The Influences of Element Addition on Compression Properties of β-Ti-Mo (Nb)-based Alloys

The influences of the third element addition on the compression properties of Ti-12Mo-based and Ti-33Nb-based alloys was investigated, with emphasis on the influences of element addition on Young’s modulus, superelastic strain and yield strength of water-cooling samples. It is found that the water-cooling Ti-12Mo alloy has a lower modulus, lower yield strength, and higher hyperelastic strain than that of the furnace-cooled alloy of the same composition. Al and Sn can reduce the modulus and the yield strength, and increase the superelastic strain in the above two water-cooling alloys. The decrease in yield strength and modulus is due to orthogonal martensite ɑ’. The enhancement of the superelastic strain originated from the phase transformation of the ɑ”↔βM under stress. The effect of Sn in water-cooling quaternaryTi-18Nb-5Mo-5Sn alloy is different from the above two ternary alloys, and it increases β Phase stability and volume in water-cooling Ti-18Nb-5Mo-5Sn alloy.


Introduction
Human allergy to Ni has led researchers to develop Ni-free β-Ti-based alloy with excellent superelasticity at human body temperature and to replace the Ni-Ti alloy [1][2][3][4] .Indeed, β-Ti-based alloys have small moduli and superelasticity and are therefore suitable for use as bio-materials [3,5] .These excellent properties are closely related to their phases and microstructures.It has been shown that the β-Ti-based alloys with both ɑ ,, (Orthorhombic martensite) and metastable β M (Body-centered cubic lattice) phases possess hyperelasticity and low modulus.On one hand, the ɑ ,, →β M transformation under the stress-inducing produces hyperelasticity.On the other hand, ɑ ,, and β M have low modulus compared to the phases of ɑ, β, and ω [6][7][8] .Therefore, appropriate phases and microstructures are necessary to obtain the reason properties.These microstructures depend on chemical compositions, heat treatments, and Thermal-mechanical composite treatments [9][10][11] .The influences of element addition on their S-S curves and the relationship of compression properties with phases and microstructures are discussed in this study.

Experimental procedures
The raw materials for sample preparation are pure Ti, Mo, Nb, Sn, Al, and Fe particles (99.99% purity, particle size of about 5-10 mm), which are melted and cast into nearly half-spherical (ball diameter of about 40 mm) alloy ingots with a mass of about 80 g under the protection of argon in a small vacuum arc furnace (KVAR-3).The alloy compositions are Ti-12Mo-2X (mass%) (X is Sn, Al, Fe, Nb) and Ti-33Nb-2X (X is Sn, Al, Fe, Mo).The alloy ingot is heated to 1, 100 o C in the box furnace for 10 minutes and then forged into a round cake shape three times.The lower forging temperature is not lower than 950 o C.
All samples used for the test are cut from the above round cake samples with an electric spark machine, and the sample size for the S-S test is 5×5×20 mm, the surface finish is 6.3 um, and the dimensional accuracy is 0.01 mm.The sample size of X-ray diffraction is 10×10×5 mm.The tested samples are heated to 950 o C in GSL 1, 600 tubular furnace for 30 minutes, and then cooled either in the furnace or into the water, different phases can be obtained.
The stress-strain(S-S) curve is tested with the E/A-QT-01 testing machine.The X-ray diffractometer is Brooke D8advance, which uses a copper target and wavelength Kɑ is 0.154 nm, diffraction angle 2θ-80 o .

Results and discussions
Pure Ti has a close-packed hexagonal (hcp) lattice at ambient temperature, i.e ɑ-Ti, and is a body-centered cubic (bcc) lattice at high temperature that is called as β-Ti.In Ti alloys, increasing the contents of the β stabilizers such as Mo, V, etc, the alloys will change from β phase to ɑ/β dual-phase when the alloys are furnace-cooling.If the alloy is from a high-temperature β phase zone is rapidly cooled to room temperature, the formed products can be either single ɑ , (hcp), or orthogonal martensitic ɑ ,, or the metastable β M phase or combining phases,e.g., ɑ , +ɑ ,, , ɑ ,, +β M [10,12]   , depending the added β stabilizer.Possibly, triangular structure ω can be produced in some situations.The water-cooling Ti-12Mo alloy possesses the dual phase of ɑ ,, /β M and the furnace-cooling has ɑ/β phases.Figure 1 is the diagram of stress versus strain for the water-cooling and furnace-cooling Ti-12Mo alloys.The former is less than the latter in modulus and has a superelastic strain and shape memory effect.The latter has no superelastic strain and larger yield stress.The differences in properties should be resulted from their phases, attributing to their different heat treatments.In Ti-base alloys, the order is ω>ɑ , >ɑ ,, >β [6][7] or ω>ɑ>ɑ , >ɑ ,, >β in Young's modulus [3] .ɑ ,, and β is the same in the modulus [2] .ɑ ,, phase is the smallest in modulus among ɑ , ,ɑ ,, and β phases [2] . , confirming the present experimental results.The hyperelastic strain of the water-cooling Ti-12Mo alloy originates from the martensitic and reverse martensitic transformations of ɑ ,, ↔β M [1]   .The furnace-cooling Ti-12Mo specimen has ɑ+β phase and the corresponding yield stress is high [3] .From Figure 2, the S-S curves of the water-cooling Ti-12Mo-2X alloys have been modified to different degrees after the third element is added.The influences of the third element on Young's modulus, superelastic strain, and yield strength are different from element to element.Young's modulus of the water-cooling Ti-12Mo-2Fe alloy increases and is the largest among the water-cooling T-12Mo-X alloys when third elements are added.Young's modulus of the water-cooling Ti-12Mo-2Al alloy decreases and is the smallest among the water-cooling T-12Mo-X alloys.The alloy with Nb has almost no effect on the modulus of the water-quenched Ti-12Mo alloy, and the alloy with Sn has a slightly lower modulus.For the hyperelastic strain, Al increases the hyperelastic strain obviously and Sn and Nb also increase the hyperelasticity of the alloy while Fe reduces the hyperelastic strain.In addition, Fe significantly enhances the yield stress, and the others decrease it.
From Figure 3, the volume of the β M phase of the Ti-12Mo alloys containing Fe increases since Fe is also β stabilizer, increasing Young's modulus.Al and Sn can are ɑ-stabilizer, which can make water-cooling Ti-12Mo alloy produce more ɑ ,, phase, causing the decrease of Young's modulus.Nb is also β stabilizer, but its effectiveness is inferior to that of Mo.It can be seen that from the XRD pattern in the water-cooling Ti-12Mo and Ti-12Mo-2Nb specimens, the relative strength of ɑ ,, and β M phase has little changed, so Young's modulus has little changed.It has been discussed that the superelasticity of water-cooling Ti-Mo alloy is induced by the ɑ ,, ↔β M of martensitic transformation and its reverse transformation under the stress-inducing.Therefore, the alloys with β M or near single β M -phase have no hyperelasticity or slight hyperelastic strain, which is true for water-cooling Ti-12Mo-2Fe because its phase composition is almost completely single β M phase.Because the occurrence of hyperelastic strain is attributed to the ɑ ,, →β M transformation induced by stress in the loading process, the number of ɑ ,, phases should be the main contribution to superelasticity, determining the magnitude of hyperelastic strain.In Figure 3, the relative intensity of the diffraction peak of ɑ ,, phase, and β M phases is different from adding-Al to adding-Sn.The former contains more ɑ ,, phase and should have a larger hyperelastic strain than the latter.The conclusions obtained from the analysis are almost the same as the test results in Figure 2.For Young's modulus, water-cooling Ti-12Mo and Ti-12Mo-2Nb alloys show almost no difference in superelasticity.The existence of ɑ ,, martensite reduces the tensile strength and yield strength [2][3] .Because the water-cooling Ti-12Mo-2Fe alloy is almost a single β M phase, it is understandable to have the highest yield strength.However, the results in Figure 2 show that the yield strength is not completely proportional to the amount of ɑ ,, martensite, which may be related to the transformation of ɑ ,, →β M .According to Kolli et al. [9] , the water-cooling Ti-33Nb alloy should have ɑ ,, /β M dual-phases and the furnace-cooling one is ɑ+β phases.There is a difference in their modulus, superelastic strain, and yield stress for the Ti-33Nb specimens with different heat treatments, whose situation is similar to the Ti-12 Mo alloy.To further clarify the relationship between mechanical properties and phases, ɑ-stabilizer or β-stabilizer is introduced to the Ti-33Nb alloy respectively.The XRD results of the water-cooling Ti-33Nb-2X (X=0, Mo, Sn, Al, Fe) specimens are shown in Figure 4.The addition of Fe and Mo elements causes the increase of β M phase since both Fe and Mo are β-stabilizers.Therefore, compared to the water-cooling Ti-33Nb alloy, the water-cooling Ti-33Nb-2X (X=Fe, Mo) specimens have large modulus and yield stress, as shown in Figure 5. Good superelasticity presents in the water-cooling Ti-33Nb-2X(X=Al, Sn) specimens since they contain more ɑ ,, phase.The above results are similar to the Ti-12Mo-2X (X=Sn, Al, Fe, Nb) alloys.From the mechanical properties of the water-cooling Ti-12Mo-2X (X=Sn, Al, Fe, Nb) and Ti-33Nb-2X(X=Sn, Al, Fe, Mo) alloys, both Sn and Al can reduce the yield strength and Young's modulus and increase the superelasticity to a certain extent.According to the above research results and analysis, Al and Sn increase ɑ ,, structure in water-cooling Ti-Mo-based and Ti-Nb-based alloys and thereby is ɑ stabilizer, but it is also reported that Sn is β stabilizer, because Sn can significantly reduce the martensitic transformation temperature and increase β stability [12] .To further understand the action mechanism of Sn, Ti-18Nb-5Mo-5Sn alloy with two phases of ɑ+β was obtained by adding 5 (wt.)%Sn into the ternary Ti-18Nb-5Mo alloy.In Figure 6, the elastic modulus of the water-cooling alloy with Sn has little change, and the superelastic strain is not as large as that of the alloy without Sn, but the yield strength of the alloy with Sn decreases.Figure 7 shows the X-ray diffraction profiles of two water-cooling alloys.There are more β M phases in the alloys with Sn, indicating that Sn in Ti-18Nb-5Mo-5Sn alloy is β stabilizer.The role of Sn systems is not the same in all Ti-based alloys.

Conclusions
The effects of the element addition on the S-S curves of Ti12Mo and Ti-33Nb-based alloys are studied.Only water-cooling alloys with orthogonal martensite structure ɑ ,, has obvious hyperelastic strain.The more ɑ ,, phases are, the greater the hyperelastic strain is.Al and Sn can increase ɑ ,, phase volume in water-cooling Ti12Mo-based and Ti-33Nb-based alloys, making the modulus and yield strength decrease and the hyperelasticity increase.The role of Sn in water-cooling quaternary alloy Ti-18Nb-5Mo-5Sn is different from that of the above two ternary alloys.Sn increases the stability and the β M phase in water-cooling Ti-18Nb-5Mo-5Sn alloy.More ɑ ,, phase in water-cooling Ti-based will be advantageous to obtaining large hyperelastic strain and small modulus.