Transformation-aggravated oxide embrittlement mechanism and carbon microalloying ductilisation strategy in refractory complex concentrated alloys

Refractory complex concentrated alloys (RCCAs) have emerged as a highly promising class of materials with great potential to replace Ni-based superalloys and legacy refractory alloys in (ultra-)high temperature structural applications ¹ . Room-temperature (RT) brittleness of various origins, among which oxygen/oxide embrittlement is particularly prevalent, has long been a critical bottleneck to practical deployment of RCCAs especially for those subsystems with sufficient high-temperature strength ² . Yet the underlying mechanisms of such embrittlement remain insufficiently understood, and existing mitigation strategies are far from complete ^3,4 . Here we unravel, for the first time, a transformation-aggravated oxide embrittlement mechanism in a Nb ₄₄ Ta ₄₄ Hf ₁₀ W ₂ RCCA, and demonstrate the effectiveness of carbon microalloying in achieving ductilisation. The premature intergranular fracture of the RCCA is attributed to the synergistic effects of (i) the internal cracking of brittle HfO ₂ oxides along the alloy grain boundaries (GBs), (ii) localized plastic deformation in the adjacent matrix, and most importantly, (iii) the degradation of interfacial registry and bonding strength between the matrix and HfO ₂ following its tetragonal-to-monoclinic transformation. Carbon microalloying generates discontinuous oxygen-soluble FCC-MC carbides rather than harmful HfO ₂ at GBs and enables coordinated plastic deformation as well as safe slip transfer across these boundaries, liberating the intrinsic ductility of the RCCA (13.3% elongation in RT tension) from the GB catastrophe. Meanwhile, the refined intragranular carbides endow the RCCA with superior high-temperature strength over traditional refractory alloys. This study provides novel yet generic mechanistic insights into oxygen embrittlement in RCCAs, breaking the long-standing black-box perception of oxides as embrittling inclusions. Furthermore, it establishes carbon microalloying as a low-cost, scalable strategy for ductilisation and concurrent strengthening in RCCAs.