Evading the intermediate temperature brittleness of a precipitation-strengthened CoNiCr alloy by grain boundary engineering

Abstract

Intermediate temperature brittleness (ITB) in alloys, characterized by brittle fracture along grain boundaries (GBs) with less than 5% elongation to fracture at 400-900 oC, diminishes work hardening and ultimate tensile strength (UTS), therefore compromises reliability of alloys. Here, in a precipitation strengthened CoNiCr alloy, through grain boundary engineering (GBE), fiber-like γʹ or topologically close packed (TCP) phase is introduced at GBs, effectively preventing GB cracking. GBE not only alters the deformation mode from dislocation pairs to stacking faults and twins, but also transforms the failure mode from GB cracking to GB void formation, which is constrained by GB bridging phases. Consequently, our GBE approaches enhance tensile ductility from ~1% to ~10% and concurrently increase yield strength from ~650 to ~770-850 MPa at 800 oC. The strategy here offers a pathway to overcome ITB and achieve strength and ductility synergy for a wide range of metallic materials at intermediate temperatures.