Effects of ductile laminate thickness, volume fraction, and orientation on fatigue-crack propagation in Ti-Al3Ti metal-intermetallic laminate composites

Abstract

Fatigue crack propagation (FCP) has been studied in a new class of materials termed metal-intermetallic laminate (MIL) composites (Ti-Al3Ti). Due to ease of fabrication and control over layer makeup, these MIL composites can be tailored to optimize the constituent properties for structural and higher performance aerospace applications. Effects of ductile reinforcement (titanium alloy) type, thickness, and volume fraction were systematically investigated in both arrester and divider orientations. Stress intensity (Kmax) values as large as 40 MPa√m were observed in the higher crack growth regime, indicating that the fracture toughness of the MIL composites is comparable to common structural metals. In both divider and arrester orientations, the overall fatigue crack growth rate showed an improvement with increasing Ti volume fraction and with increasing Ti thickness (at constant ductile-phase volume fraction). It is noted that the fatigue resistance of monolithic Al3Ti was improved by an order of magnitude by incorporating just 20 vol pct ductile Ti. In the divider orientation, toughening is obtained through plastically stretching the intact ductile Ti ligaments that bridge the crack wake, thus reducing the crack driving force. By virtue of its morphology, the arrester orientation provides toughening by trapping the crack front entirely at the metallic-intermetallic interfaces, thus requiring the crack to renucleate at each interface. Results are compared with specific crack growth rates of conventional monolithic alloys and other composite systems such as TiNb/γ-TiAl and Nb/Nb3Al. Owing to their low density (∼3.8 g/cc), Ti-Al MIL composites exhibited specific crack growth rates (da/dN vs ΔK/ρ) on par with tougher, but relatively denser, ductile metals such as Ti alloys and high-strength steels.