Non-basal Slip Research Articles

Evolution of the Plastic Deformation Mechanisms in a Compressed Mg-10Gd-2Y-0.5Zr Alloy

This paper investigates the evolution of the deformation mechanisms in a homogenized Mg-10Gd-2Y-0.5Zr alloy ingot compressed at 300-500 °C and 0.1-20 s-1. It can be found that the basal slip and mechanical twinning are the major deformation mechanisms in the alloy compressed at 300 and 0.1-20 s-1. Increasing the testing temperature to 350 °C, basal slip, non-basal slip and mechanical twinning control the plastic deformation of the alloy compressed at 0.1-20 s-1. When the testing temperatures increase further to 400-500 °C, the mechanical twinning is replaced gradually by the local shear bands which are formed by dynamic recrystallization (DRX) grains (referred as transformation bands). The transformation bands have the trend to form the typical DRX microstructure with increasing the temperatures (might be caused by increasing testing temperatures or strain rates). Besides, the transformation bands can also be found in the sample compressed at 350 °C and 20 s-1when the temperature in the deformation alloy is high enough to activate non-basal slip and form DRX grains at local zone.

Texture evolution in oriented magnesium single crystals processed by equal channel angular pressing

Magnesium single crystals with different initial orientations were processed by single-pass equal channel angular pressing (ECAP) at 503 K. The texture in the crystals after ECAP was investigated by electron backscatter diffraction and compared to texture simulations based on a visco-plastic self-consistent model. The results show that orientation of [0001] and ⟨100⟩ axes in single crystals with respect to ECAP die geometry significantly influences activation of various deformation modes and resulting texture evolution. Depending on initial orientation, {102} twinning represents an important deformation mechanism at the early stages of forming because it causes reorientation of the matrix to orientations suitable for slip activity. Simulations indicate that, in addition to ⟨a⟩ basal slip, there is an activity of ⟨c + a⟩ pyramidal slip in all crystals. The ⟨a⟩ non-basal slip systems (⟨a⟩ prismatic and ⟨a⟩ pyramidal) were active in crystals containing structure components with [0001] axis parallel to the transverse direction of the ECAP die. Occurrence of this component in resulting texture caused better deformation behavior without crack formation.

Creep and Fracture Behavior of Peak-Aged Mg-11Y-5Gd-2Zn-0.5Zr (wt pct)

The tensile-creep and creep-fracture behavior of peak-aged Mg-11Y-5Gd-2Zn-0.5Zr (wt pct) (WGZ1152) was investigated at temperatures between 523 K (250 °C) to 598 K (325 °C) (0.58 to 0.66Tm) and stresses between 30 MPa to 140 MPa. The minimum creep rate of the alloy was almost two orders of magnitude lower than that for WE54-T6 and was similar to that for HZ32-T5. The creep behavior exhibited an extended tertiary creep stage, which was believed to be associated with precipitate coarsening. The creep stress exponent value was 4.5, suggesting that dislocation creep was the rate-controlling mechanism during secondary creep. At T = 573 K (300 °C), basal slip was the dominant deformation mode. The activation energy for creep (Qavg = 221 ± 20 kJ/mol) was higher than that for self-diffusion in magnesium and was believed to be associated with the presence of second-phase particles as well as the activation of nonbasal slip and cross slip. This finding was consistent with the slip-trace analysis and surface deformation observations, which revealed that the nonbasal slip was active. The minimum creep rate and time-to-fracture followed the original and modified Monkman-Grant relationships. The microcracks and cavities nucleated preferentially at grain boundaries and at the interface between the matrix phase and the second phase. In-situ creep experiments highlighted the intergranular cracking evolution.

The activation of 〈c + a〉 non-basal slip in Magnesium alloys

The activation of 〈c + a〉 non-basal slip system can help to improving the mechanical properties of Magnesium alloys. The activation conditions of 〈c + a〉 non-basal slip system in Mg alloys are reviewed, such as the addition of lithium elements, increasing temperatures, and regions of stress concentration, and so on. Moreover, the article summarizes our results from the work on Mg alloys using equal channel angle pressing with back pressure, and points out that 〈c + a〉 non-basal slip systems also become active much easier under hydrostatic pressure, which will help to open new window to explore the basic physics of the activation of non-basal slip.

Influence of rolling temperature on static recrystallization behavior of AZ31 magnesium alloy

Poor formability of rolled magnesium (Mg) alloys extremely restricts applications in form of sheets originating from formation of strong basal texture. Recently, we found that increasing rolling temperature from 723 to 798 K for a AZ31 Mg alloy can significantly improve stretch formability due to remarkable texture weakening after annealing. In this study, static recrystallization behaviors of AZ31 alloy sheets rolled at 723 and 798 K were investigated by electron backscattered diffraction analyses at different annealing stages in order to understand the origin of high temperature rolling on texture weakening. For both sheets, similar deformation microstructures with approximately the same types and fractions of twins exist in the as-rolled condition and recrystallized grains are mainly formed at pre-existing grain boundaries due to discontinuous recrystallization during subsequent annealing. However, only the basal texture of the latter remarkably weakens due to the formation of new recrystallized grains with well-dispersed orientations. Non-basal slips enhanced during high temperature rolling at 798 K are most likely responsible for the texture randomization as a result of rotations of recrystallization nuclei.

Effects of lattice parameter changes on critical resolved shear stress and mechanical properties of magnesium binary single crystals

Abstract The effect of lattice parameter changes on the critical resolved shear stress has been investigated by using single crystals of Mg binary alloys as well as a pure Mg. A series of tensile tests has been conducted at room temperature (RT), 150 °C, 250 °C and 350 °C under the strain rate of 5 × 10−3/s. The critical resolved shear stresses (CRSS) for basal slip were then found not to depend much on test temperatures, but binary solutions exhibited higher CRSS compared to that of pure magnesium. The binary alloys of Mg–Zn and Mg–Li exhibited the highest and lowest CRSS, having the largest and smallest change in lattice parameters, respectively. A modified size misfit parameter was then proposed in this study to express the friction mechanism in HCP crystals quantitatively, which originates from the atomic size misfit between Mg and solute atoms. The yield and ultimate tensile strengths of poly crystalline Mg were also obtained to compare with those of single crystals. The yield strengths of Mg poly crystals were found to depend on the CRSS for basal slip, while the ultimate tensile strengths to depend on the CRSS of non-basal slips. The effect of axial ratio changes on texture development has also been investigated in this study. The axial ratio, (c/a) was found to either increase or decrease by adding the solute atoms of Al or Li to develop a stronger or weaker basal pole, respectively. Addition of Zn exhibited, on the other hand, no effect on the texture development.

Effects of Solute and Second-Phase Particles on the Texture of Nd-Containing Mg Alloys

Hot-rolled, binary Mg-Nd alloys with compositions ≥0.095 at. pct undergo the texture weakening phenomenon that has been reported in a number of Mg–rare earth (RE) alloys. However, alloys with compositions ≤0.01 at. pct retain a strong basal texture typical of pure Mg and other Mg alloys. Measurements of intragranular misorientation axes obtained using electron backscatter diffraction (EBSD) show that more dilute alloys contain predominantly basal $$ $$ dislocations, while richer alloys contain primarily prismatic $$ $$ dislocations. It is suggested that this change in dislocation content is related to a change in the dynamic recrystallization (DRX) mechanism. Metastable second-phase Mg x Nd1–x intermetallic particles are present within the alloys, and an annealing study indicates that the alloys undergoing texture weakening have grain sizes well predicted by classical Zener drag theory. Even though the more dilute alloys also contain second-phase particles, they are not sufficient to induce pinning. The promotion of nonbasal slip and the reduction in grain boundary mobility due to Zener drag are suggested as controlling mechanisms that promote the observed texture weakening phenomena.

Deformation Microstructures and Textures of Cast Mg-3Sn-2Ca Alloy under Uniaxial Hot Compression

The influence of deformation conditions on the microstructure and texture evolution during hot compression of Mg-3Sn-2Ca (TX32) has been studied. Cylindrical samples were compressed uniaxially at different combinations of temperatures and strain rates in the ranges 300-500 oC and 0.0003-10 s-1. The crystallographic orientation information of the as-cast and deformed specimens was obtained by EBSD micro-texture analysis. Activation of different slip systems was investigated using Schmid factor analysis and the results reinforce the importance of non-basal slip for deformation at high temperatures. Samples deformed at 500 oC/0.1 s-1 resulted in a fully recrystallized microstructure with near random crystallographic texture.

Effect of microstructure on creep behaviour of cast Mg97Y2Zn1 (at.%) alloy

Abstract The creep resistance of the Mg 97 Y 2 Zn 1 (at.%) alloy in the as-cast condition and after a isochronal thermal treatment up to 560 °C is evaluated from 200 °C to 350 °C. The as-cast alloy shows a microstructure characterized by magnesium dendrites and a Long-Period Stacking Ordered Structure in the interdendritic regions. The heat treated alloy shows a fully lamellar structure within magnesium grains. In both cases, the stress dependence of the creep rate presents two different regions. For low temperature and/or high strain rates, the creep behaviour shows a high stress exponent ( n = 11) and high activation energy. The alloy behaves as a metal matrix composite where the magnesium matrix transfers part of its load to the LPSO-phase. Moreover, the lamellar structure within the magnesium grains in the thermal treated alloy results in an additional barrier against creep deformation. At high temperature and/or low strain rates, creep is controlled by non-basal dislocation slip. The cast alloy showed higher creep resistance than the heat treated alloy at low stresses since the diffusion of Zn and Y atoms nucleate at Shockley partial dislocations inhibiting their movements.

Plastic Deformation Behavior in Magnesium Alloy Single Crystals

Zn and Al are major alloying elements of Mg alloys. Main slip system of Mg is a basal slip and the CRSS increases with Zn or Al content. According to von-Mises criterion, five kinds of independent slip systems are required for uniform deformation, so it is necessary to activate non-basal slip systems to show good ductility. However, it has not become clear the effect of Zn or Al for non-basal slip systems yet. To investigate deformation behavior of magnesium crystal by non-basal slip, Mg-Zn and Mg-Al single crystals were stretched in the [110] direction and Mg-Zn single crystals were compressed in the [0001] direction. {112}<23> second order pyramidal slip was activated in Mg-0.1at%Zn and Mg-0.5at%Al. On the other hand, {101} twin was mainly activated in Mg-1.0at%Al alloy. Yield stress due to the pyramidal slip of magnesium decreased by 0.1at%Zn addition, however they increased with addition of aluminum..

OS0310 HCP金属単結晶における非底面すべりによる変形挙動

OS0310 HCP金属単結晶における非底面すべりによる変形挙動

W012006 LPSO構造におけるキング形成と回位密度に関する転位-結晶塑性解析

Magnesium alloys with a long-period stacking ordered (LPSO) structure show excellent mechanical properties such as high strength, relatively large ductility at room temperature. These superior properties are attributed to formation of kink-bands in LPSO-phase during material processing and the kinks may be expressed by dislocation and disclination. In order to reproduce numerically kink-bands, we extended a dislocation-based crystal plasticity model for HCP crystals developed by the authors previously to a new material model suitable for LPSO-phase. In this study, considering some characteristics of LPSO-phase in which non-basal slip and twin systems are hardly activated, we consider only the activation of basal slip system. Using this model, we carry out a two-dimensional FE analysis for a Mg-based LPSO-phase under a plane strain condition. Through this numerical simulation, the generation of kink-bands is computationally reproduced and the formation process of the deformation kinks is investigated in the view of the distributions of GN dislocation and GN incompatibility expressing dislocation pairs including the disclination. From a perspective of this investigation, we assume that the generation and the accumulation of GN dislocations and GN incompatibilities play a major role in formation of kink-bands. Furthermore, we observe a large angle of crystal rotation in kink-band region and we discuss about the effect of kink-bands on material strengthening.

The role of low-angle grain boundaries in multi-temperature equal channel angular pressing of Mg–3Al–1Zn alloy

Equal channel angular pressing was used to process an AZ31B magnesium alloy (nominally Mg–3Al–1Zn in wt%) at temperatures decreasing from 200 to 150 °C. The resulting microstructure was characterized by electron backscattered diffraction to reveal the role of low-angle grain boundaries in grain refinement. It was found that low-angle grain boundaries with misorientation angles lower than 5° are surrounded by regions of increased strain gradients, which can stimulate the generation of non-basal slip dislocations during the equal channel angular pressing at temperatures of approximately 150 °C. The strain gradients in the vicinity of the grain boundaries with misorientation angles in the range of 5°–10° were less frequent or were completely absent for high-angle grain boundaries with misorientation angles higher than 10°. This article also discusses the importance of low-angle grain boundaries for the generation of non-basal 〈c+a〉 dislocations needed for successful equal channel angular pressing of AZ31B at temperature of 150 °C.

In situ analysis of the tensile and tensile-creep deformation mechanisms in rolled AZ31

A rolled AZ31 alloy was tensile tested in a scanning electron microscope at 323K (50°C), 423K (150°C), and 523K (250°C) in order to analyze the deformation mechanisms in situ. Electron backscatter diffraction was performed both before and after straining. There was a significant difference in the activity of the various deformation modes at the three test temperatures and the mechanical anisotropy was considerably reduced with temperature. At 323K (50°C) extension twinning, basal, prismatic 〈a〉, and pyramidal 〈c+a〉 slip were active. Twinning disappeared above 323K (50°C), suggesting that the critical resolved shear stress (CRSS) of non-basal systems becomes less than that of twinning at T<423K (150°C). Plasticity was controlled at high temperature by a combination of basal and prismatic 〈a〉 slip. From 423K (150°C) to 523K (250°C), a transition occurs in the dominant deformation mechanism from basal+prismatic 〈a〉 to mainly prismatic 〈a〉 slip. This is consistent with a decrease of the CRSS of non-basal slip systems with increasing temperature. These results suggest that the observed drop in normal anisotropy with increasing temperature is likely to be the consequence of an increase in non-basal slip activity. In situ tensile-creep experiments, performed at approximately the yield stress at 423K (150°C), indicated that less slip and more grain boundary cracking occurs during creep deformation compared with the higher-stress tensile experiments.

Microstructure and Texture Evolution during Hot Rolling of Mg-9Li-7Al-1Sn Alloy for Aerospace Application

In the present work, Mg - 9wt. % Li - 7% wt. Al – 1 wt. % Sn (LAT971) alloy was cast and hot rolled at ~573K. Phase analysis of LAT971 revealed the presence of dual phase structure namely Mg-rich α- and Li-rich β-phase. After hot rolling, it was observed that dynamic recrystallization led to refinement of the α-phase grain structure. Significant crystallographic texture evolution, characterized by electron backscatterd diffraction, revealed increased activity of the non-basal (101 ̅0) slip plane after conventional hot rolling process.

Microstructure and properties of Mg-12Gd-3Y-0.5Zr alloys processed by ECAP and extrusion

Equal channel angular pressing (ECAP) processing and conventional extrusion (Ex) were applied to the Mg-12Gd-3Y-0.5Zr (wt%) magnesium alloy in order to evaluate the potential improvement in the mechanical properties. The ECAP experiment was conducted at 380 °C in a die having an included angle of 90° between two channels by the Bc route with the billet rotated by 90° about its longitudinal axis. Subsequently, some billets were processed by conventional extrusion at 300 °C. The microstructures were examined by X-ray diffraction (XRD), optical microscopy and transmission electron microscopy (TEM). The experimental results indicate that the grain size is refined effectively, but the basal planes are highly inclined (about 40°) from the extrusion axis introduced by ECAP, which impairs the grain boundary strengthening effect. The conventional extrusion, following the ECAP, can modify the grains in hard orientation. Based on grain boundary strengthening due to ECAP and texture strengthening due to Ex, the strength is improved effectively. The enhanced activity of the non-basal slips, due to the refined grains and the reduction in c/a ratio, is responsible for good ductility and high strain hardening rate in samples obtained by the two-step process.

Anomaly of the Work Hardening of Zn-Cu Single Crystals Oriented for Slip in Secondary Systems

Anomaly of the Work Hardening of Zn-Cu Single Crystals Oriented for Slip in Secondary SystemsThe copper alloyed (up to 1.5%) zinc single crystals oriented for slip in non-basal systems (orientation close to &lt; 1120 &gt;) were subjected to compression test within a range of temperatures of 77-293K. It has been stated, that Zn-Cu crystals exhibit characteristic anomalies of the thermal dependence of yield stress and of the strain hardening exponent. Both of them are related to the change in type and sequence of active non-basal slip systems: pyramidal of the 1storder {1011} &lt; 1123 &gt; (Py-1) and pyramidal of the 2ndorder {1122} &lt; 1123 &gt; (Py-2). The temperature anomaly of the yield stress results from the change of the slip from Py-2 systems to simultaneous slip in the Py-2 and Py-1 (Py-2 + Py-1) systems, occurring in the preyielding stage. On the other hand, sequential activation of pyramidal systems taking place in advanced plastic stage (i.e. the first Py-2 and next Py-2 + Py-1 systems) is responsible for temperature anomaly of strain hardening exponent. Increase in copper addition favors the activity of Py-2 systems at the expense of Py-1 slip, what leads to a drastic differences in plastic behavior of zinc single crystals.

Role of Solute in the Texture Modification During Hot Deformation of Mg-Rare Earth Alloys

Although conventional Mg alloys develop strong crystallographic textures during deformation that persist during annealing, the addition of rare earth (RE) elements can induce comparably weaker textures. The texture weakening effect is explored using hot-rolled Mg-Y alloys of a single phase to focus on the possibility of solute effects. Of the studied compositions, the richer alloys (≥0.17 at. pct) show the weakening effect, whereas the most dilute alloy (≤0.03 at. pct) does not. Electron backscattered diffraction (EBSD) analysis of intragranular misorientation axes (IGMA) indicate that the geometrically necessary dislocation (GND) content in dilute, hot-rolled alloys contain primarily basal 〈a〉 dislocations. At higher concentrations, the dislocations are predominantly prismatic 〈a〉 type. This change in the GND content suggests a change in dynamic recrystallization (DRX) mode. For example, nonbasal cross slip has been associated with continuous DRX. Furthermore, nonbasal slip might also promote more homogenous shear banding/twinning. Both of these mechanisms have been shown previously to give rise to more randomly oriented nuclei during DRX. Energy dispersive X-ray spectroscopy performed through transmission electron microscopy shows that Mg-Y exhibits significant grain boundary solute segregation, consistent with recent observations of solute clustering. Slow grain growth may be explained by solute drag. It is hypothesized that limited grain boundary mobility suppresses conventional discontinuous DRX, which has been shown to retain the deformation texture. The promotion of nonbasal slip and suppression of grain boundary mobility are proposed as solid solution-based mechanisms responsible for the observed texture weakening phenomenon in Mg rare earth alloys.

Influence of grain size on deformation mechanisms in rolled Mg–3Al–3Sn alloy at room temperature

► Deformation mechanisms for rolled AT33 alloy strongly depend on grain sizes. ► For grains of ∼6–22 μm, tensile deformation is mainly dominated by dislocation-slip. ► For grains of ∼22–41 μm, however, which turns to be mediated by deformation-twinning. ► H–P slope for slip-controlled-plasticity decreases due to non-basal slip activation. ► While it for twinning-mediated-plasticity increase owing to increase in twin numbers. The deformation mechanisms for rolled Mg–3Al–3Sn alloys were strongly dependent on grain sizes at room temperature. For grain sizes of ∼6–22 μm, the deformation in tension was mainly dominated by dislocation-slip, which, however, turned to be mediated by deformation-twinning for those of ∼22–41 μm. Moreover, as deformation proceeded, Hall–Petch slopes for slip-controlled-plasticity ( k S ) decreased gradually due to continuous activation of non-basal slips, while that for twinning-mediated-plasticity ( k T ) increased rapidly owing to increase in twin number and density, corresponding to grains below and above the medium-size of ∼22 μm respectively.

Mechanical Anisotropy of Extruded Mg-10Gd-2Y-0.5Zr Alloy

Tensile and compressive tests were performed on extruded Mg-10Gd-2Y-0.5Zr (mass fraction, %) alloy specimens with different tilt angles relative to extrusion direction. The microstructures were examined by X-ray diffraction (XRD), optical microscopy and scanning electron microscopy (SEM). Calculations of the orientation factors for basal slip were done for that existed texture. The results show that the alloy doesn’t show tensile-compressive yield strength asymmetry, and the highest flow stress is appeared along the extrusion direction. Meanwhile the extruded textures are much more randomized. The reason may be that the addition of rare-earth is benefit to activate the non-basal slips, especially pyramidal &lt;c+a&gt;slip. The reduction of c/a ratio is helpful to enhance the symmetry of the Mg crystal, which decreases the critical resolved shear stress of &lt;c+a&gt; slip.