Dislocation Glide Research Articles

In-situ synchrotron high energy X-ray diffraction study on the deformation mechanisms of D019-α2 phase during high-temperature compression in a TiAl alloy

The α2 phase with an ordered hexagonal structure has a strong mechanical anisotropy. The limited plasticity interacting with crystallographic texture formation undoubtedly complicate the deformation mechanisms of the α2 phase. Taking advantage of in-situ synchrotron high energy X-ray diffraction (HEXRD), this study unveils the origin of the α2 texture and its correlation with plastic deformation. Upon loading at 800 °C, the dislocation glide in the γ phase initiates at a stress of 370 MPa and that in the α2 phase at a stress of 670 MPa. Nevertheless, the texture evolution of the α2 phase sets in in parallel with that of the γ phase, and these textures are preliminarily established at a stress of 650 MPa. A main <11 2‾ 0> fiber texture with the c axis of the α2 unit cell aligned vertically to the compression axis has been unexpectedly evolved. This texture forms primarily via rigid body rotation and is correlated with glide and twin activation in the neighboring γ phase. The preferential orientation primarily favors double prismatic glide in α2 and in addition triggers the tensile twin activation. The grain rotation caused by the tensile twin slightly affects the α2 transverse texture evolution. The results unveil a complex interaction between the α2 phase deformability, texture evolution and the combined elastic-plastic deformation of the phases γ and α2 in TiAl alloys.

Mechanical behavior of powdered iron aluminide Fe – 28Al manufactured by direct powder forging

This study examines the impact of direct powder forging of powders on the composition, structure, and mechanical properties of iron aluminide Fe–28 at. % Al. During the synthesis and forging of Fe3Al powders, a homogeneous A2 phase is formed at a temperature of 1100 °C. Porosity after forging is 2–2.5 %. Residual pores are predominantly planar in shape and are located at the boundaries of the powder particles. Annealing at 1300 °C improves the quality of interparticle boundaries and all samples exhibit transcrystalline fracture mechanism. Samples forged at 1100 °C and annealed at 1300 °C show maximum strength σbend = 1050 MPa and fracture toughness K1c = 32.3 MPa·m1/2. The yield strength demonstrates anomalous temperature sensitivity with a maximum of 400 °C and at 500 °C. Samples tested at 600 °C show a decrease in yield strength, but a high enough yield point σy ∼400 MPa and a high strengthening rate are very important for high-temperature creep resistance. In creep experiments at a load of 120 MPa at 600 °C, the strain rate varies in the range of 10−7–10−6 s−1, the value of the rate sensitivity n ≈ 4. The main mechanism of creep is dislocation glide.

Overcoming the intrinsic brittleness of high-strength Al2O3–GdAlO3 ceramics through refined eutectic microstructure

High-strength ceramic materials are known for their exceptional mechanical properties; however, they are often plagued by brittleness, limiting their applications. Because of the inherent difficulty of dislocation glide and multiplication in ceramics, efforts to overcome the brittleness of ceramics by activating plastic deformation have faced challenges. This work demonstrates that Al2O3–GdAlO3 (Gadolinium Aluminum Perovskite: GAP) eutectic micropillars with submicron-scale fibrous microstructures exhibit remarkable plastic deformability. They displayed engineering plastic strains of up to 5% even at 25 °C, while the micropillars of Al2O3 or GAP single crystals exhibited brittle fracture similar to conventional high-strength ceramics. The plasticity in Al2O3–GAP eutectic was attributed to the activation of primary prismatic slip and secondary basal slip in the Al2O3 phase, which is typically considered inactive at room temperature. These findings suggest that plastic deformability can be achieved in high-strength ceramic materials by fabricating refined eutectic microstructures.

Temperature dependence of the deformation behavior and mechanical properties of a Fe–Mn–Al–C low-density steel for cryogenic application

The temperature dependence of the deformation behavior and mechanical properties of a Fe–20Mn–6Al-0.6C-0.15Si austenitic low-density steel was studied by tensile test and Charpy impact test at −196 °C, −77 °C and room temperature (RT), followed by microstructure analyses. The results indicate that the decreased tensile temperature, which is corresponding to reduced stacking fault energy (SFE), promotes the planar glide of dislocations. This led to larger fraction of low-angle grain boundaries (LAGBs) at lower tensile temperature. Consequently, both the tensile strength and elongation were improved as temperature decreased. At lower impact test temperature, the load and energy of crack initiation were larger, indicating a higher resistance to crack initiation. However, the transition from stable to unstable crack propagation occurred more rapidly at lower temperature, resulting in lower crack propagation energy and reduced resistance to crack propagation. This is because less region experienced planar glide. Due to the rapid unstable crack propagation, the Charpy absorbed energy at −196 °C was the lowest. The smaller fracture surface area, larger fibrous zone and more obvious ductile fracture characteristics in radial zone suggest that the toughness is better at higher temperature. Additionally, the presence of secondary cracks at RT delayed fracture, therefore benefitting toughness.

Exceptional elevated-temperature properties of a Laves phase-strengthened CoNiCrFe high-entropy alloy

This work reported a Laves phase-strengthened high-entropy alloy, featured by high-density Laves phase particles uniformly distributed within a face-centered cubic matrix. The pinning effect from these fine Laves phase particles on matrix grain growth and dislocation gliding results in extraordinary microstructure stability and mechanical properties. Additionally, the alloy demonstrates superior oxidation resistance, attributed to the formation of a Cr/Si-rich oxide film and beneficial effects from the Laves phase.

Ultrahigh‐Pressure Structural Modification in BiCuSeO Ceramics: Dense Dislocations and Exceptional Thermoelectric Performance

AbstractDislocations as line defects in crystalline solids play a crucial role in controlling the mechanical and functional properties of materials. Yet, for functional ceramic oxides, it is very difficult to introduce dense dislocations because of the strong chemical bonds. In this work, the introduction of high‐density dislocations is demonstrated by ultrahigh‐pressure sintering into a typical ceramic oxide, BiCuSeO, for thermoelectric applications. The ultrahigh‐pressure induces shear stresses that surpass the critical strength for dislocation nucleation, followed by dislocation glide and profuse multiplication, leading to a high dislocation density of ≈9.1 × 1016 m−2 in Bi0.96Pb0.04CuSeO ceramic. These dislocations greatly suppress the phonon transport to reduce the lattice thermal conductivity, reaching 0.13 Wm−1 K−1 at 767 K and resulting in a record‐high zT of 1.69 in this oxide thermoelectric ceramic. This study demonstrates the feasibility of generating dense dislocations in ceramic oxides via ultrahigh‐pressure sintering for tuning functional properties.

Nanoindentation creep response of Ti–6Al–4V ELI alloy manufactured via laser powder bed fusion

The anisotropic creep response in both the XY and XZ planes of Ti–6Al–4V ELI manufactured by powder bed fusion (PBF) was examined under nanoindentation creep loading at room temperature, ranging from nm to μm scales. The stress exponent values of 4.06–4.30, rationalised through threshold stress, indicate that the creep behaviour is primarily dominated by dislocation gliding. Creep displacement results show that the anisotropic creep behaviour in the XY and XZ planes of the as-built, and heat treatment enhances the creep resistance of the XZ plane, while there is no significant difference in creep displacement between the as-built and heat-treated XY planes. Due to the higher dislocation density and compressive residual stress in the XY plane compared to the XZ plane, the creep resistance is higher in the XY plane for the as-built. It is highlighted that compressive residual stress is more relieved in the XY plane than in the XZ plane through heat treatment. The heat treatment results in improved creep resistance due to the formation of the Widmanstätten structure and β precipitates, which can impede dislocation movement under creep loading. The combination of residual stress and microstructural effects leads to anisotropic creep behaviour, suggesting that the anisotropy is inherited from the additive manufacturing process at micron scales.

Characteristic deformation microstructure evolution and deformation mechanisms in face-centered cubic high/medium entropy alloys

Face-centered cubic (FCC) high/medium entropy alloys (HEAs/MEAs), novel multi-principal element alloys, are known to exhibit exceptional mechanical properties at room temperature; however, the origin is still elusive. Here, we report the deformation microstructure evolutions in a tensile-deformed Co20Cr40Ni40 representative MEA and Co60Ni40 alloy, a conventional binary alloy for comparison. These FCC alloys have high/low friction stresses, fundamental resistance to dislocation glide in solid solutions, respectively, and share similar other material properties, including stacking fault energy. The Co20Cr40Ni40 MEA exhibited higher yield strength and work-hardening ability than in the Co60Ni40 alloy. Deformation microstructures in the Co60Ni40 alloy were marked by the presence of coarse dislocation cells (DCs) regardless of grain orientation and a few deformation twins (DTs) in grains with the tensile axis (TA) near <1 1 1>. In contrast, the MEA developed three distinct deformation microstructures depending on grain orientations: fine DCs in grains with the TA near <1 0 0>, planar dislocation structure (PDS) in grains with other orientations, and a high density of DTs along with PDS in grains oriented <1 1 1>. Three-dimensional electron tomography revealed that PDS in the MEA confined dislocations within specific {1 1 1} planes, indicating suppression of cross-slip of screw dislocations and dynamic recovery. In-situ X-ray diffraction during tensile deformation showed a higher dislocation density in the MEA than in the Co60Ni40 alloy. These findings demonstrate that FCC HEAs/MEAs with high friction stresses naturally develop unique deformation microstructures which is beneficial for realizing superior mechanical properties compared to conventional materials.

Anisotropy of plastic flow in Zr-2.5Nb pressure tube material analysed using a viscoplastic self-consistent approach

Plastic anisotropy is observed during plastic flow in samples from the axial and transverse directions of Zr-2.5Nb pressure tubes used in CANDU11CANada Deuterium Uranium - Pressurized Heavy Water power reactors. nuclear reactors tested in uniaxial tension. Plastic anisotropy was also measured in shear by testing in torsion ‘mini’ pressure tubes from the same material. Room temperature results from these experiments were analysed using a visco-plastic self-consistent model that takes into account the crystallographic texture of the material and allows for the single crystal work hardening behaviour to be described by means of a deformation law specific to each slip system.The visco-plastic self-consistent model was used to derive: (i) the evolution with strain of the critical resolved shear stress values consistent with prismatic, basal and pyramidal dislocation glide, (ii) the evolution of slip system activity as a function of strain, and (iii) the values of Hill's plastic anisotropy coefficients that are consistent with the observed anisotropy of yielding and their dependence on accumulated strain. The model also allows for the prediction of the components of the flow stress tensor that cannot be measured experimentally and their dependence on the work hardening behaviour of pressure tube material. Moreover, the yield surface after different amounts of plastic strain was calculated using the visco-plastic self-consistent model, compared to the one that results from using the Hill's anisotropy coefficients. Our work exposes the limitations of the Hill ellipsoid for describing plastic yield when microstructural evolution is present.

Dislocation evolution in anisotropic deformation of GaN under nanoindentation

The exceptional performance of GaN semiconductors in lasers, wireless communication, and energy storage systems makes them crucial for future multi-functional devices. However, during the polishing of GaN wafers, abrasive particles can induce subsurface damage, compromising device performance. This study investigates dislocation loops in GaN single crystal to understand dislocation nucleation and glide under external stress. Using nanoindentation for compressive stress, we confirmed multiple slip system activation via transmission electron microscopy after pop-in. We also performed molecular dynamics to simulate the nucleation and multiplication of U-shaped dislocation loops. Furthermore, we developed a theoretical model using Peierls–Nabarro stress to quantify GaN's critical shear stress. Raman spectroscopy was also used to analyze shear stress on U-shaped loops, supporting our model. This study provides insights into GaN dislocation dynamics under mechanical stress, aiding in wafer defect evaluation during machining and offering guidance for dislocation evolution.

Influence of enhanced Laves phase shape and distribution on atomic-scale frictional wear mechanisms in nickel-based single crystal alloys

This study aims to simulate the influence of different shapes and distribution states of Laves phases on the friction-wear behavior of nickel-based alloys using molecular dynamics (MD). The investigation systematically examined the mechanical properties, friction coefficient, number of worn atoms, dislocations, temperature, and other micro-deformation behaviors of materials incorporating horizontally and vertically distributed short rod-shaped, spherical, and short strip-shaped Laves phases. The presence of the Laves phase significantly impedes temperature transfer, defect motion, and atomic displacement in the workpiece, resulting in reduced dislocation glide rate and shorter average dislocation lengths. High dislocation densities accumulate at the Laves/γ phase interface, enhancing surface wear resistance. The short rod-shaped Laves phase, due to its large surface area at the Laves/γ interface, impedes defect motion more effectively than spherical and short strip-shaped phases. dislocation tangle, higher friction force, fewer worn atoms, a higher friction coefficient, and improved wear resistance. However, vertically distributed short strip-shaped and short rod-shaped Laves phases exhibit less effective defect interaction, resulting in increased wear and significant deformation. The spherical Laves phase, with its geometric symmetry, shows consistent wear resistance regardless of distribution state. Short rod-shaped Laves phase provides the best reinforcement due to its effective defect motion impedance, while the spherical Laves phase offers stable performance across different distribution states, making it the most suitable shape for Laves phase reinforcement.

The role of processing temperature for achieving superplastic properties in an Al-3Mg-0.2Sc alloy processed by high-pressure torsion

Experiments were conducted to systematically assess the superplastic properties and the microstructural changes of an Al-3Mg-0.2Sc alloy processed by 10 HPT revolutions at room temperature (RT ≈ 300 K) or at 450 K after subsequent tensile testing at temperatures from 473 to 723 K over a wide range of strain rates. The HPT processing at RT led to the development of elongated grains with an average size of ∼140 nm whereas the grain structures were equiaxed and slightly larger (∼150 nm) after HPT at 450 K. After HPT processing at RT, the Al-Mg-Sc alloy exhibited true superplastic flow at low homologous temperatures and attained a maximum elongation of ∼850 % at 523 K. Nevertheless, the elongations decreased at temperatures T ≥ 623 K and an elongation higher than 400 % was only achieved at 673 K for a strain rate of ε˙ = 4.5 × 10−3 s−1. The material processed by HPT at 450 K displayed superior microstructural stability and substantially higher superplastic ductilities. Elongations of >1100 % were attained at 673 K for strain rates from 3.3 × 10−4 to 1.0 × 10−1 s−1 and a record elongation of ∼1880 % for an HPT-processed metal was achieved at 1.5 × 10−2 s−1 at 673 K. High strain rate superplasticity was also reached for an extended range of temperatures and strain rates. Analysis of the data confirms a stress exponent of ∼2 which is consistent with superplastic flow by grain boundary sliding accommodated by dislocation glide and climb.

Pyramidal dislocation driven martensitic nucleation: A step toward consilience of deformation scenario in fcc materials (II)

Dislocation glides and their interactions with other defects are central to our understanding of deformation mechanism enhancing plasticity and damage tolerance. Martensitic transformation (MT) is a fascinating phenomenon overcoming strength-elongation trade-off and thus tailoring structure-property architecture. However, dislocation plasticity of Fe-based hexagonal close-packed (hcp) martensite is still challenging due to its metastability, further complicated by controversies surrounding pyramidal dislocation even in pure hcp metals. To resolve the uncertainties, we address the pyramidal-dislocation-driven plasticity in Fe-based hcp martensite by employing in-situ transmission electron microscopy (TEM) and high-resolution (HR) annular bright field (ABF) imaging together with dislocation-contrast analyses. We show that the activation and dissociation of pyramidal dislocation govern the initial stage of plastic deformation, while subsequent cross-slip by the cooperative motion of dissociated pyramidal dislocations plays a key role in nucleation of new hcp martensite. By incorporating current dislocation model into previous models coupled with stacking-fault-energy concept, we propose a synthesized deformation scenario that offers a comprehensive perspective on plasticity and transformability.

Effect of compositional undulation on the mechanical behavior of atomic-scale planar-faulted Ni-Mo-W films

Ni exhibits exceptional resistance to heat and corrosion, making it a sought-after material for harsh environment applications. The introduction of low interfacial energy planar defects (e.g., nanotwins and stacking faults) can greatly enhance the mechanical properties of Ni at elevated temperatures, but this strategy generally remains difficult due to Ni's high stacking fault energy (120–130 mJ/m2). Here, we apply HRSTEM, APT characterization, and DFT calculation to discover that inhomogeneous distribution of Mo atoms can facilitate the nucleation and stabilization of atomic-scale stacking faults in Ni-Mo-W thin films, in addition to the increase in global concentration. Micro-tensile experiments confirm the exceptionally high tensile strength up to 3 GPa. Post-mortem TEM analysis reveals that de-faulting followed by selective activation of dislocation emission and glide are the strength-limiting deformation mechanism. This work provides a practical strategy for designing ultrahigh strength, stable nanostructured materials by local chemical undulation.

Crystallographic orientation dependence in creep deformation of micron-thick silicon films for 3-D microstructures

In this study, the steady-state creep deformation properties of 5 μm-thick single crystal silicon (Si) films at elevated temperatures were investigated using punch creep-forming tests for the design of three-dimensional (3-D) microstructured MEMS. The relationship between the creep strain rate and stress of the Si films with {100}, {110}, and {111} planes at temperatures of 1223–1323 K was derived using finite element analysis following punch creep-forming tests, and a power-law creep constitutive equation for Si films was determined. The steady-state creep properties of the Si film samples varied clearly with crystallographic orientations. Among the three crystallographic orientations, the creep strain rate of the micron-thick Si films was the fastest for the {011} plane, followed by the {111} and {001} planes, in the applied stress range of 110 MPa to 220 MPa. This is consistent with the increasing order of magnitude of the thermal activation energy. The crystallographic orientation dependence of the steady-state creep properties of Si films is discussed based on the mobility of dislocation gliding per unit lattice and scanning electron microscope observations. The sample-size dependence of the creep strain rate of Si with the {001} plane was clearly observed between the micron- and millimeter-thick samples, which was attributed to the difference in the frequency factor of the power-law creep. However, the thermal-activation energy remained constant. This study successfully revealed the steady-state creep properties of Si films and provided useful engineering data for the design of 3-D Si-MEMS fabrication by the plastic processing of Si films.

Size-dependent mechanical responses of twinned Nanocrystalline HfNbZrTi refractory high-entropy alloy

Atomistic simulations are performed to study the size-dependent mechanical responses of HfNbZrTi refractory high-entropy alloy (RHEA) containing ultrafine grains and highly oriented twin boundaries (TBs). The strength and flow stress of nanocrystalline RHEA (NC-RHEA) under tensile loadings are explored versus decreasing grain size d. The transition from classical Hall-Petch (HP) strengthening to inverse HP softening at a critical grain size dc = 5.91 nm is attributed to the change of plastic deformation mechanisms from dislocation emission and phase transformation to grain boundary (GB) activities. Besides, the intragranular TBs considerably enhance the strength of nanotwinned RHEA (NT-RHEA); the enhancing effect reduces with decreasing twin thickness λ. As the volume fraction of GB increases with decreasing d, GB activities dominate the plasticity of NT-RHEA and cause comparable mechanical properties with NC-RHEA. Moreover, the influences of dislocation glide, phase transformation and twinning on the mechanical properties of RHEA are quantified and separately analyzed to further verify our simulation results. Findings of this study not only promote insights into the nanostructure-property relation of HfNbZrTi, but also shed the light on performance enhancement through nanostructural design.

The lattice friction stress driven temperature-dependent tensile deformation behaviors of CoNiCr2 eutectic medium-entropy alloy

A heterogeneous CoNiCr2 eutectic medium-entropy alloy (EMEA), comprising soft face-centered cubic (FCC) and hard body-centered cubic (BCC) lamellae, associated with minor acicular hexagonal close-packed (HCP) phase precipitated in BCC phase, was synthesized towards excellent tensile strength and ductility synergy. The tensile mechanical properties demonstrated that this alloy was temperature-dependent, i.e., when the testing temperature reduced from room temperature (RT) to liquid nitrogen temperature (LNT), the yield strength, ultimate strength, and uniform elongation were enhanced from 449 MPa, 821 MPa, and 5.0% to 702 MPa, 1174 MPa, and 8.4%, respectively. The prominent elevation of yield strength at LNT mainly resulted from the dramatically enhanced lattice friction stress (σ0) and the FCC-BCC interfacial strengthening, while the improved ductility was attributed to the superior crack-arrest capability of FCC matrix stemmed from the accumulation of stacking faults (SFs) and enhanced σ0 at LNT. Additionally, although the deformation mechanisms were dominated by planar dislocation glides and SFs at both temperatures, the initiation of premature cracks in the BCC phase due to the inferior deformation capability at LNT constrained the better strength-ductility trade-off. The cracks in the BCC phase tended to propagate along the BCC-HCP interfaces because of the strain incompatibility. Furthermore, the sub-nanoscale L12 particles in the FCC matrix could not only strengthen this alloy but also improve the stacking fault energy leading to no deformation twinning even at LNT. This work may provide a guide for the design of remarkable strength and ductility synergy EMEAs combined with outstanding castability for applications at cryogenic temperatures.

A crystal plasticity-based creep model considering the concurrent evolution of point defect, dislocation, grain boundary, and void

Creep poses a significant threat to the integrity and longevity of structural components at high-temperature. The most current understanding of creep mainly focuses on the coupled dynamics of point defects and dislocation, which may well describe the first and second stage of creep. However, the behavior of the three stages of creep is jointly controlled by point defect (vacancy) diffusion, dislocation glide, dislocation climb, grain boundary (GB) sliding, and void evolution. A critical knowledge gap still exists regarding how these different creep mechanisms are simultaneously coupled during the three stages of creep. In this work, a multi-physical mechanisms-based crystal plasticity model is proposed to consider the concurrent evolution of point defect, dislocation, GB, and void based on a unified thermodynamic framework. In-situ scanning electron microscope creep experiments and macroscopic creep experiments of Ti-6Al-4V were conducted to validate our model. The in-situ creep experiment directly revealed the GB sliding creep failure behavior of Ti-6Al-4V for the first time. The proposed model well predicts both the microscopic and macroscopic experimental behavior of creep. The contribution of different microstructure evolutions is discussed, and a phase diagram of the dominated creep mechanism is obtained. An in-depth analysis was conducted on the coupling effects and microstructure characteristics of different creep mechanisms. This work not only deepens our understanding of the micro creep mechanism but also offers valuable insights for designing materials with specific microstructures to enhance their creep resistance.

Slip to twinning to slip transition in polycrystalline BCC-Fe: Effect of grain size

A slip-to-twinning-to-slip transition concerning grain size has not been reported in the literature on BCC-Fe so far. In the present investigation, we report slip-to-twinning and twinning-to-slip transitions at 8 nm and 20 nm grain sizes, respectively. For this purpose, we used molecular dynamics simulations to study ⟨110⟩ quasi-3D polycrystals of BCC-Fe with grain sizes ranging from 3 nm to 102 nm at 10 K. Hall-Petch and inverse Hall-Petch relations were obtained depending on the grain size. Grain boundaries (GBs) were converted to lower energy boundaries through twin-GB interactions. Nucleation of twin boundary through GB decomposition was observed even under the constraints of triple junctions. Various dislocation mechanisms were observed, like delayed slip transfer, slip absorption, dislocations from a twin boundary, and formation of debris/point defects/tubular-type cylindrical rods from the gliding dislocations. Dislocation activity was observed to increase with increasing grain size. Luster and Morris parameter was used to understand dislocation-GB interactions. Intergranular fracture was observed.

The Stress State in an Elastic Disk Due to a Temperature Variation in One Sector

Abstract Consider a linear elastic infinite disk, a sector of which, of arbitrary opening angle 2β, is subjected to a uniform temperature increase ΔT with respect to the complementary portion. An analytical solution is sought, imagining that the disk is first cut along the interface with the heated sector, now free to expand; then the two parts are re-joined and the thermal mismatch is annihilated by arrays of glide dislocations, distributed along the interfaces. A sequence of approximate solutions is found as the length of the arrays of reconciling dislocations is increased, characterized by a logarithmic stress singularity at the sector tip. However, modulo the particular case 2β=π, the stress grows unboundedly when the length of the dislocation arrays tends to infinity. This is in agreement with the predictions from dimensional analysis, because for the infinite disk problem there is no internal length scale. If the disk is finite in size, its radius R represents an additional length scale enriching the class of solutions, but the analytical treatment results much more complicated. Therefore, we propose to correlate the solution of this problem with that of an infinite disk for which the length of the arrays of reconciling dislocations is finite and depends upon R. An excellent agreement with numerical experiments in abaqus is thus found. An approach of this type can be useful in many engineering problems for which the limit condition of infinite body, though leading to analytic simplifications, could imply spurious results.