Symposium : S
Shape memory materials for smart systems III
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| 08:50 | Beginning of the Symposium | |
| Welcome to Symposium S - Shape Memory Alloys I : Manfred Kohl | ||
| 09:00 | Shape setting of SMAs for smart structure applications Authors : P.Šittner Institute of Physics of the ASCR, Prague, Czech Republic Resume : Shape setting of shape memory alloy elements is an important step in the development of smart structures incorporating SMAs, namely NiTi. The outcome of the shape setting is twofold, NiTi component displays functional properties (e.g. superelasticity) and its parent austenite shape is memorized. This is due to the microstructure changes brought about by the thermomechanically driven lattice recovery processes triggered at high temperatures. The success in achieving desired shape and functional properties (including their stability during subsequent thermomechanical cycling) depends on the: i) alloy composition, ii) hot/cold work, iii) phase composition, iv) annealing temperature, v) time and vi) force experienced by the element during the high temperature exposure. When developing smart structures, researchers can use various (ii-vi) to control the outcome of the shape setting and achieve thus the desired shape and functionality of the SMA element used in the smart structure. We have recently performed dedicated in-situ experiments (mechanical, electrical resistivity, X-ray and TEM) during nonconventional shape setting of thin NiTi wires by short pulses of DC electric current. These studies brought about some new insight into the shape setting procedure, particularly to the nature and kinetics of the lattice recovery processes responsible for it. The results will be discussed in this work and new opportunities for smart structure development will be suggested. | 1 1 |
| 09:30 | The Effect of Thermal Boundary Layers on SMA Actuators Authors : Stephen Furst, John Crews, and Stefan Seelecke Dept. Mech. & Aero. Eng. North Carolina State University Raleigh, NC USA Resume : Under realistic (non-adiabatic) thermal boundary conditions,
electrically driven SMA actuators develop inhomogeneous temperature
profiles characterized by a thermal boundary layer. Due to the
thermo-machanical coupling in SMAs, this has an immediate effect on
the mechanical performance, leading to a loss of stroke of up to 30%
compared to the homogeneous case. The paper presents an experimental
study of the behavior of SMA wires of various lengths in tensile tests
reflecting the influence of the thermal boundary layers on the
resulting stress-strain hysteresis. These experiments are compared to
fully thermo-mechanically coupled simulations with COMSOL using a
mesoscopic free energy model for the SMA behavior. | 1 2 |
| 09:45 | Measurement and analysis of local strain heterogeneities in superelastic Shape Memory Alloy by Digital Image Correlation Authors : Nadine Bourgeois, LPMM-CNRS, Université de Metz, Ile du Saulcy, 57045 Metz, France Fodil Meraghni, LPMM-CNRS, ENSAM, 4 Rue Augustin Fresnel, Metz-Technopôle, 57078 Metz, France Tarak Ben Zineb, LEMTA-CNRS, ESSTIN UHP, 2 rue Jean Lamour, 54500 Vandoeuvre lès Nancy, France Resume : This work is focused on the measurement of the local strain heterogeneities in a multicrystal CuAlBe Shape Memory Alloy (SMA). Displacement and strain fields are measured by the Digital Image Correlation technique on a SMA thin sample submitted to uniaxial tension. The tested SMA is superelastic: austenite-martensite transformation is hence reversible.
Polycrystalline sample with one grain in the thickness and grain size between 500 m and 1 mm. is used. Crystallographic orientation of each grain is measured by Electron Back Scattered Diffraction technique. The specimen is etched to reveal the grain boundaries. Several tests are carried out on the same specimen, in front of a long distance microscope with or without speckle pattern (spray of paint).
Evolutions of the martensitic transformation are investigated and analysed in terms of the strain heterogeneities. In copper SMA, the important anisotropy of the elastic moduli tensor involves high intergranular elastic strain heterogeneities. The phase transformation may induce a release of these heterogeneities through displacement discontinuities. Accordingly, martensite variants induce steps on the specimen surface. These steps interfere with the speckle patterns and would perturb the correlation process. However, the strain fields were measured for greater load levels, when the material runs through the phase transformation. The study shows that strains in one grain are disturbed by the neighbouring grains and this disturbance has a strong effect on the variant activation. Experimental boundary conditions have also a large influence on the transformation. The study shows that the martensitic transformation mainly occurs in localized bands involving the well crystallographic oriented grains with respect to the load direction. | 1 3 |
| 10:00 | Coffee Break | |
| Shape Memory Alloys II : Petr Sittner | ||
| 10:30 | Nitinol wire production from pre-alloyed Spark Plasma Sintered powders Authors : J. Butler1, P. Tiernan1, A. A. Gandhi1, S. A. M. Tofail1* 1Materials and Surface Science Institute, University of Limerick, Limerick, Ireland Resume : Spark plasma sintering (SPS) is a relatively new cost-effective processing route where powder is compacted and consolidated by passing high current, low voltage spark pulsed DC current directly through the powder compact and graphite die under uniaxial pressure. A very high heating and cooling rate is possible which ensures the powders are consolidated and densified without coarsening. This fast processing also limits diffusion mechanisms, making possible the formation and the stabilization of microstructures which are not attainable by other common powder metallurgy routes such as Hot Isostatic Pressing. Hence, SPS is ideal for producing new materials without having to go down the more expensive ingot melting route. For this study the shape memory alloy Nitinol is spark plasma sintered and extruded to wire. Equaiatomic NiTi powders were sintered into small billets with less than 2 % porosity, put into a protective sleeve and isothermally extruded into 3 mm diameter wire. Characterisation by Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Energy Dispersive X-Ray (EDX), microhardness and tensile testing showed that the extruded wire has similar properties to commercially produced NiTi wire with all remaining pores being submicron. The results show that SPS followed by extrusion is a quick and efficient method of obtaining a commercial grade NiTi material. This result opens the possibility of developing new NiTi alloy materials in SPS promoting a cost effective research route. SPS on an industrial scale is forecast to be a cost effective processing route in the near future, so research work with SPS is currently becoming more prevalent. | 2 1 |
| 10:45 | Crystallization behavior and shape memory characteristics of Ti-Ni-Cu alloy ribbons Authors : Tae-hyun Nam, Min-soo Kim, Young-min Jeon(Gyeongsang National University, Korea) Yeon-wook Kim(Keimyung University, Korea) Eunsoo Choi(Hongik University) Resume : Amorphous Ti-(50-x)Ni-xCu (at.%) (x = 20, 25, 27, 30) alloy ribbons were prepared by melt spinning and then their crystallization behavior was investigated by X-ray diffraction and differential scanning calorimetry. ΔT (a temperature gap between Tg and Tx) increased from 33 K to 47K and wavenumber (Qp) decreased from 29.44 nm−1 to 29.29 nm−1 with increasing Cu content from 20 at% to 30 at%. Activation energy for crystallization decreased from 188.5 kJ/mol to 170.6 kJ/mol with increasing Cu content from 20 at% to 25 at%, then it keeps almost constant. Crystallization occurred in two-stage; amorphous - B2 - TiCu2 in Ti-Ni-Cu alloys with Cu content less than 25 at%, while it occurs in three-stage; amorphous - B2 - TiCu - TiCu2 in Ti-Ni-Cu alloys with Cu content more than 27 at%. Some of amorphous Ti-30Ni-20Cu(at%) ribbons were cold rolled by 30%, then their crystallization behavior was investigated. Although cold working did not bring about deformation-induced crystallization, it reduced activation energy for crystallization from 198.8 kJ/mol to 121.7 kJ/mol and lowered exothermic peak temperature(Tp) for crystallization from 755 K ~ 769 K to 730 K ~ 751 K, suggesting that it assists crystallization of amorphous Ti-30Ni-20Cu(at%) ribbons. Cold working induced inhomogeneous grain size distribution after crystallization, reduced average grain size, widened transformation range and raised martensitic transformation temperatures, which were ascribed to a strain field introduced during cold working. | 2 2 |
| 11:00 | Martensitic transformation and mechanical behaviour of Porous Ti-50.0 at%Ni alloy, fabricated by self-Propagating High Temperature Synthesis at different temperatures Authors : N.Resnina, S. Belyaev, A. Voronkov, V. Mozgunov, A. Krivosheev, I Ostapov Saint-Petersburg State University Resume : Ti-50.0 at%Ni alloy was fabricated by self-propagating high-temperature synthesis at different temperatures from range of 340 – 550 C. Influence of pre-heating temperature of SHS process and post-fabrication annealing on crystalline structure, kinetics of martensitic transformation and mechanical properties were studied. It was observed that independent on preheating temperature concentration of nickel distributed in TiNi phase heterogeneously that resulted in existence of TiNi phase with different concentration. Only about 20 % of the sample volume underwent a B2 – B19’ martensitic transformation at different temperature ranges, another 80 % of the sample was TiNi phase containing more than 52 % of Ni concentration, Ti2Ni and Ti3Ni4 particles except samples, fabricated at 480 and 500 C, where a Ti3Ni4 phase was not observed. It was found that post-fabricated annealing influenced the volume fraction of Ti3Ni4 particles that led to redistribution of Ni concentration and, as a result, increase a sample volume undergoing a phase transition and martensitic transformation temperatures. It was observed that pre-heating temperature influenced the strength and strain up to break of the samples because of existence or absence of Ti3Ni4 particles. It was found that porous TiNi alloy annealed at 300 – 500 C had good prerequisites for pseudoelastic behavior. | 2 3 |
| 11:15 | Surface Oxidation of NiTi Authors : Abdus Mahmud1,2, Yinong Liu1, Hong Yang1 and Tae-hyun Nam3 1 School of Mechanical Engineering, The University of Western Australia, Crawley, WA 6009 Australia 2 School of Mechanical Engineering, Universiti Sains Malaysia, 14300 Seberang Perai Selatan, Malaysia 3 School of Materials Science and Engineering, Gyeongsang National University, 900 Gazwadong, Jinju, Gyeongnam 660-701, Korea Resume : This study investigated the surface oxidation of NiTi at elevated temperatures. Heat treatment in air is found to cause selective oxidation of the Ti content, thus subsequent parting of the alloy on the surface, and titanium depletion in the interior of the alloy. The oxidised surface consists of three distinctive layers, the outer layer of TiO2, a composite layer of Ni(Ti) and TiO2, and a thin inner layer of TiNi3. The Ti-depleted zone is formed within the matrix of the parent alloy underneath the TiNi3 layer. During the oxidation process, the thicknesses of the outer TiO2 layer and the inner TiNi3 layer remained unchanged with time, whilst the composite layer continued to expand at the expense of the parent alloy. A diffusion-based mechanism is proposed to explain the process of the selective oxidation and the morphology of the oxidised surface layers formed. The Ti-depleted layer, though retaining its B2 structure, exhibits no martensitic transformation, thus no shape memory effect. The formation of these layers has adverse effects on the shape memory and pseudoelastic properties of the alloy. | 2 4 |
| 11:30 | Comparison of mechanical and tribological of TiC-NiTi and TiC-TiB2-NiTi coatings Authors : J. Ventayol (1), N. Cinca (1), I.G. Cano (1) A. Isalgue (2), J. Fernandez (1), J.M. Guilemany (1) (1)Thermal Spray Center (CPT). University of Barcelona. Faculty of Chemistry. CMEM. Marti i Franques, 1. 08028 Barcelona (Spain) (2) Applied Physics Department. Politechnical University of Catalonia. Campus Nord B4, Jordi Girona 1. 08034 Barcelona. (Spain) Resume : The TiC and TiB2 are widely used to increase wear resistance of coatings. This work compares the mechanical and tribological properties for the coatings containing TiC or TiC-TiB2. The feedstock powders have been obtained by the SHS technology. This process produces uniformly distributed ceramic particles of 3-4 µm in the metallic matrix. After crushing and sieving, the 20 – 63 micrometers have been chosen for plasma spray. The cross sections of the coatings have been characterized by SEM-EDS. The ASTM C-633 has been used to check the adherence of the coating interface. Wear tests following ASTM G99-03 and ASTM G65-00 have been performed to characterize the wear resistance of both kind of coatings.
The coatings containing TiB2 exhibit lower adhesion and higher wear rates compared with the TiC coatings. This behavior is related with the degradation of the TiB2 during the thermal spray process. | 2 5 |
| 11:45 | Texture Evolution in Hot Rolled Ni-Ti-Hf Alloys Authors : K. S. Suresh and Satyam Suwas Department of Materials Engineering, Indian Institute of Science, Bangalore 560 012. India. Resume : Ni-Ti-Hf ternary Shape memory alloys have attracted considerable attention due to their high transformation temperature. The addition of Hf to NiTi system increases both its strength as well as it makes it brittle. So, the processing of these Hf based alloys becomes difficult. It has been sown that the crystallographic texture strongly affects the shape recovery strain as well as the performance of the system. Up to date, all the reported texture studies mainly focus on binary system. So, an attempt has been made to understand the deformation mechanism of NiTiHf system through texture measurement. Three different Hf compositions were selected for the present investigation, namely 10%, 12% and 15% Hf. All the alloys were vacuum arc melted and solutionized at 1000C before processing. Hot rolling was carried out at 925C with 10% reduction per pass up to a true strain of 2. X-ray diffraction patterns of the as cast and solutionized material show only B19’ monoclinic phase. However, the R-phase has been found to get stabilized with increase in deformation. The hot rolled strips were characterized for X-ray texture measurement. The texture development with deformation for all the three compositions are reported in terms of pole figures for the B19’ monoclinic phase. | 2 6 |
| 12:00 | Characterization of the surface layers alloyed by Silicon, Molybdenum or Tantalum in an TiNi alloy treated by pulsed electron beams Authors : L.L. Meisner, A.I. Lotkov, M.G. Dement’eva, S.N. Meisner, A.A. Neyman, G.V. Prosorova. Resume : The results of X-ray diffraction analysis of nonequilibrium structural and elastic stress states in TiNi surface layers irradiated by low-energy electron beams is presented. It is found that a surface layer with a mixed 2D– and 3D– submicrocrystalline structure is formed on the irradiated side of the TiNi specimens, and the volume fractions of the two structure types depend on the beam energy parameters and number of pulses. The B2 phase synthesized in the layer is characterized by lattice microstrain due to stresses of the first and second kinds. The main mechanism of the stress relaxation is partial B2B19’ martensite transformation. The thickness of the layer in which the relaxation processes develop through the B2B19’ martensite transformation is 1015 m.
It is shown that the pulsed electron-beam treatment of the TiNi surface layers coated by the Si, Mo, or Ta lead to a change of the structural-phase conditions within the surface TiNi substrate. The outer layer with the B2 lattice parameter bigger then this one in the initial B2 phase is formed beneath the TiNi substrate surface coated by Mo or Ta. This one indicates that the ternary B2 phase (TiNi–Mo) is formed after the electron-beam impact. It was found that Si, Mo, or Ta are partially or fully dissolved within the surface layer of TiNi with thickness of 2–4 m when the electron-beam energy density was E1=15 J/cm2. and E2=30 J/cm2, correspondently. | 2 7 |
| 12:15 | Lunch Break | |
| Ferromagnetic Shape Memory Alloys I : Franca Albertini | ||
| 14:00 | Inverse Magnetoplasticity and Power Generation using Ni-Mn-Ga Authors : P. Mullner, P. Lindquist, K. Wilson, A. Rothenbuhler Resume : Inverse magnetoplasticity is the change of magnetization induced by deformation. For magnetic shape-memory alloys, the strain and the change of magnetization are proportional to the fraction of individual martensite variants. The variation of the magnetization varies the stray field energy, which may be harvested via induction in a coil. To repeatedly operate an energy harvesting device, the strain must be reversed, e.g. via a magnetic bias field. The efficiency of energy conversion is limited by mechanical losses due to internal friction (twin boundary motion) and by self-induction. At low frequency, the mechanical losses are proportional to frequency and self-induction is negligible. The electrical power generated in the coil is proportional to the square of the frequency and, thus, the efficiency of energy conversion increases linearly with frequency. At higher frequency, self-induction becomes dominant and limits the efficiency. Cyclic deformation experiments at low frequency confirmed the quadratic dependence of power. Individual loading cycles display as a set of narrow voltage peaks. Each peak can be related to a discontinuity of the stress-strain curve. These results indicate a strong dependence of the induced voltage on the twin microstructure and the mechanical properties of the transducer. We will present results of cyclic deformation experiments in the frequency range up to 0.5 kHz and discuss energy conversion in this frequency range based on details of the induced voltage signal and of the stress-strain hysteresis. | 3 1 |
| 14:30 | Field and stress induced twin boundary motion in NiMnGa Authors : Andreas Neudert, Yiu-Wai Lai, Rudolf Schäfer, Jeffrey McCord IFW Dresden, Helmholtzstr. 20, 01069 Dresden, Germany Resume : Twin boundaries in NiMnGa can be moved by applying either magnetic fields or mechanical stresses, which favour one of the two variants next to the twin boundary. We have used polarized light microscopy to study the two different ways of twin boundary motion in bulk NiMnGa. By placing magnetic indicator films on top of the sample surface we were able to detect the magnetic domain structure of the sample. Without the indicator films, the different variants can be seen by using polarized light illumination.
We found qualitative differences in the resulting magnetic state after a twin boundary has been moved by magnetic field or mechanical stress. By applying a magnetic field along the easy axis of one variant this variant is magnetically saturated and upon reducing the magnetic field the domain state consists of wide anti-parallel domains with 180° domain walls. Moving the twin boundary by applying mechanical stress results in a different domain state. Here the magnetization rotates by about 90° as the twin boundary passes through the area and we found the domain state to consist mainly of smaller patchy domains. The domain state with wide anti-parallel domains can be reached by demagnetizing the sample in an ac magnetic field. This suggests that the magnetic state after stress induced twin boundary motion is not in its global energy minima but rather trapped in a higher energy state. | 3 2 |
| 14:45 | Crack growth of 10M Ni-Mn-Ga material in cyclic mechanical loading Authors : I. Aaltio, Y. Ge, H. Pulkkinen, A. Sjöberg, O. Söderberg, X.W. Liu, and S-P. Hannula; Aalto University, School of Science and Technology, Department of Materials Science and Engineering, P.O. Box 16200, FI-00076 Aalto, Finland Resume : The 10M-martensitic Ni-Mn-Ga single crystal materials are usually applied in the magneto-mechanical actuators. Therefore, it is important to know the possible effect of the long-term cyclic shape changes on their structure and behavior. This can be evaluated also with the mechanical fatigue testing. In the present study, the 10M samples with different compositions were applied to strain-controlled uniaxial mechanical cycling in the multivariant state at ambient temperature. The experiments revealed distinctive changes of the twin variant structure, especially in the mobile twin area, density of twin boundaries, and in the tendency for fatigue crack growth. Characterization of the crack surface showed that the cracks grow of specific crystallographic planes. | 3 3 |
| 15:00 | Production and characterization of large scale single crystals made of ferromagnetic shape memory alloy Ni-Mn-Ga Authors : L. Sturz1, A. Drevermann1, U. Hecht1 E. Pagounis2, M. Laufenberg2 1: Access e.V., Intzestr. 5, D-52072 Aachen, Germany 2: ETO MAGNETIC GmbH, Hardtring 8, D-78333 Stockach, Germany Resume : Ferromagnetic shape memory alloys (MSMA) are well-known smart materials for actuation applications, due to their large shape change in applied magnetic fields. Off-stochiometric Ni-Mn-Ga single crystals of MSMA exhibit a magnetic-field induced strain of about 10%. In the past decades a lot of effort has been put into understanding and optimizing the properties of Ni-Mn-Ga alloys in terms of the magnetic shape memory effect, starting with the production of single crystals and going all the way to heat treatment and magneto-mechanical training.
Here we focus on and present the production and characterization of single crystal Ni-Mn-Ga alloys with off-stochiometric composition. The molten material was cast into preheated ceramic shells and directionally solidified in a Bridgman-type process. Cylindrical single crystal bars of diameters up to 40mm and a length of 110mm were obtained by using suitable single crystal selectors. Only small misorientations between the single crystal c-axis and the bar axis were found in these bars. The crystals were characterized in terms of composition, segregation, crystal orientation and microstructural features by means of SEM/EDS, EBSD and light microscopy. Additionally as-cast and heat-treated materials were compared in detail. | 3 4 |
| 15:15 | Deformation studies of Ni55Fe19Ga26 ferromagnetic shape memory alloy Authors : Aniruddha Biswas and Madangopal Krishnan Material Science Division, Bhabha Atomic Research Centre, Mumbai-400085, India Resume : Ni-Fe-Ga-based alloys form a new class of ductile Ferromagnetic Shape Memory Alloys (FSMA). These alloys with compositions close to the Ni2FeGa stoichiometry show a thermoelastic martensitic transformation and a high Magnetic Field Induced Strain (MFIS), comparable to the more established Ni-Mn-Ga-based FSMAs. However, the latter have poor ductility and can only be utilized in single-crystal form. The problem of brittleness of the Ni-Mn-Ga-based alloys can largely be overcome in case of Ni–Fe–Ga-based ones by introducing a face-centred-cubic -phase. This makes the alloys amenable to thermo-mechanical processing and thus opens up the possibility of achieving a favourable texture in polycrystalline materials.
Ni–Fe–Ga-based alloys exhibit a two-phase microstructure at certain compositions and annealing conditions and offer sufficient ductility to enable hot deformation processing. The crystal structure and phase fraction of martensites in Ni-Fe-Ga alloys depend on their composition, processing, and thermal history. The current study explores thermo-mechanical processing of this FSM alloy using an off-stoichiometric Ni55Fe19Ga26 alloy containing the ductile -phase. The alloy is prepared by vacuum arc melting and solutionized at 1080oC for 24 h. The alloy is further aged at temperatures above (750oC) and below (600oC) the B2- L21 ordering temperature. Aging treatments were observed to depress Ms to below room temperature (RT). Scanning electron microscopy and electron probe microanalysis showed that aging caused the precipitation of the ordered ’-phase as an envelope surrounding the -phase. The effect of aging in depressing the MS was utilized in retaining L21 phase at room temperature for texture analyses. The Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) investigations of the solutionized sample show that the martensite is stable at RT. The martensite was determined to be an internally-twinned tetragonal non-modulated type. Traces of modulated 14M martensite and the parent L21 phase are also found along with the face-centered-cubic g-phase.
Though the cubic g-phase is the one that imparts ductility to this alloy, micro hardness measurements at room-temperature show very little difference between the martensite and the g-phase. In addition, hot-hardness experiments of the matrix and the second phase were carried out to compare micro hardness at elevated temperatures. Hot-rolling experiment of Ni55Fe19Ga26 alloy was successfully carried out at 950oC and 1000oC, both with and without mild steel jacket. Typically, a vacuum arc-melted button of 60 gm weight is hot-rolled with a total reduction of thickness from 10mm to 1 mm. The quality of the rolled products is superior when mild steel jacket is used. An interlayer of thin niobium / tantalum foil is used to avoid any interaction with the mild steel jacket. The deformation microstructure of the hot-rolled products was carried out in detail. The microstructure shows the reduction of grain size of L21 phase and elongation of the -phase along the rolling direction. By and large, the hot deformed microstructure is free of cracks unlike what is expected from a brittle phase like L21. Texture analyses were carried out both for the L21 and the -phase independently using x-ray diffraction techniques. Microtexture analysis of the deformed microstructure was carried out by Electron Back-Scattered Diffraction technique. This article will present the evolution of the two-phase microstructure of Ni-Fe-Ga alloys upon hot deformation. | 3 5 |
| 15:30 | Coffee Break | |
| Ferromagnetic Shape Memory Alloys II : Peter Mullner | ||
| 16:00 | Kinetic arrest behavior in martensitic transformation of NiCoMnSn metamagnetic shape memory alloy Authors : R.Y. Umetsu,a) K. Ito,b) W. Ito,b) K. Koyama,c) T. Kanomata,d) R. Kainuma,b) and K. Ishida b) a) Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Sendai 980-8577, Japan b) Department of Materials Science, Graduate School of Engineering, Tohoku University, 6-6-02 Aoba, Sendai 980-8579, Japan c) High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan d) Faculty of Engineering, Tohoku Gakuin University, Tagajo 985-8537, Japan Resume : Magnetic-field induced shape memory alloys have been now intensively investigated, and a drastic change in magnetization associated to the reverse martensitic transformation has been confirmed in various Ni-based Heusler alloys. A kinetic arrest phenomenon, in which martensitic transformation is interrupted at a certain temperature and does not proceed with further cooling, has been reported in NiCoMnIn, NiMnIn and NiCoMnGa alloys. In order to clarify the origin of this phenomenon, in the present study, we investigate the metamagnetic behavior of NiCoMnSn alloy.
A polycrystalline Ni37.0Co11.0Mn42.5Sn9.5 alloy was made by induction melting, and then annealed at 1173 K for one day. Magnetic measurements were carried out by SQUID magnetometer and an extraction-type magnetometer using a superconducting magnet with magnetic field up to 18 T.
From the thermomagnetization curves measured under the magnetic fields of 0.05 and 5 T, it is shown that the transformation temperature decreases about 67 K by applying magnetic field of 5 T. This value is in good agreement estimated from the Clausius-Clapeyron relation with using the value of the difference of the magnetization and the entropy change. In the cooling process under applying magnetic field of 5 T, martensitic transformation does not occur down to low temperature region because the ferromagnetic austenite phase is stabilized by the applied magnetic field and mobility of habit plane becomes low. | 4 1 |
| 16:15 | Effect of temperature on the isothermal martensitic transformation in the “arrested” state of Ni-Mn-In-Co metamagnetic shape memory alloy Authors : S. Kustov, D. Salas, E. Cesari Departament de Física, Universitat de les Illes Balears, Cra Valldemossa, km 7.5, E07122, Palma de Mallorca, Spain Resume : It has been shown recently that the direct martensitic transformation (MT) in metamagnetic shape memory alloys is essentially isothermal, resulting in a pronounced isothermal accumulation of martensite [1]. Without polarizing magnetic field (and, hence, during a complete MT) the rate of isothermal accumulation of martensite was found to be proportional to the MT rate during an uninterrupted cooling. As a consequence, isothermal accumulation of martensite is restricted to the range of the MT. We report here the results of studying the isothermal direct MT in an “arrested” state of a Ni-Mn-In-Co alloy. Experimental results on the kinetics of MT are obtained by means of resistance measurements under polarizing fields up to 3 T. We show that the same regularity of the isothermal accumulation of martensite holds for the “arrested” state of the MT, namely, isothermal accumulation of martensite occurs only over the temperature range wherein the temperature-induced MT rate remains non-zero.
We associate the present results with recent observation of a rapid decrease with TC-Tm (the Curie and MT temperatures, respectively) of the absolute value of the entropy change during the MT. Our results indicate that the “arrest” of the MT is not a “kinetic” one, as has been usually claimed, but has a thermodynamic origin. To confirm this conclusion, we additionally study isothermal accumulation of martensite under step-like variation of polarizing field, which can be considered as producing nearly constant thermodynamic driving force, and found its very weak temperature dependence even well below the temperature of the “arrest” of the MT.
[1] S. Kustov, I. Golovin, M.L. Corró, E. Cesari, Journal of Applied Physics, in print. | 4 2 |
| 16:30 | Remarkable pressure effects on Co-doped NiMnGa multifunctional alloys Authors : F. Albertini*, J. Kamarad^, Z. Arnold^, S. Fabbrici*, A. Paoluzi*, L. Righi° *IMEM-CNR, Parco Area delle Scienze 37/A, I-43010 Parma, Italy °Dipartimento di Chimica GIAF, Università degli Studi di Parma, Via G.P Usberti 17/A, I-43100 Parma, Italy ^Institute of Physics, ASCR, v.v.i., Na Slovance 2, Prague 8, Czech Republic Resume : NiMnGa alloys display several “giant” effects (e.g. magnetocaloric, magnetoelastic, magnetoresistive) due the interplay between magnetic and structural degrees of freedom. They can be finely tuned by means of suitable changes in stoichiometry (Ni50+xMn25+yGa25+z, x+y+z=0) and/or element substitutions. We have recently shown that for Mn-rich NiMnGa alloys, Co substitution produces remarkable changes in magnetism and structure, giving rise to a peculiar phase diagram showing reverse magnetostrucutural transformations. Such transformation, characterized by a strong and negative field dependence of the critical temperature (|dTm/dH| up to 10 K/T), a huge difference in the saturation magnetization and in the unit-cell volumes between the two phases (DM up to 50 Am2/Kg and DV/V up to 1% , respectively ), make these materials of primary interest for their possible applications in multifunctional devices [1]. In this contribution, we present the effects of hydrostatic pressure (up to 1 GPa) on magnetism and martensitic transformation in Ni50-xCoxMn25+yGa25-y (5 | 4 3 |
| 16:45 | Ageing effects on structural and magnetic transformations in a NiCoMnGa alloy Authors : C.Seguí and E. Cesari Dept. Física, Univ. Illes Balears. Palma de Mallorca (Spain) Resume : Partial substitution of Ni by Co in Mn-rich Ni-Mn-Ga alloys has been found to modify the magnetic ordering of the phases, improving in this way the possibility to obtain large magnetization difference between austenite and martensite, an essential requirement to induce the reverse martensitic transformation by application of a magnetic field. Particularly, the Ni50-xCoxMnyGa50-y alloys undergo, for Co content below x=9, structural transformation between ferromagnetic austenite and paramagnetic martensite, thus leading to enhanced magnetization difference values.
The martensitic transformation temperatures as well as the martensite and austenite Curie temperatures depend on composition, but significant changes can be brought about by selected thermal treatments. In this work, the composition is selected as Ni42Co8Mn32Ga18 in order to obtain concurrent martensitic transformation and austenite Curie temperature, and the effect of quench and subsequent ageing on the structural and magnetic transitions is studied. Aside from the monotonic transformation temperatures change, which is mostly attributed to atomic ordering taking place upon post-quench ageing, the results show the effect of the relative position of the structural and magnetic ordering reactions on the transformation entropy change | 4 4 |
| 17:00 | Influence of grain size and training temperature on strain of polycrystalline Ni50Mn29Ga21 samples Authors : C. Hürrich1, M. Pötschke1, S. Roth1, B. Rellinghaus1 and L. Schultz1,2 1IFW Dresden, Institute for Metallic Materials, P. O. Box 270116, 01069 Dresden, Germany 2 Dresden University of Technology, Department of Mechanical Engineering, Institut for Materials Science, 01062 Dresden, Germany Resume : The Ni-Mn-Ga alloys arose great interest for application as a magnetic shape memory material. This effect is caused by reorientation of twin variants by an external magnetic field. So far most of the experiments were concentrated on single crystals. But, this effect can also be realised in polycrystals which can be prepared much more efficiently. Here, polycrystalline samples were prepared by directional solidification with a <100> fibre texture of the high temperature cubic phase parallel to the heat flow. Afterwards a heat treatment was applied for chemical homogenization and stress relaxation in the austenitic state. Then the samples were heated up to the austenitic state and cooled down under load. The microstructure was analysed by Electron Back Scatter Diffraction (EBSD) before and after that treatment. Mechanical training at room temperature and 40 °C was tracked by recording stress-strain curves. With increasing the number of training cycles the strain also increases. The different training temperatures were investigated on samples with different grain sizes.
This work is supported by DFG within SPP 1239. | 4 5 |
| 17:15 | Thermal and microstructural stability on ageing high-temperature Ni-Mn-Ga alloys Authors : R. Santamarta, E. Cesari, J. Pons Departament de Física, Universitat de les Illes Balears, E- 07122 Palma de Mallorca, Spain J. Muntasell, J. Font Departament de Física i Enginyeria Nuclear, Universitat Politècnica de Catalunya, E-08028 Barcelona, Spain P. Ochin Institute de Chimie et des Matériaux Paris Est, F-94407 Vitry-sur-Seine, France Resume : The thermal and microstructural evolution under ageing at temperatures up to 770 K has been studied on several polycrystalline high temperature Ni-Mn-Ga shape memory alloys, all of them with an excess of Ni and higher e/a ratio as compared to the stoichiometric Ni2MnGa. [1]
Within this set of alloys, those with higher content of Ni exhibit a very poor thermal stability, caused either by chemical decomposition or by fast precipitation of Ni-rich phases, which can promote loosing most of the transformation even after few cycles in the DSC. Two types of precipitates have been observed: lath-like precipitates with composition not very far from that of the matrix and ellipsoidal L12 (gamma’) precipitates with chemical composition rather different from the matrix. Some of these alloys show a reasonable stability until precipitation in the matrix starts; after that, the martensitic transformation degrades rather quickly.
On the other hand, Mn-rich alloys with Ni close to 50 at% can exhibit a stable martensitic transformation without presence of precipitates even after 7·10^7 s (27 months) at 770 K. Therefore, the thermal stability of some compositions of the Ni-Mn-Ga system is comparable to the most stable high-temperature shape memory alloys.
[1] R. Santamarta, E. Cesari, J. Muntasell, J. Font, J. Pons, P. Ochin. Intermetallics (2010), in press. | 4 6 |
| 17:30 | Effect of Porosity on the Magneto Mechanical Properties of Magnetic Shape Memory Foam Authors : C. Witherspoon, P. Zheng, M. Chmielus, D. C. Dunand, P. Mullner Resume : As single crystals, magnetic shape memory alloys display magnetic-field-induced strain of up to 10%. However, high production costs, time consumption, and chemical segregation, are inherent disadvantages of single crystals. Polycrystalline materials do not have the drawbacks of single crystals but display a much smaller magnetic field induced strain. It has been shown that adding porosity can cause polycrystalline magnetic shape memory foam to achieve magnetic-field-induced strains as high as 8.7%. Presented here is a systematic study of the effect of porosity on the magneto mechanical properties of polycrystalline magnetic shape memory foams. Individual polycrystalline foam samples where subjected to successive etching experiments to increase the porosity. After each etching experiment, the magnetic-field-induced strain was measured and thermo-magneto-mechanical training was performed. Consistently, the magnetic-field-induced strain increased with increasing porosity, in the extreme case by a factor of 10 with a porosity increase of only 2%. | 4 7 |
| 17:45 | Role of Si in improving the shape recovery of Fe-Mn-Si-Cr-Ni shape memory alloys Authors : Bikas C. Maji and Madangopal Krishnan Materials Science Division, Bhabha Atomic Research Centre, Mumbai – 400 085, INDIA Resume : Addition of Si in Fe-Mn-Si based alloys has been seen to be beneficial for the improvement of shape recovery. The most often cited reason in literature for this effect of silicon is the lowering of the paramagnetic to antiferromagnetic Neel transformation temperature (TN) of austenite phase to below the austenite to martensite transformation start temperature (MS): stress inducing of epsilon martensite, thus, becoming easier. However, there are several reports which show large amounts of epsilon martensite can be stress induced even when MS < TN. Therefore, the exact role Si plays for enhancing shape recovery is not very clear. In the present study an attempt was made to determine the role of Si in improving shape recovery and the optimum amount required to obtain highest recovery in Fe-Mn-Si-Cr-Ni shape memory alloys.
Fe-14Mn-9Cr-5Ni-xSi (x=1 to 10, all in wt.%) alloy buttons were prepared by non-consumable vacuum arc melting. These were hot rolled at 950-1000oC into strips of 0.5 mm and further solution heat treated at 1000oC for 1 h. It was observed during hot rolling that alloys containing Si > 7% tend to crack. Therefore, only alloys of Si content upto 7% have been used to evaluate shape recovery. Microstructural characterization using X-ray diffraction, optical and scanning electron microscopy shows that austenite is stable upto 6% Si, beyond which the microstructure is a two-phase one containing delta-ferrite and austenite. The microstructures of alloys containing > 7% Si is composed of a delta-ferrite matrix within which are plate shaped Fe5Ni3Si2 type phase. The presence of this brittle intermetallic phase may be reason why alloys containing > 7% Si could not be hot rolled. The shape recovery of the alloys was evaluated using a simple bend test: a prestrain of 2-8% was given and followed with a recovery annealing for 10 min. at 600oC. The amount of shape recovery was observed to increase monotonically with amount till about 6% Si. The 7% Si alloy showed low shape recovery, possibly on account of the large fraction of delta-ferrite in its microstructure. Mechanical tests were carried out at room temperature to determine the strength of these alloys. It is observed that the yield strength of the alloys only vary marginally with Si content (160 to 220 MPa). Experiments on the differential scanning calorimeter revealed that while the MS practically unaffected by increasing Si content (varies within 30 ± 5 oC), the enthalpy of transformation decreases. This indicates that the amount of athermal epsilon martensite reduces with higher Si content. This fact was also confirmed through quantitative X-ray Rietveld phase analysis of the solutionized samples. The amount of stress induced epsilon martensite after 6% prestraining were determined by X-ray analysis. It was found that stress induced martensite is highest in the 6% Si alloy, which also shows the highest shape recovery. Poor shape recovery with Si contents less than 3% was primarily due to the formation of alfa martensite through the epsilon to alfa transformation. Stacking fault energy (SFE) of the alloys was estimated from the stacking fault probablilities determined through Warren-Averbach method of X-ray line profile analysis. The SFE was found to decrease from 200mJ/m2 to 20mJ/m2 with Si content. In addition, it was estimated that the shear modulus decreases 75 to 60 GPa. Based on the results of this study, it is safe to make the following conclusions: i) shape memory in these alloys is essentially due to stress induced martensite; ii) nucleation of stress induced martensite and consequently shape recovery is enhanced by Si which decreases the SFE and the shear modulus; iii) the decrease in athermal martensite is probably related to reduction in population of pre-existing embryos with Si addition; iv) poor shape recovery at Si content <2% and >6% is due to the formation of alfa martensite through epsilon to alfa transformation and the large volume fraction of delta-ferrite, respectively. | 4 8 |
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