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 pre-pared 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 prop-erly 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 crit-ical 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.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.

S.Y.Liu;J.Y.Zhang;J.Kuang;X.Y.Bao;D.D.Zhang;C.L.Zhang;J.K.Yan;G.Liu;J.Sun

State Key Laboratory for Mechanical Behavior of Materials,Xi'an Jiaotong University,Xi'an 710049,China

材料科学技术（英文版）

[1]S.Y.Liu;J.Y.Zhang;J.Kuang;X.Y.Bao;D.D.Zhang;C.L.Zhang;J.K.Yan;G.Liu;J.Sun-.Designing hetero-structured ultra-strong and ductile Zr-2.5Nb alloys:Utilizing the grain size-dependent martensite transformation during quenching)[J].材料科学技术（英文版）,2022(30):198-211

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