STRUCTURE AND SELECTED CORROSION PROPERTIES OF Ni45Mn39Sn11Co5 ALLOY. Kateřina SKOTNICOVÁ, Stanislav LASEK, Radim KOCICH

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1 STRUCTURE AND SELECTED CORROSION PROPERTIES OF Ni45Mn39Sn11Co5 ALLOY Kateřina SKOTNICOVÁ, Stanislav LASEK, Radim KOCICH VŠB-TU Ostrava, RMTVC, 17. listopadu 21, Ostrava, Poruba. Abstract The structure characteristics of specimens were investigated using optical light microscopy and scanning electron microscopy equipped with EDX detector. The specimen in as-cast state evinced the dendritic structure, which gradually transformed into regular grains after thermal treatment. The structure of specimen after 24 hours of heat treatment at 900 C was formed by a martensitic phase and by the residue of dendritic grains with the increased content of cobalt and decreased content of tin. On the basis of potentiodynamic polarization method the resistance to corrosion of the alloy Ni45Mn39Sn11Co5 was evaluated in the as-cast state and after heat treatment (annealing at 900 C, during 2, 8, 18 and 24 hours). The sensitivity to pitting was compared mainly by values of depassivation and repassivation potentials of the tested alloy (in 0.1 M NaCl water solution). The resistance to this localized corrosion was slightly lower than for pure nickel (curves between iron and nickel). After heat treatment the resistance to pitting had increased and dimensions of pits decreased. Polarization resistance had higher values after heat treatment and corresponding rates of corrosion were very low, like in a passive state. The corrosion resistance has been described with respect to chemical composition of matrix and secondary phases and/or particles. Key words: Ni45Mn39Sn11Co5 alloy, heat treatment, structure, polarization test, pitting, 1. INTRODUCTION Ni-Mn-based meta- shape memory alloys show a drastic change in magnetization accompanied by martensitic transformation from a ferro phase (austenite) to a para phase, and they have received much attention as high-performance multi-ferroic materials. The martensitic transformation temperatures in these alloys are drastically decreased by application of a field, and field induced reverse transformation (MFIRT) occurs in martensite phase in the vicinity of the martensitic transformation temperatures. The shape memory effect (SME) induced by a field, i.e. meta shape memory effect (MMSME), has been confirmed in Ni45Co5Mn36.7In13.3 and Ni43Co7Mn39Sn11 at room temperature [1-3]. It is reported that the MT temperature of FSMAs can be tuned by altering the ingredient of the alloys or by substituting one element by another element, and it is very sensitive to the values of the valence electron concentration e/a (electrons per atom) [4-5]. New Heusler alloys are investigated for their shape memory effect that is related to the austenite martensite phase transformation. The recovery of shape occurs when changing from a lower symmetry of martensite structure to a symmetrical cubic phase (austenite) by increasing the temperature. The examples of Heusler shape memory materials are Ni 2 MnZ- based alloys (Z = Sn, In, Ga) or Mn 2 NiGa [6]. Expensive Ga can be replaced by cheaper Sn for shape memory applications. Alloy with Sn10 has cubic L2 1 structure and martensitic transformation in ferro state. Investigation was performed to characterize the corrosion behaviour of polycrystalline Ni 48 Mn 30 Ga 22 and Ni 50 Mn 30 Ga 20 alloys with cubic austenite, or tetragonal martensite state [7]. Both alloys exhibit low corrosion rates and spontaneous anodic passivation in alkaline solutions (ph 8-11). In acidic media the alloys tend towards more active state with an increase of corrosion current density. XPS studies showed that passive films comprised NiOOH, MnO 2 and Ga 2 O 3 in air. Passive films formed in neutral media (ph 5-8.5) were composed of Ni(OH) 2, NiOOH and Ga 2 O 3 in the outer region and of NiO,MnO 2 a MnO in the metal side region. Under all conditions studied, mainly in acidic solutions, the martensite alloy (Ni 48 Mn 30 Ga 22 ) was

2 significantly more reactive than the austenite Ni 48 Mn 30 Ga 22. This can be attributed to a much higher density of surface defects in terms of a large number of twin boundaries acting as energetically favoured sites for corrosion attack. SEM analysis of the martensite surface confirmed a preferential corrosion of the twin boundaries. In this work we discuss the effect of heat-treatment conditions on the microstructural characteristics and the resistance of Ni45Mn39Sn11Co5 alloy to pitting corrosion. 2. EXPERIMENTAL 2.1 Materials and samples Polycrystalline Ni45Mn39Sn11Co5 (at.%) alloy was prepared by induction melting of the appropriate quantities of high purity Ni, Mn, Sn and Co under an argon atmosphere, which were cast into a graphite cylinder mold with diameter of 10 mm. The ingot was cut into small pieces for micro-structural analysis and corrosion testing of the as-cast and annealed structure of specimens. The specimens were annealed in an evacuated and sealed quartz tube at 900 C for 2, 8, 18 and 24 hours in order to achieve a homogenization, and quenched in ice-cold water. The structure of as-cast and annealed specimens was investigated by light optical microscopy (LOM) and SEM equipped with EDS detector. Small samples with dimensions 10 x 4 x 3 mm (semi-disc shape) were prepared for corrosion tests, by precise cutting from the rod 10 mm. The samples were tested after heat treatment (900 C/2h-24h) and reference sample was used in the as-cast state. Chemical composition has been determined by X-ray microanalysis. Different standard potential of elements and electrochemical properties of phases can play role in formation of micro-galvanic cells and non-uniform corrosion under certain conditions. The values of standard potentials (E o ) of elements used for alloy preparation: Mn V, Ni V, Sn V, Co V (in HNE scale, hydrogen normalized electrode). Phases rich in Mn can act as less noble anodic places and they can accelerate the corrosion. 2.2 Electrochemical corrosion tests The polarization tests were conducted at room temperature (20 C) in 0.1 mol/l NaCl water solution (200 ml) with free access of air. The nominal exposed area of each immersed sample was 1.0 cm 2. Polarization tests began with determination of corrosion potential E cor. The potentiodynamic measurement [8] started with the potential E cor 50 mv and potential scanned in anodic direction at the rate of 1 mv/s. When the density of anodic current reached J = 1 ma/cm 2, polarisation direction was changed at return potential. Registered anodic polarizatioin curves made the basis for determination of typical parameters describing the resistance to uniform and pitting corrosion, i.e.: corrosion potential E cor, depassivation potential E d, repassivation potential E r, polarisation resistance R p, corrosion current density i cor, average rate of corrosion (r c ). Measurements were carried out with use of the system VoltaLab PGP201 and PC. Saturated calomel electrode (SCE) served as the reference electrode, whereas platinum wire electrode was used as the auxiliary electrode. 3. RESULTS 3.1 Micro-structural analysis The typical microstructures of the as-cast Ni45Mn39Sn11Co5 alloy investigated by LOM and SEM are shown in Figure 1. Note that this alloy has solidified into at least two phases. The contrast developed in the back-scattered electron micrographs and EDS microanalysis suggests significant compositional differences between these phases (Fig. 1c-d) due to a variation of Mn/Co/Sn. The solidification begins with the dendrites of primary phase having a Mn/Co/Sn ratio of 38/3/14. This solidification is followed by the formation of a second phase with the Mn/Co/Sn ratio of 45/8/4. The micro-structural changes, which occur as a function

3 of annealing time at 900 C are documented in Figure 2, and back-scattered SEM micrographs are given in Figure 3. Rapid conversion of this multiphase microstructure to a single phase microstructure takes place owing to the initial small length scale from the cast process and the large range of solid solubility below the solidus temperature [4-5]. a) b) Fig. 1 The LOM (a) and SEM (b) micro-structures of the as-cast Ni45Mn39Sn11Co5 alloy. The composition exhibits multiphase solidification behaviour. c) d) Fig. 2 Microstructures of the Ni45Mn39Sn11Co5 alloy after annealing: a) 2 h; b) 8 h; c) 18 h; d) 24 h The micro-structure of annealed specimens is formed by martensitic phase and the initial dendritic phase. The content of dendritic phase significantly decreases with the increasing time of annealing and dendrites

4 Current density J [A/m 2 ], log J, Current density J [A/cm 2 ], , Brno, Czech Republic, EU are refined. The structure of this alloy was, however, not homogenized completely after 24-hour annealing. According to micro-analysis the matrix consists of Mn40Ni44Sn12Co4 (at %), secondary phases have nominal composition Mn44Ni41Sn2Co12. Chemical composition of matrix and particles (rest of interdendridic phase) was not changed by heat treatment. After exposition to 900 C/18 h and 900 C /24 h the secondary phases were detected in smaller amount in comparison with shorter time of heat exposition (900 C/2h and 900 C/8h). Fig. 3 Microstructures of the Ni45Mn39Sn11Co5 alloy after annealing. SEM micrographs taken in back scattered mode: a) 2 h; b) 24 h. Martensitic structure of matrix and graphite inclusions from mould. 3.2 Corrosion tests The typical registered polarization curves (loops) for pitting corrosion are documented in Fig. 4. The relatively smaller resistance to pitting corrosion was found on as-cast sample. After annealing at 900 C, corrosion resistance improved. Differences in pitting parameters among samples after performed heat treatment are quite small, Tab.1, in relation with small changes in chemical composition and structure of samples C/2 h C/8 h C/18 h C/24 h 1 as-cast C/2 h C/8 h C/18 h C/24 h Potential E [mv] SCE Potential E [mv] SCE Fig. 4 Comparison of potentiodynamic polarization curves and pitting resistance of the alloy Ni45Mn39Sn11Co5 after heat treatment. Polarization loops measured at 1-st (a) and 2-nd (b) cycle. Examples of surface appearance after corrosion test are documented in Fig. 5. Under test conditions the dark grey spots were also formed. The larger pits were formed on the cast sample.

5 Table 1 Results of potentio-dynamic polarization test of the alloy Ni45Mn39Sn11Co5 (at %) sample Heat E cor E d E v E r R p j cor r c note note No. treatment mv mv mv mv kω.cm 2 μa/cm 2 μm/r cycle 1 as-cast C/2h C/8h C/18h C/24h slight non non non pits pits 2,0 mm spots spots 1,0 mm Fig. 5 Surface of the sample after 900 C/24h (a) and 900 C/18h (b) and after corrosion test 4. DISCUSSION The solidification behaviour of Ni-Mn-Sn-Co alloy is multiphase. The formed phases differ in composition from each other mostly in the ratio of Mn/Sn/Co. The length scale of the compositional inhomogeneity depends on the solidification condition. It is generally accepted that finer micro-structural length scale (faster cooling rate) is preferred to minimize homogenization times of as-cast structures [2]. The as-cast structure of this alloy consists of dendritic phase with the increased content of Sn and decreased content of Mn and Co in comparison with the spaces between the arms of dendrites. The heat treatments led to the progressive transformation of dendritic structure to regular shaped grains of martensitic phase. After 24-hours of annealing, the structure consists mostly of the martensitic phase with the minor amount of initial dendritic phase. It is evident from Fig. 6 that properties of alloy change in the dependence on the annealing time. The as-cast structure evinces the highest magnetization, whereas the specimens after 24-hours of annealing are nearly para due to mostly martensitic structure. When the content of dendritic phase is reduced by the increasing annealing time and the specimens are chemically homogenized (formation of martensitic structure after cooling), the magnetization decreases. The measured values of parameters (Table 1) of the alloy were placed between elements Ni and Fe. The lower values of potentials and polarization resistance in the second cycle were caused by pitting and damage of surface at the first cycle. Approximate values of R p are higher for the alloy after heat treatment.

6 The values of corrosion current density (j cor ) and calculated rate of corrosion (r c ) correspond to a passive state before pitting initiation. Heterogeneous minor phases and/or particles can initiate pitting. Magnetic field can cause non-uniform corrosion. Fig. 6 M-H loops at 25 C for Ni45Mn39Sn11Co5 alloy in as-cast state and after annealing 5. CONCLUSIONS Progressive Ni45Mn39Sn11Co5 (at %) alloy has shape memory effect that can be influenced or controlled by field. As-cast structure was homogenized by the heat treatment (900 C/2-24h/water) and the changes in structure were observed by light and scanning electron microscopy. Micro-analysis of the matrix and secondary phases (particles) has revealed the differences in composition (matrix Mn40Ni44Sn12Co4 and rest of Mn44Ni41Sn2Co12). The resistance to pitting corrosion was evaluated on the basis of potentiodynamic cyclic method. Higher values of resistance to pitting potentials were found after thermal treatment in comparison with heterogeneous as-cast state. ACKNOWLEDGEMENTS This paper was created in the project No. CZ.1.05/2.1.00/ "Regional Materials Science and Technology Centre" within the frame of the operation programme "Research and Development for Innovations" financed by the Structural Funds and from the state budget of the Czech Republic. REFERENCES [1] KAINUMA, R., IMANO, Y., ITO, W., MORITO, H. et al. Meta shape memory effect in a Heusler-type Ni43Co7Mn39Sn11 polycrystalline alloy. Applied Physics Letters, 2006, vol. 88, No.19. [2] KRENKE, T., ACET, M., WASSERMANN, E.F. Martensitic transitions and the nature of ferromagnetism in the austenitic and martensitic states of Ni-Mn-Sn alloys. Physical Review B, 2005, vol.75, pp : 1-7. [3] SRIVASTAVA, V., SONG, Y., BHATTI, K. and JAMES, R. D. The direct conversion of heat to electricity using multiferroic alloys. Advanced Energy Materials, 2011, vol. 1, pp [4] LIU, H.S., ZHANG, C.L., HAN, Z.D., et. al. The effect of Co doping on the entropy changes in Ni44 xcoxmn45sn11 alloys. Journal of Alloys and Compounds, 2009, vol. 467, pp [5] SCHLAGEL, D. L., MCCALLUM, R.W., LOGRASSO, T.A. Influence of solidification microstructure on the properties of Ni Mn Sn Heusler alloys. Journal of Alloys and Compounds, 2008, vol. 463, pp [6] COLL, R., ESCODA, I., et. al. Martensitic transformation in Mn-Ni-Sn Heusler alloys. J. Therm and Calorim, 2010, vol. 99, p [7] GEBERT, A., ROTH, S., OSWALD, S., SCHULTZ, L.: Corrosion studies of Ni-Mn-Ga alloys for shape memory applications. Corrosion Science, 2009, vol. 51, p [8] ASTM G 61: Standard Test Method for Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys. Annual Book of ASTM Standards, Vol , Metal Corrosion, 2001, pp