Dislocation energetics in alpha titanium
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Stress relaxation measurements in commercially pure α-Ti (A-55) were performed at a plastic strain of 2×10−3 in the temperature range 300° to 500°K. The long range component of the flow stress (τinternal) was measured as a function of temperature over the entire range studied. Contrary to the normal assumption of its temperature independence, this parameter was observed to increase approximately 80 pct as the temperature declined from 500° to 300°K. Measurements of the temperature and strain rate dependence of the effective stress (τ*) indicated that two distinct thermally activated dislocation motion processes control this mode of deformation in the temperature range studied. A low temperature process governs dislocation motion below 380°K and is characterized byH(τ*→0)=0.3 ev. The high temperature process controls plastic flow above 400°K and is characterized byH(τ*→0)≅1.1 ev. The preexponential factor appearing in the Arrhenius equation\((\dot \gamma _0 )\) was also determined for both processes and found to be a sensitive function of the effective stress. Activation volume (ν*): effective stress relationships for both processes were also obtained. The experimental findings quoted above will be discussed in terms of our current understanding of thermodynamics of plastic deformation and hypotheses will be introduced to account for the observed large variations of τinternal with temperature and\(\dot \gamma _0 \) with τ*. Mechanistic suggestions for both high and low temperature processes will also be offered.
KeywordsActivation Volume Metallurgical Transaction Volume Strain Aging Yield Drop Strain Rate Change
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