Heat treatment effects on the metastable microstructure, mechanical property and corrosion behavior of Al-added CoCrFeMnNi alloys fabricated by laser powder bed fusion

Abstract

• Micro-cracks are along the GBs in PBF-LB Al-HEAs due to the residual liquid film therein with an increased solidification temperature range by Al addition. • Al/Ni/Mn decorated dislocation cells exist in the as-built Al-HEAs and massive BCC phases precipitate at the dislocation cell/grain boundaries upon heat treatment. • Al-HEA850 exhibits the highest strength and the lowest corrosion resistance due to the massive precipitates. With the development of aerospace and transportation, high-strength structural materials manufactured by additive manufacturing techniques get more attention, which allows the production of counterparts with complex structures. This work investigates Al-added CoCrFeMnNi high-entropy alloys (Al-HEAs) prepared by laser powder bed fusion (PBF-LB), adding 4.4 wt.% Al reducing approximately 7% density. The contribution of post-heat-treatment to microstructure, mechanical properties, and corrosion behaviors are explored. Hot cracking along with grain boundaries in the as-built PBF-LB Al-HEAs is determined, which comes from the residual liquid film as a larger solidification temperature range by adding Al. The PBF-LB Al-HEAs mainly consist of a face-centered cubic (FCC) matrix with Al/Ni/Mn decorated dislocation cells therein and a minor body-centered cubic (BCC) phase. Upon 850 °C annealing treatment, massive BCC phases (ordered NiAl and disordered Cr-rich precipitates) generate at the dislocation cell/grain boundaries and the dislocation cells are still retained. However, the volume fraction of BCC phases and the dislocation cells vanish after 1150 °C solution treatment. As a result, Al-HEA850 shows an over 1000 MPa yield strength with nearly no ductility (<3%); the Al-HEA1150 exhibits considerable strength-ductility properties. Meanwhile, the Al-HEA850 demonstrates the worst pitting corrosion resistance due to the preferential dissolution of the NiAl precipitates in chloride-containing solutions. After comparatively deliberating the evolution of strength-ductility and localized corrosion, we built a framework about the effects of the heat treatment on the mechanical property and degradation behavior in additively manufactured Al-added high-strength HEAs.