Fatigue of Beta Processed and Beta Heat-treated Titanium Alloys

This publication reviews most of the available literature on the fatigue properties of β annealed Ti-6Al-4V and titanium alloys with similar microstructures. The focus is on β processed and β heat-treated alloys because β annealed Ti-6Al-4V has been selected for highly loaded and fatigue-critical structures, including the main wing-carry-through bulkheads and vertical tail stubs, of advanced high-performance military aircraft.

An important aspect of the review is a concise survey of fatigue life assessment methods and the required types of fatigue data. This survey provides the background to recommendations for further research, especially on the fatigue behaviour of β annealed Ti-6Al-4V under realistic fatigue load histories, including the essential topic of short/small fatigue crack growth. Such research is required for independent fatigue life assessments that conform to the aircraft manufacturer’s design requirements, and also for life reassessments that most probably will have to be made during the service life of the aircraft.

Contents.-   1 Introduction.-   1.1 Survey of fatigue life assessment methods.-   2  Metallurgy and microstructure.-   2.1 The general metallurgy of titanium alloys.-   2.2 Conventional thermomechanical processing of alloys like Ti-6Al-4V.-   2.3 β processing and/or β heat-treatment.-   2.4 Crystallography of the β → α phase transition.-   2.5 Microstructure of β annealed Ti-6Al-4V ELI thick plate.-   3  Fatigue crack initiation.-   3.1 Microstructural initiation sites in near-α and α – β alloys: literature.-   3.2 Microstructural initiation sites: β annealed Ti-6Al-4V ELI thick plate .-   3.3 Metallurgical defects.-   4  Fatigue initiation lives.-   4.1 Microstructural factors.-   4.2 Fatigue life − microstructure trends.-   4.2.1 Unnotched (smooth specimen) fatigue.-   4.2.2 Notched fatigue.-   4.3 Summary.-   5  Short/small fatigue crack growth.-   5.1 Introduction.-   5.1.1 Significance of short/small cracks.-   5.1.2 Definitions of short and small cracks.-  5.1.3 Size criteria.-   5.2 Short/small fatigue crack growth data from the literature.-   5.2.1 Coarse-grained fully lamellar IMI 685.-   5.2.2 Effects of microstructure: fully lamellar.-   5.2.3 Effects of microstructure: different types of microstructures.-   5.3 Summary.-   6 Long/large fatigue crack growth.-   6.1 Introduction.-   6.2 Fatigue thresholds.-   6.2.1 Fully lamellar microstructures.-   6.2.2 Different types of microstructures.-   6.3 Regions I and II fatigue crack growth in fully lamellar microstructures.-   6.4 Regions I and II fatigue crack growth in different types of microstructures.-   6.5  Region II bilinear log da/dN – log ΔK fatigue crack growth.-   6.6  Summary.-   7 Concluding remarks .-   8  References.