A new experimental ODS(oxide dispersion strengthened) Ni-base alloy of Alloy92 was developed for Gas Turbine Applications. The purpose of this work is to characterize the secondary recrystallization behaviour and to investigate factors affecting grain growth in the alloy.
ODS alloys attract great attention as advanced high temperature materials, because they can retain useful strength up to a relatively high fraction of their melting points. ODS alloys are produced by mechanical alloying and hot extrusion process. The high strength of the ODS alloy at elevated temperatures originates from the dispersed $Y_2O_3$ particles. Production of a coarse elongated microstructure is very important in ODS superalloy for additional creep-rupture strength. The formation of long grains with high grain aspect ratio (GAR) is possible during the secondary recrystallization by a process of directional recrystallization or zone annealing. The microstructure of secondary recrystallized billet varied with as-extrusion condition.
The transmission electron micrographs of the as-extruded alloys showed extremely fine grains of 0.35㎛ to 0.65㎛. It has been found that the optimum grain size for producing a coarse elongated structure ranged from about 0.45㎛ to 0.55㎛ in Alloy92. When the primary grain size was less than 0.45㎛, a coarse elongated structure was not developed during the secondary recrystallization. However, an application of preannealing heat treatment at 1135℃ for 40 minutes for the fine grained sample (0.35㎛) produced the desirable coarse elogated structure. It appeared that the effect of preannealing heat treatment have produced an optimum primary grain size for a coarse elongated structure. X-ray measurements on the as-extruded and preannealed bars showed that the application of preannealing treatment also increased by about 10% the number of grains orienting in the [011] direction which is the extrusion direction.
It was also found that the contamination of oxygen and insufficient MA processing time was harmful in production of a coarse elongated grain structure in the Alloy 92.
