Phase prediction, densification, and microstructure of AlCrFeNi(TiO2)x high entropy alloy composite fabricated by spark plasma sintering

This study incorporates the knowledge of empirical calculation and computational concepts using Thermo-Calc as a design guide to develop AlCrFeNi high entropy alloy (HEA). Subsequently, a novel AlCrFeNi HEA composite consisting of varying weight percentages (wt%) of TiO2 reinforcements is synthesized using mechanical alloying (MA) and spark plasma sintering (SPS) techniques. Our work investigated the role of TiO2 reinforcement on the densification behavior, phase changes, microstructure, and properties of AlCrFeNi HEA. The phase diagram was constructed using Thermo-Calc software. Powder densification during SPS was analyzed using obtained sintering data. The samples were characterized using Xray Diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques. The density and micro-hardness were evaluated. The result shows that the densification during SPS is dependent on kinetic mechanisms such as melt diffusion, surface diffusion, and plastic flow. Phases of the disordered BCC phase (rich in FeCr) and ordered B2 phase (rich in NiAl) were found in the AlCrFeNi HEA. The phases were consistent with the prediction using thermo-calc software. The BCC phases in the AlCrFeNi are retained regardless of the addition of TiO2 particles but with the diffusion of Ti with O atoms fitting into the interstitial sites in the BCC and B2 lattice. The microstructure verified the homogeneous distribution of the TiO2-rich phase within the BCC and B2 phases of the AlCrFeNi HEAs. The density decreases with the adding TiO2. The hardness of reinforced AlCrFeNi is enhanced from 537.24 Hv to 752.74 Hv with an increment of TiO2 reinforcement from 0 to 8 wt%. TiO2 particles acted as impediments to the mobility of dislocation, thereby increasing the resistance to deformation of HEA composites during micro indentation.