Effects of heat treatment on crystallization behavior, microstructure and the resulting microhardness of a (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 bulk metallic glass composite

NOC-17624; No of Pages 6 Journal of Non-Crystalline Solids xxx (2015) xxx–xxx

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Effects of heat treatment on crystallization behavior, microstructure and the resulting microhardness of a (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 bulk metallic glass composite Amir Seiffodini ⁎, Soheil Zaremehrjardi Department of Mining and Metallurgical Engineering, Yazd University, Safayieh, Daneshgah Blvd, University Main Campus, P.O. Box 89195-741, Yazd, Iran

a r t i c l e

i n f o

Article history: Received 11 July 2015 Received in revised form 7 October 2015 Accepted 11 October 2015 Available online xxxx Keywords: Bulk metallic glass; Heat treatment; Microhardness; Liquid phase separation; Composite

a b s t r a c t The influence of heat treatments on crystallization behavior, microstructure and the resulting microhardness of an as-quenched (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 bulk metallic glass (BMG) composite was investigated. (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 alloy shows liquid phase separation (LPHS) during solidification and possesses amorphous composite structure in as cast state. Amorphous composite was annealed at different temperatures as Tp1 (725 K) and Tp2 (772 K) by DSC, and effect of crystallization of the amorphous composite on hardness was investigated by XRD, SEM, EDS, DSC and micro-hardness test. In the present study, an attempt has been made to gain knowledge of the crystallization sequence of each separated phase and the effect of their crystallization on the microhardness of (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 alloy. The first peak (TX1, 725 K) is attributed to the primary crystallization of the separated phases while the second one (T X2, 755 K) is ascribed to the crystallization of the amorphous matrix. Annealing at first and second crystallization peaks reduces hardness of separated phases and increases hardness of the matrix phase. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Metallic glasses have been one of the extensive research subjects over the past twenty years. The unique properties such as high strength and hardness, special damping property, soft magnetic characteristics, and oxidation/corrosion resistance enable this new category of metallic materials to be applied for structural or functional applications [1–3]. The definition of amorphous material is a general term which is referred to as solid state with non-periodical atomic arrangement. Contrary to crystalline counterparts, amorphous solids such as metallic glasses do not have long distance order. Amorphous material has an area which has ordered region not higher than 1 nm [4–7]. In contrast to crystalline alloys, where dislocations are the basic carriers of plasticity, the deformation of metallic glasses is characterized by the formation of shear bands, their rapid propagation, and the sudden fracture of the sample. Metallic glasses have been shown to exhibit the phenomenon of strain softening, while the crystalline materials undergo strain hardening [8]. Shear softening and formation of shear bands in metallic glasses have been attributed to a local decrease in the viscosity of the glass [8,9]. The dependence of the plastic flow in BMGs on the normal stress, shear plane, has been reported in some studies [8–12]. Although, computer simulation shows metallic glasses can display the super high plastic strain in the individual shear band on the microscopic scale [13,14], ⁎ Corresponding author. E-mail address: [email protected] (A. Seiffodini).

and shows high mechanical properties such as high yield and ultimate strength are the benefit of using BMGs with a thoroughly disordered atomic structure, their applicability is limited by the lack of global plasticity resulting from shear bands localization and it has apparently brittle fracture model. Because of this reason, the most attention has been attracted to improving toughness and hardness of amorphous material. Thus, developing a composite microstructure is done in order to overcome this restricted plasticity [15,2,16]. Extensive research has been devoted to the mechanical properties of BMGs, in an attempt to improve the plasticity of MGs via many methods mainly include (1) ex-situ second phase particles reinforced BMG composites; (2) in-situ formed BMG composites; (3) high Poisson ratio BMGs; (4) dual phases BMGs with phase separation; (5) interpenetrating phase composites; (6) porous BMG; (7) sample size effect and (8) BMG surface modification [17]. The purpose of all of the methods is to increase the density of shear bands in order to dissipate the deformation energy into much larger volume and to eliminate the stress concentration, therefore to improve the plasticity [18]. As a composite metallic glass, phaseseparated alloy has been demonstrated to exhibit much enhanced compressive plasticity [19,8]. Recently, new types of immiscible metallic materials, called two-phase metallic amorphous alloys, were developed from a variety of immiscible alloys [20–23]. Phase separation will result in some regions with different chemical compositions, different cooperation numbers and different nearest atomic numbers. Finally, there will be inhomogeneous distribution of hardness (modulus). The formed two amorphous phases have different critical shear stress (CSS) to facilitate

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Please cite this article as: A. Seiffodini, S. Zaremehrjardi, Effects of heat treatment on crystallization behavior, microstructure and the resulting microhardness of a (Fe0.9Ni0.1)77Mo5P9C7.5..., J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.10.023

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the shear bands. Extensive researches on finding other phase-separating alloy systems to yield two-phase glassy structures have been carried out [20–28]. Fe-based BMGs are more attractive for engineering application due to the combination of ultrahigh strength, excellent corrosion resistance and relatively low material cost [19,29]. Fe-based amorphous material likes other amorphous material which has low ductility. Since the 1990s, widely researches have been done to improve the ductility of BMGs [30,31] using Controlled annealing method. During Controlled annealing of metallic glasses, nanoscale crystalline phases can be formed in the amorphous phase due to the redistribution of the solute elements at the amorphous/crystalline interface [20]. But, most of the BMG matrixes have the tendency of brittleness [32–34]; because the structural relaxation resulted from the heating process as well as the precipitated brittle intermetallic phases will both deteriorate the mechanical properties. Therefore, the non-satisfying results restrict the widely application of this method. Our previous researches have demonstrated that the appropriate addition of Ni to the Fe77Mo5P9C7.5B1.5 alloy shifts the composition closer to the eutectic, lowers the liquidus temperature, and significantly enhances the GFA and the plastic strain of the alloys [35,36]. Our previous research showed occurrence of liquid phase separation (LPHS) in (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 alloy [37] and the subject is currently being further investigated. Microstructure of (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 alloy is consisting of two amorphous phases with different crystallization temperatures. In the present study, an attempt has been made to gain knowledge of the crystallization sequence of each separated phase and the effect of their crystallization on the mechanical properties of (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 alloy. A detailed investigation concerning the evolution of microstructure, phase and composition, mechanical properties (in terms of microhardness) was undertaken to determine the utility of the improving ductility with Controlled annealing. Investigations were done with the use of XRD, SEM, EDS, DSC and microhardness measurements methods. 2. Experimental procedure Multi-component master alloy with nominal composition of (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 was prepared by induction melting in a quartz crucible under purified argon atmosphere. The starting materials were commercially pure Fe (99.9%), Ni (99.99%), Mo (99.9%), and crystalline B (99.5%), as well as in-house prepared Fe–P (containing 14 wt.% P) and Fe–C (containing 5.18 wt.% C) pre-alloys. The ingots were re-melted several times for homogenization. Pieces of the master alloys were inductively re-melted in a quartz crucible and injected into a graphite mold with a diameter of 2.5 mm. The length of the cast samples was 60 mm. The reason for choosing the graphite mold was to provide enough time in order to coalescence of L2 droplets during liquid phase separation and formation of larger and detectable structure. The structure of the cylindrical rods was investigated by X-ray Diffraction (XRD, Philips X'Pert PRO) using Co kα radiation (λ = 1.78897 Å). For the qualitative determination of phase composition of the crystallized alloy samples the JCPDS-PDF database was used. The thermal

behavior related to glass transition, crystallization and melting events of the alloys was investigated using a differential scanning calorimeter (PerkinElmer, DSC-7) under a flow of purified argon, at a heating rate of 20 K/min and a cooling rate of 40 K/min. Due to their similarity in terms of the crystallization temperature and achieving high accuracy, the as-cast rods were heated by means of differential scanning calorimetry (DSC) to first peak crystallization temperature (Tp1 = 725 K) and second peak crystallization temperature (Tp2 = 772 K) at a heating rate of 20 K/min. Cross section of specimens which were cast has been polished carefully. After being etched by Nital (2% HNO3 + 98% ethanol), the lateral surfaces of the samples were examined with OM microscope and a scanning electron microscope (Hitachi TM1000 tabletop SEM) linked with energy dispersive X-ray spectrometry (EDS). Volume fraction of phases was estimated by Micro-structural Image Processor (MIP) software (produced by the University of Ferdowsi Mashhad, Iran). The Vickers hardness was measured using a computer controlled (Future-Tech FM-FM700) hardness tester. The tests were performed using a typical diamond indenter in the form of pyramid with square base and an angle of 136° between opposite faces applying a load of 0.49 N for 10 s. The diagonal of the imprints as well as the hardness were calculated using a Digital Video Measuring System. For the indentations, the samples were embedded in epoxy resin and the measured surface was carefully polished to mirror-like appearance using diamond paste. At least 10 indentations were performed to verify the accuracy of the indentation data. Prior to hardness test the specimens were carefully polished to mirror-like appearance using diamond paste. 3. Result and discussion The cross section of specimens before annealing treatment with a composition of (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 is shown in Fig. 1(a), that is etched by Nital. This figure obviously shows three various phases involved in bright matrix, gray phases and black dendrite phases that were corroded by etching. Fig. 1(b) shows analyzed image of specimens by MIP. Volume fraction of bright matrix phase, gray phase and black dendrite phase has been calculated (numbered in Table 1). The XRD pattern of the as-cast alloy (Fig. 1(c)) exhibits a broad diffraction maximum without any crystalline reflection indicating that the 2.5 mm diameter rod is fully amorphous within the sensitivity limit of XRD [8]. Fig. 1(a) shows an OM image of the cross-sectional surface of the as-cast (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 bulk metallic glass composite with moderate volume fraction of gray second glassy particles. Owing to the existence of temperature gradient in the radial direction of the cylindrical sample, the liquid phase separated droplets migrate radially and thereby undergo collision and coalescence [38]. As it is seen in Table 1, volume fraction of dendrite and other phases together constitutes less than 2 vol.% that is less than the sensitivity limit of XRD device. But volume fraction of gray and matrix phases is in order 39.3 vol.% and 58.7 vol.%. Thus, these phases must be detectable by X-ray Diffraction Pattern. High volume fraction of matrix and gray phases and lack of crystalline peak in XRD pattern denote that both

Fig. 1. (a) OM image of the cross-sectional surfaces of the as-cast (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 bulk metallic glass composite with 2.5 mm diameter, (b) the inset shows analyzed image of Fig. 1(a) by MIP software, (c) shows the corresponding XRD pattern of the alloy.

Please cite this article as: A. Seiffodini, S. Zaremehrjardi, Effects of heat treatment on crystallization behavior, microstructure and the resulting microhardness of a (Fe0.9Ni0.1)77Mo5P9C7.5..., J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.10.023

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Table 1 Volume fraction of phases existing in Fig. 1(a). Phases

Volume fraction (vol.%)

Bright matrix Gray Black crystalline dendrite Other phases

58.8 39.3 1.3 0.6

phases are amorphous. Other reasons for existing liquid phase separation (LPHS) noticeable in Fig. 1 are; 1– it clearly shows separation of the second liquid phases with round shape and different sizes which are typical features of liquid phase separation alloys [39,40], 2– coupling of separated round shape phases in some part of the sample that is the specific character of liquid phase separation (LPHS) as a result of Marangoni or gravity forces in double-phase liquid zone for decreasing interface energy, and 3– formation of a composite consisting of two amorphous structures with uniformly dispersed crystalline dendrite phases in matrix and separated round shape phases upon casting. These reasons denote these phases separated during liquid state are maintained in liquid structure (amorphous structure) after solidification due to high cooling rate. Fig. 2 shows the DSC trace of the as-cast (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 alloy, measured at 20 K/min heating rate. The curve reveals a distinct glass transition followed by a super cooled liquid region before crystallization, in agreement with the XRD patterns (Fig. 1(c)), which showed the presence of a glassy matrix for the as-cast (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 rod with 2.5 mm diameter. The (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 alloy shows a small endothermic event at 953 K, but far lower than the melting temperature. Most probably this event is associated with an allotropic transformation of one of the crystallized phase(s) upon heating the sample from its glassy state. The DSC curve shown in Fig. 2 reveals the existence of four exothermic peaks corresponding to various crystallization processes that initiation of each step is distinguished by TX1–TX2–TX3–TX4 on DSC curve. Amorphous alloy is a kind of metastable material (included higher energy level than crystalline state), and it has the tendency to crystallize during heating process [8]. This kind of properties can be applied to synthesize in situ composites containing precipitated phases with the length of 10 nm. For studying effects of each crystallization step on microhardness, amorphous samples with a composition of (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 and microstructure which were

Fig. 3. SEM image of the matrix and separated phases of (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 alloy annealed at Tp1, point #1 is located in matrix, and point #2 is located in the separated phase.

interpreted in Fig. 1 were heated by DSC device until different temperatures, (725 K) and Tp2 (772 K). Fig. 3 shows a cross-sectional SEM image of graphite mold cast rod with a diameter of 2.5 mm which was annealed at Tp1. It is obvious that separated phases were crystallized much more than matrix due to annealing treatment at temperature of Tp1. Separation in liquid phase (monotectic evolution) makes two liquid phases L1 and L2 with a composition different from primary composition of liquid L. Table 2 displays the Energy-Dispersive X-ray spectroscopy data (EDS) of matrix and separated phase of samples annealed at Tp1 and Tp2, indicating the occurrence of phase separation, were shown according to points distinguished in Figs. 3 and 6, (numbered in Table 2). Fig. 3 shows that annealing at 725 K (Tp1) has induced the primary crystallization of the separated phases. Upon decreasing temperature, the molar partition ratio will change with the free energy, and the solute distribution in the two coexistent liquids automatically assembles to minimize the free energy of the two-liquid system. Table 2 proves that less volume fraction of Fe and higher amounts of alloy elements exist in amorphous separated phase than matrix, compared with Fe, Ni has a smaller negative mixing enthalpy with the metalloid elements P and B, and a higher positive mixing enthalpy with C (the values are summarized in Table 4 [41]). This could result in a structure with weaker atomic interaction and thus decreases the overall activation barrier for crystallization to occur, causing a decrease in Tx [42]. So the separated phase has less resistance against crystallization than matrix, and therefore less crystallization temperature (Tx). Table 3 summarizes the effects of annealing at Tp1 and Tp2 on the resulting micro-hardness. The separated phase shows higher hardness than matrix in as-cast state. The atomic clusters form in liquid

Table 2 Elemental analysis of matrix and separated phase of (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 alloy annealed at Tp1 and Tp2 according to Figs. 3 and 6.

Fig. 2. DSC trace of the as-cast (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 metallic glass alloy performed at a heating rate of 20 K/min.

1 2 3 4

%Fe

%Mo

%P

%Ni

%C

62.4 56.8 53.9 49.7

7.2 7.2 6.1 6.4

18.4 23.4 18.6 21.1

7.2 7.5 6.5 7

4.8 5.1 15 15.8

Please cite this article as: A. Seiffodini, S. Zaremehrjardi, Effects of heat treatment on crystallization behavior, microstructure and the resulting microhardness of a (Fe0.9Ni0.1)77Mo5P9C7.5..., J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.10.023

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Table 3 Micro-hardness results of (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 alloy before and after annealing at Tp1 and Tp2.The measurement errors are within ±5 HV.

As cast Annealed at Tp1 Annealed at Tp2

Matrix HV

Separated phases HV

888 1000 1107

990 978 837

alloy among the elements with a large electronegativity. Therefore there are more atomic clusters with strong bonding between the constituent elements in the matrix compared to separated phases that can be attributed to the mixing enthalpies with large negative values (the values are summarized in Table 4 [41]). The driving force for long distance diffusion of atom and rearrangement is not big enough for diffusion, rearrangement of atom and annihilation of vacancy in solidification processing in matrix compare with separated phase. The other factor that favors the hardness of separated phase is higher metalloid content [43]. Energy dispersive X-ray spectrometry (EDS) analysis shows higher content of metalloid in separated phases compare with matrix (Table 2). According to Table 3, by heat treatment of sample at Tp1 hardness of matrix and separated phase is changed respectively by +13% and −1%. Fig. 4 shows X-ray diffraction patterns of studied alloy after annealing at 725 K (Tp1). The XRD pattern shows a superposition of a broad diffuse background and a set of feeble peaks, which implies that partly amorphous-crystalline composite obtained and the amorphous phase is the majority in this composite. Fig. 3 shows that crystallization starts first at the separation phases with annealing at 725 K (Tp1). The crystals which are identified after a heat treatment until 725 K (Tp1 ) are: Fe23 (C,B)6 (ICCD PDF2 12–0570) and (Fe,Ni)FCC (ICCD PDF2 47-1417). The primary precipitation of the Fe23(C,B) 6 type phase having a complex FCC structure with a large lattice parameter of 1.12 nm including 96 atoms from the network-like glassy structure requires long-range atomic rearrangements to be able to be formed. In contrast, (Fe,Ni)FCC has simple FCC structure that short-range atomic rearrangements are enough in order that it can be formed [24,26]. Thus, forming of (Fe,Ni)FCC is easier than Fe23(C,B)6 phase having complex structure at low temperature. When sample was annealed at Tp1, more fraction of softer phases ((Fe,Ni)FCC) than Fe23(C,B)6 having higher hardness than amorphous phases [27] were formed. Because ((Fe,Ni)FCC phases are softer than Fe 23 (C,B)6 phase [28], distribution and composite construction effect of these crystalline phases were decreased, causing that the hardness of separated phase was approximately reduced. Stress relaxation could be also responsible for the reduction in hardness. It is suggested that residual stresses, due to differential cooling during processing, play a major role. Indeed, residual stresses up to 900 MPa can be generated in bulk metallic glasses because of their very low thermal conductivity and their large thermal expansion coefficient [44]. It is likely that the compressive stresses induced on the surface after cooling are responsible for a relatively high hardness. During annealing atoms rearrangement in short range and stress relaxation happened in the structural relaxation period of separation phases and it would bring about the decrease of hardness [28,29].

Fig. 4. XRD pattern of (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 alloy after annealing at 725 K (Tp1).

Fig. 5 shows micro-structure of sample annealed at Tp1 that it demonstrates existence of three phases namely; amorphous matrix, dendrites and separated phases. Micro-structure of separated phase demonstrates that some parts of the amorphous separated phase were crystallized and the remaining parts were kept in an amorphous state. According to less volume fraction of amorphous separated phase (39.3 vol.%) which it is not completely crystallize, XRD pattern of sample annealed at Tp1 (Fig. 4) demonstrates a few peak and low fraction of crystallization. Contrary to lack of noticeable crystallization in the matrix phase after annealing treatment at a temperature of Tp1, results of micro-hardness show increasing hardness of the matrix phase by 13% (Table 3). In BMGs, there is no periodicity of a lattice. But there are also lots of atomic clusters in BMGs and they are retained from molten pool but they are metastable. If there is enough driving force for a short distance diffusion of atoms, a structural relaxation could have happened in the matrix and the atoms in the local structure (atomic clusters) would like to diffuse farther to decrease the energy of the alloy system in the relaxation. Because the diffusion of atoms would happen in a short distance instead of long distance which is controlled by the annealing temperature [45], the structure of the amorphous composite matrix

Table 4 The mixing enthalpies ΔHmix (kJ/mol) for atomic pairs between the constituent elements (data taken from [41]). –

Ni

Mo

P

C

B

Fe Ni Mo P C

−2

−2 −7

−31 −26 −45

+40 +51 +23 −4.5

−11 −9 −19 +0.5 −10

Fig. 5. SEM image of matrix and separated phases of (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 alloy annealed at Tp1.

Please cite this article as: A. Seiffodini, S. Zaremehrjardi, Effects of heat treatment on crystallization behavior, microstructure and the resulting microhardness of a (Fe0.9Ni0.1)77Mo5P9C7.5..., J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.10.023

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Fig. 6. SEM image of matrix and separated phase of (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 alloy annealed at Tp2, point #3 is located in matrix, and point #4 is located in separated phase.

under this temperature 725 K (Tp1) does not change obviously (Fig. 5). Atoms rearrangement in short range and stress relaxation happened in the amorphous matrix and it would bring about the decrease of hardness. But as it is seen in Table 3, the hardness of matrix increased by 13% with the annealing at 725 K (Tp1). Therefore, the stress relaxation in structural relaxation may not be the only factor which controls the change of hardness. On the other hand, the aggregation and disappearance of atomic clusters greatly influence the hardness of amorphous composite matrix in the annealing process. Increase in hardness of the matrix after annealing treatment at temperature of Tp1 can be related to two reasons. First, there is mixing enthalpies with negative values between the constituent elements. The enthalpies of mixing for the different atomic pairs in Fe–Ni–Mo–P–B–C are shown in Table 4 [41]. So, connecting of bonds between constituent elements reduces system energy. Annealing treatment at temperature of Tp1 does not provide required driving force for long-range rearrangement in matrix to form crystalline stable phases. In contrast, when short distance diffusion of atoms in the amorphous phase is provided, metastable clusters (quasi-crystal-like structure) which have small size, and are so hard because of existing mixing enthalpies with negative values between the constituent

Fig. 7. XRD pattern of (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 alloy after annealing at 772 K (Tp2).

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elements are formed [29]. This quasi-crystal-like structure, as has been shown by Hirata et al., can be considered as an intermediate state appearing prior to nano-crystallization [46]. The quasi-crystal-like structure is also related to the medium range order (MRO) locally forming the glassy state [47]. In the sample annealed at 725 K, the formation of quasicrystals, which have harder structure than amorphous matrix, could be responsible for the increase in hardness of the matrix. This effect has been reported in the literature for various systems, for example in a Febased alloy where the precipitation of quasi-crystalline particles was used to increase the tensile strength of the alloy [48,49]. Second effect that can contribute to the increasing hardness of the matrix before the onset of its crystallization is interpreted as a result of the atomic reordering associated with the annihilation of free volume frozen in the amorphous structure during the casting process, i.e., structural relaxation [50]. Fig. 6 shows SEM image of sample annealed at Tp2. At higher heat treatment temperatures, that is, Tp2 the amorphous matrix changed from the original smooth shape (Fig. 1(a) (before heat treatment) and Figs. 3 and 5 (annealed at Tp1)) to silk like structures which are shown in Fig. 6. It was demonstrated that initiation of crystallization was started in matrix phase when the temperature was ascended until Tp2. It could be some nanocrystalline grains precipitated from amorphous matrix. It can be concluded that there should be a long distance diffusion of atoms in the amorphous matrix phase to form crystalline phases [45]. The XRD pattern of the sample annealed at Tp2 (Fig. 7) shows that annealing process at temperature Tp2 obviously causes formation of the more crystalline phases. The relatively smooth baseline of the diffraction curve indicates a remarkable decrease in the volume fraction of the amorphous phase in the matrix and specimen undergoes serious crystallization, leading to the appearance of several more crystalline peaks. Sample which annealed at temperature of Tp2 was intensely crystallized, and the number of XRD pattern peaks was increased in a sample annealed at Tp1 rather than Tp2. Besides the Fe23(C,B)6 and (Fe,Ni)FCC phases (which were already found after annealing at TX1), additional FeNi2P (ICCD PDF2 51-1367) phase is now identified. When annealing temperature is raised up to 772 K (Tp2), the atoms in matrix can diffuse in a longer distance. If there is enough driving force for a long distance diffusion of atoms, the clusters would form new phases. Therefore, the atomic clusters disappear gradually and the Fe23(C,B)6 and FeNi2P phases form in the matrix. According to high volume fraction of matrix phase (58.7 vol.%) and initiation of crystallization in matrix, XRD pattern of sample annealed at Tp2 (Fig. 7) shows high fraction of crystallization. Fraction of crystalline phases such as Fe23(C,B)6 and FeNi2P which have more elaborate structure than (Fe,Ni)FCC phases was increased when temperature increased until Tp2 and conditions became more appropriate for long distance diffusion. Because structure of Fe23(C,B)6 and FeNi2P phases are complicated, and their speed of growing is low, their structure becomes finer (Fig. 6), causing that the hardness is increased when annealed at temperature of Tp2. It should be noted that the increase of hardness with the annealing temperature does not necessarily occur in all families of metallic glasses since the lack of atomic ordering in the amorphous structure precludes dislocation activity, thus often resulting in larger hardness values in the glass when compared to crystalline phases with analogous compositions. However, it is known that Fe–B phases are particularly brittle and therefore possess an ultrahigh hardness [51,38]. When annealing temperature is raised up to 772 K (Tp2), separated phase was intensely crystallized as which is shown in Fig. 6. The lamellar phases are formed and further growth of existing particles (such as Fe23(C,B)6 and γ-(Fe,Ni)) occurs, the different diffraction peaks become thinner (Fig. 7), indicating the growth of the crystals. Thus strengthening effects of dispersed metastable phases or microcrystals on the separated phase are weakened. As a result, crystal/ crystal structure dominated (Fig. 6), and the conditions become appropriate for growing shear bands and crack which decreased hardness drastically [52].

Please cite this article as: A. Seiffodini, S. Zaremehrjardi, Effects of heat treatment on crystallization behavior, microstructure and the resulting microhardness of a (Fe0.9Ni0.1)77Mo5P9C7.5..., J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.10.023

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4. Conclusion (Fe0.9Ni0.1)77Mo5P9C7.5B1.5 alloy shows liquid phase separation (LPHS) during solidification and possesses composite structure in as cast state. Both separated and matrix phases (L1 and L2) have high glass forming ability and show amorphous structure in as cast state. Each phase has different composition and so possesses different crystallization temperature and hardness. The first peak (TX1, 725 K) is attributed to the primary crystallization of the separated phases while the second one (TX2, 755 K) is ascribed to the crystallization of the amorphous matrix. Increasing hardness of the matrix during annealing at Tp1 is attributed to the precipitation of quasi-crystalline particles and annihilation of free volume. Stress relaxation and formation of soft γ(Fe,Ni) phases could be responsible for the reduction in hardness of separated phases during annealing at Tp1. When annealing temperature raised up to 772 K (Tp2), the Fe23(C,B)6 and FeNi2P phases form in the matrix causing the hardness to increase. Separated phase that was intensely crystallized with annealing at Tp2 crystal/crystal structure was dominated, and the conditions become appropriate for growing shear bands and crack which decreased hardness drastically. Acknowledgments The work was supported by the German Science Foundation (DFG) through grant STO 873/2-1. One of the authors (A. S.) gratefully acknowledges the Iran Ministry of Science, Research and Technology for its financial support and the efficient cooperation with the Institute for Complex Materials at the Leibniz Institute for Solid State and Materials Research (IFW) Dresden. References [1] B.J. Park, H.J. Chang, D.H. Kim, W.T. Kim, In situ formation of two amorphous phases by liquid phase separation in Y–Ti–Al–Co alloy, Appl. Phys. Lett. (2004) (6353-6355-85). [2] E.S. Park, J.S. Kyeong, D.H. Kim, Phase separation and improved plasticity by modulated heterogeneity in Cu–(Zr,Hf)–(Gd,Y)–Al metallic glasses, Scr. Mater. 57 (2007) 49–52. [3] Z.F. Zhang, F.F. Wu, G. He, J. Eckert, Mechanical properties, damage and fracture mechanisms of bulk metallic glass materials, Mater. Sci. Technol. 23 (2007) 747. [4] R. Zallen, Physics of Amorphous Materials, PWN, Warsaw, 1994 (in Polish). [5] W. Johnson, Bulk glass-forming metallic alloys: science and technology, MRS Bull. (1999). [6] M. Soi,ski, Magnetic materials in engineering, COSiW SEP, Warsaw, 2001 (in Polish). [7] H.K. Lachowicz, Amorphous Magnetic Materials: Fabrication Mathods, Properties and Technical Applications, Proceedings of I Polish Seminar of Amorphous Magnetic Materials, Warsaw, 1983. [8] C. Suryanarayana, A. Inoue, Bulk Metallic Glasses, CRC Press, Boca Raton, London, New York, 2011. [9] G.S. Yu, J.G. Lin, W. Li, S.F. Li, Effect of pressure sensitivity index on the deformation behavior of Zr41.2Ti13.8Cu12.5Ni10Be22.5 metallic glasses, J. Alloys Compd. 482 (2009) 366–370. [10] C.A. Schuh, T.G. Nieh, A nanoindentation study of serrated flow in bulk metallic glasses, Acta Mater. 51 (2003) 87–99. [11] V. Keryvin, Indentation of bulk metallic glasses: relationships between shear bands observed around the prints and hardness, Acta Mater. 55 (2007) 2565–2578. [12] C. Tang, Y. Li, K. Zeng, Characterization of mechanical properties of a Zr-based metallic glass by indentation techniques, Mater. Sci. Eng. A 384 (2004) 215–223. [13] H. Chen, Y. He, G.J. Shiflet, Poon, Deformation-induced nanocrystal formation in shear bands of amorphous alloys, SJ. Nat. 367 (1994) 541. [14] C.A. Pampillo, Flow and fracture in amorphous alloys, J. Mater. Sci. 10 (1975) 1194. [15] M.W. Chen, A. Inoue, W. Zhang, T. Sakurai, Extraordinary plasticity of ductile bulk metallic glasses, Phys. Rev. Lett. 96 (2006) 245502. [16] B.J. Park, H.J. Chang, D.H. Kim, W.T. Kim, K. Chattopadhyay, T.A. Abinandanan, Phase separating bulk metallic glass: a hierarchical composite, Phys. Rev. Lett. 96 (2006) 245503–245507. [17] J. Eckert, J. Das, S. Pauly, C. Duhamel, Mechanical properties of bulk metallic glasses and composites, J. Mater. Res. 22 (2) (2007) 285–301. [18] L.Q. Xing, Y. Li, K.T. Ramesh, J. Li, T.C. Hufnagel, Enhanced plastic strain in Zr-based bulk amorphous alloys, Phys. Rev. B 64 (2001) 180201. [19] S.F. Guo, L. Liu, X. Lin, Formation of magnetic Fe-based bulk metallic glass under low vacuum, J. Alloys Compd. 478 (2009) 226.

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Please cite this article as: A. Seiffodini, S. Zaremehrjardi, Effects of heat treatment on crystallization behavior, microstructure and the resulting microhardness of a (Fe0.9Ni0.1)77Mo5P9C7.5..., J. Non-Cryst. Solids (2015), http://dx.doi.org/10.1016/j.jnoncrysol.2015.10.023