Calvin V. Hyatt

Effects of growth rate and composition on the microstructure of directionally solidified NiMnGa alloys

Magnetic shape memory (MSM) alloys give recoverable strain when subjected to an applied magnetic field. The strongest MSM effect has been observed in single crystals. The magnitude of the effect and the consistency of behavior over the entire volume of a sample is strongly dependent on the solute and phase distributions in crystals. Samples of stoichiometric and off-stoichiometric Ni2MnGa magnetic shape memory alloys were directionally solidified by a seedless Bridgman method using different rates of growth. The growth conditions used resulted in oriented polycrystals exhibiting a coarse cellular structure. Significant macro-segregation was observed, with the top of the ingot enriched in Mn and the bottom enriched in Ga. Micro-segregation also occurred, resulting in Mn-rich intercellular eutectic or eutectoid structures, and coarse intra- and inter-cellular Mn-rich particles. An increase in the pulling rate during the directional solidification process resulted in finer cellular and eutectic / eutectoid structures, as well as finer particles.

Magnetic properties of single crystals of Ni-Mn-Ga magnetic shape memory alloys

The magnetic shape memory (MSM) effect occurs in some ferromagnetic martensitic alloys at temperatures below the martensite finish temperature and involves the re-orientation of martensite variants by twin boundary motion, in response to an applied stress and/or magnetic field. The driving force for twin boundary motion is the magnetic anisotropy. In this study, magnetization measurements as a function of magnetic field were made on several oriented single crystals of Ni-Mn-Ga alloys using a vibrating sample magnetometer. The magnetization versus magnetic field curves were characteristic of magnetically soft materials with magnetic anisotropy consistent with literature estimates for the different martensite structures observed in Ni-Mn-Ga alloys. Differences in the slope of the curves were due to the martensite structure, the relative proportion of martensite variants present, and their respective easy and hard axis orientations. Thermo-magneto-mechanical training was applied in an attempt to transform multi-variant specimens to single variant martensite. Training of the orthorhombic 7M martensites was sufficient to produce a near single variant of martensite, while the tetragonal 5M martensite responded well to training and produced a single-variant state. The strength of the uniaxial magnetic anisotropy constant for single-variant tetragonal 5M martensite, Ni52.9Mn27.3Ga19.8, was calculated to be Ku=1.8 x 105 J/m3, consistent with literature values. To obtain single-variant martensites, heat-treatment of the specimens prior to thermo-magneto-mechanical training is necessary.

Microstructure and Solidification Behavior of Ni-Mn-Ga Magnetic Shape Memory Alloys

In order to understand the solidification behavior of Ni-Mn-Ga alloys, ingots with different compositions were prepared by arc melting. Two series of compositions were investigated: Ni100-2xMnxGax (15≤x ≤30) and Ni50Mn50-yGay (0≤y≤50). The microstructures obtained were observed and the compositions of the phases occurring in the ingots were identified by energy dispersive spectroscopy in the scanning electron microscope. Based on these observations, three solidification paths were identified: direct solidification of γ-Ni from the liquid, direct solidification of β-NiMnGa from the liquid, and solidification of β-NiMnGa phase via a peritectic reaction. It was found that the γ-Ni liquidus surface covers a large area of the ternary phase diagram. The γ-Ni liquidus boundary is located between Ni50Mn25Ga25 and Ni45Mn27.5Ga27.5 in the equal Mn and Ga alloy series, and between Ni50Mn5Ga45 and Ni50Mn10Ga40 in the 50 at.% Ni alloy series. The alloys with compositions close to the stoichiometric Ni2MnGa composition that show the magnetic shape memory effect are all covered by the γ-Ni liquidus surface. The β-NiMnGa liquidus surface covers the remaining alloy compositions.

Comparison of martensite transformation temperatures in a NiMnGa alloy determined with hot/cold stage optical microscopy and differential scanning calorimetry

The martensite transformation temperatures of both as-grown and heat-treated specimens removed from a Bridgman grown boule of off-stoichiometric Ni2MnGa were determined by differential scanning calorimetry (DSC) and hot/cold stage microscopy. The work showed that martensite start and austenite finish transformation temperatures determined by the hot/cold stage microscope technique were in agreement with those determined by the DSC method. The hot/cold stage microscope technique was shown to be useful for characterizing variations of transformation temperature across a specimen. The results revealed that residual stress, deformation and boule composition variations produce artefacts in DSC traces which need to be identified, understood and controlled. Transmission electron microscope results suggest that the possible contribution of a premartensitic transformation to the high temperature edge of the martensite peak on DSC scans needs further investigation.