The mechanical property evolution and grain boundary accommodation during hydride transformation in commercial pure titanium

The mechanical property evolution of titanium surface during hydrogen charging was investigated by nanoindentation tests. The nanohardness increases while elastic modulus decreases due to the formation of hydride precipitation. By mean of interrupted in-situ electron backscatter diffraction (EBSD) measurements, the microstructure evolution of hydrogenated surfaces with different grain sizes was examined. The more prevalent grain-boundary accommodation in fine grain sample leads to the faster hydride transformation from metastable γ to stable δ hydride than coarse grain sample. The formation of intergranular hydride pair is regarded as one type of grain-boundary accommodation mode. Adjacent hydrides prefer to be induced at grain boundaries with both low c-axis misorientation angle θc (θc < 30o) and high θc (75o < θc < 85o). The synergistic shear of Shockley partial dislocation and the c-axis dilatation misfit of hydride transformation are the fundamental reasons for the above two types of favorable grain boundaries, respectively.