Dynamic strain ageing in stoichiometric rutile single crystals

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Abstract
Stoichiometric rutile single crystals have been deformed under uniaxial compression between 800 and 1700 K in oxygen at 1 atm pressure. Four equivalent {101} slip systems operated in the crystals, which could be plastically deformed above 875 K. The distinctive feature of the mechanical behaviour of the crystals is the occurrence of a serrated flow due to a double dynamic-strain-ageing (DSA) effect in the temperature range 1150–1450 K at strain rates between 10−5 and 10−3 s−1. Detailed study of this effect suggests that each of the two DSA maxima is associated with a different point defect having a different mobility. The activation energies for migration of both defects have been determined as 2.8 ± 0.3 eV and 3.8 ± 0.4 eV respectively; the latter value is higher than that reported for non-stoichiometric point defects in rutile. Point defects causing DSA also give rise to ageing-dependent yield drops after stress-relaxation experiments at temperatures below those at which the serrated flow occurs. Study of the corresponding ageing law suggests a prominent contribution of electrostatic effects to the interaction energy between mobile dislocations and diffusing point defects. Transmission electron microscope observations of foils cut from crystals deformed at different temperatures confirm that glide motion of the dislocations dominates at temperatures up to that at which DSA occurs; screw dislocations are more mobile than edge dislocations.

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