Heat treatment influence on the microstructure and mechanical properties of AlCrFeNiTi_0.5 high entropy alloys

The influence of heat treatments and cooling rates on the microstructure and mechanical properties of AlCrFeNiTi_0.5 high-entropy alloys were investigated using a variety of techniques such as DTA, XRD, SEM, Vickers microhardness, and compression tests. Both AlCrFeNi and AlCrFeNiTi_0.5 alloys solidified dendritically with a dendrite core composed of nanosize disordered BCC (Cr+Fe) precipitates embedded in an ordered BCC (B2-NiAl) matrix and interdendritic region composed of radiating lamellar eutectic (EHEA). However, three additional phases were found in the as-cast AlCrFeNiTi_0.5 alloy: Fe_0.2Ti_0.8 -BCC, Fe₂Cr - FCC, and traces of Heusler Ni₂AlTi. This in turn resulted in increased strength while maintaning reasonable ductility of the AlCrFeNiTi_0.5 alloy compared to AlCrFeNi. Heat treatment at 650 °C increased the amount of the hard and brittle disordered BCC and ordered B2 phases, while significantly reducing the ductile FCC2 (Fe₂Cr) phase, as well as causing coarsening of the dendrite core. These changes led to increased hardness and brittleness of the alloy when compared to the as-cast state. Heat treatment at 850 °C led to coarsening of the dendrite core microstructure and decreased quantities of the hard and brittle disordered BCC and ordered B2 phases, while simultaneously increasing the ductile Heusler phase which led to an increase in ductility but less overall strength. Heat treatment at 1200 °C resulted in morphological changes of the dendrite core from precipitates to coarse cells and significantly increased the formation of the Heusler phase, which is still hard but is relatively ductile. This resulted in both increased strength and ductility of AlCrFeNiTi_0.5 alloy when heat treated at 1200 °C. Increasing the cooling rates during solidification of the AlCrFeNiTi_0.5 alloy from 0.35 K/s to 10 K/s had only minor effects on the as-cast microstructure and microhardness. However, increasing the cooling rate to 10⁵ K/s led to a dramatic morphological change of the grain cross-section which composed of four different microstructure morphologies to featureless grains at the SEM resolution used in this investigation as well as a large increase in microhardness. Although the AlCrFeNiTi_0.5 alloy contained large quantities of Cr, Fe, and Ti (which are known to enhance the formation of σ phase, especially after heat-treatment between 650 and 850 °C), no σ phase was observed after heat treatment. We attribute this behavior to the absence of Co in the alloy.