STRUCTURE PROPERTY STUDIES IN FERRO-MAGNETIC SHAPE MEMORY Ni-Mn-Ga-X ALLOYS

Abstract

Shape Memory alloys based on Fe, Ni, Cu and Ti systems are finding an ever-increasing application in devices due to a large recoverable strain caused by conversion of variants in the martensite phase. Martensitic transformations are typically first order transitions without atom diffusion. Such transformations are influenced by external physical parameters such as temperature; applied uniaxial stress and hydrostatic pressure. In ferromagnetic systems, strains under magnetic fields or the magnetostriction due to realignments of magnetic domains is a common phenomenon with small strains usually of the order of —I 0-6. In contrast, magnetic field induced large reversible strain results in recoverable shape changes in some martensite alloys known as Ferro Magnetic Shape Memory (FMSM) alloys. Off-stoichiometric Ni2MnGa has shown the Magnetic Field Induced Strains (MFIS or 2) going an order of magnitude higher than that observed commonly in magnetostrictive materials. The FMSM alloys have an additional advantage of displaying the phenomenon without contact and with a faster response time over the heating methods applicable to the conventional shape memory alloys. A range of applications envisaged for the FMSM alloys emerges from their ability to deliver a large MFIS ( optimized ?) including those in sensors and actuators in smart devices, force delivery (optimized E.?) in retractable missile wings and energy packet ( optimized E.22) for SONAR transducers. These materials may find a place in some of the applications where conventional magnetostrictive materials (e.g. in low frequency higher surveillance range sonar transducers in place of Terfenol-D) or a conventional shape memory alloys (faster response actuators and transducers like retractable wings in place of Nitinol) are employed. Presently the FMSM elements are available commercially and a few demonstration devices have already been made (www.adaptamat.com). Functional properties of Ferro-Magnetic Shape Memory (FMSM) alloys i.e., a large reversible change in dimensions under a magnetic field, emanates from the reorientation of martensite variants under an external field. Reorientation of the variants requires suitable ferromagnetic martensite variants with low de-twinning energies to enable movement of boundaries with the forces generated by the magnetic field. Large magnetization, adequate anisotropy and martensite variants with a low de-twinning energy are preliminary requirements for FMSM alloys. In addition, these alloys should possess other engineering properties such as ease of fabrication, ductility, environmental stability etc. While a range of alloys show the FMSM phenomenon, alloys based on Ni2MnGa compositions have been investigated in literature extensively as Ni-Mn-Ga alloys show large FMSM strains around room temperatures (Ullakko et al, 1996; Kakeshita and Ullakko, 2002). The mechanical properties of the Ni-Mn-Ga alloys is however poor. Improvements in the mechanical properties are attempted through both alloy modification as well as improved processing. The alternate Heusler alloys such as Co-Ni-Al, Ni-Fe-Ga are also being investigated to overcome the limitations of the Ni-Mn-Ga alloys (Wuttig et al, 2000; Oikawa et al, 2001). The process improvements result in microstructural refinements accommodative of stresses generated during the FMSM phenomenon (Li et al, 2004). The alloy development includes modifications within the Heusler phase or generation of beneficial secondary phases, thereby necessitating multi-component alloy systems with different components providing one or more beneficial aspects to the basic alloy system. The X2YZ type Heusler alloys consists of the transition element X and Y, and also, trivalent element Z with majority of atomic magnetization concentrated at Y atom positions and a very little contribution reported from other atoms. The present study aims at modification of Heusler alloys -by addition of quaternary element M by preparing Ni-Mn-Ga-M alloys and investigating their structural and magnetic behaviour. Existing reports indicate that Nis0Mn306a20 is the optimal composition of the Heusler alloy maximizing the ferromagnetic martensite transformation temperature and so, this composition was chosen as the base composition for modification. The quaternary additions from the following groups were investigated: I. Transition element II. Trivalent element III. Other elements different from those of group I and II ii The transition elements in group I like Fe and Co, which are in between Ni and Mn, have been selected. Substitutions for either Ni or Mn have been intended. Group-II and Group-III additions are elements substituting for Ga. The addition of trivalent elements like Al and B from group II has been tried. B-atoms were expected to go into interstices and change the behaviour of the parent Heusler alloy completely. B-additions were therefore made along with Al addition and alloys with B—additions were 5 component alloys. The additions outside groups I and II are categorized as Group III additions and the elements added may have low solubility in the L21 phase, generating thereby grain boundary phases, in order to improve the mechanical properties. Chapter-I contains the introductory remarks about the role of the ferromagnetic shape memory alloys in the present generation of smart devices and their potential in the context of engineering applications. Chapter-2 describes the available literatures on the basics of the characterization and' the terminology along with the FMSM alloy system with special emphasis on the primary- system viz. Ni-Mn-Ga alloys. Finally, the gap in current status of knowledge about FMSM alloys have been identified and. the problem under investigation has been formulated. Chapter-3 presents the experimental techniques and procedures employed for preparation and characterization of the alloys. The preparation technique includes the alloy melting of and subsequent heat treatment of the bulk alloys. The rapid quenching method adopted for select alloy compositions is also elucidated. The chapter also highlights various characterization techniques and specimen preparations and methods for data analysis relevant to this study. As stated earlier, a base composition Ni50Mn30Ga20 (atom %) was chosen and accordingly quaternary alloys with replacement for the specific elements were prepared by repeated vacuum arc melting of high purity constituent elements. The alloys melted were homogenized for different durations by vacuum heat-treatment. The alloys thus homogenized were characterized as bulk Alloys. As the alloys with boron are normally easy glass formers, iii alloys with trivalent aditions of Al and/or B additions were processed through rapid quenching route additionally to see the effect of processing technique. The alloys thus prepared were characterized for their chemical composition, structure and microstructure and were evaluated for chemical, magnetic and thermal properties. Alloy chemistry was analyzed by chemical method as well as energy dispersive and wave length dispersive analysis facilities available with electron microscope. Diffraction pattern using X ray diffractometer for bulk alloys and electron diffraction for ribbon specimens were used for determining crystal structure. Optical and electron microscopy were employed for microstructure characterizations. The differential scanning calorimetry measurements gave the transformation temperatures. Magnetic hysteresis loops were recorded at room temperature using vibrating sample magnetometer. In addition, thermo-magnetic measurements were also made on VSM wherein magnetizations under a constant magnetic field at different temperatures were recorded to complement the transformation temperatures through thermal techniques. Alloys with Fe —additions were analyzed through Mossbauer spectroscopy to understand the site occupancy of Fe atoms. The measurements so made were correlated to arrive at a coherent knowledge base. Chapter-4 presents the results of studies with the intention to substitute Ni/Mn atoms in Ni50Mn30Ga20 alloy by Group-I additions. Alloys with compositions Ni40MioMn30Ga20 (transition element M=Fe or Co) with intended substitutions of 10 at% M at Ni sites were prepared by vacuum arc melting and heat-treated at 1273K for 3 hrs and gas quenched in order to investigate the effect of addition of Fe and Co. The as-cast alloys contained -small amount of secondary phases. While alloy Ni40Co10Mn30Ga20 showed single-phase structure after heat-treatment, the alloy Ni40Fe10Mn30Ga20 showed the presence of traces of a secondary phase in x ray diffraction studies. The microstructural studies however showed signs of the ploughing out the secondary phase during specimen polishing. The diffraction patterns revealed a cubic L21 (Heusler) structure at room temperature for the alloy Ni40Fe10Mn30Ga20 and a modulated martensite (5M) structure for the alloy Ni40Co1oMn30Ga20 after heat-treatment. The obtained phase structures were consistent with the martensite transformation temperatures evaluated for the alloys through two independent techniques of thermal analysis iv by differential scanning calorimetry (DSC) and thermo-magnetic measurements on a vibrating sample magnetometer (VSM). The room temperature saturation magnetization improved with addition of ferromagnetic elements Fe or Co respectively over the reported values for ternary base composition. In view of the L21 structure of Fe containing alloy and the martensite, a one-to-one correlation was difficult. The lowering of martensite transformation temperatures may be explained due to a reduction in electron to atom ratio with Fe/Co additions. The Mossbaeur spectrum of the Fe containing alloy indicated the presence of Fe atoms at paramagnetic as well as ferromagnetic sites. In combination with the magnetic measurements, the ferromagnetic spectrum could be explained with Fe atoms placed at Mn sites. Based on the studies conducted on Ni4oKoMn3oGazo (M=Fe/Co), a systematic study for the effect of varying amounts of Fe with intended substitutions for Ni or Mn were undertaken. Series of alloy compositions Ni5oFe,Mn30_,Ga20 (x=2.5, 5 and 15) and Ni45Fe5Mn30Ga20 were prepared. The heat—treated Ni40Fe1 oMn3oGa20 has indicated the presence of a trace secondary- phase, thereby suggesting that the heat-treatment temperature of 1273 K is in two-phase field. The heat treatment was therefore modified to a two stage heat-treatment i.e. 1273 K for 72h and 1073K for 48h followed by water quenching. The alloys with low Fe content of 2.5 and 5 at% were single-phase martensite structure whereas alloy with 15 at% Fe showed cubic L21 austenite with about 10 at% Fe along with a second phase depleted in Mn and Ga. The two-, phase structure showed that the limit of solubility of Fe in the base L21 structure is below 15 at% Fe. The room temperature magnetization increased with increasing Fe and the measurements at 173K for the martensite phase for all the compositions indicated the saturation magnetization to follow no specific pattern with Fe. The martensite transformation temperatures measured by DSC and VSM techniques did not follow a consistently increasing or decreasing trend with increasing Fe. The Mossbaeur spectrum of the alloys showed the two ferromagnetic sub-spectra for the single phase alloy Ni45Fe5Mn30Ga20 These measurements indicated that Fe went to Ga sites initially and at higher Fe contents but at intermediate level of addition, it went to Mn and then to Ni sites. The Fe atoms are thus located in different chemical environment at different substitution levels. Chapter-5 presents the studies carried out with Group II and III additions substituting for Ga in Ni50Mn30Ga20 alloy. For the Group-II additions, the initial investigations with Ni5oMn30Gal5A15 alloys showed that the marten site transformation may have modified to two stage sequence with Al additions. Moreover the martensite transformation temperatures were lowered with the Al addition even though the average electron to atom ratio, a key parameter to the stability of the martensite is unchanged. Additions of smaller sized atom B were contemplated for compact structures. Envisaging a large replacement of the largest atom Ga with small B atoms may form interstitial alloy rather than substitutional alloys, compositions Ni50Mn30Ga1sA14B and Ni5oMn3oGaisAl IR' have been prepared and investigated. The addition of B did not change the phase structure, but the magnetizations were drastically reduced even at I at% B. The grain sizes were refined as revealed by the peak broadening of the x-ray diffractograms. With B and Al as possible glass formers, alternative technique of rapid quenching using melt spinning was tried to understand the effects of the addition on the processing aspects. The melt spun alloys showed a direct formation of beneficial 7M martensite phase along with varied amount of the amorphous phases of different compositions. The martensite obtained was very fine and the sub martensite structure showing the orientation relationship was also seen. For Group-III additions, many additives have been reported earlier, however one of the main aims of this study was to understand the effect of modifications in Heusler structure with the additions rather than obtaining secondary phases modifying other functional properties. The rare earth additions were selected as these additions are-known to improve the magnetic properties especially in rare earth- transition element permanent magnets through 3d-4f electronic interactions if present in the Heusler structure. I at% addition of a light rare earth element Sm and heavy rare earth element Dy for substituting the largest atom of Ga have been selected. Ni5oMn3oGai9Dy1 and Ni50Mn30Ga19Sm I accordingly were prepared and heat-treated at 1273 K for 72h and 1073K for 48h followed by water quenching. EPMA studies of both the alloys showed the matrix phase with rare earth elements below detectable limits, which were detected only in second phase. The change in martensite temperatures and the work hardening behaviour during mechanical testing, however; have indicated the vi possibility of small amount of the rare earths in the matrix phase. Magnetization of the alloys lowered with additions in line with lower phase fraction of the matrix phase. The Curie temperature of the alloys lowered and the martensite transformation showed an increase. The modified matrix compositions due to formation of the second phase, showed an increased Mn/Ga ratio over the bulk composition. The studies with the group III elements has shown that these additions though partitioning mainly to second phase, may be retained in very small levels to modify the structure and properties of the L21/ related martensite phases. Both the transformation temperatures as well as specific magnetization were lower for the heavy rare earth Sm additions. The beneficial effects of the rare earth additions may therefore be through modifications of the secondary functional properties such as ductility, mechanical strength etc. through modified L21/ martensite phases as well as the secondary phases. These additions may be useful in the wider context involving the other functional aspects. Chapter 6 outlines the broad conclusions of the present investigation and the emerging trends for future studies.