Effect of long-term natural aging on microstructure and room temperature superplastic behavior of UFG/FG Zn-Al alloys processed by ECAP

M. Demirtas, H. Yanar, G. Purcek
Received: 15 September 2018; Revised: 12 October 2018; Accepted: 14 October 2018
Citation: M. Demirtas, H. Yanar, G. Purcek. Effect of long-term natural aging on microstructure and room temperature superplastic behavior of UFG/FG Zn-Al alloys processed by ECAP. Letters on Materials, 2018, 8(4s) 532-537
BibTex   DOI: 10.22226/2410-3535-2018-4-532-537

Abstract

ECAP-processed Zn-22Al and Zn-0.3Al alloys were subjected to long-term natural aging up to 1100 days to evaluate the effect of long-term natural aging on their microstructures and superplastic behaviors. Decrease in the maximum elongation of Zn-22Al alloy is quite low comparing to its ECAP-processed condition while Zn-0.3Al alloy loses more than half of its superplastic elongation at the end of the natural aging.Two potential superplastic compositions of Zn-Al alloy systems, Zn-22Al and Zn-0.3Al alloys, were chosen and processed by equal-channel angular pressing/extrusion (ECAP/E) in order to achieve high strain rate (HSR) superplasticity at room temperature (RT). ECAP-processed samples of both alloys were then subjected to long-term natural aging up to 1100 days to evaluate the effect of long-term natural aging on their microstructures and superplastic behaviors. Before natural aging, the maximum elongations to failure at RT were 400% for ultrafine-grained (UFG) Zn-22Al at the strain rate of 5×10-2 s-1 and 1000% for fine-grained (FG) Zn-0.3Al at the strain rate of 1×10-4 s-1. Long-term natural aging did not cause a significant change in the elongation of UFG Zn-22Al alloy with 355% maximum elongation. However, optimum strain rate giving the maximum elongation decreased to 3×10-3 s-1. On the other hand, Zn-0.3Al alloy lost more than half of its superplastic elongation and showed an elongation to failure of 435 % at the end of the natural aging period of 1100 days. Microstructural analyses show that grain boundary corrosion occurred in dilute Zn-0.3Al alloy during the natural aging process. Corroded grain boundaries resulted in cavity nucleation during the tensile tests and some of these cavities attained large sizes and caused premature failure.

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