Environmentally assisted cracking on titanium alloys

Titanium alloys have been widely used as the structural components in gas turbine engines for aerospace applications, owing to the combined benefits of the high specific strength, good corrosion resistance and excellent fatigue properties at the temperature range 300-600 °C. Failure associated with AgCl-induced HSSCC was firstly identified in 1966, on a titanium compressor disc in a spin test. However, the reaction chemistry and cracking mechanisms have been rarely studied and still remain unknown. This PhD project, sponsored by Rolls-Royce plc, aims to investigate the environmentally assisted cracking of titanium alloys, specially Ti-6246 and Ti-64. The main motivation of this study is to explore AgCl corrosion in Ti-6246, regarding the trigger conditions, reaction sequences and cracking mechanisms. Given the complex operating conditions in engines, there exist several stress/temperature windows. It is thus crucial to understand the role of each regime in the overall cracking kinetics. In addition, the fatigue crack growth behaviours and striation formation were studied in Ti-64 at different test frequencies, to examine the effect of oxygen accessibility on striation formation. Chemical analyses were performed by SEM/STEM-EDX, high resolution TEM and XRD, and oxide-Ag-oxide multilayered structures were found above and within the crack. The oxide mainly consisted of a mixture of TiO2, SnO2 and Al2O3. A proposed model of reaction sequence was established based on experimental observations and thermodynamic analysis. Hydrogen embrittlement (HE) was suggested as the possible responsible cracking mechanism, although the possibility of solid metal embrittlement (SME) could not be ruled out because of the presence of metallic Ag. With regards to the different stress/temperature windows in service, it was revealed that overall cracking rate was controlled by the HSSCC regime. However, adding extra moisture at every cold cycle of the cyclic tests was found to cause the most severe cracking with pitting, implying the occurrence of aqueous HCl attack. The fatigue crack growth rates of Ti-64 corned-cracked specimens, tested at different frequencies, were characterised by potential drop method and striation counting. The crack propagation and striation formation showed no profound dependence on the test frequency. The dislocation structures underneath the striation profiles were compared between a hollow HCF sample partially tested in vacuum and the corned-cracked specimens tested in air. There was no observation of significant change in slip activities, because the striations on the hollow sample were believed to be formed in air. Future work was suggested to complete this study.Open Acces