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

Based on ternary Ni45Ti51.8Fe3.2 (at %) shape memory alloy (SMA), Nb and Ta elements are added to an NiTiFe SMA by replacing Ni element, and consequently quaternary Ni44Ti51.8Fe3.2Nb1 and Ni44Ti51.8Fe3.2Ta1 (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.


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.

Materials and Methods
On the basis of ternary Ni 45 Ti 51.8 Fe 3.2 (at %) SMA, Nb and Ta elements were added by replacing the Ni element, and consequently quaternary Ni 44 Ti 51.8 Fe 3.2 Nb 1 and Ni 44 Ti 51.8 Fe 3.2 Ta 1 (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:HNO 3 :H 2 O = 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% C 2 H 5 OH and 10% HClO 4 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.observed in the case of Ni44Ti51.8Fe3.2Nb1and Ni44Ti51.8Fe3.2Ta1SMAs.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 Ni45Ti51.8Fe3.2 and Ni44Ti51.8Fe3.2Nb1SMAs, Ni44Ti51.8Fe3.2Ta1SMA 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 Ni44Ti51.8Fe3.2Ta1SMA, plenty of dislocations need to be enhanced so as to guarantee the compatiblity of plastic deformation.

Figure 1
Figure 1 shows the microstructures of Ni 45 Ti 51.8 Fe 3.2 , Ni 44 Ti 51.8 Fe 3.2 Nb 1 , and Ni 44 Ti 51.8 Fe 3.2 Ta 1 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 Ni 45 Ti 51.8 Fe 3.2 , Ni 44 Ti 51.8 Fe 3.2 Nb 1 , and Ni 44 Ti 51.8 Fe 3.2 Ta 1 SMAs are illustrated in Figure 2. It is evident that the three NiTi-based SMAs are composed of B2 austenite and Ti 2 Ni precipitate.In order to gain an in-depth insight into the microstructures of the three NiTi-based SMAs, TEM micrographs of Ni 45 Ti 51.8 Fe 3.2 , Ni 44 Ti 51.8 Fe 3.2 Nb 1 , and Ni 44 Ti 51.8 Fe 3.2 Ta 1 SMAs are shown in Figures 3-5, respectively.It can be observed from Figure 3 that Ni 45 Ti 51.8 Fe 3.2 SMA consists of B2 austenite matrix and Ti 2 Ni precipitate.It can be seen in Figure 4 that in terms of Ni 44 Ti 51.8 Fe 3.2 Nb 1 SMA, Ti 2 Ni and β-Nb precipitates occur in the matrix of B2 austenite.It can be seen in Figure 5 that the matrix of Ni 44 Ti 51.8 Fe 3.2 Ta 1 SMA belongs to B2 austenite, where both Ti 2 Ni and Ni 3 Ti precipitates can be observed.

Figure 6
Figure 6 illustrates the stress-strain curves of Ni 45 Ti 51.8 Fe 3.2 , Ni 44 Ti 51.8 Fe 3.2 Nb 1 , and Ni 44 Ti 51.8 Fe 3.2 Ta 1 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, Ni 44 Ti 51.8 Fe 3.2 Nb 1 SMA possesses a higher yield strength than Ni 44 Ti 51.8 Fe 3.2 Ta 1 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 Ni 44 Ti 51.8 Fe 3.2 Nb 1 and Ni 44 Ti 51.8 Fe 3.2 Ta 1 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 Ni 45 Ti 51.8 Fe 3.2 and Ni 44 Ti 51.8 Fe 3.2 Nb 1 SMAs, Ni 44 Ti 51.8 Fe 3.2 Ta 1 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 Ni 44 Ti 51.8 Fe 3.2 Ta 1 SMA, plenty of dislocations need to be enhanced so as to guarantee the compatiblity of plastic deformation.

Figure 7
Figure7shows the DSC curves of Ni45Ti51.8Fe3.2,Ni44Ti51.8Fe3.2Nb1,and Ni44Ti51.8Fe3.2Ta1SMAs.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 B19ʹ martensite (M).However, all three NiTi-based SMAs exhibit a two-step phase transformation during heating.First, they are converted from B19ʹ 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 Ni44Ti51.8Fe3.2Nb1SMA, the addition of the Nb element results in the severe diminishment of the martensite and austenite transformation temperatures.

Figure 7
Figure 7 shows the DSC curves of Ni 45 Ti 51.8 Fe 3.2 , Ni 44 Ti 51.8 Fe 3.2 Nb 1 , and Ni 44 Ti 51.8 Fe 3.2 Ta 1 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 Ni 44 Ti 51.8 Fe 3.2 Nb 1 SMA, the addition of the Nb element results in the severe diminishment of the martensite and austenite transformation temperatures.Metals 2017, 7, 309 8 of 10