Phase transformation in additively manufactured Co-Cr-Mo alloy after solution and aging heat treatment

Abstract

The unique structure and solute distribution of CoCrMo alloys produced using Laser Powder Bed Fusion (L-PBF) technique require custom heat-treating processes to achieve the targeted phase distribution and mechanical properties. The rise of multi-material additively manufactured structures also prompts further study at temperatures beyond standard aging and solution treatment conditions. In the present study, the phase transformation behavior, precipitate distribution, and γ-ε phase boundary orientation relationships of as-printed and solution heat-treated CoCrMo samples after aging heat treatment at 940 °C were investigated. The results show a higher ε-phase fraction of 71.2% in directly aged samples compared to 49.2% in solution heat-treated samples. This discrepancy is attributed to band-like isothermal transformation, coherent grain face massive transformation, and recrystallization. Carbide nucleation sites shifted from disintegrated cellular structures in directly aged samples to grain boundaries and twin interfaces in solution-treated samples. The presence of transgranular ε-phase bands and adjacent grains with analogous orientations observed after direct aging were linked to the weak 〈110〉 // BD and 〈111〉 // BD texture after printing. Tensile tests demonstrated an increase in tensile strength from 1162 MPa to 1261 MPa after direct aging, while significant enhancement in ductility from 16.4% to 26.1% was attained after solution heat treatment. While direct aging is more suitable for applications requiring high strength and wear resistance, solution heat treatment is advantageous when a consistent morphology and stress-free isotropic structures are desired.