Electron probe microanalysis
(EPMA) was performed on JEOL JXA-8100 at an accelerating
voltage of 20 kV, with a 2 108 A beam current. Grain
boundary precipitates was characterized using a Tecnai G2 F30STWIN
transmission electron microscopy (TEM) operating at
300 kV. Electron transparent specimen was generated by twin-jet
electropolishing in a solution of 80 vol.% methanol þ20 vol.% nitric
acid.