Abstract
This study deals with the influence of microstructure on the fatigue crack growth resistance of αβ titanium alloys: Ti-6Al-4V ELI (Extra Low Interstitial) that may compete with the conventional Ti-6Al-4V alloy in the manufacture of high performance aircraft. Six different microstructures have been considered: the as-received bimodal microstructures and five distinct fully lamellar microstructures. The characteristic parameters of these microstructures were determined and crack growth tests were performed with crack closure measurements in order to evaluate the shielding effect induced by closure. A comparison of crack growth rates, fracture surfaces, and crack path was carried out for the different microstructures. The results outline a transition between two propagation regimes from a microstructure-sensitive to microstructure-insensitive propagation.
Highlights
Titanium alloys are nowadays widely used in structural parts in the aeronautic industry especially due to their high strength to weight ratio
In addition the five lamellar microstructures studied showed a nearly similar the behavior with a superior resistance up to ∆K = 50 MPa m as compared to the bimodal microstructure
The first point is that, even by taking into account the shielding induced by crack closure when expressing the growth rates as a function of effective stress intensity range, it appeared that the lamellar microstructure still presented a higher resistance to crack propagation than the bimodal microstructure
Summary
Titanium alloys are nowadays widely used in structural parts in the aeronautic industry especially due to their high strength to weight ratio. It is well established that the change from a bimodal microstructure, composed of nodules and lamellae colonies, to a fully lamellar microstructure leads to a significant improvement in crack propagation resistance [1,2,3,4,5]. This improvement is explained by the more tortuous path of the crack through the microstructure and a higher roughness of the crack surface, leading to slower macroscopic crack propagation rates [6,7,8]. The influence of microstructural parameters (prior β grain and colony) of this lamellar structure has been studied [7,8,9] and shows an improvement of damage resistance with increasing prior β grain or colony size
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