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

•A heterogeneous structured Zr-2.5Nb alloys were fabricated successfully utilizing the grain size-dependent martensite transformation.•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%).•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 dc = 3.3 μm.