Designing hetero-structured ultra-strong and ductile Zr-2.5Nb alloys: Utilizing the grain size-dependent martensite transformation during quenching

To further improve the service performance of Zr-2.5Nb alloy worked as pressure tubes in pressurized heavy water reactors, more investigation about the microstructure and thermomechanical processing route of Zr-2.5Nb alloy need to be conducted. In this work, a hetero-structured Zr-2.5Nb alloy was prepared by applying a novel technique. Microstructure analysis reveals that the alloy exhibits a grain size-dependent martensite substructure transition during post-rolling quenching. The hetero-structure consists of equiaxed primary α grains and the lamellae groups containing both parallel α' dislocation martensite and α' twin martensite. Compared with the previously reported Zr-Nb alloys, the present Zr-2.5Nb alloys manifest the highest yield strength (∼710 MPa), together with a high ultimate tensile strength (∼844 MPa) and good ductility (∼17.1%). The enhanced mechanical properties are found to arise from the properly controlled fraction/size of the two types of martensite, which not only significantly strengthens the alloy but also contributes to a stronger strain hardening. A model based on the grain-size-dependent critical resolved shear stress for dislocation slip and twinning has been proposed to explain the α' martensite substructures transition at a critical grain size d_c = 3.3 μm. Below this size, the critical resolved shear stress (CRSS) for twinning is higher than that for the <c + a> slip. Thus, the α' dislocation martensite is more favorable to form. Otherwise, the α' twin martensite would exhibit a high activity. The present work indicates that making use of the grain size-dependent martensite transformation to tailor the hetero-structure in Zr alloys is an effective strategy to overcome the strength–ductility trade-off in the material.