Mingbo Yang

Investigation on the Microstructure Characterization and Aging Response of Rapidly Solidified Mg-6wt%Zn-5wt%Ca Alloy Produced by Atomization-Twin Roll Quenching Technology

The Mg-6Zn-5Ca (wt%) alloy flakes were produced via atomization-twin roll quenching technology and analyzed by transmission electron microscopy (TEM)/energy-dispersive spectroscopy (EDS) (selected area diffraction pattern) and microhardness testing. The results showed that the alloy exhibited a relatively high hardness of 97.1 ± 5.3 Hv in the as-quenched state, and a significant age-hardening response and the maximum hardness was 120.8 ± 4.1 Hv after annealing at 200°C for 1 h, which mainly resulted from the precipitation of dispersive Ca2Mg6Zn3 phases that were distributed within the grains according to the transmission electron microscopy/selected area diffraction pattern. The stable Mg2Ca and a few Ca2Mg6Zn3 precipitates with relatively larger size were observed at the grain boundary, which were beneficial for a high hardness (87.9 ± 4.4 Hv) of the alloy experienced aging at 400°C.

Multiscale Modeling of Polycrystalline NiTi Shape Memory Alloy under Various Plastic Deformation Conditions by Coupling Microstructure Evolution and Macroscopic Mechanical Response

Numerical modeling of microstructure evolution in various regions during uniaxial compression and canning compression of NiTi shape memory alloy (SMA) are studied through combined macroscopic and microscopic finite element simulation in order to investigate plastic deformation of NiTi SMA at 400 °C. In this approach, the macroscale material behavior is modeled with a relatively coarse finite element mesh, and then the corresponding deformation history in some selected regions in this mesh is extracted by the sub-model technique of finite element code ABAQUS and subsequently used as boundary conditions for the microscale simulation by means of crystal plasticity finite element method (CPFEM). Simulation results show that NiTi SMA exhibits an inhomogeneous plastic deformation at the microscale. Moreover, regions that suffered canning compression sustain more homogeneous plastic deformation by comparison with the corresponding regions subjected to uniaxial compression. The mitigation of inhomogeneous plastic deformation contributes to reducing the statistically stored dislocation (SSD) density in polycrystalline aggregation and also to reducing the difference of stress level in various regions of deformed NiTi SMA sample, and therefore sustaining large plastic deformation in the canning compression process.

Phase constitutions of rapid solidification/powder metallurgy Mg–Zn–Ca–RE alloys

Mg-6Zn-5Ca-3Ce and Mg-6Zn-5Ca-3Ce-0.5La (wt.%) alloys in the form of rods were prepared by hot extrusion of rapidly solidified powders, produced via atomization of the alloy melt and subsequent splat-quenching between a pair of water-cooled copper rollers. The microstructures and phase constitutions of the alloys were investigated. The results showed that the thermally stable MgxZnyREz phase with a small amount of Ca was present in the alloys, and its formation may be due to the dissolving of Ca in MgxZnyREz phase. For the Mg-6Zn-5Ca-3Ce alloy, the grain size ranged from 350 nm to 1 μm. With an addition of 0.5 wt.% La, the microstructure of the alloy was refined, for which the grain size ranged from 300 nm to 500 nm, mainly resulting from the increase in both the number of RE-containing intermetallics and the melting point of the MgxZnyREz phase with a small amount of Ca.