Microstructural instability of L-PBF Co-28Cr-6Mo alloy at elevated temperatures

Abstract

In relation to the production of complex aircraft components, the additive manufacturing (AM) of Co-28Cr-6Mo alloy is highly discussed as it could bring many benefits regarding new designs, components lightening and shortening the design-to-product period. However, such components must undergo strict requirements for mechanical properties and material stability, even at elevated temperatures which might occur during part operation. So far, little is known about the thermal stability of Co-28Cr-6Mo alloy when produced by laser powder bed fusion (L-PBF), one of the mostly used AM technologies. In the present study, we reveal the instability of this material already at temperatures above 673 K. Material hardening accompanied with a significant loss of plasticity is caused by the breakdown of fine cellular microstructure with intercellular Mo segregation, precipitation of σ-phase and phase transformation of the matrix. Detail characterization by transmission electron microscopy was carried out for the maximum hardness state obtained after annealing the alloy at 1173 K for 5 h. The important findings of this study, representing an operational risk for L-PBF Co-28Cr-6Mo alloy, call for subsequent research into suitable heat treatment that would induce the stability of the material.