Abstract
Microstructural evolutions and resulting mechanical properties have been investigated in the near-β Ti-17 alloy following long- term ageing heat-treatment up to 6000 h at 450 °C. The initial microstructure was bimodal lamellar, consisting of two populations of α grains (αlam-primary and αsecondary) in a β phase matrix. Two microstructures were obtained either via controlled heat- treatments from the β phase field - in order to generate significant differences in the grain fraction, size, density and spatial distribution - or sampled from a part submitted to an industrial processing route. High energy XRD reveals that whatever the initial microstructure, the amount of α phase increases significantly after 1000 h long-term ageing. Complementary SEM and image analysis characterizations enable to deduce that this evolution is the consequence of αsecondary growth and/or coarsening. Also, TEM observations and EDX analysis show that the Mo and Cr contents of the β phase increase and that α2 nano-precipitates form within the αlam-primary grains. Considering the mechanical properties, long-term ageing leads to an increase in the yield and ultimate tensile strength, as well as a decrease in the elongation at failure, at an extent which depends on the ageing time.
Highlights
Due to their excellent specific mechanical properties, titanium alloys are widely used for the manufacturing of aeronautical parts
The present study focuses on the microstructural evolutions during long-term ageing of bimodal lamellar microstructures, consisting of two populations of α grains in a β phase matrix, in the near-β Ti-17 titanium alloy
Microstructural characterizations were performed by electron microscopy, complemented by image analysis and chemical analysis, while the mechanical properties were evaluated by tensile testing
Summary
Due to their excellent specific mechanical properties, titanium alloys are widely used for the manufacturing of aeronautical parts. The present study focuses on the microstructural evolutions during long-term ageing of bimodal lamellar microstructures, consisting of two populations of α grains in a β phase matrix, in the near-β Ti-17 titanium alloy. Such thermal long exposure follows the usual thermal cycle performed on parts after forming steps, which are usually referred to as “ageing” treatments. Even though the formation of silicides is not expected for Ti-17 alloy, the one of α2 phase could occur, but its presence before long-term ageing should be checked This latter point is further investigated in the present study. Microstructural characterizations were performed by electron microscopy, complemented by image analysis and chemical analysis, while the mechanical properties were evaluated by tensile testing
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