A HIGH-RESOLUTION-ELECTRON-MICROSCOPY STUDY OF A GAMMA GAMMA'-ALPHA DIRECTIONALLY SOLIDIFIED EUTECTIC ALLOY .2. AS-AGING TREATMENT/

A master alloy with eutectic compositions of Ni-30.26Mo-6.08Al-1.43 V (wt%) has been directionally solidified (DS) into gamma/gamma'-alpha a lloy. The microstructural as-ageing treatment was studied by means of high resolution electron microscopy (HREM). A majority of alpha fibres still display the Bain orientation relationship with the gamma'/gamma matrix. In a few cases, however, the so-called Nishiyama-Wasserman (N W) orientation relationship is found in specimens aged at 850 degrees C for 2000 h. Different microdomain structures of the gamma phase, cor responding to different ageing temperatures, were revealed. Orthorhomb ic Ni3Mo phase, with a size of tens of nanometres, was found to precip itate inside alpha fibres after ageing at both 850 and 650 degrees C. Occasionally, an gamma'-Ni3Al phase with lamellar twin structure was a lso found to coexist with Ni3Mo precipitate inside the alpha fibres. T he orientation relationships between the precipitates and the alpha fi bres were determined. Energy dispersive X-ray analysis (EDX) showed th at the precipitation of Ni3Mo and Ni3Al is due to solid solution of Ni and Al in the alpha fibres.