**1. Introduction**

The development of biomaterials has created several significant benefits for the general population over the last few decades, including dental implants, prosthetics, artificial arteries, and contact lenses [1]. These benefits have been either for the purpose of correcting problems or have been esthetic in nature [2].

Various materials have been used as biomaterials. Titanium alloys [3, 4] are among these materials because they have excellent mechanical strength/density ratio, excellent corrosion resistance, and biocompatibility.

The mechanical properties [5], corrosion, and wear resistance [6] of a material are largely dictated by the microstructure. Titanium alloys are favorable because a wide spectrum of microstructures can be obtained, depending on the chemical composition and the processing. This makes titanium alloys advantageous because they allow the desired microstructure to be obtained for specific requirements, as low elastic modulus, for example, [7, 8].

Titanium exists in two allotropic forms. At low temperatures, the so-called alpha phase (with hexagonal compact crystalline structure) is presented, and the beta phase (with body-centered cubic crystalline structure) emerges above 883°C. Some elements called beta stabilizers, such as niobium, molybdenum, iron, vanadium, and nickel, lead to a decrease in beta transus temperature (the transition from alpha to beta phase) when added to the forming titanium alloys and may stabilize this phase at room temperature. These phases determine a classification of titanium alloys: alloys containing only α stabilizers at this phase are known as α alloys, alloys containing 1–2% beta stabilizers and about 5–10% of beta phase are called near-α, alloys containing increased amounts of beta stabilizers resulting in 10–30% of beta phase are called α + β alloys, and alloys containing increased amounts of beta stabilizers where this phase can be retained by quick cooling are known as β metastable phases (these alloys decay into α + β in aging treatments) [9]. Beta transus temperature fulfills a central role in the microstructure's evolution, and so it is of great importance to determine the type of technological processing and heat treatment, which also includes doping with oxygen or nitrogen, whose elements present in the crystalline lattice can significantly affect the properties of the same [10].

In addition to interfering significantly in the structure and microstructure of titanium alloys, substitutional elements interfere in mechanical properties such as elastic modulus and hardness to the extent that they stabilize phases with different properties [11, 12]. The α phase, with a greater packaging factor of atoms, has a greater elastic modulus in relation to phase β, due to greater proximity between the atoms. Regarding toughness, it is usually greater in α phase, which contains fewer plans of slipping in relation to β phase, making the material harder and brittle [13]. However, depending on the alloy and the processing performed, alloys with a β structure have similar hardness [14].

concentration is, the greater the amount of this intermetallic is. By the decay of the β-transus curve, it is concluded that the addition of nickel favors the stabilization of this phase. The eutectoid reaction β ! α + Ti2Ni occurs at 765°C. By the lever rule, in the field β + Ti2Ni, the amount of Ti2Ni is lower in relation to α + Ti2Ni field, in

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

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

For example, for an alloy with 10 wt% nickel that underwent a homogenization heat treatment at 1000°C, followed by slow cooling, in equilibrium conditions, at this temperature, the alloy presents the β phase of titanium. With cooling, it begins to precipitate the intermetallic phase Ti2Ni. At a temperature just above 765°C, the precipitates of Ti2Ni proeutectoid phase are found in a β matrix of eutectoid composition and just below 765°C and are found in the eutectoid matrix, with a granular microstructure of α and Ti2Ni phases intercalated. From there, the intermetallic

Theoretically, the phases present in these alloys at room temperature are α + Ti2Ni; however, when the eutectoid reaction is suppressed, as in the case where the cooling is not done in perfect equilibrium condition, there is the possibility of β-phase retention at room temperature, the higher the nickel concentration is [24]. According to Ti-Ni binary diagram, a eutectic L ! β + Ti2Ni to 942°C occurs at a concentration of 28.4 wt% of nickel, <700°C in relation to the melting point of pure titanium (1670°C), thus decreasing the melting point and conforming temperature. Therefore, in the case of alloys with 15 and 20 wt% of nickel, at some point during the melting, there is the possibility of developing a microstructure composed of preeutectic precipitates of the β phase in a eutectic matrix, although the melting is not

The Ti-Ni alloys were prepared using commercially pure titanium (Cp-Ti) (99.7% purity, Aldrich Inc., USA) and nickel (99.5% purity, Camacam Industrial, Brazil). The ingots were obtained by arc melting, using a water-cooled copper crucible and an argon-inert atmosphere. The melting was held five times to ensure the homogeneity of the produced ingots. Samples were prepared with 5, 10, 15, and 20 wt% of nickel. Chemical analysis of the samples was performed using an inductively coupled plasma optical emission spectrometry (ICP-OES), Varian, Visa

Ti2Ni composes both precipitates and the eutectoid matrix.

the same concentration.

*Phase diagram of the Ti-Ni system [23].*

**Figure 1.**

done under equilibrium conditions.

**13**

The presence of atoms in interstitial positions of the lattice (e.g., nitrogen and oxygen) strongly influences some properties, such as mechanical resistance and ductility [15]. These elements act as stabilizers in titanium's α and α' phases, increasing the temperature of β-transus [16, 17].

Nickel causes several adverse reactions in the human body, such as carcinogenic, genotoxic, mutagenic, cytotoxic, and allergenic effects [18]. However, when nickel is connected to titanium, these problems are minimized [19]. In a study of corrosion of titanium alloys with varied concentrations of nickel in Ringer's solution, it was found that the potential of disruption occurs from 29 wt% of nickel and decreases rapidly, indicating that alloys below this concentration present promising applications regarding corrosion [20, 21]. A later study on Hank's biocorrosion solution concluded that the daily release of nickel would be hundreds of times smaller than that contained in water intake [22]. In addition, the microhardness of these alloys is similar to enamel (310–390 HVN), which is suitable for use in dental screws [18].

Thus, it is of fundamental importance to understand the role of substitutional nickel in the structure, microstructure, and selected mechanical properties in Ti-Ni system alloys. To this purpose, Ti-Ni alloys containing 5, 10, 15, and 20 wt% of nickel were melted in an arc-melting furnace, mechanically processed by hot rolling, and subjected to heat treatment in a vacuum to cause homogenization and stress relief.

### **2. Materials and methods**

#### **2.1 Ti-Ni alloys**

The phase diagram of the Ti-Ni system is presented in **Figure 1**. This phase diagram shows that alloys considered in this chapter (5, 10, 15, and 20 wt% of nickel) present the phases α and β (both of titanium) and the intermetallic phase Ti2Ni, at high temperatures [23]. By the lever rule, the higher the nickel

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

**Figure 1.** *Phase diagram of the Ti-Ni system [23].*

elements called beta stabilizers, such as niobium, molybdenum, iron, vanadium, and nickel, lead to a decrease in beta transus temperature (the transition from alpha to beta phase) when added to the forming titanium alloys and may stabilize this phase at room temperature. These phases determine a classification of titanium alloys: alloys containing only α stabilizers at this phase are known as α alloys, alloys containing 1–2% beta stabilizers and about 5–10% of beta phase are called near-α, alloys containing increased amounts of beta stabilizers resulting in 10–30% of beta phase are called α + β alloys, and alloys containing increased amounts of beta stabilizers where this phase can be retained by quick cooling are known as β metastable phases (these alloys decay into α + β in aging treatments) [9]. Beta transus temperature fulfills a central role in the microstructure's evolution, and so it is of great importance to determine the type of technological processing and heat treatment, which also includes doping with oxygen or nitrogen, whose elements present in the crystalline lattice can significantly affect the properties of the same [10]. In addition to interfering significantly in the structure and microstructure of titanium alloys, substitutional elements interfere in mechanical properties such as elastic modulus and hardness to the extent that they stabilize phases with different properties [11, 12]. The α phase, with a greater packaging factor of atoms, has a greater elastic modulus in relation to phase β, due to greater proximity between the atoms. Regarding toughness, it is usually greater in α phase, which contains fewer plans of slipping in relation to β phase, making the material harder and brittle [13]. However, depending on the alloy and the processing performed, alloys with a β

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

The presence of atoms in interstitial positions of the lattice (e.g., nitrogen and oxygen) strongly influences some properties, such as mechanical resistance and ductility [15]. These elements act as stabilizers in titanium's α and α' phases,

Nickel causes several adverse reactions in the human body, such as carcinogenic, genotoxic, mutagenic, cytotoxic, and allergenic effects [18]. However, when nickel is connected to titanium, these problems are minimized [19]. In a study of corrosion of titanium alloys with varied concentrations of nickel in Ringer's solution, it was found that the potential of disruption occurs from 29 wt% of nickel and decreases rapidly, indicating that alloys below this concentration present promising applications regarding corrosion [20, 21]. A later study on Hank's biocorrosion solution concluded that the daily release of nickel would be hundreds of times smaller than that contained in water intake [22]. In addition, the microhardness of these alloys is similar to enamel (310–390 HVN), which is suitable for use in dental screws [18]. Thus, it is of fundamental importance to understand the role of substitutional nickel in the structure, microstructure, and selected mechanical properties in Ti-Ni system alloys. To this purpose, Ti-Ni alloys containing 5, 10, 15, and 20 wt% of nickel were melted in an arc-melting furnace, mechanically processed by hot rolling, and subjected to heat treatment in a vacuum to cause homogenization and

The phase diagram of the Ti-Ni system is presented in **Figure 1**. This phase diagram shows that alloys considered in this chapter (5, 10, 15, and 20 wt% of nickel) present the phases α and β (both of titanium) and the intermetallic phase

Ti2Ni, at high temperatures [23]. By the lever rule, the higher the nickel

structure have similar hardness [14].

stress relief.

**2.1 Ti-Ni alloys**

**12**

**2. Materials and methods**

increasing the temperature of β-transus [16, 17].

concentration is, the greater the amount of this intermetallic is. By the decay of the β-transus curve, it is concluded that the addition of nickel favors the stabilization of this phase. The eutectoid reaction β ! α + Ti2Ni occurs at 765°C. By the lever rule, in the field β + Ti2Ni, the amount of Ti2Ni is lower in relation to α + Ti2Ni field, in the same concentration.

For example, for an alloy with 10 wt% nickel that underwent a homogenization heat treatment at 1000°C, followed by slow cooling, in equilibrium conditions, at this temperature, the alloy presents the β phase of titanium. With cooling, it begins to precipitate the intermetallic phase Ti2Ni. At a temperature just above 765°C, the precipitates of Ti2Ni proeutectoid phase are found in a β matrix of eutectoid composition and just below 765°C and are found in the eutectoid matrix, with a granular microstructure of α and Ti2Ni phases intercalated. From there, the intermetallic Ti2Ni composes both precipitates and the eutectoid matrix.

Theoretically, the phases present in these alloys at room temperature are α + Ti2Ni; however, when the eutectoid reaction is suppressed, as in the case where the cooling is not done in perfect equilibrium condition, there is the possibility of β-phase retention at room temperature, the higher the nickel concentration is [24].

According to Ti-Ni binary diagram, a eutectic L ! β + Ti2Ni to 942°C occurs at a concentration of 28.4 wt% of nickel, <700°C in relation to the melting point of pure titanium (1670°C), thus decreasing the melting point and conforming temperature. Therefore, in the case of alloys with 15 and 20 wt% of nickel, at some point during the melting, there is the possibility of developing a microstructure composed of preeutectic precipitates of the β phase in a eutectic matrix, although the melting is not done under equilibrium conditions.

The Ti-Ni alloys were prepared using commercially pure titanium (Cp-Ti) (99.7% purity, Aldrich Inc., USA) and nickel (99.5% purity, Camacam Industrial, Brazil). The ingots were obtained by arc melting, using a water-cooled copper crucible and an argon-inert atmosphere. The melting was held five times to ensure the homogeneity of the produced ingots. Samples were prepared with 5, 10, 15, and 20 wt% of nickel. Chemical analysis of the samples was performed using an inductively coupled plasma optical emission spectrometry (ICP-OES), Varian, Visa

model, from the sample's dissolution in acid medium. The presence of interstitial elements was carried out using a LECO TC-400 model gas analyzer. **Table 1** shows the chemical analysis of the produced samples.

the samples is important because it allows tests that require symmetrical samples, in addition to causing changes in microstructure and some properties that are of interest in the analysis. The samples were hot rolled as their mechanical processing. After slow cooling, a new heat treatment was performed for strain relief and recrystallization of microstructures as there were internal stresses and deformed microstructures due to an aggressive hot-rolling process. This treatment was

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

The structural characterization of the samples was carried out by X-ray diffraction (XRD) measurements on a Rigaku diffractometer (D/Max-2100PC model). This included a Cu-Kα radiation of wavelength of 1.544 Å, 20 mA current, and a potential of 40 kV. The fixed mode, with step of 0.02° and time of 1.6 s, in the range of 10–90°, was used. The used method to data acquisition was the powder

For all samples of the Ti-Ni alloys used in this chapter and in all processing conditions, the microstructure was evaluated by both optical and scanning electron microscopy. To obtain the images, an Olympus model BX51M optical microscope with a camera connected to a computer and a SEM Carl Zeiss EVO/LS15 model with

Vickers microhardness tests were performed using a Shimadzu HMV model-2 microdurometer connected to a computer. The load used was 1.941 N for a time of 60 s. Following ASTM standards, almost 20 indentations were made in different regions of the sample [26, 27]. The dynamic elastic modulus was measured using Sonelastic® equipment (ATCP, Brazil). A boost in sample, the frequencies of vibration (through the Sonelastic® software), and damping were calculated and linked with the modulus of elasticity, according to standard ASTM E1876-01 [28].

**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

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.

performed at 1143 K for 24 h in a vacuum.

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

**2.2 Sample's characterization**

the SmartSEM software were used.

**3. Results and discussion**

**15**

**3.1 Structural characterization**

and according to the system's phase diagram [23].

method [25].

It can be verified that the obtained ingots show the correct stoichiometry of nickel and a low amount of impurities and interstitial elements, displaying the good quality of prepared alloys.

To check the homogeneity of the produced alloys, the samples were subjected to a mapping of the elements that comprise the alloys using an electron dispersion spectrometry (EDS) technique that used an Oxford Instruments Incax-act model detector coupled to the scanning electron microscope (SEM) Carl Zeiss EVO/LS15 model. **Figure 2** shows the mapping of the elements titanium and nickel obtained by EDS in the produced alloys after melting. In all cases, homogeneous distribution of the elements titanium and nickel can be observed, showing that the melting was complete, without the presence of aggregates or segregates.

After melting, the ingots were submitted to a homogenization heat treatment that consisted of slowly heating the samples up to 1173 K for 24 h in a vacuum of 10<sup>6</sup> Torr, followed by slow cooling in the furnace. The mechanical processing of


#### **Table 1.**

*Chemical analysis of the Ti-Ni alloys.*

**Figure 2.** *EDS mapping of the elements Ti and Ni that make up the Ti-Ni alloys.*

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

the samples is important because it allows tests that require symmetrical samples, in addition to causing changes in microstructure and some properties that are of interest in the analysis. The samples were hot rolled as their mechanical processing.

After slow cooling, a new heat treatment was performed for strain relief and recrystallization of microstructures as there were internal stresses and deformed microstructures due to an aggressive hot-rolling process. This treatment was performed at 1143 K for 24 h in a vacuum.

## **2.2 Sample's characterization**

model, from the sample's dissolution in acid medium. The presence of interstitial elements was carried out using a LECO TC-400 model gas analyzer. **Table 1** shows

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

It can be verified that the obtained ingots show the correct stoichiometry of nickel and a low amount of impurities and interstitial elements, displaying the good

To check the homogeneity of the produced alloys, the samples were subjected to a mapping of the elements that comprise the alloys using an electron dispersion spectrometry (EDS) technique that used an Oxford Instruments Incax-act model detector coupled to the scanning electron microscope (SEM) Carl Zeiss EVO/LS15 model. **Figure 2** shows the mapping of the elements titanium and nickel obtained by EDS in the produced alloys after melting. In all cases, homogeneous distribution of the elements titanium and nickel can be observed, showing that the melting was

After melting, the ingots were submitted to a homogenization heat treatment that consisted of slowly heating the samples up to 1173 K for 24 h in a vacuum of 10<sup>6</sup> Torr, followed by slow cooling in the furnace. The mechanical processing of

Ti-5Ni 0.07 0.005 5.36 0.020 0.008 0.001 0.124 0.109 0.012 Ti-10Ni 0.10 0.018 10.03 0.034 <0.001 <0.001 0.380 0.179 0.007 Ti-15Ni 0.12 0.035 15.83 0.052 <0.001 <0.001 0.023 0.290 0.058 Ti-20Ni 0.12 0.028 19.80 0.072 <0.001 <0.001 0.160 0.300 0.076

**Cr (wt%)**

**Cu (wt%)**

**C (wt%)**

**O (wt%)**

**N (wt%)**

**Mn (wt%)**

the chemical analysis of the produced samples.

complete, without the presence of aggregates or segregates.

**Ni (wt%)**

*EDS mapping of the elements Ti and Ni that make up the Ti-Ni alloys.*

quality of prepared alloys.

**Alloy Fe**

**Table 1.**

**Figure 2.**

**14**

**(wt%)**

*Chemical analysis of the Ti-Ni alloys.*

**Al (wt%)**

The structural characterization of the samples was carried out by X-ray diffraction (XRD) measurements on a Rigaku diffractometer (D/Max-2100PC model). This included a Cu-Kα radiation of wavelength of 1.544 Å, 20 mA current, and a potential of 40 kV. The fixed mode, with step of 0.02° and time of 1.6 s, in the range of 10–90°, was used. The used method to data acquisition was the powder method [25].

For all samples of the Ti-Ni alloys used in this chapter and in all processing conditions, the microstructure was evaluated by both optical and scanning electron microscopy. To obtain the images, an Olympus model BX51M optical microscope with a camera connected to a computer and a SEM Carl Zeiss EVO/LS15 model with the SmartSEM software were used.

Vickers microhardness tests were performed using a Shimadzu HMV model-2 microdurometer connected to a computer. The load used was 1.941 N for a time of 60 s. Following ASTM standards, almost 20 indentations were made in different regions of the sample [26, 27]. The dynamic elastic modulus was measured using Sonelastic® equipment (ATCP, Brazil). A boost in sample, the frequencies of vibration (through the Sonelastic® software), and damping were calculated and linked with the modulus of elasticity, according to standard ASTM E1876-01 [28].
