Solid Solution strengthening of high purity niobium alloys
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the past ten years, substitutional solid solution hardening in bcc alloys has been the subject of several studies. Interpretation of the resulting data is complicated by two factors. First, the low temperature flow stress of these alloys consists of both thermally activated and athermal components. I Quite often little attempt has been made to separate the two components and to analyze them individually. Secondly, it has been recently shown that small amounts of interstitial impurities can have significant effects on the deformation properties of bcc substitutional alloys, at least at low temperatures. ~'4 Thus, examination of the mechanism of substitutional hardening requires the near absence of interstitial impurities, lest they mask the substitutional solute effects. Although thermally activated solution hardening has been examined in "purified" substitutional systems,a'4 only the very recent study by Jaxs examined the athermal flow stress in bcc alloys of high interstitial purity. Jax observed the athermal yield stress of selected Nb-Mo, Nb-Pd, and Nb-W alloys employing ultrahigh vacuum degassed crystals. The resultant data indicated that the yield stress was influenced by interstitial content and increased in approximately a linear manner with substitutional concentration. However, interpretation of the rate of hardening in terms of atomic size or modulus misfit was complicated by the relatively large magnitude of both the atomic size and modulus misfit between solute and solvent atoms; as a result, no completely satisfactory correlation was found. The present study examines the athermal yield stress of three high purity bcc substitutional alloy systems which consist of: a) Nb-Ta (small modulus misfit and nearly no difference in atomic sizes), b) Nb-Hf (small modulus misfit plus large atomic size misfit), and c) Nb-W (large modulus misfit plus large atomic size misfit).
Two zone p a s s e s at 7.5 c m / h were used. The o r i e n t a tion of the c r y s t a l s t e s t e d a r e shown in Fig. 1. Following c r y s t a l growth, the alloy single c r y s t a l s were d e g a s s e d at a p p r o x i m a t e l y 2300~ for 8 h with an u l t i m a t e d y n a m i c v a c u u m of 4 to 7 • 10 -1~ t o r r . The ultrahigh v a c u u m a p p a r a t u s is a modified v e r s i o n of that d e s i g n e d by Smialek and Mitchell 6 with the s a m ples b e i n g heated by induction in a s m a l l t u b u l a r c h a m b e r pumped by a 20 1/s ion pump. D e t a i l s a r e d e s c r i b e d e l s e w h e r e . 7 The t e m p e r a t u r e of the s p e c i m e n s was m e a s u r e d s h o r t l y after i n i t i a l induction h e a t i n g and e m i s s i v i t y c o r r e c t i o n s were applied to obtain the t r u e t e m p e r a t u r e . C h e m i c a l a n a l y s i s i n d i c a t e d that the i n t e r s t i t i a l content (C + O + N) is about 400 at. ppm. Since such an i n t e r s t i t i a l c o n c e n t r a t i o n is n e a r the l i m i t s of the a n a l y s i s technique, it is p o s s i b l e that the a
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