Multi-dimensional morphology of hydride diffusion layer and associated sequential twinning in commercial pure titanium

The dominant hydride precipitates have been well demonstrated to follow two types of orientation relationships (ORs) with Ti matrix: OR1 with {0001}//{001}, <12¯10>//<110> and OR2 with {0001}//{11¯1}, <12¯10>//<110>. Within the grains with special orientations, the complicated interactions of different hydride variants inside Ti-hydride diffusion layer are characterized in this work. For OR1 hydride layer, the orientations of {101¯0} plane parallel to the sample surface and a-axis parallel to the normal direction prefer multiple OR1 variants. The orientations favorable for OR2 hydride layer are: {101¯3} plane parallel to sample surface corresponding to the layer with one OR2 variant dominated and c-axis parallel to the surface normal with multiple OR2 variant layer preferred. Furthermore, {101¯2} extension twins and {112¯2} contraction twins are activated to accommodate the OR2 hydride-induced surface expansion and local misfit strain. The stimulation of these two twins is also orientation-dependent: {101¯2} and {112¯2} twins are observed in the grains with c-axis parallel to and deviated from the surface normal, respectively. The further variant selection for each twin mode is performed through shear accommodation of hydride-twin pairs.