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Title: Diffraction experiments on superelastic beta titanium alloys
Author: Joris, Oliver Pieter Johnathan
ISNI:       0000 0004 6059 1041
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2015
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This thesis investigates superelasticity in metastable beta titanium alloys that contain substantial additions of Mo, principally. Superelasticity arises from a reversible transformation from the β to the orthorhombic alpha double prime (α") phase during loading, which was studied principally using in situ synchrotron X-ray diffraction at the I12 beamline at the Diamond synchrotron. Superelastic β-Ti alloys have the potential to be low weight, economical alternatives to NiTi, Gum metal and Ti-2448 for biomedical, military and aerospace applications. The cubic to orthorhombic stress induced martensitic phase transformation is reversible but has an associated permanent deformation. Reducing the permanent deformation associated with cyclic strain is key to the commercial use of these alloys. The effect of β stability on the SE recovery of Ti-Mo, Ti-Mo-Al and Ti-Mo-O alloys has been investigated. Superelastic behaviour was recreated using cyclic strain whilst being examined under in-situ synchrotron X-ray diffraction. Study of the superelastic phase transformation was carried out in-situ due to the reversible nature of the transformation causing the superelastic phase to largely disappear upon unloading. The superelastic behaviour is shown to be sensitive to β stability and in turn composition and temperature. The addition of aluminium and oxygen can be used to enhance both superelastic recovery and strength. The alloys were designed using Morinaga's orbital design approach combined with Laheurte's average valence electron values, tailoring the bond order (Bo) and electronegativity (Md) in order to alter the method of deformation and phase stability. The third alloy design factors considered concern the effect of alloying on the C' modulus of the β, the ω stability and martensite transformation temperatures. Together these methods, whilst semi-empirical, provide a rational basis for alloy design. Both the lowered stiffness and lower apparent stiffness associated with the design method and the transformation respectively could lead to lowering the stiffness of beta Ti alloys towards that of cortical bone. This would reduce the stiffness mismatch that promotes bone re-absorption around surgical implants, reducing the need for implant replacements. Also, the recent developments in eradicating the residual strain associated with the transformation has led to interest from the aerospace industry for possible (high temperature) damping applications. The Ti-Mo binary mechanical curves show a correlation between the apparent yield stress and composition. An increase in Mo concentration from 7.2 at.% was shown to decrease the yield stress to a minimum at 8.2 at.% Mo, after which the yield stress increased. Out of the 5 binary samples, Ti-8.7Mo at.% showed the best superelastic recovery with a recovery of 1.58% strain for a total strain of 2.3%. The yield stress minima is indicative of the composition at which Ms is closest to room temperature. Ti-8.2Mo at.% has the lowest apparent yield stress and Ti-8.7Mo at.% has the largest SE recovery; this correlates well with the theory that the best superelastic behaviour should be observed for an alloy whose composition places it just above As at room temperature. Al additions, which promote the α phase, were found to significantly improve the superelastic behaviour. Al also acts to suppress the ω phase. The ternary alloy Ti-8.1Mo-5Al at.% showed a 95% strain recovery from a total strain of 1.6% at room temperature. Oxygen behaved as a solution strengthener increasing the yield stress and the associated elastic recovery without impeding the austenitic strain recovery transformation. The addition therefore improved both the apparent yield stress and the SE recovery.
Supervisor: Dye, David Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral