Regulating the formation ability and mechanical properties of high-entropy transition metal carbides by carbon stoichiometry

Abstract

• High-entropy transition metal carbides with regulatable carbon stoichiometry (HE TMC x ) achieve the promoting of formation ability from carbon stoichiometry deviations. • Bulk (Zr 0.25 Hf 0.25 Ta 0.25 Nb 0.25 )C x (SPS-ZHTNC x ) with carbon stoichiometry deviations show promoted sintering. • Bulk (Zr 0.25 Hf 0.25 Ta 0.25 Nb 0.25 )C x (SPS-ZHTNC x ) present tunable mechanical properties. Tremendous efforts have been dedicated to promote the formation ability of high-entropy transition metal carbides. However, the majority of methods for the synthesis of high-entropy transition metal carbides still face the challenges of high temperature, low efficiency, additional longtime post-treatment and uncontrollable properties. To cope with these challenges, high-entropy transition metal carbides with regulatable carbon stoichiometry (HE TMC x ) were designed and synthesized, achieving improved ability for single phase solid solutions formation, promoting of sintering and controllable mechanical properties. Two typical composition series, i.e., easily synthesized (Zr 0.25 Hf 0.25 Ta 0.25 Nb 0.25 )C (ZHTNC) and difficultly synthesized (Zr 0.25 Hf 0.25 Ta 0.25 Ti 0.25 )C (ZHTTC) are selected to demonstrate the promoting formation ability of single phase solid solutions from carbon stoichiometry deviations. Single phase high-entropy ZHTTC, which has been proven difficult in forming a single phase solid solution, can be prepared with the decrease of C/TM ratio under 2000 °C; while the high-entropy ZHTNC, which has been proven easy in forming a single phase solid solution, can be synthesized at lower temperatures with the decrease of C/TM ratio. The synergistic effect of entropy stabilization and reduced chemical bond strength gaining from carbon stoichiometry deviations is responsible for the formation of single phase solid solutions and the promoted sintering of HE TMC x . For example, the relative density of bulk (Zr 0.25 Hf 0.25 Ta 0.25 Nb 0.25 )C x (SPS-ZHTNC x ) increases from 90.98% to 94.25% with decreasing the C/TM atomic ratio from 0.9 to 0.74. More importantly, the room temperature flexural strength, fracture toughness and brittleness index of SPS-ZHTNC x can be tuned in the range of 384 MPa–419 MPa, 4.41 MPa⋅m 1/2 –4.73 MPa⋅m 1/2 and 3.679 μm −1/2 –4.083 μm −1/2 , respectively. Thus, the HE TMC x prepared by adjusting the ratio of carbon to refractory transition metal oxides have great potential for achieving low temperature synthesis, promoted sintering and tunable properties.