Characteristics of martensitic and strain-glass transitions of the Fe-substituted TiNi shape memory alloys probed by transport and thermal measurements

Balakrishnan Ramachandran,Yung-Kang Kuo,Chen Chien,Shyi-Kaan Wu,Pei-Chi Chang

doi:10.1038/s41598-017-16574-0

Balakrishnan Ramachandran, Yung-Kang Kuo + Show 3 more

Open Access

https://doi.org/10.1038/s41598-017-16574-0

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Journal: Scientific Reports	Publication Date: Nov 27, 2017
Citations: 19	License type: open-access

Affiliation: National Taiwan University

Abstract

The electrical resistivity, Seebeck coefficient, thermal conductivity, and specific heat of Ti50Ni50-xFex (x = 2.0–10.0 at.%) shape memory alloys (SMAs) were measured to investigate the influence of point defects (Fe) on the martensitic transformation characteristics. Our results show that the Ti50Ni48Fe2 and Ti50Ni47Fe3 SMAs have a two-step martensitic transformation (B2 → R and R → B19′), while the Ti50Ni46Fe4, Ti50Ni44.5Fe5.5, and Ti50Ni44Fe6 SMAs display a one-step martensitic transition (B2 → R). However, the compounds Ti50Ni42Fe8 and Ti50Ni40Fe10 show strain glass features (frozen strain-ordered state). Importantly, the induced point defects significantly alter the martensitic transformation characteristics, namely transition temperature and width of thermal hysteresis during the transition. This can be explained by the stabilization of austenite B2 phase upon Fe substitution, which ultimately leads to the decrease in enthalpy that associated to the martensitic transition. To determine the boundary composition that separates the R-phase and strain glass systems in this series of SMAs, a Ni-rich specimen Ti49Ni45Fe6 was fabricated. Remarkably, a slight change in Ti/Ni ratio converts Ti49Ni45Fe6 SMA into a strain glass system. Overall, the evolution of phase transformation in the Fe-substituted TiNi SMAs is presumably caused by the changes in local lattice structure via the induced local strain fields by Fe point defects.

Highlights

MethodsSamples of Ti50Ni50-xFex (x = 2.0–10.0 at.%) SMAs were fabricated using a vacuum arc re-melter, which described elsewhere[32,33]
Our study revealed that the Ti50Ni48Fe2 and Ti50Ni47Fe3 SMAs undergo a two-step martensitic transition (B2 → R and R → B19′), which is distinct from a transition (B2 → B19′) that occurs in the parent TiNi
With further Fe substitution (4.0 ≤ x ≤ 6.0), a one-step B2 → R transition was observed for the Ti50Ni46Fe4, Ti50Ni44.5Fe5.5, and Ti50Ni44Fe6 SMAs, accompanied with a complete destabilization of the B19′ phase

Summary

Methods

Samples of Ti50Ni50-xFex (x = 2.0–10.0 at.%) SMAs were fabricated using a vacuum arc re-melter, which described elsewhere[32,33]. High-purity raw materials consisting of titanium (4 N), nickel (4 N), and iron (3 N) were melted six times using the re-melter to form ingots of the Ti50Ni50-xFex samples. The sample plates were solution heat-treated at 1173 K for 1 h, followed by water quenching to cool the samples to room temperature (RT). The surface oxide layer of the samples was removed using an etching solution of HF:HNO3:H2O (1:5:20 volumes). The Ni-rich Ti49Ni45Fe6 sample was prepared to explore the influence of excess Ni on the boundary composition (x ≤ 6.0) of the Ti50Ni50-xFex SMAs19,20

Results

Discussion

Conclusion