title: Extraordinary strain hardening by gradient structure.
abstract: Gradient structures have evolved over millions of years through natural selection and optimization in many biological systems such as bones and plant stems, where the structures change gradually from the surface to interior. The advantage of gradient structures is their maximization of physical and mechanical performance while minimizing material cost. Here we report that the gradient structure in engineering materials such as metals renders a unique extra strain hardening, which leads to high ductility. The grain-size gradient under uniaxial tension induces a macroscopic strain gradient and converts the applied uniaxial stress to multiaxial stresses due to the evolution of incompatible deformation along the gradient depth. Thereby the accumulation and interaction of dislocations are promoted, resulting in an extra strain hardening and an obvious strain hardening rate up-turn. Such extraordinary strain hardening, which is inherent to gradient structures and does not exist in homogeneous materials, provides a hitherto unknown strategy to develop strong and ductile materials by architecting heterogeneous nanostructures.
authors.affiliation: State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China; xlwu@imech.ac.cn ytzhu@ncsu.edu.
authors.forename: XiaoLei
authors.initials: X
authors.lastname: Wu
authors.affiliation: State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China;
authors.forename: Ping
authors.initials: P
authors.lastname: Jiang
authors.affiliation: State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China;
authors.forename: Liu
authors.initials: L
authors.lastname: Chen
authors.affiliation: State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China;
authors.forename: Fuping
authors.initials: F
authors.lastname: Yuan
authors.affiliation: Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695; andSchool of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China xlwu@imech.ac.cn ytzhu@ncsu.edu.
authors.forename: Yuntian T
authors.initials: YT
authors.lastname: Zhu
mesh_terms: D055592:Biophysical Phenomena; D006244:Hardness; D008422:Materials Testing; D008670:Metals / Q000737:chemistry*; D046529:Microscopy, Electron, Transmission / Q000379:methods; D049329:Nanostructures / Q000737:chemistry; D011312:Pressure; D013314:Stress, Mechanical; D013499:Surface Properties; D013696:Temperature; D013718:Tensile Strength
chemical_list: D008670:Metals
keywords: gradient structured metal; nanocrystalline metal