Yu Sun

Hot deformation behavior and microstructure evolution of as-cast AZ91D magnesium alloy without pre-homogenization treatment

The deformation behavior and the microstructure evolution of as-cast AZ91D magnesium alloy without pre-homogenization treatment were systematically investigated. The flow stress behavior was studied by compression tests in strain rate range of 0.001–1.000xa0s−1 and deformation temperature range of 220–380xa0°C with a maximum deformation strain of 60xa0%. The dependence of flow stress on deformation temperature and strain rate was described by hyperbolic sine constitutive equation. Through regression analysis, the average apparent activation energy and coefficient of strain rate sensitivity were estimated to be 181.98xa0kJ·mol−1 and 0.14, respectively. The results also reveal that the variation of peak stress depends on strain rate and deformation temperature. Microstructure observation shows that, at temperatures higher than 300xa0°C and strain rates lower than 0.01xa0s−1, DRX developed extensively at the grain boundaries and in the core of coarse grains, resulting in a more homogeneous microstructure. Furthermore, the effects of strain, deformation temperature, strain rate, and eutectic β phase on the microstructure evolution of as-cast AZ91D magnesium alloy were discussed.

Investigation on Cracking Behavior and Development of a Fracture Model of Ti-47Al-2Nb-2Cr Alloy During Hot Deformation

When alloys are strained toxa0a certain limit under the plastic deformation, ductile fracture occurs. Whether the required deformation can be put into effect before fracture appearsxa0has become a serious concern. A combination ofxa0compressionxa0tests, ductile fracture criteria as well as finite element simulation (FEM) werexa0employed in this study to establish the fracture initiation prediction model of Ti-47Al-2Nb-2Cr alloy. To obtain the critical fracture strain, axa0technique was proposed to calculate the data for deformed cylindrical samples through isothermal compression tests in the temperature range between 1223 and 1423xa0K with strain rates ranging from 0.001 to 1xa0s−1. Five commonly used ductile fracture criteria were compared by FE simulation of compressing tests. It was found that the Brozzo criterion wasxa0found to be suitable and accordingly a failure criterion, considering temperature and strain ratio was established. Hot working parameters (critical fracture value and critical strain) in deformation could be obtained through the finite element simulation, revealing thatxa0this newly established failure criterion could accurately predict cracking initiation in compressing deformationxa0of Ti-47Al-2Nb-2Cr alloy ingot.