Effect of Primary Grain Sizeon the Secondary Recrystallization of Mechanically Alloyed Oxide Dispersion Strengthened Ni-Base Superalloy

Abstract

Oxide dispersion strengthened (ODS) superalloys produced by mechanical alloying (MA) have attracted great attention as advanced high-temperature materials, because they can retain useful strength up to a relatively high fraction of their melting points. Mechanical alloying is a dry, high-energy ball-milling process that produces composite metal powders with controlled, extremely fine micostructures. Mechanically alloyed powders are sealed in a steel can and are consolidated by extrusion and then subjected to conventional hot and cold working processes, m The asextruded ODS superalloys usually have an equiaxed, finegrained structure resulting from the primary recrystallization during hot extrusion. A final annealing at very high temperature is required to develop a stable, coarse grain structure by secondary recrystallization, one that is suitable for the most demanding stress-rupture applications. The attractive high-temperature strengths of ODS superalloys are primarily due to the presence of uniformly dispersed fine oxide particles. These oxide particles produce a dispersion strengthening effect by acting as barriers to dislocation motion.tEl In addition to the dispersion strengthening from oxide particles, the other important microstructural features affecting the elevated temperature strength of ODS superalloys are the grain size and the grain shape represented by the grain aspect ratio (GAR), which is a ratio of grain length, L, to grain width, I47. The accumulation of creep damage on transverse boundaries of ODS superalloys