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 alloy92 used in the present work was Ni-8Cr-6.5Al-6W-3Ta-1.5Mo-3Re-5Co-1Ti-0.15Zr-0.01B-0.05C-$1.0Y_{2}O_{3}$.
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 treament.
Alloy92 was subjected to the stationary gradient annealing (maximum temperature 1290℃ and isothermal annealing in order to determine the recrystallization behavior for the extruded bars. The temperature gradient was about 30℃/cm in the recrystallization temperature range. For the stationary gradient annealing, the critical secondary recrystallization temperature was about 1237℃. Several factors (thermal conductivity, recrystallization time, maximum hot zone temperature, furnace moving rate and the sample shape) were considered to examine the secondary recrystallization behavior of Alloy92.
Under the constant zone travelling rate of 9cm/hr, the grain aspect ratio of about 18 was obtained at above 1280℃ for the round bar and 1250℃ pointed-bar. The effect of zone travelling rate at a constant heat treatment temperature on the recrystallization behavior was also investigated. The round and pointed bars exhibited the grain aspect ratio of 18 below 10 and 16cm/hr zone travelling rate, respectively. From the results of the above investigation, it is concluded that the pointed shape is beneficial and effective for the better secondary recrystallization of Alloy92.
