Influence of hydride-induced microstructure modification on mechanical properties of metastable beta titanium alloy Ti 10V-2Fe-3Al

Abstract

The five-step thermohydrogen treatment (THT) “hydride-induced recrystallization of the β phase” (HIRB) aiming at improvement of the high cycle fatigue limit was successfully developed and implemented for the high-strength β titanium alloy Ti 10V-2Fe-3Al. The possibility of establishing an advantageous microstructure modification depends sensitively on the THT process parameters, particularly in terms of recrystallization and dehydrogenation. The benefit of the HIRB-THT process results from the reduced volume fraction of the α phase at β grain boundaries, the disappearance of the equiaxed primary α phase associated with the increased driving force for the secondary α phase as well as the grain hardening by fine β grains. The formation of hydrogen-induced ternary phases, such as TiFeH and TiFeH2, is the microstructural key feature to induce recrystallization of the β matrix. The hydride formation is based on the hydrogen-induced redistribution of alloying elements. The peak-aged THT microstructure shows an improvement of fatigue crack initiation resistance by about 16 MPa and causes an increase in tensile strength by about 60 MPa, which is accompanied by an expected decrease in tensile ductility. The raise of fatigue life is not strongly pronounced by the hydride-induced microstructural modification as expected. It can be concluded that the observed preferred β crystal orientation after recrystallization influences strongly the fatigue limit because of the corresponding anisotropic behavior.