Nanoscale Near-Surface Microstructures and High Temperature Fatigue of Laser-Shock Peened and Deep Rolled Ti-6Al-4V

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Abstract

Mechanical surface treatments such as laser-shock peening or deep rolling are known to induce highly strain-hardened nanoscale microstructures as well as deep compressive residual stress layers into surface regions of fan blade titanium alloys such as Ti-6Al-4V.1,2） The benefit of such treatments is well documented for cyclic loading at room temperature. In addition, it appears that the induced microstructures are stable enough to provide significant lifetime and strength improvements even at temperatures up to 550 ℃ which is far above the operating temperature of such components. This study seeks to provide basic understanding for the strengthening mechanisms of deep rolled/lasershock peened and high temperature fatigued Ti-6Al-4V under stress-control. For this purpose, the stability of near-surface microstructures is investigated by TEM (transmission electron microscopy) after step-wise thermal exposure by in situ heating and after isothermal stress-controlled fatigue up to 550 ℃. Furthermore, for structural assessment of the softening or hardening behaviour, the plastic strain amplitude is recorded as a function of temperature and number of cycles. The resulting stress-life behaviour clearly indicates that laser-shock peening and deep rolling are both extremely efficient methods for fatigue strength enhancement of solution treated and overaged Ti-6Al-4V, even at elevated temperatures up to 550 ℃, despite a very pronounced relaxation of compressive residual stresses.

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