Enhanced interfacial structure of (HfZrTiTaNb)C high-entropy ceramic joint brazed using FeCoCrNiTi0.2 alloy filler

Abstract

To fabricate large-sized and complex-shaped ceramic components for service at elevated temperatures, (HfZrTiTaNb)C high-entropy ceramic (HEC) joints with the improved joint strength of 292 MPa were successfully realized by using a FeCoCrNiTi0.2 high-entropy alloy (HEA) filler at 1430 °C. The reliable joint relied on a direct metallurgical bonding with semi-coherent interfaces between HEC' carbides and HEA' alloy, independent of compounds. The high-entropy interface was confirmed by the habit planes of (020)HEC' // (020)HEA' and the calculated lattice misfit of 1.58%, which represented small internal stresses at bonding interfaces in the joints. The formation of HEC and HEA phases originated from the direct phase transition of HEC induced by active Ti and the residual of liquid HEA, respectively, and they maintained their original lattice structures, nanohardness, and elastic modulus. The joints possessed undiminished strength values when tested at 1000 °C, benefiting from high-entropy microstructures throughout the interfacial region. The failure location migrated from the HEC near the HEC' to the HEA'. In addition, the interfacial formation strategy was investigated by altering HEA thicknesses and bonding times. The HEC' was invariably present in the joint, while preventing phase separation of liquid HEA upon cooling to form HEC'/HEA'/HEC' structure required the HEA thickness of at least 300 µm and the resulting abundant liquid filling. With extended bonding times of up to 60 min, the depletion of the HEA' layer would decrease joint strength by 140 MPa. This work provides a new perspective on joint design toward ∼1000 °C high-temperature applications.