Lian-xi Hu

Microstructure and mechanical properties of AZ61 magnesium alloy prepared by repetitive upsetting-extrusion

Abstract The process of repetitive upsetting-extrusion (RUE) was used to achieve severe plastic deformation (SPD) for an as-cast AZ61 magnesium alloy in temperature range of 285−380 °C. The microstructure and mechanical properties of the as-cast and RUE processed AZ61 alloys were investigated. The results indicated that homogeneous fine-grained structure with mean grain size of 3.5 μm was obtained as the accumulated true strain in the axial direction increased to 4.28 after three RUE passes at 285 °C. The dominant reason of grain refinement was considered the dynamic recrystallization induced by strain localization. It was also found that the microstructural evolution was affected by temperature and accumulated deformation. The mechanical properties of RUE processed AZ61 alloys were significantly improved owing to grain refinement. Furthermore, the relationship between deformation parameters and mechanical properties of AZ61 alloy prepared by RUE processing was revealed by tensile tests carried out at room temperature.

Grain refining and property improvement of AZ31 Mg alloy by hot rolling

Abstract Hot rolling of AZ31 Mg alloy was performed by using as-cast alloy ingot as the starting material. The microstructures and mechanical properties of the as-rolled alloy subjected to various rolling passes were investigated. The results show that the grain size of the alloy can be refined steadily with increasing rolling passes by dynamic recrystallization. With the steady refining of the grain size, both the mechanical strength and the plasticity of the alloy are improved correspondingly. In particular, when the grain size is reduced to about 5 μm after 5 rolling passes, the yield strength, ultimate tensile strength and tensile fracture strain of the alloy are 211 MPa, 280 MPa and 0.28 in the transverse direction, they are 200 MPa, 268 MPa and 0.32 in the rolling direction, respectively.

Development of flow stress model for hot deformation of Ti-47%Al alloy

Abstract The hot deformation behavior of a γ-TiAl based alloy (Ti-47%Al, mole fraction) was investigated by isothermal compression tests performed at elevated temperature of 900-1 200°C and strain rate of 0.001–0.02 s −1 . The effect of temperature, strain rate and strain on the flow stress of the alloy was evaluated. The higher the deformation temperature and the lower the strain rate, the smaller the deformation resistance. The stress exponent, n , and the apparent activation energy, Q , were determined as 2.6 and 321.2 kJ/mol by the sine hyperbolic law, respectively. Based on the experimental results by the orthogonal method, a flow stress model for hot deformation was established by stepwise regression analysis. Then the effectiveness of the flow stress model was confirmed by other experimental data different from those experimental data used to establish the model. And it was proved that the flow stress model can well predict the mechanical behavior and flow stress of the alloy during hot deformation.

Dynamic recrystallization kinetics of as-cast AZ91D alloy

Abstract The flow behavior and dynamic recrystallization (DRX) behavior of an as-cast AZ91D alloy were investigated systematically by applying the isothermal compression tests in temperature range of 220–380 °C and strain rate range of 0.001–1 s −1 . The effect of temperature and strain rate on the DRX behavior was discussed. The results indicate that the nucleation and growth of dynamic recrystallized grains easily occur at higher temperatures and lower strain rates. To evaluate the evolution of dynamic recrystallization, the DRX kinetics model was proposed based on the experimental data of true stress-true strain curves. It was revealed that the volume fraction of dynamic recrystallized grains increased with increasing strain in terms of S-curves. A good agreement between the proposed DRX kinetics model and microstructure observation results validates the accuracy of DRX kinetics model for AZ91D alloy.

Effect of hot rolling on grain refining and mechanical properties of AZ40 magnesium alloy

AZ40 Mg alloy thin sheets were prepared by multi-pass hot rolling with the hot-extruded alloy as the starting material. The effect of hot rolling on the microstructure, mechanical properties, and fracture behavior of the alloy was investigated. The results show that the microstructure homogeneity can be improved and the grain size is refined steadily by dynamic recrystallization with increasing rolling passes. As a consequence, the mechanical properties of the as-rolled sheets are improved significantly as compared with the starting as-extruded alloy. By 5 or more rolling passes, the average grain size is reduced to no more than 10 μm, and the yield strength and the tensile elongation of the sheets prepared achieve as high as more than 175 MPa and 20%, respectively, in both the rolling and the transverse direction.

Microstructures and properties of cold drawn and annealed submicron crystalline Cu-5%Cr alloy

Abstract The microstructures and properties after cold drawing and subsequent annealing of submicron crystalline Cu-5%Cr (mass fraction) alloy were investigated. The results show that, the microstructure of submicron crystalline Cu-5%Cr can be further refined by cold drawing. After cold drawing, the grains of Cu-5%Cr alloy with grain size of 400–500 nm can be refined to be cellular structures and subgrains with size of 100–200 nm. Both strength and ductility of Cu-5%Cr alloy can be enhanced by cold drawing, and the optimal mechanical properties can be achieved with drawing deformation increasing. It is suggested that dislocation glide is still the main mechanism in plastic deformation of submicron crystalline Cu-5%Cr, but grain boundary slide and diffusion may play more and more important roles with drawing deformation increasing. When the cold drawn Cu-5%Cr wires are annealed at 550 °C, fine recrystal grains with grain size of 200–300 nm can be obtained. Furthermore, there are lots of fine Cr particles precipitated during annealing, by which the recrystallization softening temperatures of the cold drawn Cu-5%Cr wires can be increased to 480–560 °C. Due to the fact that Cr particles have the effect of restricting Cu grains growth, a favorable structural thermal stability of the submicron crystalline Cu-5%Cr can be achieved, and the submicron grained microstructure can be retained at high temperature annealing.

Nanocrystalline Mg and Mg alloy powders by hydriding-dehydriding processing

Abstract The process of mechanically assisted hydriding and subsequent thermal dehydriding was proposed to produce nanocrystalline Mg and Mg alloy powders using pure Mg and Mg-5.5%Zn-0.6%Zr (mass fraction) (ZK60 Mg) alloy as the starting materal. The hydriding was achieved by room-temperature reaction milling in hydrogen. The dehydriding was carried out by vacuum annealing of the as-milled powders. The microstructure and morphology of both the as-milled and subsequently dehydrided powders were characterized by X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM), respectively. The results show that, by reaction milling in hydrogen, both Mg and ZK60 Mg alloy can be fully hydrided to form nanocrystalline MgH 2 with an average grain size of 10 nm. After subsequent thermal dehydriding at 300 °C, the MgH 2 can be turned into Mg again, and the newly formed Mg grains are nanocrystallines, with an average grain size of 25 nm.

MC simulation in microstructure evolution and grain growth during desorption-recombination processing of NdFeB alloy

Abstract Based on the mechanism of grain growth during desorption-recombination process of NdFeB alloy, Monte Carlo (MC) physical model was introduced to simulate microstructure evolution during grain growth process of Nd2Fe14B phase at various processing temperatures. Furthermore, the grain growth kinetics was studied. The analysis shows that the grain growth can be well simulated, and the average growth exponent n≈0.8 for the entire time domain by fitting the experimental data with the theoretical model, higher than the normal value of 0.5.

Desorption behaviour and microstructure change of nanostructured hydrided AZ31 Mg alloy powders

Abstract In order to optimize the dehydriding process for producing nanocrystalline Mg alloy powders by hydriding-dehydriding treatment, nano-structured as-hydrided Mg-3%Al-1%Zn (AZ31 Mg) (mass fraction) alloy powders were thermally dehydrided at various temperatures from 275 to 375 °C. The kinetics of hydrogen desorption was examined by hydrogen discharge measurement during dehydriding. The microstructure of the as-hydrided and the subsequently fully dehydrided alloy powders was investigated by X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM), respectively. Both the desorption kinetics and the grain size of the alloy after complete dehydriding were found to be strongly dependent on the processing temperature. The higher the temperature, the faster the desorption, and the more significant the grain growth. When the desorption temperature was raised from 300 to 375 °C, the time to achieve complete dehydriding was shortened from 190 to 20 min, and the average grain size increased correspondingly from 20 to 58 nm.

Hydriding and microstructure nanocrystallization of ZK60 Mg alloy by reaction milling in hydrogen

Abstract The hydriding of as-cast Mg-5.5%Zn-0.6%Zr (ZK60 Mg) (mass fraction) alloy was achieved by room-temperature reaction milling in hydrogen, with the mechanical energy serving as the driving force for the process. The hydriding progress during milling was examined by hydrogen absorption measurement, and the microstructure change was characterized by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), respectively. The results show that, by room-temperature reaction milling in hydrogen, the as-cast ZK60 Mg alloy can be fully hydrided to form a nanocrystalline MgH 2 single-phase microstructure. In particular, the average grain size of the MgH 2 phase obtained by room-temperature reaction milling in hydrogen for 16.2 h is about 8–10 nm, and the average particle size of the as-milled hydrided powders is 2–3 μm.