The purposes of this study are to develop and characterize new advanced mechanically alloyed ODS(Oxide Dispersion Strengthened) Ni-base superalloys exhibiting the better strength capability at the intermediate and elevated temperature than MA 6000 alloy. The chemical composition(wt%) of the Alloy 92 used in the present work was Ni - 8Cr - 6.5Al - 6W - 3Ta - 1.5Re - 5Co - 1Ti - 0.15Zr - 0.01B - 0.05C - 1.0$Y_2O_3$.
The high temperature strength of ODS material is known to be limited by grain boundary sliding and transverse grain boundary rupture at the elevated temperature. These effects can be minimized by developing the materials with coarse elongated grain structure parallel to the applied stress axis. The elongated grain structure can be obtained by zone travelling recrystallization heat treatment.
The primary and secondary recrystallization behaviour of an experimental oxide dispersion strengthened Ni-base superalloy(Alloy 92)which contains a high volume fraction of gamma prime(65 vol%)has been characterized. The as-extruded material consisted of extremely fine grains(0.5㎛), which were experienced the primary recrystallization during the hot extrusion process. The highly elongated coarse grains were developed through secondary recrystallization by isothermal and zone annealing treatment of extruded alloy in order to enhance the high temperature creep resistance. Elongated grains possessed [022] texture. Grain aspect ratio increased with increasing the hot zone temperature up to 1300℃ and with decreasing furnace travel speed down to 3cm/hr.
The elevated temperature(above 800℃) tensile strength of Alloy 92 is higher than that of MA 6000. The 100hr rupture strength in the longitudinal direction is about 280MPa at 982℃(1800℉) and 650MPa 760℃(1400℉). The stress exponent at 760℃ is about 35. The alloy displays normal three-stage creep behaviour. The microstructural stability of the alloy under stress rupture conditions was invesigated in terms of coarsening/coalescence and rafting of gamma prime precipitates.
