The effect of post-deformation aging on superelastic properties of Ni50.9Ti thin wires attaining micro and nano-substructure
Highlights
► Precipitation of Ni4Ti3 during aging at 450 °C is hindered after severe cold work. ► Nanostructure formation improves the superelastic behavior of Ni-rich NiTi. ► Deformations less than 0.4 result in work hardening and reduce plateau strain. ► Deformations more than 0.4 yield in high stress and strain values of upper plateau.
Introduction
Functional properties such as shape memory and superelasticity accompanied with good biocompatibility and high corrosion resistance have lead to great utilization of NiTi alloys in various applications [1], [2], [3], [4]. It is well known that shape memory and superelastic effect of NiTi alloys are based on thermo-elastic martensitic transformations. The well-known phases in binary NiTi alloys are B2 (austenite), R phase (intermediate martensitic phase) and B19′ (martensite).
Fine and coherent Ni4Ti3 precipitates form during aging of Ni-rich NiTi alloys. Ni4Ti3 precipitates affect both the transformation sequences, temperatures and mechanical properties of NiTi alloys [5], [6], [7]. Effect of aging treatment on superelastic behavior, sequences and transition temperatures of martensitic transformations have been studied extensively [5], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. It is well established that aging and thermomechanical treatments can improve thermal shape memory and superelastic properties of Ni-rich NiTi alloys [3], [8], [24]. However, there is less knowledge about the influence of aging treatment in deformed NiTi alloys (post-deformation aging) on the martensitic transformations and superelastic properties of Ni-rich NiTi alloys.
Many industrial producers of NiTi alloys have their own practical information about the effect of cold work amount on mechanical behavior and transformation characteristics. However, information about the effect of cold work amount on superelastic behavior is scarce in open literature [25], [26], [27], [28], [29]. It is established that cold deformation up to 40% lead to work hardening while more increase in the cold work degree does not affect the mechanical properties significantly [30]. Recent researches have illustrated that deformations about 60% and more would lead to partial amorphisation that could result in nanocrystalline structures after post-deformation annealing [31], [32], [33], [34]. Moreover, it has been reported that grain sizes less than 100 nm impedes the Ni4Ti3 precipitation [35].
Cold work degree has a sharp influence on superelastic properties of NiTi alloys due to work hardening effect. Moreover, grain size refinement and development of precipitates during post-deformation heat treatment seems to be highly dependent on cold work amount. Accordingly, present study concentrates on the effect of cold work amount prior to post-deformation aging/annealing on superelastic properties of Ni50.9Ti shape memory alloy.
Section snippets
Experimental
A NiTi wire with nominal composition of 50.9 at.% Ni was used in this study. The as-received wire was in cold drawn state with a diameter of 340 ± 0.5 μm. The wire was cut into 15 cm length samples. The samples were encapsulated in a quartz tube filled with high purity argon gas and they were annealed at 850 °C for 1 h. In addition, some Ti foils were used inside the tube as a getter to purify the atmosphere. Surface of the samples were chemically etched out to decrease the oxidation-affected layer
Results and discussions
Fig. 1 shows stress–strain curves of the samples with different cold work degrees.
As shown in Fig. 1 the cold drawing of the NiTi wires results in reducing the residual strain after unloading. Furthermore, applying cold work leads to annihilation of stress plateau, which is a basic characteristic of functional superelastic NiTi alloys. It is well known that cold deformation of NiTi alloys results in formation and stabilization of stress induced martensitic phase. Thus, the cold drawn wires do
Conclusion
Strain value of upper plateau during loading of cold drawn wires after post-deformation heat treatment at 450 °C for 45 min reduces with increasing the cold work value up to 0.4. While applying cold work values higher than 0.4 leads to increasing of strain value of upper plateau. Results show that cold drawing values higher than 0.4 causes the formation of equiaxed nanocrystalline structure, which suppresses the precipitation of Ni4Ti3 particles.
R phase transformation occurs in annealed and cold
Acknowledgments
The authors acknowledge Professor Gunther Eggeler, Dr. Jan Frenzel and Dr. Tobias Simon from Ruhr University of Bochum, Germany for FIB and TEM investigations.
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