Effect of phase transformation on ductility of additively manufactured Co-28Cr-6Mo alloy: An in situ synchrotron X-ray diffraction study during mechanical testing

The influence of strain-induced martensitic transformation on the mechanical properties of a Co-28Cr-6Mo alloy produced using the direct metal laser sintering additive manufacturing technique was analyzed. Both the as-built and solution heat-treated (ST) specimens were subjected to conventional tensile tests. Furthermore, X-ray diffraction spectra were simultaneously acquired using a synchrotron light source. In addition, electron backscatter diffraction analysis was performed on the specimens before and after tensile testing. Both techniques revealed the occurrence of the strain-induced gamma-face-centered cubic -> epsilon-hexagonal close-packed phase transformation. The higher microstrain and preferential grain orientation of the as-built specimen favored the onset of the phase transformation. Moreover, the ductility of the ST specimen was significantly higher than that of the as-built specimen. This behavior was attributed to the isotropic properties of the randomly-oriented and equiaxed grains, increase in resistance to necking, and reduction in internal stresses caused by the solution heat treatment.