Fatigue testing and microstructural characterization of tungsten heavy alloy Densimet 185

A rotating target consisting of helium-cooled tungsten has been chosen for the European Spallation Source (ESS) facility to be built in Lund. Thermo-mechanical cycling due to the incidence of the proton beam every 2 s on any given tungsten slab in the rotating wheel could lead to crack formation and failure over the lifetime of the target. This work reports tensile and fatigue data obtained at room temperature for the Densimet 185 alloy in the non-irradiated condition. Methods for extracting relevant parameters from fatigue curves with small sets of data are discussed. Fatigue results show a large spread of data for which the application of such methods is challenging. Stress controlled fatigue testing was carried out in this study with mean stress approaching zero and amplitudes in the range 250 to 450 MPa, with 50 MPa increments. A frequency of 25Hz was employed and the fatigue tests lasted until failure was registered or until the upper limit of 2 x 10(6) cycles was reached. No failure due to fatigue occurred in specimens subjected to stress amplitudes below 300 MPa. Microstructural and fractographic studies on the fatigue samples using Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) showed that the samples had low porosity, large and nearly spherical tungsten grains, and with a fairly uniform distribution of the ductile phase rich in nickel andiron. However, bonding between tungsten grains in some areas was found to be inadequate. Intergranular fracture was predominant in the specimens at room temperature. Data for the D185 alloy are compared to those for IT180 and D176 alloys obtained in a previous study and strategies for improving the fatigue strength are discussed. (C) 2013 Elsevier Ltd. All rights reserved.