Hardness and Stress-Strain Curves of Al-Zn-Mg-Cu Alloy Single Crystals

Abstract

The present paper describes the variations in mechanical properties of Al Zn Mg Cu alloy single crystals with various aging times. The cylindrical single crystals of 7475 aluminum alloys were produced at 1033 K by the Bridgman method, where the composition of initial material and the shape of carbon mold were modified for the growth of quaternary crystals. Specimens appropriate for tensile and hardness testing were obtained from the single crystal rod homogenized at 823 K for 900 ks using a spark cutting method. Subsequently, they were aged at 393 K for 3.6, 86.4, 900 and 2880 ks after quenching in ice water from 773 K. In the stress strain curves of the alloy single crystals, the yield stress increased remarkably with the increase of aging time. Also, a decrease of elongation for the aged specimens was seen accompanying a decrease in the rate of work hardening after yielding. Moreover, some serrations occurred particularly in the final plastic stage on the curves. Hardness of the alloy single crystals agreed with those of polycrystals due to the occurrence of multiple slips during loading. However, the alloy single crystals exhibited a marked decrease in yield strength in comparison to the polycrystals. In single crystals, a single glide was found to occur. If we compare the increase of critical resolved shear stress (CRSS) derived from two different theories of precipitation hardening with the experimental value, the present results were roughly in agreement with the theoretical values except for the specimen aged for 3.6 ks. This fact indicated that the maximum strength of Al Zn Mg Cu alloy single crystals was achieved in the transition from the mechanism in which dislocations cut through the precipitated particles to the bypass mechanism of Orowan with the growth in particle size.