Evaluation of critical strain for crack nucleation of magnesium di-boride superconductor using indentation method

To fulfill the promises of MgB₂ and MgB_2-xC _x in superconducting electrical transportation, the components made by these materials need to undergo plastic work process, for which the knowledge of their mechanical behavior (i.e. elastoplastic and strength properties) is critical. This study proposes the use of a convenient method based on indentation to evaluate both the elastoplastic and strength properties of MgB₂ and MgB_2-xC_x. Through the reverse analysis using dual sharp indenters, we first extract the yield stress and work hardening exponent quantitatively. Next, the critical stress for crack nucleation (fracture strength) is estimated from a combination of experiment and numerical simulation of indentation stress field using the deduced elastoplastic properties. The critical fracture strain of MgB₂ is evaluated to be reasonably close to that from conventional tensile test. We further show that in MgB_2-xC_x, the material stiffness, yield stress and fracture strength are degraded due to carbon additive, while the work hardening exponent is slightly higher. Therefore, the addition of C does not improve the mechanical properties of MgB₂. The findings shed some useful light on the workability of MgB₂, and may become a basis for understanding the feasibility of fabrication and usage of superconductor (where the mechanical and electrical properties are coupled).