Effect of Annealing Temperature on Annealing Twins and Shape Memory Effect in Hot-Forged Co-23.9Ni-5.6Si Alloy
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NTRODUCTION
RECENT studies showed that the shape memory effect (SME) in Co-Ni-based alloys undergoing face-centered cubic (fcc) Ð hexagonal close-packed (hcp) martensitic transformation also resulted from the stress-induced hcp martensitic transformation and its reverse transformation,[1–4] as the Fe-Mn-Si-based shape memory alloys do.[5–8] Due to their same SME origin, theoretically, a good SME comparable to that in Fe-Mn-Si-based alloys can be also expected in the Co-Ni-based alloys. Unfortunately, up to now the recovery strain in the solution-treated Co-Ni-based alloys was only about 1 pct,[9,10] far lower than that of 2 to 4 pct in solution-treated Fe-Mn-Sibased alloys.[11,12] After some special treatments, such as ausforming,[13,14] thermomechanical training,[8,15,16]
X. YANG, D. WANG, Q. LIAO, H.B. PENG, and Y.H. WEN are with the College of Manufacturing Science and Engineering, Sichuan University, Chengdu 610065, P.R. China. Contact e-mails: [email protected], [email protected] S.L. WANG is with the Analytical and Testing Center, Sichuan University, Chengdu 610065, P.R. China. B.J. WU is with the Key Lab of Nuclear Reactor System Design Technology, Nuclear Power Institute of China, Chengdu 610041, P.R. China. Manuscript submitted January 9, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
aging,[17,18] the recovery strain in Fe-Mn-Si-based alloys can be further improved to 5 to 6 pct. However, the recovery strain in Co-Ni-based alloys only reached 2 to 3 pct even after the above special treatments.[1,2] Obviously, it still deserves to develop some methods for further improving the SME in the Co-Ni-based alloys. Zhou et al. first tried to improve the SME in Co-31Ni (in wt pct, here and after) binary alloys by Si alloying based on the fact that the Si alloying can remarkably enhance the SME in Fe-Mn binary alloys.[10] Disappointedly, the shape recovery rate (SRR) in the solution-treated Co-31.4Ni-4.7Si alloy only reached 13 pct when deformed at room temperature (RT) by 2 pct. Our recent studies showed that although the SRR in the solution-treated Co-31Ni-2.99Si alloy was only 20 pct when deformed at RT by 2.4 pct, it was significantly improved to 59 pct when deformed at 77 K.[3] Our studies further revealed that the yield strength was still low in the Co-31Ni-2.99Si alloy due to the rather low addition of Si.[2] Moreover, the 2.99 wt pct Si addition greatly lowered the forward martensitic transformation temperature (Ms) of Co-31Ni alloy to below 77 K.[1–3] As the result of the low yield strength and too low Ms temperature, the intrusion of slip deformation must be preferential at RT rather than the occurrence of stress-induced hcp martensitic transformation. This was the reason why the SME in the solution-treated Co-31Ni-2.99Si alloy was poor at RT. When deformed at 77 K, both the yield strength of parent phase and the
chemical force for the stress-induced hcp martensitic transformation increased. Consequently, more deformation will be assumed by the stress-induced hcp martensitic transformation instead of
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