*2.5. Microstructural Observation*

The number of crystal grains across the wall thickness *t*<sup>0</sup> of the starting materials was evaluated. The cross sections of the starting materials were mechanically polished after resin filling. Kikuchi patterns of the starting materials were obtained using a field emission scanning electron microscope (FE-SEM) (JSM-6500F, JEOL, Tokyo, Japan) with electron backscatter diffraction (EBSD) (OIM7, TSL, Kanagawa, Japan). The Inverse pole figure (IPF) maps were drawn, and the average grain size *g* was measured using an OIM AnalysisTM (ver.8, TSL, Kanagawa, Japan). A boundary with an orientation of 15◦ or more was defined as a grain boundary. The weighted mean of the grain area

was calculated, and the average diameter of crystal grains was calculated as the grain size. The grain numbers across the wall thicknesses of the starting materials were calculated by dividing the wall thicknesses by the average grain size as *t*0/*g*.
