Abstract
The aim of this research work was to study phase evolution, microstructural development and resulting mechanical properties in Ni46Al12Co18Cr8Fe12Mo4, at. % high entropy alloys (HEAs) synthesized via spark plasma sintering (SPS) of 50 hr mechanically alloyed (MAed) powder with varied sintering parameters. Detailed microstructural analysis of the consolidated samples revealed that sintering at 1100 °C resulted in improved densification, i.e. up to 99% and enhanced grain growth compared to the samples sintered at lower temperatures. Phase analysis of the sintered HEAs envisaged formation of face centered cubic (FCC) structured single-phase solid solution with minor amount of brittle sigma (σ) phases. Phase diagram generated from Thermo-Calc shows that volume fraction of the σ phases decreases continuously with temperature and further they dissolve at 1180 °C, which is in close agreement with experimental results of differential thermal analysis (DTA) performed on the sintered samples. Moreover, microhardness of the consolidated HEAs decreased gradually with increase in sintering temperature, ascribed to lower volume fraction of the brittle σ phases at higher temperature. Further, nanoindentation test was conducted to estimate the hardness of the individual phases and it was found that nanohardness of σ phase is approximately 1.7 times higher than that of the solid solution phase. The HEA sintered at 1100 °C showed an excellent strength-ductility synergy, with a compressive yield strength of 1121 ± 21 MPa and the elongation to failure up to 20 ± 1%, attributed to the presence of ductile FCC phase with minor amount of brittle σ phase. The present study demonstrates the potential of SPS technique to synthesize Ni base HEA with excellent thermal stability and superior mechanical properties for structural applications.
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