Evolution of Ni(sub 3) Precipitation Kinetics, Morphology and Spatial Correlations in Binary Ni-X Alloys Aged Under Externally Applied Stress

摘要

Experiments were performed on cylindrically symmetric specimens with the axis of the applied compressive stress parallel to (100). The geometries employed included doubly tapered (DT) specimens approx. 10 mm long with diameters varying from approx. 3 to 6 mm and right cylindrical (RC) specimens approx. 4 mm in diameter and approx. 3 mm in length. The DT geometry was chosen because microstructural evolution under stress could be investigated without requiring a unique specimen for each stress. We chose to vary the diameter of the cross section by factor of approximately 2, so that the stress varied by about factor of 4. The experiments on the RC specimens were done, in part, to evaluate the possible effects of the stress gradient that is always present in the DT samples. The state of stress is multiaxial near the periphery of the DT specimens, but the microstructures were examined near the axis of each specimen where the stress is uniaxial. A single crystal Ni-Al alloy approx. 25 mm in diameter and 80 mm long, grown by the Bridgman method, was purchased from Alloy Preparation Facility of the Ames Laboratory. The nominal composition of the alloy was 13.36 at. % Al, but during solidification some variation of the Al concentration along the entire length of the bulk crystal is inevitable.