High‐Entropy Ti, Zr, Hf, Ta Multiphase Diboride with Deformation Resistance up to 2000 °C

A multiphase high‐entropy diboride (Ti 0.25 Ta 0.25 Hf 0.25 Zr 0.25 )B 2 is obtained by spark plasma sintering from a mixture of single‐metal diborides. The as‐prepared material at the microscale can be defined as a composite where grains of a Ta‐rich/Ti‐poor complex diboride phase are the reinforcement and grains of Ta‐poor/Ti‐rich complex diboride are the matrix. However, at the nanoscale, the grains are heterogeneous, composed of regions with a multitude of complex diboride compositions. The interface between nanoregions is compositionally graded and has an irregular shape. The four‐metal diboride shows a deformation‐resistant mechanism under bending load. A strengthening process is active, increasing the room temperature bending strength (326 MPa) by ≈50% at 1800 °C (488 MPa). A ductile behavior with a deformation strain of ≈7.5% is observed at 2000 °C while bending strength (407 MPa) is ≈25% above the value at room temperature. At 2000 °C, observation of dislocations propagating from one compositional nanoregion to another and with a different density suggests dislocation contribution, first of all, to plasticity. The peculiar heterogeneity of this material at nano‐ and microscales is considered the reason for the remarkable mechanical response to bending load at different temperatures.