Characterization and amorphous pha formation of mechanically alloyed Co 60Fe 5Ni 5Ti 25B 5powders
Baris Avar a ,*,Sadan Ozcan b
a Department of Metallurgical and Materials Engineering,Bulent Ecevit University,Incivez,67100Zonguldak,Turkey b
SNTG Lab.,Department of Physics Engineering,Hacettepe University,Beytepe,06800Ankara,Turkey
a r t i c l e i n f o
Article history:
Received 22April 2015Received in revid form 8July 2015
Accepted 23July 2015
Available online 30July 2015Keywords:
Mechanical alloying Microstructure Amorphization
Magnetic properties
a b s t r a c t
In this work,the multicomponent Co 60Fe 5Ni 5Ti 25B 5(at.%)alloy powders were synthesized from commercially available pure elemental powders by using a mechanical alloying (MA)process under argon gas atmosphere.The changes in structural,morphological,thermal and magnetic properties of the procesd powders during MA were examined by X-ray diffraction (XRD),scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX),differential thermal analysis (DTA)and vibrating sample magnetometer (VSM).The results showed that the amorphization occurred after 3.5h of milling,and the amorphous pha was stable up to 580 C,where crystallization occurred.The SEM obrvations indicated that different morphologies were obtained during the MA stages.In addition,the EDX mapping con firmed the uniform distribution of elemental content.Magnetic results indicated that all the samples exhibited soft-ferromagnetic behavior.The evolution of the saturation magnetization (Ms),the coercivity (Hc)and the squareness ratio (Ms/Mr)during milling process were discusd with respect to micro-structural changes.The in fluence of the annealing on magnetic hysteresis was also studied.The Ms,Hc and Ms/Mr values of about 53.4emu/g,7.6Oe and 0.01,respectively was obtained after 7h of milling.
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1.Introduction
billabongCo-bad amorphous alloys have been extensively studied due to their good glass-forming ability and their excellent soft magnetic properties such as low coercivity,large saturation magnetizations,and low hysteresis loss.The properties make them as an ideal candidate for some applications like magnetic nsors,memories,and power devices [1e 9].It is known that various Co-bad amorphous alloys can be obtained by veral methods:electro-deposition,gas condensation,rapid solidi fication and mechanical alloying.Among them,mechanical alloying (MA)has advantages of relatively low-cost equipment,simplicity,low-temperature pro-cessing,great flexibility in the lection of the processing param-eters,and ability to produce large quantities of material with the same physical properties [10].Also,the mechanically alloyed powders can easily be consolidated into a bulk shape and heat-treated to obtain the desired microstructure and properties.Furthermore,since MA processing is carried out entirely in the solid state,pha diagram restrictions,such as immiscibility in the liquid
and solid states,do not apply to the MA process,and therefore the alloying capabilities is wider than t
he solidi fication methods [11].It has been noted that with MA it is easier to obtain the materials far from their thermodynamic equilibrium.In particular,extended solid solutions,nanocrystalline,quasicrystalline,and amorphous alloys with a large difference in melting temperatures of its ingredient can be obtained via MA [12].Besides,the limits and drawbacks of the MA process is the introduction of contamination from milling vials and grinding balls,which can affect the thermal stability and some of the physical properties of the amorphous powder [13].A number of studies have been reported where MA has been successfully applied to produce Co-bad amorphous alloy systems with various compositions.For example,Corrias et al.[14]applied MA to Co e B mixtures containing 20to 50at%B,where only Co 80B 20and Co 67B 33alloys were fully amorphous.El-Eskandarany et al.[3]reported that a single amorphous pha of Co 75Ti 25alloy powders was obtained after a short MA time (11ks).They also reported the formation of a single glassy pha of Co 71Ti 24B 5alloy powders after 130ks of milling [4].Wu et al.[5,6]developed the ferromagnetic Co-bad amorphous alloys by replacing some of the Co with (Fe,Nb)or (Zr,Ti)elements,and found that the substitution reduces the thermal stability of the
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E-mail address:barisavar@ , (B.
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Journal of Alloys and Compounds
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dx.doi/10.1016/j.jallcom.2015.07.268
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Journal of Alloys and Compounds 650(2015)53e 58
amorphous powders.However,Bolarin-Miro et al.[15]succeeded in producing an amorphous pha in
Co60Cr30Mo10,obtained upon milling Cr80Mo20solid solution with Co,the solid solution being obtained as well upon MA of elemental powders.Recently,Tagh-vaei et al.[7]have investigated the formation of the amorphous pha in the mechanically alloyed Co40Fe22Ta8B30powders.On another work,Moreno et al.[9]investigated the synthesis of Co62Nb6Zr2B30amorphous alloy by MA,using different types of boron powders in the starting mixture.It is clear that amorphous pha formation by MA is very nsitive to milling conditions, atomic size of the constituents and thermodynamic properties of the alloying system.
In this study we aimed at obtaining the amorphous Co e(Fe,Ni)e Ti e B powders by MA,and at investigating the influence of Fe and Ni atoms on the formation of the alloy.The chon composition Co60Fe5Ni5Ti25B5(at.%)for this study is quite different from the alloys studied in the previous works.The changes in the structural, morphological,thermal and magnetic properties of the mechani-cally alloyed Co60Fe5Ni5Ti25B5powders were investigated by X-ray diffraction(XRD),scanning electron microscopy with energy-
dispersive X-ray spectroscopy(SEM/EDX),differential thermal analysis(DTA)and vibrating sample magnetometer(VSM), respectively.
2.Experimental procedure
Pure elemental powders of cobalt(Sigma e Aldrich,<150m m,
99.99%),iron(Riedel-de Haen,<212m m,99%),nickel(Sigma-
e Aldrich,<150m m,99.99%),titanium(Aldrich,<150m m,99.7%) and crystalline boron(Aldrich,<1cm,99.7%)were mixed to give a nominal composition o
f Co60Fe5Ni5Ti25B5(at.%).MA was performed in a planetary ball mill(Retsch-PM100CM),under argon atmo-sphere,usin
g hardened steel balls and vial.The ball-to-powder weight ratio was10:1and the rotation speed was500rpm.Eac
h 30min of MA was followed by a pau of15min to avoid excessive heating during MA.Also,rotational direction of the instrument was changed counter-clockwi/clockwi after every45-min intervals to increa the efficiency.The MA process was interrupted at lected times(0.5,1.5,3.5,5,and7h)and a small amount of powder was removed for further characterization.The structural evolution and the pha identification were investigated by using XRD with a Panalytical Empyrean diffractometer with Cu K a radi-ation generated at45kV and40mA.
The XRD analys were per-formed from25 to80 (2q)with a step size of0.013 and a count time of48.1950s per step.Due to the small quantity of sample,a zero-background wafer sample holder was ud.The morpholog-ical change of the powder particles was examined by using SEM/ EDX with a FEI-Quanta FEG450microscope operated at12.3kV in condary electron mode.The average powder particle size was estimated from the statistical t of50particles on each SEM micrograph by image tool software.Thermal behavior of the me-chanically alloyed powders were analyzed by using DTA with a SII Exstar TG/DTA7200inflowing argon atmosphere,a temperature range of200e1150 C at a heating rate of20e C/min.The magnetic characterizations were carried out by using VSM module of Physical Properties Measurement System(PPMS-Quantum Design)at25 C within±15kOe magneticfield ranges.
3.Results and discussion
3.1.XRD analysis
The XRD patterns of the unmilled and milled Co60Fe5Ni5Ti25B5 powders are shown in Fig.1.The unmilled powder mixture(0h) displayed all the expected diffraction peaks of the constituent elements,except boron becau of its low atomic scattering factor. Afterfirst stage of milling(0.5h),the
diffraction peaks began to broaden and the height of them became lower due to reduction of particle size and introduction of lattice strain as well as stacking faults which are induced by the vere plastic deformation[16].As milling progresd to1.5h,the diffraction peaks became more broadened and much lower.After prolonging the milling time to 3.5h,all the crystalline peaks disappeared and only a broad diffu diffraction maximum appeared at2q z44.5 ,indicating the for-mation of an amorphous pha within the resolution of XRD.This fast amorphization reaction can be explained by the large negative heat of mixing between Co and other constituent elements[17]. However,milling up to7h,the amorphous pha remained un-changed,and after7h of milling,no crystalline peak appeared in the XRD pattern.This could be due to less possibility of excessive heating of the vial during MA,which can lead to the pha transformations.
3.2.Morphology and particle size
diskpartThe changes in morphology and size of the powder particles during the different stages of the MA process were analyzed by SEM technique.Fig.2shows the morphology of the powder particles before and after veral milling times.The average values of particle size estimated from the SEM micrograph are illustrated in Fig.3.As can be en,different morphologies and particle sizes were prent during MA stages.The unmilled powder particles were mostly irregular in shape and size,and
partly in a spherical-like morphology(Fig.2a).At the very beginning of the milling,the powder particles wereflattened with a layered structure as a result of the compressive force of the ball-powder-ball collisions(Fig.2b), and with prolonged milling time,the powder particles agglomer-ated to form bigger particles as a result of intensive fracture and cold welding(Fig.2c).So,it can be obrved from Fig.3that at this stage of milling the average particle size incread,and reached a maximum value of about140m m after1.5h of milling(Stage I).In this short milling period,the main mechanism was the plastic deformation and the cold welding,which caud the agglomera-tion of the particles.As it is known,during MA process,the powder particles are continuously beingflattened,cold welded and frac-tured[10e12].With further milling,the fracture mechanism became dominant,which resulted in a general decrea in size with milling time(Stage II).Also,at this stage the homogeneity of the powders incread and the powder particles became regular
in Fig.1.XRD patterns of Co60Fe5Ni5Ti25B5powders as a function of milling time.
B.Avar,S.Ozcan/Journal of Alloys and Compounds650(2015)53e58 54
shape with an average size in the range of 20e 30m m (Fig.2d).After 7h of milling,the shape of the particles became finer,and thor-oughly spherical-like morphology signi fies the completion of the solid state amorphization reaction [7,18e 20](Fig.2e).However,as stated by Zerniz et al.[13],the homogeneity of the powder particles at a macroscopic scale is the result of equilibrium between frac-turing and cold welding mechanisms.Fig.4shows the EDX map-ping analysis related to 5h milled sample.The elemental analysis of the sample clearly revealed the uniform and homogeneous distri-bution of each constituent element in the particle.Also,no impurity elements were detected in the sample from EDX elemental mapping.
3.3.Thermal analysis
In order to investigate the thermal stability of the as-milled powders,DTA was performed at a constant heating rate of 20e C/min under argon gas atmosphere.Fig.5shows the DTA curves of the as-milled powders after different milling times.
Except for the 0.5and 1.5h of milling powders,the other milled powders showed a large and broad exothermic peak at the tem-perature range of 510e 650 C,followed by an endothermic peak at about 1095 C.As en in Fig.5,the weak exothermic peak centered at around 580 C appeared in the curve of the 3.5h milled powder,which indicated the crystallization of an amorphous pha.With milling up to 5h,the exothermic peak became more pronounced and sharp,suggesting an increa in the volume fraction of the amorphous pha.To determine the crystallization products,the 5h milled amorphous powder were annealed below and above the exothermic peak (400 C and 800 C),and then cooled rapidly to freeze the microstructure for subquent XRD analysis.Fig.6shows the XRD patterns of the annealed samples compared with the as-milled alloy for 5h.As can be en from this figure,the XRD pro-file of the as-milled sample revealed a broad peak between 41 and 48 2q indicating that the sample was of XRD-amorphous micro-structure.However,it can be en that the main broad peak of the as-milled sample annealed at 400 C was slightly split.Additionally,two other Bragg peaks emed to be prent,centered at around 58 and 75 2q values.This indicates the beginning of partial crystallization of the amorphous pha.Accurate identi fication of the crystalline phas could not be carried out by XRD due to their low volume fraction.During subquent annealing at 800 C,diffusion of atoms takes place in the crystallization process and the remained amorphous pha crystallizes into thermodynamically stable phas,namely Co 3Ti and C
oFe phas.This suggests that the exothermic reaction in the DTA curve helped the formation of Co 3Ti and CoFe phas.However,bad on the binary pha diagrams (Co e Ti and Co e Fe),the endothermic peaks at around 1095 C corresponds to the melting of the Co 3Ti pha [21].
3.4.Magnetic properties
Fig.7shows the hysteresis curves (magnetization,M versus applied magnetic field,H)of Co 60Fe 5Ni 5Ti 25B 5powders after different milling times.The M ÀH curves exhibited a sigmoidal shape,which is usual in nanostructured samples with small mag-netic domains.The small hysteresis loss are the properties generally desired in soft magnetic materials [13,20,22,23].It was also obrved that the maximum available field of 15kOe was
not
萋萋Fig.2.SEM micrograph of the mechanically alloyed Co 60Fe 5Ni 5Ti 25B 5powders for different milling times:(a)0h,(b)0.5h,(c)1.5h,(d)5h,and (e)7
h.
Fig.3.Average particle size variation during the MA process.
B.Avar,S.Ozcan /Journal of Alloys and Compounds 650(2015)53e 5855
enough to completely saturate the 0.5and 1.5h milled powders.This was probably becau of the large demagnetizing effect of the flattened powders at low milling times (Fig.2)[8].Magnetic properties such as saturation magnetization (Ms),coercivity (Hc)and remanence-to-saturation ratio (Mr/Ms)were obtained from the hysteresis curves.
Fig.8shows the variation of Ms and Hc with respect to different milling times.It can be en that the Ms decreas slightly from 108to 104emu/g at the beginning of the milling process (0.5e 1.5h),then decreas rapidly to 57emu/g as the milling time increas from 1.5to 3.5h,and then levels off 53.4emu/g on further milling time.The initial reduction in Ms could be attributed to the disso-lution of non-ferromagnetic atoms into the microstructure of the powders.The addition of the non-ferromagnetic elements like Ti and B lesned the proportion of Co,Fe and Ni magnetic atoms,thus reducing the density of magnetic atom interactions [8,19].Addi-tionally,the reduction in the Ms might have been becau of
the
Fig.4.EDX elemental mapping of the mechanically alloyed Co 60Fe 5Ni 5Ti 25B 5amorphous powder after 5h of
milling.
Fig.5.DTA curves of the as-milled失败者英文
响晴
powders.
Fig.6.XRD patterns of the as-milled for 5h and subquently annealed at 400 C and 800 C
powders.
pokeyFig.7.Hysteresis curves of the as-milled powders.
B.Avar,S.Ozcan /Journal of Alloys and Compounds 650(2015)53e 58
56
csr是什么paramagnetic slight contribution of Ti.However,the decrea in Ms after 1.5h of milling could be due to the transformation of the crystalline phas to amorphous phas.Towards the end of the milling process,the Ms remained nearly constant becau of the completion of the mechanically induced solid-state reaction [4,22,24].As reported in Refs.[18,20],the considerable amount of magnetic atoms located in strongly disordered grain boundaries can reduce the Ms.Correlating with the SEM results,the Ms also decreas with the reduction in the particle size becau of the increa in the magnetic disordering at the surface [19,25].
As it can also be en from Fig.8,the Hc increas from 129to 159Oe after 1.5h of milling,then decreas rapidly,and reaches a value of 7.6Oe after 7h of milling.The increa in Hc in the early stage of milling could be related to the plastic deformation effect,prence of residual stress (strain)a
nd different type of defects (dislocations).In addition,the surface roughness (irregularities)and the non-spherical shape of the particles could also contribute to enhance the Hc [8,13,20,23].With continuous milling time,the decrea in Hc could be due to the formation of amorphous pha.Furthermore,the variation of Hc could be attributed to the particle size variation during the MA process (Fig.3).
Fig.9shows the variation of the magnetic squareness ratio (Mr/Ms)as a function of milling time.The Mr/Ms slightly incread in
the first hour of milling,then decread continuously with further milling.Such a behavior is quite similar to already obrved coer-civity changes (Fig.8).As en from Fig.9,all Mr/Ms values are ranging from 0.01to 0.1,which is far less than the value expected for randomly packed single domain particles [26].For certain ap-plications such as a magnetic recording medium,the required value of the Mr/Ms is clo to 1,while for certain applications such as the core of transformer/electromagnet,the preferred value is clo to 0.This is becau of the Mr/Ms value clo to 1or 0indicate that most of the magnetic material retain its magnetization or not even after removing the magnetic field [27].
However,applying heat-treatment to the mechanically alloyed powders did not improve the magnetic properties.Fig.10shows the effect of annealing on the magnetic properties of 5h milled amorphous powder.As en from the hysteresis curves,the Ms ro after annealing at 400 C due to the slight increa in ferromagnetic ordering.This might be attributed to the prence of unreacted elemental fine particles of Co in the annealed sample [3].By annealing the sample at 800 C,the Ms decread and the Hc incread due to the crystallization of the amorphous pha,where crystalline phas were obrved in the sample annealed above the exothermic peak by XRD (Fig.6).Similar results were reported for mechanically alloyed Co 75Ti 25and Co 65Si 15B 14Fe 4Ni 2powders [3,28].
4.Conclusions
英语免费在线翻译
In the current study,MA method was ud to obtain amorphous pha in the multicomponent Co 60Fe 5Ni 5Ti 25B 5alloy.The struc-tural,thermal and magnetic properties were investigated as a function of milling time.The results showed that complete amorphization was feasible after 3.5h of milling.This fast amorphization reaction could be explained by the large negative heat of mixing between Co and other constituent elements.From SEM obrvations,different morphologies were obtained during the MA stages.It was also found that the particle size incread initially,reaching a maximum amount and then showed a decreasing trend at prolonged milling times.However,the obtai
ned amorphous powders exhibited a single exothermic peak centered at around 580 C,and an endothermic peak at about 1095 C.The exothermic peak is due to the crystallization of the
amorphous
Fig.8.Saturation magnetization (Ms)and coercivity (Hc)as a function of milling
time.
Fig.9.The remanence-to-saturation ratio (Mr/Ms)as a function of milling
time.
Fig.10.Hysteresis curves of the 5h milled powder with annealing at 400 C and 800 C.
B.Avar,S.Ozcan /Journal of Alloys and Compounds 650(2015)53e 5857音标培训
