title: Direct observation of hierarchical nucleation of martensite and size-dependent superelasticity in shape memory alloys.
abstract: Martensitic transformation usually creates hierarchical internal structures beyond mere change of the atomic crystal structure. Multi-stage nucleation is thus required, where nucleation (level-1) of the underlying atomic crystal lattice does not have to be immediately followed by the nucleation of higher-order superstructures (level-2 and above), such as polysynthetic laths. Using in situ transmission electron microscopy (TEM), we directly observe the nucleation of the level-2 superstructure in a Cu-Al-Ni single crystal under compression, with critical super-nuclei size L2c around 500 nm. When the sample size D decreases below L2c, the superelasticity behavior changes from a flat stress plateau to a continuously rising stress-strain curve. Such size dependence definitely would impact the application of shape memory alloys in miniaturized MEMS/NEMS devices.
authors.affiliation: State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China. dingxd@mail.xjtu.edu.cn junsun@mail.xjtu.edu.cn.
authors.forename: Lifeng
authors.initials: L
authors.lastname: Liu
authors.forename: Xiangdong
authors.initials: X
authors.lastname: Ding
authors.forename: Ju
authors.initials: J
authors.lastname: Li
authors.forename: Turab
authors.initials: T
authors.lastname: Lookman
authors.forename: Jun
authors.initials: J
authors.lastname: Sun
mesh_terms: D000497:Alloys / Q000737:chemistry*; D000535:Aluminum / Q000737:chemistry*; D003300:Copper / Q000737:chemistry*; D003722:Dental Alloys / Q000737:chemistry*; D009532:Nickel / Q000737:chemistry*
chemical_list: D000497:Alloys; D003722:Dental Alloys; C093824:Martensite; D003300:Copper; D009532:Nickel; D000535:Aluminum