Characterising the microstructure and properties of aerospace β-21S titanium alloy during thermal ageing

The commercial titanium alloy !-21S is becoming an increasingly attractive option for hightemperature applications, mostly due to its creep and oxidation resistance. Its thermal properties allegedly permit safe operation at temperatures up to 650°C, making it a popular choice for aircraft engine components. Despite this, it has been reported that the mechanical properties of aircraft components manufactured from !-21s are changing unfavourably when exposed to temperatures up to an estimated 550°C – a range in which the alloy is supposedly thermally stable. Consequently, it is necessary to investigate the alloy's behaviour up to 550°C, and monitor changes to its microstructure and mechanical properties over time. This study investigated the age-hardening response of !-21S when exposed to temperatures of 425°C and 525°C, for varying time periods up to 9 days. The response was characterised using microscopy techniques to analyse the microstructure, which was correlated with hardness measurements. Hardness testing indicated that the alloy’s age-hardening behaviour was highly sensitive to different temperatures. Ageing at 425°C resulted in a peak hardness of 504 HV after 9 days. Conversely, at 525°C, a lower peak hardness of 429 HV was achieved but after only 1 day. This corresponded to hardness increases of 75% and 48% from the un-aged condition, respectively. Development of the ageing microstructure showed similar trends at both temperatures; fine precipitates nucleated and grew with prolonged ageing, in clusters at grain boundaries and interiors. This trend continued at 425°C until precipitates dominated the microstructure, while ageing at 525°C for over 4h produced a basket-weave microstructure of acicular precipitates. The relationship between hardness and microstructure was based on the volume fraction and distribution of precipitates. Maximum hardening was associated with finer, more uniformly distributed precipitates creating effective barriers for dislocation movement, achieved at lower temperatures. It was speculated that indirect decomposition of ! into the intermediate phase $ at lower temperatures promoted homogenous distribution of . Coarser and longer precipitates observed at 525°C resulted in less effective hardening but improved thermal stability, with a relatively consistent microstructure and hardness maintained after ageing for 4h. Increases in hardness are inversely proportional to ductility, and further mechanical testing is required to quantify changes to ductility in aged !-21S. By nature, reduced ductility can increase the risk of embrittlement cracking, which is critical for aged alloys in applications such as aircraft where safety is of high consideration. It is not recommended for severely hardened alloys to be used in environments where they will be susceptible to cracking, without extensive testing to first gauge their suitability for the application.