2.2. Microstructural analysis
The morphologies of grains and precipitates in BM, HAZ and WNZ were investigated. Optical microscopy (OM) observation was performed using Leica DFC 295 in polar light mode. Electron backscatter diffraction (EBSD) analyses were performed using ZEISS Sigma 300 field emission gun scanning electron microscope with a scanning step size of 0.15 μm. The EBSD data was analyzed by Channel 5 software to acquire microstructure statistics information. Boundaries with misorientations less than 15° were defined as low-angle grain boundary (LAGB) and those with misorientations greater than 15° were defined as high-angle grain boundary (HAGB), respectively. The scanning electron microscopy (SEM) observation of fracture surface were also operated by ZEISS Sigma 300 at 20 KV. Transmission electron microscopy (TEM) and high resolution transmission electron microscope (HRTEM) observations were carried out on FEI Tecnai G2 F20.