**3.2 Microstructural characterization**

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

For the sample with 5 wt% Ni, according to the phase diagram (**Figure 1**), there must be a certain amount of Ti2Ni intermetallic phase, whose peak was not 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 4.** *Optical micrographs for 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

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

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.

elements [36].

**16**

**Figure 3.**

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

and Ti2Ni. This microstructure was considered hypoeutectoid because the concentration of 5 wt% of nickel was slightly lower than the concentration of about 6%, where the eutectoid reaction occurs [31]. The presence of the intermetallic phase that contained higher concentrations of nickel was evident in the micrographs for backscattered electrons, where the lighter area comprised the region with the highest average atomic number and the nickel had a higher atomic number com-

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

For the alloy with 10 wt% of nickel, observed by optical micrographs (**Figure 4**), the microstructure consisted of precipitates of titanium proeutectoid α phase (clear) in a matrix of eutectoid microstructure (dark region) comprised of alternating lamellae of α and Ti2Ni phases, also known as pearlite [38]. According to the phase diagram (**Figure 1**), the eutectoid reaction occurred at a nickel concentration of 6 wt%, and thus the alloy presented hypereutectoid composition, and the precipitates would be Ti2Ni or Ti4Ni2O, in view of the small amount of nickel. Furthermore, α phase precipitates characterize an alloy of hypoeutectoid composition. The high quantities of oxygen and nitrogen (stabilizers of α phase) can cause change in the composition of this reaction [39]. The increase of the concentration of nickel, from the composition of about 6 wt%, does not act as a β-phase stabilizer, but causes an increase in the temperature of the formation of intermetallic phase Ti2Ni. The images obtained by SEM (**Figure 5**) allowed more details to be obtained. At the junction of three lines where grain and leaving these needles of the α phase microstructure perpendicular to each other did originate in the grain boundaries when phase change occurred. This microstructure is similar to a Ti-24Al-11Nb alloy, whose intermetallic phase Ti3Al locates on the grain boundaries and metallic matrix [39]. Also, according to Liu et al. [29], the diffusion of oxygen in grain boundaries is higher than in its interior, and the precipitates of Ti4Ni2O are formed over these. Such morphology is proven by the images obtained by backscattering electrons, where lighter points indicate the inter-

Unlike the Ti10Ni samples in the initial conditions of processing, in samples of Ti15Ni, a proeutectoid of Ti2Ni presents precipitates which probably reacted with oxygen, thus forming Ti4Ni2O, comparing the amount of nickel and precipitates. This microstructure is similar to that shown by Chern lin et al. [20] and was expected due to the higher concentration of nickel in a matrix with the α phase in accordance with the X-ray diffractograms for this sample and the system's phase diagram. The optical micrographs show lighter regions that are proeutectoid of Ti4Ni2O precipitates in α + Ti2Ni array. The same morphology was observed in micrographs obtained by a SEM, both for secondary electrons as backscattered, shown in **Figure 5**. Dendritic structures were also obtained by Xu et al. in Ti with

**Figure 6** shows the values of microhardness and elastic modulus according to the amount of nickel in the homogenized condition after hot rolling. Due to experimental conditions, the microhardness and elastic modulus measurements were made only after homogenized condition after hot-rolling conditions. There is no relationship between these properties because they involve different processes: microhardness is a measure of resistance to a plastic deformation; and elastic modulus depends on the binding force between the atoms. A factor that increases microhardness is the increase in the concentration of the substitutional element because atoms of different sizes in the crystalline lattice cause deformation in it, creating obstacles to the movement of dislocations [13]. There is also no direct

pared to titanium.

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

metallic phase between the lamellae of the α phase.

20 wt% Ni alloy procured by laser melting [24].

**3.3 Mechanical characterization**

**19**

**Figure 5.** *SEM micrographs for the Ti-Ni alloys after melting, (a) secondary and (b) backscattering electrons.*

However, the dark regions between the lamellae of the α phase clearly were a eutectoid microstructure characterized by two phases, alternating between α and Ti2Ni itself. This eutectoid microstructure was like the microstructure of the sintered Ti-3Ni alloy shown in small expansion, but a larger magnification electron microscopy clearly showed this eutectoid microstructure [35]. In the SEM micrographs (**Figure 5**), between the lamellae of the α phase, the presence of a eutectoid microstructure characterized by two phases was observed, alternating between α

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

and Ti2Ni. This microstructure was considered hypoeutectoid because the concentration of 5 wt% of nickel was slightly lower than the concentration of about 6%, where the eutectoid reaction occurs [31]. The presence of the intermetallic phase that contained higher concentrations of nickel was evident in the micrographs for backscattered electrons, where the lighter area comprised the region with the highest average atomic number and the nickel had a higher atomic number compared to titanium.

For the alloy with 10 wt% of nickel, observed by optical micrographs (**Figure 4**), the microstructure consisted of precipitates of titanium proeutectoid α phase (clear) in a matrix of eutectoid microstructure (dark region) comprised of alternating lamellae of α and Ti2Ni phases, also known as pearlite [38]. According to the phase diagram (**Figure 1**), the eutectoid reaction occurred at a nickel concentration of 6 wt%, and thus the alloy presented hypereutectoid composition, and the precipitates would be Ti2Ni or Ti4Ni2O, in view of the small amount of nickel. Furthermore, α phase precipitates characterize an alloy of hypoeutectoid composition. The high quantities of oxygen and nitrogen (stabilizers of α phase) can cause change in the composition of this reaction [39]. The increase of the concentration of nickel, from the composition of about 6 wt%, does not act as a β-phase stabilizer, but causes an increase in the temperature of the formation of intermetallic phase Ti2Ni. The images obtained by SEM (**Figure 5**) allowed more details to be obtained. At the junction of three lines where grain and leaving these needles of the α phase microstructure perpendicular to each other did originate in the grain boundaries when phase change occurred. This microstructure is similar to a Ti-24Al-11Nb alloy, whose intermetallic phase Ti3Al locates on the grain boundaries and metallic matrix [39]. Also, according to Liu et al. [29], the diffusion of oxygen in grain boundaries is higher than in its interior, and the precipitates of Ti4Ni2O are formed over these. Such morphology is proven by the images obtained by backscattering electrons, where lighter points indicate the intermetallic phase between the lamellae of the α phase.

Unlike the Ti10Ni samples in the initial conditions of processing, in samples of Ti15Ni, a proeutectoid of Ti2Ni presents precipitates which probably reacted with oxygen, thus forming Ti4Ni2O, comparing the amount of nickel and precipitates. This microstructure is similar to that shown by Chern lin et al. [20] and was expected due to the higher concentration of nickel in a matrix with the α phase in accordance with the X-ray diffractograms for this sample and the system's phase diagram. The optical micrographs show lighter regions that are proeutectoid of Ti4Ni2O precipitates in α + Ti2Ni array. The same morphology was observed in micrographs obtained by a SEM, both for secondary electrons as backscattered, shown in **Figure 5**. Dendritic structures were also obtained by Xu et al. in Ti with 20 wt% Ni alloy procured by laser melting [24].

#### **3.3 Mechanical characterization**

**Figure 6** shows the values of microhardness and elastic modulus according to the amount of nickel in the homogenized condition after hot rolling. Due to experimental conditions, the microhardness and elastic modulus measurements were made only after homogenized condition after hot-rolling conditions. There is no relationship between these properties because they involve different processes: microhardness is a measure of resistance to a plastic deformation; and elastic modulus depends on the binding force between the atoms. A factor that increases microhardness is the increase in the concentration of the substitutional element because atoms of different sizes in the crystalline lattice cause deformation in it, creating obstacles to the movement of dislocations [13]. There is also no direct

However, the dark regions between the lamellae of the α phase clearly were a eutectoid microstructure characterized by two phases, alternating between α and Ti2Ni itself. This eutectoid microstructure was like the microstructure of the sintered Ti-3Ni alloy shown in small expansion, but a larger magnification electron microscopy clearly showed this eutectoid microstructure [35]. In the SEM micrographs (**Figure 5**), between the lamellae of the α phase, the presence of a eutectoid microstructure characterized by two phases was observed, alternating between α

*SEM micrographs for the Ti-Ni alloys after melting, (a) secondary and (b) backscattering electrons.*

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

**Figure 5.**

**18**

**4. Summary**

mechanical properties.

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

with the amount of nickel.

**Acknowledgements**

**Author details**

Bauru, SP, Brazil

**21**

Branch, Bauru, SP, Brazil

provided the original work is properly cited.

The Ti-Ni samples obtained by arc melting were adequately prepared regarding

The alloys showed the presence of α, Ti2Ni, and Ti2Ni4O, due to the reaction of the intermetallic Ti2Ni with oxygen. The microhardness results showed values in accordance with those presented in the literature, suitable for the use of fixation devices. There is no clear variation of the microhardness due to the amount of nickel because there were several factors involved, and the microstructures were very diversified and complex. The dynamic elastic modulus was slightly above the Cp-Ti due to the addition of a new intermetallic phase but did not vary significantly

The authors would like to acknowledge the Brazilian agencies Capes, for D. Cascadan's fellowship, CNPq (grants #481313/2012-5 and #307.279/2013-8), and

1 Laboratório de Anelasticidade e Biomateriais, UNESP—Univ. Estadual Paulista,

2 IBTN/Br—Institute of Biomaterials, Tribocorrosion and Nanomedicine-Brazilian

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

FAPESP (grant #2015/25.562-7) for their financial support.

Daniela Cascadan1,2 and Carlos Roberto Grandini1,2\*

\*Address all correspondence to: carlos.r.grandini@unesp.br

stoichiometry and homogeneity and characterized by XRD, SEM, and selected

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

#### **Figure 6.**

*Microhardness and elastic modulus values for Ti-Ni alloy samples in homogenized condition, after hot rolling.*

relationship between such quantities and the amount of nickel because this is not the only factor involved.

A large variation in microhardness values was observed from 250 to 590 HV. Several factors influence a material's hardness: the concentration of substitutional and interstitial elements, microstructure, size of grain boundaries, types of phases, crystallographic orientation in which deformation occurs (since it involves a plastic deformation), and the type of processing [13]. The Ti-Ni alloy samples used in this chapter presented considerable variations of these factors.

To analyze whether these values were in agreement with those found in the literature, it was used specifically by Chern Lin et al., with similar preparation of Ti-Ni alloys by arc melting and nickel concentration ranging from 18 to 28.4 wt%. The authors obtained microhardness values ranging from 300 to 390 HV [20]. In the present study, the microhardness ranged from 345 to 390 HV, values aligned approximately with Lin's study. For Ti-Ni alloys with nickel concentration varying from 10 to 20 wt% obtained by laser fusion of metal powders [32], the microhardness ranged from 270 to 510 HV due to the increase of Ti2Ni precipitates. Although the phases are different in relation to this study and other metallic atoms are present, the values are approximate with Lin's in relation to the other conditions, although the experimental parameters are not explicit in both cited studies.

For the Ti15Ni alloy, a high value for the microhardness was observed. As mentioned earlier, the increase of the concentration of nickel, from the composition of about 6 wt%, does not act as a β-phase stabilizer but causes an increase in the temperature of the formation of intermetallic phase Ti2Ni that can be responsible for this increase in the hardness value [21]. In samples of Ti15Ni, a proeutectoid of Ti2Ni presents precipitates which probably reacted with oxygen, thus forming Ti4Ni2O, comparing the amount of nickel and precipitates. The formation of Ti4Ni2O phase is another component to increase the hardness of the Ti15Ni alloy [15, 21].

The elastic modulus for Cp-Ti is around 95–105 GPa [9]. Thus, the addition of nickel caused a small increase in this property, probably from the addition of a new fcc phase referring to intermetallic Ti2Ni. However, as shown in **Figure 6**, it was not observed as a proportional ratio of the elastic modulus with the nickel concentration.

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