Determination of strain rate sensitivity index in superplastic Ti-6Al-4V
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II.
Tensile
Surface Condition
As-machined Etched Annealed--4 h 600 ~ 700 ~ 800 ~
Ductility of Weldable Beryllium*
Powder-Source
Percent Elongation at Fracture]' Low Strain Rate High Strain Rate (10-4 s l) (10~s-I) 1.4 1.5
0.5 2.2
1.2 1.5 1.6
1.1 2.0 1.9
* Specification P-31664-6. ]'Accuracy of measurements estimated at _ 0.2 pet elongation.
4. S. Beitscher: RFP-1205, RockwellInternational, Rocky Flats, Golden, CO, December 1l, 1968. 5. EvaluationTest Methods for Beryllium, Materials Advisory Board, Division of Engineering, National Academy of Sciences, National Research Council, Washington, DC, MAB-205M,Committee on Beryllium Metallurgy, T. L. Johnston, Chairman, March, 1966.
Determination of Strain Rate Sensitivity Index in Superplastic Ti-6AI-4V
(P-31664-6") of powder-source beryllium. Impact en* Rocky Flats specification for a weldable grade of beryllium, 1.5 pct Be maximum, similar to Brush Wellman S-100and average grain size less than 35/Lm. ergies increased by a factor of 2 to 3 from the as-machined level of 0.05 j / m m 2 to 0.09 to 0.17 j / m m 2 after etching or annealing. The tensile data are shown in Table II and indicate that at the higher strain rate, machining damage removal increased the ductility by a similar amount (a factor of 2 to 4). At the lower rate, there was an insignificant increase in elongation upon etching. There was a small and systematic increase in ductility at low strain rate with annealing temperature from 600 to 800 ~ but even at 800 ~ the ductility was only an insignificant 0.2 pct elongation higher than that of the as-machined specimens. Increased strain rate increased the ductility in 3 of the 5 conditions tested, but the largest effect observed was in the as-machined specimens in which increased rate reduced the ductility by two-thirds (from 1.4 to 0.5 pct elongation). These data indicate that the high strain-rate tests were more sensitive to surface damage than were the low strain-rate tests. It is evident that high strain-rate tests are more reliable for evaluating machining practice and damage removal methods for beryllium components subjected to sudden loads. The mechanism responsible for the increased machining damage effect that was observed at high strain rate is not known. The effect points to a strain-rate sensitivity of cracking in beryllium. Whether this sensitivity is caused by twins, residual stress or some other result of machining damage is not clear. Further investigation of the effects of strain rate and testing temperature on different grades of beryllium may elucidate the observed behavior and furthermore, lead to a better understanding of the mechanisms responsible for machining damage in beryllium. This work was performed under a contract with the U.S. Department of Energy. 1. S. Beitscher: Beryllium: Science and Technology, D. R. Floyd and J. N. Lowe, eds., vol. 2, p. 197, Plenum Press, New York, 1979. 2. J. E. Hanafee: UCRL-52287,Lawrence LivermoreLaboratory, Livermore, CA, June 14, 1977. 3. S. Beitscher: RFP-2838, Rockwell International, Roc
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