Study of fatigue properties of forged BT25 titanium alloy based on fractographic and quantitative microstructural analysis

Abstract

The high cycle fatigue (HCF) properties of forged BT25 titanium alloy was examined by fractographic and quantitative microstructural analysis. Two different height reductions (50 and 80%) were applied to the isothermal compression of BT25 alloy with initial lamellar microstructure. Fatigue limits obtained through up and down method were 440 and 450 MPa respectively. Microstructural analysis showed that the globularisation volume fraction of the two microstructures (MA represents the microstructure of the alloy with the height reduction of 50%, while MB is used for 80%) were 62?72 and 84?13% respectively. There was more lamellar a in which Feret ratio was greater than 3?5 remained in MA, 16?96%. Hence, a larger globularisation volume fraction is more beneficial for resistance to fatigue failure. The fracture surfaces of the failed testing specimens were examined and the fatigue crack initiation mechanisms were discussed. Cracks initiated in grains by means of constituting extrusions through combined glide and dislocation annihilation in slip bands, while those initiating along the a/b phase interface arose from discordant deformation of a and b phases with different Young's modulus.

High-cycle fatigue behaviour and mechanism analysis of a forged TiZr-based alloy

“…Severe local stress concentration is produced due to extensive dislocation pile-up at α/β interfaces, which make the α/β interfaces become preferential nucleation sites of cracks in titanium alloys. 29–31 It is also demonstrated that α phase as the softer phase yields first and dislocation sources are activated to accommodate strain.…”

Research Progress on the Creep Resistance of High-Temperature Titanium Alloys: A Review

“…The near-α high-temperature titanium alloys take into account the high creep strength of α titanium alloys and the high-temperature instantaneous strength of α + β titanium alloys. The representative alloys are Ti6242S [74] and Ti1100 [75] (U.S.A.); IMI829 [76] and IMI834 [9] (U.K.); BT25 [77], BT18y [78], and BT36 [79] (Russia); and Ti55 [80] and Ti60 [81] (China). The main features of the burn-resistant titanium alloys is the very high Mo equivalent, and the content of β stable elements is as high as 40 wt.%.…”