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An Investigation Into the Susceptibility of Tantalum and Vanadium to Slow Strain-Rate Embrittlement Due to Oxygen, Nitrogen and Carbon.

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Final rept. 1 Apr 79-30 Jun 80,

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The phenomena of slow strain-rate embrittlement was studied in three metal-interstitial systems, tantalum-oxygen, vanadium-oxygen and tantalum-nitrogen. The embrittlement of tantalum by oxygen was investigated on specimens containing four levels of oxygen -- 0.33, 0.50, 0.85 and1.30 atomic percent. All of the compositions showed embrittlement between 500-1000K in specimens deformed at epsilon 8.8 x 10 to the 4th power sec to the minus one power. Associated with the embrittling phenomenon, and occurring in the same temperature range where the ductility is dropping, and three dynamic strain-aging effects a a yield stress plateau from 300-600K, b serrated yielding from 453-600K and an increasing work hardening rate from 300-600K Scanning electron microscope studies show that failures were by microvoid coalescence at low temperatures changing to a predominantly intergranular mode in the embrittlement range. Study of the vanadium-oxygen system also showed an embrittlement region between 600-875K. This embrittlement was accompanied by a yield stress plateau between 400-600K, an increasing work hardening rate between 300-650K and serrated yielding from 525K to 650K. A high level of titanium impurity limited testing to low oxygen concentrations. Tests on the tantalum-nitrogen system concluded that nitrogen did not produce slow strain-rate embrittlement in this metal although it served as a potent room temperature strengthener. The interval in which embrittlement is expected in this metal is at a sufficiently high temperature that dynamic recovery and sofening take place. Author

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