The Mechanical Properties Of NiTi/a-CuZrAl Nano-multilayed Thin Films

Metallic glasses alloys have been widely investigated for decades due to their unusual mechanical properties,such as high strength,large elastic strain,superior wear and corrosion resistance.However,metallic glasses alloys undergo plastic deformation via fast propagation of shear bands which leads to the strain localization and successive poor ductility or premature failure.This is the major hurdle of metallic glasses to structural applications.Recently,it has been suggested that transforming component within amorphous matrix could reduce stress/strain concentration and inhibit the extension of shear bands to improve the plasticity of metallic glasses.NiTi shape memory alloy is a well-known martensitic material.The stress induced martensite phase transformation of NiTi generates 6-8 % homogeneous shear strain that can ruduce stress/strain concentration of adjacent area rapidly.In addition,martensite possesses high strength that provides prominent work hardening ability to suppress the stain localization.Therefore,it is reasonable to hypothesize that the composties of NiTi and amorphous may utilize the phase transformation properties of NiTi to change the extension of shear bands,resulting in possibly enhanced ductility and delayed premature failure.It is thereby likely to obtain NiTi/amorphous compsites with improved mechanical properties.We fabricated different modulation period and modulation ratio a-NiTi(amorphous)/a-CuZrAl(amorphous)multilayered thin films.Subsequently,using the different crystallization temperature cunningly,this paper got the NiTi(crystalline)/a-CuZrAl(amorphous)multilayerd thin films by annealing.Because the NiTi layers are phase transformation to provide plasticity,so the NiTi /a-CuZrAl multilayers are transformting component/amorphous composites.In addition,in order to compare the effect to the deformation mechanisms of amorphous between transformting component and dislocation gliding component,we fabricated NiTiNbFe(crystalline)/a-CuZrAl(amorphous)multilayers by adding Nb and Fe element to reducing the phase transformation temperature of NiTi that are dislocation to provide plasticity instead of phase transformation.Thus,the NiTiNbFe /a-CuZrAl multilayers are dislocation gliding component/amorphous composites.The result shows that although the nanoindentation hardness of single layered NiTi film is only 1/2 of NiTiNbFe,the nanoindentation hardness of NiTi/a-CuZrAl multilayers is higher than NiTiNbFe/a-CuZrAl multilayers that doesn't accord with the mix-theory of composites.The phase transformation behavior of NiTi is confirmed by in situ tensile tests glazing angle XRD.The results of strain rate sensitivity(SRS)m show that transformating NiTi could improve the m of amorphous CuZrAl.It implies that the work hardening ability of amorphous CuZrAl is enhanced and amorphous possess greater plasticity,higher breaking strength and hardness.In summary,we fabricated NiTi/a-CuZrAl multilayers that possess enhanced mechanical properties by using the homogeneous shear property of transformting NiTi(shape memory alloy)to improve the work hardening ability and delay the failure of amorphous CuZrAl.Results of this thesis may deepen our understading on the strengthening and plastic deformation mechanisms of transformting component/amorphous composites,and may provide insightsfor the design of new metallic glasses composites.