A Concept of High Temperature Alloy Design Based on Creep Curve Data Analysis

Abstract

Creep curve analysis based on the modified θ concept provides three material constants, α, A and B, which can specify the creep process of alloys quantitatively: α is the rate constant of deformation, A is small when strain hardening is significant, and B is large when microstructural degradation is substantial. In order to utilize these material constants for high temperature alloy design, the following problems must be solved: the correlation between the material constants and the important creep properties (minimum creep rate ε_m and rupture life t_r), and the method to control the material constants.
The creep properties are related to the material constants by the following simple equations:
ε_m=2α√AB, t_r=[ln(ε_r/B)]/α
These equations contain only one more parameter (rupture strain ε_r) in addition to the three material constants. This simplicity is of practical benefit in alloy design on the basis of the modified θ concept. The material constants can be controlled by several metallurgical means. For example, reduction of diffusivity in matrix and precipitation of second phase lower the rate constant α, resulting in the improvement of both ε_m, and t_r. Coarsening of grains in matrix and stabilization of precipitates reduce the weakening constant B, and consequently improve mainly ε_m. Though the present knowledge of the material constants control is still limited, further creep curve analysis on various other materials will enable us to construct a comprehensive alloy design system.