Thermally activated slip deformation between 0·7 and 77 K in high-purity iron single crystals

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Abstract
High-purity iron single crystals of ·2 mm diameter with a residual resistivity ratio of ∼3500 have been produced by the strain-annealing method from a poly-crystalline rod purified by zone refining in hydrogen gas and in vacuo. These single crystals could be deformed at helium temperature up to several percent strain without exhibiting twinning. Temperature and strain-rate change tests were performed in tension between 0·7 and 77 K using a 3He cryostat for single crystals with orientations of x=−1° and 29°. The strain-rate sensitivity decreases with lowering temperature but seems to have a non-vanishing value as the temperature approaches zero. Activation parameters, enthalpy ΔH and volume v∗, were obtained by the conventional procedure. At high temperatures the exponent, ΔH/kT, for a constant nominal shear strain-rate of 8×10−5 s−1 is almost constant (∼26) but below 30 K it decreases down to a very small value (about 2 at 1 K). This deviation from the Arrhenius rate equation of deformation has been interpreted as a quantum effect of the vibrational mode of the dislocation in overcoming the Peierls potential.