The flow stress of ultra-high-purity molybdenum single crystals

The paper investigates the dependence of the flow stress σ of ultra-high-purity molybdenum single crystals on temperature T, strain rate, and crystallographic orientation of the crystal axis. The cyclic-deformation technique developed by Mughrabi and Ackermann allowed a complete set of flow-stress data (covering the temperature range 125 to 460 K at 15 different shear strain rates, varying from 5.9 x 10-7 to 1.7 x 10-3 s-1) to be obtained on one and the same specimen. The two crystals investigated, with Schmid factors μ{110} = 0.50 and μ{112} = 0.43, or μ{110} 0.40 and μ{112} = 0.43, respectively, had residual resistivity ratios exceeding 2.5 x 105, the main impurity being W. The data are in excellent agreement with the theory of flow-stress control by kink-pair formation and kink migration. It is shown that for both orientations the elementary glide steps of the a0 left angle 111 right angle /2 screw dislocations controlling the flow stress occur on {112} planes. The formation energy of a pair of isolated kinks on a {112} plane is 1.27 eV, in perfect agreement with the activation energy of the so-called γ-relaxation as determined by internal-friction measurements. The apparent kink mass and the kink diffusivity could be determined, too. The present results leave no doubt that the ''lump'' observed in the σ-T relationship of high-purity b.c.c. metals is due neither to a change in the glide mechanism nor to a special form of the Peierls potential but is a natural consequence of the dependence of the kink-pair formation enthalpy on the resolved shear stress. (orig.)