Microstructural and Mechanical Property Development in Metastable Beta Titanium Alloys

Titanium alloys represent an ever increasing proportion of the materials employed in aerostructural applications. Metastable beta alloys in particular, offer high specific strength and good corrosion resistance that allow them to compete with steels. Ti-5Al-5Mo-5V-3Cr and Ti-10V-2Fe-3Al are two such alloys used in the main landing gear of large commercial aircraft. Thermomechanical processing of metastable beta alloys is critical in obtaining the desired microstructure, which in turn governs the mechanical properties. This therefore demands a thorough understanding of the relationship between processing, microstructure and mechanical properties in order to optimise the final product and process route. This project characterises the microstructural and mechanical property variation within forged Ti-5553. Microstructural variables are quantified in order to examine their relative influence on mechanical properties. This reaffirmed the importance of microstructural control during materials processing. Gradual changes in primary alpha morphology, beta volume fraction and grain structure were observed throughout the forgings. However, it was also found that the size of secondary alpha precipitates could fluctuate rapidly over relatively short distances. The effect on mechanical properties was significant enough to completely reverse the general trends exhibited over the entire forging. It was also found that the heat treatment response varied with orientation. It would appear that unspecified microstructural variables limited the maximum achievable properties in certain orientations, preventing the heat treatment from further affecting them. However, changes in work hardening behaviour were observed which increased the proof stress while leaving the tensile strength essentially unchanged. The influence of subtransus thermomechanical processing on the microstructural evolution of Ti-10-2-3 was also investigated. Flow curves exhibited an initial peak at low strain followed by extensive flow softening. Microstructural analysis would suggest that the fragmentation and globularisation of acicular alpha particles is at least partially responsible for this softening effect. The use of torsion tests demonstrated that non-linear strain paths may not represent an efficient means of globularising primary alpha. The Burger’s Orientation Relationship (BOR) was found to break down at a linear strain of about 0.5. However, the process of ‘strain reversal’ could partially restore this up to an original linear strain of around 0.8. Solution treatment and ageing revealed that more highly strained regions were less responsive to age hardening.