size dependent structure and magnetocaloric properties of fe based glass forming alloy powders

Size-dependent structure and magnetocaloric properties of Fe-based glass-forming alloy powders Qiang Luo, Fengxia Ye, Changjun Huang, Jin Jiao, Anisur Rahman, Peng Yu, Jie Li, and Jun Shen Citation: AIP Advances 6, 045002 (2016); doi: 10.1063/1.4945754 View online: http://dx.doi.org/10.1063/1.4945754 View Table of Contents: http://aip.scitation.org/toc/adv/6/4 Published by the American Institute of Physics , AIP ADVANCES 6, 045002 (2016) Size-dependent structure and magnetocaloric properties of Fe-based glass-forming alloy powders Qiang Luo,1 Fengxia Ye,1,2 Changjun Huang,1 Jin Jiao,1 Anisur Rahman,1 Peng Yu,2 Jie Li,3 and Jun Shen1,a School of Materials Science and Engineering, Tongji University, Shanghai 201804, China Chongqing Key Laboratory of Photo-Electric Functional Materials, College of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 401331, China Laboratory for Microstructures, Shanghai University, Shanghai 200444, China (Received January 2016; accepted 28 March 2016; published online April 2016) We investigated the influence of particle size on the microstructure and magnetocaloric effect of Fe-based alloy powders (11 µm to 100 µm in diameter) The degree of structure order varies with the powder size The 11 µm to 18 µm powders show the largest peak magnetic entropy change (MEC) Increasing the degree of structure order tends to decrease the maximum MEC Nevertheless, enhancement of refrigerant capacity and MEC (above 70 K) is achieved when the crystalline phase content is ∼50% (above 75 µm) in the 75 µm to 100 µm powders Exponent n of the field dependence of MEC increases with the decrease in powder size above 22.5 K The size dependence of the structure and properties is associated with the fact that a larger particle has a slower cooling rate and takes a longer time to form medium-to-long range ordered structures C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4945754] Magnetic refrigeration based on the magnetocaloric effect has been considered a potential alternative to conventional gas compression/expansion refrigeration because of its significant advantages, such as environmental friendliness and relatively high efficiency.1–8 This unique refrigeration method has stimulated considerable research on exploring materials with excellent magnetocaloric performance from ultralow (1 mK) to room temperature.1–4 In particular, several crystalline materials have been reported to exhibit a giant magnetic entropy change (MEC) resulting from the first-order magnetic–crystallographic phase transition.1–6 However, the giant MEC usually emerges in a narrow temperature range and accompanies significant hysteresis, which reduces the refrigerant capacity (RC) Meanwhile, increasing attention has been focused on the MEC of amorphous metals, such as Fe-based, Co(Ni)-based, rare earth-based amorphous alloys (including ribbons, bulk samples, and even wires), and their composites.8–17 These alloys usually show second-order magnetic transition; nevertheless, their amorphous nature brings about additional benefits for magnetic refrigeration, such as high electrical resistivity, outstanding mechanical and chemical properties, and facile tunability of the transition temperature.8–17 Compared with the rare earth-based amorphous alloys, the Fe-based amorphous alloys have lower material cost and superior soft magnetic properties Many studies have investigated the composition dependence of the MEC and Curie temperature of different Fe-based amorphous alloy series, such as Fe90 − x Zr10B x , Fe88 − 2x Co x Ni x Zr7B4Cu1, Fe80 − x B12Cr8La x , and Fe80 − y B12Cr8Ce y 14–20 Franco et al revealed that MEC is characterized by a master curve for a series of amorphous alloys with similar compositions.21 Despite intensive studies, information on the physical origins underlying the influence of microstructure, such as the short-to-medium range ordered local structures and the degree of structure order, on the MEC of these alloys is deficient a Corresponding author Prof Jun Shen; Tel.: +86 21 69581009; Fax: +86 2133515214; E-mail address: Junshen@tongji edu.cn (J Shen) 2158-3226/2016/6(4)/045002/9 6, 045002-1 © Author(s) 2016 045002-2 Luo et al AIP Advances 6, 045002 (2016) In this article, we report on the microstructure and the magnetic and MEC properties of Fe-based alloy powders with different sizes from 10 µm to 100 µm in diameter fabricated by the one-step preparation process to reveal the relationship between degree of structure order and MEC performance With the increase in size, the microstructure of the powders evolves from fully amorphous state to composite structure with nanocrystalline phases embedded in the amorphous matrix to amorphous matrix with a large number of microsized crystalline grains Enhancement of RC is observed only when the crystalline phase (consisting of microsized grains) fraction is ∼50%, but not in other nanocrystalline/amorphous composites The degree of structure disorder has a strong influence on the temperature and field dependence of MEC Fe-based metallic powders with nominal composition of Fe49.7Cr18Mn1.9Mo7.4W1.6B15.2C3.8Si2.4 were produced by high-pressure argon (Ar) gas atomization method Then, the powders were divided into different size ranges using the conventional sieve method The microstructure of the Fe-based alloy powders was characterized using X-ray diffraction (XRD; D/max2550VB3+/PC) with Cu Kα radiation, scanning electron microscopy (SEM; Quanta 200 FEG) equipped with energy-dispersive spectroscopy, and transmission electron microscopy (TEM; JEM-2100F) operated at 200 kV The TEM samples were prepared using the focused ion beam approach The thermal stability of the powders was examined using a differential scanning calorimeter (DSC; TA, MDSC-Q100) in an Ar atmosphere at a rate of 20 K/min The magnetic properties were measured using the Physical Properties Measurement System, PPMS 6000 of Quantum Design Company The nanoindentation measurements were conducted in a Triboindenter (Hysitron Inc.) with a Berkovich diamond tip Fused silica was used as a reference sample for the initial tip calibration procedure The indentations were performed in a load-control mode to a load limit of 10 mN with a loading rate of 0.25 µm/s Figs 1(a) to 1(c) show the SEM morphology of the Fe-based alloy powders with different size ranges The figures show that most of the powders are roughly spherical with a smooth surface and only a few are rod-shaped or irregularly shaped We ignored the few irregularly shaped particles FIG The morphology of the powders with different dimension ranges: (a) 48-100 µm, (b) 38-48 µm, (c) < 38 µm (d) XRD patterns of the Fe-based amorphous powders with different diameter ranges 045002-3 Luo et al AIP Advances 6, 045002 (2016) and simply considered all the particles to be spherical Moreover, we only explored the size effect Fig 1(d) shows the XRD patterns of the five groups of powders Only a broad diffuse peak is observed in the pattern of the powders with diameter below 38 µm, indicating their amorphous structure For the powders between 41 µm and 50 µm, a few weak peaks appear, which implies that several nanocrystalline phases are embedded in the amorphous matrix Further increasing the powder size results in the decrease in the amorphous phase content and emergence of several new crystalline phases because of the slower cooling rate with the increase in size Above 75 µm, a significant fraction of crystalline phases is detected from the intensified and sharper peaks in the XRD pattern Therefore, the degree of structure order depends strongly on the powder size Fig 2(a) shows the DSC curves of the five groups of powders A common feature of all the traces is that the samples exhibit an obvious broad endothermic reaction due to glass transition and two highly exothermic peaks due to crystallization The XRD pattern of the annealed sample (not shown here) revealed that primary crystallization products mainly consist of α-Fe and Cr23C6 phases Although having similar glass transition and crystallization temperatures (∼579 ◦C and 629 ◦C, respectively) for the five groups of powders, crystallization enthalpy decreases with the increase in size From the normalized DSC patterns, the values of crystallization enthalpy are determined to be 33.9, 58.3, 65.1, 66.1, and 68.3 J/g in sequence for the five groups of powders with the decrease in size, from which the amorphous phase ratio is estimated to be 0.5, 0.85, 0.95, 0.97, and 1, respectively A larger particle has a slower cooling rate; thus, the atoms take a longer time to rearrange the structure to reach more ordered states SEM backscattered electron and TEM experiments for several groups of powders were conducted to obtain an in-depth understanding of the microstructure and its effect on the MEC of these powders Fig 2(b) shows that several microsized crystalline grains (larger than 10 µm) are dispersed FIG (a) DSC curves of the gas atomized Fe-based amorphous powders SEM back scattered electron images of the Fe-based amorphous powders: (b) 100-75 µm, (c) 60-50 µm, (d) 37-30 µm 045002-4 Luo et al AIP Advances 6, 045002 (2016) in the amorphous matrix for the 75 µm to 100 µm powders A similar clear contrast of crystalline grains (less than 10 µm) is also detected in the 50 µm to 60 µm powders (Fig 2(c)) The larger average size of the crystalline clusters (grains) in larger powder is due to the longer time it takes for atoms to rearrange to form a long range order in the powder For the powders between 30 µm and 37 µm, no contrast of the crystalline phases in most particles can be observed Meanwhile, in the few largest particles of this group, bright spots with several hundred nanometers related to crystalline phases can be observed (Fig 2(d)) For the powders below 18 µm, no discernible features can be observed (not shown here), indicating a fully glassy nature The aforementioned microstructure features can be further corroborated by the TEM images Fig shows the high-resolution TEM images of three particles with diameters of 10, 50, and 100 µm The high-resolution TEM image of the 10 µm ball shows a uniform contrast, indicating a homogeneous amorphous structure (Fig 3(a)) Meanwhile, several nanocrystalline phases are clearly detected in the 50 µm ball (Fig 3(b)) By contrast, a large quantity of crystalline phases can be observed in the 100 µm particle (Figs 3(c) and 3(d)) These observations are consistent with the selected area electron diffraction patterns shown in the insets of Figs 3(a) and 3(c) Fig shows the temperature dependence of field-cooled (FC) magnetization of all the powders, which was measured on heating after initially cooling to K under 500 Oe The zero FC branch was also measured after initially cooling from 300 K to K in zero field for the 11 µm to 18 µm powders (the insets), which shows only a slight difference with their FC branch The magnetization of the 30 µm to 37 µm powders is almost the same as that of the 11 µm to 18 µm FIG TEM images of the particles with diameters of (a) 10 µm, (b) 50 µm, (c) 100 µm The insets are the corresponding selected area electron diffraction patterns (d) The enlarged part of marked area with red circle in image (c) 045002-5 Luo et al AIP Advances 6, 045002 (2016) FIG Temperature dependence of the FC magnetization curves, the inset shows the ZFC and FC magnetization curves of the 11-18 µm powders powders The existence of a few nanocrystalline phases results in a slight reduction of magnetization (compared with the 11 µm to 18 µm powders) in the entire investigated temperature range However, magnetization increases when the powder size increases to 50 µm to 60 µm because of the increase in the number and size of nanoclusters and their interactions The 75 µm to 100 µm powders show maximum magnetization because of the magnetic contribution from the crystalline phases Despite the different magnetization processes, all the powders show almost the same Curie temperature of ∼16.3 K, indicating the dominant role of the amorphous matrix in the magnetic transition process Notably, the 75 µm to 100 µm powders show an additional broad transition process between 70 K and 100 K that originate from the crystalline phases These observations indicate that the degree of structure order in the powders alters the magnetic exchange interactions and magnetization process significantly These observations can be further confirmed by the isothermal magnetization curves, from which the MEC can be calculated based on the Maxwell relations A typical set of isothermal magnetization curves for the 11 µm to 18 µm powders from K to 95 K are displayed in Fig 5(a) A temperature interval of K was adopted in our M−H isotherm measurements, and the field sweeping rate was sufficiently slow to ensure the isothermal condition Fig 5(b) compares the isothermal M−H curves for the five groups of powders at and 95 K (the inset), which show obvious differences At K, the finest powders show the largest magnetization values in the entire investigated field range and have the fastest increase pace of magnetization in the low field range The 41 µm to 50 µm powders have the lowest values in the entire investigated field range Notably, no proportional relationship between magnetization and powder size can be observed, which implies that the structure order influences the overall magnetization process in a complex manner The temperature-dependent MEC under a field variation of T for the five groups of powders is shown in Fig 6(a) The peak values of -∆Sm are 0.68, 0.77, 0.74, 0.81, and 0.89 J·kg−1 K−1 for the powders within the ranges of 100 µm to 75 µm, 60 µm to 50 µm, 41 µm to 50 µm, 37 µm to 30 µm, and 18 µm to 11 µm, respectively Generally, the existence of (nano)crystalline phases leads to a reduction of the peak MEC (compared with the 11 µm to 18 µm sample) because these crystalline grains (clusters) obstruct the magnetization process of the amorphous matrix by inducing 045002-6 Luo et al AIP Advances 6, 045002 (2016) FIG (a)A set of isothermal magnetization curves for the 11-18 µm powders.(b) Comparison of the isothermal magnetization curves of the five groups of powders at K and 95 K (the inset) more magnetic anisotropy and frustration effects A similar reduction of the peak MEC after crystallization has been observed in Gd-based and Fe-based amorphous alloys.22,23 The MEC shape of the powders does not change much, except for that of the 100 µm to 75 µm sample, which has a broader composite structure The full width at half maximum (∆THM) of MEC can be determined to be approximately 74, 74, 74, 80, and 100 K in sequence for the five groups of powders with the increase in powder size Notably, the MEC of the 100 µm to 75 µm powders have the largest values above 70 K because of the contribution of the crystalline phases From the MEC curve, the RC can be determined by numerically integrating the area under the ∆Sm –T curve using the temperatures at half maximum of the peak as the integration limits The RC value initially decreases and then increases with the increase in powder size The RC values are determined to be 52, 47, 44, 49, and 045002-7 Luo et al AIP Advances 6, 045002 (2016) FIG (a) MEC for all the investigated powders under a field change of 0-5 T (b) MEC of the 11-18 µm powders under different field changes (c) Field dependence of the MEC of the 11-18 µm powders under a field change of T at selected temperatures.(d) Temperature dependence of the local exponent n for the powders with different size ranges (e) Typical load-displacement curves for the Fe-based alloy powders (f) The size dependence of the reduced modulus and hardness 55 J kg−1 for the five groups of powders with the increase in size First, a slight decrease in the RC is related to several (nano)crystalline phases embedded in the amorphous matrix and a high degree of medium range ordered structure in the amorphous phase In a series of nanoperm Fe–Co–Nb–B alloys, the RC value was observed to decrease with the increase in crystalline fraction.24 In addition, the effect of the medium range ordered structure on the MEC and RC values of MGs has been observed from the larger MEC and RC values in a Gd-based amorphous microwire than those of its bulk counterpart.25 The obvious increase in the RC in the 100 µm to 75 µm sample is due to the broadened peak and enhanced MEC above 70 K The increased MEC values (above 70 K) compared with that of the 11 µm to 18 µm powders mainly derived from the crystalline phases (α-Fe and Fe–C phases) These results are similar to the observations in a Nd–Pr–Fe ribbon, where the two-phase composite structure results in the enhancement of the RC.26 The difference is that the Nd–Pr–Fe sample has a nanocrystalline/amorphous composite structure, whereas the 100 µm to 75 µm powders have multicrystalline phases with a grain size of micrometers For materials with a second-order phase transition, the mean field picture predicted the followpk pk ing field dependence of the peak MEC:27 |∆SM | ∝ H n , where |∆SM | is the maximum MEC and n = 2/3 at the Curie point Experimentally, n of the soft Fe-based and Gd-based amorphous alloys deviates from the predicted value.8–16 At present, no analytical expression that can be used to predict 045002-8 Luo et al AIP Advances 6, 045002 (2016) the behavior for temperatures below the Curie temperature exists We analyzed the field dependence of -∆Sm using the relation |∆SM | ∝ H n in the temperature region of K to 95 K to obtain a deeper insight into the MEC of the powders used in the present study and into the effects of structure order The MEC under different field variations for 11 µm to 18 µm powders is shown in Fig 6(b) as an example The figure shows that the shape remains almost the same for different field changes, and the height and width of -∆Sm increase with the increase in field changes Notably, the power law is well satisfied in the entire temperature range (Fig 6(c)) Fig 6(d) shows that, above the Curie temperature, n increases with the increase in temperature The n value does not reach because of the limited temperature range investigated The Curie temperature of the crystalline phases (such as α-Fe) in the powders is higher than the maximum temperature achieved in the experiments A similar minimum value of n ∼ 0.84 near 22.5 K is observed for all the five groups of powders This value is comparable with n ∼ 0.89 of the mechanically alloyed Fe-based powders, but larger than 0.75 of the pure amorphous ribbons.14,16,24 Comparing the five curves shown in Fig 6(d), we determined that the 11 µm to 18 µm and 30 µm to 37 µm powders show almost the same field dependencies of MEC between 20 K and 95 K A slight decrease is observed for the 41 µm to 50 µm powders because of the influence of a few nanocrystalline phases A large reduction of n is observed for the 50 µm to 60 µm and 75 µm to 100 µm powders, revealing a significant influence of the crystalline phases on the field dependence of MEC The deformation behavior of the Fe-based alloy powders was further investigated by nanoindentation Fig 6(e) shows the typical load–displacement (p−h) curves for the powders All the powders exhibit almost continuous plastic deformation during the loading process However, sparse and slight serrations (one or two pop-in event(s) for each curve) can also be found, as seen in other metallic glasses.28 Generally, the serrated flow is considered to be related to the formation and propagation of the shear bands during plastic deformation, which induce sudden release of elastic energy around the shear bands The sparse and slight serrated flow in Fe-based amorphous metals could be due to the small temporal and spatial scales of shear banding The figure shows the existence of a trend that smaller powders have a larger first pop-in stress The hardness and reduced modulus obtained from the load–displacement curves are shown in Fig 6(f) For powders between 18 µm and 30 µm in diameter, the average hardness and reduced modulus decrease to 11.9 and 76.6 GPa, respectively A clear trend of “the larger the harder” is observed in Fig 6(f) This trend arises from the contribution of the crystallization effect and free volume change of the amorphous matrix on the mechanical behavior In particular, the existence of the Fe–C and FeCrWMoB phases with high hardness results in the largest average hardness and reduced modulus of the 75 µm to 100 µm powders The microstructure, magnetocaloric performance, and mechanical properties of Fe-based alloy powders are strongly related to their size The field dependence of the MEC shows a power dependence for all the magnetic regimes The n value of the peak MEC is ∼0.84 for all the groups of powders, which is larger than the predicted value from the mean field picture The multiphase character of the investigated powders can explain the temperature dependence of magnetization and n The finest powders show the largest peak MEC, whereas the largest 75 µm to 100 µm powders show the largest RC and MEC (above 70 K) Thus, tuning the size and fraction of the crystalline phase is an effective way to tune the mechanical behavior and magnetocaloric performance of amorphous/crystalline composites, including the field and temperature dependence of MEC The use of the gas atomization method simplifies the fabrication process and avoids the prolonged thermal annealing at high temperatures to produce amorphous/crystalline composites Optimizing the structural and functional performances of glass-forming alloy powders and their coatings by tuning the powder size is also of considerable importance for the commercial application of Fe-based amorphous powders in other fields of anticorrosive and antiwear coatings, soft magnetic materials, and wastewater treatment ACKNOWLEDGMENTS The authors thank Xilei, Bian in Shanghai University for some experimental assistance.This work is supported by National Natural Science Foundation of China (Grant Nos 51371127 and 51274151) and Shanghai Natural Science Foundation (Grant No.13ZR1462400) 045002-9 Luo et al AIP Advances 6, 045002 (2016) V.K Pecharsky and K.A Gschneider, Jr., Phys.Rev.Lett 78, 4494 (1997) O Tegus, E Brück, K.H.J Buschow, and F.R de Boer, Nature 415, 150 (2002) F X Hu, B G Shen, J R Sun, and Z H Cheng, Phys Rev B 64, 012409 (2001) K.A Gschneider, V.K Pecharsky, and A.O Tsokol, Rep.Prog.Phys 68, 1479 (2005) S Fujieda, A Fujita, K Fukamichi, N Hirano, and S Nagaya, J Alloys Compd 408, 1165 (2006) B G Shen, J R Sun, F X Hu, H W Zhang, and Z H Cheng, Adv Mater 21, (2009) M Pasquale, C P Sasso, L H Lewis, L Giudici, T Lograsso, and D Schlagel, Phys Rev B 72, 094435 (2005) Q Luo, D Q Zhao, M X Pan, and W H 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