Strain rate history effects and observations of dislocation substructure in aluminum single crystals following dynamic deformation

Abstract

Single crystals of aluminum are deformed in shear at a number of constant strain rates in the range 10 −5–1600 s −1 and to strains of about 20%. These constant-rate tests are supplemented by a series of jump tests in which a sharp increment in strain rate is imposed during the quasi-static straining. Thin foil samples of the strained specimens are examined by transmission electron microscopy (TEM). The constant-strain-rate tests show a linear increase in flow stress with the logarithm of strain rate up to 500 s −1, followed by a more rapid increase at higher strain rates. TeM observations after deformation at quasistatic strain rates reveal a cellular dislocation arrangement that develops into well-outlined subgrains as strain increases. Cells are also visible after dynamic deformation, but these cells are about one-quarter of the size of those produced by quasi-static deformation and their walls are not as well defined. Furthermore, results indicate that, as the cell size is reduced, both the flow stress and the strain rate sensitivity increase. The jump tests show a pronounced strain rate history effect and their results permit the calculation of an activation volume applicable to quasi-static straining. Results of this calculatin, which is based on macroscopic measurements and on the theory of thermally activated deformation of metals, are compared with an activation volume based on the dislocation densities measured after deformation.