The significance of strain weakening and self-annealing in a superplastic Bi-Sn eutectic alloy processed by high-pressure torsion

A cast Bi-Sn eutectic alloy was processed by high-pressure torsion (HPT) at room temperature and stored at room temperature for durations of up to 91 days in order to investigate the effect of self-annealing. The HPT processing produces grain refinement but hardness measurements show there is strain softening with lower microhardness values than in the initial as-cast material and with hardness values that decrease with increasing amounts of imposed torsional strain. This softening is attributed to a loss of precipitates within the Sn and Bi phases during the processing operation. In self-annealing at room temperature, the microhardness increases significantly due to reprecipitation and there is also a minor increase in grain size with increasing time of storage. It is demonstrated by tensile testing that the HPT-processed Bi-Sn alloy exhibits superplastic behavior with elongations of up to >1000% after storage for 35 days and with an associated strain rate sensitivity close to ~0.5. Grain boundary sliding plays an important role in superplastic flow and it is shown that maximum sliding occurs on the Bi-Bi interfaces where this is consistent with estimates of the coefficients for grain boundary diffusion in the Bi-Sn alloy.

Bi-Sn alloy, grain boundary sliding, hardness, high-pressure torsion, superplasticity

10.1016/j.actamat.2019.11.064

1359-6454

245-256

Wang, Chuan Ting

80ad9adf-a9df-49ed-934d-fa64505b920d

He, Yong

479033f6-82b3-4004-88bd-a105203a1c0b

Langdon, Terence G

86e69b4f-e16d-4830-bf8a-5a9c11f0de86

15 February 2020

Wang, Chuan Ting

80ad9adf-a9df-49ed-934d-fa64505b920d

He, Yong

479033f6-82b3-4004-88bd-a105203a1c0b

Langdon, Terence G

86e69b4f-e16d-4830-bf8a-5a9c11f0de86

Wang, Chuan Ting, He, Yong and Langdon, Terence G (2020) The significance of strain weakening and self-annealing in a superplastic Bi-Sn eutectic alloy processed by high-pressure torsion. Acta Materialia, 185, 245-256. (doi:10.1016/j.actamat.2019.11.064).

Record type: Article

Abstract

Text

CT Wang-BiSn-text - Accepted Manuscript

Available under License Creative Commons Attribution Non-commercial No Derivatives.

Download (84kB)

Text

CTWang-Figures - Accepted Manuscript

Available under License Creative Commons Attribution Non-commercial No Derivatives.

Download (1MB)

Text

CTWang-Supplementary Materials - Accepted Manuscript

Available under License Creative Commons Attribution Non-commercial No Derivatives.

Download (1MB)

More information

Accepted/In Press date: 27 November 2019

e-pub ahead of print date: 29 November 2019

Published date: 15 February 2020

Additional Information: Funding Information: This work was supported in part by the National Science Foundation of the United States under Grant no. DMR-1160966 and in part by the European Research Council under ERC Grant Agreement No. 267464-SPDMETALS . One of the authors (CTW) is grateful for funding from the National Natural Science Foundation of China [ NSFC51601095 ] and the National Natural Science Foundation of Jiangshu China [BK20160832]. The authors thank Dr N.N. Liang of NJUST for assistance with the FIB operation. Publisher Copyright: © 2019 Acta Materialia Inc.

Keywords: Bi-Sn alloy, grain boundary sliding, hardness, high-pressure torsion, superplasticity