Microstructures and compressive properties of AlxCoCrFeNi high entropy alloys prepared by arc melting and directional solidification

The AlxCoCrFeNi (molar radio, x=0.6 and 1.2) high entropy alloys (HEAs) were prepared by arc melting and directional solidification at the withdrawal rate of 150 μm/s. All microstructures were characterized by x-ray diffraction, optical microscopy and scanning electron microscopy with an energy-dispersive spectrometer. Strong similarities in phase constituent were observed between the as-cast samples and directionally solidified samples. The Al0.6CoCrFeNi HEA and Al1.2CoCrFeNi HEA fabricated by two different techniques respectively consisted of Cr-Fe-Co enriched FCC phase + Al-Ni enriched BCC phase and Al-Ni enriched B2 phase + Cr-Fe-Co enriched A2 phase. It was micromorphology found that directional solidification could not only make the microstructures arranged regularly but also coarsen the grains. This has been attributed to the preferred grain orientation and lower cooling rate during directional solidification process. Compression testing showed that the compressive ductility of directionally solidified samples decreased obviously. The ultimate compressive strength of Al0.6CoCrFeNi HEA increased from 1 675 MPa to 1 903 MPa, but the strength of Al1.2CoCrFeNi HEA decreased from 2 183 MPa to 1 463 MPa. The difference in strength has been suggested to be the result of micropores in the matrix.

Multi-phase field simulation of competitive grain growth for directional solidification

The multi-phase field model of grain competitive growth during directional solidification of alloy was established, solving multi-phase field models for thin interface layer thickness conditions, grain boundary evolution and grain elimination during the competitive growth of SCN-0.24wt% camphor model alloy bi-crystals were investigated, the effects of different crystal orientations and pulling velocities on grain boundary microstructure evolution were quantitatively analyzed. The results show that in the competitive growth of convergent bi-crystals, when favorably oriented dendrites are in the same direction as the heat flow and the pulling speed is too large, the orientation angle of the bi-crystal from small to large is the normal elimination phenomenon of the favorably oriented dendrite blocking the unfavorably oriented dendrite, and the grain boundary is along the growth direction of the favorably oriented dendrite, and when the pulling speed becomes small, the grain boundary shows the anomalous elimination phenomenon of the unfavorably oriented dendrite eliminating the favorably oriented dendrite. In the process of competitive growth of divergent bi-crystal, when the growth direction of favorably oriented dendrites is the same as the heat flow direction and the orientation angle of unfavorably oriented grains is small, the frequency of new spindles of favorably oriented grains is significantly higher than that of unfavorably oriented grains, and as the orientation angle of unfavorably oriented dendrites becomes larger, the unfavorably oriented grains are more likely to have stable secondary dendritic arms, which in turn develop new primary dendritic arms to occupy the liquid phase grain boundary space, but the grain boundary direction is still parallel to favorably oriented dendrites. In addition, the tertiary dendritic arms on the developed secondary dendritic arms may also be blocked by the surrounding lateral branches from further developing into nascent main axes, this blocking of the tertiary dendritic arms has a random nature, which can have an impact on the generation of nascent primary main axes in the grain boundaries.