Powder metallurgical processing of equiatomic AlCoCrFeNi high entropy alloy: Microstructure and mechanical properties

Phase formation, microstructural evolution and the mechanical properties of novel multi-component equiatomic AlCoCrFeNi high entropy alloy synthesized by high energy ball milling followed by spark plasma sintering have been reported here. The microstructure of the mechanically alloyed (MA) powder and sintered samples were studied using X-ray diffraction, scanning electron and transmission electron microscopy, whereas the detailed investigation of the mechanical properties of the sintered samples were measured using micro and nano hardness techniques. The fracture toughness measurements were performed by applying single edge V notch beam (SEVNB) technique. The MA powder shows the presence of FCC (tau) and BCC (kappa) solid solution phases. Extended ball milling (up to 60 h) does not change the phases present in MA powder. The sintered pellets show phase-separated microstructure consisting of Al-Ni rich L1(2) phase, alpha' and tetragonal Cr-Fe-Co based sigma phase along with Al-Ni-Co-Fe FCC solid solution phase (epsilon) for sample sintered from 973 to 1273 K. The experimental evidences indicate that BCC (kappa) solid solution undergoes eutectoid transformation during sintering leading to the formation of L1(2) ordered alpha' and sigma phases, whereas FCC (tau) phase remains unaltered with a slight change in the lattice parameter. The hardness of the sample increases with sintering temperature and a sudden rise in hardness is observed 1173 K. The sample sintered at 1273 K shows the highest hardness of similar to 8 GPa. The elastic modulus mapping clearly indicates the presence of three phases having elastic moduli of about 300, 220 and 160 GPa. The fracture toughness obtained using SEVNB test shows a maximum value of 3.9 MPa m(1/2), which is attributed to the presence of brittle nanosized sigma phase precipitates. It is proposed that significant increase in the fraction of sigma phase precipitates and eutectoid transformation of the tau phase contribute to increase in hardness along with better densification at higher sintering temperatures.