Development of novel Ti-Nb based alloys for biomedical applications
Manogar, B. (2021). Development of novel Ti-Nb based alloys for biomedical applications (Thesis, Master of Engineering (ME)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14589
Permanent Research Commons link: https://hdl.handle.net/10289/14589
Since the advent of industrialisation, the interaction between humans and machines have become imminent. In doing so, unfortunate accidents are bound to occur, resulting in severe injury or loss. On the other hand, with the advancement of modern medicine, many pathological diseases have been eradicated. This has caused the overall life expectancy to increase. As a result, the population of the elderly is increasing day by day. Subsequently, due to old age, accidents and poor lifestyle, joints and bone tend to experience premature fatigue. In order to overcome this issue, patients prefer to use implant materials to restore the functionality of the injured part of the body. A few common examples where implant materials are employed are in total knee and hip replacement surgery and spinal fixation. Since there is a structural (load bearing) component to these implants, among various classes of materials such as ceramics and plastics, metallic biomaterials are deemed to be an appropriate fit. Among the metallic materials, stainless steel (SS 316L), Co-Cr alloys and Ti alloys are the primary choice. SS 316L and Co-Cr alloys have been used as structural implant materials for a couple of decades. However, it is important that the material does not induce any immune response or in any way affect the homeostasis of the patient. Taking this into consideration, Ti has been confirmed to be the most favourable choice for structural implant materials as it is biocompatible and has high specific strength i.e. it possesses high strength to weight ratio. This becomes very critical especially in terms of the comfort of the patient. Ti-Nb based alloys are considered to be effective in employing as an implant material. Thus, two systems of Ti-Nb based ternary alloys were developed by adding manganese (Mn) and iron (Fe). Among various Ti alloys, Ti-Nb alloys have been scarcely explored and elements such as Mn and Fe, becomes especially significant in terms of availability, ease of processing and the biocompatible nature. Powder metallurgy is chosen as the processing route owing to its capability to incorporate a wide range of alloying elements, the near net shape products prevent material wastage and the possibility to control the porosity. Using Mn and Fe becomes critical while processing via powder metallurgy as the primary means of densification is diffusion and both Mn and Fe have high diffusivity with Ti. Since both Ti-Nb-Mn and Ti-Nb-Fe systems have not been explored to a larger extent, 8 compositions from Ti-Nb-Mn and 12 compositions from Ti-Nb-Fe were designed. So, 8 Ti-Nb-Mn alloys and 12 Ti-Nb-Fe alloys along with C.P. Ti as reference were warm compacted at 250oC, vacuum sintered at 1300oC and characterised for various physical and mechanical properties. The relative green density were found to be above 85% and relative sintered density ≥ 95%. Hence, the maximum porosity was about 5% and the least was around 2%. Warm compaction of the alloys helped to achieve a higher green density when compared to conventional cold pressing of elemental powders. Based on the microstructural analysis by optical and electron microscopy, most of the alloys were characterised by Widmänstatten microstructures with varying levels of refinement in the α+β lamellae and the interlamellar spacing due to the effect of adding β stabilisers. Since all the alloys have achieved a relative density above 95%, most of the alloys consists of closed pores. The size and distribution however varied depending on the density values. Mechanical properties were measured by conducting a uniaxial tensile test and Rockwell hardness test. It was found that with the addition of alloying elements leads to solid solution strengthening and as a result the strength increases with the addition of Nb, Fe and Mn. The hardness also follows a similar trend with most of the alloys, except a few that were influenced by porosity. Among Ti-Nb-Mn alloys, Ti-6Nb-8Mn exhibited the highest UTS of 1090 MPa and Ti-5Nb-5Mn exhibited the highest elongation to failure. For the Ti-Nb-Fe system, Ti-9Nb-6Fe possesses the highest UTS and Ti-2Nb-3Fe showed the highest elongation to failure. Among both the system, Ti-Nb-Mn alloys possessed high strength and relatively very low ductility, whereas it Ti-Nb-Fe alloys had a good combination of strength and ductility. In order to improve the performance of the alloys to be used as implant materials, the near β and metastable β alloys can be subjected to thermo-mechanical processing or heat treatment.
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