Comparing fatigue behavior of titanium and nickel-based alloys

Abstract

Abstract Titanium alloys are in competition with nickel-based alloys particularly in aerospace applications, where the lower density of titanium alloys is of advantage. For these applications fatigue strength is of primary importance. When discussing the influence of microstructure on fatigue resistance, it is important to distinguish between fatigue crack initiation, the propagation of microcracks and the propagation of macrocracks. In terms of microstructure, this paper addresses those parameters, which can be readily varied during the processing of a part: i.e. in titanium alloys the choice between a duplex (bi-modal) and a fully lamellar microstructure, in a given nickel alloy it is usually the grain size and the population of the hardening γ′ precipitates. Titanium alloys with duplex microstructures are found to be superior to lamellar structures with respect to fatigue at high stress amplitudes (LCF regime) and to microcrack propagation. At low stress amplitudes and with respect to macrocrack propagation, the tendency is reversed. In nickel alloys, a dominating effect of crack nucleation at hard particles is observed, which masks the positive effects of increased yield stress by grain refinement or γ′ modifications. Both variables also have little influence on microcrack propagation, while increasing the grain size tends to reduce the propagation rate of macrocracks like in most other metallic materials.