Void-strengthening in aluminum and its nature

Abstract

Temperature and strain rate dependence of yield strength were used to analyze the nature of aluminum strengthened by the formation of voids. Aluminum rods 99.999% pure were quenched and heat treated to form voids with an approximate density of 10 13–10 14 voids/cm 3. Voids in selected samples were observed by electron microscopy. The yield strength of the void strengthened samples was measured at various temperatures from 77 to 593 K and at two strain rates, 3.33 × 10 −2/s and 1.67 × 10 −3/s. Tests at room temperature and at 77 K were made at various strain rates. The similarity of Coulomb's approach to void strengthening and that of Orowan stress with Ashby's dipole criterion was observed, and it was determined that the extent and effectiveness of void strengthening depends primarily on void density and much less on void size. The amount of strengthening obtained at room temperature was found to be consistent with the estimates. Compared to annealed aluminum, void strengthened aluminum is more susceptible to the instability of plastic flow at low temperatures; its temperature dependence of yield strength varies in different temperature ranges and shows a higher strain rate sensitivity of yield strength.