(A) positron annihilation study of defects in high purity Ti

Abstract

The positron annihilation technique is used for the study of defect formation and annealing characteristics in Ti under the various degree of plastic deformation and subsequent annealing heat treatment. Ti specimens of extra high purity (99.99 \%) are deformed by cold-rolling from 4 \% up to 55 \% of thickness reduction and then subjected to the isochronal annealing from $100^\circ C$ up to $800^\circ C$ at an interval of $50^\circ C$ for 1 hour. At each step of isochronal annealing temperature, specimens are subject to the positron annihilation and the hardness measurements. Results of the investigations show that the positron annihilation responds most sensitively on defects in a low deformation region, but saturated above 12 \%. These defects are regarded as vacancy-type defects which play a significant role in the annealing of defects in the recovery stage (100-$250^\circ C$). In a relatively higher deformation region, however, the hardness responds more sensitively. Defects in this region are regarded as dislocation-related defects like dislocations and twinnings, which contribute mainly to the annealing of defects in the recrystallization stage (350-$550^\circ C$). It is observed that purity of Ti affects the clear appearance of the recovery stage and the saturation of defects in much higher deformation region. Activation energies of the recovery and the recrystallization stage for 30 \% cold-rolled specimens which are determined by the combination of isochronal and isothermal annealing curve are 0.59 eV and 1.19 eV, respectively. For heavily cold-rolled Ti specimen, R parameter rises stepwisely with the isochronal annealing above $400^\circ C$, and this suggests that defects related to twins contribute more than vacancies to the positron trapping. Optical and TEM micrographs show the existence of dislocations and twins in heavily deformed deformed (30 \%) Ti specimens until they are finally annealed out at temperature above the recrystallization stage. T...