Cyclic deformation characteristics of the metastable β-type Ti–40Nb alloy

Abstract

The present study investigates the cyclic deformation behavior of a new metastable β-type Ti–40Nb alloy (wt%) in two different microstructural conditions. Severe cold rolling followed by recrystallization annealing were performed on cast and homogenized Ti–40Nb rods to obtain a single β-phase microstructure at room temperature. Additional aging at 573 K was carried out to generate isothermal ω-phase precipitates in the β-matrix. Fatigue tests at cycles up to 2 × 106 and a stress ratio of R = −1 were realized with samples with electrochemically polished surfaces. EBSD analysis was carried out after the fatigue tests. Furthermore, detailed fractographic investigations as well as TEM analysis were executed. Results revealed significant differences in the cyclic deformation behavior and a higher fatigue strength for the aged condition. Reasons for superior fatigue properties of the aged condition are a pronounced precipitation hardening effect of the ω-phase as well as a complete suppression of stress-induced martensite formation and deformation twinning due to a barrier function of the ω-precipitates. In the vicinity of the fatigue crack tip, where the localized plastic zone dominates the damage evolution, precipitation-depleted channels can be observed. Within these ω-depleted channels dislocations have an increased mobility, allowing highly localized plastic deformation. Signs of other deformation features with increasing distance to the fatigue crack are not observed. EBSD analysis of the recrystallized samples with initial single β-phase showed on the contrary changes in the microstructure caused by the cyclic loading and the high instability of the β-phase. A dominant development of stress-induced α’’-martensite towards a full martensitic microstructure dependent on the applied stress amplitudes and cycles as well as additional isolated {332} <113> twinning were detected.