Dislocation structure, deformation structure, strengthening mechanisms, and embrittlement causes of ordered Ni4Mo alloy

The following experiments were performed for samples aged at 725 and 7750C for up to 100,000 minutes. (1) The long-range order parameter and the domain size as a function of aging time were measured. (2) The microstructure for various aging treatments was characterized using optical microscopy, SEM, and TEM. (3) The Tensile yield strength, elongation and microhardness were measured at 250C as a function of aging time. (4) The fracture surface of the tensile samples were characterized by SEM. (5) The fracture surfaces for one aging treatment were analyzed by Auger spectroscopy. Yield strength and elongation were measured at temperature from 300 to 10000C. Upon aging, the strength approximately doubled, but the alloy became very brittle (e.g., less than 4% elongation), fracturing intergranularly along the high-angle, prior α boundaries. It was found that the degree of long-range order is much more important in strengthening than is the domain size. The dislocation structure of deformed Ni4Mo consisted mainly of overlapping {111} intrinsic stacking faults, which were bounded by widely separated 1/6 <112>, single Burgers vector, type dislocations. Upon sufficient deformation, these bands thickened into twins. The main strengthening factors are the development of long-range order, which limits the slip systems, and the development of a dislocation structure consisting of widely separated partial dislocations, which cannot cross slip