Effect of thermomechanical processing on the mechanical properties of cost-effective powder metallurgy titanium-based materials
Romero Villarreal, C. (2020). Effect of thermomechanical processing on the mechanical properties of cost-effective powder metallurgy titanium-based materials (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/13537
Permanent Research Commons link: https://hdl.handle.net/10289/13537
Despite their excellent balance of properties, titanium alloys are still a niche material whose range of applications is limited by the high cost of their manufacturing. With recent developments, powder metallurgy (PM) approaches, in particular the blended elemental (BE) route, are able to reduce considerably the cost of manufacturing while keeping a reasonable performance, mostly in terms of tensile strength and ductility. The cost reduction of the BE approach derives from the simplification of the process by removing a large amount of re-melting and ingot breakdown operations, regardless of the use of the approach as a net-shape technology. However, in terms of fatigue behaviour, the performance of these alloys is generally worse than conventional processes unless additional steps are applied. This thesis aims to study the mechanical behaviour, with emphasis on the fatigue behaviour, of two cost-effective titanium alloys in order to find the key processing, microstructural and texture parameters that control their performance, with the goal of achieving similar properties to conventional wrought alloys with a reduction of the cost. These alloys, a conventional Ti-6Al-4V composition and a non-conventional low-cost Ti-5Fe composition, were processed following the blended elemental approach and thermomechanical processing, via hot extrusion, was subsequently applied. A variety of microstructures and textures were achieved under different extrusion conditions, which were further modified by applying additional heat treatments. The tensile behaviour of both Ti-6Al-4V and Ti-5Fe alloys is strongly dependent on the processing conditions. Fully lamellar microstructures (from processing in the β phase) showed similar strength levels but considerably lower ductility compared to alloys with bimodal microstructures and stronger texture (from processing in the α+β phase). In the case of Ti-5Fe, moreover, there was a Fe partitioning effect that, when the processing temperature is low enough, leads to a sharp increase in both the strength and the ability to deform past necking took place. Heat treatments in Ti-6Al-4V alloys were useful in order to increase the strength as well as to keep the ductility similar or even increase it to higher values. The tensile behaviour of Ti-5Fe alloys was more tuneable, achieving higher differences in strength and ductility with the as-extruded conditions, even though this happened by trading-off. Overall, Ti-6Al-4V alloys outperformed Ti-5Fe alloys as their strength was typically higher (associated to their higher amount of alloying elements) as well as their ductility, due to their higher ability to deform past necking, which was very limited for Ti-5Fe alloys in most cases. The fatigue behaviour of the alloys was found to as dependent on the chemistry as on the microstructure and the texture. While the Ti-6Al-4V alloy is not affected by the residual porosity left after thermomechanical processing, the Ti-5Fe alloy shows pores in the region where fatigue damage begins. In the Ti-6Al-4V alloy, it was found that texturing had a much stronger effect on the fatigue strength of the alloy than microstructural refinement. Therefore, the PM Ti-6Al-4V alloy processed in the α+β phase had a higher fatigue strength than alloys processed in the β phase, regardless if the latter has fully lamellar or acicular microstructure. In the case of the Ti-5Fe alloy, two conditions processed in the α+β phase were studied, and it was found that the Fe concentration in transformed β (which is higher when there is a larger amount of primary α) had a critical effect on the fatigue behaviour, overriding the effect of texture or microstructural refinement. This leads to the Ti-5Fe alloy processed at a lower temperature in the α+β phase showing a much lower fatigue strength compared with the alloy processed just below the β transus, despite the fact that its texture and microstructure were, in theory, optimal for fatigue behaviour. It is concluded that, in order to process cost-effective Ti alloys with fatigue performance comparable to wrought alloys with conventional compositions (with fatigue strength in the range of 650 MPa), thermomechanical processing in the α+β phase must be applied to make use of the strong texturing of the alloy. However, in the case of alloys containing β-eutectoid elements like Fe, the range of temperatures is limited as the partitioning of the alloying element results in severe reduction of the fatigue strength.
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