Aerospace engineering demands the use of materials of extra superior mechanical properties coupled with excellent physical resilience. So far, no other materials match such a demanding definition better than titanium-based alloys. Titanium alloys have been demonstrated to serve as critical load bearing elements at severe environments. As such, interest has sparked with much focus being on the factors that make them quite resilient at severe conditions. Researchers have particularly focused on five typical types of microstructures of titanium alloy under different hot working conditions, i.e., equiaxed microstructure, bimodal microstructure, basketweave microstructure, widmanstätten microstructure and tri-modal microstructure.
The tri-modal microstructure consisting of equiaxed alpha (αp), lamellar alpha (αl) and beta (β) transformed matrix has been reported to possess a possibility to reach the excellent combination of strength-ductility-toughness properties. To this end, much has been published; however, the cyclic stress response, property feature, internal fracture mechanisms and low-cycle fatigue (LCF) property-microstructural parameter relationship of tri-modal microstructure are still unknown.
Recently, Northwestern Polytechnical University researchers led by Professor Pengfei Gao from the State Key Laboratory of Solidification Processing investigated the LCF behavior and property characteristics of tri-modal microstructure systematically based on the cyclic stress response and fractography analysis. The team also aspired to quantitatively study the effects of microstructural parameters on LCF property of tri-modal microstructure. Their work is currently published in the research journal, Materials Science & Engineering A.
In brief, they started by obtaining tri-modal microstructures by utilizing a three-step heat treatment schedule. Next, the heat-treated samples were subjected to fatigue tests and axial total strain measurements. The resultant deformed sampled were then characterized using scanning electron microscopy. The main material used for the study was TA15 titanium alloy.
The authors observed that at different strain amplitude (εta) levels, the cyclic hardening/softening was governed by the competition (εta < 0.9%) or superposition (εta ≥ 0.9%) effect of the variations of back stress and friction stress. Further, fractography showed remarkably different features at different εta levels. When εta < 0.9%, only one fatigue crack initiation site activated by the dislocation pile-ups at αp/βt and αl/βt interfaces was reported. Moreover, narrow fatigue striation space in fatigue crack propagation region reported implied a relatively slower crack propagation.
In summary, the study presented an in-depth experimental investigation of the low-cycle fatigue behavior and property feature of TA15 titanium alloy with a tri-modal microstructure. The observed divisional LCF behavior and fracture features highlighted a two-part linear Coffin-Manson relationship. They suggested that increasing equiaxed α content could delay the fatigue crack nucleation and propagation due to its positive effect for improving deformation compatibility. Altogether, the results presented will act as a guide on material selection enabling realization of optimum properties of such materials.
P.F. Gao, Z.N. Lei, Y.K. Li, M. Zhan. Low-cycle fatigue behavior and property of TA15 titanium alloy with tri-modal microstructure. Materials Science & Engineering A, volume 736 (2018), page 1–11.Go To Materials Science & Engineering A