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Effect of Zirconium, Niobium and Chromium on Structure and Properties of Ni-Ti Alloy

The Ni-Ti alloy belongs to the group of materials with shape memory. This alloy has found its application mainly due to the excellent properties associated with the shape memory effect. It is necessary to keep the ratio between nickel and titanium approximately equimolar. In the alloy there could exist many intermetallic phases – NiTi, Ti2Ni, Ni3Ti, Ni4Ti3 and Ni3Ti2 and Ni8Ti5 [1, 2]. The intermetallic phase NiTi is responsible for the ability of shape memory. In addition to this phase, there is always a Ti2Ni intermetallic phase, which is undesirable because of its hardness and brittleness. In the NiTi phase, the phase transformation could be indicated between the high-temperature modification (austenite) and the lowtemperature modification (martensite). It could be indicated by thermal energy or mechanical energy [3-5]. The superelasticity is the most important property of this alloy. The material is above the austenite finish temperature (austenite phase). After mechanical deformation austenite is changed to martensite. Martensite is not stable in these conditions and it is changed back to austenite [6, 7]. Manufacturing is difficult because of the need for the exact chemical composition of a product. It is necessary to keep the exact ratio between nickel and titanium because of the temperature of the phase transformation. For example 0.1 at. % of nickel could decrease the martensite start temperature for 10 °C. The problem is also with the reactivity of titanium. Titanium has a high affinity to oxygen, nitrogen and carbon, so it is necessary to do the production under vacuum or inert atmosphere. The most common techniques of production are vacuum induction melting (VIM) and vacuum arc remelting (VAR). In VIM, the ingot has a good homogeneity, but there is a danger of contamination due to the usage of a crucible. The crucible is made of graphite and it reacts with titanium and products carbide of titanium. In VAR, the crucible is made of copper, so there is less contamination, but the problem is with homogeneity [8, 9]. An alternative to melting metallurgy is powder metallurgy. The biggest advantage of this technique is the opportunity of making a product with an exact chemical composition. Reactive sintering method is used for the production of Ni-Ti alloys. The principle of this method is very easy. The powders of nickel and titanium are mixed in the exact chemical composition, then pressed by define pressure and heated up to the temperature of initiation the reaction between nickel and titanium. This temperature is lower than the melting temperature of the phase NiTi. This reaction is very exothermic. The propagation of heat is very fast and it causes the reaction in a whole bulk. This process runs in two stages. A slow diffusion part in which the temperature is lower than in the second part. The second part is faster and is called selfpropagating high-temperature synthesis (SHS). The advantages of this method are low price and low energy for production, but the negatives of this method are uncontrollable porosity and an incomplete reaction of all elemental particles is possible [1, 10]. There are also the alternatives to reactive sintering such as hot isostatic pressing (HIP), spark plasma sintering (SPS) or metal injection moulding (MIM), but the reactive sintering powder metallurgy was chosen in this work [1, 10].


Introduction
The Ni-Ti alloy belongs to the group of materials with shape memory. This alloy has found its application mainly due to the excellent properties associated with the shape memory effect. It is necessary to keep the ratio between nickel and titanium approximately equimolar. In the alloy there could exist many intermetallic phases -NiTi, Ti2Ni, Ni3Ti, Ni4Ti3 and Ni3Ti2 and Ni8Ti5 [1,2]. The intermetallic phase NiTi is responsible for the ability of shape memory. In addition to this phase, there is always a Ti2Ni intermetallic phase, which is undesirable because of its hardness and brittleness. In the NiTi phase, the phase transformation could be indicated between the high-temperature modification (austenite) and the lowtemperature modification (martensite). It could be indicated by thermal energy or mechanical energy [3][4][5]. The superelasticity is the most important property of this alloy. The material is above the austenite finish temperature (austenite phase). After mechanical deformation austenite is changed to martensite. Martensite is not stable in these conditions and it is changed back to austenite [6,7].
Manufacturing is difficult because of the need for the exact chemical composition of a product. It is necessary to keep the exact ratio between nickel and titanium because of the temperature of the phase transformation. For example 0.1 at. % of nickel could decrease the martensite start temperature for 10 °C. The problem is also with the reactivity of titanium. Titanium has a high affinity to oxygen, nitrogen and carbon, so it is necessary to do the production under vacuum or inert atmosphere. The most common techniques of production are vacuum induction melting (VIM) and vacuum arc remelting (VAR). In VIM, the ingot has a good homogeneity, but there is a danger of contamination due to the usage of a crucible. The crucible is made of graphite and it reacts with titanium and products carbide of titanium. In VAR, the crucible is made of copper, so there is less contamination, but the problem is with homogeneity [8,9].
An alternative to melting metallurgy is powder metallurgy. The biggest advantage of this technique is the opportunity of making a product with an exact chemical composition. Reactive sintering method is used for the production of Ni-Ti alloys. The principle of this method is very easy. The powders of nickel and titanium are mixed in the exact chemical composition, then pressed by define pressure and heated up to the temperature of initiation the reaction between nickel and titanium. This temperature is lower than the melting temperature of the phase NiTi. This reaction is very exothermic. The propagation of heat is very fast and it causes the reaction in a whole bulk. This process runs in two stages. A slow diffusion part in which the temperature is lower than in the second part. The second part is faster and is called selfpropagating high-temperature synthesis (SHS). The advantages of this method are low price and low energy for production, but the negatives of this method are uncontrollable porosity and an incomplete reaction of all elemental particles is possible [1,10].
There are also the alternatives to reactive sintering such as hot isostatic pressing (HIP), spark plasma sintering (SPS) or metal injection moulding (MIM), but the reactive sintering powder metallurgy was chosen in this work [1, 10].

Experimental
The dependence of microstructure, hardness and temperatures of phase transformation are described in this work. The elemental powders with given purities and particle sizes (nickel: 99.99 wt. %, <150 µm; titanium: 99.50 wt. %, <44 µm; chromium 99 wt. %, <44 µm; niobium and zirconium 99 wt. %) were blended to the powder mixtures with chemical compositions of Ni52.4Ti44.6X3 and Ni53.5Ti45.5X1 where X represents alloying element. The powder mixtures were uniaxially cold compressed at a pressure of 450 MPa for 5 minutes to cylindrical green body samples using LabTest 5.250SP1indexed on: http://www.scopus.com VM universal loading machine. The reactive sintering was carried out in evacuated silica ampoules which were placed in the preheated electric resistance furnace at 1100 °C for 20 minutes. After cooling in air, the samples were ground by sandpapers with silicon carbide abrasive elements and polished by the diamond paste D2. The microstructure was etched in Kroll´s reagent (10 ml HF, 5 ml HNO3 and 85 ml H2O). The phase composition was identified by X-ray diffraction (XRD) PANalytical X'Pert Pro diffractometer with a copper anode. The microstructure was observed by the scanning electron microscope TESCAN VEGA 3 LMU equipped with the OXFORD Instruments X-max 20 mm 2 SDD EDS analyser for determination of chemical composition of individual intermetallic phases. The hardness of the samples was measured with a load of 10 kg (HV 10) and microhardness (using Microhardness tester FM-700) of the NiTi phase with a load of 10 g (HV 0.01). The transformation temperatures of sintered products (MS martensite start, MF martensite finish, AS austenite start, AF austenite finish) were measured by differential scanning calorimetry (DSC) from the temperature of -10 °C to 200 °C with the heating rate of 10 °C·min -1 .

Results and discussion
In the previous work, successful preparation of many intermetallics was investigated by powder metallurgy methods and specifically by reactive sintering. These methods are used mainly for intermetallic compounds with poor casting properties, high melting temperature or in the case when the melt reacts with the material of melting crucible. The Ti-Al, Fe-Al, Ni-Ti based systems belong to the group of materials with problems described above and the research is focused currently on the description of the reaction mechanism between elemental powders and intermetallic phases, influence of sintering conditions like temperature, heating rate, used particles sizes of powders or addition alloying elements on properties of sintered products [11][12][13][14][15]. The low addition of alloying elements (1 and 3 wt. % of chromium, niobium and zirconium) does not change phase composition, the area fraction of the undesirable Ti2Ni phase and generally microstructure considerably in comparison with the binary NiTi46 alloy. Only in the sample with chromium, there is a little decrease of the amount of the Ti2Ni phase. According to XRD analysis, all sintered samples contain the NiTi phase (cubic structure) and the Ti2Ni phase, in the samples with addition of niobium there is the NiTi phase with monoclinic structure moreover and all mentioned phases and the Ni4Ti3 phase were formed in the NiTiZr1 and NiTiZr3 alloys. The phase compositions, the area fraction of the Ti2Ni phase and chemical compositions of the NiTi and Ti2Ni phases measured by EDS are summarized in Tab. 1. The Ni4Ti3 phase was identified only by XRD in the NiTiZr samples but it was not observed in the microstructure in the NiTiZr samples, therefore its chemical composition is not placed in Tab. 1. Two intermetallic phases (NiTi and Ti2Ni) were observed in microstructures. The differences in chemical composition of individual phases are well visible in Fig.  1 a-f which were taken in back-scattered electrons mode of SEM. From chemical compositions of phases it is evident that the alloying elements are dissolved in the NiTi phase preferably. The alloying elements zirconium and niobium substitute mostly titanium in intermetallic phases and only chromium by higher addition replaces nickel in the Ti2Ni phase. The shape memory behavior was not detected in the samples with the addition of chromium. According to paper [16], the decrease of the temperature of the austenite-martensite phase transformation was expected. The disappearance of the transformation temperatures was mentioned in the case of the NiTiCr5 alloy prepared by reactive sintering [17]. However, the austenite finish temperature (approx. 49 °C) was reported in another research [18]. It is a question why the reactively sintered NiTiCr samples do not show shape memory effects. It is possible that only 1.3 wt. % of chromium decreases the temperature of austenite finish below the temperature of -10 °C or chromium has another effect and it leads to a loss of shape memory effects in our experiments. The samples alloyed by niobium have the transformation temperatures higher than the NiTi46 alloy and these temperatures decrease with the increasing amount of niobium. The addition of zirconium causes the increase of the transformation temperature. The DSC curves of the Ni52.4Ti44.6Cr3 and Ni52.4Ti44.6Zr3 are shown in Fig. 2  The results of hardness (HV 10) and microhardness of the NiTi phase (HV 0.01) measurement are shown in Fig.  3. In Ni53.5Ti45.5Cr1 and Ni52.4Ti44.6Cr3 there is the clear increasing evolution of both values with increasing amount of chromium in the sample. When looking at the results of the NiTiCr5 alloy [17], there is comparable hardness of the NiTiCr3 and NiTiCr5 alloys and microhardness of NiTiCr alloys is still only increasing. The increase of microhardness of the NiTi phase slows down with the higher amount of chromium in the samples. In case of samples alloyed by niobium and zirconium, there is a trend of hardness little ambiguous because after addition of 1 wt. %, hardness increases. However after 3 wt. % amount of the alloying elements, hardness decreases below the value of the binary NiTi46 [19]. The microhardness of the NiTi phase is decreasing only with the increasing amount of niobium and zirconium in the alloys. The decrease of the mechanical properties continues by addition of 5 wt. % [14, 17].

Fig. 3 Hardness and microhardness of NiTiX alloys
Generally, reactive sintering enables to prepare the ternary NiTiX alloys without contamination from the melting crucible like in VIM. The presence of the undesirable Ti2Ni phase is the problem which appears by melting metallurgy and powder metallurgy too and the addition of ternary alloying elements does not solve it completely.

Conclusions
In this work, the influence of alloying elements (chromium, niobium and zirconium) was researched. The samples consist of the NiTi and Ti2Ni phases. The only alloying element of chromium reduced the area fraction of the Ti2Ni phase for 1.8 %, the decrease was negligible in case of other alloying elements. The disappearance of the shape memory effects represents the problem of the NiTiCr samples. On the contrary, using zirconium and niobium enables to manage the temperature of the phase transformation between the austenite and martensite structure of the NiTi phase. Mechanical properties -hardness and microhardness of the NiTi phase were increased by the addition of chromium. Niobium and zirconium decreased microhardness of the NiTi phase.