Interstitial solution hardening in tantalum single crystals

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Abstract
The strong temperature dependence of the flow stress of body-centred cubic metals at temperatures below ∼0·1 Tm has generally been attributed to the inherent lattice friction of the b.c.c. structure (the Peierls stress). To test this hypothesis, very high purity (with regard to interstitials) tantalum single crystals were prepared. The purification treatment greatly reduced the temperature dependence of the flow stress as well as the a thermal component of the yield stress of Ta which had been given six zone-refining passes only. Small, controlled additions of interstitial oxygen, nitrogen and carbon (10-1000 at. p.p.m.) were introduced into the pure Ta. Yield and flow stresses were found to increase rapidly with interstitial content over the temperature (4·2 to 550°k) and composition ranges investigated. Solution hardening could be divided into two regions : from 0 to ∼ 300 at. p.p.m. the yield stress increases as the square root of concentration and is strongly temperature dependent; the rate of solution hardening is greatest for nitrogen, followed by carbon and then oxygen. In the region from ∼ 300 to 1000 at. p.p.m., the yield stress increases linearly with concentration and the rate of solution hardening is independent of temperature and the type of interstitial. Although interstitial impurity elements can make a major contribution to the temperature dependence of the flow stress, it appears that the residual temperature dependence after purification is due to the Peierls stress. The authors wish to thank Professor R. Gibala and Dr. K. V. Ravi for their interest and stimulating discussions. This work was supported by the Air Force Office of Scientific Research under grant number 68-1501.