Effects of Grain Size on the Dislocation Density and Flow Stress of Niobium

摘要

Effects of grain size from 32 to 435 micron on the dislocation density and distribution and on the yield and flow stresses of 125-micron-thick niobium foil pulled in tension at room temperature (300K) were investigated. For a given grain size, the dislocation density initially increased in an approximately linear manner with strain; for a given strain, the dislocation density was proportional to the reciprocal of the grain size. The tensile flow stress Sigma increased with the square root of the dislocation density, yielding the relation Sigma = Sigma f + 0.88 Gb rho 1/2 where Sigma f was independent of grain size when the grain diameter was less than the foil thickness. A significant part of Sigma f was identified with the thermally activated nucleation of kinks. The effect of grain size on the lower yield and flow stresses of the fine-grained niobium is explained in terms of a work-hardening model. A cellular network of dislocations developed for strains of the order of 10%. The strain at which the network was first well defined increased with grain size. Once the cell was formed, the cell size tended to decrease with further straining. Cell size at a given strain was proportional to the square root of the grain diameter and was approximately equal to the average dislocation spacing, assuming a random distribution. Flow stress increased linearly with the reciprocal of the cell size. (Author)