A high-resolution-electron-microscopy study of γ/γ′-α directionally solidified eutectic alloy - Part II As-ageing treatment

A master alloy with eutectic compositions of Ni-30.26Mo-6.08Al-1.43 V (wt%) has been directionally solidified (DS) into γ/γ′-α alloy. The microstructural as-ageing treatment was studied by means of high resolution electron microscopy (HREM). A majority of α fibres still display the Bain orientation relationship with the γ′/γ matrix. In a few cases, however, the so-called Nishiyama-Wasserman (NW) orientation relationship is found in specimens aged at 850 °C for 2000 h. Different microdomain structures of the γ phase, corresponding to different ageing temperatures, were revealed. Orthorhombic Ni3Mo phase, with a size of tens of nanometres, was found to precipitate inside α fibres after ageing at both 850 and 650 °C. Occasionally, an γ′-Ni3Al phase with lamellar twin structure was also found to coexist with Ni3Mo precipitate inside the α fibres. The orientation relationships between the precipitates and the α fibres were determined. Energy dispersive X-ray analysis (EDX) showed that the precipitation of Ni3Mo and Ni3Al is due to solid solution of Ni and Al in the α fibres.