INFLUENCE OF Sn ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF Ti-Mo-Nb ALLOYS FOR ORTHOPAEDIC APPLICATIONS

Metastable ß-Ti alloys intended for orthopaedic implants typically possess undesirable α ′, α ″, ω precipitates, which increase the elastic modulus. Non-toxic Sn was reported as an effective suppressor of α ′, α ″ and ω precipitates. Furthermore, increasing Sn content was reported to decrease the elastic modulus. In this study, the cluster plus glue atom (CPGA) model was used to develop structurally stable ß -Ti alloys through the addition of Sn. Arc melting was conducted to fabricate the


Starting materials
The starting materials used in this study were angular shaped Ti powder (-45 μm) with 99.5% purity, angular shaped Nb powder (-5 μm) with a purity of 99.8%, spherical shaped Mo powder (-150 μm) with a purity of 99.9%, and spherical shaped Sn powder (-150 μm) with a purity of 99.9%. The Ti, and Nb powders were supplied by Thermo Scientific, while the Mo and Sn powders were supplied by Alfa Aesar.

Alloy design and fabrication
Three compositions in the Ti-Mo-Nb-Sn system were formulated using the CPGA model using cluster formula [(Mo1-x,Snx)(Ti)14](Nb)1 where x=0.4 and 0.5. The alloys were then prepared using 100g powders (per alloy) of varying chemical compositions in accordance with the alloy make-up in Table 1. The starting powders were mixed and then compacted into 45 mm diameter button-shaped green compacts using a uniaxial cold compacting machine at 250 bars. The compacts were then melted in an arc furnace with a non-consumable tungsten electrode and water-cooled copper crucible under argon atmosphere, where Ti getters were used to minimise oxidation. The samples were homogenised by inverting and re-melting them at least three times.

Microstructure, chemical composition, and phase analysis
A Leica DMI 5000M optical microscope and a Joel JSM-6510 scanning electron microscope (SEM) equipped with Energy-Dispersive X-ray Spectroscopy (EDS) was used to study the microstructure and chemical composition of the samples. Precision-cut samples from as-cast ingots were mounted, ground, and polished in accordance with the ASTM E3-11 standard guide for the preparation of metallographic specimens. Colloidal silica (3 μm) was used for the final polishing. Etching of the samples was performed using Kroll's reagent, which was composed of 3 ml HNO3, 2 ml HF, and 100 ml distilled water. Phase identification was analysed using a PANalytical Empyrean diffractometer system operated at 45 kV and 40 mA with Cu Kα as an X-ay source with λ1 = 0.1540598 nm, λ2 = 1.544426 nm and a scan range of 20 to 90 degrees 2θ.

Mechanical testing
Vickers micro-hardness measurements were conducted using a Vickers micro-hardness tester (FM-700, micro-indenter with a square base pyramid tip). A load of 500 g and dwell time of 15 s were used, and five indentations were made on each sample. Tensile testing of the alloys was performed on an Instron 1342 mechanical testing machine in accordance with the ASTM E8/E8M standard test method for tension testing of metallic materials. Figure 1 shows the dimensions of the tensile specimen. Testing was done at a speed of 0.5 mm/sec using three test specimen per alloy.  (Figures 2(b) and (c)) revealed sub-grain structures within the primary grains upon etching. The alloy with a low Sn content ([(Mo0.6Sn0.4) (Ti)14] (Nb)1) presented the most sub-grain structures. As the Sn content was increased to 0.5 atoms, the structures were seen to diminish. The appearance of such structures in Ti alloys is known to occur during work-hardened alloy recovery (Raganya, 2020), but have also been reported in as-cast metastable β-type Ti alloys, and were attributed to compositional differences Ozan et al., 2017).

Figure 2: Optical micrographs of the alloys (a) [(Mo)(Ti)14] (Nb)1, (b) [(Mo0.6Sn0.4) (Ti)14] (Nb)1and (c) [(Mo0.5Sn0.5) (Ti)14] (Nb)1 x100
The chemical compositions of the samples are shown in Table 2. The results suggest that the measured compositions were very close to the nominal compositions.  consequently, EDS mapping of the as-cast alloys was performed to assess for chemical homogeneity. The EDS mapping in Figure 3 shows that all of the elements were evenly distributed, with no substantial enrichment or depletion in the alloys.

CONCLUSIONS
In this study, we used the cluster-plus-glue-atom (CPGA) model with cluster formula [(Mo1-x,Snx)(Ti)14](Nb)1 to investigate the effect of Sn on the microstructure, and mechanical properties of Ti-Mo-Nb alloys. Based on the findings, the following can be concluded: • The microstructures of the formulated alloys consisted of equiaxed β grains with no evidence of any secondary phases.
• XRD analysis showed that substituting 0.4 atoms of Mo with Sn atoms in the cluster shell resulted in the formation of secondary α″ phases. However, further increasing the Sn atom to 0.5 atoms resulted in a highly stable β phase.
• Increasing the content of Sn atoms in the cluster shell resulted in a decrease in the elastic modulus and micro-hardness of the alloy. The [(Mo0.5Sn0.5)(Ti)14](Nb)1 obtained the lowest hardness and elastic modulus values of 372 HV and 49 GPa.