The evolution of microstructure in cold worked TiNi alloy with the degree of the thickness reduction has been studied by TEM. The moirphological aspect of the martensite changes gradually as follows: "self-accommodation"-"accommodated+coalescent bands"-"cross-hatch plates"-"fine curved lamellar". whereas the substructure of martensite varies correspondingly as follows:'<011) Type II twin"-"<011>. Type II twin+(001) stacking faults+dislocation-"<011> Type II twin +(001) compound twin+dislocatons"-"(001) compound twin+dislocation+amphorous phase". In-situ TEM observation indicates the mechanism of linear superelasticity of the cold rooled TiNi alloy is due to the appearance and disapperance of subtwins upon loaidng and unloading.
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