Basal Plane Slip Research Articles

Effect of different extrusion parameters on the microstructure and mechanical properties of Mg-4Sm-3Gd-2Yb-0.5Zr alloy

The effect of different extrusion parameters on the microstructure and mechanical properties of Mg-4Sm-3Gd-2Yb-0.5Zr (SGY2) alloy was investigated. It was observed that under different extrusion parameters, unDRXed grains of SGY2 alloy exhibited a pronounced basal plane texture, specifically <011¯0>//ED, while the texture of DRXed grains was relatively dispersed. Under the condition of 420 °C and an extrusion ratio of 9.4 (420 °C-ER9.4), the basal plane texture strength of unDRXed grains in SGY2 alloy was the highest. Furthermore, SGY2 alloy at different extrusion parameters exhibited recrystallization mechanisms mainly characterized by continuous dynamic recrystallization (CDRX), with some DRXed grains and deformed grains experiencing discontinuous dynamic recrystallization (DDRX). Additionally, at the 420 °C-ER9.4, the second phase particles in the as-extruded SGY2 alloy were smaller in size and exhibited a dispersed distribution. Under this condition, a significant amount of dislocation accumulation, dislocation bypassing, and dislocation tangling phenomena were observed in the SGY2 alloy. The primary deformation mechanism of unDRXed grains in the SGY2 alloy at the 420 °C-ER9.4 may involve prismatic plane 〈a〉, pyramidal plane 〈a〉, and pyramidal plane 〈c + a〉 slip, thereby activating a significant amount of dislocations. Compared to other extrusion conditions, this condition is more prone to activate non-basal plane slip. The as-extruded SGY2 alloy exhibited superior mechanical properties, with ultimate tensile strength (UTS) and yield strength (YS) of 332 MPa and 278 MPa, respectively. This is mainly attributed to the extremely fine grains, with many DRXed grains having grain sizes smaller than 1 µm, and higher density grain boundaries produced under this condition. Additionally, the unDRXed grains contain a high density of dislocations with small Schmid factor (SF), thus effectively inhibiting basal plane slip and strengthening the alloy to some extent. Similarly, the increased presence of second phase particles will also contribute to strengthening the alloy matrix through precipitation hardening.

Distribution of basal plane dislocations in 4-degree off-axis 4H-SiC single crystals

The basal plane slip model in 4H-SiC was developed to investigate the effects of off-axis angles on total resolved shear stress. The results showed that the TRSS changed from 6-fold to 4-fold symmetry with the increasing off-axis angles.

The effect of orientation on the deformation behavior of Cr2AlC

The MAX phases are a group of ternary carbides and nitrides with potential for use in advanced high temperature applications. Numerous studies have shown their main deformation mechanism to be basal plane slip, even in extreme orientations, yet the fundamentals of this mechanism and dependencies on size and applied stress state remain inconclusive. Based on similar studies in Ti3SiC2, Ti3AlC2 and Ti2AlC, the current work investigated the onset of basal plane slip as a function of loading orientation by compressing single crystal micropillars of Cr2AlC. The results suggest clear changes in the critical resolved shear stress with loading orientation (non-Schmid effects), and attempts were made to rationalize this behavior by comparison with models of dislocation activity. On this basis, it is proposed that external influences on dislocation mobility are likely the governing factor in the observed non-Schmid effects in the MAX phases.

Fabrication, microstructure and mechanical properties of TiAl matrix composite reinforced by submicro/nano-Ti2AlC

In the present work, using pre-alloyed Ti-48Al-2Cr-2Nb powders and graphene nanosheets as raw materials, the TiAl matrix composite reinforced by submicro/nano-Ti2AlC was fabricated via spark plasma sintering and subsequent heat-treatment. The microstructure characterization results have indicated that submicro-Ti2AlC and nano-Ti2AlC exhibit the coherent atomic correspondence with the full lamellar TiAl matrix. Furthermore, The Ti2AlC/TiAl composite has excellent balance relationship of room-temperature compression strength (∼2171 ± 3 MPa) and fracture strain (∼31 ± 1%). The submicro-Ti2AlC could effectively hinder the twin propagation and inhibit the dislocations motion, which is benefit to improve the strength of the Ti2AlC/TiAl composite. In addition, the nano-Ti2AlC could not only cause the dislocation multiplication in γ-TiAl by the activation of pyramidal dislocations, but also induce the formation of deformation twins in γ-TiAl by the basal-plane slip, resulting in the improvement of plastic deformation capacity in the Ti2AlC/TiAl composite.

Basal Plane Dislocation Slip Band Characterization and Epitaxial Propagation in 4H SiC

Correlation of X-ray topography and production line defect inspection tools has demonstrated the capability of in-line tools to differentiate between geometrically comparable basal plane slip bands (BPSB) and bar stacking faults (BSF) on 4H SiC wafers. BPSBs were found to propagate through epitaxial growth at high rates and with similar photoluminescence signatures to post-epitaxy BSFs. Molten KOH etching post-epitaxy provided evidence of distinguishing features between BPSBs and BSFs, suggesting that the defects were indeed correctly identified by in-line defect inspection tools pre-epitaxy.

Characterization of Prismatic Slip in SiC Crystals by Chemical Etching Method

In 4H-silicon carbide crystals, basal plane slip is the predominant deformation mechanism. However, prismatic slip is often observed in single crystals grown by the physical vapor transport method as the diameter expands to 6 inches or larger. Thermal modeling has shown that occurrence of prismatic slip is attributed to increased radial thermal gradients. While X-ray topography can be used to characterize the presence and extent of prismatic slip, the feasibility of using the chemical etching method to assess the extent of prismatic slip in an industrial setting is investigated. The distribution of scallop shaped etch pits oriented along the directions that correspond to prismatic dislocations, correlate well with the results of the thermal model that predicts the occurrence of prismatic slip dislocations. This capability of the etch pit method to characterize prismatic slip can be used to manage radial thermal gradients during PVT growth.

Shedding new light on the dislocation-mediated plasticity in wurtzite ZnO single crystals by photoindentation

Dislocation-mediated plasticity in inorganic semiconductors and oxides has attracted increasing research interest because of the promising mechanical and functional properties tuned by dislocations. In this study, we investigated the effects of light illumination on the dislocation-mediated plasticity in hexagonal wurtzite ZnO, a representative third-generation semiconductor material. A (0001) 45o off sample was specially designed to preferentially activate the basal slip on (0001) plane. Three types of nanoindentation tests were performed under four different light conditions (550 nm, 334 nm, 405 nm, and darkness), including low-load (60 μN) pop-in tests, high-load (500 μN) nanoindentation tests, and nanoindentation creep tests. The maximum shear stresses at pop-in were found to approximate the theoretical shear strength regardless of the light conditions. The activation volume at pop-ins was calculated to be larger in light than in darkness. Cross-sectional transmission electron microscope images taken from beneath the indentation imprints showed that all indentation-induced dislocations were located beneath the indentation imprint in a thin-plate shape along one basal slip plane. These indentation-induced dislocations could spread much deeper in darkness than in light, revealing the suppressive effect of light on dislocation behavior. An analytical model was adopted to estimate the elastoplastic stress field beneath the indenter. It was found that dislocation glide ceased at a higher stress level in light, indicating the increase in the Peierls barrier under light illumination. Furthermore, nanoindentation creep tests showed the suppression of both indentation depth and creep rate by light. Nanoindentation creep also yielded a larger activation volume in light than in darkness.

Improved impact toughness of laminate heterogeneous AZ31/GW103K alloys by interface delamination

An AZ31/GW103K (AG) laminate heterostructured material was fabricated by diffusion bonding. The impact resistance of AG was evaluated by Charpy notched impact testing and the fracture mechanism was systematically studied. The impact toughness of AG is nearly twice of the homogeneous structured AZ31 or GW103K. The delamination can effectively prevent crack initiation and propagation under the high-speed impact, which significantly improves the overall impact toughness by consuming high impact energy. Simultaneously, the cracks crossing over the Al2(Gd, Y) zone at the interface consume considerable impact energy. Besides, the conventional thermal strain, composition diffusion, and the mechanical incompatibility of AG can also consume energy by coordinating plastic deformation. During impact, the primary deformation mechanism of the GW103K layer is {0001} basal plane slip. While the AZ31 layer is relatively complicated, primarily of twinning, multiple dislocation slip, and stacking faults.

High-strain-rate superplasticity and microstructural evolution in ECAP-processed Mg–6.5Y–1.2Er–1.6Zn–0.5Ag alloy

A warm equal-channel angular pressing (ECAP) procedure was applied to a new Mg–6.5Y–1.2Er–1.6Zn (WEZ612)–0.5Ag alloy with a long-period stacking ordered (LPSO) phase. The microstructure, superplasticity, and deformation mechanism of the WEZ612–0.5Ag alloy were then systematically investigated. Three different precipitation phases formed in the alloy with trace Ag addition, namely LPSO, γ, and nanoprecipitate phases. A remarkable elongation of 726% was attained at 623 K and a strain rate of 0.01 s−1. The strain rate sensitivity index (m) and activation energy (Q) under these deformation conditions were 0.461 and 133.08 kJ mol−1, respectively. These m and Q values confirm that the primary deformation mechanism of the WEZ612–0.5Ag alloy at 623 K was grain boundary sliding assisted by lattice diffusion. The highly stable LPSO phases, γ precipitates, and nanoprecipitates synergistically improved the superplasticity of the alloy by retarding crack propagation and grain growth and by promoting the basal plane slip.

Microstructure evolution, strengthening mechanisms and deformation behavior of high-ductility Mg−3Zn−1Y−1Mn alloy at different extrusion temperatures

Hot extrusion was performed on as-cast Mg−3Zn−1Y−1Mn alloy to further improve the mechanical properties and investigate its microstructure evolution and deformation mechanism at different thermal deformation temperatures. The experimental results suggested that the alloy at the extrusion temperature of 330 °C had the optimal mechanical properties, and the tensile strength and elongation of the alloy were 270 MPa and 16.8%, respectively. The strength improvement of the as-extruded alloy primarily arose from three aspects, including fine-grain strengthening due to dynamic re-crystallization (DRX), dislocation strengthening not offset by DRX, and second phase strengthening. Continuous dynamic re-crystallization (CDRX) and dynamic recovery (DRV) occurred simultaneously at the extrusion temperature of 300 °C. The discontinuous dynamic re-crystallization (DDRX) occurred at the extrusion temperature of 330 and 360 °C. However, the contribution of {0001} basal plane slip, {1010} prismatic type I and {1120} prismatic type II slips to plastic deformation was significantly affected by the extrusion temperature. The {1120} prismatic type II slip was found to be the most difficult one to initiate among the three slip systems.

Plasticity of the Nb-rich μ-Co7Nb6 phase at room temperature and 600°C

The µ phase is a common precipitation phase in superalloys and it exists in a wide composition and temperature range. As such, we study the influences of composition and temperature on its plasticity by micropillar compression tests and transmission electron microscopy. The micropillars of the μ-Co7Nb6 phase deform plastically by basal slip at room temperature and 600 °C. At room temperature, the Co-49Nb and Co-52Nb micropillars show high yield stresses and an abrupt large strain burst at the onset of yielding regardless of orientation, whereas the Co-54Nb micropillars oriented for basal slip yield at much lower stresses and show intermittent small strain bursts during plastic deformation. While the Co-49Nb micropillars deform by full dislocation slip on the basal plane at room temperature, the Co-54Nb micropillars deform by partial dislocation slip on the basal plane. At 600 °C, the Co-49Nb micropillars oriented for basal slip show stable and continuous plasticity and their critical resolved shear stresses decrease dramatically. In contrast to the full dislocation slip at room temperature, the plastic deformation of the Co-49Nb micropillars occurs via partial dislocation slip on the basal plane at 600 °C. Based on the geometric γ-surfaces for all potential basal slip planes, we explore where and why the glide of full and partial dislocations on the basal plane occurs in the μ-Co7Nb6 phase.

The influence of alloying on slip intermittency and the implications for dwell fatigue in titanium

Dwell fatigue, the reduction in fatigue life experienced by titanium alloys due to holds at stresses as low as 60% of yield, has been implicated in several uncontained jet engine failures. Dislocation slip has long been observed to be an intermittent, scale-bridging phenomenon, similar to that seen in earthquakes but at the nanoscale, leading to the speculation that large stress bursts might promote the initial opening of a crack. Here we observe such stress bursts at the scale of individual grains in situ, using high energy X-ray diffraction microscopy in Ti–7Al–O alloys. This shows that the detrimental effect of precipitation of ordered Ti3Al is to increase the magnitude of rare pri〈a〉 and bas〈a〉 slip bursts associated with slip localisation. By contrast, the addition of trace O interstitials is beneficial, reducing the magnitude of slip bursts and promoting a higher frequency of smaller events. This is further evidence that the formation of long paths for easy basal plane slip localisation should be avoided when engineering titanium alloys against dwell fatigue.

A Comparative Study on Mechanical and Corrosion Behaviours of α/(α + β) Mg-Li Alloys Subjected to Ultrasonic Nanocrystal Surface Modification

Ultrasonic nanocrystal surface modification (UNSM) was applied to hot-rolled Mg-Li alloys (LAE361 and LA106). The microstructure, mechanical properties, deformation mechanisms, and corrosion resistance properties of these alloys after UNSM treatment were systematically studied. Significant improvement in surface hardness and decrease in surface roughness were achieved by UNSM treatment. Meanwhile, the basal texture intensity of the Mg-Li alloys reduced significantly, and several deformation twins appeared on the surface layer. The α phase of the surface layer underwent twin deformation and basal plane slip. The fibre textures in the β phase of LA106 Mg-Li alloy changed from γ and η to α and ε, which mainly resulted in the dislocation slip. More importantly, UNSM treatment exhibited enhanced strength and improved plasticity of LAE361 and LA106 Mg-Li alloys. The corrosion current density of LAE361 Mg-Li alloy reduced approximately 29.3% by UNSM treatment, while it increased the corrosion current density of LA106 Mg-Li alloy by 189.7%. These studies show that the application of UNSM to improve the corrosion resistance of duplex phases of LA106 Mg-Li alloy needs further investigation.

Mechanisms of strain localization and nucleation of earthquake faulting by grain-scale processes at the middle crustal level

Abstract The mechanism of strain localization is the key to our understanding of the transition from steady-state to unstable flow, and therefore of earthquake faulting in the middle crust. In this study, biotite grains in mylonitic gneisses along the Jinzhou detachment fault zone, Liaodong peninsula, northeast China, acted as a preexisting weak phase that had important influences on deformation of mid-crustal rocks. High phase strength contrasts between biotite and other mineral phases resulted in stress concentrations at the tips of biotite grains and induced semi-brittle deformation of neighboring quartz and feldspar grains. As a consequence, the biotite grains became interconnected to form zones of weakness, while basal plane slip and grain boundary sliding operated in biotite grains and fine-grained biotite-feldspar-quartz aggregates, respectively. The zones filled with biotite grains and fine-grained quartz-feldspar aggregates continued to propagate and coalesce during the deformation. These processes led to transition from load-bearing (i.e., coarse plagioclase grains) framework to interconnected weak phase (i.e., biotite grains and finegrained feldspar aggregates) domination, that further led to the formation of initial strain localization zones (SLZs). With the propagation and linkage of the SLZs, high stress concentrations at the tips of the SLZs led to nucleation of rupture along the SLZs. As a consequence, there occurred an abrupt increase in strain rates that resulted in transition from stable to unstable slip within the SLZs. The processes were accompanied by occurrence of mid-crustal earthquake faulting and formation of pseudotachylites along the SLZs.

Characterization of prismatic slip in PVT-grown AlN crystals

The most frequently observed deformation mechanism in wurtzite aluminum nitride (AlN) grown by the physical vapor transport (PVT) method is basal plane slip. However, prismatic slip also takes place in such crystals. In this study, prismatic slip is observed in a 50 mm commercial AlN substrate wafer, leading to a sixfold symmetric BPD pattern in the wafer. Prismatic dislocations cross slip on to basal plane and multiply there, increasing the dislocation density of the wafer. A radial thermal model is established to explore the origin of the unevenly distributed dislocations. The unevenly distributed resolved shear stress across the whole radial area of the boule during PVT growth caused by radial thermal gradients offers driving force for the activation of the three sets of prismatic slip systems, producing nonuniform dislocation distribution in the boule and thus the wafer. Both experimental observations and theoretical calculations of the critical resolved shear stress (CRSS) correlate well with the model results, revealing that controlling radial thermal gradients is critical for lowering dislocation density and enhancing the crystal quality using PVT growth method.

Residual stress evolution enhanced martensite phase transition and texture development in cryogenic-tempered WC-Co ultra-coarse grained cemented carbide

The aim of this study is to clarify the effect of residual stress on the microstructure, martensite phase transition of Co binder phase, texture development of WC hard phase and mechanical properties enhancement in cryogenic-tempered WC-6wt.%Co ultra-coarse grained cemented carbides. Sin2(ψ)-2θ method was used to analyze the residual stress, and EBSD was used to reveal the phase transition of Co phase and texture of WC phase. The results showed that the density, grain size and carbon balance of cemented carbide revealed little difference after cryogenic-tempering treatment. The mean compressive residual stress of WC phase increased by 60.84% after cryogenic treatment, while tempering treatment resulted in a significant relaxation of the residual stress by 101.97% and even transformed the compressive residual stress to tensile stress. EBSD analysis showed that the mean ratio of hcp-to-fcc Co increased from 1.75% to 4.22% and 11.61%. It is difficult to distinguish the effect of residual stress increase and the temperature decrease on the martensite transition during the cooling process. However, the occurrence of martensite phase transition during tempering is not temperature-dependent but highly related to the residual stress evolution. After cryogenic-tempering treatment, {0001}<11 2‾ 0> preferred orientation of WC phase was concentrated, which was related to the basal plane slip and grain deflection of WC grains during the evolution of residual stress. Due to the martensite phase transition strengthening of Co and the preferred orientation of WC, the mean Vickers hardness increased from 1068.4 HV30 to 1149.4 HV30. The mean transverse rupture strength firstly decreased from 2747.5 MPa to 2515.3 MPa, and then increased to 2705.0 MPa, which was related to the increase and relief of residual stress during the post treatment. Furthermore, mean fracture toughness showed a slight increase from 22.5 MPa m1/2 to 23.4 MPa m1/2. The simultaneous increase of fracture toughness with hardness can be attributed to the residual stress evolution and martensite phase transition.

Prismatic Slip in AlN Crystals Grown By PVT

Aluminum nitride (AlN) single crystals are desired as substrates for nitride based electronic and optoelectronic applications[1]. A low density of dislocations and other structural defects in bulk AlN substrates is in demand to grow high-quality heteroepitaxial epilayers. Physical vapor transport (PVT) grown AlN crystals possess low density dislocations and other structural defects[2]. In AlN crystals grown by PVT, basal plane slip is the most frequently observed deformation mechanism. However, prismatic slip takes place as well in such crystals [3, 4].When the diameter of AlN wafers expands to 50 mm and larger, managing the thermal gradients in the PVT growth chamber becomes critical for reducing thermal stresses that cause deformation. Recent synchrotron X-ray topography [5] studies of 50 mm diameter AlN wafers showed that while most wafers contained few to no basal plane dislocations (BPDs), some wafers possessed a 6-fold BPD dislocation pattern [2] which was aligned along the <11-20> directions. This configuration indicated that prismatic slip had likely occurred. Screw typed prismatic dislocations cross slipped from the prismatic plane onto the basal plane, and underwent basal plane slip, leading to dislocation multiplication. To investigate the origins of prismatic slip, a radial thermal gradient model was established showing[6] the resolved shear stress across the entire area of the crystal boule during growth. The results from the model were compared with experimental observations to investigate the role of radial thermal gradients in the nucleation of prismatic slip in AlN during bulk growth. This insight will help guide efforts to minimize structural defects in PVT AlN.

Prismatic Slip in AlN Crystals Grown By PVT

Basal plane slip is the most frequently observed deformation mechanism in hexagonal aluminum nitride (AlN) crystals grown by the physical vapor transport (PVT) method. However, prismatic slip can also take place in such crystals. In this study, slip in the three prismatic slip systems with nonuniform distributions, observed in a commercial 50 mm AlN substrate wafer, was investigated. The nonuniformity was attributed to the distribution of resolved shear stress in each prismatic slip system caused by radial thermal gradients in the growing crystal boule. A radial thermal model was established to quantitively estimate the thermal stress across the area of the crystal boule during PVT growth. The model results correlated well with both the experimental observations and theoretical calculations of the critical resolved shear stress, revealing that radial thermal gradients play a key role in activating prismatic slip during AlN bulk growth.

Role of twinning and texture on fatigue resistance enhancement of Mg-6Gd-3Y-1Nd-0.5Zr alloys

The fatigue crack propagation behavior of rolled magnesium alloys was studied and compared with as-cast alloys to unravel a relation between the early crack propagation and the fatigue resistance at the microstructural level. Grain structures results indicated that the geometric compatibility of grains and stress component on basal slip planes determine the propagation path of cracks. Twins are usually unfavorable for crack propagation, and the texture changes the stress component distribution in grains under the condition of unchanged loading. Both twins and textures increase the difficulty of crack propagation and improve the fatigue resistance at higher stress level.

The Role of Crystallographic Texture and Basal Plane Slip on Microstructurally Short Fatigue Crack Initiation and Propagation in Forged Billet and Rolled Bar Ti-6Al-4V Alloy

The Role of Crystallographic Texture and Basal Plane Slip on Microstructurally Short Fatigue Crack Initiation and Propagation in Forged Billet and Rolled Bar Ti-6Al-4V Alloy