Corrosion fatigue crack initiation in ultrafine-grained near-a titanium alloy PT7M prepared by Rotary Swaging

Abstract

The study focuses on corrosion fatigue processes taking place in an ultrafine-grained (UFG) near-a-titanium alloy Ti-2.5Al-2.6Zr (Russian industrial name PT7M) used in nuclear engineering. UFG structure formed with Rotary Swaging is found to increase resistance to corrosion fatigue. Parameters of the Basquin's equation are defined and the slope of the fatigue curve Sa-lg(N) is shown to depend (nonmonotonic dependence) on the UFG alloy annealing temperature. This effect can be explained with the patterns of microstructural evolution in a UFG alloy PT7M during annealing: (1) reduced density of lattice dislocations, (2) precipitation and dissolution of zirconium nanoparticles, (3) release of a''-phase particles causing internal stress fields along interphase (a-a'')-boundaries, and (4) intensive grain growth at elevated annealing temperatures. It is shown that the fatigue crack closure effect manifested as changing internal stress fields determined using XRD method may be observed in UFG titanium alloys.