In Situ TEM Study Of The Size Effect On The Martensitic Transformation In Nanoscaled NiTi Alloy

NiTi shape memory alloys have superior properties, such as superelasticity andshape memory effect. These superios mechanical properties and goodbiocompatibility make it an excellent candidate for application in aerospace,biomedical and microelectro mechanical systems (MEMS). Recently, development insemiconductor and micro fabrication leads to miniaturization of MEMS. At thisdimension, the size of component is close to the main characteristics size of NiTi.Therefore, the effect of size, both of grain and physical dimension, on the martensitictransfomation must be taken into account to ensure the reliability for applications ofNiTi in MEMS and NEMS. Consider in this, it is theoretically and practicallyimportant to investigate the size effect on martensitic transformation in nanoscaledNiTi alloys.The “dumbbell” shaped NiTi samples with different thicknesses were fabricatedby a series of sample processing technology, including mechanically ground, twin-jetelectrochemically polishing, sample transfer and focused ion beam (FIB) cutting. Thestress-induced martensitic transformation behaviors were in situ observed duringtensile deformation using a self-designed TEM tensile apparatus accompanied with abi-axialtilt heating stage inside a transmission electron microscope (TEM). NiTi thinfilm was prepared by standard TEM sample preparation process. The evolution ofthermally-induced martensitic transformation with the film thickness was in situstudied by a cooling stage inside a TEM. Electron energy-loss spectroscopy (EELS)technique was used to measure the thickness of the nanoscaled NiTi strip and the NiTifilm. Energy dispersive spectrometer (EDS) analysis was performed to measure thecomposition especially the evolution of oxygen content with the film thickness andthe intrinsic reason of the size effect was analyzed. The main findingsare summarizedbelow:(1) For these ultrathin NiTi miniature strips of40–83nm in thickness, stress-inducedmartensitic transformation was first observed in the83nm thick strip, however, nomartensitic plate was observed to nucleate in the strip of40nm in thickness evenwhen tensile to fracture, exhibited an apparent ‘‘size effect’’.(2) EDS composition analysis indicating that the size effect can be attributed to theeffect of damaged surfaces, including a Ga+-impregnated amorphous layer caused bysample fabrication using FIB and natural oxidation affected layers. In this regard, the‘‘size effect’’is not intrinsic to the martensitic transformation.(3) Ga+-impregnated amorphouslayer and surface oxidation causes the formation ofnon-transforming surface layers, which hinder the martensitic transformation. In addition, depletion of Ti due to oxidation causes Ni enrichment in the NiTi matrix,which also suppresses the martensitic transformation. The volume fraction of thesurface oxidation layer and amorphouslayer increased with the decreasing ofthickness of NiTi strips and the the martensitic transformation was completelysuppressed in the strip of40nm in thickness.(4) A cold-rolled Ti–50.8at.%Ni sheet was annealed at500oC for30min forrecrystallization and a NiTi nanocrystals with a mean grain size of180nm wasprepared. DSC measurement for the transformation behavior of the bulk NiTinanocrystals showed a two-stage transformation (B2→R→B19′) upon cooling.(5) The two-stage transformation was also observed in nanoscale NiTi thin film, butthe transformation temprature show a strong hysteresis than the bulk material. Thehysteresis of transformation temprature increase with the decreasing of the thin filmthickness. When the thickness of thin film reduce to100nm, the hysteresis of RSandMSare26and36oC respectively; When the thickness of thin film reduce to60nm,the hysteresis of RSand MSare43and80oC respectively. The critical thickness forexhibiting the B2→R transformation was~22nm, whereas the critical thickness forthe R→B19′transformation was~50nm, indicating an apparent ‘‘size effect’’.(6) With the decreasing of the thin film thickness, the content of oxygen increasegradually. Experimental results indicate that the surface of the thin film occursnaturally oxidized. Ti has high chemical affinity to oxygen. This may lead to depletionof Ti in the matrix and Ni enrichment, which will lower the transformation temprature.The martensitic transformation was completely suppressed when the thickness of thethin film was decreased to the critical value (~50nm).The discovery and the mechanism of size effect on martensitic transformation inNiTi alloys have practical significance for design, critical dimensions and reliabilityof NiTi-based micro-components, which offer important theoretical and experimentalevidence to insure the successful application of MEMS.