In-situ investigation of martensitic transformation toughening with electron backscatter diffraction and nano-indentation

Quantitative and direct evidence for tetragonal to monoclinic martensitic transformation toughening was revealed by electron backscatter diffraction (EBSD) and in-situ nanoindentation, using plasma-sprayed 3 mol% Y2O3-ZrO2 coatings. On the basis of EBSD phase distribution, four zooms with different phase compositions and microstructure were selected. The tetragonal grains, which were surrounded by large pores, completely transformed into a monoclinic phase, but were then crushed when subjected to a loading of 10 mN. Moreover, the critical excitation stress σc for the martensitic transformation was estimated to be about 4.2 GPa. According to the displacement curves, the ratio of reduced modulus to hardness (Er/H), which directly indicates the toughness of a material, was quantitively calculated. This experimentally demonstrated that both the elastic and plastic deformation capacity of the partially transformed grain were significantly improved, compared with the untransformed tetragonal grains. These findings will provide a fundamental insight into martensitic transformation toughening.