Structure and mechanical properties of a low-density AlCrFeTi medium entropy alloy produced by spark plasma sintering

Abstract

Bulk equiatomic AlCrFeTi medium entropy alloy was produced by mechanical alloying (MA) and spark plasma sintering (SPS). Powders with a homogeneous chemical composition composed of amorphous and bcc phases were obtained due to MA. Bulk, almost porosity free specimen of the alloy with the measured density of 5.53 ± 0.07 g × cm−3 was obtained after SPS. A mixture of a (Fe, Ti)-rich C14 Laves phase, a Cr-rich bcc phase, and an Al-rich L12 phase was found in the as-sintered specimens. The average size of grains/particles of the different phases was 115 ± 100 nm. Annealing at 1000°С resulted in some coarsening of the microstructure to the grain/particle size of 140 ± 120 nm and in an increase in the fraction of the bcc and L12 phases, yet the microstructure was found to be relatively stable. After SPS the AlCrFeTi alloy had a high microhardness of 1090 ± 120 HV, which decreased after annealing to 870 ± 70 HV. Compression tests have meanwhile revealed low ductility of the alloy after SPS – the alloy exhibited brittle fracture at room temperature and became ductile at only 800 °C. After annealing the alloy demonstrated reasonable ductility already at 700 °C. Also, the annealed alloy had a remarkable specific yield strength of 258 kPa × m3/kg at 700 °C. The experimental data on the phase composition of the alloy was compared with the Thermo-Calc predictions, and the relationships between the structure and properties of the alloy were discussed.