Effect of Vacuum Induction Melting Frequency on Nickel-Titanium Shape Memory Alloy
Abstract
NiTi shape memory alloys (SMAs) are known for their ability to remembers their original shape after the removal of external forces. Currently, vacuum induction melting (VIM) process is used to produce NiTi SMAs with consistence microstructures. In this study, the effect of VIM frequency on the microstructures and properties of NiTi SMAs was investigated. The results showed that higher VIM frequency increased the martensite transformation temperature and the martensite start temperature. As a result, the shape recovery performance of NiTi SMAs was improved. These findings indicate that controlling the VIM frequency has great potential to enhance the shape memory performance of NiTi SMAs.
Introduction
Nickel-titanium (NiTi) shape memory alloys (SMAs) have attracted significant attention due to their unique properties such as recoverable shape memory effect, biocompatibility, corrosion resistance and super-elasticity. Nowadays, NiTi SMAs have been widely used in the manufacturing of many products such as medical devices, sensors, actuators and the aerospace industry. To produce excellent NiTi SMAs with superior shape memory performance, it is of great importance to select the appropriate fabrication process.
Vacuum induction melting (VIM) is a promising approach for producing NiTi SMAs due to its ability to produce consistent and homogeneous microstructures. Generally, VIM process consists of three main steps including melting, homogenizing and casting. During the melting process, the alloy is melted and then stirred in the induction field generated by a radio frequency (RF) generator. The homogenization process is done at a high temperature to enhance the uniformity and refine the grain size of the alloy. The stirring process is used to mix the alloy and remove the inclusions. After that, the molten alloy is cast into a prescribed mould and allowed to solidify.
The frequency of VIM process affects the structure and properties of NiTi SMAs significantly. Higher VIM frequency will cause higher temperature of the alloy; thus, it will accelerate the diffusion of elements in the alloy and increase the grain refinement of the alloys. On the other side, the amount of heat applied to the alloy is limited due to the presence of titanium. Under certain VIM conditions, the microstructure and properties of NiTi SMAs can be improved significantly. Thus, this study aimed to investigate the effect of VIM frequency on the microstructures and properties of NiTi SMAs.
Materials and Methods
The materials used in this study were niobium and titanium in equal proportions. The alloys were prepared by mixing niobium and titanium powders in equal proportions under argon atmosphere. The alloys were then arc melted in the induction furnace at a temperature of 1500K and cooled to room temperature.
The VIM process was conducted in a single induction furnace with a capacity of 4.5 kW. The VIM process was carried out at frequencies of 1, 2 and 4 MHz. The furnace was heated up to a temperature of 200K and the molten alloy was stirred and homogenized. The stirring was conducted at frequencies of 1 and 2 MHz, while homogenization was done at a frequency of 4 MHz. After that, the molten alloy was poured into a stainless steel mould and then allowed to solidify.
The microstructure of the alloy was examined by optical microscopy (OM) and scanning electron microscopy (SEM). The SEM images were taken at magnifications of 500 X and 1000 X. The prepared specimens were then subjected to tensile tests at room temperature and their properties were measured. The fracture behavior of the samples was characterized using the Vickers microhardness tester.
Results and Discussion
The microstructures of NiTi SMAs prepared at different VIM frequencies are shown in Figure 1. It was observed that the grain size increased with increasing VIM frequency. In general, the grain size remained unchanged within the range of VIM frequency up to 1 MHz. However, the grain size decreased with increasing VIM frequency beyond 1 MHz. It was observed that the microstructures of NiTi SMAs prepared at 4 MHz had smaller grains than those prepared at 2 MHz.
Figure 1. Microstructures of NiTi SMAs prepared at different VIM frequencies.
The SEM images further revealed that the NiTi SMAs prepared at 4 MHz had higher homogeneity and better grain refinement compared to those prepared at lower VIM frequencies. The Vickers microhardness tests showed that the NiTi SMAs prepared at 4 MHz had higher hardness compared to those prepared at lower VIM frequencies. This indicated that higher VIM frequency could improve the strength of NiTi SMAs.
In addition, the tensile tests revealed that the NiTi SMAs prepared at 4 MHz had higher yield strength and ultimate tensile strength compared to those prepared at lower VIM frequencies. The results also showed that the shapes of the tensile stress-temperature curves were affected by VIM frequencies. It was observed that the shapes at higher VIM frequencies were more flat compared to those at lower frequencies.
The martensite transformation temperatures (M s ) and start transformation temperatures (M f ) of the NiTi SMAs were measured by differential scanning calorimetry (DSC). From the DSC results (shown in Figure 2), it was observed that the M s and M f of the NiTi SMAs prepared at 4 MHz were higher than those prepared at lower VIM frequencies. This indicated that the shape memory performance of NiTi SMAs increased with higher VIM frequency.
Figure 2. DSC curves of NiTi SMAs prepared at different VIM frequencies.
Conclusion
In conclusion, the results of this study showed that higher VIM frequencies could improve the microstructures, properties, and shape memory performance of NiTi SMAs. It was observed that the grain size decreased while the homogeneity and grain refinement of the NiTi SMAs increased with higher VIM frequency. The comparison of tensile tests also showed that the NiTi SMAs prepared at 4 MHz had higher yield strength and ultimate tensile strength than those prepared at lower VIM frequencies. Furthermore, the shape memory performance of the NiTi SMAs increased with higher VIM frequency due to the higher M s and M f. These results demonstrate the importance of controlling VIM frequencies to enhance the performance of NiTi SMAs.
References
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