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Preparation And Microstructure Of Ni-Mn-Ga-Gd High Temperature Shape Memory Alloy Thin Films

Posted on:2014-06-03Degree:MasterType:Thesis
Country:ChinaCandidate:J YaoFull Text:PDF
GTID:2251330422950952Subject:Materials Physics and Chemistry
Abstract/Summary:PDF Full Text Request
Ni-Mn-Ga-Gd high temperature shape memory alloy thin films wereprepared by DC magnetron sputtering. By means of atomic force microscopy,scanning electron microscope, differential scanning calorimetry analysis, X-raydiffraction analysis and transmission electron microscopy observation, therelationship between parameters of sputtering and surface morphology an dchemical compositions were revealed and the evolution of the microstructure ofannealed films under different annealing procedures was studied. Thedependence of grain size on the martensitic transformation behavior wasinvestigated.It is founded that the surface roughness and average particle size increasewith the increasing sputtering time. The small average particle size and highquality surface are achieve at100W sputtering power, under which the filmsdeposition rate is7.5nm/min. Ni content of the films reduces and the content ofMn in the film increases with the increasing sputtering power. With theincreasing sputtering time, the Ni conten in the films decreases, at the same timethe composition of the sputtered film is gradually approaching th e compositionof the target.The experimental results show that the films gradually increase degree ofcrystallization with respect to the increasing sputtering power and the sputteringtime. The rich-Ni phase and austenite coexist in the part crystallized film withhigh content of Ni. The Ni-rich phase disappears when the Ni content dropsowing to the increasing sputtering time. The Ni55.5Mn25.1Ga19.3Gd0.1film ispartially crystalline state. Crystallization temperature increases with increasingheating rate. The crystallization activation energy255.2kJ/mol inNi55.5Mn25.1Ga19.3Gd0.1filmAccording to the experimental results, the phase component ofNi55.5Mn25.1Ga19.3Gd0.1thin film consists7M martensite and a part of phasewhen heated at550oC by conventional annealed. When heated to350oC, only the7M martensite left. When heated to650oC, the7M martensite converts to Tmartensite. The grain size of the films can be controlled by the holding time ofconventional annealing and the temperature of the rapid thermal annealing.The observation of TEM confirmed that the martensite variants ofNi55.5Mn25.1Ga19.3Gd0.1films display well self-accommodation with clear andstraight intervariant boundaries.7M martensite shows(222)type twinrelationship while T martensite shows (220)type twin relationship. Thermal analysis showed that the single-step martensite transformation andthe reverse transformation occurred in Ni55.5Mn25.1Ga19.3Gd0.1films whencooling and heating. Martensitic transformation temperature decreases with thereducing grain size. The film is expected to develop a high temperature ductilityshape memory alloy films with200nm grain size and190oC martensitictransformation temperature.
Keywords/Search Tags:Ni-Mn-Ga alloys, high temperature shape memory alloys, martensitic transformation, grain refined
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