Creep Behavior of Refractory Alloys in Ultrahigh Vacuum.

Abstract

SUMMARYSeveral Cb, Ta, Mo, and W base alloys have been creep tested with both constant and continuously increasing loads in a vacuum environment of < 1~1 0 -~ t o r r at temperatures and stresses chosen to provide between 1/2 and 1 percent creep i n times up to 20 000 hours. Isostatic design data a r e presented in the form of a.Larson-Miller plot for 1/2 o r 1 percent creep life for each of these refractory alloys. Elevated temperature tension test data from the tantalum base alloy T-111 showed a strain aging reaction in the 1400' to 2200' F (760' to 1204' C) range which produced an unusual transient creep behavior. Analysis of the isostatic steady state creep data for this material showed that the minimum creep rate can be described by the expression 5 of13. 17,-90 000/RT ?-in the temperature and stress ranges of 1600' to 2600' F (870' to 1427' C) and 500 to 45 000 psi ( 0 . 3 4~1 0~ to 31. OX107 N/m2). These values of activation energy and s t r e s s exponent suggest that the isostatic steady state creep of T-111 is governed by a diffusion controlled microcreep mechanism rather than by nonconservative dislocation motion. The creep tests which were run on T-111 alloy with continuously increasing loads provided continuously increasing creep rates which were comparable to the isostatic steady state creep rates at equivalent s t r e s s levels.Methods have been developed for prediction of the 1 percent creep life under varying s t r e s s conditions from isostatic creep test data, and comparison of these predictions with the experimental results shows the dependence of analytical techniques on the character of the isostatic creep curves.Based on work done under NASA contracts NAS 3-2545 and NAS 3-9439.

Microstructural and mechanical property changes in the Ta-base T-111 alloy following thermal aging

“…The large difference between the YS and UTS of the samples indicate a high level of work-hardening (which is also apparent in the stress-strain curves) and is consistent with the data from open literature. Sheffler et al [2] studied the strain aging behavior of T-111 and reported that the high strength (relative to pure Ta) may be caused by a complex atmosphere-dislocation interaction rather than by simple interstitial dislocation pinning. During aging, as the interstitial elements are redistributed and precipitated, the complex atmosphere-dislocation interactions weaken and the UTS drops.…”

“…Of the possible Group V (V, Nb, Ta) and VI (Cr, Mo, W) refractory alloys, Ta-base T-111 (Ta + 8 wt% W + 2Hf) offers excellent formability and ductility at low temperatures, while showing good high temperature strength and alkali metal compatibility. Though tensile properties [1][2][3][4][5][6][7][8][9][10], creep testing [11][12][13][14], corrosion testing [15][16][17], and weldability studies [18][19][20] of T-111 have been investigated for different levels of cold work and annealing, bringing the technology to varying levels of maturity, the effects of aging on the mechanical properties is not as well understood. Earlier work on aging of T-111 under vacuum and liquid metal environments has shown conflicting results related to degradation in material ductility following exposure.…”

Effects of alloy composition in alleviating embrittlement problems associated with the tantalum alloy T-111

“…Dynamic strain-aging has been observed previously in T-111 by Sheffler, Sawyer, and Steigerwald (ref. 8). Based on a comparison of the activation energy for strain aging and the activation energy for diffusion of each of the interstitial impurities in T-111, Sheffler, Sawyer, and Steigerwald concluded that oxygen is the critical interstitial specie responsible for dynamic strain aging i n T-111.…”