Microstructural Development and Ternary Interdiffusion in Ni-Mn-Ga Alloys
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wing research interest in recent years due to a variety of potential applications based on novel functional properties. Among various Heusler alloys, Ni-based Ni2MnGa and its off-stoichiometric alloys were among the earliest discovered alloys to show large magnetic-induced strains[1] and paved the idea of controlling material LE ZHOU, Doctoral Graduate Research Assistant, and YONGHO SOHN, Professor and Associate Director, are with the Department of Materials Science and Engineering and Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, FL 32816-2455. Contact e-mail: [email protected] CATHERINE KAMMERER, formerly Doctoral Graduate Research Assistant with the Department of Materials Science and Engineering and Advanced Materials Processing and Analysis Center, University of Central Florida, is now Principal, Material and Processes Engineer, with Aerojet Rocketdyne, Jupiter, FL 33478. ANIT GIRI, Senior Scientist, is with TKC Global (Contractor to US Army Research Laboratory), Herndon, VA 20171. KYU CHO, Materials Engineer, is with the Weapons and Materials Research Directorate, US Army Research Laboratory, Aberdeen Proving Ground, Aberdeen, MD 21005-5069. Manuscript submitted June 25, 2015. Article published online September 10, 2015 5572—VOLUME 46A, DECEMBER 2015
response with magnetic fields. In general, Ni-Mn-Ga alloys undergo phase transformation from partially ordered B2 (cP2, Pm3m) phase to fully ordered L21 (cF16; Fm3m) phase, and then to a low-temperature martensitic phase.[2] The application of a magnetic field can cause the rearrangement of the twin variants in the martensitic phase and lead to large strain in the crystal.[3,4] Additionally, magnetic fields can also directly induce phase transformations, and thus cause a large magnetization change, which causes the large entropy change that is the magnetocaloric effect.[5–7] Because of these properties, Ni-Mn-Ga alloys can be applied to magnetic actuators or refrigerators. Extensive investigations on the magnetic properties, martensitic transformation, and crystallography of twin variants in Ni-Mn-Ga alloys have been carried out, but with limited focus on the phase equilibria or diffusion kinetics within the alloys. Recently, two-phase shape memory alloys (martensite + c) have been developed by increasing the Ni content or alloying a fourth element. The c (cF4; Fm3m) phase is beneficial for ductility and thermal cycling stability of the alloys.[8] Precipitation of cubic Ni3Ga (cP4, Pm3m) during thin film deposition with high Ni content has been reported.[9] Accurate METALLURGICAL AND MATERIALS TRANSACTIONS A
information on phase equilibrium of the ternary Ni-Mn-Ga alloys is important for understanding their processing-properties relations and for designing higher order alloys with enhanced properties. Diffusion-controlled processing techniques are being studied to develop fabrication technologies of shape memory alloys including Ni-Mn-Ga alloys. Ni-Mn-Ga microtubes have been successfully produced through interdiffusion of M
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