In-Situ SEM Fracture Analysis of W-Eurofer Brazed Joints Under Three-Point Bending Test Configuration
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NTRODUCTION
One of the main issues still under investigation regarding the development of the next generation of fusion power plants is the development of feasible joining technologies between some materials of the first wall (FW) and divertor components.[1] The first wall consists of a tungsten layer that faces the plasma of the reactor chamber. This protects the inner components or materials from the harsh conditions inside the vessel, such as high energetic particle exposure or plasma transition phenomena.[2] This layer is supported by a reduced activation ferritic-martensitic steel (Eurofer), which is the main structural material in the in-vessel reactor.[3] The development of a high-quality joint between tungsten and Eurofer supposes overcoming some joinability aspects such as the high mismatch in the thermal expansion coefficients of the base materials (aW = 4.5 9 10 6 K 1[4] and aE = 12.7 9 10 6 K 1[5]). A high metallic continuity is necessary to remove the heat generated by the plasma through the first wall and transmit it to the coolant. It must also act as a structural component that fulfills all the mechanical requirements according to the design criteria in terms of mechanical strength,[2] thermal fatigue[6] and creep resistance.[7]
J. DE PRADO, M. SA´NCHEZ, G. ARBIZU, and A. UREN˜A, are with the Materials Science and Engineering Area, ESCET, Rey Juan Carlos University C/ Tulipa´n, s/n, Mo´stoles 28933 Madrid, Spain. Contact e-mail: [email protected] Manuscript submitted December 5, 2019. Article published online April 29, 2020 3488—VOLUME 51A, JULY 2020
However, in addition to engineering considerations, it is crucial to understand the fracture behavior of this dissimilar joint under different load conditions.[8–10] This fact is particularly important in this scenario because of the heterogeneous nature of the joint and the complex loads of the reactor environment and conditions.[11] An analysis of the fracture mechanisms is commonly undertaken after the mechanical characterization of the joint to understand the main fracture behavior during the test.[11,12] However, this analysis comprises a static study of the fracture, where some aspects of the mechanism are missed, such as initiation of cracks, the propagation path of the crack through the determined phases and interface integrity during the test. An in-situ scanning electron microscopy (SEM) bending test allows obtaining a dynamic analysis of the fracture mechanism during the test, which gives information on the above-mentioned effects. This characterization study is commonly applied to coatings and bulk materials,[13,14] but its application to joints is not widely done. The complex stress distribution in the sample subjected to a bending test could make a deep discussion of the data obtained difficult, especially in a complex sample such as a heterogeneous joint where the mechanical properties of both the base materials and braze are different. Finite element simulation is commonly applied to this type of joint to analyze the residual stresses generat
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