Author: Soon-Chul Ur
Title: A Study of Normal Grain Growth, Secondary Recrystallization, and High Temperature Properties in Mechanically Alloyed NiAl
ABSTRACT
An ODS intermetallic, NiAl, has been produced by the mechanical alloying (MA) process. The resulting powders were characterized using a modified TGA by several techniques in order to gain an understanding of the MA process. The powders have been consolidated by hot pressing and hot extrusion under several different conditions. Various thermal treatments have been applied to modify the consolidated microstructures. Two types of grain coarsening mechanism and the resultant high temperature properties in ODS MA NiA1 have been studied. Normal grain growth leads to enhanced coarsening of the dispersoid and a consequent reduction in high temperature mechanical properties. Secondary recrystallization (SRx) can be made to occur in this material under certain processing conditions. The occurrence of SRx has been enhanced by providing an additional driving force in the form of stored energy of cold work. SRx leads to a pronounced grain coarsening and high grain aspect ratio without concurrent dispersoid coarsening, which results in improved creep resistance. The apparent activation energy and the stress exponent for creep in SRx MA NiA1 indicate that creep is controlled by dislocation creep. However, a grain size dependence on creep has been shown, indicating that grain boundary sliding mechanism accommodated by dislocation creep contributes to the overall deformation in MA NiAl. Threshold stress becomes negligible in SRx microstructures. This is attributed to dispersoid coarsening and a decrease in grain boundary area.