Effect of Al incorporation for Co on the gamma-beta phase boundary of rapidly solidified CoNiAl ferromagnetic shape memory alloys Satnam Singh, R.K. Roy, B. Mahato, M. Ghosh, A. Mitra, A.K. Panda n Material Science and Technology Division, CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India article info Article history: Received 18 March 2014 Received in revised form 13 May 2014 Accepted 30 May 2014 Available online 10 June 2014 Keywords: CoNiAl alloy Rapid-solidification Martensitic transformation Magneto-strain. abstract The alloys Co 64x Ni 36 Al x (at%, X ¼23,24,26 and 28) prepared through melt spinning technique revealed β- and γ-phase structure at room temperature. A monotonous increase in β-phase and simultaneous decrease in γ-phase has been observed with increasing Al content. The synergistic competition between β- and γ-phase influence β(B2)-L1 0 martensitic transformation. Therefore transformation broadening has been observed with Al content. Consequently, martensitic transformation temperature decreases and austenite transformation temperature increases. The γ-rich alloys showed magnetically soft phase, with high initial susceptibility and enhanced saturation magnetization. These alloys also revealed large magneto-strain values which are also supported by micro-structural morphology. & 2014 Elsevier B.V. All rights reserved. 1. Introduction In recent years different stoichiometric variations [1,2] and heat treatment [3,4] in NiMnGa ferromagnetic shape memory alloys (FSMAs) have been investigated to enhance the FSMA properties. Though this alloy system deliver high magneto-strain[5], but its high brittleness limits its applicability as actuator material. Alternate Ni- based alloy systems like NiFeGa, NiMnAl have been therefore explored [6]. The quest for enhanced properties also lead to investigations on Fe-base systems like FePd and FePt [6]. Though these alloy systems are ductile but they are expensive enough to be used as actuator materials at commercial scale. The Co-based FSMAs are gaining prominence owing to use of relatively cheaper elements than Fe-based systems. A typical CoNiAl system is under intensive investigation due to its ductility and preparation intricacies pertaining to low melting point Ga element in NiMnGa development. The ductility in CoNiAl system is delivered by the presence of γ-phase in combination with other phases [7]. In this alloy the martensitic transformation occurs between L1 0 and β-phase (B2), wherein former is tetragonal martensite while later is bcc phase [7,8]. The B2-L1 0 martensite transformation is displayed by a limited composition range and at low temperature [7]. In CoNiAl alloys the martensite transforma- tion temperature depends on alloy composition [9], preparation history of the alloy [10]and electron concentration (e/a ratio)[9]. At constant Al content, increasing Co decreases martensite trans- formation while increases Curie temperature [11]. However increasing Ni at constant Al raised martensite transformation and reduces Curie temperature [12]. This investigation reported high Curie temperature and saturation magnetization in L1 0 martensite phase as compared to β-phase. Kositsyna et al. reported rise in martensite transformation temperature in the range of 303–333 K and 373–383 K for 1 at% replacement of either Co by Ni or Al by Ni, respectively[13]. Most of the research is focused on stoichiometric variation in CoNiAl system at constant Al concentration [11,12]. Moreover the alloys studied were mostly in the form of ingots, annealed and water quenched materials. The present investigation is an attempt to investigate the influence of Al incorporation for the Co on the material processed in the form of ribbons through melt spinning technique. 2. Experimental Pure elements (purity 499.95%) were melted in vacuum arc melting furnace to prepare master alloys with nominal composi- tions series of Co 64 x Ni 36 Al x (X ¼ 23, 24, 26 and 28 at%). The alloys were induction melted and melt-spun into ribbons. The prepared ribbons were annealed at 400 1C for 2 h for relieving quenched-in stresses. The prepared ribbons are designated as Al 23 , Al 24 and so on depending on their Al content. Phase identification of the prepared melt spun ribbons was carried out using an X-ray diffractogram (Bruker D8) with Cu-Kα radiation. Temperature variation of electrical resistivity was measured using Quantum Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials http://dx.doi.org/10.1016/j.jmmm.2014.05.053 0304-8853/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ91 657 2345002; fax: þ91 657 2345213. E-mail addresses: akpanda@nmlindia.org, akpanda2_in@rediffmail.com (A.K. Panda). Journal of Magnetism and Magnetic Materials 368 (2014) 379–383