Enhanced thermal stability and mechanical properties of an additively manufactured CoCrNiFeMn high entropy alloy

Abstract

• CoCrNiFeMn high entropy alloy was manufactured by selective laser melting. • Unique microstructure (e.g. cellular structure) formed during printing process. • It exhibited enhanced thermal stability even after annealing at 1373 K. • It showed remarkable tensile properties from cryogenic to elevated temperatures. • These properties were due to innately stable dislocation cellular structures. High entropy alloys (HEAs), particularly CoCrNiFeMn system, have emerged as a transformative class of high-performance alloys due to their exceptional mechanical and functional properties. However, traditional manufacturing methods for HEAs are limited by inefficiencies and high costs, restricting their widespread applications. Additive manufacturing (AM), specifically laser powder bed fusion (LPBF), offers a promising alternative by enabling the fabrication of HEAs with unique microstructures and enhanced properties. This study investigates the thermal stability and mechanical performance of LPBF-printed CoCrNiFeMn HEA across a wide temperature range. The as-built LPBF HEA with a hierarchically heterogeneous microstructure, featured by columnar grains and ultrafine dislocation cellular structure, demonstrates exceptional thermal stability, with minimal hardness reduction and no apparent recrystallisation even after prolonged exposure to high temperatures (up to 1373 K), in stark contrast to the significant property degradation observed in conventionally processed HEAs. This stability is attributed to the unique dislocation cellular structures and the intrinsic thermal self-stabilizing effects induced by the LPBF process and the inhibition of recrystallisation due to the low stored energy and columnar grain morphology. The LPBF-fabricated HEA also exhibits outstanding strength-ductility synergy across a broad temperature spectrum, with cryogenic deformation enhancing both strength and ductility due to the activation of deformation twinning. At elevated temperatures, the alloy undergoes a slight reduction in strength but retains good ductility, except at 873 K, where a sharp decline in ductility is observed likely due to grain boundary decohesion and porosity-related crack initiation manifested by the cleavage fracture surface and the cracks at grain boundaries. These findings provide new insights into the temperature-dependent mechanical behavior of AM HEAs, highlight the critical role of dislocation cellular structures in achieving superior thermal and mechanical performance, and underscore the potential of additively manufactured HEAs with tailored microstructures for extreme environments.

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