Abstract
NiTi wires were brazed together via eutectic reaction between NiTi and Nb powder deposited at the wire contact region. Phase transformation and deformation behavior of the NiTi-Nb eutectic microstructure were investigated using transmission electron microscopy (TEM) and cyclic loading-unloading tests. Results show that R phase and B19′ martensite transformation are induced by plastic deformation. R phase transformation, which significantly contributes to superelasticity, preferentially occurs at the interfaces between NiTi and eutectic region. Round-shaped Nb-rich phase with rod-like and lamellar-type eutectics are observed in eutectic regions. These phases appear to affect the deformation behavior of the brazed NiTi-Nb region via five distinct stages in stress-strain curves: (I) R phase reorientation, (II) R phase transformation from parent phase, (III) elastic deformation of reoriented martensite accompanied by the plastic deformation of Nb-rich phase and lamellar NiTi-Nb eutectic, (IV) B19′ martensitic transformation, and (V) plastic deformation of the specimen.
Highlights
NiTi wires were brazed together via eutectic reaction between NiTi and Nb powder deposited at the wire contact region
The NiTi wires were bonded using following six steps: (i) weight ratio of NiTi and Nb was designed as 1.5, (ii) 0.5 g NiTi and the appropriate amount of Nb powder were blended and tumbled for 2 h, (iii) the blended NiTi/Nb powder and 0.04 g PVA was added to 0.6 ml H2O to create a slurry, (iv) slurry at contact area between two parallel partially overlapping NiTi wires was deposited using a syringe, (v) thereafter water and PVA were removed by heating at 250 °C for 1 h in air, and (vi) NiTi/Nb powder were melt by heating to 1180 °C and kept/maintained the liquid eutectic at this temperature for 4 min to bond the wires in a high vacuum (10−5 torr) furnace with pure titanium getters
Following conclusions can be highlighted: (1) B2 to B19′ phase transformation is found to be retarded after heat treatment at 520 °C for 30 min for both NiTi wire and eutectic region
Summary
NiTi wires were brazed together via eutectic reaction between NiTi and Nb powder deposited at the wire contact region. B2 → R phase transformation takes place in near-equiatomic NiTi alloy due to the precipitation-induced inhomogeneity in both composition and internal stress fields of matrix[4]. Hao et al.[6] reported large elastic strain and high strengh in bulk Ni41Ti39Nb20 (at.%), which resulted from the stress-induced B19′ phase transformation during tensile deformation. For Ni47Ti44Nb9 SMA, soft β -Nb eutectic phase formed during solidification process effectively stabilizes austenite phase by absorbing large strain energy during the formation of stress-induced martensite. Martensite phase transformation and its reorientation are deemed to be the key factors in improving the shape memory effect and superelasticity in NiTiNb alloys
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