Remarkable Improvement of Shape-Memory Effect in a Co-31Ni-3Si Alloy by Ausforming
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INTRODUCTION
CO-NI alloys with a transformation of gamma (c) fi epsilon martensite (e) have drawn attention in the past decades.[1–11] On one hand, the Fe-Mn-Si-based alloys show good shape-memory effect (SME) due to the c fi e transformation.[12–22] On the other hand, both the c and the e in the Co-Ni alloys are ferromagnetic,[3,4] but both of them are paramagnetic in the Fe-Mn-Si-based alloys.[15] In addition, the start temperature of e reverse transformation can be above 573 K (300 C) by adjusting the Ni content.[1,5] Therefore, although they are expensive compared with low-cost Fe-Mn-Si-based alloys, the Co-Ni alloys combining both the ferromagnetic property and the SME will find a potentially wider application in the field of the magnetic sensors and actuators than Fe-Mn-Si-based alloys. Also, contactless induction heating techniques can be used readily in recovering the deformation because the heating efficiency becomes higher due to the high permeability.[23] Unfortunately, the SME in the Co-Ni-based alloys is much poorer than that of the Fe-Mn-Si-based alloys even after they were subjected to the thermal cycling and alloying.[1,6] Based on the fact that the Fe-Mn-Si-based alloys show much better SME than binary Fe-Mn alloys due to the addition of Si,[20] Zhou et al. investigated the effect of Si alloying on the shape-memory effect in the binary Co-Ni alloys.[6] The result showed that the addition of Si JIANGWEI SUN and ZHIWEI YAN, Postgraduate Students, HUABEI PENG, Lecturer, and YUHUA WEN, Professor, are with the College of Manufacturing Science and Engineering, Sichuan University, Chengdu 610065, P.R. China. Contact e-mail: wenyh@ scu.edu.cn SHANLING WANG, Professor, is with the Analytical & Testing Center, Sichuan University, Chengdu 610065, P.R. China. Manuscript submitted October 8, 2014. Article published online 31 January 2015 1550—VOLUME 46A, APRIL 2015
only slightly improved the SME in the Co-Ni alloys. A maximal recovery strain of only 0.45 pct was obtained in Co-31.4Ni-4.7Si alloy (wt. pct here and after),[6] far lower than that of solution-treated Fe-Mn-Si alloys (~2 pct).[12–14] As far as we know, a major breakthrough in the SME improvement has not been made in the CoNi alloys. Some studies showed that ausforming, pre-straining austenite at a certain high temperature, can remarkably improve the SME in the Fe-Mn-Si alloys.[12–14] Rong et al. investigated the effect of ausforming temperature on the SME in an Fe-29.9Mn-6.0Si alloy.[12] The results showed that after pre-straining by 2.5 pct at 973 K (700 C), its maximal recovery strain was improved to 3.8 pct from 1.7 pct in solution-treated state. Wang et al. found that the SME of Fe-28Mn-6Si-5Cr alloy was substantially improved by ausforming at 973 K (700C), and the optimal strain in the ausforming was about 9 pct.[13] Considering the same martensitic transformation in the Co-Ni and Fe-Mn-Si-based alloys, it can be expected that the SME will also be enhanced by the ausforming. So far, no papers reported the effect of the ausforming on the SME in the Co-N
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