Non-basal Slip Research Articles

On the High Formability of AZ31-0.5Ca Magnesium Alloy.

In this work, we investigated the effect of Ca on the formability of the AZ31 Mg alloy. For this purpose, the microstructure, texture, mechanical properties and formability of AZ31 Mg alloy samples containing 0.5 wt. % Ca (AZ31-0.5Ca) were studied. For comparison, the performance of Ca-Free AZ31 alloy samples with similar grain size was also investigated. In addition, formability of this alloy was reached at a high punch speed. The results of this work showed that the addition of 0.5 wt. % Ca can enhance the formability of the AZ31 alloy, which was three times greater than that of the Ca-Free AZ31 alloy. The improved formability was attributed to the formation of (Mg,Al)2Ca particles (~1 μm), which, in turn, contribute to reducing the intensity of the strong basal texture during the primary processing of the alloy. The in-grain misorientation axis analysis determined by electron back-scattered diffraction and critical resolved shear stress calculations carried out by the viscoplastic self-consistent model showed that the non-basal slip systems could be activated in the AZ31-0.5Ca alloy.

Study on mechanical behaviors and theoretical critical shear strength of cold-rolled AZ31 alloy with different Li additions

The as-extruded Mg-xLi-3Al-1Zn (x = 1, 3, 5; wt%) sheets (LAZ131, LAZ331 and LAZ531 respectively) were performed with different reductions in multi-pass cold-rolling process. The changed mechanical behaviors of cold-rolled AZ31 alloys with Li additions were investigated via the electron-backscattered diffraction (EBSD) and texture. The results indicated that the main deformation modes of LAZ131 alloy were twinning and basal slip during cold rolling process. However, for the LAZ331 and LAZ531 alloy, the prismatic slip, {112̅2} pyramidal slip, {101̅2} extension twining, {101̅1} contraction twinning sequentially dominated in the deformation with increased rolling reduction. Then, the theoretical critical shear strength (τmax) (reflecting the critical resolved shear stress, CRSS) of Mg and Mg-Li crystal was characterized by first-principles calculation. The direct consequences from calculations indicated that the value ofτmax decreased from 1571 MPa to 1276 MPa in pyramidal slip systems, while that increased from 114 MPa to 177 MPa in basal slip, due to doping Li to Mg single crystal. The decrease in τmax ratio between non-basal and basal may promote an easy activation of non-basal slip and show different deformation modes of cold-rolled AZ31 alloys with different Li additions.

Effect of temperature on microstructure and texture evolution of Mg-Zn-Er alloy during hot compression

Abstract The microstructure and texture evolutions in Mg-Zn-Er alloy during hot compression were investigated by using optical microscope (OM), field emission scanning electron microscope (EBSD) and transmission electron microscope (TEM). The results indicate that the temperature plays an important role in dynamic recrystallization (DRX) mechanism. The twin dynamic recrystallization (TDRX) is induced at a strain of 0.6 because of the activation of non-basal slip «a+c» dislocations at 200 °C. Meanwhile, the continuous DRX (CDRX) occurs at 350 °C, which is identified by the typical necklace-like structure around the residual initial grains. The DRX contributes to the modification of texture significantly. The tension twins are responsible for the weak texture at 200 °C. Meanwhile, the decrease in the basal texture is ascribed to the DRX sites which transfer from twin boundaries to initial grain boundaries as the temperature is increased from 200 to 350 °C.

Microstructure and texture development during solid consolidation recycling of Ti-6Al-4V

Ti-6Al-4V machining chips were recycled using equal channel angular pressing (ECAP). The microstructural and texture evolution of the recycled Ti-6Al-4V have been investigated using scanning electron microscopy, electron backscattered diffraction and transmission electron microscopy. For samples consolidated at 500 °C with a back-pressure of 100 MPa, the as-pressed density reached up to 99.9% after 8 passes. Pronounced grain refinement was also observed with increasing number of passes. The morphology of the grains has been changed from elongated and coarse to equiaxed and ultrafine as the number of passes increases. Strong textures were also introduced during multiple passes via Bc route. Texture has been developed with basal planes parallel to the inclination direction which is at 21° of the extrusion direction. After 4 and 8 passes, basal planes were rotated towards the transverse direction. No oxide can be detected at the chip-chip boundaries when the Ti-Al-4 V machining chips was consolidated at 500 °C. When the sample was processed at 550 °C, significant grain growth and clear oxide layers at the chip-chip interface were observed. In addition, the c-axis were rotated towards the longitudinal direction due to the non-basal slip activity. TEM observation revealed the <a→+c→> dislocations presence in the 550 °C ECAP-processed sample.

Grain refinement, texture, and mechanical properties of a magnesium alloy after radial-shear rolling

Microstructure (including its uniformity), texture, and mechanical properties of Mg-Al-Zn-Mn alloy billets after radial-shear rolling (RSR) were investigated. RSR with the temperature in the interval 420–140 °C and true strain up to 2.63 was shown to lead to grain refinement down to 1.5–3.5 μm (as determined by optical microscopy). The microstructure was found to be reasonably uniform over the transversal cross-sections of the billets. The transmission electron microscopy analysis also revealed submicron sized grains and the formation of two types of particles: Mg17Al12 particles 300–500 nm in size and MgAl nanoparticles of different shapes 20–40 nm in size. Increments in strain and step-wise decrease in deformation temperature with the number of RSR passes was found to lead to a transformation of the initial basal texture to the prismatic texture. The RSR of the Mg-Al-Zn-Mn magnesium alloy was demonstrated to result in significant strength and good ductility owing to a fortunate interplay between fine grain structure and favorable texture promoting activity of non-basal slip systems.

Deformation mechanisms and recrystallization behavior of Mg-3Al-1Zn and Mg-1Gd alloys deformed by electroplastic-asymmetric rolling

AZ31 Mg and Mg-1Gd alloys were subjected to electroplastic rolling (ER) and electroplastic-asymmetric rolling (EASR). Due to the additional shear stress in EASR, different deformation behaviors, including twinning, dislocations and DRX, were witnessed in EASR. For AZ31, significant grain refinement was achieved in the EASR sample. The refined grains with weakened texture contributed to the largely enhanced tensile ductility of the EASR sample. While for Mg-1Gd, the formation of extension twins was booming and propagated throughout the EASR sample, which was considered to be attributed to the high participation of non-basal slip during deformation. Extension twins and their related recrystallization strongly influenced the texture evolution and mechanical properties of the EASR sample, so that both the tensile ductility and strength of the EASR sample were improved.

Effect of Ag, In and AgIn Alloying Additions on Microstructure and Texture of Mg-3Al-1Zn Alloy during Multi-Pass Warm Rolling

In the present investigation the rolling response, microstructure and texture evolution of four Mg alloys during multi-pass warm rolling were evaluated. The nominal composition of the base alloy (alloy-1) was Mg-3Al-1Zn. The alloy-2, 3 and 4 were developed by separate additions of non-rare earth elements Ag and In, and a master alloy 85Ag15In (wt.%) to make target compositions Mg-3Al-1Zn-0.5x, (x = Ag ,In, AgIn). Samples from all four alloys were subjected to multi-pass warm rolling at 300 °C to accumulative reductions of 50, 75 and 90% with 8 minutes inter-pass annealing. For all four alloys, crack free sheets of less than 1 mm thickness were produced successfully with true strain corresponding to 90% reduction. The as-cast microstructures revealed second phase particles at grain boundaries and grains interiors for all alloys. A slight scatter in the size of the deformed grains was observed for alloy-1, 2 and 3 after rolling reductions of 50, 75 and 90%. However, a sustained decrease in grain size with increasing the rolling reductions was only observed in alloy-4, despite inter-pass annealing. XRD macro-texture results of alloy-2 and 3 presented very strong basal texture showing almost concentric contours around normal direction (ND). Such strong sheet texture is attributed to a preferential alignment of basal planes parallel to the sheet surface. On the other hand alloy-1 and alloy-4 revealed a weaker texture with basal poles spread more towards transverse direction (TD) as compared to rolling direction (RD) and may be due to the activation of some &lt;c+a&gt; non-basal slip and twinning in addition to basal slip under the same processing parameters.

Effect of alloying elements on room temperature stretch formability in Mg alloys

The effect of alloying elements on room temperature stretch formability and its deformation mechanism was investigated using pure magnesium and its binary alloys containing an element of Ag, Al, Ca, Li, Mn, Pb, Sn, Y and Zn, respectively. All of the alloys as well as pure magnesium were produced by extrusion and had a similar average grain size. The alloying elements clearly affected stretch formability as measured by Erichsen testing. The addition of manganese or yttrium element played a role in improving formability; on the other hand, most of the alloying elements did not improve formability, as exhibited by their limited dome height values, which were similar to or lower than that of pure magnesium. This resulted from their differences in deformation mode. An intragranular misorientation axis analysis using electron back-scatter diffraction results revealed that the Mg-Y alloy had a high fraction of grains amenable to non-basal dislocation slips, which is a helpful deformation mode for improving formability. In contrast, pure magnesium and the Mg-Mn alloy, which had a high limited dome height, did not have a high fraction of such grains. Grain boundary sliding compensated for the lack of <c>-component, such as activating non-basal dislocation slips.

Effect of aluminium on microstructure, mechanical property and texture evolution of dual phase Mg-8Li alloy in different processing conditions

The present study investigates the possibility of enhancing the strength with ductility of dual-phase magnesium (Mg)-8 lithium (Li) alloy by the combination of alloying addition aluminium (Al) and suitable thermo-mechanical processing. Microstructural evolution, phase analysis and texture studies were performed for Mg-8Li-xAl (x=0, 2,4 and 6) alloys with the help of scanning electron microscopy (SEM) and X-ray diffraction (XRD). It is understood from the texture studies that the addition of Al to the Mg-8Li alloys activates the non-basal slip at room temperature. In turn, it facilitates the recovery process, hence a substantial improvement in plastic deformation after annealing of the alloys is observed. This is attributed to non-basal slip activity at room temperature. The presence of fine intermetallic compounds in the annealed Mg-8Li-xAl (x=4 and 6) alloys leads to the higher ultimate strength (193±7MPa and 267±9MPa) and ductility (20% and 17%), respectively.

Effects of carbon nanotubes on twin and texture evolution of magnesium matrix composite during compression process

A compression test is performed to investigate the effects of carbon nanotubes (CNTs) on twin and texture evolution of the 1.0 wt.% CNTs reinforced magnesium matrix (CNTs/Mg) composite. The compression process consists of elastic, yield and hardening stages, and the number of twin significantly increases at the yield stage. During the compression process, the crystal rotation and non-basal slip caused by twin shear alternately dominate the deformation of the 1.0 wt.% CNTs/Mg composite. The CNTs existing at grain boundaries lead to stress concentration, and the stress concentration stimulates the formation of twins at the yield stage. The formed twins interact with dislocations, result in crystal rotation and promotion of the non-basal slip system at the hardening stage under the effect of twin shear, thus, improve the toughness of the CNTs/Mg composite.

Microstructure, mechanical properties and stretch formability of Mg-3Al-0.5Ca-0.2Gd alloy processed at various finish rolling temperatures

Effects of various finish rolling temperatures on the microstructure, texture, mechanical properties and stretch formability of rolled and annealed Mg-3Al-0.5Ca-0.2Gd (wt%) alloy were investigated in this paper, and it was found that compared with grain size and second phase particles, the basal textures, tensile properties and stretch formability Mg-3Al-0.5Ca-0.2Gd alloy are more sensitive to the increasing finishing rolling temperature. For the rolled and annealed Mg-3Al-0.5Ca-0.2Gd alloy, their grains barely grow up and second phase particles are slightly coarsened, while their basal poles are obviously weakened and tilted with increasing finish rolling temperature. Consequently, the weakened and RD-tilted basal textures are beneficial to the gradually improved elongation and stretch formability of Mg-3Al-0.5Ca-0.2Gd alloy. It is investigated that the gradually activated non-basal slips, e. g. 〈c 〉, 〈c + a〉 dislocations due to the increasing finish rolling temperature could contribute to the weakened RD-tilted textures in rolled and annealed Mg-3Al-0.5Ca-0.2Gd alloy.

Influence of quasicrystal I-phase on twinning of extruded Mg-Zn-Y alloys under compression

The interaction of the I-phase with twins during compression has been studied in a Mg-6Zn-1Y (wt.%) alloy using the combination of Synchrotron Radiation Diffraction and Acoustic Emission experiments during compression tests. The I-phase occurs as coarse particles at grain boundaries and nanosized precipitates within magnesium grains. Their interaction with twins depends on their shape and size. The presence of coarse I-phase particles with higher Young Modulus requires an additional stress for the activation of twinning since they induce tensile residual stresses in the magnesium matrix. Besides twinning, the basal and non-basal slip systems are also activated. Non-basal slip is observed after macroscopic yield stress within twins. The fine I-phase precipitates within grains interacts with the dislocations and the twins. In the latter case a back-stress arises, which acts against the compressive applied stress. This back stress hinders the twin growth more than the effect of the coarse particles.

Combination of in-situ diffraction experiments and acoustic emission testing to understand the compression behavior of Mg-Y-Zn alloys containing LPSO phase under different loading conditions

The effect of the orientation of the non-recrystallized grains (non-DRX) and the LPSO phase on plasticity in extruded MgY2Zn1 alloy with a bimodal grain structure have been studied in-situ using the combination of synchrotron diffraction and acoustic emission techniques during compression tests. The adaptive sequential k-means (ASK) procedure was applied to analyze the acoustic emission signal. This method can successful separate the signal for each possible deformation systems. Combining both techniques, the deformation mechanisms that take place during the compression tests under different loading directions have been distinguish. Independently of the loading direction, the beginning of the macroscopic plasticity is always controlled by the activation of basal slip system in the DRX grains. However, highly oriented non-DRX grains and LPSO phase have a strong influence over the compressive deformation of the MgY2Zn1 alloy. The basal planes in the non-DRX grains were oriented parallel to the extrusion direction (ED). Thus, the activation of the {101¯2}〈101¯1¯〉 extension twinning was found to be significant only in the ED mode. In the other two loading cases, TD and 45, the activation of non-basal slip systems has been detected. On the other hand, the reinforcing effect of the elongated LPSO phase is the most effective, if the loading axis is aligned with the fiber direction (extrusion direction) since the load transfer mechanism is more effective. In this loading case, the LPSO can be plastically deformed due to kinking. This deformation mode has been isolated by the ASK procedure and characterized by in-situ scanning electron microscopy.

Improved Formability of Mg-AZ80 Alloy under a High Strain Rate in Expanding-Ring Experiments.

Magnesium alloys offer a favored alternative to steels and aluminum alloys due to their low density and relatively high specific strength. Their application potentials are, however, impeded by poor formability at room temperature. In the current work, improved formability for the commercial magnesium AZ80 alloy was attained through the application of the high-rate electro-magnetic forming (EMF) technique. With the EMF system, elongation of 0.2 was achieved while only 0.11 is obtained through quasistatic loading. Systematic microstructural and textural investigations prior, during and post deformation under high strain-rate experiments were carried out using electron back-scattered diffraction (EBSD) and other microscopic techniques. The analysis indicates that enhanced elongation is achieved as a result of the combination of deformation, comprising basal and non-basal slip systems, twinning and dynamic recrystallization. An adopted EMF-forming technique is tested which results in enhanced elongation without failure and a higher degree of dynamically annealed microstructure.

Deformation Mechanisms and Formability Window for As-Cast Mg-6Al-2Ca-1Sn-0.3Sr Alloy (MRI 230D)

The hot deformation characteristics of MRI 230D alloy have been evaluated in the temperature range 260-500 °C and strain rate range 0.0003-10 s−1, on the basis of processing map. The processing map exhibited two domains in the ranges: (1) 300-370 °C and 0.0003-0.001 s−1 and (2) 370-480 °C and 0.0003-0.1 s−1. Dynamic recrystallization occurs in the both domains with basal slip dominating in the first domain along with climb as recovery process and second-order pyramidal slip dominating in the second with the recovery by cross-slip. In Domains (1) and (2), the apparent activation energy values estimated using the kinetic rate equation are 143 and 206 kJ/mole, respectively, the first one being close to that for lattice self-diffusion confirming climb. It is recommended that the alloy is best processed at 450 °C and strain rates less than 0.1 s−1, where non-basal slip and cross-slip occur extensively to impart excellent workability. The alloy exhibits flow instability in the form of adiabatic shear band formation and flow localization at lower temperatures and higher strain rates. Forging of a cup-shaped component was performed under various conditions, and the results validated the predictions of the processing map on the workability domains as well as the instability regimes.

Weakened anisotropy of mechanical properties in rolled ZK60 magnesium alloy sheets with elevated deformation temperature

The rolling direction (RD) and the transverse direction (TD) samples were obtained from an as-rolled ZK60 magnesium alloy sheet with strong anisotropy of initial texture and their mechanical properties were tested at various deformation temperatures. Meanwhile, the microstructure and texture of these samples after fracture were investigated. Results revealed that a higher flow stress along the RD than that along the TD at room temperature were ascribed to the strong anisotropy of transitional texture, and this texture effect was remarkably weakened with the increase of deformation temperature. Deformation structure was dominant at 100°C, and was replaced by dynamic recrystallization structure when the deformation temperature increased to 200°C and 300°C. The texture presented a strong texture (transitional texture in the RD sample and basal texture in the TD sample) at 100°C, but its intensity visibly decreased and texture components became more disperse at 200°C and 300°C. These microstructure and texture results were employed in conjunction with calculated results to argue that raising deformation temperature could increase the activity of non-basal slip by tailoring the relative critical resolved shear stress of each deformation mode and finally result in low texture effect on mechanical anisotropy.

Role of pre-width reduction in deformation behavior of AZ31B alloy during break-down rolling and finish rolling

Present study aimed at investigating the effect of introducing pre-width reduction (Pre-WR) on microstructure evolution and rolling formability variation during both the break-down rolling and finish rolling of AZ31B alloy. Different rolling routes relevant to the above two rolling processes were performed and compared. Results show that pre-induced twins by pre-rolling (8% thickness reduction) can significantly reduce the edge-cracking depth level during subsequent super-high reduction rolling (SHRR) (80% thickness reduction) through replacing the formation of shear bands by the promotion of the twinning-induced recrystallization to coordinate the severe deformation. However, the dominant behavior of twinning and twinning-induced DRX within the edge region induced a unique partial recrystallization with coarse grains and further a poor rolling formability in SHRR. Introducing more {101¯2} tensile twins at the edge by Inter-WR (a Pre-WR type with 10% width reduction) before the SHRR can significantly refine the grains and obtain almost a recrystallized homogeneous microstructure. It can also weaken the basal texture of the edge material and further decrease the number/depth level of edge crack by enhancing the activity of both non-basal slips and dominant continuous dynamic recrystallization (CDRX). During finish rolling, heavy twins and shear bands resulted in the larger edge crack level. The improvement of Pre-WR on rolling formability can be inherited to the strip finishing rolling process by evidently reducing the twins and shear bands.

Role of loading direction on compressive deformation behavior of extruded ZK60 alloy plate in a wide range of temperature

The plastic flow behavior of extruded Mg-6Zn-0.6Zr (ZK60) alloy plate in T5 treated condition was characterized in the extrusion direction (ED) and along plate normal (ND) in the temperature range of 25 °C–500 °C, with a view to study the effect of loading direction. Cylindrical specimens were compressed parallel to ED and along ND, and flow curves and specimen shapes were recorded. The flow behavior at temperatures >200 °C in the strain rate range 0.0003–10 s−1 was analyzed using the approach of processing map. The plate texture could be described as basal planes parallel to the plate surface with <101¯0> as ED. The ED specimens deformed at temperatures <175 °C showed limited plastic strain leading to shear cracks, which were attributed to the restriction of basal slip and activation of limited non-basal slip. The ND specimens showed more plastic flow due to the occurrence of {101l} slip in <112¯3> direction in the absence of both basal and prismatic slip. The processing map for the ED specimens exhibited three domains in the ranges: (1) 230 °C–375 °C and 0.0003 s−1–0.002 s−1, (2) 410 °C–500 °C and 0.0003 s−1–0.008 s−1, and (3) 370 °C–430 °C and 2 s−1–10 s−1. In Domains 1 and 3, dynamic recrystallization (DRX) occurred due to prismatic slip and recovery by lattice-diffusion controlled climb, and first-order pyramidal slip and recovery by grain-boundary self-diffusion controlled climb, respectively. In the second domain, DRX occurred by second-order pyramidal slip and cross slip, which was followed by grain boundary sliding leading to superplasticity. The processing map for ND specimens exhibited an additional domain at lower temperatures and strain rates (225 °C–350 °C and 0.0003 s−1–0.1 s−1) which was due to easy occurrence of {101l} <112¯3> slip with lattice self-diffusion controlled climb. The map for ND orientation was free from flow instability regime unlike that for ED and this was the result of extensive occurrence of slip on {101l} planes in <112¯3> direction.

Effect of tensile direction on mechanical properties and microstructural evolutions of rolled Mg-Al-Zn-Sn magnesium alloy sheets at room and elevated temperatures

In the present work, Mg-9Al-1Zn-0.4Sn (AZT910) magnesium alloy sheets with a fine-grained structure (∼5 μm) and tilting basal texture were prepared by a 16-passes rolling process. Effect of tensile directions on properties and microstructural evolutions of the AZT910 alloy sheet at different temperatures, i.e. room temperature, 200 and 300 °C, respectively, were investigated by using three kinds of tensile specimens (rolling direction (RD), transverse direction (TD) and 45° towards RD). Experimental results showed that tensile properties exhibited a strong dependence on tensile directions at room temperature and 200 °C, while a negligible dependence at 300 °C. The anisotropic tensile properties imply that the deformation system of rolled AZT910 sheet consists of basal and non-basal slips at room temperature, and is a combination of basal slip, non-basal slip and grain boundary sliding/rotation (GBS) at 200 °C. The decrease of intensity and spreading of basal poles during tensile test and isotropic tensile properties indicates that GBS becomes the dominate deformation mechanism at 300 °C. The present work provides reference for the study of deformation behavior of novel superplastic magnesium alloy, and hence is of great significance for further application of wrought magnesium alloy.

Development and comparison of processing maps of Mg-3Sn-1Ca alloy from data obtained in tension versus compression

The hot workability of extruded Mg-3Sn-1Ca alloy has been evaluated by developing processing maps with flow stress data from compression and tensile tests with a view to find the effect of the applied state-of-stress. The processing maps developed at a strain of 0.2 are essentially similar irrespective of the mode of deformation – compression or tension, and exhibit three domains in the temperature ranges: (1) 350 – 425 °C, and (2) 450 – 550 °C and (3) 400 – 500 °C, the first two occurring at lower strain rates and the third occurring at higher strain rates. In all the three domains, dynamic recrystallization occurs and is caused by non-basal slip and controlled by lattice self-diffusion in the first and second domains and grain boundary self-diffusion in the third domain. The state-of-stress imposed on the specimen (compression or tension) does not have any significant effect on the processing maps.