Enhanced tensile yield strength in laser additively manufactured Al0.3CoCrFeNi high entropy alloy

Abstract

A precipitation strengthenable high entropy alloy (HEA), Al0.3CoCrFeNi, was processed via laser-based additive manufacturing (AM), using the laser engineered net shaping (LENS) process. The as LENS processed HEA exhibited twice the tensile yield strength, as compared to the conventionally arc-melted and solution treated HEA of the same composition, with a tensile ductility greater than 20%. Subsequent heat-treatments of the AM HEA alloy led to further enhancement of the yield strength while maintaining good tensile ductility. The microstructure of these AM alloys was investigated by coupling transmission electron microscopy (TEM) and atom probe tomography (APT). The near doubling of the yield strength in case of the as AM processed HEA samples, which were devoid of second phase intermetallic precipitates, has been rationalized based on the formation of nanometer-scale Al–Ni rich solute clusters due to the re-heating of the deposited layers during AM. The enhanced yield strength due to these solute clusters has been estimated using a simple cluster-dislocation interaction model involving the coherency strain fields of these nano-clusters. The even higher yield strength in case of the heat-treated AM HEA samples has been quantitatively rationalized employing precipitation strengthening models, based on nanometer scale L12 (gamma prime) precipitates.