Excellent high-temperature strength of (HfZrTiTaNb)C high-entropy carbide diffusion-bonded joint via <i>in-situ</i> alloying of Ni/Nb/Ni composite interlayer

To obtain high-entropy carbide (HEC) joints with excellent high-temperature performance, a (HfZrTiTaNb)C HEC joint featuring a direct diffusion-bonded interface with a Nb-based interlayer was successfully fabricated at relatively low temperatures of 1150–1250 °C for 60 min under 10 MPa. Starting from a modified Ni/Nb/Ni composite interlayer with a Nb content of &gt; 64 at%, an alloyed Nb₂Ni layer was constructed <i>in situ</i> by accelerating the directional diffusion of Ni atoms from the high-entropy interface into the remaining pure Nb through the Ni‒Nb eutectic liquid. Moreover, the excess liquid phase was squeezed out of the bonding region, ensuring the absence of Ni-based compounds. Leveraging the intrinsic interfacial stability and sluggish diffusion effect, the HEC with its original lattice structure, was capable of developing diffusion bonding with the Nb₂Ni layer instead of interacting with the liquid phase. The high reliability of the HEC/Nb₂Ni bonded interface was confirmed by the coherence of (1<inline-formula id="M1"> <math id="mathml_M1" display="inline" overflow="scroll"><mrow class="MJX-TeXAtom-ORD"><mover><mtext>1</mtext><mo stretchy="false">¯</mo></mover></mrow></math></inline-formula>3)_HEC//<inline-formula id="Z-20241231163915"> <math id="mathml_Z-20241231163915" display="inline" overflow="scroll"><mo stretchy="false">(</mo><mn>141</mn><msub><mo stretchy="false">)</mo><mrow class="MJX-TeXAtom-ORD"><msub><mrow class="MJX-TeXAtom-ORD"><mrow class="MJX-TeXAtom-ORD"><mi mathvariant="normal">N</mi><mi mathvariant="normal">b</mi></mrow></mrow><mn>2</mn></msub><mrow class="MJX-TeXAtom-ORD"><mrow class="MJX-TeXAtom-ORD"><mi mathvariant="normal">N</mi><mi mathvariant="normal">i</mi></mrow></mrow></mrow></msub></math></inline-formula> and a calculated lattice misfit of 0.044. The HEC joint had a high room-temperature strength of 174 MPa because of the homogenous Nb₂Ni layer, which exhibited nanohardness (15.2±1.5 GPa) and an elastic modulus of 219.9±17.5 GPa. Furthermore, the strength of the HEC joint did not decrease at 1000 °C, increasing by ~49% over that of HEC/Ni/HEC diffusion-bonded joints, which have stringent surface flatness requirements. This suggested that the HEC/Nb₂Ni interface had excellent resistance to high-temperature softening, even though it was invariably the initial failure location. This work is informative for designing bonding structures and preparing HEC components.