Microstructure and tensile property of a precipitation strengthened high entropy alloy processed by selective laser melting and post heat treatment

The microstructure and tensile property of Al_0.2Co_1.5CrFeNi_1.5Ti_0.3 high entropy alloy (HEA) processed by selective laser melting (SLM) have been studied in comparisons to that fabricated by traditional cast & wrought (C&W) route. The C&W samples were annealed and then aged at 750 ℃ for 50 h to promote precipitation strengthening of L1₂ structured phase in FCC matrix. By contrast, SLM samples were aged directly at 750 ℃ for 50 h from the as-built condition, which possessed sub-micron dendritic segregation, internal stress and nano oxides; the result was a microstructure with not only L1₂ particles dispersion, but also L2₁ phase was formed at subgrain boundaries, nano oxides and dislocations were present in the matrix. The tensile properties of this HEA were significantly enhanced by the SLM and direct ageing process; the room temperature yield strength (1235 MPa) and ultimate tensile strength (1550 MPa) achieved in this work are the highest to-date for HEAs subjected to laser additive manufacturing processes. Comparing to those of C&W samples, the SLM samples at room temperature and 500 °C exhibited additional yield strengths of 284 MPa and 229 MPa, and additional ultimate tensile strengths of 240 MPa and 275 MPa, respectively. The degree of L1₂ phase precipitation strengthening was similar for both SLM and C&W samples subjected to the same ageing treatment, interestingly, the additional microstructure features of SLM sample provided extra strengthening contributions; detailed analysis on microstructure evolution and strengthening factors are presented and discussed in this article.