Microstructural and micromechanical assessment of aged ultra-fast sintered functionally graded iron/tungsten composites

Abstract Functionally graded (FG) iron/tungsten (Fe/W) composites are considered for stress-relieving interlayers in tungsten-steel joints, required in future fusion reactors. The macroscopic gradation of the two materials allows relaxation of thermally-induced stresses and hence extend the lifetime of the cyclic-loaded dissimilar materials joints. While many properties, e.g. thermal expansion and strength, of the as-manufactured Fe/W composites are promising with respect to the anticipated application, the temperature-induced microstructural changes and their effect on the material properties remain largely unexplored. Given that the thermodynamic system of Fe W contains two types of intermetallic phases, understanding the microstructural changes in the FG Fe/W composites are crucial for long-term operation of fusion reactors. In the present work, the microstructure of ultra-fast sintered Fe/W composites containing 50 and 75 vol% tungsten is studied via electron microscopy (SEM) and X-ray diffraction (XRD) in as-manufactured and thermal aged conditions (300, 500, and 800 °C for up to 72 h). The hardness and modulus of selected composites are measured via nanoindentation, and the fracture toughness of the Fe W interfaces is tested via notched micro-cantilever bending tests. The results from microstructural and micromechanical analyses are discussed, and the materials are evaluated for their application in fusion reactors based on the microstructure-to-property relationship.