Effect of Oxygen on Static Recrystallization Behaviors of Biomedical Ti-Nb-Zr Alloys

Abstract

Titanium alloys that are used in biomedical applications must possess biocompatibility and a low elastic modulus so that they protect host bone tissue without causing stress shielding. As the elastic modulus of beta Ti alloys is close to that of bone (10–30 GPa), these alloys are considered potential orthopedic implant materials. The elastic modulus of the single β-phase Ti-39Nb-6Zr (TNZ40) alloy is approximately 40 GPa, whereas the strength is lower than that of other types of Ti alloys. Interstitial oxygen in a Ti matrix is well known to improve the matrix strength by solid-solution hardening. The desired mechanical properties can be optimized using a thermo-mechanical procedure to maintain a low elastic modulus. In order to enhance the strength, TNZ40 alloys were fabricated with different amounts of oxygen. The TNZ-0.16O and TNZ-0.26O alloys were cold swaged into 11 mm diameter bars, subjected to solution treatment at 900 °C and 950 °C for 2 h, and furnace-cooled to room temperature. As a result, recrystallized grains were clearly observed in the β matrix. The TNZ-0.26O alloy that was cold-worked by swaging followed by solution treatment at 900 °C exhibited the best mechanical properties (Vickers hardness: 247 HV, ultimate tensile strength: 777 MPa, elongation at rupture: 18.6%, and compressive strength: 1187 MPa). This study reports the effects of oxygen content on the recrystallization behavior and mechanical properties of these alloys.

1. Introduction

As human lifespans increase globally, the world is entering an era of aging in which the proportion of the elderly population increases. The demand for medical devices is expected to steadily rise, and interest in medical biomaterials is increasing [1,2,3]. When biomaterials are implanted in the human body, they must not cause stress shielding, in which the biomaterials suppress stress from being transmitted to the bone tissue. Studies are focusing on developing materials that include innocuous alloying elements that do not cause side effects such as allergic reactions and tissue inflammation [4,5].

The elastic modulus of a titanium alloy is not much different than that of bone tissue, and the alloy exhibits excellent bonding strength. Ti alloys are widely used as biomaterials owing to favorable properties, such as high corrosion resistance, low density, and elastic modulus [6,7,8,9,10,11,12,13]. Many studies have been focusing on the development of β-Ti alloys, which have better properties in terms of biomechanics and biochemical compatibility [14,15,16,17,18,19]. Poorly biocompatible materials can cause inflammatory reactions, corrode in the body, and leak metal ions, resulting in loss of osseointegration. In addition, if there is a large difference in elastic modulus between the bone and the implant material, most of the stress is transferred to the implant material, and the stress transferred to the bone is reduced, resulting in a stress-shielding effect. The bone is not stressed by the implant material, and bone loss occurs around the implant material. If this stress-shielding effect occurs for a long period of time, a large amount of bone loss causes many holes in the bone, resulting in low bone density and weak bone strength, leading to osteoporosis, a disease that increases the likelihood of fracture.

In order to improve the stress transfer between human bone tissue and implants, which is particularly important for biomaterials, it is important to develop materials that have an elastic modulus close to the elastic modulus of human bone tissue. Therefore, there is an increasing demand for the development of β-phase beta titanium alloys, which have a relatively low elastic modulus among titanium alloys. However, β-phase titanium alloys have a low modulus of elasticity, but there is a problem that their strength is lower than that of other alpha or alpha + beta Ti materials. Since the low modulus of elasticity is accompanied by a decrease in strength, it is necessary to study how to improve the compatibility of these opposing properties via microstructure control and mechanical processes. Typical β-Ti alloys have a lower elastic modulus than those of other biomaterials (e.g., stainless steel, cobalt-chrome, and polymer materials), and researchers are actively reporting on the development of β-Ti alloys such as Ti-Nb, Ti-Mo, Ti-Zr, Ti-Ta, and Ti-Nb-Zr that include biocompatible β-stabilizer elements such as Nb, Mo, Zr, and Ta [20,21,22,23,24,25,26,27,28].

The β-stabilizer element Nb is known to be effective in reducing the elastic modulus when added to titanium and is a biocompatible material that does not cause any harmful reactions in the human body [29]. In addition, as a β-isomorphous element, it stabilizes so that it exists in a β-alloy structure even at room temperature and does not cause phase decomposition during solution heat treatments. Zr is a biocompatible alloying element that has excellent corrosion resistance and neutral properties when used in Ti alloys. Furthermore, it has been reported that when Zr and Nb are added together, the β phase-stabilization effect is enhanced [30,31,32].

For this study, the Ti-39Nb-6Zr (TNZ40) alloy, which is a β-Ti alloy with a large amount of Nb and Zr content, was selected. This alloy has an elastic modulus of approximately 40 GPa and a single β phase [23,24]. The TNZ40 alloy has better cold formability than α + β Ti alloys, but it has lower strength, and research is needed to resolve this. To increase the strength of Ti alloys, interstitial elements such as O, N, and C are typically added, and strength improvements can be expected via the solid solution hardening mechanism [33]. However, studies are not actively being conducted on improving the mechanical properties of alloys by performing crystal grain refinement based on recrystallization via mechanical and thermal processes [34,35,36,37,38,39,40].

Therefore, in order to improve the strength of the TNZ40 alloy used in this study, TNZ40 alloys with different amounts of oxygen content were fabricated, and experiments were performed in which mechanical and thermal process conditions were designed to improve mechanical properties. The aim was to analyze changes in the microstructures and mechanical properties according to the oxygen content and process conditions.

2. Material and Methods

To manufacture the alloys used in this study, vacuum arc remelting was performed on Ti, Nb, and Zr raw materials to produce ingots containing Ti-39Nb-6Zr-0.16O (wt.%) and Ti-39Nb-6Zr-0.26O (wt.%) alloy compositions. These ingots had a diameter of Φ16. Cold swaging was performed to produce bars with a diameter of Φ11, which were then heat treated at 900 °C and 950 °C for 2 h. The bars were then furnace-cooled (FC) to produce the final specimens (referred to below as Ti-39Nb-6Zr-0.16O SW+ST900 °C (TNZ-0.16O SS900 °C), Ti-39Nb-6Zr-0.26O SW+ST900 °C (TNZ-0.26O SS900 °C), Ti-39Nb-6Zr-0.16O SW+ST950 °C (TNZ-0.16O SS950 °C), and Ti-39Nb-6Zr-0.26O SW+ST950 °C (TNZ-0.26O SS950 °C)). Four types of specimens were used in the experiment, and the microstructure and mechanical properties were analyzed comparatively to see how the microstructure and mechanical properties are affected by the difference in oxygen content as a function of the solution heat treatment temperature after cold swaging. The specimen fabrication process is shown in Figure 1.

To observe the microstructure of each specimen, the specimens were micro-polished to 0.04 µm and then etched with an etching solution (60 mL H₂O₂, 30 mL H₂O, and 10 mL HF). To observe the microstructure, optical microscopy (OM) (BX53M, Olympus, Tokyo, Japan) was used. The samples were micro-polished to 0.04 µm to measure grain size, and grain orientation images were compared using field emission scanning electron microscopy (FE-SEM) and electron backscatter diffraction (EBSD) (JSM-7100F, JEOL, Tokyo, Japan) analysis. In general, SEM-EBSD analysis can be used to determine IPF, IQ, high-angle and low-angle grain boundaries, Kernel average misorientation (KAM), grain orientation spread (GOS), etc. The KAM value is the average value of the crystal rotation (crystal orientation difference) between the targeted measurement point and the surrounding measurement points, and the higher the value, the more strain is present in the material. GOS is used to determine the recrystallization region or to know the recrystallization fraction. The KAM and GOS values tend to increase as the dislocation density or internal strain energy accumulated in the specimen increases.

The presence of the phases was confirmed by X-ray diffraction (XRD) (XRD-7000, Brucker D8, Brucker, Ettlingen, German). To determine mechanical properties, a Vickers hardness tester (HM-200, Mitutoyo, Kawasaki, Japan) was used to measure hardness by pressing 12 points for 10 s with a load of 1 kgf. A dynamic universal materials testing machine (BESTUM-10MD, Ssaul Bestech, Seoul, Republic of Korea) was used for tensile and compression tests. The tensile samples had dimensions of 25 mm gauge length and Φ6 diameter according to ASTM E8 standard. The tensile tests were then performed at room temperature by applying tension at a rate of 10⁻³/s until fracture occurred. The compression sample had a gauge length of 9 mm and a diameter of Φ6. The compression test was then performed at room temperature by applying compression at a strain rate of 10⁻³/s until the strain reached 60%.

3. Results and Discussion

3.1. Analysis of Microstructure and Static Recrystallization

Images of the specimens that were observed with the optical microscope are shown in Figure 2. Sub-grains were observed within the large parent-phase grains of both specimens that experienced the SS900 °C conditions. For the SS950 °C conditions, it can be seen that the large parent-phase grain boundary disappeared, and the sub-grains were composed of main grains.

All specimens have a high internal deformation energy due to the accumulation of many internal dislocations due to cold swaging. This high internal deformation energy is directed to a lower energy state by recrystallization behavior due to the thermal energy supplied by successive solution treatment, i.e., the thermal energy and internal deformation energy of the solution treatment become the driving force for recrystallization nucleation and growth. Static recrystallization occurs in which new grains with lower energy are generated via dislocation annihilation by dislocation movement.

Figure 3 shows an image quality (IQ) map of TNZ-0.16O SS900 °C and TNZ-0.16O SS950 °C that was captured by EBSD, as well as an inverse pole figure (IPF) map that shows the high-angle grain boundary in black and the small-angle grain boundary in white. By analyzing the microstructure change process from TNZ-0.16O SS900 °C to TNZ-0.16O SS950 °C as shown in the IQ map, the microstructure of TNZ-0.16O SS900 °C simultaneously contains coarse-sized parent-phase grains and small-sized grains that have high-angle grain boundaries and low-angle grain boundaries within coarse-sized grains. It was determined that in the case of the fine-sized grains, recrystallization occurred due to the process that was performed. In TNZ-0.16O SS950 °C, the coarse-sized grain boundary disappeared, and it was composed of relatively equiaxed recrystallized grains with high-angle grain boundaries. As for the change in the average grain size, there was a reduction from 90 µm (min. 22 µm and max. 389 µm) in TNZ-0.16O SS900 °C to 80 µm (min. 13 µm and max. 365 µm) in TNZ-0.16O SS950 °C.

IQ and IPF maps of the microstructures and textural characteristics of TNZ-0.26O SS900 °C and TNZ-0.26O SS950 °C are shown in Figure 4. The microstructure of TNZ-0.26O SS900 °C consisted of coarse grains and small-sized recrystallized grains. In TNZ-0.26O SS950 °C, recrystallization occurred in most areas, and it consists of recrystallized grains and larger grains in which the size of the recrystallized grain has grown. It was observed that the areas where recrystallization occurred and their distribution were more even in TNZ-0.16O than in TNZ-0.26O, and the size and shape were uniform. The change in grain size was slight, with TNZ-0.26O SS900 °C having an average grain size of 84 µm (min. 16 µm and max. 460 µm) and TNZ-0.26O SS950 °C having an average grain size of 83 µm (min. 17 µm and max. 353 µm). From this, it was determined that the larger amount of oxygen that is dissolved in the crystal lattice is a factor that affects the occurrence of recrystallization and grain-growth behavior changes, and the change in the size of the average crystal grain was not large.

Overall, the microstructure of the TNZ-0.16O SW+ST950 °C specimen is composed of an equiaxed structure with a more uniform shape and size than the microstructure of the TNZ-0.26O SW+ST950 °C specimen and no coarse-sized crystal grains were observed. It was observed that the microstructure of the TNZ-0.26O SW+ST950 °C specimen was distributed with more uniformly shaped and sized equiaxed crystal grains than the TNZ-0.16O SW+ST950 °C specimen, and some regions had coarse-sized crystal grains. The oxygen atom, which has a very strong affinity for titanium, must be controlled very carefully in titanium materials. It was found that the static recrystallization phenomenon, depending on the oxygen content, resulted in inhomogeneous recrystallization grain size and distribution and that more oxygen employed in the crystal lattice delayed the recrystallization occurrence, resulting in a lower recrystallization fraction. It was found that controlling this inhomogeneous microstructure is necessary for the stability of the alloy’s mechanical performance.

Figure 5 shows the IPF maps of TNZ-0.16O and TNZ-0.26O for each process, as well as the low-angle grain boundary fraction in which the misorientation between two grains is less than 15° and the high-angle grain boundary fraction in which the misorientation between two grain is 15° or more. There is also a grain orientation spread (GOS) map that shows the classified recrystallized regions. The GOS is the average value of orientation spread between all the points in a grain. The grains with low GOS values are considered recrystallized grains scanned with EBSD data [41]. In Figure 5, if the grain has a GOS value of 2 or less, it means that the grain is a recrystallized grain and is shown in blue. The regions with GOS values of 2 or greater are non-recrystallized grains, and they are shown in red. In TNZ-0.16O, the coarse-sized grains are non-recrystallized grains, and it can be seen that these regions decreased as the heat-treatment temperature increased. In TNZ-0.26O as well, the non-recrystallized grain regions decreased as the heat-treatment temperature increased, and the ratio of recrystallized regions was smaller than that of TNZ-0.16O.

The shapes of the microstructures of each specimen observed in Figure 3 and Figure 4 were different. It found that the difference in oxygen content is thought to affect recrystallization behavior. The material had high internal strain energy due to the cold swaging that was performed initially during processing. It can be seen that the heat energy that was obtained from the subsequent solution heat treatment was the driving force for recrystallized grain nucleation, and in order to move toward a stable energy state, recrystallization occurred, and the material changed to a low energy state. The nucleation and growth of new grains occurred during solution heat treatment and above after cold swaging. It can be concluded that static recrystallization occurred in this study’s specimens due to the subsequent heat treatment. It can be inferred that recrystallization was delayed in the specimens with higher oxygen content because there were more obstacles to the recovery process, which relieved the accumulated strains.

Line graphs of all the specimen high-angle grain boundaries and recrystallized fractions, which are the areas where the GOS values were 2 or less, are shown in Figure 6. When the recrystallized fractions of the specimens are compared according to the process conditions, the recrystallized fraction was 76% in TNZ-0.16O SS900 °C, 71% in TNZ-0.26O SS900 °C, 91% in TNZ-0.16O SS950 °C, and 80% in TNZ-0.26O SS950 °C, indicating that TNZ-0.16O had a higher recrystallized fraction than TNZ-0.26O. In addition, the slope of the recrystallized fraction of TNZ-0.16O, according to the increase in the heat-treatment temperature, was steeper than that of TNZ-0.26O. Here, it can be concluded that the steep slope occurred because there was more active recrystallization due to the lower oxygen content.

Moreover, as the heat-treatment temperature increased, the recrystallized fraction increased, but the high-angle grain boundary fraction decreased. The change in the high-angle grain boundary fraction was 67% in TNZ-0.16O SS900 °C, 51% in TNZ-0.26O SS900 °C, 55% in TNZ-0.16O SS950 °C, and 24% in TNZ-0.26O SS950 °C. The reduction in the high-angle grain boundary fraction at the higher heat-treatment temperature occurred because recrystallization occurred in most areas of all specimens, and the recrystallized grains grew. Because of this, the total grain boundary area decreased.

The slope of the high-angle grain boundary fraction was steeper in TNZ-0.26O. It was determined that the reason for this is because TNZ-0.16O consisted of equiaxed grains, whereas TNZ-0.26O had a microstructure consisting of grains of non-uniform size and shape when the microstructures of the two specimens were compared; therefore, the range of change in the high-angle grain boundary fraction was larger. Also, as shown in Figure 5, in the case of TNZ-0.16O, static recrystallization occurred quickly throughout the entire area, and the recrystallization region rapidly grew as the temperature increased from 900 to 950 °C. The growth of the recrystallized grains was relatively slow, and the high-angle grain boundary fraction exhibited a gradual slope (Figure 6b). However, in the case of TNZ-0.26O, which had a high oxygen content, the diffusion barrier caused by the oxygen atoms and the area where static recrystallization did not occur was larger than in the case of TNZ-0.16O. For this reason, a larger amount of heat energy from the solution heat treatment can be used to promote the growth of static recrystallized grains; therefore, as shown in Figure 6b, the high-angle grain boundary fraction decreased significantly as the temperature increased from 900 to 950 °C.

When the solution treatment is first performed at a temperature above the β transformation temperature, and then cold swaging is applied, the dislocation density increases and plastic deformation bands are formed in the region where the dislocation is integrated. As a result, brittleness increases, so high compressive strength can be obtained, but an appropriate elongation and yield strength value cannot be secured. Therefore, after cold swaging, a solution treatment was performed at a temperature above the β transformation temperature. A static recrystallization phenomenon occurred during the solution treatment process, and recrystallization initiation and growth were observed in the crystal grains.

The static recrystallization fraction increased as the heat treatment temperature increased, and the recrystallization fraction of the TNZ-0.16O specimen was higher than that of the TNZ-0.26O specimen. This means that the higher the oxygen content, the higher the dissolved oxygen in the β crystal lattice, which further hinders the movement of the dislocation. Therefore, less recrystallization was generated, and non-uniform recrystallization occurred.

The specimen XRD diffraction pattern results according to the oxygen content in each process are shown in Figure 7. It can be seen that only β-phase peaks were detected in both specimens for all processes, and no second phase other than the β phase was created. In addition, the β-phase peak intensity was slightly stronger in TNZ-0.16O than in TNZ-0.26O, and as the heat-treatment temperature increased, the peak intensity increased. It was determined that TNZ-0.16O, which includes a small amount of oxygen (an α-stabilizer element), exhibited strong peak intensity because it has a β phase in a stable energy state due to the larger amount of recrystallization.

The recrystallization behavior was found to be different depending on the oxygen content, with the specimens with lower oxygen content showing a more homogeneous region of recrystallization. For the TZN-0.26O specimen, grains of inhomogeneous size grew as the heat treatment temperature increased. It is believed that oxygen is unevenly employed in the region and recrystallization occurs in localized areas, i.e., the parent grain disappears and the recrystallized grain becomes dominant, but uneven recrystallization and growth occurs.

3.2. Mechanical Properties

The measured Vickers hardness values of the specimens for all process conditions are shown in Figure 8. In all conditions, TNZ-0.26O had a higher hardness value than TNZ-0.16O due to the solid solution hardening effect. In a comparison of the hardness for each specimen, the hardness of TNZ-0.16O SS900 °C was 220 HV, and the hardness of TNZ-0.16O SS950 °C was 225 HV. When the solution heat-treatment temperature was high, the average grain size was reduced (90 µm → 80 µm) despite the increase in temperature, and the hardness increased slightly due to the changes in the microstructure, which consisted of equiaxed grains. The hardness of TNZ-0.26O SS900 °C was 247 HV, and the hardness of TNZ-0.26O SS950 °C was 242 HV. Unlike the trend of change in the hardness values of TNZ-0.16O, the hardness was lower at a higher solution heat-treatment temperature. Comparing the aforementioned microstructure shapes, it was determined that the reduction in the average grain size was small, and the hardness value was reduced slightly even as the temperature increased from 900 to 950 °C due to the non-uniform size and shape of the TNZ-0.26O grains.

Figure 9 and Figure 10 and

Table 1. Ultimate tensile strength, yield strength and elongation of Ti-39Nb-6Zr-0.16O SS900 °C, Ti-39Nb-6Zr-0.16O SS950 °C, Ti-39Nb-6Zr-0.26O SS900 °C, and Ti-39Nb-6Zr-0.26O SS950 °C.

Specimens	Ti-39Nb-6Zr-0.16O			Ti-39Nb-6Zr-0.26O
	Ultimate Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)	Ultimate Tensile Strength (MPa)	Yield Strength (MPa)	Elongation (%)
SW+ST900 °C	737 ± 5	734 ± 4	10.2 ± 2.2	777 ± 10	775 ± 3	18.6 ± 1.4
SW+ST950 °C	773 ± 3	764 ± 8	15.8 ± 1.5	755 ± 7	741 ± 6	17.4 ± 1.2

and

Table 2. Compressive yield strength and compressive strength at strain 30% of Ti-39Nb-6Zr-0.16O SS900 °C, Ti-39Nb-6Zr-0.16O SS950 °C, Ti-39Nb-6Zr-0.26O SS900 °C, and Ti-39Nb-6Zr-0.26O SS950 °C.

Specimens	Ti-39Nb-6Zr-0.16O		Ti-39Nb-6Zr-0.26O
	Compressive Yield Strength (MPa)	Compressive Strength (MPa) (at Strain 30%)	Compressive Yield Strength (MPa)	Compressive Strength (MPa) (at Strain 30%)
SW+ST900 °C	774 ± 9	1086 ± 15	796 ± 13	1187 ± 21
SW+ST950 °C	777 ± 12	1092 ± 17	788 ± 14	1155 ± 15

show the strain–stress curves and the numerical data obtained by performing room temperature tension and compression tests. The trend of results in the tensile and compressive strength is similar to that of the Vickers hardness. TNZ-0.260, which has a higher oxygen content than TNZ-0.16O, showed excellent overall tension and compression properties. The tension and compression properties of TNZ-0.16O improved at SS950 °C compared to SS900 °C, and this specimen exhibited an excellent elongation (15.8%). In TNZ-0.16O SS950 °C, it can be seen that the coarse parent-phase grains disappear, and that it consists of uniformly distributed equiaxed grains, giving it improved tensile and compressive strength.

In the TNZ-0.16O specimen, the average grain size decreased from 90 to 80 µm due to the increase in the solution heat-treatment temperature, and it showed a higher tensile strength, yield strength, and elongation at SS950 °C. However, the TNZ-0.26O specimen exhibited higher tension and compression properties at SS900 °C than at SS950 °C. More recrystallization occurred in TNZ-0.26O SS950 °C, but the grain size and shape were non-uniform, and the tensile strength (777 MPa → 755 MPa) and compressive strength (1187 MPa → 1155 MPa) were reduced.

As mentioned above, in order to control the microstructure of the Ti-39Nb-6Zr (TNZ40) β-Ti alloy and improve its mechanical properties, controlling the amount of oxygen, which is an α-stabilizer element, makes it possible to control the static recrystallization behavior, recrystallization fraction, and uniformity of grain-size distribution, thereby improving tension and compression properties at room temperature.

4. Conclusions

This study observed changes in the recrystallization behavior of Ti-39Nb-6Zr-0.16O (wt.%) and Ti-39Nb-6Zr-0.26O (wt.%) alloys, and the following conclusions were drawn.

(1) It was confirmed that static recrystallization, which creates new grains within coarse parent grains, occurred when solution heat treatments were performed at 900 °C and above after cold swaging a Ti-39Nb-6Zr alloy that had oxygen added. The microstructure of the Ti-39Nb-6Zr alloy with oxygen added at 0.16 wt.% consisted of equiaxed grains, and there were improvements in tensile strength (737 MPa → 773 MPa), elongation (10.2% → 15.8%) as the heat-treatment temperature increased, due to the grain refinement effect.

(2) It was confirmed that the Ti-39Nb-6Zr-0.26O alloy exhibited higher mechanical properties than Ti-39Nb-6Zr-0.16O due to the solid-solution hardening effect, but its recrystallized fraction was lower because the recrystallization behavior was delayed due to the greater amount of dissolved oxygen in the crystal lattice. Unlike the microstructure of Ti-39Nb-6Zr-0.16O, which consisted of equiaxed grains, the microstructure of Ti-39Nb-6Zr-0.26O exhibited a non-uniform grain size and distribution, and as the heat-treatment temperature increased, its mechanical properties decreased slightly.

(3) The solid-solution hardening caused by the addition of oxygen, which is an interstitial element, and the crystal grain refinement caused by static recrystallization had a combined effect, and it was found that the SW+ST900 °C conditions for the Ti-39Nb-6Zr-0.26O alloy were the optimal process conditions. These conditions resulted in excellent mechanical properties, including a tensile strength of 777 MPa, an elongation of 18.6%, and a compressive strength of 1187 MPa.