B. Saberpour, A. K. Taheri, J. M. Akhgar,
Volume 28, Issue 1 (6-2009)
Abstract
An Al-Mg-Si-Cu alloy was subjected to tensile testing, both under solutionized and ECAE-ed conditions, using strain rates of 10-4 s-1 to 10-1 s-1 at temperatures of 25 ˚C to 325 ˚C to investigate the dynamic strain aging (DSA) phenomenon in the alloy. Negative strain rate sensitivity (m) and increasing ultimate tensile stress were observed in the DSA region with increasing temperature . Regarding the activation energy of the phenomenon, it was suggested that the process is controlled by the interaction of Mg atoms with mobile dislocations at lower temperatures of DSA occurrence while at higher temperatures, the aggregation of Mg atoms and precipitates of a second phase decreases the amount of Mg atoms in the solid solution, resulting in the inverse DSA effect. Moreover, it was shown that at temperatures greater than 250 ˚C, the ratio of post-uniform to uniform elongation increases with increasing temperature or with decreasing strain rate due to the solute drag of Mg atoms in the Al matrix. Processing the alloy by ECAE transferred the negative m values to lower temperatures and decreased the tendency to DSA at higher temperatures. Calculating the mentioned ratio for the ECAE-ed specimens revealed that the post-uniform elongation dominates the uniform elongation at all examined temperatures and strain rates. Also, it was found that for ECAE-ed specimens, the ratio is not so sensitive to variations of temperature and strain rate.
M. T. Asadi Khanouki,
Volume 39, Issue 3 (12-2020)
Abstract
In this study, the influence of temperature and strain rate on plastic flow of a Zr-based bulk metallic glass (BMG) during the three-point bending test was studied to find a correlation between strain rate sensitivity (m) and flow behavior. The flexural stress-deflection curves revealed two distinct types of dynamics, serrated and non-serrated flow, related to temperature and strain rate. The serrated flow which appeared at temperatures higher than a critical value or strain rates lower than a critical value, was simultaneously due to activation of shear transformation zones (STZs) and time-dependent structural relaxations. Further results indicated negative and positive values of m at temperatures above and below 0.4 Tg, respectively. The main reason for negative strain rate sensitivity was insufficient time of structural relaxation at high strain rates which lead to generation of free volume inside shear bands making the BMG softer. Comparison of STZ activation energy with activation energy for the onset of serration indicated almost equal values and there was a close relationship between serrated flow and STZ operation.
