The deformation and fracture properties of polycrystalline molybdenum

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Abstract
A series of experiments on polycrystalline molybdenum are described which were designed to provide a quantitative test of a theory of the deformation and fracture of b.c.c. transition metals proposed by Cottrell (1958). The parameters σi and k y in the Petch relation σy=σi+k y d −1/2 between lower yield stress, σy, and grain diameter, 2d, were measured at room temperature and at 195°K which is just above the ductile-brittle transition temperature. These values of σi and k y were used to calculate the dependence of ductile-brittle transition temperature on grain size that is expected from the theory. Increasing the grain diameter from 0.034 to 0.39 mm was found experimentally to lower the transition temperature by 14°K, whereas the theory predicts an increase of about 80°K. It was also found that the stress for ductile cleavage fracture at 195°K was not directly proportional to d −1/2 as is predicted by the theory.