Low-cycle fatigue properties of a titanium alloy exhibiting nonlinear elastic deformation behavior

Strain-controlled low-cycle fatigue (LCF) behavior of a metastable β-type titanium alloy Ti–24Nb–4Zr–8Sn (wt.%, termed Ti2448) was investigated at room temperature under different cycle strains. The results showed that the alloy possesses good LCF resistance because applied total cycle strains up to 4.5% are endured mainly by the recoverable elastic strain. The study also revealed a dependence of the elastic deformation behavior on the loading condition: nonlinear elasticity with large recoverable strains in uniaxial tension but almost linear elasticity with higher elastic modulus and lower recoverable strain in uniaxial compression. The above elastic asymmetry explains the facts that both fatigue life in LCF and fatigue strength in high-cycle fatigue are worse in tension–compression cycles than in tension–tension cycles. Microstructure analyses of these fatigue-tested specimens showed that the β phase is elastically stable at cycle stresses up to ∼400 MPa whereas further increasing the applied cycle stress and strain results in small amounts of the ω phase and the α′′ martensite. These two stress-induced phase transformations are competing and exclude each other across a wide range of cycle stress.