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Influence of sintering temperature on microstructure, magnetic properties of vacuum sintered Co (-Zn)-Ni-Al alloys
Accepted Manuscript Influence of sintering temperature on microstructure, magnetic properties of vacuum sintered Co (-Zn)-Ni-Al alloys G. Johnsy Arputhavalli, S. Agilan, P. Saravanan PII: DOI: Reference:
S0167-577X(18)31363-6 https://doi.org/10.1016/j.matlet.2018.08.152 MLBLUE 24863
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
25 April 2018 28 July 2018 30 August 2018
Please cite this article as: G. Johnsy Arputhavalli, S. Agilan, P. Saravanan, Influence of sintering temperature on microstructure, magnetic properties of vacuum sintered Co (-Zn)-Ni-Al alloys, Materials Letters (2018), doi: https:// doi.org/10.1016/j.matlet.2018.08.152
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of sintering temperature on microstructure, magnetic properties of vacuum sintered Co (-Zn)-Ni-Al alloys G. Johnsy Arputhavallia,*, S. Agilanb, P. Saravananc a
Department of Physics, Karunya Institute of Technology & Sciences, Coimbatore 641114, India b Department of Physics, Coimbatore Institute of Technology, Coimbatore 641114, India c
Defence Metallurgical Research Laboratory, Hyderabad 500 058, India *
E-mail.id: [email protected]
Abstract Co38Ni35Al27 and Co35Zn10Ni32Al23 ferromagnetic shape memory alloys were prepared via facile powder metallurgy and vacuum sintering technique. The influence of sintering temperature on phase transformation behaviour, microstructure and magnetic properties were investigated. The rise in peak intensity of A2-phase shows that the alloy is rich in martensiticphase in the polycrystalline cubic B2-phase structure and Zn incorporation leads to the reduction in austenite-phase. The needle shaped cobalt precipitates on predominant martensite A2-phase, shows Widmanstatten-like structure in the samples sintered at 673 K. The saturation magnetization was found to be 87.92 and 96.2 emu/g for Co38Ni35Al27 and Co35Zn10Ni32Al23 alloy. The existence of non-interacting single-domain particle were observed and proved using Stoner-Wohlfarth model.
Keywords: Co-Ni-Al alloys; Vacuum Sintering; Microstructure
1. Introduction Co-Ni-Al alloy is a new and interesting group of materials in the ferromagnetic shape memory alloy (FSMA), which is known for its actuation by the application of magnetic field. Its notable properties such as large magnetic field induced strain (MFIS), large magnetoresistance, good ductility, higher transformation stresses and enhanced resistance to cracking[1,2] indicates that it is a significant alloy for sensors and actuator based applications. On the other hand, the different type of martensite-phases that exists in Co-NiAl has not yet been fully studied, as it depends on the choice of method of preparation. The high cobalt content based Co-Ni-Al alloy has two phase structures. The B2 crystallized matrix structure (β-phase) contains precipitates of A2-phase (γ-phase, which has disordered 1
FCC cobalt structure). The microstructure behaviour of Co38Ni35Al27 composition shows its ductility by mixed-matrix structure (β + γ phase). Polycrystalline Co–Ni–Al alloys are considered as an alternative to single crystalline Ni–Mn–Ga alloys for magnetic shape memory (MSM) application [3-5]. Therefore for the present study, the Co-Ni-Al alloy has been chosen because of its unique ductile behaviour and soft magnetic nature [6] which is appropriate for MSM application. Also, the Zn incorporated alloy (Co-Zn-Ni-Al) will further increase the ductile nature and have good corrosion resistance [7].
Compared with other conventional casting techniques, powder metallurgy is one of the facile and well-known techniques to produce a high-quality sample which can be tuned close to final dimensions with a refined microstructure. The vacuum sintering is an efficient technique to produce cost effective alloys compared with other sintering methods (Hot Isostatic Press, Spark Plasma Sintering etc...). Hence vacuum sintering technique has been chosen for this study, as it can easily diffuse bond with neighbouring particles at low temperature to obtain refined microstructure, less porosity and high density sample [8]. The ball milling process provides very fine microstructure with the extended solid solubility limits in the alloys. In the present study, the influence of sintering temperature (Ts) on microstructure, phase analysis and magnetic properties have been studied for the Co38Ni35Al27 and Co35Zn10Ni32Al23 alloy system which were prepared by powder metallurgy technique. 2. Experimental details High purity (99.99%) raw materials of cobalt, nickel, aluminium and zinc powders were initially mixed together to required stoichiometric compositions of Co38Ni35Al27 and Co35Zn10Ni32Al23 (in at%). They were ball milled for 12 hrs in a high-energy planetary mill and compacted into pellets with a diameter of 10mm and thickness of 5mm using a Hydraulic press under an axial pressure of 600 MPa for about 15 mins. The compacts were then put in an alumina boat and sintered at 353 K, 473 K and 673 K in a vacuum furnace (10-3 Pa) for about 8 hours followed by quenching. The phase structures of alloy samples were characterized using PANalytical X'PERT, X-ray diffractometer. The microstructure investigations were performed using Joel Scanning Electron Microscope (SEM). The nanocrystalline structure of alloys were analysed using JEOL JEM 2100 HR-TEM combined with selected area electron diffraction (SAED) at room temperature. The magnetic properties
2
of alloy samples were evaluated using Vibrating Sample Magnetometer (ADE make, model EV9) to a maximum field of 20 kOe. 3. Results and Discussion The XRD pattern of Co38Ni35Al27 and Co35Zn10Ni32Al23 alloy shown in Fig.1 (a) and (b) represents the polycrystalline face centred cubic (fcc) B2-phase structure. The phase structure is of uniform orientation which represents the austenite-phase with lattice parameter a = 11.39 Å and is of good agreement with SPS sintered alloy [9]. In all the sintered alloys, highintensity plane was observed at 44.5° (110), representing austenite B2-phase cubic structure [10, 11]. Few other peaks observed at 44.3, 47.4°, 51.8° and 76.4° corresponds to L10(111), B2(200), A2(200), A2(220), show the mixture of austenite and martensite face-centered cubic structure. These characteristic peaks of Co-Ni-Al alloy match very well with the standard JCPDS data (file no- 654244). The peak which is not indexed in graph attributes to intermartensite phase.
Fig. 1 (a) XRD patterns of Co38Ni35Al27 alloy and (b) Co35Zn10Ni32Al23 alloy sintered at 353 K, 473 K and 673 K.
At 673 K, the B2(110) peak gets split into B2(110) and L10(111) peaks representing the phase transformation of parent fcc B2-phase structure into mixed-martensitic face centred tetragonal (fct) L10-phase structure. The intensities of the B2(200) peak is suppressed while the A2(200) and A2(220) peaks are enhanced which represents the alloy is rich in martensitic-phase with diminishing parent austenite-phase, confirming the ductile nature of the alloy [12]. Also, A2(200) and A2(220) peaks represent the presence of twinned 3
martensite structure. The minor peak shift of about 0.2 degree is due to internal stress that happens in the alloys during increase in Ts. In Co35Zn10Ni32Al23 alloy, the reduction in peak intensity and slight shift in peaks indicates the misfit of Zn atom to Co, Ni, and Al lattice-site. Also, the incorporation of Zn content leads to reduction in austenite-phase. It is noted that as the Ts increases, the crystallite size of alloy decreases. The SEM images of Co38Ni35Al27 and Co35Zn10Ni32Al23 alloys at 673 K shown in Fig. 2 (a), (b), has many nano-sized twinned needle-like Co precipitates, formed perpendicular to the surface, which are of 5 to 10 nanometer thickness and 2 to 3 micrometer in length. The nanosized A2-phase Co precipitates (needle like structure) exhibits face centered cubic structure. These nano-sized A2-phase precipitates found in austenite matrix B2-phase also exhibit Widmanstatten-like structure [13]. Similar to the above observation, worm like precipitates were observed in martensitic matrix for the arc melted Co-Ni-Al alloy [5]. The analogy between structure model and topographical image from the various investigations determines Ts and method of preparation. The morphology of Co precipitates grows along (111) direction due to misfit between B2 matrix (a mixture of cubic and tetragonal phase) and fcc Co precipitates (A2-phase).
Fig. 2 (a) SEM images of Co38Ni35Al27 alloy and (b) Co35Zn10Ni32Al23 alloy sintered at 673 K, (c) TEM images of Co38Ni35Al27 alloy and (d) Co35Zn10Ni32Al23 alloy sintered at 673 K, (e) SAED patterns for Co38Ni35Al27 alloy and (f) Co35Zn10Ni32Al23 alloy sintered at 673 K. 4
Fig. 2 (c) and (d) shows the TEM image of Co38Ni35Al27 and Co35Zn10Ni32Al23 alloy. The SAED pattern shown in Fig. 2 (e) and (f), illustrates the diffraction rings of Co38Ni35Al27 alloy L10(111), B2(200), A2(200), A2(220) of and B2(100), A2(111), B2(200), A2(200), A2(220) for Co35Zn10Ni32Al23 alloy. The spotted rings analysed in the pattern confirms that the alloys are in polycrystalline and the grains are in nanocrystalline form, which are in the range of 20 to 30 nm. The presence of bright spots in SAED pattern is due to the formation of twinned- phase structure which indicates the improvement in crystallinity during alloying and sintering.
Magnetic hysteresis loops for Co38Ni35Al27 alloys and Co35Zn10Ni32Al23 alloys sintered at different Ts are shown in Fig. 3 (a) and (b). Saturation magnetization (Ms), retentivity (Mr), coercivity(Hc) and relative permiability obtained from the M-H curve are listed in Table1. Table 1. Magnetic properties of Co38Ni35Al27 and Co35Zn10Ni32Al23 alloys sintered at different temperatures. Sample
Sintered
Ms
Hc
Mr
Relative
Crystalline
temperat
(emu/g)
(Oe)
(emu/g)
permeability
size D (nm)
ures
(µr)
Co38Ni35Al27
353 K
111.9
189.4
10.94
15,684.71
43.82
Co35Zn10Ni32Al23
353 K
113.7
191.8
11.15
13,455.41
43.82
Co38Ni35Al27
473 K
113.14
173.3
8.77
16,074.84
43.81
Co35Zn10Ni32Al23
473 K
114.2
180.3
12.13
15,605.10
43.82
Co38Ni35Al27
673 K
87.92
162.2
7.64
18,949.05
29.2
Co35Zn10Ni32Al23
673 K
96.2
160
7.85
16,082.80
29.21
In the Fig. 3 (c) and (d), Ms of both the alloy system shows varying tendency as the Ts rises. The decreased value may be due to strong vibration of magnetic variants, disturbing the alignment of magnetic moment vector towards the field direction at high temperature. Another reason may be due to lattice defects and the presence of impurities during milling or sintering [14]. The Ms value of Co35Zn10Ni32Al23 alloy is found to be increased when compared with Co38Ni35Al27 alloys, which is due to grain refinement, thereby reducing magnetocrystalline anisotropy [15]. The alloys represent soft ferromagnetic nature, having minimum Hc value of 160 Oe and 162 Oe for Co38Ni35Al27 and Co35Zn10Ni32Al23 alloy sintered at 673 K. The decrease in Hc value 5
signifies that there is a smooth transformation of magnetic variants from cubic austenite-sites to mixed sites resulting in decrease of austenite-sites. This transformation strongly influences
the
magnetocrystalline
anisotropy
of
martensite-phase
[16].
The
magnetocrystalline anisotropy constant (K1) in the martensite-phase is four times greater than austenite-phase; therefore martensite transformation takes place in the FSMAs [17].
Fig. 3 (a) M-H curves for Co38Ni35Al27 alloy and (b) Co35Zn10Ni32Al23 alloy sintered at 353 K, 473 K and 673 K. (c) variation of Ms and Hc as a function of Ts for Co38Ni35Al27 alloy and (d) Co35Zn10Ni32Al23 alloy. (e) Ms as a function of 1/H2 for Co35Zn10Ni32Al23 and (insight) Co38Ni35Al27 alloy sintered at 673 K. The Hc depends on the crystallite size and Ms
Hc = 3 (kTc K1/aM s ) * (1/D)
(1)
where k is Boltzmann constant, TC is Curie temperature, K1 is magneto-crystalline anisotropy,
6
a is lattice constant and D is crystallite size [18]. The magnetic exchange length Lex can be expressed as (2)
Lex = (A/K1 )
where A is exchange stiffness constant. The crystallite size for the two alloy systems sintered at 673K was found to be 29.2 nm which is less than the Lex. This could be due to the diminishing of domain wall effect and each grain acting as a single domain.
In order to confirm that the alloy sintered at 673 K are in single-domain state, estimation of Ms based on the Stoner-Wohlfarth (S-W) model [19] was made and results are presented in Fig. 3 (e). The graph of S-W model shows linear relationship between measured magnetization and applied field as a function of 1/ H2. The estimated values for Ms are found to be 87.92 and 96.2 emu/g for Co38Ni35Al27 and Co35Zn10Ni32Al23 alloy respectively. This confirms the existence of single-domain nature.
4. Conclusion The vacuum sintered alloys at 673 K have been studied for their phase analysis, microstructure and magnetic properties. The XRD pattern of both the alloy reveals that they have highly oriented martensite-phase with cubic B2-phase structure. The presence of intermartensite phase with decreased crystallite size of 29.2 nm is observed. The SEM image shows that nano-sized twinned needle-like Co precipitates (A2-phase) in the austenite B2phase. The soft magnetic single domain nature was confirmed using S-W model. The study exhibits that the alloy sintered at 673 K is significant for actuator based applications. References [1] P.Z. Li, H.E. Karaca, Y.I. Chumlyakov, J. Alloys Compd. 718 (2017) 326–334. [2] J. Ju, S. Lou, C. Yan, L. Yang, T. Li, S. Hao, X. Wang, H. Liu, Journal of Elec Mater. 46 (2017) 2540-2547. [3] Y. Tanaka, T. Oikawa, Y. Sutou, T.Omori, R. Kainuma, K. Ishida, Mater. Sci. Eng. A. 438–440 (2006) 1054–1060. [4] B. Bartova, D. Schryvers, Z. Yang, S. Ignacova, P.Sittner, Script. Mater. 57 (2007) 37-40. [5] J. Li, J. Li, Mater. Lett. 68 (2012) 40–43. [6] F. Dağdelen, T. Malkoç, M. Kök, E. Ercan, Eur. Phys. J. Plus. 131 (2016) 196-201. [7] H.T. Naeema, K.S. Mohammeda, K.R. Ahmada, Mater Res. 17 (2014) 1663-1676. 7
[8] S.H. Chang, S.H. Chen, K.T. Huang, Mater. Trans. 54 (2013) 1857-1862. [9] G. J. Arputhavalli, S. Agilan, R. Johnson, Springer Proceedings in Physics. 189 (2017) 391-400, DOI: 10.1007/978-3-319-44890-9_36. [10] X. Jin, Y. Zhou, L. Zhang, X. Du, B. Li, Mater. Lett. (2018), doi: https://doi.org/10.1016/j.matlet.2018.01.017. [11] L. Zhang, D. Zhou, B. Li, Mater. Lett. 216 (2018) 252–255. [12] E. Panchenko, A. Eftifeeva, Y. Chumlyakov, G. Gerstein, H.J. Maier, Script. Mater. 150 (2018) 18–21. [13] R. Kainuma, M. Ise, C.C. Jia, H. Ohtani, K. Ishida, Intermetallics. 4 (1996) 151–158. [14] M.D. Chermahini, S. Sharafi, H. Shokrollahi, M. Zandrahimi, A. Shafyei, J. Alloys Compd. 484 (2009) 54–58. [15] R. Hamzaoui, O. Elkedim, E. Gaffet, Mater. Sci. Eng. A. 381 (2004) 363–371. [16] V. Sanchez-Alarcos, J.I Perez-Landazabal, C. Gomez-Polo, V. Recarte, J. Magn. Magn. Mater. 320 (2008) 160–163. [17] L. Manosa, X. Moya, A. Planes, T. Krenke, M. Acet, E. F. Wassermann, Mater. Sci. Eng. A. 481–482 (2008) 49–56. [18] A.H. Bahramai, S. Sharafi, H.A. Baghbaderani, Adv. Powder Technol. 24 (2013) 235– 241. [19] R.C. O’Handley, Modern Magnetic Materials: Principles and Applications, Wiley Interscience, New York, (2000) 327–1314.
8
9
HIGHLIGHTS
Co(-Zn)-Ni-Al alloys exhibit polycrystalline mixed-phase face centered cubic structure. The alloys show nano-sized twinned needle-like Co precipitates (A2 phase) in B2 matrix. The alloys exhibit single domain state, estimated using the Stoner-Wohlfarth model.
10
S0167-577X(18)31363-6 https://doi.org/10.1016/j.matlet.2018.08.152 MLBLUE 24863
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
25 April 2018 28 July 2018 30 August 2018
Please cite this article as: G. Johnsy Arputhavalli, S. Agilan, P. Saravanan, Influence of sintering temperature on microstructure, magnetic properties of vacuum sintered Co (-Zn)-Ni-Al alloys, Materials Letters (2018), doi: https:// doi.org/10.1016/j.matlet.2018.08.152
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Influence of sintering temperature on microstructure, magnetic properties of vacuum sintered Co (-Zn)-Ni-Al alloys G. Johnsy Arputhavallia,*, S. Agilanb, P. Saravananc a
Department of Physics, Karunya Institute of Technology & Sciences, Coimbatore 641114, India b Department of Physics, Coimbatore Institute of Technology, Coimbatore 641114, India c
Defence Metallurgical Research Laboratory, Hyderabad 500 058, India *
E-mail.id: [email protected]
Abstract Co38Ni35Al27 and Co35Zn10Ni32Al23 ferromagnetic shape memory alloys were prepared via facile powder metallurgy and vacuum sintering technique. The influence of sintering temperature on phase transformation behaviour, microstructure and magnetic properties were investigated. The rise in peak intensity of A2-phase shows that the alloy is rich in martensiticphase in the polycrystalline cubic B2-phase structure and Zn incorporation leads to the reduction in austenite-phase. The needle shaped cobalt precipitates on predominant martensite A2-phase, shows Widmanstatten-like structure in the samples sintered at 673 K. The saturation magnetization was found to be 87.92 and 96.2 emu/g for Co38Ni35Al27 and Co35Zn10Ni32Al23 alloy. The existence of non-interacting single-domain particle were observed and proved using Stoner-Wohlfarth model.
Keywords: Co-Ni-Al alloys; Vacuum Sintering; Microstructure
1. Introduction Co-Ni-Al alloy is a new and interesting group of materials in the ferromagnetic shape memory alloy (FSMA), which is known for its actuation by the application of magnetic field. Its notable properties such as large magnetic field induced strain (MFIS), large magnetoresistance, good ductility, higher transformation stresses and enhanced resistance to cracking[1,2] indicates that it is a significant alloy for sensors and actuator based applications. On the other hand, the different type of martensite-phases that exists in Co-NiAl has not yet been fully studied, as it depends on the choice of method of preparation. The high cobalt content based Co-Ni-Al alloy has two phase structures. The B2 crystallized matrix structure (β-phase) contains precipitates of A2-phase (γ-phase, which has disordered 1
FCC cobalt structure). The microstructure behaviour of Co38Ni35Al27 composition shows its ductility by mixed-matrix structure (β + γ phase). Polycrystalline Co–Ni–Al alloys are considered as an alternative to single crystalline Ni–Mn–Ga alloys for magnetic shape memory (MSM) application [3-5]. Therefore for the present study, the Co-Ni-Al alloy has been chosen because of its unique ductile behaviour and soft magnetic nature [6] which is appropriate for MSM application. Also, the Zn incorporated alloy (Co-Zn-Ni-Al) will further increase the ductile nature and have good corrosion resistance [7].
Compared with other conventional casting techniques, powder metallurgy is one of the facile and well-known techniques to produce a high-quality sample which can be tuned close to final dimensions with a refined microstructure. The vacuum sintering is an efficient technique to produce cost effective alloys compared with other sintering methods (Hot Isostatic Press, Spark Plasma Sintering etc...). Hence vacuum sintering technique has been chosen for this study, as it can easily diffuse bond with neighbouring particles at low temperature to obtain refined microstructure, less porosity and high density sample [8]. The ball milling process provides very fine microstructure with the extended solid solubility limits in the alloys. In the present study, the influence of sintering temperature (Ts) on microstructure, phase analysis and magnetic properties have been studied for the Co38Ni35Al27 and Co35Zn10Ni32Al23 alloy system which were prepared by powder metallurgy technique. 2. Experimental details High purity (99.99%) raw materials of cobalt, nickel, aluminium and zinc powders were initially mixed together to required stoichiometric compositions of Co38Ni35Al27 and Co35Zn10Ni32Al23 (in at%). They were ball milled for 12 hrs in a high-energy planetary mill and compacted into pellets with a diameter of 10mm and thickness of 5mm using a Hydraulic press under an axial pressure of 600 MPa for about 15 mins. The compacts were then put in an alumina boat and sintered at 353 K, 473 K and 673 K in a vacuum furnace (10-3 Pa) for about 8 hours followed by quenching. The phase structures of alloy samples were characterized using PANalytical X'PERT, X-ray diffractometer. The microstructure investigations were performed using Joel Scanning Electron Microscope (SEM). The nanocrystalline structure of alloys were analysed using JEOL JEM 2100 HR-TEM combined with selected area electron diffraction (SAED) at room temperature. The magnetic properties
2
of alloy samples were evaluated using Vibrating Sample Magnetometer (ADE make, model EV9) to a maximum field of 20 kOe. 3. Results and Discussion The XRD pattern of Co38Ni35Al27 and Co35Zn10Ni32Al23 alloy shown in Fig.1 (a) and (b) represents the polycrystalline face centred cubic (fcc) B2-phase structure. The phase structure is of uniform orientation which represents the austenite-phase with lattice parameter a = 11.39 Å and is of good agreement with SPS sintered alloy [9]. In all the sintered alloys, highintensity plane was observed at 44.5° (110), representing austenite B2-phase cubic structure [10, 11]. Few other peaks observed at 44.3, 47.4°, 51.8° and 76.4° corresponds to L10(111), B2(200), A2(200), A2(220), show the mixture of austenite and martensite face-centered cubic structure. These characteristic peaks of Co-Ni-Al alloy match very well with the standard JCPDS data (file no- 654244). The peak which is not indexed in graph attributes to intermartensite phase.
Fig. 1 (a) XRD patterns of Co38Ni35Al27 alloy and (b) Co35Zn10Ni32Al23 alloy sintered at 353 K, 473 K and 673 K.
At 673 K, the B2(110) peak gets split into B2(110) and L10(111) peaks representing the phase transformation of parent fcc B2-phase structure into mixed-martensitic face centred tetragonal (fct) L10-phase structure. The intensities of the B2(200) peak is suppressed while the A2(200) and A2(220) peaks are enhanced which represents the alloy is rich in martensitic-phase with diminishing parent austenite-phase, confirming the ductile nature of the alloy [12]. Also, A2(200) and A2(220) peaks represent the presence of twinned 3
martensite structure. The minor peak shift of about 0.2 degree is due to internal stress that happens in the alloys during increase in Ts. In Co35Zn10Ni32Al23 alloy, the reduction in peak intensity and slight shift in peaks indicates the misfit of Zn atom to Co, Ni, and Al lattice-site. Also, the incorporation of Zn content leads to reduction in austenite-phase. It is noted that as the Ts increases, the crystallite size of alloy decreases. The SEM images of Co38Ni35Al27 and Co35Zn10Ni32Al23 alloys at 673 K shown in Fig. 2 (a), (b), has many nano-sized twinned needle-like Co precipitates, formed perpendicular to the surface, which are of 5 to 10 nanometer thickness and 2 to 3 micrometer in length. The nanosized A2-phase Co precipitates (needle like structure) exhibits face centered cubic structure. These nano-sized A2-phase precipitates found in austenite matrix B2-phase also exhibit Widmanstatten-like structure [13]. Similar to the above observation, worm like precipitates were observed in martensitic matrix for the arc melted Co-Ni-Al alloy [5]. The analogy between structure model and topographical image from the various investigations determines Ts and method of preparation. The morphology of Co precipitates grows along (111) direction due to misfit between B2 matrix (a mixture of cubic and tetragonal phase) and fcc Co precipitates (A2-phase).
Fig. 2 (a) SEM images of Co38Ni35Al27 alloy and (b) Co35Zn10Ni32Al23 alloy sintered at 673 K, (c) TEM images of Co38Ni35Al27 alloy and (d) Co35Zn10Ni32Al23 alloy sintered at 673 K, (e) SAED patterns for Co38Ni35Al27 alloy and (f) Co35Zn10Ni32Al23 alloy sintered at 673 K. 4
Fig. 2 (c) and (d) shows the TEM image of Co38Ni35Al27 and Co35Zn10Ni32Al23 alloy. The SAED pattern shown in Fig. 2 (e) and (f), illustrates the diffraction rings of Co38Ni35Al27 alloy L10(111), B2(200), A2(200), A2(220) of and B2(100), A2(111), B2(200), A2(200), A2(220) for Co35Zn10Ni32Al23 alloy. The spotted rings analysed in the pattern confirms that the alloys are in polycrystalline and the grains are in nanocrystalline form, which are in the range of 20 to 30 nm. The presence of bright spots in SAED pattern is due to the formation of twinned- phase structure which indicates the improvement in crystallinity during alloying and sintering.
Magnetic hysteresis loops for Co38Ni35Al27 alloys and Co35Zn10Ni32Al23 alloys sintered at different Ts are shown in Fig. 3 (a) and (b). Saturation magnetization (Ms), retentivity (Mr), coercivity(Hc) and relative permiability obtained from the M-H curve are listed in Table1. Table 1. Magnetic properties of Co38Ni35Al27 and Co35Zn10Ni32Al23 alloys sintered at different temperatures. Sample
Sintered
Ms
Hc
Mr
Relative
Crystalline
temperat
(emu/g)
(Oe)
(emu/g)
permeability
size D (nm)
ures
(µr)
Co38Ni35Al27
353 K
111.9
189.4
10.94
15,684.71
43.82
Co35Zn10Ni32Al23
353 K
113.7
191.8
11.15
13,455.41
43.82
Co38Ni35Al27
473 K
113.14
173.3
8.77
16,074.84
43.81
Co35Zn10Ni32Al23
473 K
114.2
180.3
12.13
15,605.10
43.82
Co38Ni35Al27
673 K
87.92
162.2
7.64
18,949.05
29.2
Co35Zn10Ni32Al23
673 K
96.2
160
7.85
16,082.80
29.21
In the Fig. 3 (c) and (d), Ms of both the alloy system shows varying tendency as the Ts rises. The decreased value may be due to strong vibration of magnetic variants, disturbing the alignment of magnetic moment vector towards the field direction at high temperature. Another reason may be due to lattice defects and the presence of impurities during milling or sintering [14]. The Ms value of Co35Zn10Ni32Al23 alloy is found to be increased when compared with Co38Ni35Al27 alloys, which is due to grain refinement, thereby reducing magnetocrystalline anisotropy [15]. The alloys represent soft ferromagnetic nature, having minimum Hc value of 160 Oe and 162 Oe for Co38Ni35Al27 and Co35Zn10Ni32Al23 alloy sintered at 673 K. The decrease in Hc value 5
signifies that there is a smooth transformation of magnetic variants from cubic austenite-sites to mixed sites resulting in decrease of austenite-sites. This transformation strongly influences
the
magnetocrystalline
anisotropy
of
martensite-phase
[16].
The
magnetocrystalline anisotropy constant (K1) in the martensite-phase is four times greater than austenite-phase; therefore martensite transformation takes place in the FSMAs [17].
Fig. 3 (a) M-H curves for Co38Ni35Al27 alloy and (b) Co35Zn10Ni32Al23 alloy sintered at 353 K, 473 K and 673 K. (c) variation of Ms and Hc as a function of Ts for Co38Ni35Al27 alloy and (d) Co35Zn10Ni32Al23 alloy. (e) Ms as a function of 1/H2 for Co35Zn10Ni32Al23 and (insight) Co38Ni35Al27 alloy sintered at 673 K. The Hc depends on the crystallite size and Ms
Hc = 3 (kTc K1/aM s ) * (1/D)
(1)
where k is Boltzmann constant, TC is Curie temperature, K1 is magneto-crystalline anisotropy,
6
a is lattice constant and D is crystallite size [18]. The magnetic exchange length Lex can be expressed as (2)
Lex = (A/K1 )
where A is exchange stiffness constant. The crystallite size for the two alloy systems sintered at 673K was found to be 29.2 nm which is less than the Lex. This could be due to the diminishing of domain wall effect and each grain acting as a single domain.
In order to confirm that the alloy sintered at 673 K are in single-domain state, estimation of Ms based on the Stoner-Wohlfarth (S-W) model [19] was made and results are presented in Fig. 3 (e). The graph of S-W model shows linear relationship between measured magnetization and applied field as a function of 1/ H2. The estimated values for Ms are found to be 87.92 and 96.2 emu/g for Co38Ni35Al27 and Co35Zn10Ni32Al23 alloy respectively. This confirms the existence of single-domain nature.
4. Conclusion The vacuum sintered alloys at 673 K have been studied for their phase analysis, microstructure and magnetic properties. The XRD pattern of both the alloy reveals that they have highly oriented martensite-phase with cubic B2-phase structure. The presence of intermartensite phase with decreased crystallite size of 29.2 nm is observed. The SEM image shows that nano-sized twinned needle-like Co precipitates (A2-phase) in the austenite B2phase. The soft magnetic single domain nature was confirmed using S-W model. The study exhibits that the alloy sintered at 673 K is significant for actuator based applications. References [1] P.Z. Li, H.E. Karaca, Y.I. Chumlyakov, J. Alloys Compd. 718 (2017) 326–334. [2] J. Ju, S. Lou, C. Yan, L. Yang, T. Li, S. Hao, X. Wang, H. Liu, Journal of Elec Mater. 46 (2017) 2540-2547. [3] Y. Tanaka, T. Oikawa, Y. Sutou, T.Omori, R. Kainuma, K. Ishida, Mater. Sci. Eng. A. 438–440 (2006) 1054–1060. [4] B. Bartova, D. Schryvers, Z. Yang, S. Ignacova, P.Sittner, Script. Mater. 57 (2007) 37-40. [5] J. Li, J. Li, Mater. Lett. 68 (2012) 40–43. [6] F. Dağdelen, T. Malkoç, M. Kök, E. Ercan, Eur. Phys. J. Plus. 131 (2016) 196-201. [7] H.T. Naeema, K.S. Mohammeda, K.R. Ahmada, Mater Res. 17 (2014) 1663-1676. 7
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HIGHLIGHTS
Co(-Zn)-Ni-Al alloys exhibit polycrystalline mixed-phase face centered cubic structure. The alloys show nano-sized twinned needle-like Co precipitates (A2 phase) in B2 matrix. The alloys exhibit single domain state, estimated using the Stoner-Wohlfarth model.
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