Structure, phase transformation, and hardness of NiTiHfNd alloys

Ni50Ti29Hf21−xNdx (x = 0, 1, 2 at.%) and Ni50Ti29−xHf21Ndx (x = 1, 2 at.%) alloys were fabricated via arc melting. For the first time, the influence of Nd addition on structure, phase transformation, and hardness of NiTiHf alloy was investigated experimentally. It is found that the NiTiHfNd alloys consist of NiTiHf matrix and Nd-rich precipitates. Ni50Ti29Hf21 alloy demonstrates a martensitic transformation temperature as high as 314.1 °C, a thermal hysteresis as narrow as 37.7 °C, and a Vickers hardness as high as 500 HV. Nd addition obviously decreases the martensitic transformation temperature of NiTiHf alloys but still maintains a relatively narrow thermal hysteresis and a relatively high Vickers hardness compared with most other components of NiTiHf-based alloys.


Introduction
NiTi-based alloys can occur phase transformation between martensite and austenite, thus show so-called shape memory effect. Consequently, they have been applied in various fields, especially in mechanical engineering and medical applications [1]. It is well known that the phase transformation behaviors and the mechanical properties of NiTi-based alloys are strongly dependent on composition, alloying, precipitates, and heat treatment history [2]. Thus, alloying has been frequently utilized for tuning the phase transformation properties of NiTi-based alloys [3]. To date, transition metals such as Fe [4], Cu [5], Nb [6], Ta [7], Zr [8], Hf [9], Au [10], and Pd [11] have been adopted as alloying elements of NiTi-based alloys and the influence on the structure, phase transformation behavior, and mechanical properties have been comprehensively studied. These studies demonstrated that most transition metals lower the phase transformation temperatures, but some transition metals such as Zr, Hf, Au, and Pd can increase the phase transformation temperatures to higher than 100 °C [8] Therefore, NiTiZr, NiTiHf, NiTiAu, and NiTiPd are considered as candidates for high-temperature applications [2]. However, Au and Pd are noble metals, which largely limit the large-scale applications of NiTiAu and NiTiPd because of their high cost. Considering the high phase transformation temperatures and low cost, NiTiHf is a favorite candidate for high-temperature applications [12]. However, NiTiHf alloys always exhibits a wide thermal hysteresis (> 50 °C) in previous reports, which is not beneficial to fast actuation applications (such as MEMSs and robotics) [12,13]. Thus, quaternary NiTiHf-based alloys such as NiTiHfTa are designed for tuning the phase transformation behaviors and the mechanical properties through structural modifications. However, the thermal hysteresis of Ni 49 Ti 36 Hf 15−x Ta x (x = 0, 3, 6, 9, 12) alloys is still wide, from 50.0 to 80.8 °C [14].
Rare earth (RE) elements such as Ce [15], Gd [16], Dy [17], Nd [18], La [19] and Pr [20] have also been added to binary NiTi alloys to tune the phase transformation properties. The addition of Ce, Gd, and Dy to NiTi alloy increased the phase transformation temperatures because of some new-type second precipitates that exist in the NiTiRE alloys, which change the Ni/Ti ratio of the matrix [21]. However, the La or Pr addition to NiTi alloy decreased the phase transformation temperatures because of stress around NiLa or NiPr precipitates, which provides resistance to martensite transformation [19,20]. Recently, the influence of Y addition on the structure, the phase transformation, and the mechanical properties of NiTiHf alloys have been systematically investigated [22]. The Y addition only slightly decreases the phase transformation temperatures, whereas the thermal hysteresis is still wide and remains almost constant at ~ 50 °C when the Y content changes. Nd is another widely applied RE element, such as in glassy laser materials. However, no studies have been conducted on the Nd addition to NiTiHf alloy. Therefore, the influence of Nd addition on the structure, the phase transformation, and the mechanical properties of NiTiHf alloys remains unclear.
In this work, quaternary NiTiHfNd alloys with Nd content of 0, 1, 2 at.% were fabricated via arc melting, and the structure, phase transformation behavior, and the hardness were studied experimentally following our previously published studies [18][19][20]. It is found that Nd addition obviously decreases the martensitic transformation temperature of NiTiHf alloys but still maintains a relatively narrow thermal hysteresis and a relatively high Vickers hardness compared with most other components of NiTiHf-based alloys.

Materials and methods
NiTiHfNd alloys were fabricated by melting 50 g of raw materials (99.9 mass% Ti, 99.7 mass% Ni, 99.9 mass% Hf, and 99.95 mass% Nd) with predetermined nominal compositions in a vacuum non-consumable arc melting furnace by using a water-cooled copper crucible. The NiTi-HfNd alloys are denoted by Ni 50 Ti 29 Hf 21−x Nd x (x = 0, 1, 2) and Ni 50 Ti 29−x Hf 21 Nd x (x = 1, 2), respectively. Arc melting was repeated four times to ensure the uniformity of the composition. The as-fabricated alloys were spark-cut from the ingots and solution-treated at 850 °C for one hour in a vacuum quartz tube furnace. Thereafter, the specimens were mechanically and lightly polished to obtain a plain surface for the structure, phase transformation, and hardness tests.
The structure and chemical composition of NiTiHfNd alloys were examined using a scanning electron microscope (SEM, TM3030, Hitachi, Tokyo, Japan) equipped with an energy dispersive X-ray spectroscope (EDS, SwiftED3000, Oxford Instruments, Oxford, UK). All SEM images were taken at 15 kV in backscattering mode. The phase transformation behaviors of NiTiHfNd alloys were measured using a differential scanning calorimeter (DSC, Q2000, TA Instrument, New Castle, USA) with a temperature range of 0-500 °C and a heating and cooling rate of 10 °C/min. The mass of each sample for the DSC measurement was about 10 mg. The hardness of NiTiHfNd alloys was tested using a Vickers hardness tester (HXD-1000TMC/ LCD, Shanghai Optical Instrument, Shanghai, China) with a loading of 200 g and a holding of 15 s for each indentation. The values of hardness are the average of at least 10 measurements for each sample.

Results and discussion
The morphologies of NiTiHfNd alloys are observed using SEM and the backscattering SEM images are depicted in Fig. 1. For NiTiHf alloy without Nd, the SEM image (Fig. 1a) shows a featureless morphology and no precipitate can be found, indicating a pure NiTiHf alloy. For NiTiHfNd alloys, the SEM images ( Fig. 1b-e) clearly show precipitates (dark particles, marked by white arrows) and grey matrix morphologies. Obviously, the quantities and size of the precipitates increase with the increase in Nd content, as indicated by a comparison of Fig. 1b, d with Fig. 1c, e. To identify the chemical compositions of the precipitates and the matrix, EDS was employed and obtained data are summarized in Table 1. The ratio of Ni/Ti/Hf in the matrix of each alloy is very close to each other and close to 50:29:21, which means the Nd addition almost does not affect the chemical composition of the matrix. This result confirms that the solid solubility limit of Nd in the matrix is very low, which should be attributed to the large atom size of Nd [22], whereas the ratio of Nd/Ni/Ti/Hf in the precipitates of each alloy is about 82:10:5:3, which can be regarded as Nd-rich precipitates. Thus, EDS data can confirm that the matrix of NiTiHfNd alloys is ternary NiTiHf, whereas the precipitates are Nd-rich precipitates with Ni, Ti, Hf solute. This structure is very similar to that of NiTiHfTa [14] and NiTiHfY [22] alloys. But this structure is very different from ternary NiTiLa [19] or NiTiPr [20] alloys, which consist of the NiTi matrix and near-equiatomic NiLa or NiPr precipitates.
The phase transformation behaviors of NiTiHfNd alloys are examined using DSC and the DSC curves are depicted in Fig. 2. Each DSC curve shows only one peak during heating and cooling, which indicates a one-step B2 ↔ B19' phase transformation [2,12]. This phase transformation behavior is in agreement with ternary NiTiCu [5], NiTiTa [7], NiTiPd [11], NiTiCe [15], NiTiDy [17] as well as quaternary NiTiHfTa [14] and NiTiHfY [22] alloys. The martensitic transformation start temperature (M s ), the martensitic transformation finish temperature (M f ), the austenite transformation start temperature (A s ), and the austenite transformation finish temperature (A f ) were determined from the DSC curves by tangent-intersection method (shown in Fig. 2), and are summarized in Table 2. Meanwhile, the martensitic transformation peak temperature (M p ), the austenite transformation peak temperature (A p ) and some literature data are summarized in Table 2 in comparison with this work. The M s of Ni 50 Ti 29 Hf 21 alloy reaches as high as 314.1 °C, which is the highest M s for NiTiHf-based and NiTi alloys shown in Table 2 = 1, 2), which indicates that Hf content is more sensitive than Ti content for the M s of NiTiHf-based alloys [2]. The M s of Ni 50 Ti 29−x Hf 21 Nd x is higher than that of Ni 50 Ti 29 Hf 21−x Nd x at same Nd content because of higher Hf content. The thermal hysteresis (H M ) of the Ni 50 Ti 29 Hf 21 alloy (37.7 °C) is notably narrower than that of most NiTiHf-based alloys as shown in Table 2. This result confirms a decreased tendency with the increase in Hf content when Hf content is above 10 at.% for ternary NiTiHf alloys [12]. The Nd addition generally demonstrates an increasing tendency for the H M of  It is well known that the phase transformation temperatures are strongly dependent on Ni content in binary NiTi alloy when Ni content is higher than 50 at.% [23]. Previous reports show that the addition of Hf content above 10 at.% almost linearly increases the M s of NiTi alloys at a rate of over 20 °C/at.% Hf, when Ni content is not higher than 50 at.% [12,24], also as shown in Table 2. Meanwhile, the M s is not notably affected by a change in Ni content for NiTiHf alloys, but it dropped steeply when Ni content increased beyond the equiatomic (50 at.%) composition [12]. Previous reports showed that the addition of RE element in NiTi alloy has evident effects on the martensitic transformation, which is because of either the change of the Ni/Ti ratio of the matrix [15][16][17] or the stress around NiRE precipitates [19,20]. As shown in Table 1, Ni content in the matrix of NiTiHfNd alloys is very close to each other and not higher than 50 at.%. Thus, the decrease in the M s of NiTiHfNd alloys is not due to Ni content. Whereas, SEM observations ( Fig. 1) suggest that the size and quantity of the Nd-rich precipitates increased with the increase in Nd content. Thus, we propose that stress around Nd-rich precipitates is responsible for the decrease in the M s with the increase in Nd content in NiTiHfNd alloys. While, increasing Hf contents improves the crystallographic compatibility and thus results in lower hysteresis widths [25].
Hardness that is closely related to material strength has been regarded as an indicative parameter of the mechanical properties of NiTi-based alloys [13,22,24,27,28]. Thus, the hardness value of NiTiHfNd alloys was measured and is shown in Fig. 3. The hardness value of Ni 50 Ti 29 Hf 21 alloy is 500 HV, which is much larger than that (261 HV) of binary Ni 50.3 Ti 49.7 alloy. This result can be explained by the solid solution being strengthened by bigger Hf  [24], precipitates [13], thermal treatment [28], and quaternary alloying [22], as shown in Fig. 3. SEM observation (Fig. 1) confirms that the size of Nd-rich precipitates increases with the increase in Nd content in NiTiHfNd alloys. Consequently, material strength, as observed from the hardness results, decreases with the increase in Nd content in NiTiHfNd alloys [13].

Conclusions
This work expands our previous studies on the ternary NiTiRE alloy to the quaternary NiTiHfRE alloy. The influence of the addition of RE element Nd on the structure, phase transformation behavior, and hardness of NiTiHf was investigated experimentally by SEM, DSC and Vickers hardness tester. The structure of the NiTiHfNd alloys consists of the NiTiHf matrix and Nd-rich precipitates. The NiTiHfNd alloy undergoes a one-step phase transformation during heating and cooling. Ni 50 Ti 29 Hf 21 alloy demonstrates a martensitic transformation temperature as high as 314.1 °C, a thermal hysteresis as narrow as 37.7 °C, and a Vickers hardness as high as 500 HV. The martensitic transformation start temperature decreases gradually with the increase in Nd content due to stress around Nd-rich precipitates provides resistance to the martensitic transformation. However, NiTiHfNd alloys still maintain a relatively narrow thermal hysteresis and relatively high Vickers hardness compared with most other components of NiTiHf-based high-temperature alloys.