GENERATION OF LONG TIME CREEP DATA ON REFRACTORY ALLOYS AT ELEVATED TEMPERATURES

摘要

Four separate studies of various aspects of the vacuum creep behavior of two tantalum alloys (T-lll and ASTAR 811C) and of pure CVD tungsten are reported. The first part of the program involved a study of the influence of high temperature pre-exposure to vacuum or to liquid lithium on the subsequent creep behavior T-lll alloy. Results of this study revealed significant effects of pre-exposure on the] % creep life of T-lll, with life reductions of about 3 orders of magnitude being observed in extreme cases. These effects are interpreted in terms of variations in the grain size and residual oxygen in solid solution caused by the exposures. The second part of this study involved an investigation of the creep behavior of T-lll under conditions of continuously increasing stress and decreasing temperature which simulated the conditions anticipated in radioisotope capsule service. Results of this study showed that such test conditions produced a creep curve having a very unusual shape, and led to the identification of a new creep design parameter for this type of service. Methods were also investigated for the prediction of long time variable stress and temperature creep behavior from short time results. The third area of investigation was a study of the influence of heat treatment on the microstructure and creep behavior of ASTAR 811C. Results of this study established tentative design data for ASTAR 811C annealed for 1/2 hour at 3600°F (2255°K) and for 1 hour at 3000°F (1922°K) and led to a hypothesis concerning the role of the carbide in strengthening this alloy. A third heat treatment cycle, 100 hours at 3000°F (1922°K) was defined for this alloy which maximized the advantages and minimized the disadvantages associated with the other two treatments. The fourth part of the program was directed toward a preliminary characterization of the % creep life of CVD tungsten as obtained from two different sources. The creep data obtained in this study was insufficient to define a significant difference between materials from these two sources.