Evolution of interface voids and columnar grains of the FeCrAl/RAFMs HIP bonding joint

Abstract

Hot Isostatic Pressing (HIP) diffusion bonding of FeCrAl and reduced-activation ferritic/martensitic steels (RAFMs) is a novel method for preparing the tritium permeation barrier (TPB) in nuclear fusion reactors. This study primarily investigates the formation mechanisms and evolution of interfacial voids and columnar grains under different bonding temperatures and post-welding heat treatment (PWHT) conditions. It is found that increasing bonding temperature facilitates element diffusion, leading to the closure of interfacial voids. Decarbonization of RAFMs and the increase in Cr content facilitates the formation of interfacial columnar grains, with C playing a crucial role in this process due to its lower diffusion activation energy than Cr. Although appropriate PWHT can reduce the size of interfacial columnar grains, it cannot entirely diminish them. The FeCrAl/IPFs joint effectively prevents the formation of brittle (Cr, C) compounds and interfacial AlN defects. After HIP (1050 °C) + PWHT (750 °C), the joint achieves high tensile strength and ductility both at room temperature and 300 °C. The enhancement in ductility is primarily attributed to the high Schmid factor of interfacial columnar grains, which facilitated the activation of the primary slip system under tensile load, resulting in a high elongation of joints.