Microstructure Evolution Of Mg Research Articles

Hot tensile deformation behavior and microstructure evolution of Mg–Mn–Ce alloy

Magnesium-manganese series alloys, as materials with very low-density value, have excellent mechanical properties and corrosion resistance, and have broad prospects in automotive, aerospace, biomedical and other fields. In this work, the thermal deformation behavior and microstructural evolution of Mg–Mn–Ce alloys were investigated through uniaxial hot stretching experiments on Mg–Mn–Ce alloy plates under different deformation conditions, with the main focus on the plastic deformation mechanism of Mg–Mn–Ce alloys under different forming conditions. The unified viscoplastic constitutive equation for coupled damage is developed and compared with different types of constitutive equation models. Remarkably, the decrease in peak stress with increasing temperature or decreasing strain rate in Mg–Mn–Ce alloys is due to the fact that the internal slip system of the material is more easily activated with increasing temperature. As the temperature increases, the dislocations within the material decrease, which is due to the fact that the temperature increase makes the DRX action progressively stronger than the dislocation action, and the DRX leads to a significant coarsening of the grains at 573 K. In addition, the fracture behavior of the alloy under different deformation conditions has also been investigated, and the fracture pattern of Mg–Mn–Ce alloys transitions from pronounced cleavage fracture to the appearance of dimples and finally to ductile fracture with a large number of dimples as the temperature increases.

Recrystallization behavior and microstructure evolution of Mg–5Bi–3Al alloy during very high-speed extrusion

In our previous study, we extruded Mg–5Bi–3Al (BA53, wt.%) alloy at a very high speed of 70 m/min, and the high-speed-extruded alloy exhibited an unusual fine grain structure. Since dynamic recrystallization (DRX) behavior determines the microstructure and corresponding mechanical properties of the extruded alloy, understanding its origin is crucial for further improvement and optimization of alloys. Herein, the DRX behavior during high-speed extrusion of the recently developed BA53 alloy was investigated in detail by analyzing the microstructure and texture of the extrusion butt along the extrusion path obtained by water quenching the remaining part of the billet immediately after extrusion. During the initial stages of extrusion with a high extrusion speed of 70 m/min and a high temperature of 400 °C, tension twinning predominantly occurred in grains with a high Schmid factor (SF), switching the c-axis of the grains toward the transverse direction of the butt and causing the formation of a ring basal texture. In the intermediate stage, the microstructural changes were dominated by continuous and discontinuous DRX, resulting in the formation of fine recrystallized grains and orientation distribution originated from the parent grain. DRX was completed in the late stage wherein unrecrystallized grains with low SF for basal slip contributed to the late-stage recrystallization, strengthening the ring basal texture. Additionally, DRXed grains with favorable orientation for basal slip underwent lattice rotation, contributing to the formation of the obvious ring basal texture. Finally, preferential grain growth occurred after the DRX, leading to an increase in texture intensity and grain size of the extruded alloy. Therefore, the fine grain structure and ring basal texture obtained through high-speed extrusion were primarily attributed to the activated discontinuous DRX facilitated by a high strain rate and temperature.

Microstructural evolution of Mg–14Gd–0.4Zr alloy during compressive creep

The present work reports the creep behavior and microstructural evolution of the sand-cast Mg–14Gd–0.4Zr alloy (wt.%) prepared by the differential pressure casting machine. Their compressive creep tests at 250 °C were performed under various applied stresses (i.e., 60, 80 and 100 MPa). Among them, the sand-cast Mg–14Gd–0.4Zr samples examined under 250 °C/80 MPa for 39 and 95 h, respectively, were chosen to systemically analyze their creep mechanisms using high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). The obtained results showed that the enhancement of creep resistance can be mainly attributed to the coherent β' and β'F phases with an alternate distribution, effectively impeding the basal <a> dislocations movement. However, with the creep time increasing, the fine β'+β'F precipitate chains coarsened and transformed to semi-coherent β1 phase and even to large incoherent β phase (surrounded by precipitate-free areas) in grain interiors. The precipitate-free zones (PFZs) at grain boundaries (GBs) were formed, and they could expand during creep deformation. Apart from the main cross-slip of basal and prismatic <a> dislocations, <c + a> type dislocations were activated and tended to distribute near the GBs. The aforementioned phenomena induced the stress concentrations, consequently leading to the increment of the creep strain.

Critical role of dynamic precipitation on enhanced creep resistance in Mg–Zn alloy by Mn substitution for Zr

Mg alloys with excellent creep resistance are able to withstand fatigue and damage under high temperature and complex load. Adding alloying elements rather than rare earth elements, such as Mn, can further reduce the cost and thus facilities the widespread applications of Mg alloys. In this paper, the individual effect of Mn and Zr elements on the dynamic microstructural evolution of Mg–Zn alloys during creep is evaluated, on the basis of identical initial microstructure before creep. And the essential origin of Mn substitution for Zr improving the creep resistance of Mg–Zn alloys by one order of magnitude is revealed. Experimental investigations and first principle calculations reveal that prior-precipitation of α-Mn particles serve as the heterogeneous nucleation sites for the precipitation of β1′ phases, and Mn partitioning in MgZn2 phases promote the nucleation and thermal stability of β1′ precipitates during creep. High number density of fine precipitates can effectively inhibit the occurrence of continuous dynamical recrystallization (CDRX), pin dislocations and impede the activation of cross-slip, which are the main reasons for the enhanced creep resistance in ZM60 alloy. As Zr does not participate in the formation of precipitates in ZK60 alloy, the MgZn2 phases with low thermal stability are prone to growth and coarsening during long-term creep. Large number of fine CDRXed grains and sparse coarse precipitates in ZK60 alloy cannot effectively impede the dislocation movement, which contributes to the occurrence of cross-slip and deteriorate the creep resistance. This work provides new insights for designing and fabricating outstanding heat-resistance Mg alloys.

Microstructure Evolution in Mg&lt;sub&gt;98.6&lt;/sub&gt;Y&lt;sub&gt;1&lt;/sub&gt;Zn&lt;sub&gt;0.4&lt;/sub&gt; Alloys and the Development by Hot Deformation Examined by Synchrotron Radiation Small- and Wide-Angle Scattering

Synchrotron radiation small- and wide-angle scattering measurements have been performed for Mg98.6–Y1–Zn0.4 alloys. In the early stage of phase transformation from supersaturated solid solutions, isotropic scattering suggesting segregation at grain boundaries was observed. It grew with temperature during heating the sample at a constant rate of 0.133 K/s. Above 600 K where introduction of stacking faults is expected, needle-like scattering became visible, which represents platelet shape segregation of cluster layers called cluster arranged layer (CAL). The layer eventually developed to form multiple layers, cluster arranged nano plates (CANaP) at higher temperatures. Microstructure change by hot rolling after the heat treatments has been examined from a viewpoint of kink-deformed microstructures.

Remarkable age-hardening response and microstructural evolution of Mg–7Sn alloy

The poor age-hardening response of Mg alloys limited their development and applications. In this study, the age-hardening response of Mg–7Sn alloy has been improved significantly by a 40 MPa compressive stress aging at 220 °C. Phase transition from metastable βˈ (Mg3Sn) phase to β (Mg2Sn) phase occurs during the stress aging. The βˈ with L12 structure has an orientation relationship with the Mg matrix of (11¯0)β＇//(112¯0)α and [1¯1¯0]β＇//⌈0001¯⌉α. The precipitated sequence in the stress-aged alloy is S.S.S. (supersaturated solid solution)→GP→βˈ(Mg3Sn)→β(Mg2Sn). The stress-aged alloy exhibits a high peak-aged hardness (∼71 HV) and short time to reach the peak hardness (12 h) as compared with the aged alloy without the stress. Interestingly, a long-time aging platform with 71 HV for 96 h is observed during stress aging. The formation of the platform is related to a balance of hardening and softening arisen from microstructure evolution during stress aging. The stress aging provides a promising heat treatment way to improve aging hardening response of Mg alloys.

Effects of bimodal basal texture on bending stress and microstructure evolution of Mg−2.6Nd−0.55Zn−0.5Zr alloys

AbstractThe extrusion direction (ED) and transverse direction (TD) samples were cut from Mg−2.6Nd− 0.55Zn−0.5Zr alloy sheet, and the effects of bimodal basal texture on the bending stress and microstructure evolution were investigated. Results showed that the ED sample with weak basal texture possessed much lower bending stress than the TD sample. This anisotropy is attributed to the role of different textures on the evolution of deformation mechanisms and microstructures. For the ED sample with weak basal texture, the dominant deformation mechanisms were basal slip in the majority with minor supplement of tension twin in the inner region while basal slip and prismatic slip in the outer region. In comparison, the TD sample with strong basal texture activated more tension twin in the inner region and more prismatic slip in the outer region. The activation of tension twin promoted the development of 〈0002〉//compression direction type texture with the confined ±30° angle range in the inner region and reduced the emergence propensity of high dislocation density and low angle grain boundaries. The massive activation of tension twin allowed the TD sample to maintain a high strain-hardening capacity, which was the fundamental reason for the high bending stress in this sample.

Hot compressive deformation behaviour and constitutive equations of Mg–Pb–Al–1B–0.4Sc alloy

ABSTRACT The hot compression features and microstructural evolution of Mg–Pb–Al–1B–0.4Sc alloy deformed at the strain rate range of 0.001–1 s−1 and the temperature range of 493–613 K were investigated by isothermal compression tests. The results show that the flow stress is significantly influenced by deformation conditions. The true stress decreases with the raised temperature or the reduced strain rate. The Zuzin-type model and Arrhenius-type constitutive model were employed to predict the flow behaviour of Mg–Pb–Al–1B–0.4Sc alloy. The values of and in the Arrhenius-type model are 6.353% and 9.595, respectively, which are smaller than the 13.653% and 24.662 of the Zuzin-type model, indicating that the Arrhenius-type model is more suitable for predicting the flow stress of this alloy. The processing maps and microstructure show that the optimum strain rates and temperatures are 0.001–0.015 s−1 and 550–613 K, respectively.

Effect of compression ratio on microstructure evolution of Mg–10%Al–1%Zn–1%Si alloy prepared by SIMA process

A new Mg–10%Al–1%Zn–1%Si alloy with non-dendritic microstructure was prepared by strain induced melt activation (SIMA) process. The effect of compression ratio on the evolution of semisolid microstructure of the experimental alloy was investigated. The results indicate that the average size of α-Mg grains decreases and spheroidizing tendency becomes more obvious with the compression ratios increasing from 0 to 40%. In addition, the eutectic Mg2Si phase in the Mg–10%Al–1%Zn–1%Si alloy transforms completely from the initial fishbone shape to globular shape by SIMA process. With the increasing of compression ratio, the morphology and average size of Mg2Si phases do not change obviously. The morphology modification mechanism of Mg2Si phase in Mg–10%Al–1%Zn– 1%Si alloy by SIMA process was also studied.

Microstructural evolution of Mg–6Zn–0.6Zr alloy during high strain rate compression deformation

In this study, the microstructural evolution and single magnesium cell deformation behaviour of extruded Mg–6Zn–0.6Zr alloy were investigated along the extrusion direction with strain rate at 1800 s−1 using a split Hopkinson pressure bar apparatus. The microstructural evolution and deformation mechanism were studied by X-ray diffraction, electron back scatter diffraction, and transmission electron microscopy. Analysis of the microstructural evolution shows that <100> tension twinning and (0001)<110> basal plane slip were dominant when the strain was less than 6%, and <113> pyramidal plane<c+a> slip was the major deformation mechanism when the strain exceeded 6%. In particular, the transformation process of single magnesium crystal cell during deformation was studied in detail.

Grain refinement and the microstructure evolution of Mg–9Al–5Ca alloy solidified under permanent magnetic stirring

To achieve the grain refinement of the as-cast Mg–9Al–5Ca alloy and understand the microstructure evolution during the grain refinement process, the permanent magnetic stirring(PMS) was applied to the solidification process of the alloy and the water-quenched experiment was carried out. The experimental results show that, the as-cast microstructure of Mg–9Al–5Ca alloy can be refined by PMS, and the better effect of grain refinement could be obtained with the increase of stirring rate. After applied PMS, the cooling of the melt is improved and the whole solidification time of the alloy is reduced. During the solidification process with PMS, there are more dendrite fragments produced in the melt and more initial dendrite grains at mold wall detached due to the forced convection, and then the grains would gradually grow into the fine and rose-like grains, which lead to the grain refinement of the as-cast alloy.

Effect of Hydrostatic Pressure on LPSO Kinking and Microstructure Evolution of Mg–11Gd–4Y–2Zn–0.5Zr Alloy

Two different kinds of hot compressions, namely normal-compression and can-compression, were performed on the Mg–11Gd–4Y–2Zn–0.5Zr alloy, featured with long period stacking ordered (LPSO) phase. The kinking behavior of LPSO phase and microstructure evolution was investigated to clarify the effect of levels of imposed hydrostatic pressure. The results suggest that the LPSO phases including both the intragranular 14H-LPSO phase and intergranular 18R-LPSO phase suffer severe kinking behavior under higher hydrostatic pressure induced by can-compression, which is firstly characterized with more kinking times and smaller relative kinking width. The main reason for such enhanced LPSO kinking during can-compression may be mainly ascribed to the higher dislocation density under a higher level of hydrostatic pressure. Meanwhile, a competitive relationship between the kink behaviors of intergranular 18R-LPSO phase and intragranular 14H-LPSO phase was observed. That is, the intergranular 18R-LPSO phase only kinks obviously on the condition that the surrounded intragranular 14H-LPSO phase scarcely kinks. In contrast to the distinctive kinking of LPSO phase, the dynamic recrystallization (DRX) mechanism shows less dependence on the hydrostatic pressure. Resultantly, similar DRX fractions and crystallographic texture were attained for two compression processes owing to the similar operation of deformation mode.

Microstructure and Mechanical Properties of Mg–2.0Gd–1.2Y–1.0Zn–0.2Zr Alloy

The microstructural evolution of Mg–2.0Gd–1.2Y–1.0Zn–0.2Zr (GWZK) alloy during homogenization and subsequent aging heat-treatment and the tensile properties of the peak-aged sample at various temperatures have been investigated. The phase transformation during homogenization, static precipitation during aging, and deformation mechanisms at 250 °C are systematically analyzed through XRD, TEM and EBSD measurements. The peak-aged GWZK alloy has good combination of strength and ductility at elevated temperatures owing to its high thermal stability by the LPSO phases distributed along the grain boundaries. The change (19 MPa) in the yield strength of peak-aged samples between RT and 250 °C is mainly related to easy gliding of dislocations on the non-basal planes due to the decrease in critical resolved shear stress of slip. Two strengthening models are applied to quantitatively describe the effect of $${\left\{11\stackrel{-}{2}0\right\}}_{\alpha }$$ precipitate plates on Orowan strengthening corresponding to RT and 250 °C. Ignoring changes in solid solution strengthening and grain boundary strengthening of the peak-aged sample caused by increasing temperature, the calculated reduction value (16.5 MPa) in macroscopic yield strength from RT (based on the basal slip mode) to 250 °C (based on the pyramidal slip mode) is in good agreement with the measured reduction value (19 MPa).

Ex-situ EBSD analysis of hot deformation behavior and microstructural evolution of Mg–1Al–6Y alloy via uniaxial compression

The microstructure and texture evolution of as-extruded Mg-0.96 wt%Al-5.82 wt%Y (denoted as Mg–1Al–6Y) alloy with strain at a temperature of 350 °C and a strain rate of 0.1 s−1 were studied using ex-situ electron backscatter diffraction (EBSD). Moreover, the deformation mechanism was systematically analyzed. The results shown that weak <2-1-11> rare-earth texture and no twinning were observed in the as-extruded Mg–1Al–6Y alloy. The hot compression curve exhibited obvious work-hardening characteristics with a yield strength of 98 MPa. In the initial stage of deformation, {10–12} extension twinning was formed in some grains. As the strain increased, the number of {10–12} extension twinning and texture intensity gradually decreased, whereas the amount of low-angle grain boundaries increased. The Schmid factor (SF) of pyramidal slip remained at ~0.4. In addition, the TEM characterization revealed that as the strain increased, dislocations multiplied, and the pyramidal <c+a> dislocation was always observed. Therefore, the reduction of twinning and activation of pyramidal <c+a> slip lead to the dominance of slip during the deformation. Hence, work hardening appeared in the compression curve of Mg–1Al–6Y alloy.

The Influence of Holding Time on the Microstructure Evolution of Mg–10Zn–6.8Gd–4Y Alloy during Semi-Solid Isothermal Heat Treatment

A semi-solid microstructure of Mg–10Zn–6.8Gd–4Y alloys is acquired via an isothermal heat treatment process, and the effects of the holding time on the microstructure evolution of Mg–10Zn–6.8Gd–4Y alloys are investigated. The results show that the microstructure of the cast alloy is composed of primary α-Mg dendritic grains with a eutectic structure (W-phase and eutectic Mg) distributed at the grain boundaries. The primary α-Mg dendritic grains grow in size with increasing holding time, and they tend to grow into more globular structures in the initial stage; they then become a bit more dendritic, as small branches grow from the grain boundaries after holding the sample at 580 °C for 10 min. Meanwhile, the interdiffusion of magnesium atoms within the eutectic region, and between the primary α-Mg and eutectic structure, leads to the formation of fine and relatively globular eutectic Mg grains in the eutectic structure after holding for 10 min. The eutectic Mg grains begin to grow, coarsen, coalesce, or be swallowed by the surrounding primary grains, causing fluctuations of the general grain size. Over the whole isothermal heat treatment process, two mechanisms—coalescence and Ostwald ripening—dominate the grain coarsening.

Effect of Cooling Rate on Solidification Behavior and Microstructure Evolution of As-Cast Mg–5Al–2Ca–2Sm Alloy

In the present work, the Mg–5Al–2Ca–2Sm alloy was fabricated in stepped type sand mold, the cooling rate varied from 0.3 to 3.5 °C/s. The solidification behavior and microstructure evolution of Mg–5Al–2Ca–2Sm alloy were carried out by computer-aided cooling curve thermal analysis method, optical microscope (OM), X-ray diffraction analysis, scanning electric microscope and transmission electron microscope. The experimental results showed that the nucleation temperature of α-Mg phase decreased with increasing cooling rate. In addition, the grain size of α-Mg phase in Mg–5Al–2Ca–2Sm alloy were 95.47 ± 1.2 μm, 88.65 ± 1.5 μm, 71.24 ± 1.7 μm and 42.35 ± 1.3 μm, which were responds to the cooling rates of 0.3 °C/s, 0.5 °C/s, 1.2 °C/s and 3.5 °C/s, respectively. There were both Al2Sm (particle structures) and (Mg, Al)2Ca (lamella structure) phases in the Mg–5Al–2Ca–2Sm alloy under different cooling rates. However, there were Mg2Ca (blocky structure) in the samples cooled in 0.5 °C/s and 0.3 °C/s. The solidification sequence of precipitated phase in Mg–5Al–2Ca–2Sm alloy could be obtained as: Al2Sm → α-Mg → (Mg, Al)2Ca → Mg2Ca. Furthermore, the volume fraction of precipitated phase increased with the cooling rate. The volume fraction of precipitated phase in Mg–5Al–2Ca–2Sm alloy were 11.86 ± 0.7%, 13.05 ± 1.2%, 16.19 ± 0.8% and 20.77 ± 0.9%, which were responds to the cooling rates of 0.3 °C/s, 0.5 °C/s, 1.2 °C/s and 3.5 °C/s, respectively.

Hot deformation behavior and microstructural evolution of Mg–Zn–Ca–La alloys

Abstract

Characterization of Microstructure and Mechanical Properties of Mg–8Li–3Al–1Y Alloy Subjected to Different Rolling Processes

The mechanical properties and microstructure evolution of Mg–8Li–3Al–1Y alloy undergoing different rolling processes were systematically investigated. X-ray diffraction, optical microscope, scanning electron microscopy, transmission electron microscopy as well as electron backscattered diffraction were used for tracking the microstructure evolution. Tensile testing was employed to characterize the mechanical properties. After hot rolling, the MgLi2Al precipitated in β-Li matrix due to the transformation reaction: β-Li → β-Li + MgLi2Al + α-Mg. As for the alloy subjected to annealed hot rolling, β-Li phase was clearly recrystallized while recrystallization rarely occurred in α-Mg phase. With regard to the microstructure undergoing cold rolling, plenty of dislocations and dislocation walls were easily observed. In addition, the microstructure of alloys subjected to annealed cold rolling revealed the formation of new fresh α-Mg grains in β-Li phase due to the precipitation reaction. The mechanical properties and fracture modes of Mg–8Li–3Al–1Y alloys can be effectively tuned by different rolling processes.

Effect of cooling rate and Al content on solidification behavior and microstructure evolution of as-cast Mg–Al–Ca–Sm alloys

In the present study, the Mg–xAl–2Ca–2Sm (x = 3, 5, 9 and 15) alloys were fabricated in sand mold with stepped type, and the cooling rates varied from 0.3 to 3.5 °C s−1. The solidification behavior and microstructure evolution of Mg–xAl–2Ca–2Sm alloys were carried out by computer-aided cooling curve thermal analysis method, optical microscope, scanning electron microscope, X-ray diffraction analysis and transmission electron microscope, respectively. The experimental results showed that the nucleation temperature and grain size of α-Mg phase decreased with increasing the cooling rate and Al content. However, the volume fraction of precipitated phase increases with cooling rate and Al content. In the Mg–3Al–2Ca–2Sm alloy cooled at 3.5 °C s−1, there were Al2Sm (rod and particle structures), Al11Sm3 (acicular structure) and (Mg, Al)2Ca (lamella structure) phases. But the Al11Sm3 phase and (Mg, Al)2Ca phase will gradually transform to the Al2Sm phase and Al2Ca phase, respectively. In addition, there were both Al2Sm (particle structures), Al2Ca (fish-bone structure) and β-Mg17Al12 (reticular structure) phases in Mg–15Al–2Ca–2Sm alloy cooled ranging from 3.5 to 0.3 °C s−1. In the Mg–xAl–2Ca–2Sm alloys, the Al2Sm phase mainly distribute in the grains, but all the Al11Sm3, (Mg, Al)2Ca, Al2Ca and β-Mg17Al12 phases distributed in the boundary of grains. The solidification sequence of precipitated phase in Mg–xAl–2Ca–2Sm alloys can be obtained: Al2Sm → α-Mg → Al2Ca → β-Mg17Al12.

Effect of applied pressure on microstructures of squeeze cast Mg–15Gd–1Zn–0.4Zr alloy

Abstract Microstructural evolution of Mg–15Gd–1Zn–0.4Zr (GZ151K, wt%) alloys, cast under 0 MPa (gravity cast) and 6 MPa (squeeze cast), were comparatively studied. It is found that the grain size of squeeze cast GZ151K alloy with applied stress 6 MPa is much smaller than that of the gravity cast counterpart. Moreover, the squeezing pressure hinders the transition from β' precipitates to β1 precipitates during subsequent aging process, leading to reduced volume fraction of β1 precipitates in the squeeze cast alloy. Thus, the relatively lower volume fraction of β1 precipitates in the squeeze cast GZ151K results in higher hardness increment and stronger precipitation hardening effect.