9812211 Baker The aim of this research is to explore whether the vacancy- hardening model that explains the yield strength anomaly in FeAl is applicable to other B2 compounds that show a similar temperature dependence of yield strength. The model is general and could be applicable to a wide variety of crystal structures, which show a yield anomaly. The research involves single crystal growth, mechanical testing and slipline analysis, and transmission electron microscopy (TEM) of single crystals of CoTi, CoHf and CoZr. Measurements of vacancy concentrations are made for different annealing temperatures using the Simmons/Balluffi method, whereby the density measured by dilatometry is compared to that measured by x-rays. The activation enthalpies for both vacancy formation and vacancy migration are determined and compared with values obtained from the vacancy-hardening model. In order to determine the dislocation/vacancy interactions associated with the yield strength anomaly, TEM in-situ straining experiments are performed both as a function of temperature and on crystals containing different quenched-in vacancy concentrations. Mechanical testing includes determination of the tension/compression asymmetry of the yield stress as a function of temperature in single crystals of different orientations. The compression tests are performed in a wide range of strain rates with the very high strain rate tests being performed at Los Alamos National Laboratory. Specialized mechanical tests using a Gleeble test machine, which allows very rapid heating and quenching, are performed at Oak Ridge National Laboratory. This apparatus enables yield strength measurement to be performed at the yield strength peak temperature with different vacancy concentrations present. TEM examination of slip-plane- sectioned specimens rapidly-cooled after testing in the Gleeble allows dislocation structures truly representative of high temperature testing to be examined. %%% Intermeta llic alloys exhibit unique mechanical properties that could enable their use in high temperature structural applications. This research should advance understanding of the mechanical behavior of this important class of metal alloys.
