ABSTRACT
Even low-volume fractions of dispersoid particles can have a pronounced effect on the high-temperature strength of metallic materials: with decreasing applied stress, the creep rates of dispersion-strengthened materials fall rapidly below those of their dispersoid-free counterpart, resulting in immeasurably small strain rates at some finite stress (Figure la). The stress exponent in this range can assume unphysically large values, e.g., well above 20. Therefore the creep data are generally best described by defining a "threshold stress" below which the rate of creep deformation is considered to be negligible. The existence of a threshold stress is also reflected in the stress rupture behavior (Figure 1 b). Although rupture life can be exceedingly long below this threshold, it decreases drastically when the applied stresses start to exceed the threshold. For this reason, the threshold stress is a design-limiting strength property. It is this characteristic feature of dispersion-strengthened materials with which the present section is concerned.
