Microstructure, Mechanical Property, and Phase Transformation of Quaternary NiTiFeNb and NiTiFeTa Shape Memory Alloys

Abstract

Based on ternary Ni₄₅Ti_51.8Fe_3.2 (at %) shape memory alloy (SMA), Nb and Ta elements are added to an NiTiFe SMA by replacing Ni element, and consequently quaternary Ni₄₄Ti_51.8Fe_3.2Nb₁ and Ni₄₄Ti_51.8Fe_3.2Ta₁ (at %) SMAs are fabricated. The microstructure, mechanical property, and phase transformation of NiTiFeNb and NiTiFeTa SMAs are further investigated. Ti₂Ni and β-Nb phases can be observed in NiTiFeNb SMA, whereas Ti₂Ni and Ni₃Ti phases can be captured in NiTiFeTa SMA. As compared to NiTiFe SMA, quaternary NiTiFeNb and NiTiFeTa SMAs possess the higher strength, since solution strengthening plays a considerable role. NiTiFeNb and NiTiFeTa SMAs exhibit a one-step transformation from B2 austenite to B19’ martensite during cooling, but they experience a two-step transformation of B19’-R-B2 during heating.

1. Introduction

NiTi shape memory alloys (SMAs) have deserved increasing attention in the domain of engineering since they possess shape memory effect and superelasticity. In general, phase transformation temperature and mechanical property are the two critical factors influencing the application of NiTi SMA in engineering [1,2,3]. In particular, a third element can be added to the binary NiTi SMA so as to change its phase transformation temperature and mechanical property [4,5]. As a consequence, typical ternary NiTi-based SMAs, such as NiTiCu, NiTiNb, and NiTiFe, have gone toward the engineering application. For instance, NiTiCu SMA possesses a narrow transformation temperature hysteresis, so it can be used in actuators [6,7,8]. However, NiTiNb SMA possesses a broad transformation temperature hysteresis, so it is suitable for pipe coupling [9,10,11]. NiTiFe SMA is typically used for coupling pipe since it possesses a lower martensite transformation start temperature [12,13,14]. NiTiPt, NiTiPd, NiTiZr, and NiTiHf SMAs become candidates for high temperature SMAs since they possess a higher reverse transformation temperature [15,16,17,18]. So far, quaternary NiTi-based SMAs have deserved more attention. Some quaternary SMAs have also become potential candidates for high temperature SMAs, such as TiNiPdCu [19], NiTiHfCu [20], NiTiHfZr [21], and NiTiHfTa [22] SMAs. In addition, some new quaternary NiTi-based SMAs, which possess special properties, have been put forward as well. These new quaternary NiTi-based SMAs include NiTiHfPd SMA with high strength [23,24], TiNiCuPd SMA with near-zero hysteresis [25], and NiTiCuV SMA with high thermal stabilization [26]. However, only a few studies have reported the investigations of quaternary NiTi-based SMAs.

In the present study, Nb and Ta elements were added to NiTiFe SMA in order to prepare NiTiFeNb and NiTiFeTa SMAs. Furthermore, the microstructure, mechanical property, and phase transformation of NiTiFeNb and NiTiFeTa SMAs were investigated.

2. Materials and Methods

On the basis of ternary Ni₄₅Ti_51.8Fe_3.2 (at %) SMA, Nb and Ta elements were added by replacing the Ni element, and consequently quaternary Ni₄₄Ti_51.8Fe_3.2Nb₁ and Ni₄₄Ti_51.8Fe_3.2Ta₁ (at %) SMAs were fabricated via the vacuum arc melting method. Then, three as-cast NiTi-based SMAs were heated to 1000 °C and maintained for 12 h. Subsequently, the three heat-treated NiTi-based SMA samples were quenched with ice water. NiTi-based SMA samples, whose height and diameter were 6 mm and 4 mm, respectively, were removed from the heat-treated NiTi-based SMA ingots using electro-discharge machining (EDM) for the purpose of compression tests. The compression tests were performed with INSTRON-5500R equipment (Instron Corporation, Norwood, MA, USA) at room temperature, where the compression strain rate was determined as 0.001 s⁻¹.

Differential scanning calorimetry (DSC) was used to determine the phase transformation temperatures of the three heat-treated NiTi-based SMA specimens by using a Pyris Diamond type differential scanning calorimeter (Perkin Elmer Inc., Waltham, MA, USA). Therein, DSC measurement temperature ranged from −150 °C to 150 °C, and the heating and cooling rates were determined as 10 °C/min.

Optical microscopy (OM) as well as transmission electron microscopy (TEM) was employed to characterize the microstructures of the three heat-treated NiTi-based SMA samples. The samples for the OM experiment were etched in a solution whose composition was determined as HF:HNO₃:H₂O = 1:2:10. Then, the OM experiment was performed using an OLYMPUS311 type optical microscope (Olympus Corporation Tokyo, Japan). The TEM experiment was carried out on an FEI TECNAI G2 F30 microscope (FEI Corporation, Hillsboro, OR, USA) with a side-entry and double-tilt specimen stage with an angular range of ± 40° at an accelerating voltage of 300 kV. Foils for TEM observation were mechanically ground to 70 μm and then thinned by twin-jet polishing in an electrolyte containing 90% C₂H₅OH and 10% HClO₄ by volume fraction.

Employing a Philips X’Pert Pro diffractometer (Royal Dutch Philips Electronics Ltd. Amsterdam, Netherlands) with CuKα radiation at ambient temperature, X-ray diffraction (XRD) analysis was used to identify the phase composition of the three heat-treated NiTi-based SMA samples. The samples were scanned over 2θ, which ranges from 20° to 90° by continuous scanning based on a tube voltage of 40 kV and a tube current of 40 mA.

3. Results and Discussion

3.1. Microstructure Analysis of NiTi-based SMAs

Figure 1 shows the microstructures of Ni₄₅Ti_51.8Fe_3.2, Ni₄₄Ti_51.8Fe_3.2Nb₁, and Ni₄₄Ti_51.8Fe_3.2Ta₁ SMAs determined by OM. The microstructures of the three NiTi-based SMAs are dominated by equiaxed grains rather than dendrites. In addition, there are some precipitates in the grain interior as well as at the grain boundary.

To further determine the phase composition of the three NiTi-based SMAs, XRD maps of Ni₄₅Ti_51.8Fe_3.2, Ni₄₄Ti_51.8Fe_3.2Nb₁, and Ni₄₄Ti_51.8Fe_3.2Ta₁ SMAs are illustrated in Figure 2. It is evident that the three NiTi-based SMAs are composed of B2 austenite and Ti₂Ni precipitate.

In order to gain an in-depth insight into the microstructures of the three NiTi-based SMAs, TEM micrographs of Ni₄₅Ti_51.8Fe_3.2, Ni₄₄Ti_51.8Fe_3.2Nb₁, and Ni₄₄Ti_51.8Fe_3.2Ta₁ SMAs are shown in Figure 3, Figure 4 and Figure 5, respectively. It can be observed from Figure 3 that Ni₄₅Ti_51.8Fe_3.2 SMA consists of B2 austenite matrix and Ti₂Ni precipitate. It can be seen in Figure 4 that in terms of Ni₄₄Ti_51.8Fe_3.2Nb₁ SMA, Ti₂Ni and β-Nb precipitates occur in the matrix of B2 austenite. It can be seen in Figure 5 that the matrix of Ni₄₄Ti_51.8Fe_3.2Ta₁ SMA belongs to B2 austenite, where both Ti₂Ni and Ni₃Ti precipitates can be observed.

3.2. Mechanical Property of NiTi-based SMAs

Figure 6 illustrates the stress-strain curves of Ni₄₅Ti_51.8Fe_3.2, Ni₄₄Ti_51.8Fe_3.2Nb₁, and Ni₄₄Ti_51.8Fe_3.2Ta₁ SMAs under uniaxial compression. It is observed that the addition of Nb and Ta elements results in the increasing yield strength of NiTi-based SMAs, but also leads to the decreasing plasticity of NiTi-based SMAs. Furthermore, Ni₄₄Ti_51.8Fe_3.2Nb₁ SMA possesses a higher yield strength than Ni₄₄Ti_51.8Fe_3.2Ta₁ SMA. In other words, the Nb element plays a more substantial role in strengthening NiTi-based SMAs compared with the Ta element. According to the aforementioned microstructural analysis, no metallic compounds of Nb and Ta elements are observed in the case of Ni₄₄Ti_51.8Fe_3.2Nb₁ and Ni₄₄Ti_51.8Fe_3.2Ta₁ SMAs. Consequently, it can be concluded that Nb and Ta exist in the solid solution of B2 austenite as the solute atoms. In particular, compared to Ni₄₅Ti_51.8Fe_3.2 and Ni₄₄Ti_51.8Fe_3.2Nb₁ SMAs, Ni₄₄Ti_51.8Fe_3.2Ta₁ SMA exhibits a steady strain hardening ability during plastic deformation. It is well known that the strain hardening ability of metal materials is related closely to the dislocation density. In general, the strain hardening ability increases with increasing dislocation denstiy. Therefore, as for Ni₄₄Ti_51.8Fe_3.2Ta₁ SMA, plenty of dislocations need to be enhanced so as to guarantee the compatiblity of plastic deformation.

3.3. Phase Transformation of NiTi-based SMAs

Figure 7 shows the DSC curves of Ni₄₅Ti_51.8Fe_3.2, Ni₄₄Ti_51.8Fe_3.2Nb₁, and Ni₄₄Ti_51.8Fe_3.2Ta₁ SMAs. All three NiTi-based SMAs exhibit a one-step phase transformation during cooling. The one-step phase transformation deals with the transformation from B2 austenite (A) to B19’ martensite (M). However, all three NiTi-based SMAs exhibit a two-step phase transformation during heating. First, they are converted from B19’ martensite to an R-phase. Subsequently, they are transformed from the R-phase into B2 austenite. It can be noted that the addition of Nb and Ta elements does not change the phase transformation path of NiTiFe SMA, but it does have a certain effect on the transformation temperatures of NiTiFe SMA. As a consequence, all of the transformation temperatures are diminished. In particular, as for Ni₄₄Ti_51.8Fe_3.2Nb₁ SMA, the addition of the Nb element results in the severe diminishment of the martensite and austenite transformation temperatures.

Compared to Ni₄₅Ti_51.8Fe_3.2 SMA, Ni₄₄Ti_51.8Fe_3.2Nb₁ and Ni₄₄Ti_51.8Fe_3.2Ta₁ SMAs possess a lower phase transformation temperature. In particular, in the case of Ni₄₄Ti_51.8Fe_3.2Nb₁ SMA, the addition of the Nb element influences the phase transformation temperature considerably, and consequently all the phase transformation temperatures are lowered substantially compared with Ni₄₅Ti_51.8Fe_3.2 SMA. In general, for the purpose of lowering the martensite transformation start temperature M_s, the transformation resistance should be enhanced when B2 austenite is transformed into B19’ martensite. Simultaneously, in order to enhance the austenite transformation start temperature A_s, the mechanical driving force should be diminished when B19’ martensite is transformed into B2 austenite.

It can be deduced that the addition of Nb and Ta elements contributes to the increase of the elastic strain energy of B2 austenite. Consequently, the elastic strain energy becomes the resistance, which prevents B2 austenite from being transformed into B19’ martensite, so that the martensite phase transformation temperature is lowered. Conversely, the elastic strain energy kept in the martensite interface becomes the mechanical driving force, which facilitates the transformation from B19’ martensite to B2 austenite, so that the austenite phase transformation temperature is enhanced.

4. Conclusions

Based on ternary Ni₄₅Ti_51.8Fe_3.2 (at %) SMA, two new quaternary Ni₄₄Ti_51.8Fe_3.2Nb₁ and Ni₄₄Ti_51.8Fe_3.2Ta₁ (at %) SMAs were designed and fabricated by adding Nb and Ta elements to replace the Ni element. Furthermore, the microstructure, mechanical property, and phase transformation of NiTiFeNb and NiTiFeTa SMAs were investigated. The following conclusions are drawn:

(1)

The microstructures of three NiTi-based SMAs are dominated by equiaxed grains rather than dendrites. Ni₄₅Ti_51.8Fe_3.2 SMA comprises a B2 austenite matrix and a Ti₂Ni precipitate. In the case of Ni₄₄Ti_51.8Fe_3.2Nb₁ SMA, Ti₂Ni and β-Nb precipitates occur in the matrix of B2 austenite. The matrix of Ni₄₄Ti_51.8Fe_3.2Ta₁ SMA belongs to B2 austenite, where Ti₂Ni and Ni₃Ti precipitates can be observed.

(2)

The addition of Nb and Ta elements results in the increasing yield strength of NiTi-based SMA, but leads to the decreasing plasticity of NiTi-based SMA. Furthermore, Ni₄₄Ti_51.8Fe_3.2Nb₁ SMA possesses a higher yield strength than Ni₄₄Ti_51.8Fe_3.2Ta₁ SMA, where the Nb element plays a more substantial role in strengthening NiTi-based SMAs compared with the Ta element. Nb and Ta exist in the solid solution of B2 austenite as the solute atoms, since no metallic compounds of Nb and Ta elements are observed in the case of Ni₄₄Ti_51.8Fe_3.2Nb₁. In particular, Ni₄₄Ti_51.8Fe_3.2Ta₁ SMA exhibits a steady strain hardening ability during plastic deformation.

(3)

All three NiTi-based SMAs exhibit a one-step phase transformation during cooling, which is involved in the transformation from B2 austenite to B19’ martensite. However, all three NiTi-based SMAs exhibit a two-step phase transformation during heating, where they are converted from B19’ martensite to the R-phase and subsequently they are transformed from the R-phase into the B2 phase. The addition of Nb and Ta elements does not change the phase transformation path of NiTiFe SMA, but it does have a certain effect on the transformation temperatures of NiTiFe SMA. As a consequence, all the transformation temperatures are diminished.