Tension Twinning Research Articles

Effects of Anisotropy on the Microstructural Characteristics and Mechanical Behavior of Shock-Loaded of AZ31 Alloy

Effects of anisotropy on the microstructural characteristics and mechanical behavior of shock loaded of AZ31 magnesium alloy have been investigated. Using electron backscatter diffraction, tension twinning was observed in both shock loading directions along the normal (ND) and rolling directions (RD). Compression tests were carried out along ND and RD in both as-received and post-shock conditions. It indicated that the RD samples show a more notable hardening behavior compared to the as-received conditions. Moreover, it is postulated here that detwinningresults in a drop of strain-hardening rate for the ND samples under post shock reload conditions and tension twinning formed during the shock wave loading process leads to a significant moving left of the peak strain hardening rate for the RD samples under post shock reload conditions.

Orientational Analysis on Static Recrystallization at Tension Twins in AZ31 Magnesium Alloy

The deformation depends on twinning in Mg alloy, and twins will be the dominant recrystallization nucleation site. Tension twinning proceeds much more easily than compression twinning since its volume fraction is much higher than that of compression twins, which may have a promotive effect on the recrystallization to a certain degree. Based on the previous research on the static recrystallization at compression twins, the evolution of microstructure and texture in AZ31 magnesium alloy during its static recystallization at tension twins was further investigated; and the orientational characteristics of new grains formed at tension twins in the early stage of static recrystallization were analyzed by EBSD technique. The results showed that the strong basal texture was retained and weakened with no new texture component being detected during annealing. New grains were observed to nucleate preferentially at the intersections of tension twin variants or the intersection between tension twins and compression twins. Their orientations are relatively random and are strongly scattered from those of original tension twins or compression twins. A comparison of the recrystallization at tension twins and compression twins was further made.

Deformation Structure of Unidirectionally Compressed Ultrafine-Grained Mg-3Al-1Zn Alloy

Ultrafine-grained (UFG) Mg-3Al-1Zn (AZ31) alloys with gain sizes ranging from 0.46 to 3.22 μm were prepared by equal channel angular pressing (ECAP) and annealing. The deformation structure of UFG AZ31 alloy resulting from uniaxial compression was studied by optical and electron microscopy. The deformation was noted to proceed with the development of shear bands (SBs), which has not been reported in an UFG hcp metal. Characterization of these SBs was performed, and comparison with the SBs formed in UFG bcc and fcc metals was given. \( \{ 10\bar{1}2\} \) tension twins inside SBs were found in all specimens compressed, irrespective of the grain size. Discussion on the limiting grain size of twinning in the UFG AZ31 alloy is also given.

Room-temperature compressive deformation behavior of Mg–Zn–Ca alloy processed by equal channel angular pressing

The ultra-fine grained (UFG) Mg–5.25 wt.% Zn–0.6 wt.% Ca alloy was processed by equal channel angular pressing (ECAP), and the coarse grained (CG) alloy was prepared by subsequent annealing treatment. The microstructure, texture evolution and mechanical properties of the UFG and CG alloys during compressive deformation at ambient temperature were investigated. The results indicated that the dislocation slip was dominant in the UFG Mg alloy, and the compressive yield stress (CYS) of the UFG alloy was higher than that of the CG alloy due to grain refinement. In the CG Mg alloy, the activation of tension twinning also occurred in addition to basal slip, the work hardening rate was increased because of twin-induced additional hardening. Dynamic recrystallization (DRX) was observed in the CG alloy compressed to large strains at ambient temperature. Most of the {0 0 0 2} planes in both UFG and CG Mg alloys were rotated gradually to be perpendicular to the compressive direction (CD).

Deformation twinning in polycrystalline Co during room temperature dynamic plastic deformation

Strain rate has an important influence on the deformation of metals. Dynamic plastic deformation (DPD) causes a face-centered cubic polycrystalline Cu to deform by twinning. In this paper, we report that for hexagonal close-packed polycrystalline Co, deformation twinning during DPD is similar to that during normal compression. Various tension and compression twins, as well as double twins, take place during deformation. The results also show that the tension and compression twinning can take place in one grain.

ANALYSIS OF THE STATIC RECRYSTALLIZATION AT TENSION TWINS IN AZ31 MAGNESIUM ALLOY

Due to the poor plasticity of magnesium alloys at room temperature(about 15%), twinning plays an important role in the deformation of magnesium alloys,and twins will be the dominant recrystallization nucleation sites.There are at least two types of twinning in magnesium:the {10(?)2}-type tension twinning and the {10(?)1}-type compression twinning.Tension twinning proceeds much more easily than compression twinning since its volume fraction is much higher than that of compression twins,which may have a promotion effect on the recrystallization to a certain degree. Based on the previous research on the static recrystallization at compression twins,the evolution of microstructure and texture in AZ31 magnesium alloy during its static recystallization at tension twins was futher investigated;and the orientational characteristics of new grains formed at tension twins in the early stage of static recrystallization were analyzed by EBSD technique.The results showed that tension twins played only a subordinate role in recrystallization nucleation and suppressed recrystallization rate,thus failed to refine grain size effectively.The strong basal texture was retained and weakened with no new texture component being detected during annealing.New grains were observed to nucleate preferentially at the intersections of tension twin variants or the intersections between tension twins and compression twins.Their orientations were relative random and are strongly scattered from those of original tension twins or compression twins.A comparison of the recrystallization at tension twins and compression twins was made.

Anisotropy of dynamic behavior of extruded AZ31 magnesium alloy

Abstract Textures always exert an intensive influence on the deformation and fracture of magnesium alloys. A hot extruded AZ31 alloy with intensive texture was selected for impact tests with the impact direction parallel to the respective normal, extrusion and transverse directions (ND, ED and TD) of the initial extrusion plate. The results showed that the stress–strain response of the alloy is highly anisotropic and sensitive to the strain rate. The maximum flow stress can reach nearly as high as 600 MPa for TD impact. Microstructure responses show that when impacted along normal direction, {1 0 −1 1}–{1 0 −1 2} double twins is common in large grains, however, small grains show {1 0 −1 2} tension twins. After impacted along transverse direction, grains of the alloy are evidently refined and the texture changes from {0 0 0 1}〈1 1 −2 0〉 to that with {0 0 0 1} pole in TD direction. The stress–strain, microstructure and orientation responses could well be interpreted with the Schmid factor distributions.

Twinning characteristics in textured AZ31 alloy under impact loading along specified direction

A plate of hot extruded AZ31 alloy with some large grains was selected for impact tests. The occurrence of twinning was investigated. It was found that when impacted along the normal direction (ND) of the plate, {10–11}–{10–12} double twinning is commonly observed in large grains, and {10–12} tension twinning can also be activated although their c-axis is close to the compression direction. In small grains, {10–12} tension twins are favored in which case the extension strain component is parallel to the c-axis. In addition, twins with < 11–20> 75° misorientation to the parent matrix were also observed.

Twinning in weld HAZ of ZK21 commercial magnesium alloy

The microstructure and properties of Mg ZK21 laser beam weld without filler were researched using optical microscopy (OM), electron microscopy and mechanical test. The results show that the fracture strain of the joints after laser beam welding reduces by about 10.7% at room temperature. By means of laser beam welding, the fusion zones contain tensile RS, while the base material far away from the fusion line is under balancing compressive RS. The microstructures of the weld were characterized by a narrow heat affected zone and twins. Significant {10 ▪ 2} tension twins occur in the weld HAZ during laser welding processing. Due to the influence of temperature field and stress on morphologies, most of twins form twinning bands, which are nearly parallel to the welding direction.

Microstructural evolution of Mg–Al–Ca–Sr alloy during creep

To provide new insight into microstructural development during creep, sequential EBSD (electron backscatter diffraction) orientation maps were obtained from fixed surface areas at intervals in creep tests of a high-pressure die-cast Mg–8.5Al–1Ca–0.3Sr alloy. Samples were tested at 200 °C at various stress levels to achieve power-law creep behavior. In all specimens, steady-state creep was marked by the formation of tension twins. At intermediate stress levels, specimen failure was initiated by the formation of defects at boundaries with high crystallographic misorientation. However, at high stress levels, the failure mechanism was assisted by the crack formation at interdendritic boundaries with low crystallographic misorientation.

Dependence of deformation mechanisms on grain orientations and their changes calculated based on Sachs model in magnesium alloy AZ31

During deformation, the orientation of a grain influences not only the deformation mechanisms (slip or twinning) and the specific selection of activated slip or twinning systems for that grain, but also the kinetics of different types of transformation. Schmid factor analysis was applied to determine the orientation dependency of deformation mechanisms in magnesium alloys AZ31 in this work. The orientation changes after the operation of the specific deformation mechanisms were also calculated based on Sachs model. It was found that different deformation mechanisms proceeded differently according to theoretical predictions. Basal slip occurred when basal planes of grains were tilted toward ND around TD. Prismatic slip dominated when basal planes were approximately perpendicular to TD. Calculation results also indicated that the operating of pyramidal 〈a〉 slip can not be neglected. {10 \( \bar 1 \)2} twinning was favorable when basal planes were approximately normal to RD and {10 \( \bar 1 \)1} twinning was analyzed to be related to the grains with basal orientations. The operating of 〈a + c〉 slip could greatly suppress the activating of twinning by our Schmid factor analysis. Basal orientations with TD and RD scattering can favor basal slip and tension twinning, respectively, after the operation of compression twinning based on the Schmid factor calculations.

Electron Backscatter Diffraction Mapping of Microstructural Evolution of Pure Magnesium during Creep

To provide new insight into microstructural development during creep, sequential electron backscatter diffraction (EBSD) orientation maps were obtained from fixed surface areas during interrupted creep tests of extruded pure magnesium. Samples were tested under two distinctly different conditions, i.e., low-temperature power-law creep and high-temperature power-law creep. Three aspects of microstructural evolution during creep were observed. In both specimens, transient creep was marked by the elimination of tension twins formed during the extrusion process. As steady-state creep rates were obtained, both specimens exhibited an ever-decreasing contribution by basal slip accompanied by an ever-increasing contribution by prismatic slip. In the low-temperature specimen, prismatic slip was manifested by the formation of a network of new fine subgrains along original grain boundaries, resulting in a bimodal grain size distribution. However, the high-temperature specimen developed linear prismatic slip lines while maintaining its original grain size.

Analyses on Compression Twins in Magnesium

The orientation relationships between the compression twins and their matrices in magnesium were investigated using EBSD technique. Schmid factor analyses were applied to determine the orientation dependency of compression twins and the condition for double twinning. Results show that thin compression twin bands of {1011) type lose easily their exact twin relation of 56°(1120) to ∼40° due to the instability of twin orientation and immobility of twin boundaries. And the basal orientation with TD-rotation tends to induce tension twinning in compression twins.

A Preliminary Analysis on Compression Twins in Magnesium

Basal slip and tension twinning are dominant deformation mechanisms of polycrystalline magnesium at low temperature. However, fracture originates mainly from compression twins or shear bands developed from compression twins. This work compared firstly the morphological difference of two types of twins. Then, the dependence of different deformation mechanisms on initial orientations is computed by Schmid factor analysis and compared with measured matrix orientations of twins. Finally, orientation relationships of compression twins with matrices are determined using EBSD technique and compared with theoretical value.

Large-scale atomistic study of core structures and energetics of dislocations in hexagonal close packed metals

Using two different kinds of many-body potentials as well as the Lennard-Jones potential for hexagonal close packed metals, we have found that c+a edge dislocations with dislocation lines along 100 can split onto the basal plane, forming a non-planar sessile structure. The `type I' undissociated dislocation core, observed in previous papers, is shown to be stable only for small simulations. The observed dissociated core structure has a large distorted region that we interpret as a (111) twin nucleus, which may help the formation of (111) tension twins. We also find that this core structure is lower in energy than the previously observed glissile `type II' configuration, dissociated on the (112) plane. The sessile splitting of the core gives rise to non-Schmid behaviour, with the twin nucleus expanding under c-axis tension, and contracting under compression.

Defect Structure of Strained Heteroepitaxial In(1—x)Al(x)P Layers Deposited by MOVPE on (001) GaAs Substrates

Weak-beam, large angle convergent beam electron diffraction and high resolution transmission electron microscope experiments have revealed, that after strain relaxation due to plastic deformation dislocation networks can be observed in In(1—x)Al(x)P heteroepitaxial layers grown on (001) GaAs substrates under compressive stress. The 60° slip dislocations are mostly dissociated into partials of Shockley type whereas in the particular case of layers grown under tension twins are predominantly formed by successive nucleation and slip of 90° Shockley partials on adjacent {111} glide planes lying inclined to the (001) surface. When a few 90° Shockley partials pile up during extension of twins, then planar incoherent twin boundaries with {112} coincidence planes have been formed during strain relaxation. Due to the space group symmetry ((InAl)P belongs to the space group F4-3m) there is a striking asymmetry in defect formation, i.e. defect nucleation and slip on the planes (111) and (1-1-1) slip of the [1-10] zone are preferred to nucleation and slip on the {111} planes of the [110] zone. Apparently, the occupacy of the atomic sites in the dislocation core with either group-III or group-V atoms is responsible for this behaviour. The nature of the defects implies that their spontaneous nucleation should have taken place at the growing surface. Under tensile strain the 90° Shockley partial is nucleated first and the 30° one trails. Under compressive strain this sequence is reversed. It is evident, for dissociated dislocations lying at the interface always the 30° partial, i.e. the partial with less mobility or with higher friction force, is detained near or directly in the interface. Thus, in layers grown under tension the stacking fault associated with the dissociated 60° dislocation lies inside the GaAs substrate. For layers grown under compression it is located inside the ternary layer.

Investigation of preferred orientation on zinc by galvanomagnetic effects

The mechanical responses of hexagonal close-packed (HCP) materials are highly anisotropic, as well as strain rate and temperature-dependent, because of inherently asymmetric slip and twinning in constituent crystalline grains. How slip and twinning are affected by strain rate/temperature and how they alter the deformation response of HCP materials are still not well understood. To provide greater insights, a novel crystal plasticity model is proposed to describe the mechanical behavior and microstructure evolution of single-crystal and polycrystalline HCP materials under different loading conditions. A new flow rule is developed, which incorporates the effects of strain rate and temperature on the activation of specific deformation mechanisms. Both dislocation density-based and empirical hardening laws are adopted to describe the overall hardening contributed by slip, twinning, and their interaction, taking into consideration loading conditions and the difference between tension and compression twinning. The proposed model is implemented in the commercial FE package ABAQUS via a user-defined subroutine. The mechanical response and microstructure evolution of single-crystal titanium with different orientations, and polycrystalline titanium comprising grains with 350 random orientations, subjected to deformation at different strain rates and temperatures, are numerically explored. The numerical results display good agreement with published experimental data. The simulations indicate that an increase in strain rate or decrease in temperature will retard the activation of slip systems, thereby promoting twinning instead. A higher twinning volume fraction leads to a larger amount of lattice reorientation, more pronounced hardening, and higher stress levels. This effort has yielded a greater understanding of the influence of strain rate and temperature on the mechanical response of HCP materials and facilitated mesoscale simulations that couple slip and twinning.