Dependence of mechanical strength on grain structure in the γ′ and oxide dispersion-strengthened nickelbase superalloy PM 3030

The mechanical strength of the oxide dispersion-strengthened (ODS) nickelbase superalloy PM 3030 during deformation at constant strain rate (CSR) is discussed with special emphasis placed on its dependence on grain structure. In the fine-grain state the material shows a very high strength at low temperatures due to Hall-Petch type strengthening. However, the 0.2 % offset yield strength sigma(0.2) falls off sharply at temperatures above 700 degrees C. Increase of grain size by isothermal annealing leads to a reduction of sigma(0.2) at low temperatures, but also to an increase of creep resistance at higher temperatures. Coarse and elongated grain structures essentially eliminate grain-boundary sliding and reduce diffusion-controlled void formation and, therefore, exhibit a superior strength level at elevated temperatures. The dependence Of sigma(0.2) on grain structure results in a cross-over of the sigma(0.2) vs. T-curves within a narrow temperature range where all grain variants exhibit similar mechanical strengths. In accord with Dieter [1] this region may be called "equicohesion range" of a given material with identical chemical composition and particle structure.