Dependence of Microstructures and Mechanical Properties on the Growth Rate and Composition in Directionally Solidified Mg-Gd Alloys

Microstructures and mechanical properties of directionally solidified Mg-xGd (5.21, 7.96 and 9.58 wt.%) alloys were investigated at a wide range of growth rates (V = 10-200 μm/s) under the constant temperature gradient (G = 30 K/mm). The results showed that when the growth rate was 10 μm/s, different interface morphologies were observed in three tested alloys: cellular morphology for Mg-5.21Gd alloy, a mixed morphology of cellular structure and dendritic structure for Mg-7.96Gd alloy and dendrite morphology for Mg-9.58Gd alloy, respectively. Upon further increasing the growth rate, only dendrite morphology was exhibited in all experimental alloys. The microstructural parameters (λ1, λ2) decreased with increasing the growth rate for all the experimental alloy, and the measured λ1 and λ2 values were in good agreement with Trivedi model and Kattamis-Flemings model, respectively. Vickers hardness and the ultimate tensile strength increased with the increase of the growth rate and Gd content, while the elongation decreased gradually. Furthermore, the relationships between the hardness, ultimate tensile strength, the growth rate and the microstructural parameters were discussed and compared with the previous experimental results.

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.