Continuation of twins across grain boundary in extruded Mg–3Al–1Zn alloy

Abstract

Paired twins in an extruded Mg–3Al–1Zn alloy are investigated by using electron backscattered diffraction in the current paper. The results show that these paired twins are discovered at low misorientation grain boundaries. The twin variant (1–102)[?1101] is operated in the paired twins. Additionally, a macroscopic angle exists in the paired twins and is determined by the c axis misorientation of the grains.     

Microstructure and mechanical behaviour of asymmetric extruded Mg–3Al–1Zn alloy sheets

Abstract

Microstructural evolution and mechanical responses of Mg–3Al–1Zn (AZ31) sheet processed by the asymmetric extrusion (ASE) and conventional extrusion (CE) are examined. Mechanical properties of ASE sheets were remarkably enhanced compared with CE samples. This is attributed to the subdivision of the asymmetric extrusion die along the flow passage equipped with a chamfer on one side, which would trigger the angular spread of the basal texture by introducing an asymmetry shear deformation. Moreover, subsequent annealed ASE specimens show a significant weakening of the basal texture and a combination of the superior stretch formability.     

Microstructure and mechanical properties of variable-plane-rolled Mg–3Al–1Zn alloy

A flawless bulk AZ31 magnesium alloy with extensive mechanical twins was produced by variable-plane rolling, in which the sample was rotated 90° around its longitudinal axis between passes. The unique orientation relationship between the parent grains and the twin grains favours twinning during variable-plane rolling, which leads to the formation of extensive twins. Tensile testing revealed an excellent balance of mechanical properties, with a yield strength of 280 MPa and 15.5% elongation to failure. The significant strengthening originates from the effective blockage of glide dislocations by numerous conventional grain boundaries and twin boundaries. A weak double-peak (slightly off-basal) texture is formed during variable-plane rolling, which helps in achieving the desired level of ductility.     

Precipitation hardening of Zr-modified Mg–Ca–Zn alloy

The microstructure and mechanical properties of Mg–Ca–Zn alloys with 1 wt.% Zr were investigated in as-cast and heat-treated conditions. A substantial decrease in grain size (from 65 μm for the Mg–Ca–Zn base alloy to 22 μm) was observed. The alloy was solution treated at 410 °C for up to 96 h followed by aging at 175 °C for up to 24 h. Conventional techniques, X-ray diffraction, EM + EDS, and TEM were used to characterize the microstructure of the alloy. The microstructure obtained after heat treatment had equiaxed grains with evenly distributed binary phase Zn₂Zr. The binary Mg₂Ca and ternary Mg₂Ca₆Zn₃ phases were identified in the matrix and at grain boundaries surrounded by precipitate-depleted zones (PDZs). The thermal stability of the Zr-modified alloys was examined by microhardness measurements conducted after prolonged exposures of the alloys to elevated temperatures. It was found that Zr is a structure-stabilizing factor. Its influence was associated with the formation of Zn₂Zr phase that does not undergo coarsening at the elevated temperatures used (due to the low diffusivity of Zr). The nanoscale mechanical properties of grain boundary PDZs were analyzed using combined nanoindentation and atomic force microscopy. These mechanical properties were then correlated to the composition and precipitate distribution in PDZs. An increase in the solution treatment duration from 10 to 96 h at 410 °C resulted in expansion of PDZs from ~0.75 to ~3 μm, while the following aging at 175 °C for up to 24 h did not lead to a detectable change in PDZs. The analysis indicates that the lowest hardness was found in the region where Zn₂Zr precipitates density was low, regardless of the solute concentration.     

Microstructure and mechanical properties of as aged Mg–3Sn–1Al and Mg–3Sn–2Zn–1Al alloy

Effect of Zn on the microstructure, age hardening response and mechanical properties of Mg–3Sn–1Al alloy which is immediately aged at 180°C after extrusion process (T5) was investigated. It was found that the Zn can refine the microstructure, remarkably improve the aging response with the peak hardness increases to 75 HV and the time to peak hardness reduces from ～110 to ～60 h, which is attributed to the solid solution hardening of Al, Zn and an amount of finer Mg₂Sn precipitates. The as aged Mg–3Sn–2Zn–1Al alloy exhibits better mechanical property at room temperature or 150°C than that of Mg–3Sn–1Al alloy, which is ascribed to the fine grained microstructure and thermally stable Mg₂Sn particles dispersed at grain boundaries and in the matrix.     

Microstructure and tensile properties of as extruded and as aged Mg–Al–Zn–Mn–Sn alloy

The effect of 0–4 wt-% Sn addition on the microstructure and tensile properties of AZ80 alloys was investigated. The results indicated that Mg₂Sn particles were barely formed during the extrusion process until the content of Sn is >2 wt-%. The dislocation density in alloys after extrusion declined with the addition of Sn due to the promotion of dynamic recrystallisation after adding Sn. In aging treatment, Mg₁₇Al₁₂ precipitates were promoted by Sn and the phases distributed uniformly at low density level of dislocation. The AZ80-2 wt-% Sn alloy possessed the excellent tensile properties in as extruded and as aged state.     

Hot tensile behavior of an extruded Al–Zn–Mg–Cu alloy in the solid and in the semi-solid state

This is the first reported research into the tensile behavior of as-deformed Al–Zn–Mg–Cu alloy in the semi-solid state. Tensile tests of extruded 7075 aluminium alloy were carried out in the high temperature solid and semi-solid states. Based on the tensile results and microstructural examination, the tensile behavior can be divided into three stages according to the effect of liquid: one behaves in predominantly ductile character between 400 and about 520 °C (f_l ∼ 0.31%), one is governed by both of solid and liquid between 520 and 550 °C (f_l ∼ 2%), and almost completely dominated by liquid above ∼550 °C. A brittle temperature range (519–550 °C) is proposed, in which the as-deformed Al–Zn–Mg–Cu alloy exhibits large crack probability. An equation based on ultimate tensile stress and temperature is proposed.     

Microstructure and mechanical properties of high-performance Mg–Y–Er–Zn extruded alloy

The Mg–8Y–1Er–2Zn (wt.%) alloy with high strength, plasticity and heat-resistance was prepared by the hot extrusion technique and the following aging treatment. The microstructure and mechanical properties were investigated. The results show that long period stacking ordered (LPSO) phase is different from common inermetallics, and the former can be bent by plastic deformation and presents good combination with the Mg matrix. The good mechanical properties of as-extruded alloy are mainly attributed to the lamellar strips with 18R LPSO structure as well as the microstructure refinement. Aging treatment at 220 °C can further improve the strength but not at the expense of plasticity. The ultimate tensile strength (UTS) and elongation to failure (ε) of as-extruded alloy at peak hardness are 390 MPa and 18% at room temperature, and 322 MPa and 30% at 250 °C, respectively. The formation of fine α-Mg recrystallization grains with high number density of 14H LPSO structure is mainly responsible for the superior mechanical properties of extruded alloy after peak-aging.     

Geometrical compatibility factor analysis of paired extension twins in extruded Mg–3Al–1Zn alloys

Recently, geometrical compatibility factor (m′) has been used to characterize local strain compatibility between paired twins formed in rolled Mg alloys with basal texture. High m′ and m′ rank were present in most cases, suggesting that m′ plays an important role in twin pair formation. The present study aims to extend the understanding of the effects of m′ and grain boundary (GB) misorientation angle on twin pair formation in an extruded Mg alloy with basal fiber texture. 106 sets of paired twins were extracted from electron backscatter diffraction (EBSD) data and were analyzed. The results show that m′ of paired twins distributes in a wide range (− 0.5 to 1) in extruded Mg alloys due to the presence of a large fraction of high angle GBs. About 60% of twin pairs have the first m′ rank, implying that m′ is still an important factor for variant selection of paired twins in extruded Mg alloys. The distribution of m′ with SF rank was also analyzed, showing that more than half of twin pairs are located in the left-top corner with m′ > 0.6 and SF either rank one or two, which confirms that simultaneously high SF and high m′ is favorable for twin pair formation in extruded Mg alloys.     

High speed rolling of Mg–3Al–1Zn alloy: texture and microstructure analysis

Abstract

High speed rolling (HSR) of 1000 m min^?1 was employed to successfully roll AZ31 alloy in one pass with 65% reduction in thickness at 300 and 450°C. The rollability, texture and microstructure after HSR, in comparison with low speed rolling (15 m min^?1), improved significantly. It is suggested that the double peak and weaker basal texture obtained after HSR are attributed to the activation of compression and double twins. After annealing, the double peak basal texture is replaced by a single peak one, which may be due to preferential grain growth of basal grains.     

Effect of equivalent strain and redundant shear strain on microstructure and texture evolution during hot rolling in Mg–3Al–1Zn alloys

Equivalent strain and redundant shear strain distribution in the roll bite during normal rolling were calculated by a numerical integration method combined with the experimental method. The microstructural parameters, such as length of high-angle grain boundaries (HAGB), length of low-angle grain boundaries (LAGB) per unit area, and (0002) basal texture in surface layer and center layer were measured quantitatively by EBSD or X-ray diffraction. The effect of equivalent strain and redundant shear strain on the microstructure and (0002) basal texture evolution in AZ31 alloy during hot rolling were examined. As a result, it was found that the formation of the HAGB depends on the equivalent strain, while the formation of the LAGB is strongly affected by the redundant shear strain, which restrains the formation of the LAGB. The experimental results also suggest that the redundant shear may have little effect on improving DRX and weakening the (0002) basal texture intensity when the redundant shear strain is in small to moderate range (≤0.8).     

Superplasticity and grain boundary sliding characteristics in two stage deformation of Mg–3Al–1Zn alloy sheet

A ‘Two-Stage Deformation Method’ was proposed to enhance the superplasticity of Mg–3Al–1Zn (AZ31) alloy sheet. This method exploited the capability of the material to undergo dynamic recrystallization (DRX) at optimum DRX conditions of 250 °C and constant strain rate of 1×10⁻⁴ s⁻¹. Stage I was aimed at refining the coarse microstructure of the as-received alloy to result in fine equiaxial grains measuring less than 10 μm, which deformed by grain boundary sliding accommodated by intragranular slip. Subsequently, Stage II was performed at a higher deformation temperature, whereby viscous glide mechanism accommodated by lattice diffusion was predominant. By altering the deformation mechanisms at different strain levels, elongation-to-failure of 320 and 360% was attained at 400 and 450 °C, respectively.     

Effect of strain on cavity development during Al–Zn–Mg–Cu alloy superplastic flow

Cavitation behaviour has been investigated in an Al–Zn–Mg–Cu alloy with an average grain size of 10?µm during superplastic deformation. The superplastic tensile tests were interrupted at different true strains at 530°C and 3?×?10^?4?s^?1. The results showed that cavity nucleation occurred above a critical strain in the optimum loading condition. It was easy for cavities to form at the triple junction due to the stress concentration caused by cooperative grain boundary sliding. Since the tensile stress was higher in the middle of the sample, the cavities were arranged in a straight line parallel to the tensile axis in the centre of the sample. A more appropriate cavity growth equation considering the critical strain was proposed to describe the cavitation behaviour.     

Preparation and characterization of As-cast and hot-rolled Mg–3Al–0.5Mn–0.5Zn–1MM alloy

Mg–3Al–0.5Mn–0.5Zn–1MM alloy was prepared by metal mould casting method. The as-cast ingot was homogenized and then hot-rolled at 673 K with total thickness reduction of 65%. Microstructure and mechanical properties of the as-cast and hot-rolled samples were investigated. The results showed that the as-cast sample mainly consisted of α-Mg, β-Mg₁₇Al₁₂, Al₁₀Ce₂Mn₇, and Al₁₁RE₃ (RE = La and Ce) phases. The average grain size of the sample homogenized at 673 K was about 240 μm, and it was greatly refined to about 7 μm by dynamic recrystallization for the hot-rolled sample. The ultimate tensile strength and 0.2% yield strength of the hot-rolled sample were 300 MPa and 230 MPa, respectively. They were enhanced by 55% and 400% correspondingly compared with those of the as-cast sample. The improvement of the strengths was attributed to the refined grains, breakup of the precipitates and increase of the dislocation density.     

Influence of strain and strain rate on microstructural evolution during superplasticity of Mg–Al–Zn sheet

Strain-induced abnormal grain growth was observed along the gage length during high-temperature uniaxial tensile testing of rolled Mg–Al–Zn (AZ31) sheet. Effective strain and strain rates in biaxial forming of AZ31 sheets also affected the nature of grain growth in the formed sheet. For the uniaxial testing done at 400 °C and a strain rate of 10^?1 s^?1, abnormal grain growth was prevalent in the gage sections that experienced true strain values between 0.2 and 1.0. Biaxial forming of AZ31 at 5 × 10^?2 s^?1 and 400 °C also exhibited abnormal grain growth at the cross sections which experienced a true strain of 1.7. Uniaxially tested sample at 400 °C and a strain rate of 10^?3 s^?1, however, showed no abnormal grain growth in the gage sections which experienced true local strain values ranging from 1.0 to 2.3. The normalized flow stress versus temperature and grain size compensated strain rate plot showed that the deformation kinetics of the current AZ31 alloy was similar to that reported in the literature for AZ31 alloys. Orientation image microscopy (OIM) was used to study the texture evolution, grain size, and grain boundary misorientation during uniaxial and biaxial forming. Influence of deformation parameters, namely strain rate, strain, and temperature on grain growth and refinement were discussed with the help of OIM results.     

Strain rate and orientation-dependent strain hardening of Mg–9Li–Al using crystal plasticity

A crystal plasticity finite element model was proposed considering slip and twinning interactions. The grain morphology and crystallographic orientations were introduced into the model to describe the microstructure of duplex polycrystalline Mg–9Li–Al. The activation of the slip systems and the strain localisation with respect to initial grain orientations were investigated. In addition, the effects of phase distributions and volume fractions on the macroscopic responses and on strain hardening rates were analysed. The results show that the strain hardening is rate-dependent but the texture is less sensitive to strain rate. The distribution of a phase and its volume fraction play primary roles in governing the mechanical response.     

Effect of rolling strain on microstructure and tensile properties of dual-phase Mg–8Li–3Al–2Zn–0.5Y alloy

This study was conducted to discuss the effect of rolling strain on microstructure and tensile properties of dual-phase Mg–8Li–3Al–2Zn–0.5Y (wt%) alloy, which was prepared by casting, and then homogenized and rolled at 200?°C. The rolling process was conducted with 10% reduction per pass and five different accumulated strains, varying from 10% to 70%. The results indicate that the as-cast and as-rolled Mg–8Li–3Al–2Zn–0.5Y alloys are composed of α-Mg, β-Li, AlLi and Al₂Y phases. After rolling process, anisotropic microstructure was observed. α-Mg phase got elongated in both rolling direction and transverse direction with the addition of rolling strain. Consequently, the strength of the alloy in both directions was notably improved whereas the elongation declined, mainly caused by strain hardening and dispersion strengthening. The tensile properties of the as-rolled alloys in the RD, no matter the YS, UTS or the elongation, are higher than those of the TD due to their larger deformation strain and significant anisotropy in the hcp α-Mg phase. In addition, the fracture and strengthening mechanism of the tested alloys were also investigated systematically.     

Hot shear deformation constitutive analysis and processing map of extruded Mg–12Li–1Zn bcc alloy

The hot shear deformation behavior of an extruded Mg–12Li–1Zn alloy was studied by shear punch test (SPT) in the temperature range 200–300 °C, and in the shear strain rate range 1.2 × 10⁻³–6.0 × 10⁻² s⁻¹. Based on the constitutive analysis of the SPT data, it was found that a sine hyperbolic function could properly describe the hot shear deformation behavior of the material. The activation energy of 108 kJ mol⁻¹ calculated from sine hyperbolic function together with the power-law stress exponents of 3.6–4.7 is indicative of lattice-diffusion-controlled dislocation climb mechanism as an operative deformation mechanism. As a new approach, the shear processing map was developed in order to determine the optimum processing condition, which was found to be 300 °C and 1.2 × 10⁻³ s⁻¹. Domains of the processing map are also interpreted on the basis of the associated microstructural observations. It was found that the post-deformation microstructure is sensitive to the Zener–Hollomon parameter, so that DRX was encouraged with decreasing Z-value.