Predictive M‐Site Design of MAX‐Derived CoNi Composites: Dual‐Scale TaC + L1 2 Strengthening Enabled by Rare Metal Ta

ABSTRACT In MAX‐phase‐derived metal–matrix composites, the M‐site element simultaneously governs carbide chemistry and matrix precipitate stability, yet it has traditionally been selected empirically rather than from a predictive criterion linking M‐site choice to carbide chemistry, matrix alloying, and L1 2 stability; moreover, the reaction pathway, phase evolution, and strengthening response of a given M 2 AlC–CoNi system cannot be assumed a priori from previously studied Ti‐ or V‐based composites. Here, we address this gap through a first‐principles screening of nine candidate M‐site elements by antiphase boundary energy and L1 2 ‐forming tendency, which identifies the rare metal Ta as the optimum. This prediction is realized using the MAX‐phase Ta 2 AlC as a reactive precursor in a CoNi matrix: sintering decomposes Ta 2 AlC, and a CALPHAD‐guided heat treatment yields a stable dual‐scale architecture of uniformly dispersed semi‐coherent TaC particles (∼9.6 vol%) together with coherent L1 2 ‐(Co, Ni) 3 (Al, Ta) nanoprecipitates (∼34 vol%) triggered by the released Al and Ta. The composite achieves a compressive yield strength of 1684 ± 30 MPa at room temperature and retains 816 ± 42 MPa at 900°C (48% of the room‐temperature value). The dominant strengthening contribution (∼55%) arises from Ta‐enhanced L1 2 order strengthening ( γ APB ≈ 302 mJ m −2 ), complemented by Orowan dispersion strengthening from TaC (∼19%). This work establishes the MAX M‐site element as a first‐principles‐predictable design variable: the screening correctly identifies Ta, and the resulting dual‐scale strengthening validates this predictive approach—offering a transferable framework for M‐site‐guided design of in‐situ‐reinforced composites.