Yoichi Kishi

Martensitic transformation in Co2NiGa ferromagnetic shape memory alloys

Abstract This investigation of the martensitic transformation in ferromagnetic Co2NiGa shape memory alloys shows that, like in the Ni2MnGa system, the martensite start temperature is proportional to the valence electron concentration. Al is increasing the transition temperatures when it substitutes Ga and is decreasing them when it substitutes Ni.

Microstructures and magnetic properties of rapidly solidified CoNiGa ferromagnetic shape memory alloys

The transformation characteristics of CoNiGa ribbons resemble those of bulk alloys. However, the transformation temperatures in the ribbons are higher than those of the bulk suggesting stress-induced martensites. TEM observations reveal arch typical twins and precursor tweed structures. The twins can be wavy suggesting a small boundary energy.

Cobalt-base ferromagnetic shape memory alloys

Single crystalline and polycrystalline CoxNiyGa100-(x+y), 41 < xCo< 62 and 19.3 < yNi < 32.7, Ferromagnetic Shape Memory Alloys have been produced in the range of the Heusler-type composition. Elasto-mechanical properties have been analyzed for the annealed and quenched states, respectively. The mechanical spectroscopy data show the occurrence of martensitic phase transformation with the transition range and characteristics depending on the state and the composition of the alloys. For XCo approximately equals 49 +/- 1 at percent, the Ni/Ga ratio was shown to be in direct relationship with the transition temperature range, from an Ms of -100 degrees C for Ni/Ga approximately equals (21/29) to a +150 degrees C for a Ni/Ga ratio of about (26/25). For Ga approximately equals 27 +/- 0.4 at percent, the Co/Ni ratio is in indirect relationship with the transition temperature, with an Ms of -125 degrees C for a (53/19) ratio to a +175 degrees C for a ratio of about (49/26). Optical and electron microscopy shows that a typical thermoelastic martensitic transformation occurs. The L21 Structurbericht parent phase transforms into monoclinic or orthohombic martensitic upon cooling. The formation of a Co-rich phase has been observed for alloys with lower Ga content and is considered to be one of the reasons for the difference in the transformation range for annealed and quenched alloys.

Ferromagnetic NiMnGa and CoNiGa Shape Memory Alloy Films

This paper reports results on Heusler-type NiMnGa and CoNiGa ferromagnetic shape memory alloy films deposited by magnetron sputtering and pulsed laser deposition respectively, on silicon cantilever-type substrates. NiMnGa FMSMA films deposited by magnetron sputtering on silicon substrates show good ferromagnetic and shape memory properties if the substrate is heated above 350°C or if the films deposited at RT are annealed above 600°C. The growth of the film is columnar. TEM examination shows typical martensitic variants while the SAED pattern suggests an orthorhombic structure. CoNiGa FMSMA films deposited at 550°C by PLD are crystalline. Mechanical spectroscopy data proves the presence of a typical martensitic transformation that develops below room temperature: the phase transitions manifest themselves by typical changes of the modulus defect and displacement vs. temperature characteristics similar to those associated with the martensitic transformation in known shape memory alloy films. The change in the elastic modulus as a function of the magnetic field suggests that in both NiMnGa and CoNiGa films the spontaneous magnetization is clamped by the magnetic anisotropy field.

Magnetic Tweed Contrast In Ferromagnetic Shape Memory Alloys

Recently, ferromagnetic martensite, specifically ferromagnetic shape memory alloys (FMSMAs), have received renewed attention because of their large domain motion induced magnetostrictive strains. In the low temperature phase of these alloys, 90◦ magnetic domain walls are simultaneously elastic twin boundaries. Above the temperature at which these alloys undergo a near second order martensitic transformation, their microstructure as observed by two-beam TEM methods, consists of the well known tweed contrast. This structure represents a random mixture of incipient twins of the martensitic phase. The transformation hysteresis of Co-Ni-Ga FMSMAs is quite small. In addition, the alloy is elastically soft similar to others that undergo a near second order martensitic transformation. Therefore, it is possible that magnetic tweed be observed in this alloy. This paper reports on its observation by conventional TEM, electron diffraction and Lorentz microscopy. Disciplines Materials Science and Engineering | Metallurgy Comments This article is from Microscopy and Microanalysis 9 (2003): pp. 584—585, doi:10.1017/S143192760344292X Authors Marc De Graef, S. Venkateswaran, Yoichi Kishi, Thomas A. Lograsso, D. Viehland, and Manfred Wuttig This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/ameslab_pubs/140 MAGNETIC TWEED CONTRAST IN FERROMAGNETIC SHAPE MEMORY ALLOYS M. De Graef , S. Venkateswaran , Y. Kishi, T.A. Lograsso, D. Viehland, and M. Wuttig 1 Dept. of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213 2 Dept. of Materials Science and Engineering, University of Maryland, College Park, MD 20742 3 Ames Laboratory, Iowa State University, Ames, IA 50011 4 Dept. of Materials Science and Engineering, Virginia Tech, Blacksburg, VA 24060 Recently, ferromagnetic martensite, specifically ferromagnetic shape memory alloys (FMSMAs), have received renewed attention because of their large domain motion induced magnetostrictive strains. In the low temperature phase of these alloys, 90◦ magnetic domain walls are simultaneously elastic twin boundaries. Above the temperature at which these alloys undergo a near second order martensitic transformation, their microstructure as observed by two-beam TEM methods, consists of the well known tweed contrast. This structure represents a random mixture of incipient twins of the martensitic phase. The transformation hysteresis of Co-Ni-Ga FMSMAs is quite small. In addition, the alloy is elastically soft similar to others that undergo a near second order martensitic transformation. Therefore, it is possible that magnetic tweed be observed in this alloy. This paper reports on its observation by conventional TEM, electron diffraction and Lorentz microscopy. The TEM studies were carried out on a JEOL 4000EX microscope operating at 400 kV. This system is equipped with a Gatan imaging filter (GIF) which compensates for the magnification loss due to the increased focal length of the Lorentz imaging mode and also permits removal of inelastically scattered electrons to enhance the image contrast. Imaging of ferroelastic or structural domains was performed using the conventional bright field mode, whereas imaging of the ferromagnetic domain structure was performed using Lorentz microscopy (both Fresnel and Foucault imaging modes). Fig. 1 shows a bright-field image of the (400) bend contour of Co 0.50Ni0.205Ga0.295, taken at room temperature. The accompanying diffraction pattern was taken near the [110] zone axis orientation, and exhibits pairs of diffuse streaks in apparent [112] directions, typical for tweed modulated structures. The image reveals ferroelastic or structural domains. Tweed-like contrast can be seen, with a characteristic length scale of about 10 nm. In this image, the cross-hatched tweed patterns are clearly evident on both sides of the bend contour. Zone axis patterns acquired for the [111] orientation reveal that the diffuse streaks lie along 〈110〉-type directions. Fig. 2 shows again a bright field bend contour in the in-focus (b), under-focus (a), and over-focus (c) condition. While the bright band (arrowed) does not show any tweed contrast features in the in-focus image, the defocused images clearly reveal tweed-like contrast on a length scale of several tens of nanometers. Since the contrast only appears in the out-of-focus images, it must be caused by magnetic contributions to the electron phase shift. This has been confirmed using Foucault images, for which the displaced aperture again causes contrast to appear in the featureless region of Fig. 2b. Using the Transport-of-Intensity formalism [1] we have reconstructed the phase of the electron wave (shown in Fig. 2d). The components of the gradient of the phase are shown in (e) and (f), and reveal more clearly the modulated magnetic microstructure. Tweed-like striations identical to those shown in Fig. 1 have previously been observed by TEM at temperatures above the transformation temperature, M S, which upon cooling below MS condense into micron size twin structures, sometimes through an adaptive metastable phase. The known tweed structure in paramagnetic austensites is elastically driven. It can be anticipated that magneto-elastically driven magnetic tweed will equally condense into ferromagnetic martensite. The dual, magnetic and elastic, nature of the tweed demonstrated in Figs. 1 and 2 must be the result of magnetoelastic coupling. We will present theoretical considerations in support of the existence of magnetic tweed. References 1. D. Paganin and K.A. Nugent, Phys. Rev. Lett. 80 (1998) 2586. 2. This work was supported by the U.S. National Science Foundation under grant numbers DMR0095586 and DMR0095166, the Office of Naval Research, contracts No. MURI N000140110761, N000149910837 and N000140010849, and the Office of Basic Energy Sciences, Materials Sciences Division, of the U.S. Department of Energy under Contract Nos. W-7405-ENG-82 and DE-FG0201ER45893.