Influence of microstructure of (α+β) Ti‐6.2.4.6 alloy on high‐cycle fatigue and tensile test behaviour

Tensile and gigacyclic fatigue behaviour of Ti–6Al–2Sn–4Zr–6Mo alloy were investigated as a function of lamellar primary α‐ and β‐transformed microstructures. Three thermomechanical processes (TP1, TP2, TP3) were selected to produce different combinations of microstructural features on two slightly different compositions of the alloy (A and B). Ultrasonic fatigue tests were performed in air and liquid nitrogen at a frequency of 20 kHz (R = −1, T = 300 and 77 K), giving fatigue tests up to 10⁹ cycles. Microstructural features and the fracture initiation dependence on the primary α lamellar phase were observed by SEM and/or characterized by quantitative image analysis. It has been found that the microstructure of alloy B produced by TP1 represents a better compromise between resistance to initiation and resistance to microcrack growth. Quicker initiation occurs in coarser α‐platelets (TP2, alloy B), and the continuous partially transformed β matrix appears to effectively decrease the tensile and HCF resistance. The bimodal structure (TP3, alloy B) has the best resistance at room temperature, but the presence of a coarse globular phase decreases this fatigue resistance at low temperature.