The Effect of Heat Treatment on Nickel-based Oxide Dispersion Strengthened (ODS) Alloy

Abstract

Nickel-based oxide dispersion strengthening (ODS) alloys are promising materials mainly used for aerospace industries, due to their advantages such as excellent strength and oxidation resistance up to 1100oC. The studying of Ni-based ODS is significant for nuclear reactors especially for materials exposed to high temperature and corrosion environments which cannot be achieved by Fe-based ODS alloys. The purpose of this project is to study the effect of heat treatment on the microstructure as well as to determine its influence on mechanical properties. Nickel ODS with composition Ni-15Cr-4Al-3W-0.8Hf-1Y2O3 has undergone two different manufacturing procedures, namely Alloy A and Alloy B. Their microstructure, size as well as the distribution of oxide particles and gamma prime (denote as γ’) precipitates were studied and compared by TEM. Then the mechanical properties are determined through tensile and hardness testing. Alloy A is the as-HIPed condition whereas Alloy B has gone through additional solution heat treatment and aging. The degree of strengthening is highly dependent on the size of oxide particles, inter-particle spacing, oxide/matrix interface structure and distribution of γ’ precipitates. Thus, this thesis studied in detail on how they affect the mechanical strength through hardness and tensile testing. After heat treatment, the average size of oxide dispersoids decreased from ~9-13nm down to ~5-6nm, and γ’ precipitates reduced from 77nm to 42nm. Hardness of 5.4GPa, 4.3GPa and elastic modulus of 171GPa, 164GPa obtained for Alloy A and Alloy B respectively under room temperature. The performance of Alloy A from tensile test reached the highest yield strength with a value of 687MPa at room temperature, which corresponds well with the hardness results. The size, distribution of oxides and γ’ are the main factors contributing to the strength of the alloy. Therefore, it is important to carefully control those variables by applying appropriate heat treatment parameters.