Solid-solution strengthening in Ni-C alloys
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A. S. K E H
" P u r e " n i c k e l and Ni-C s o l i d - s o l u t i o n a l l o y s of v a r i o u s g r a i n s i z e s (ASTM no. 2 to 10), with eight d i f f e r e n t c a r b o n c o n c e n t r a t i o n s in the r a n g e 0.008 to 0.304 wt pct, w e r e s t r a i n e d in t e n s i o n b e t w e e n 4 ~ and 474~ at a s t r a i n r a t e of 8.3 x 10-Sper sec. The c r i t i c a l r e s o l v e d s h e a r s t r e s s (CRSS) was i n d e p e n d e n t of t e m p e r a t u r e in the r a n g e 200 ~ to 4 7 4 ~ ( a t h e r m a l region). Below 200~ the CRSS i n c r e a s e d s h a r p l y with d e c r e a s i n g t e m p e r a t u r e , the i n c r e a s e b e i n g l a r g e r for a l l o y s of high c a r b o n c o n c e n t r a t i o n . Both the t e m p e r a t u r e - d e p e n dent and the athermal alloy hardening were found to be linear functions of carbon concentration. The strain-rate sensitivities of flow stress of alloys did not change with strain and were larger for alloys of higher carbon concentrations. The Hall-Perch relation was used to calculate the CRSS of Ni-C single crystals, ~/, so that the data can be compared with existing alloy hardening theories. The data are compatible with the solid-solution theory of Friedel in which the hardening is attributed to both elastic and electrical interactions between dislocation cores and solute atoms.
ALTHOUGH
there has been considerable interest in interstitial solid-solution strengthening in bcc metals, I-4 interstitial solid-solution strengthening in fcc metals has been largely ignored. This is evident in the lack of references in more recent review articles on solidsolution hardening.S-v However, available data6'e-'2 indicate that interstitial solid-solution strengthening may be larger than substitutional solid-solution hardening. Of the six references cited above, only Sonon and Smithn m~.dPeterson and Skaggs~2 attempted to elucidate the mechanism of interstitial solid-solution hardening. Furthermore, only Peterson and Skaggs obtained data at 4~ which is so vital in determining the nature of thermally activated process involved in plastic deformation of metals. Therefore, it was decided that a compilation of deformation data which included measurements at very low temperatures and a wide range of interstitial content would contribute to the understanding of the mechanism of interstitial solid-solution hardening in fcc metals. The Ni-C system was chosen because Flinn6had previously shown that carbon had a large strengthening effect on nickel, particularly at low temperatures. Sonon and Smithn had previously shown that carbon can affect the yield and flow stresses of Ni-C alloys by increasing ~f and Kf in the Hall-Petch relationIs'14
7f = "rf + Kfl -V2 where ~'f = flow s t r e s s at c o n s t a n t s t r a i n , Jo - f r i c t i o n a l s t r e s s ,
K f = a m e a s u r e of the effect of s u r r o u n d i n g g r a i n s on the r e s i s t a n c e to flow, l = average grain diameter. Y. NAKADA,formerly with the Edgar C. Bain Laboratory for Fund
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