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Investigation of Structural Properties and Martensitic Phase Transformations in Heat-treated Ni-25.5 at. %Ta High Temperature Shape Memory Alloys

The effect of heat treatment performed at 800 °C, 900 °C and 1000 °C for 1 hour on structural and martensitic transformation properties of Ni-25.5 at.%Ta high temperature shape memory alloy has been examined. Morphological observations by SEM-EDS showed that the heat treatment affected directly microstructural properties of the alloy. Microstructure of the heat treated samples is composed of intermetallic Ni-rich Ni 8 Ta and Ta-rich NiTa 2 compound in the Ni 3 Ta matrix. In addition to these phases, orthorhombic Ni 3 Ta phase was only observed in the sample heat-treated at 1000 o C. Structural investigations of alloy the samples by XRD indicated that the martensitic crystal orientation of the samples changed with heat treatments. DSC measurements revealed that all the samples displayed high temperature shape memory behavior with martensitic transformation temperatures of above 200 o C. Vicker’s microhardness measurements showed that the microhardness of the alloy influenced dramatically by applying heat treatments, especially at 800 and 900 o C.


Introduction
Shape memory alloys (SMAs) belong to a specific class of smart materials that can remember or retain their previous shape when exposed to thermomechanical or magnetic stimuli.
Due to their unique and superior properties, these alloys have become very popular and attracted great interest in many commercial applications in recent years [1,2]. Binary Ni-Ti alloy is already the most well-known and widely used SMA group and its martensitic transformation temperature is generally below 100 °C. However, at the present time, there is a need for SMAs with higher martensitic transformation temperatures of above 100 °C for high temperature applications, and there are many industrial application areas, at which high temperature SMAs are popular, e.g. sensors and actuators in automotive industry, rocket technologies, nuclear reactor systems and safety devices [3,4]. Additionally, high-temperature SMAs should have reason for recoverable strain levels, long-term stability, plastic deformation, and adequate environmental resistance [5].
In recent years, binary Ni-Ta alloys, which have high melting temperature and exhibit excellent features in harsh environmental conditions, have attracted attention as a remarkable candidate material, especially for high temperature applications [6]. In 2008, Firstov et al. [7] discovered that the intermetallic Ni3Ta compound also exhibited shape memory behavior with a martensitic transformation temperature of above 300 o C. Thus, a new candidate material has been joined in SMA family. The martensitic transformation in intermetallic Ni3Ta compound occurs resulting in transformation of tetragonal austenite phase to monoclinic martensite phase [8]. The possible phases in the phase diagram of binary Ni-Ta system are liquid solution, Ni-rich fcc-A1, and Ta-rich bcc-A2 solid solutions and intermetallic Ni8Ta, Ni3Ta, Ni2Ta, NiTa and NiTa2 compounds [9,10]. However, there have been some uncertainties about crystal structures and properties of intermetallic phases in the Ni-Ta system. For example; intermetallic Ni3Ta is polymorphic with three different crystal structures (orthorhombic Pmmn-Ni3Ta, tetragonal I4/mmm-Ni3Ta and monoclinic P21/m-Ni3Ta) and which polymorphic phase is stable is still controversial. In addition, the mechanism of the shape memory effect exhibited by the intermetallic Ni3Ta phase is also unclear [6]. Eventually, it is very important to understand the functional behaviors, such as shape memory effect, and characteristics of intermetallic phases in the binary Ni-Ta system.
The main purpose of this work was to examine the effect of heat treatment performed at 800 °C, 900 °C and 1000 °C for 1 hour on martensitic transformation behavior, morphological and structural properties of high temperature Ni-25.5 at.% Ta SMA were systematically examined by means of DSC, XRD, SEM-EDS and Vicker's microhardness measurements. The effects of heat treatments performed at high temperatures on some physical properties of the nonstoichiometric Ni-Ta SMA, which exhibits shape memory behavior, are studied for the first time.

Materials and Methods
The Ni-25.5Ta (at.%) alloy was produced by using high purity nickel (Ni) and tantalum (Ta) powders in an arc-melting furnace under vacuum. The ingot was homogenized at 1400 o C for 4 hours in a furnace. After the homogenization process, the ingot was cut to different sizes for heat treatments to be performed at different temperatures. The samples were heat treated at 800 o C, 900 o C, and 1000 o C for 1 hour. Sample groups without heat treatment and subjected to heat treatment at 800 o C, 900 o C, and 1000 o C temperatures were labeled as NiTa-0, NiTa-800, NiTa-900 and NiTa-1000, respectively.
Transformation properties of the samples were examined by differential scanning calorimeter (SII NanoTechnology EXSTAR DSC 7000) measurements taken in a nitrogen gas atmosphere at a heating/cooling rate of 10 °C/min. Structural analysis of the samples were realized by taking X-ray diffractometer (Bruker Discover D8 XRD) patterns using Cu Kα radiation at room temperature. Morphological properties and chemical analysis of the samples were studied by carrying out scanning electron microscope (SEM, LEO EVO 40) images equipped with an energy-dispersive X-ray spectrometer (EDS). The Vicker's microhardness measurements of the samples were performed by using Emco Test DuraScan at a load of 300 g and repeated for five times for each sample. However, Ta content of grey structures in NiTa-0 sample is much higher than that in stoichiometric NiTa2 compound and it is well-known that NiTa2 is only phase with high Ta content in the binary Ni-Ta system. On account of this, it is thought that high Ta content of grey structures in the NiTa-0 sample may be a result of homogenization process. From this, it has been seen that the homogenization condition was not enough to form single phase supersaturated structure. In addition to small-sized Ta-rich NiTa2 precipitate phase in the Ni3Ta matrix ( Fig. 1a), Ni-rich Ni8Ta precipitate phases were also observed, containing 83.94 at.% Ni + 16.06 at.% Ta, at different regions of the NiTa-0 sample. Biffi et al. [11] detected intermetallic Ni8Ta and Ni2Ta precipitates in microstructure of Ni75Ta24B1 alloy homogenized at 1400 o C for 4 h. On the other hand, Firstov et al. [7] reported that the Ni8Ta and Ni2Ta precipitate phases located in the grain boundaries of the Ni3Ta matrix were disappeared after the same homogenization condition as in Ref. [11]. They did not observe intermetallic NiTa2 compounds in their alloy samples.

Results and discussion
Eventually, optimal homogenization condition for Ni-Ta alloys are needed to obtain single Ni3Ta phase structure. The surface morphology of NiTa-800 sample is presented in Fig. 1-b. The heat treatment at 800 o C led to change in the microstructure of the alloy. The microstructure of NiTa-800 sample contains a large of cracks. Firstov et al. [7] reported that intergranular cracks were formed on the surface of Ni3Ta alloy after homogenization process. However, it could be clearly seen that some secondary phase precipitates in the NiTa-800 sample located in the crack. In order to determine chemical composition of this secondary phase in the crack, point EDS analysis was made.
According to EDS analysis, its chemical composition is 30.56 at.% Ni + 69.44 at.% Ta and it is concluded that this phase located in the crack is the Ta-rich NiTa2 phase. From here, it was understood that the Ta-rich NiTa2 phase nucleated and grown in the cracks. Also, in Fig. 1-b, structural agglomerates are observed in some areas of the matrix. According to EDS analysis taken, it was determined that this agglomerate structure in Fig. 1-b chemically included 72.65 at.% Ni + 27.35 at.% Ta elements and this is close to matrix composition. As a result, these agglomerates are not any secondary phases.
SEM image of the NiTa-900 sample is shown in Fig. 1-c. It is obvious that microstructural characteristics of the NiTa-900 sample are similar to that of the NiTa-800. EDS results revealed that the chemical composition of the gray structures in the crack was 20.13 at.% Ni + 79.87 at.% Ta and these gray structures are the Ta-rich NiTa2 phase as well as observed in NiTa-800 sample.
In contrast, the agglomerates observed in NiTa-800 sample disappeared in NiTa-900 sample. In addition, intermetallic Ni8Ta phase was also detected in some regions of NiTa-900 sample. From the results of EDS spectra, it is thought that thin white structures are orthorhombic Ni3Ta phase. Aballe et al. [12] showed that prolonged aging process at 800 o C or above could be caused formation of the stable orthorhombic Ni3Ta phase in Ni-25wt.% Ta-10wt.% Cr alloy by replacing the b.c.t. Ni3Ta. Likewise, Kosorukova et al. [13] reported that as aging temperature increased the stable orthorhombic Ni3Ta phase was formed in the Ni3Ta alloy, as a result of diffusional phase transformation mechanism. Consequently, the results indicated that heat treatment performed at 1000 o C led to starts diffusional phase transformation in the Ni-25.5Ta (at.%) alloy.
The crack formation in the microstructure of the binary Ni-Ta shape memory alloy is an important issue after homogenization process. It is thought that cracks in the Ni-Ta SMA have a negative effect on its physical properties. Although there are some suggestions to hinder crack formation in the Ni-Ta SMAs [7], Yildiz [14] reported that it might be prevented from the crack formation by changing homogenization condition of Ni-Ta SMA.   orthorhombic Ni3Ta phase (PDF:03-065-2588) in addition to mentioned phases above. The 2θ peak positions of the all phases are also in accordance with the literature [7,8,15,16] and the corresponding phases are marked on patterns. Also, as expected, all samples contained austenite phase at room temperature [11,13,17]. The XRD pattern of NiTa-0 sample display in Fig. 3-a. Compared to the XRD patterns in Figs. 3-b and c, it is clear that the heat treatments performed at 800 o C and 900 °C resulted in significant changes on the crystallographic properties of the alloy.
The main martensite phase peak located at 2θ ≈ 43.8 o in the XRD pattern of NiTa-0 sample was completely disappeared. However, the peak intensity at 2θ ≈ 27.4 o , which is quite weak in the XRD pattern of the NiTa-0 sample, increases in the XRD pattern of NiTa-800 sample. As can be seen from the XRD pattern in Fig. 3-c, intensity of this peak is maximum for NiTa-900 sample.
These results indicated that the heat treatments at 800 °C and 900 o C directly affected the martensite phase orientation of alloy. Both number and intensities of the NiTa2 peaks in Fig. 3-c were also increased. This is in good agreement with the SEM observations in Figs. 1-b and c. Finally, from the XRD pattern of NiTa-1000 sample, as shown in Fig. 3-d, it can be clearly seen that the number of related phase peaks increased considerably. The maximum intensity peak in  Table 1. The DSC results in Table 1 indicate that all the samples exhibit high temperature shape memory behavior. It can be also seen that the transformation characteristics of all the samples are well consistent with works in literature [7,11]. The reverse and forward transformation temperatures were not affected strongly by heat treatment, whereas reverse and forward transformation peak intensities were influenced. The transformation peaks of NiTa-800, NiTa-900 and NiTa-1000 samples are stronger than those of NiTa-0 sample. It is well-known that the heat exchanged during martensitic transformation is related to the amount of martensite phase in the sample [11]. Therefore, it has been concluded that, as a result of heat treatment, the amount of martensite phase in NiTa-800, NiTa-900 and NiTa-1000 samples increased, compared to NiTa-    -a- Another remarkable result is that the average microhardness of NiTa-0 and NiTa-1000 samples are almost the same. Therefore, it has been concluded that the cracks in the microstructure of the alloy samples did not cause significant changes in mechanical behaviors of the samples. However, it is believed that differences in the average microhardness values of NiTa-800 and NiTa-900 samples are closely related to the structural properties of these samples. El Bougory [18] reported that microhardnesses of Ni51Ti49 and Ni47Ti49Co4 shape memory alloys altered by the effect of thermal aging and these changes were attributed to change occurring in volume fraction and size of hard precipitates in the microstructures of alloys and also the morphology of martensite plates.
Aballe et al. [12] noted that heat treatments applied to Ni-Ta-Cr alloys affected the microstructural properties of alloys, resulting in changes in the hardness of alloys. Kojima et al. [19] reported that the microhardness of Ni3Ta compounds subjected to high-energy ion irradiation changed due to changes in lattice structure of the compound. SEM-EDS observations also revealed that the NiTa-1000 sample has the stable orthorhombic Ni3Ta phase, which is a result of heat treatment at 1000 o C. It was observed that volume frictions of intermetallic Ni8Ta and Ta-rich NiTa2 phases are higher than those of the NiTa-800, NiTa-900 and NiTa-1000 samples. Thermal measurements revealed that all the samples exhibited high temperature shape memory alloy behavior and the heat treatment processes did not affect that.
Structural analysis indicated that XRD patterns of all samples were in good consistent with their SEM-EDS observations. The XRD results also demonstrated that the martensitic phase orientation of Ni-25.5Ta (at.%) high temperature SMA was very sensitive to heat treatment. It was deduced that Vicker's microhardness values of the samples changed by applying heat treatment, especially at 800 o C and 900 o C. This was attributed to change in martensitic phase orientation.