High temperature deformation of ultra-fine-grained oxide dispersion strengthened alloys

The high temperature deformation properties of two oxide dispersion strengthened (ODS) alloys, MA 754 and MA 6000, with initial grain sizes of 0.67 μm and 0.26 μm, respectively, have been studied. Tensile tests have been conducted at 1173, 1273, and 1373 K at strain rates ranging from 2 X 10^-4 to 5 s^-1. Tension creep tests were conducted on MA 6000 at 1273 K to extend the strain rate regime to 3 X 10^-8 s^-1. Microstructures of both undeformed and deformed samples have been characterized by transmission electron microscopy. MA 754 exhibits a maximum elongation of 200 pct and a maximum strain rate sensitivity of 0.30 at 1273 K. MA 6000 is superplastic, exhibiting a maximum elongation of over 300 pct and a maximum strain rate sensitivity of 0.47 at 1273 K. The microstructure of MA 754 is unstable during deformation, showing recrystallized grains and grains which have grown to 1 μm in diameter. No evidence for ordinary recrystallization is found for MA 6000, and grain growth is slight. For both alloys, strain rates less than about 1 s^-1 alter the initial microstructure and prevent grain coarsening on subsequent annealing at higher temperature. Deformation of the fine-grained MA 6000 can be described as a combination of power law creep and diffusional (Coble) creep, with a threshold stress caused by the presence of γt' particles existing only for the diffusional creep process. Structural instabilities do not permit a simple description of deformation of MA 754.