Heat treatment possibilities for an in situ βTi-TiC composite made by laser powder bed fusion

After laser powder bed fusion (LPBF) of an ultra-strong in situ TiC whisker reinforced beta-Ti composite, this paper investigates the evolution of microstructure and mechanical properties in response to heat treatment at different temperatures. Using in depth nano-SEM and TEM analyses, it is shown that ageing at 400 degrees C rounds the whiskers, annihilates the strain fields and grows Mo segregated nano-cells, but without improving the ductility. In contrast, ageing at 600 degrees C enables the transformation of metastable beta to a lamellar beta + alpha, leading to a dual phase matrix embedding TiC particles. This is in such a manner that extra ageing at 600 degrees C coalesces the nano-lamellar alpha + beta microstructure to form a coarser micro-lamellar alpha + beta matrix. This microstructure achieves 66 % of the compressive deformation of Cp-Ti, and over 1400 MPa compressive strength after 1 h of ageing at 600 degrees C. Despite this success under compression, hard and stiff TiC particles may still cause large spherical fractured voids, severely limiting the plastic deformation under tension.