The effects of hydrostatic pressure on fracture and the ductile-brittle transition in polycrystalline molybdenum

Abstract

The tensile properties of polycrystalline molybdenum were investigated in the ductile-brittle transition temperature region at atmospheric and superposed hydrostatic pressures, H, extending to 275 MNm −2. Down to 180 K, at all pressures, the yield stress, σ Y , and (below the ductility transition temperature, T T the brittle fracture stress, σ F , increased with decreasing temperature. T T was lowered from ∼209 K by ∼0.17 K per MNm −2 of superposed pressure, which had no effect on the values of the stresses for yielding or brittle fracture, whichever occurred. There was a continuity, as a function of temperature (down to 180K), between the values of the tensile yield and brittle fracture stresses and the compressive yield stress. In the temperature region immediately above T T , at any pressure, failure took place by cleavage at approximately the same stress as at T T . The application of H raised the value of the required applied stress for failure by the magnitude of H, i.e. ductile cleavage occurred at a constant value of the maximum principal stress, postulated to be the crack propagation stress. Below T T the critical stage in brittle fracture, however, apperars to be crack nucleation, as the process took place at a constant shear stress. The data are consistent with T T , at all pressures, being determined by the condition that the stresses for yielding (approximately the same as for crack nucleation) and crack propagation are equal.