Effect of grain shape on the stress-rupture property of mechanically alloyed oxide dispersion strengthened Ni-base superalloy at elevated temperature

Abstract

Oxide dispersion strengthened (ODS) superalloys produced by the mechanical alloying (MA) process attract great attention as advanced high temperature materials, because they can retain useful strength up to a relatively high fraction of their melting points. The attractive high temperature strengths of ODS superalloys are due to the presence of uniformly dispersed fine oxide particles. These oxide particles give dispersion strengthening effect by acting as barriers to dislocation motion [1]. In addition to the dispersion strengthening from oxide particles, the important microstructural features affecting elevated temperature strength of ODS superalloys are the grain size and grain shape represented by grain aspect ratio (GAR), which is a ratio of grain length, L, to grain width, W. The accumulation of creep damage on transverse boundaries of ODS superalloys can be controlled by an accommodation process, i.e. mutual sliding displacement between neighbouring grains in the longitudinal direction. The elongated coarse grains retard the accommodation process and thus enhance the high temperature creep resistance. The coarse elongated grains with a high grain aspect ratio are possibly formed during secondary recrystallization by a process of directional recrystallization or zone annealing heat treatment [2-6]. Although, the exact mechanism of secondary recrystallization behaviour is not fully understood yet, the importance of GAR has received considerable attention [7-9]. The aim of the study reported here was to show that there is a suitable range of average grain size in as-extruded materials to obtain a coarse elongated grain structure after zone annealing and that rupture life is closely related to grain structure at elevated temperature (/> 850°C). The ODS Ni-base superalloy, called alloy 92, was prepared at INCO Alloys International, Inc., by a mechanical alloying process. The mechanically alloyed powders were packed into mild steel cans which were evacuated and sealed. The sealed cans were extruded into bars of 18 mm in diameter. The nominal composition of alloy 92 (wt%) is Ni-8.0Cr-6.5A1-6.0W-3.0Ta-3.0Re1.5Mo-5.0Co-1.0Ti-0.15Zr-0.01B-0.05C -1.1Y203. The as-extruded ODS superalloys usually have an