Homogenization annealing of the 7050/6009 bimetal slab prepared by direct-chill casting was investigated and its effects on microstructural evolution, composition distribution and mechanical properties in the interfacial region of the bimetal were studied. The results show that the optimized homogenization annealing process was 460 °C for 24 h. After homogenization annealing, the Zn-rich phases and Al15(FeMn)3Si2 phases were precipitated at the interface of the bimetal. The diffusion layer thickness of homogenized bimetal increased by 30 µm from 440 to 480 °C for 24 h, while it increased by 280 µm from 12 to 36 h at 460 °C. The Vickers hardnesses at 6009 alloy side and interface of the bimetal decreased after homogenized annealing and grain coarsening was considered as the dominating softening mechanism. The hardness variation at 7050 alloy side was complicated due to the combined action of solution strengthening, dispersion strengthening and dissolution of reinforced phases.

The aging effects on the deformation and fatigue behavior of extruded Mg–8.0Gd–3.0Y–0.5Zr (GW83) magnesium (Mg) alloy were experimentally studied. The aging process significantly enhances the monotonic strengths under both tension and compression. The ductility under tension is unchanged but the elongation under compression is reduced due to the aging treatment. The cyclic stress–strain curve of the aged GW83 is much higher than that of the corresponding extruded state. The strain–life fatigue curve of the aged GW83 is similar to that of the extruded GW83, but the stress–life fatigue curve of the aged GW83 is much higher, indicating an improved fatigue strength due to the aging process. Similar to the other Mg alloys, a kink point in the strain–life fatigue curve was identified for the aged GW83 Mg alloy, and the kink point of a strain amplitude of 0.80% for the material demarcates the activation of bulk and persistent twinning/detwinning during cyclic deformation. The enhanced fatigue strength is discussed with respect to the observed fragmented persistent slip bands (PSBs) and the inhibition of microcracks by the precipitates in aged GW83.

•Compression/double twins can be easily induced under cyclic loading.•Formation of compression/double twins under cyclic loading do not lead to immediate fracture of the material.•Twin nucleation is promoted by cyclic loading whereas twin growth and twin shrinkage are inhibited.•Residual twins are trivial even at a large strain amplitude.An extruded ZK60 magnesium alloy was subjected to fully-reversed strain-controlled cyclic loading at a strain amplitude of 4.0% in the extrusion direction in ambient air. Electron backscatter diffraction (EBSD) analyses were conducted on samples taken from companion specimens terminated at different loading cycles to study the twinning–detwinning process and the evolution of the twin structures at different stages of cyclic deformation. It is observed that the twin nucleation sites are increased whereas twin growth/shrinkage is inhibited due to repeated twinning–detwinning. The enhanced twin nucleation sites are responsible for the observed increase in the number of twin lamellae and the increased twin volume fraction with loading cycles. Cyclic loading enhances formation of compression and double twins which do not result in immediate fracture of the material. With increasing number of loading cycles, more and larger sized residual tension twin lamellae can be detected by EBSD, but the total volume fraction of the residual twins is trivial.

The formability in three-dimensional rectangular cup drawing of AZ31 magnesium alloy sheet is successfully predicted by the Drucker–Prager plasticity (DP model). Numerical results show that the punch corner shoulder is under the condition of high tension bending and the die corner shoulder is on pure bending regardless of constitutive models. The failure is attributed to the competition between the biaxial tension bending induced fracture on the punch corner shoulder and the through-thickness shear induced fracture on other shoulders in the sheet forming. Taking the yield surface asymmetry into consideration, in a way of the friction angle in the coordinate of compressive and tensile strengths in the DP model, the accuracy of plastic strain is improved remarkably. The fracture locates on the punch corner shoulder where the strain reaches its fracture limit earlier than the die corner shoulder's. The predictions of forming limit and fracture locus in the DP model simulation are in good agreement with the experimental results.Highlights► DP model is applied to successfully predict the formability of AZ31 sheet. ► The strain of biaxial tension is lower than the pure shear's. ► The punch corner shoulder is under high tension bending. ► The biaxial tension bending induces fracture first.

Some large Mg–3.0Nd–0.2Zn–0.4Zr (NZ30K) magnesium alloy seamless tubes were prepared by forward extrusion. The as-extruded tubes were cooled in the air or by spraying liquid N2 after extrusion. The formability, mechanical and corrosive properties of the NZ30K magnesium alloy seamless tubes were investigated. The experimental results show that seamless NZ30K tubes with an outer diameter of 110 mm and inner diameter of 90 mm can be produced by forward extrusion and the tubes have good roundness, concentricity and straightness even without any straightening. The tensile results show that the maximum ultimate tensile strength, yield strength and elongation of the extruded tubes cooled in the air and by spraying liquid N2 are 306.3 and 314.6 MPa, 250.4 and 270.3 MPa, 14.2% and 15.6%, respectively. The corrosion rates of the as-extruded tubes cooled in the air and by spraying liquid N2 immersed in 5% NaCl solution for 3 days are 0.225 and 0.234 mg cm−2 day−1, respectively, which are a little inferior to the as-cast, T4 and T6 NZ30K alloys, but much lower than that of AZ91 alloy. Localized corrosion is suggested to be its corrosion pattern.

Tensile and high cycle fatigue properties of hot extruded ZK60 magnesium alloy have been investigated, in comparison to that of hot-extruded plus T5 heat-treated ZK60 magnesium alloy which was named as ZK60-T5. High cycle fatigue tests were carried out at a stress rate (R) of −1 and a frequency of 100 Hz using hour-glass-shaped round specimens with a gage diameter of 5.8 mm. The results show that tensile strength greatly improved and elongation is also slightly enhanced after T5 heat treatment, and the fatigue strength (at 107 cycles) of ZK60 magnesium alloy increases from 140 to 150 MPa after T5 heat treatment, i.e., the improvement of 7% in fatigue strength has been achieved. Results of microstructure observation suggest that improvement of mechanical properties of ZK60 magnesium alloy is due to precipitation strengthening phase and texture strengthening by T5 heat treatment. In addition, fatigue crack initiations of ZK60 and ZK60-T5 magnesium alloys were observed to occur from the specimen surface and crack propagation was characterized by striation-like features coupled with secondary cracks.

Mg–9Al–1Zn (AZ91) cast ingots annealed at 420 °C for 2 and 24 h were hot extruded at 270 °C, respectively. Microstructural observation showed that dynamic recrystallization (DRX) occurred in both cases resulting in fine equiaxed grains. It was revealed that the bar fabricated by extruding short-time annealed ingot had a much smaller average grain size (about 4 μm) than that extruded with a long-time annealed ingot (about 30 μm). It was found that in the former situation, large amounts of small spherical β (Mg17Al12) particles (about 0.3–0.8 μm) were broken directly from the unresolved β phases during the hot extrusion process, most of them finally distributing along the recrystallized grain boundaries. Tensile tests showed that the bars extruded from ingot with short annealing time had superior mechanical properties because of the refined microstructure and the small β particles.

•Strain gradients lead to graded microstructure and texture, affecting plasticity.•The twin volume fraction is influenced by slip-induced twinning behavior and strain compatibility among surrounding grains.•Slip-induced twinning behavior and its surrounding effects is much more profound in materials with weak texture.•The AM30 alloy with weaker texture shows better deformation compatibility than AZ31 alloy.Three-point bending represents important straining in metal forming, and is an ideal loading condition for in-situ investigation of complex microstructure evolution and plasticity of magnesium alloys. Two extruded alloys with different initial texture, AZ31 (Mg–3%Al–0.5%Zn1) and AM30 (Mg–3%Al–0.3%Mn), were studied in three-point bending with in-situ electron backscatter diffraction (EBSD) observations, to reveal the role of strain gradient and deformation compatibility in the plasticity of magnesium alloys. The results indicate that strain gradients in macro-scale from tension to compression can lead to a graded microstructure and texture evolution in the samples during bending. At the intergranular and intragranular levels, the volume fraction of twins was influenced by the slip-induced twinning behavior and strain compatibility among the surrounding grains except for Schmid factor relating to grain orientation. Slip-induced twinning behavior and its surrounding effects on the twinning volume fraction is much more profound in materials with weak textures. The AM30 alloy with a weaker texture shows better deformation compatibility (and, thus, formability) during three-point bending compared to AZ31 alloy with a stronger texture in this study.

Plastic deformation up to final rupture failure of a rolled magnesium (Mg) alloy Mg–3.0Al–1.0Zn–0.34Mn (AZ31B) under low stress triaxiality was investigated. Local strain evolution was quantified by the digital image correlation (DIC) technique analysis with tensile, combined tensile-shear, and shear specimens, corresponding to the stress triaxiality of 1/3, 1/6 and 0, respectively. Stress–strain curves show that the yield stress reduces with the decrease in the stress triaxiality, and obviously exhibits different strain hardening response. Electron backscatter diffraction (EBSD) observations reveal that the twinning behavior depends on stress triaxiality. Before fracture, double twinning is the dominant mechanism at the stress triaxiality of 1/3, while extension twinning is prevalent at the stress triaxiality of 0. Moreover, scanning electron microscopy (SEM) shows that the fracture mechanism is transformed from micro-void growth and coalescence to internal void shearing as the stress triaxiality decreases from 1/3 to 0.

•Detailed analysis of cyclic plastic deformation of ZK60 Mg alloy under uniaxial loading.•Microyielding dependent on microstructure at the peak stress prior to the loading reversal.•Constant elastic limit range of 100 MPa for ZK60 Mg alloy when twin-free.•Yielding dominantly associated with the activation of basal slips for basal a-texture.•Elastic limit dependent on twin volume fraction and loading history.A detailed analysis of the plastic deformation characteristics was performed for an extruded ZK60 magnesium alloy under uniaxial cyclic loading along the extrusion direction. The experiments used for the analysis were performed under single-step strain-controlled loading, two-step strain-controlled loading, and stress-controlled loading. An elastic limit with an offset of 10−5 plastic strain is used for the demarcation of elastic and elastic-plastic deformation. An inflection point is used to signify a transition of the dominated deformation mechanism from twinning-detwinning to dislocation slips. The macroscopic stress-strain response of the material is intrinsically related to the microstructures of the material during cyclic loading. The elastic limit range is closely related to the microstructure of the material at the peak stress prior to the loading reversal. If the microstructure at the peak stress displays a strong basal a-texture, yielding is dominantly associated with the activation of basal slips. The elastic limit range to activate basal slips for the ZK60 magnesium alloy under investigation is 100 MPa. If the microstructure at the peak stress contains tension twins, the elastic limit range during subsequent loading reversal reflects the activation stress of detwinning/retwinning process, which can be interpreted as the critical stress to activate the gliding of twin boundaries. The stability of twin boundaries is influenced by twin volume fraction, twin morphology, and cyclic hardening. Dependent on the twin volume fraction and loading history, the elastic limit range varies from 20 MPa to 100 MPa for the material under investigation.