Fatigue strength of metals and alloys modified by ion beams

Abstract

The objective of the present research is the critical analysis of test results, dedicated to the effect of ion-beam irradiation conditions upon the fatigue strength of metallic materials. The shaped specimens, simulating the gas turbine engine blade operation and directly the blades, produced of titanium and nickel alloys as well as refractory steels by machining method are used as the study objects. The arc-pulsed ion implantation and irradiation of targets by the high-power pulsed ion beam were realised by means of Raduga-2, Temp-M and MUK accelerators. The irradiation conditions were varied within the following ranges: La, B, Sm and Hf ion implantation— E=30–80 keV (ion energy), j=1–5 mA/cm 2 (ion current density), f=30 Hz (pulse frequency), D=10 16–5×10 17 ion/cm 2 (irradiation dose); high-power pulsed ion beam irradiation—ions of carbon (60–70%) and protons, E=120–300 keV, j=30–200 A/cm 2, τ=50–100 ns (pulse duration), n=3–20 pulses (number of pulses). After irradiation, some targets were subjected to vacuum annealing for 2 h at their service temperature. Fatigue tests of initial and irradiated samples and blades were performed in air at the operating temperatures (450–650 °C) on the magneto-strictional vibrobanches with the loading frequency to 5000 Hz and low-frequency vibrobenches (50–80 Hz). The target surface state prior to and after tests was studied by electron Auger spectroscopy, scanning electron microscopy and X-ray structural analysis. The fracture surface was studied by optical and electron fractography. The test results showed that the endurance limit of samples and blades, subjected to the ion-beam irradiation with the post-process vacuum annealing at the optimum conditions, could be increased by 30–180% depending on an alloy type. Using the electron fractography, it was stated that the fatigue strength increase was associated both with the crack nucleation mechanism change and its growth rate deceleration. The extremely high endurance limit values were observed for patterns and VT8M titanium alloy blades with sharp edges after the high-power ion beam irradiation (pulse number: 15–20 at not high current density, j=60±10 A/cm 2) and vacuum annealing at 500 °C for 2 h.