3D characterization of microstructural evolution and variant selection in additively manufactured Ti-6Al-4V

R. DeMott, N. Haghdadi, X.Z. Liao, S.P. Ringer, S. Primig
Journal of Materials Science 56 (2021) 14763-14782. 

This is Ryan's second paper using 3D EBSD to unravel the microstructure evolution in Ti-6Al-4V during electron powder bed fusion..

3D electron backscatter diffraction (EBSD), can provide a wealth of information about processing-structure relationships. Despite this, it has not seen widespread adoption due in part to the difficulty and long collection time of the experiment.

We apply 3D EBSD to reveal new and otherwise unavailable information about the complex microstructures and α variant selection mechanisms during electron beam melting of Ti-6Al-4V. We aim to provide a clearer understanding without ambiguity from sectioning effects of how α’ decomposes into microstructures with distinct morphologies and variant/intervariant distributions as a function of electron beam scanning strategy. We present detailed analyses of three datasets collected from three different builds with variations from raster to spot melts, resulting in highly distinct microstructures. From this data, we extract quantitative 3D information on the various intervariant boundaries networks formed.

We demonstrate that differing mechanisms during the α’ decomposition result in a shift from self-accommodating clusters in an acicular microstructure, to either the preferred growth of six variants in a basketweave microstructure or to a colony microstructure where variant selection is determined by prior-β grain boundaries.

In our discussion we propose an ‘intervariant network diagram’, a method of representing the interplay between different groups of variants and intervariant boundaries resulting from the Burger’s orientation relationship. This allows us to reveal how the selection of certain variants during the martensitic transformation and subsequent decomposition leads to the intervariant boundary networks observed.