Microstructure evolution of Mg–Al–Zn alloys during compression test at low strain and temperature

The microstructure evolutions and texture changes during the compression test were investigated using an extruded magnesium alloy with average grain sizes of 11.4 and 49.6 μm. The deformation twins were formed in all the samples; however, a comparison of the fraction of deformation twins on the effect of grain size and initial texture, i.e., the cutting position (normal or parallel to the extrusion), showed that the fine-grained alloy and/or the sample with the normal-cut to the extrusion had a lower fraction of deformation twins. On the other hand, the texture change showed different tendencies depending on the grain size and/or the initial texture. In the coarse-grained alloy, since the dominant deformation mechanism was the deformation twins, the lattice was rotated without relation to the initial texture. However, in the fine-grained alloy, even the applied strain of 0.20, the intensity peaks existed at 10-10 and the basal texture remained in the sample with the parallel- and normal-cut to the extrusion, respectively. This resulted from the difference in the fraction of deformation twins and the occurrence of partial grain boundary sliding.     

Hot deformation and processing maps of extruded ZE41A magnesium alloy

The hot deformation behavior and microstructure evolution of extruded ZE41A magnesium alloy has been studied using the processing map. The compression tests were conducted in the temperature range of 250–450 °C and the strain rate range of 0.001–1.0 s⁻¹ to establish the processing map. The dynamic recrystallization (DRX) and instability zones were identified and validated through micrographs. The observations were performed in order to describe the behavior of the material under hot forming operation in terms of material damage and micro-structural modification.     

Microstructure evolution and texture development during high-temperature uniaxial compression of magnesium alloy AZ31

Texture development in magnesium alloy AZ31 was studied by uniaxial compression tests at temperatures, strain rates and final strains ranging from 573 to 773 K, 1.0 × 10⁻³ to 5.0 × 10⁻⁵ s⁻¹ and −0.2 to −1.5, respectively. Fiber texture was formed in all of the deformation conditions. The main component of the texture varied depending on deformation conditions; it appeared about 33–38° away from the basal pole after the deformation at higher temperatures and lower strain rates. This can be attributed to the increased activity of the secondary pyramidal slip system. With a decrease in temperatures and an increase in strain rate, the tilting angle of the main component (compression plane) from the basal pole decreased down to about 20°. Construction of a basal fiber texture was detected after deformations at the lowest temperature and high strain rates.     

ZK60镁合金热压缩变形行为

为了研究ZK60镁合金的热变形行为,采用Gleebe-1500热模拟机在变形温度为423～673K、应变速率为0.001～10s-1条件下对合金进行的热压缩试验.分析合金流变应力与应变速率、变形温度之间的关系,通过引入Z参数建立合金流变应力本构方程,并观察合金变形过程中的显微组织演变.结果表明:变形温度低于473K且应变速率大于0.1s-1时试样发生宏观开裂;在变形温度较高和应变速率较低时,合金真应力-真应变曲线具有动态再结晶特征.随变形温度升高和应变速率的降低流变应力减小,热压缩后的组织中再结晶现象越明显;应变速率越高,再结晶晶粒越细小.     

Evaluation of low strain rate constitutive equation of 7075 aluminium alloy at high temperature

Abstract

The 7075 aluminium alloy is one of the most important engineering alloys utilised extensively in aircraft and transportation industries due to its high specific strength. In the present research, the flow behaviour of this alloy has been investigated using hot compression test at strain rates of 0·001, 0·01, 0·1 and 1 s^?1 and temperatures of 350, 400 and 450°C. The results reveal that dynamic softening occurred in these temperatures and strain rates. The activation energy, strain rate sensitivity and two constitutive equations (hyperbolic sine law and the power law) are derived from the results. It is shown that the hyperbolic sine law has a better agreement with the experimental results.     

镁合金AZ31高温形变机制的织构分析

利用X射线衍射和背散射电子衍射方法测定了镁合金AZ3l高温动态再结晶和超塑形变时的宏观和微观织构,分析了晶粒内部的形变机制.结果表明,在动态再结晶和超塑形变过程中,晶粒内部的滑移机制仍起重要作用,表现为再结晶晶粒出现择优取向以及一些晶粒可充分均匀形变成长条状.宏观织构的测定表明,具有不同初始织构的两类样品高温动态再结晶时,新晶粒有不同的取向择优过程,形成相似的织构;长条形变晶粒内部开动的滑移系也有一定的差异.分析了不同温度下相同的织构对应的不同塑变机理取向成像分析表明,基面织构取向的晶粒间总伴随着较高比例的小角晶界和30°(0001)的取向关系,这是六方结构的六次对称性限制了动态再结晶时(亚)晶粒间取向差的有效增大的缘故.     

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.     

Microstructure characteristic for high temperature deformation of powder metallurgy Ti–47Al–2Cr–0.2Mo alloy

Hot compression tests of a powder metallurgy (P/M) Ti–47Al–2Cr–0.2Mo (at. pct) alloy were carried out on a Gleeble-3500 simulator at the temperatures ranging from 1000 °C to 1150 °C with low strain rates ranging from 1 × 10⁻³ s⁻¹ to 1 s⁻¹. Electron back scattered diffraction (EBSD), scanning electron microscope (SEM) and transmission electron microscope (TEM) were employed to investigate the microstructure characteristic and nucleation mechanisms of dynamic recrystallization. The stress–strain curves show the typical characteristic of working hardening and flow softening. The working hardening is attributed to the dislocation movement. The flow softening is attributed to the dynamic recrystallization (DRX). The number of β phase decreases with increasing of deformation temperature and decreasing of strain rate. The ratio of dynamic recrystallization grain increases with the increasing of temperature and decreasing of strain rate. High temperature deformation mechanism of powder metallurgy Ti–47Al–2Cr–0.2Mo alloy mainly refers to twinning, dislocations motion, bending and reorientation of lamellae.     

Microstructural evolution in 42CrMo steel during compression at elevated temperatures

In order to study the workability and to optimize the processing parameters for 42CrMo steel hot forming, the hot compressive tests of 42CrMo steel were performed in the temperature range of (850-1150) °C at strain rates of (0.01-50) s^− 1 and deformation degrees of (10-60)% on Gleeble-1500 thermo-simulation machine. The effects of processing parameters, including the strain rate, forming temperature and deformation degree, on the microstructures of 42CrMo steel were investigated by metallurgical analysis. The results show that the average grain size of the deformed 42CrMo steel increases with the forming temperature and decreases with the deformation degree and strain rate. In a word, the grain size of hot compressive 42CrMo steel is dependent on the forming temperature, strain and strain rate, also on Zener-Hollomon parameter.     

Microstructural evolution in AZ91D magnesium alloy during electron beam welding

Vacuum electron beam welding can have a low heat input, which means there is a minimum heat affected zone during welding of AZ91D magnesium alloy. From the observed microstructure, the weld of the AZ91D magnesium alloy can be divided into four regions, which are the weld metal zone, a partially-melted zone adjacent to the fusion boundary, a partially-melted zone adjacent to the base metal and the base metal zone. A sharp transition from the fusion zone to the non-melted zone, especially the characteristic partial melting microstructure and nature of the alloy elements, was observed. It was found that significant partial melting had taken place in the very narrow region around the weld metal of the AZ91D magnesium alloy. The Al content of eutectic β-Mg₁₇Al₁₂ in the partially-melted zone adjacent to the fusion boundary was close to the content in the continuously precipitated eutectic β particles in the fusion zone and much lower than the eutectic β in the base metal. The fully melted eutectic β-phase coexisted with the partially melted eutectic β phase in the partially-melted zone adjacent to the base metal.     

Research on the hot deformation behavior of Ti40 alloy using processing map

The deformation behavior of a Ti40 titanium alloy was investigated with compression tests at different temperatures and strain rates to evaluate the activation energy and to establish the constitutive equation, which reveals the dependence of the flow stress on strain, strain rate and deformation temperature. The tests were carried out in the temperature range between 900 and 1100 °C and at strain rates between 0.01 and 10 s⁻¹. Hot deformation activation energy of the Ti40 alloy was calculated to be about 372.96 kJ/mol. In order to demonstrate the workability of Ti40 alloy further, the processing maps at strain of 0.5 and 0.6 were generated respectively based on the dynamic materials model. It is found that the dynamic recrystallization of Ti40 alloy occurs at the temperatures of 1050-1100 °C and strain rates of 0.01-0.1 s⁻¹, with peak efficiency of power dissipation of 64% occurring at about 1050 °C and 0.01 s⁻¹, indicating that this domain is optimum processing window for hot working. Flow instability domains were noticed at higher stain rate (≥1 s⁻¹) and stain (≥0.6), which located at the upper part of the processing maps. The evidence of deformation in these domains has been identified by the microstructure observations of Ti40 titanium alloy.     

Hot working and crystallographic texture analysis of magnesium AZ alloys

Abstract

The hot working behaviour of magnesium AZ (e.g. AZ31; Al: 3%, Zn: 1%) alloys and their associated crystallographic texture evolution is reviewed. Under hot working conditions, the stress–strain curves show flow softening at all the temperatures and strain rates indicating dynamic recrystallisation (DRX) is predominant. The mean size of the recrystallised grains in all the alloys decreases as the value of Zener–Hollomon parameter Z increases. The hot working range of the alloys dwell between 200 and 500°C and the strain rates between 10^?3 and 5 s^?1. The hot working of AZ series alloy shows discontinuous DRX as the main mechanism. Equal channel angular processing shows continuous DRX. The constitutive equation development shows a linear relationship between the stress and the Z parameter. The activation energy for the alloys ranges from 112 to 169 kJ mol^?1 and Z values range from 10 to 10 s^?1. Textural examinations show basal texture as the predominant orientation.     

Microstructure evolution of Al-Mg alloy during solidification under high pressure

The microstructure of Al-21.6Mg alloy solidified under high pressure was investigated. The results show that the amount of β-Al₃Mg₂ phase decreases with increasing pressure and a supersaturated Al(Mg) solid solution is formed under 2 GPa. The distribution of Mg in the solid solution is inhomogeneous, causing peak asymmetry of the XRD patterns under high pressures. The Mg concentration in the interdendritic region extends up to about 30 at.%, which is higher than that in the dendrite. Besides solution treatment, mechanical alloying, mechanical deformation of pre-alloy ingots and rapid solidification, solidification under high pressure is proved to be a new way to prepare supersaturated Al(Mg) solid solution.     

Deformability of a SiCw/6061Al composite during high strain rate compression at elevated temperatures

The deformability of SiCw/6061Al composite during high strain rate compression has been investigated at elevated temperatures around the solidus of the matrix alloy. The results show that the maximum deformability was obtained at 580°C which is near the solidus of the matrix. Analysis of the results indicates that the composites deformed at 580°C have the largest strain rate sensitivity (m value) and the lowest threshold stress, both of which lead to the maximum deformability. Microstructure observation shows that microcracks were formed at the interfaces in the composites deformed at 540°C and 620°C, whereas, in the composite deformed at 580°C, microcracks were rarely found because of the low stress concentration at the interfaces due to the presence of a small amount of liquid. It is suggested that the presence of an adequate amount of liquid phase gives rise to the effective accommodation required for grain boundary sliding for the composite, and thus directly affects the deformability of SiCw/6061Al composite.     

The effect of heterogeneous deformation on the hot deformation of WE54 magnesium alloy

The high temperature forming behavior of WE54 magnesium alloy is studied by means of compression and tension tests. Metallographic investigation was performed to evaluate the heterogeneous deformation of the compression samples at high temperature. Dynamic recrystallization was found to be related to the amount of deformation in the various regions of the compression sample. The compression data allowed determination of the Garofalo equation describing the hot deformation behavior. The parameters n and Q, stress exponent and activation energy, of this equation were 4.4 and 237 kJ/mol respectively. This equation was used to predict the formability behavior for the hot rolling process and also to determine the maximum forming efficiency and stability of the alloy. The optimum rolling temperature was found to be 520 °C.     

Hot deformation characterization of as-homogenized Al-Cu-Li X2A66 alloy through processing maps and microstructural evolution

Uniaxial compression tests were carried out on an Al-Cu-Li alloy at a temperature range of 300–500 °C and a strain rate range of 0.001–10 s⁻¹. Four representative instability maps based on Gegel, Alexander-Malas (A-M), Kumar-Prasad (K-P) and Murty-Rao (M-R) criteria were constructed. Through formula deduction and microstructural observation, it can be concluded that M-R criterion is more accurate than K-P criterion, and the first two criteria are better than Gegel and A-M criteria. From a power dissipation map and a M-R instability map, the optimized processing parameters are 480–500 °C/0.001–0.1 s⁻¹ and 420–480 °C/0.1-1 s⁻¹. The corresponding microstructural analysis shows that dynamic recovery and partial dynamic recrystallization are main dynamic softening mechanisms. Transmission electron microscopy observation indicated that a large number of primary coarse T₁ (Al₂CuLi) particles precipitated in the homogenized specimen. After deformation at 500 °C, most of the primary T₁ particles dissolved back into the matrix, and secondary fine T₁ particles precipitated at deformation-induced dislocations, high angle grain boundaries and other dispersed particles.     

Hot deformation behavior of an extruded Mg-Li-Zn-RE alloy

A new Mg-7.8%Li-4.6%Zn-0.96%Ce-0.85%Y-0.30%Zr alloy has been developed. α phase, β phase and RE-containing intermetallics formed in the alloy. It is found that the alloy can easily be extruded at 260 °C with σ_0.2 = 256 MPa, σ_b = 260 MPa and δ = 14%. Hot deformation behavior of the extruded alloy was studied using the processing map technique. Compression tests were conducted in the temperature range of 250-450 °C and strain rate range of 0.001-10 s⁻¹ and the flow stress data obtained from the tests were used to develop the processing map. The different efficiency domains and flow instability region corresponding to various microstructural characteristics have been identified as follows: (1) Domain I occurs in the temperature range of 250-275 °C and strain rate range of 1-10 s⁻¹, with a peak efficiency of about 50% at 250 °C/10 s⁻¹. Incomplete DRX process has occurred in β phase and DRX process hardly occurs in α phase; (2) Domain II occurs in the temperature range of 250-275 ?C and strain rate range of 0.001-0.003 s⁻¹, with a peak efficiency of about 42% at 250 °C/0.001 s ⁻¹. Incomplete DRX process has occurred in β phase and α phase; (3) Domain III occurs in the temperature range of 400-450 °C and strain rate range of 1-10 s⁻¹, with a peak efficiency of about 42% at 450 °C/10 s⁻¹. Complete DRX process has occurred in β phase and α phase. No cracking, cavity and band of flow localization are observed in flow instability region. The optimum parameters for hot working of the alloy are 250 °C/10 s⁻¹ and 250 °C/0.001 s⁻¹, at which fine dynamic recrystallization microstructure will be achieved. RE-containing intermetallics and α phase accelerate the DRX process in β phase. The softer β phase reduces the driving force for DRX process in α phase, so DRX process in α phase is retarded.     

Dynamic deformation behavior and microstructural evolution during high-speed rolling of Mg alloy having non-basal texture

The dynamic deformation behaviors and resultant microstructural variations during high-speed rolling(HSR) of a Mg alloy with a non-basal texture are investigated. To this end, AZ31 alloy samples in which the basal poles of most grains are predominantly aligned parallel to the transverse direction(TD) are subjected to hot rolling with different reductions at a rolling speed of 470 m/min. The initial grains with a TD texture are favorable for {10–12} twinning under compression along the normal direction(ND); as a result, {10–12} twins are extensively formed in the material during HSR, and this consequently results in a drastic evolution of texture from the TD texture to the ND texture and a reduction in the grain size. After the initial grains are completely twinned by the {10–12} twinning mechanism, {10–11} contraction twins and {10–11}-{10–12} double twins are formed in the {10–12} twinned grains by further deformation.Since the contraction twins and double twins have crystallographic orientations that are favorable for basal slip during HSR, dislocations easily accumulate in these twins and fine recrystallized grains nucleate in the twins to reduce the increased internal strain energy. Until a rolling reduction of 20%, {10–12}twinning is the main mechanism governing the microstructural change during HSR, and subsequently,the microstructural evolution is dominated by the formation of contraction twins and double twins and the dynamic recrystallization in these twins. With an increase in the rolling reduction, the average grain size and internal strain energy of the high-speed-rolled(HSRed) samples decrease and the basal texture evolves from the TD texture to the ND texture more effectively. As a result, the 80% HSRed sample, which is subjected to a large strain at a high strain rate in a single rolling pass, exhibits a fully recrystallized microstructure consisting of equiaxed fine grains and has an ND basal texture without a TD texture component.     

Improving tensile and compressive properties of magnesium alloy rods via a simple pre-torsion deformation

A simple method has been developed in order to improve the tensile and compressive properties of extruded AZ31 rod. The strategy is to apply pure shear strain to the extruded rods by a pre-torsion deformation. It is found that pre-torsion exhibits little influence on the shape and size of AZ31 rod, however largely enhances tensile and compressive yield strength and reduces yield asymmetry along extrusion direction. With increasing twisted angle, the compressive yield strength exhibits gradually increase, and the tensile yield strength exhibits a non-monotonous change. The changes in tensile and compressive properties are ascribed to the proliferation of dislocations, the generation of twin lamellae and the weakening of extrusion texture during pre-torsion. The corresponding mechanisms are addressed and discussed.     

TB6合金热压缩变形行为研究

本文主要研究了热变形过程中变形温度、应变速率对TB6合金组织性能的影响。研究表明流动应力随应变速率的升高而增大,随变形温度的升高而减小。而变形温度对流动应力的影响程度与应变速率的大小有关。