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Deformation of polycrystalline cobalt Sanderson, Craig Carter

Abstract

The deformation of polycrystalline cobalt has been investigated over the temperature range where the hep phase is stable (0 to 0.39 T[sub m]). The structure of cobalt following various annealing procedures has been detailed. Following heat treatment, cobalt is a two phase aggregate of fee and hep phases, A maximum of 50-60% retained fee occurs in small grained specimens (6 - 10 microns) decreasing to less than 15% retained fee for 60 micron material. The amount of retained fee phase also decreases with increasing purity. The variety of substructures arising from the rultivariant transformation are discussed. The yield stress for cobalt exhibits differing temperature dependence above and below 0.25 T[sub m]. Below 0.25 T[sub m], the 0.2% yield stress is almost temperature independent, whereas above 0.25 T[sub m]the yield stress decreases rapidly. The behaviour belov; 0.25 T[sub m] is related to the bulk transformation of retained fee phase while the decreasing stress levels observed above 0.2 5 T[sub m] are explained in terms of decreasing Peierls stress on the {1122} slip planes. The yield stress increases rapidly as the grain size is reduced. This effect is compared to similar behaviour in other hep metals that exhibit a limited number of slip systems. The ductility of cobalt is related to the retained fee phase by equations of the form ε = A (10)[sup m%fcc]. A larger fraction of retained fee phase gives rise to increased ductility. The elongation to fracture decreases as test temperature increases, reflecting obeyance of Considere's Criterion. The observed work hardening rates are high, as are the measured values for flow stress. The values are compared to data obtained for other metals that transform martensitically while undergoing deformation. Metallographic evidence is presented to substantiate the occurence of non-basal slip in cobalt above 0.25 T[sub m]. Twins having high and low shear values occur at all temperatures where the hep phase is stable. The intense surface shear resulting from transformation and continuing; dislocation production on variously oriented basal planes is discussed.

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