An investigation into the influence of microstructural condition on the superelastic behaviour in Ti-Nb-based alloys.
Creators: name: Hildyard, Emma Mary
Publisher: name: Apollo - University of Cambridge Repository
Year: 2019
Subjects: Titanium
titanium alloys
martensite
phase transformations
stress-induced phase transformation

description: Ti-Nb based alloys show superelastic and shape memory properties over a wide range of temperatures, making them attractive for industrial applications. However, within the literature there are significant variations in the reported martensite start temperatures that have not been rationalised. To investigate the source of these discrepancies a model alloy, Ti-24Nb (at.$\%$), has been studied in two microstructural conditions using a variety of analytical techniques. The evolution of the phase constitution of the material during a five step thermomechanical cycle was determined, $\textit{in situ}$, using synchrotron radiation. To expand this dataset, the load-unload cycle was repeated at different temperatures in the range of -196$^{\circ}$C to 30$^{\circ}$C. The $\alpha^{\prime\prime}$ martensite was observed to form when cold rolled material was cooled. In contrast, no evidence of the $\beta$ to $\alpha^{\prime\prime}$ transformation was observed in solution treated material, despite cooling to -196$^{\circ}$C under identical conditions. Further transformation could be induced in both samples through the application of an external stress. These results highlight that the transformation behaviour in Ti-24Nb (at.$\%$) is significantly influenced by microstructural condition. In light of these results, it was proposed that the martensitic transformation can be explained in terms of type I, II and III stresses rather than being solely reliant on the key transformation temperatures. To elucidate this, preliminary experiments investiging the effect of grain size and dislocation density on the transformation behaviour in Ti-24Nb-4Zr-8Sn (wt.$\%$) were conducted. Changes in the grain size did not alter the transformation behaviour as significantly as the dislocation density, however, further investigations are required to validate this. Four Ti-Nb alloys were subjected to the same thermomechanical cycles, $\textit{in situ}$, and the true martensitic start temperatures were determined and compared with previously reported datasets in the literature. The new total stress based mechanism was then used to rationalise why there is a significant amount of variation in the reported martensite start temperatures. Ti-Nb based alloys exhibit superelastic and shape memory properties over a wide range of temperatures, making them attractive for industrial applications. However, within the literature there are significant variations in the reported martensite start temperatures and associated behaviour that have not been fully explained. These variations are problematic to engineering design, thereby limiting the industrial uptake of these alloys. To investigate the source of these discrepancies a model alloy, Ti-24Nb (at.$\%$), has been studied in two different microstructural conditions, cold rolled and solution heat treated, using a variety of different analytical techniques. Initial characterisation of the material was performed using X-ray diffraction and electron microscopy. Information relating to the evolution of the material in response to changes in stress and temperature were obtained \textit{in situ} using synchrotron radiation. These diffraction data were used to directly characterise the phase constitution of the alloy during a five step thermomechanical cycle. To expand this dataset, this cycle was repeated a number of times to enable loading at different temperatures in the range of -196$^{\circ}$C to 30$^{\circ}$C. The $\alpha^{\prime\prime}$ martensite was observed to form when cold rolled material was cooled. In contrast, no evidence of the $\beta$ to $\alpha^{\prime\prime}$ transformation was observed in material that had been solution heat treated, despite cooling to -196$^{\circ}$C under identical conditions. Further transformation could be induced in both samples through the application of an external stress. In light of these results, it was proposed that the martensitic transformation that gives rise to superelastic behaviour can be explained in terms of type I, II and III stresses rather than being solely dependent on the key transformation temperatures. The observed results highlight that the transformation behaviour in Ti-24Nb (at.$\%$) is significantly influenced by microstructural condition. To elucidate this further, the effect of grain size and dislocation density on the transformation behaviour in Ti-24Nb-4Zr-8Sn (wt.$\%$) have been briefly investigated. The results indicated that the grain size did not alter the transformation behaviour as significantly as the dislocation density, however, these experiments were preliminary and further investigations into the microstructural features that change the superelastic properties exhibited by these alloys are required. The use of $\textit{in situ}$ synchrotron diffraction experiments are recommended to identify the true martensitic start temperature for an alloy in a given microstructural condition. Finally, the effect of niobium content on the true martensitic start temperatures and form of the stress-strain behaviour were obtained and compared with previously reported datasets in literature. The new total stress based mechanism was then used to rationalise why there is a significant amount of variation in the previously reported martensite start temperatures.
description: iCASE in collaboration with Rolls Royce