**3. Results and discussion**

#### **3.1 Structural characterization**

**Figure 3** shows the X-ray diffractograms of all samples of Ti-Ni alloys after melting. During analysis of the X-ray diffractograms of samples after melting, it was observed that the addition of at least 10% in the weight of nickel caused the appearance of other phases beyond the α phase of titanium. There is the emergence of intermetallic Ti2Ni or Ti4Ni2O phases (which have the same diffraction pattern) [29, 30], and perhaps a small amount of β phase because nickel is a β-stabilizer element. It was also observed that the higher the amount of nickel was, the greater the amount of intermetallic Ti2Ni observed in the increased intensity of the peaks and according to the system's phase diagram [23].

Cascadan et al. studied casting Ti-5Ni (wt%) [31] and Ti-10Ni (wt%) [17] concerning structural and microstructural characterization. In the case of Ti-5Ni alloy, in the XRD measurements, single α and α'phases were observed, which were corroborated by optical micrographs, showing Widmanstatten-type morphology in the samples that were subjected to quick cooling from above β transus temperature, while larger lamellar structures were observed in samples whose slow-cooling process allowed large-scale diffusion processes. In the case of Ti-10Ni alloy, the structure and microstructure of the produced alloy were analyzed by XRD and SEM, and the results showed that the alloy presents predominantly titanium α phase, with proeutectoid lamellar precipitates in eutectoid matrix of α phase and intermetallic Ti2Ni.

**3.2 Microstructural characterization**

*DOI: http://dx.doi.org/10.5772/intechopen.86717*

**Figure 4.**

**17**

*Optical micrographs for Ti-Ni alloys after melting.*

**Figure 4** shows the optical micrographs, and **Figure 5** shows the scanning electron microscopy micrographs of all samples of Ti-Ni alloys, after melting.

must be a certain amount of Ti2Ni intermetallic phase, whose peak was not

*Structure, Microstructure, and Some Selected Mechanical Properties of Ti-Ni Alloys*

For the sample with 5 wt% Ni, according to the phase diagram (**Figure 1**), there

observed in the X-ray diffractogram because it was such a small portion, and it was not displayed by optical microscopy either due to the equipment's resolution.

**Figure 3.** *X-ray diffraction for the Ti-Ni alloys after melting.*

Lin et al. produced a Ti-18Ni (wt%) alloy by arc melting and showed the same phases in relation to this paper [32]. However, Ti-Ni alloys produced from metal powders melted with a 5 kW CO2 laser presented the β phase, in addition to the α and Ti2Ni phases, which showed that arc melting is a process of higher thermodynamic equilibrium in relation to the laser melting. The same features were observed in the case of Ti-Ni alloys that were quickly solidified for the analysis of metastable microstructures. The metastable microstructure non-equilibrium conditions also allowed the β beyond the expected α and Ti2Ni phases [33]. In another type of processing, Ti-7Ni alloy samples were produced by sintering at 1200°C for 2 h with heating and cooling rates of 4°C/min. In this case, the peaks of X-ray diffraction of α and Ti2Ni phases were also observed due to the low cooling rate [34]. The same occurred with the Ti-3Ni sintered to 1300°C for 2 h and heated and cooled at 4°C/ min rate, with measurements of X-ray diffraction to 960°C [35]. However, samples of Ti-2Ni and Ti-5Ni sintered at 800 and 1100°C for 1 h with a heating rate of 10°C/ min and cooled in the furnace presented α phase, in addition to the intermetallic Ti2Ni and TiNi3 phases. In this paper, it was found that the higher the sintering temperature and the amount of Ni are, there were higher quantities of intermetallic phases due to the diffusion process that allowed the reaction between Ti and Ni elements [36].

Peak shifts were also observed, which indicated changes in the lattice and angle parameters as well as differences in their format. The asymmetry of the lattice and angle parameters signaled distortion in the crystalline lattice because of the different quantities of substitutional and interstitial elements [15]. The α phase peaks shifted to smaller angles with the increased amount of nickel. This type of displacement is related to the increase in the lattice parameter [25] because the nickel has an atomic radius of 0.078 nm, slightly higher compared to that of titanium (0.076 nm). However, the substitutional element was not the only factor that influenced the lattice parameter; the interstitial elements and mechanical processing can also influence it [37]. In the case of the range of intermetallic phases' peaks, although the elements titanium and nickel were constant, there was a displacement of the peaks due to the presence of nitrogen and oxygen in interstitial positions.

*Structure, Microstructure, and Some Selected Mechanical Properties of Ti-Ni Alloys DOI: http://dx.doi.org/10.5772/intechopen.86717*
