A tungsten-copper (W—Cu) duplex structure is the reference divertor plate design concept for the Fusion Experimental Reactor (FER). In the present study, a durability test of the W—Cu duplex structure against thermal cycling was performed. A tungsten disk was bonded to a copper disk by means of brazing or direct casting. The tungsten surface of the test piece was periodically heated by a high temperature argon plasma jet. Before and after the tests, the test pieces were examined with the aid of a scanning electron microscope and the Knoop hardness was measured. Grain boundary microcracks were observed after 200 and 1100 thermal cycles in brazed tungsten samples which contained a small amount of nickel and phosphorus. A cast tungsten specimen subjected to 2200 thermal cycles also contained microcracks. However, microcracks were not observed in a brazed tungsten sample containing an extremely small amount of impurities for thermal cycles up to 3700 times. Microcracks were observed in the brazing material of this test piece. None of the test specimens were broken. It is found that brazing is a valid bonding method and that W—Cu duplex structure, especially with high purity tungsten, is able to endure a practical number of thermal cycles.
Material properties and their influence on the behaviour of tungsten as plasma facing material