyramidal <c + a> dislocation was also investigated. It was found that the soft oriented grains had a higher basal <a> density, whereas the average GND density of <c + a> slip appeared to decrease with the Schmid factor of pyramidal dislocation. In addition, slip transfer process of the basal-basal and basal-pyramidal type was observed with high m' factors in this study.

Crack coalescence and strain accommodation in gypsum rock

Gypsum is a soft rock with low strength and high propensity to plastic deformation. Its mechanical properties are relevant in a wide range of technical application (e.g. tunnel excavation, stability assessment of underground quarries, oil and gas accumulation). The micro-mechanisms involved in the deformation of gypsum rock have for long time interested the scientific world and are still not completely understood. The peculiar crystallographic structure of this bi-hydrate salt, with water molecules layered in the salt structure, favours the development of plastic structures in the rock. The present work proposes an experimental investigation of strain accommodation mechanisms in gypsum rock, reporting data referred to uniaxial and triaxial stress conditions. The rock strain was studied with a multiscale approach, with the comparison of results from DIC and microstructural analyses.

Towards understanding twinning behavior near fracture surface in magnesium

Deformation twin is one of the most important strain accommodation mechanisms and ultimately influences the mechanical properties for magnesium and its alloys. Especially, {101¯1} twin is usually thought to be closely related to the fracture or fatigue process of magnesium alloys. In the present work, the characteristics of microstructure near fracture region of deformed magnesium alloy have been investigated by a combination of electron back-scatter diffraction (EBSD) and transmission electron microscope (TEM). It has found that a large of deformation twins occur near fraction region, including {101¯2} and {101¯1} primary twins, {101¯1}-{101¯2} double twin and {101¯1}-{101¯2}-{101¯1}-{101¯2} quadruple twin. The actual boundaries of {101¯1} twins at atomic scale consist of {101¯1} coherent twinning boundaries (TBs) and parallel basal-pyramidal (BPy/PyB) planes. The tip of {101¯1} twin can even end up with BPy/PyB interfaces only. The experimental observations also reveal that when two {101¯1} twin variants sharing a common [112¯0] zone axis approach each other, the growth of one twin is usually hindered by the boundaries of the other twin. In addition, an apparent “crossing” phenomenon is also discovered when interaction of two {101¯1} twins takes place. According to these experimental observations, the possible underlying mechanisms behind such phenomena are proposed and discussed. These finding are expected to provide an insight into understanding the twinning behavior and the relationship between twin and fracture in magnesium and other materials with hexagonal structure.

Anomalous twinning in AZ 31 magnesium alloy during electrically assisted forming

Abstract Electroplasticity has been explained by increased glide due to electron dislocation interactions and thermal effects. However, current pulse treatment of AZ 31 at 0.7 kA mm−1 within the elastic regime caused significant twinning, occurring in thin bands in the interior of grains. This is inconsistent with a purely dislocation based explanation of electroplasticity, which predicts a suppression of twinning as a strain accommodation mechanism. The presence of unexpected twinning offers insights on the relative contributions of dislocation mediated mechanisms and contributions from Joule heating to the overall effect.

Frictional Properties of Simulated Chlorite Gouge at Hydrothermal Conditions: Implications for Subduction Megathrusts

AbstractChlorite is abundant at hypocentral depths in subduction zones and is likely to play a key role in controlling megathrust slow slip and catastrophic rupture. However, no data exist on the frictional properties of chlorite(‐rich) fault rocks under the hydrothermal conditions relevant for the subduction seismogenic zone. We report results from experiments conducted under such conditions, using chlorite powders prepared from single crystal clinochlore (Mg‐chlorite), as well as limited experiments using a stack of single crystal sheets. Shear experiments were carried out at effective normal stresses (σn) of 100 to 400 MPa, pore fluid pressures (Pf) of 50 to 220 MPa, and at temperatures (T) of 22 to 600 °C, using stepped displacement rates (v) from 0.3 to 100 μm/s. The gouges are characterized by a coefficient of friction (μ) of 0.2–0.3 at T ≤ 400 °C and 0.3–0.4 at 500–600 °C, while (a‐b) values showed positive values for nearly all conditions tested, except at 300 °C. Microstructures of gouges sheared at T ≤ 300 °C show evidence for widespread comminution, compared with a lower porosity at 600 °C. Experiments using a stack of single crystal sheets showed μ ≤ 0.008 at low displacements (&lt;3 mm) followed by hardening, while microstructures are suggestive of slip along (001), folding and tear of cleavage planes, and gouge production. Our results have important implications for the mechanisms controlling megathrust fault slip under greenschist facies conditions in a subduction zone and shed new light on the strain accommodation mechanisms within sheared gouges versus single crystals composed of phyllosilicates.