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The Mechanical Properties of Tantalum with Special Reference to the Ductile-Brittle Transition

Report Number: ASD TR 61-203 Part II
Author: Murray, G. T.
Author: Burn, R. A.
Corporate Author: Materials Research Corporation
Laboratory: Metals and Ceramics Laboratory
Date of Publication: 1963-03
Pages: 45
Contract: AF 33(616)-7173
Project: 7351
Task: 735106
AD Number: AD0403074

The lower yield stress-grain size relationship was studied in tantalum for two compositions; one consisting of a total interstitial content of 72 ppm with the major constituent being oxygen, and the other containing primarily carbon (116 ppm) in which part of the carbon existed in the form of a finely dispersed carbide precipitate. These results were compared to those obtained earlier (1) on commercial tantalum containing a total interstitial content of about 200 ppm. The parameters σi and ky in the Petch equation

σyi + kyd-1/2

were of primary interest. The commercial tantalum exhibited significantly higher yield and flow stresses and correspondingly higher σi values. The carbon containing material showed the lowest yield stresses primarily because of its lower oxygen content. It was concluded that oxygen is much more effective than carbon in restricting dislocation movement. The effect of the flow stresses, however, even in the presence of the carbide phase were lower than those observed for commercial tantalum.

The ky values obtained on the 72 ppm solute content materials for a strain rate of 10-1/sec. were found to be approximately 2 x 107 c.g.s. units and thus about a factor of two larger than those found on commercial tantalum. For both materials ky did not vary with test temperature at a strain rate of 10-3/sec., however at a strain rate of 10-1/sec. ky for the 72 ppm solute material was found to vary from about 1 to 3 x 107 c.g.s. with the larger values being measured at the lower test temperature. ky values for the carbon containing material were found to be very small (~0.5 x 107 c.g.s) and did not vary appreciably with test temperature or strain rate. It was suggested that this small ky may be due to a large number of unlocked dislocations as a result of the carbide particles acting as sources.

ky and σi values were also computed by the Lüders strain technique for the room temperature tests and found to agree reasonably well with those obtained by the grain size method.

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