Phase Stability of Single Crystalline Co-Ni-Ga Shape Memory Alloy
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Phase Stability of Single Crystalline Co-Ni-Ga Shape Memory Alloy
Deborah L. Schlagel, Thomas A. Lograsso, and Alexandra. O. Pecharsky Materials and Engineering Physics Program, Ames Laboratory, Iowa State University Ames, Iowa 50014
ABSTRACT Single crystals of Co48Ni22Ga30 have been synthesized using a modified Bridgman method. The ability to solidify and retain single phase B2 austenite was found to depend not only on the starting composition and growth rate, but also the ability to maintain sufficiently high cooling rates to avoid the precipitation of a Co-rich FCC phase during post-solidification cooling. DSC measurements on the single crystal found the Ms, Mf, As, and Af to be 35.7°C, -1.8°C, 34.1°C and 72.2°C, respectively. On subsequent heating the B2 phase was found to partially decompose into the Co-rich phase at temperatures exceeding 380°C. Decomposition of the single phase B2 phase was tracked by microstructural observation, DSC, powder diffraction and low temperature heat capacity measurement. Restoration of the crystal to single phase B2 austenite required annealing of the crystal at temperatures above 1125°C followed by rapid cooling. INTRODUCTION The recent investigations of shape memory transformations in Co-Ni-Ga alloys [1-4] have reported shape memory behavior comparable to those associated with other ternary Heusler alloys such as Ni2MnGa. Initial measurements on Co-Ni-Ga alloys show similar trends to Ni2MnGa alloys in both their dependence of martensitic transformation temperatures on electron concentration and saturation magnetization [2, 5]. However, unlike Ni2MnGa alloys which are single phase over a large compositional range [6], recent phase equilibria investigations [3,4] have reported that Co2NiGa alloys with transformations near room temperature lie within a twophase field, similar to the Co–Ni–Al alloys [4,7,8]. This major difference in phase equilibria imposes significant limitations on the synthesis of single crystals as well as subsequent annealing and training of the martensitic transformation. Studies of the solidification microstructures coupled with differential thermal analysis of Co-Ni-Ga alloys have shown these alloys solidify via a peritectic reaction, resulting in two phase mixtures of a Co-rich FCC phase and an austenitic B2 ordered phase. Our results agree with Oikawa et. al. [4]. In our study we concluded that the primary formation of FCC phase can be avoided by increasing the Ga content; however, the martensitic transformation is either suppressed to temperatures far below room temperature or eliminated completely due to the lowering of the electron concentration. The synthesis of single crystals of Co-Ni-Ga alloys therefore requires balancing the gallium content of the alloy to avoid solidification of a second phase during growth while maintaining the electron concentration sufficiently high for near room temperature martensitic transformations. Such a balance has been found for the composition Co48Ni22Ga30 as shown in Figure 1.
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Figure 1. Single crys
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