Microstructure and Mechanical Properties of Ti-Nb Alloys Prepared by Mechanical Alloying and Spark Plasma Sintering
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The effect of Nb content on microstructure, mechanical properties and superelasticity was studied in Ti-Nb alloys fabricated by powder metallurgy route using mechanical alloying and spark plasma sintering. In the microstructure of the as-sintered materials, undissolved Nb particles as well as precipitations of α-phase at grain boundaries of β-grains were observed. In order to improve the homogeneity of the materials, additional heat treatment at 1250 °C for 24 h was performed. As a result, Nb particles were dissolved in the matrix and the amount of α-phase was reduced to 0.5 vol.%. Yield strength of the as-sintered alloys decreased with Nb content from 949 MPa for Ti-14Nb to 656 MPa for Ti-26Nb, as a result of the decreasing amount of α-phase precipitations. Heat treatment did not have a significant effect on mechanical properties of the alloys. A maximum recoverable strain of 3% was obtained for heat-treated Ti-14Nb, for which As and Af temperatures were − 12.4 and 2.2 °C, respectively.
Keywordsmartensitic transformation mechanical alloying sintered alloy superelasticity titanium alloys
Ti-Nb-based alloys are considered as materials for orthopedic implants due to their excellent biocompatibility (Ref 1), low elastic modulus (Ref 2) and good superelastic properties (Ref 3). The superelastic behavior of Ti-Nb alloys is connected with the thermo-elastic martensitic transformation between body-centered cubic (BCC) β parent phase and orthorhombic α” martensite phase (Ref 3). In the case of pure titanium low-temperature HCP (hexagonal close packed), α-phase transforms to β-phase at 882 °C. However, by adding elements such as Nb, Ta, Mo, the high-temperature β-phase can be stabilized in room temperature (RT) (Ref 4). In binary Ti-Nb system, an addition of 1 at.% of Nb to β-Ti decreases Ms temperature by 40 °C and an addition of 26-27 at.% allows to obtain Ms temperature slightly below room temperature (RT), which ensures optimal RT superelastic properties (Ref 5, 6). Theoretical calculation shows that alloys which contain 26 at.% of Nb maximum strain transformation along  direction can reach about 3% (Ref 3). This value can be increased by reducing the concentration of Nb or by replacing Nb by other elements such as Ta, Mo, Zr (Ref 3, 6). The extensive research is focused on shape-memory characteristic of Ti-Nb-based alloys obtained through casting and cold-working processes. For example, Kim et al. (Ref 3) reported a maximum recovery strain of 4.2% in cold-rolled Ti-26Nb alloy aged at 400 °C. Aging of superelastic Ti-Nb alloys is required in order to obtain stable superelastic behavior as a result of the formation of nanometric precipitations of athermal ω-phase which increase critical stress for slip deformation in those alloys (Ref 7, 8). Another way for increasing critical stress for slip deformation is the addition of interstitial elements such as O or N. It was confirmed that an addition of 2 at.% of oxygen to Ti-22Nb increases critical stress for slip deformation up to 890 MPa, but also reduces the maximum recovery strain to about 3% (Ref 9). Nowadays, powder metallurgy (PM) techniques such as mechanical alloying (MA) as well as additive manufacturing (AM) are gaining popularity due to the possibility of fabricating elements with complicated shapes and superior mechanical properties (Ref 10-13).
The powder metallurgy route based on mechanical alloying of pure metal powders followed by consolidation using spark plasma sintering (SPS) is a promising method of preparing a wide range of alloys, in particular when alloying elements have a wide difference in melting points. Moreover, by controlling the processing parameters such as sintering temperature or by introducing space holder materials, porous materials can be obtained, which allow controlling the elastic modulus of the material. Introducing of pore structure to the conventional NiTi SMAs allows obtaining better biocompatibility as a result of reduction in elastic modulus close to the bone values and the ability to grow cells inside the pores (Ref 14). In the case of powder metallurgy processes, the oxygen and nitrogen contaminations have a significant impact on Ms temperature of Ti-Nb-based alloys as well as superelastic properties. It was confirmed that an addition of 1 at.% of O to the Ti-Nb alloys leads to a decrease in Ms temperature by about 160 °C (Ref 15, 16). Similarly, an addition of 1 at.% of N decreases Ms temperature by about 200 °C (Ref 17). This is the reason why the martensitic transformation is not observed in sintered materials even if the Nb content indicates that Ms temperature is close to RT. For example, Lai et al. (Ref 10) show that stable RT superelastic properties can be obtained in binary Ti-Nb alloys prepared by powder metallurgy route by reducing Nb content to about 13%. Yuan et al. (Ref 18) reported high recovery strain of 5.4% in Ti-11Nb alloy containing 4 at.% of oxygen, obtained by mechanical alloying and pressureless sintering. Therefore, in order to obtain superelastic properties in binary Ti-Nb alloys obtained by powder metallurgy route, the Nb concentration has to be reduced to compensate the effect of interstitial atoms on the Ms temperature.
In the presented work, two different compositions of the alloy, containing 14 and 20 at.% of Nb, were chosen based on the typical oxygen content observed in mechanically alloyed Ti-Nb-based alloys, which is in the range 1.0-2.5 at.% (Ref 10, 19, 20). The third alloy containing 26 at.% of Nb was chosen as a reference material in which superelastic behavior is observed in the as-cast state. Because of limited information about the evolution of properties of PM-fabricated Ti-Nb alloys, the aim of the presented work is to analyze the influence of Nb content on the microstructure, mechanical properties and superelastic behavior of Ti-Nb alloys.
In the presented study, Ti (150 mesh, 99.9%) and Nb (325 mesh, 99.8%) elemental powders, supplied by Alfa Aesar, were used as initial materials. Three different compositions of alloys were prepared using mechanical alloying method—Ti-xNb (where x = 14, 20 and 26 at.%). Mechanical alloying was conducted using Fritsch Pulverisette 7 planetary ball mill with rotation speed 150 rpm and cemented tungsten carbide containers and balls. The ball-to-power weight ratio was 10:1. Time of milling was 30 h. In order to avoid extensive oxidation of powders during synthesis, all the operations with powders were conducted in a glovebox under protective argon atmosphere (O2 and H2O < 1 ppm). In the next step, powders were consolidated using spark plasma sintering (HP D5/2 FCT System) at 1300 °C for 30 min under 35 MPa pressure in argon atmosphere. Samples 20 mm in diameter and 8 mm high were obtained. Additional annealing at temperature 1250 °C for 24 h was applied in order to improve the homogeneity of sintered samples.
Results and Discussion
Powders Characterization After Milling
Mechanical properties of investigated alloys
Yield strength, MPa
949 ± 36
746 ± 19
656 ± 10
921 ± 30
624 ± 26
710 ± 14
Figure 10(b) shows compressive curves of heat-treated materials. Although the applied heat treatment leads to an increase in the homogeneity of the materials, only small changes in mechanical properties were observed. In the case of alloys containing 14 and 20 at.% of Nb, a slight decrease in YS was noted. On the other hand, for Ti-26Nb alloy, the YS increased by about 50 MPa. No changes were observed in compression strain. Slight difference in the mechanical properties between the as-sintered and heat-treated materials can be attributed to a few processes taking place during the annealing. First of all, annealing leads to grain growth, which can reduce the properties of the alloys, as described by the Hall–Petch relationship. Simultaneously, the reduction in amount of hexagonal α-phase precipitations, as well as, undissolved Nb particles, which also had influence on mechanical properties, was observed. On the other hand, the formation of nanometric precipitations of ω-phase during high-temperature annealing leads to an increase in mechanical properties (Ref 8). It is important to note that Ti-Nb alloys obtained by casting techniques exhibit much lower mechanical properties in comparison with obtained alloys. YS of casted and solution-treated alloys typically did not excess 400 MPa (Ref 8, 35), whereas this value for alloys obtained by powder metallurgy route typically exceeded 1000 MPa (Ref 19, 36). There are several possible reasons describing the increased mechanical properties of alloy obtained by powder metallurgy route in comparison with cast alloys. First of all, the oxygen and nitrogen contaminations caused the solution-hardening effect, which increases mechanical properties of the alloy, e.g., addition of 2 at.% of oxygen to Ti-22Nb alloy increases the fracture stress of the alloy to 1.37 GPa (Ref 9). Secondly, the observed α-phase precipitations in fabricated alloys also lead to an increase in the mechanical properties, as in the case of near-β Ti alloys (Ref 27). The frequently observed, in the case of mechanically alloyed materials, nanoparticles of oxides also can increase the properties of the materials (Ref 37).
The applied sintering conditions (temperature 1300 °C, pressure 35 MPa, time 30 min) allow to obtain dense samples, with porosity below 0.5 vol.%; however, additional annealing at 1250 °C for 24 h had to be applied in order to increase the homogeneity of the as-sintered materials. As a result, the concentration of Nb in the matrix reached the value close to the intended, and amount of hexagonal α-phase was reduced to about 0.5 vol.%.
Mechanical properties of fabricated alloys are higher in comparison with cast Ti-Nb alloys. YS of the as-sintered alloys decreases with Nb content from 950 MPa for Ti-14Nb to 650 MPa for Ti-26Nb, as a result of decrease in the volume fraction of α-phase with Nb content. Heat treatment has no significant effect on mechanical properties of the alloys.
Superelastic behavior at RT was observed for heat-treated Ti-14 alloy. This alloy exhibited maximum recoverable strain up to 3%. Determined for this alloy, by DSC technique, As and Af temperatures were − 12.4 and 2.2 °C, respectively.
The research was co-financed by the European Union from resources of the European Social Fund (Project No.WND-POWR.03.02.00-00-I043/16).
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