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

Intermediate temperature brittleness in alloys characterized as brittle fracture along grain boundaries (GBs) with less than 5% elongation to fracture (EF) at 600 ℃–900 ℃ diminishes work hardening, leads to sudden failure under load, and thus threatens the reliability during the service of alloys. Here, in a precipitation-strengthened CoNiCr alloy, through two grain boundary engineering (GBE) methods, fiber-like γ' or topologically close-packed phase is introduced at GBs, which effectively optimizes the grain structure and prevents GB cracking under tensile stresses. GBEs not only alter the deformation mode from dislocation pairs to stacking faults and/or deformation twins, but also transform the failure mode from GB cracking to GB void formation, because the crack propagation along GBs is constrained by GB bridging phases. Consequently, our GBE approach enhances tensile EF from ∼1% to ∼10% and concurrently increases the yield strength from ∼650 to ∼770–850 MPa at 800 ℃. A cavity growth model is then developed to illustrate the role of these bridging phases in GBs for ductility improvement. The fundamental philosophy utilized in the present work might be also applicable to other metallic materials.