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Production of an Iron Boride Reinforced Nanocomposite by Devitrifi cation of a Fe-Co-Ni Based Metallic Glass

Aytekin Hitit, Z. Özgür Yazıcı, Hakan Şahin, Pelin Öztürk, Nusrettin Barut, Buğrahan Eryeşil

Afyon Kocatepe University - Türkiye

Abstract Effects of annealing temperature and time on microhardness of Fe25Co25Ni25B17.5Si7.5 bulk metallic glass have been investigated. Samples of the alloy were annealed at 825, 900 and 950 K for annealing times between 10 and 150 min. It was found that for the annealing temperature of 825 K, FeCo and (Fe,Co,Ni)3B phases precipitate. At this annealing temperature, microhardness of the samples were found to be between 1120 and 1140 Hv. For the annealing temperature of 900 K, it was determined that (Fe,Co,Ni)3B, (Fe,Co,Ni)2B and fcc phases precipitate, and microhardness values, which are between 1129 and 963 Hv, decrease with annealing time. For the annealing temperature of 950 K, precipitation of fcc phase and (Fe,Co,Ni)2B were observed. For this annealing temperature, microhardness of the samples were determined to be between 895-972 Hv. 1. Introduction

It is known that it is possible to synthesize composites having ultrahigh hardness values by devitrification of metallic glasses which has high boron content. Ultrahigh hardness values result from precipitation of borides such as Fe2B, Fe23B6 and Fe3B[1-3]. In fact, microhardness values as high as 1800 Hv have been reported in these composites[3]. There is no information about fracture toughness values of these composites obtanined by devitrification metallic glasses. However, since they do not contain any ductile phase(s), they are expected to be brittle. Obviously, in order to obtain composites which have both ultrahigh hardness and high fracture toughness, at least one of the precipitating phases must a ductile one. When compositions of bulk metallic glasses are examined, it is seen that only Fe- and Co-based bulk metallic glasses contain high volume fraction of boron[4-7], and upon devitrification no ductile phase form in these systems. At this point, one might consider using a nickel or copper based bulk metallic glass which contain high volume fraction of boron, as a precursor to synthesize composite. Nevertheless, no such metallic glass has been reported up to date. For the metallic glass which has the composition of Fe25Co25Ni25B17.5Si7.5, it has been reported that (Fe,Co,Ni)2B and fcc phases form in the samples having

diameter larger than critical casting thickness[8]. Because one of the precipitating phases is a boride and the other one is a fcc phase, it is reasonable to expect that this metallic glass can be used as a precursor to synthesize a composite which might not only have ultrahigh hardness but also have high fracture toughness. In this study, we investigate the effect of annealing temperature and time on microhardness of Fe25Co25Ni25B17.5Si7.5 alloy. Microhardness of the alloy was determined as a function of annealing temperature and time and correlated with the crystallizing phases.

2. Experimental Procedure Alloy ingot with composition of Fe25Co25Ni25B17.5Si7.5 was prepared by arc melting the mixtures of pure Co (99.8 mass%), Fe (99.9 mass%), Co (99.9 mass%) , Ni (99.7 mass %), Si(99.9 mass%) metals and pure crystalline B (98 mass%) in a Ti-gettered high purity argon atmosphere. In order to ensure homogenity, the master alloy was melted three times. The alloy composition represents nominal atomic percentages. Bulk glassy samples of the alloy in a rod form with diameter of 1 mm and a length of 20 mm were produced by suction casting method in an arc furnace. The glass transition temperature (Tg), crystallization temperature (Tx), melting temperature (Tm) and liquidus temperature (Tl) of the alloy were determined by differential scanning calorimetry (DSC) (Netzsch STA 409 PC/PG ) at a heating rate of 0.33 K/s. Rod shaped glass samples were isothermally annealed at 825, 900 and 950 K for 10, 25, 50, 75, 100 and 150 min under flowing high purity argon atmosphere. All of the samples were examined by optical microscopy (OM) and scanning electron miscroscopy (SEM) so that annealed samples are fully amorphous before annealing. The structure of as-cast and annealed samples were examined by X-ray diffraction (XRD) (Bruker D8 Advance) with Cu-K radiation. Vickers hardness of the as-cast and the annealed samples were measured with a Vickers hardness tester (Shimadzu HMV 2L ) under a load of 2.94 N. Before the hardness measurements, samples were carefully polished with diamond paste in order to have smooth surfaces. For the as-cast and annealed samples, twenty hardness measurements were taken randomly and arithmetic mean of measurements were recorded as microhardness of the sample.

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3. Result and Discussion DSC curve of the alloy is given in Fig. 1. The DSC scan exhibits a distinct glass transition and supercooled liquid region, followed by two exothermic events which are characteristics of crystallization. The glass transition temperature (Tg) and the first crystallization temperature (Tx1) of the alloy are determined as 758 and 795 K, respectively. Also, melting temperature (Tm) and liquidus temperature (Tl) of the alloy are found to be 1249 and 1379 K, respectively.

Figure 1. DSC curves of the Fe25Co25Ni25B17.5Si7.5 alloy: (a) low temperature measurement and (b) melting behaviour. XRD patterns of the as-cast and annealed samples are given in Fig.2. For the as-cast sample, no crystalline peak is visible, which indicates that structure is fully amorphous. After annealing at 825 K for 10 min, it is found that structure of the sample contains some amount of amorphous phase. It is also determined that FeCo (space group Pm3m) and (Fe,Co,Ni)3B (space group Pnma) phases co-precipitate in the amorphous matrix. Comparison of the XRD peak intensities of the phases indicate that FeCo is the primary crystalline phase. As the annealing time is increased, volume fraction of (Fe,Co,Ni)3B phase increases significantly. This increase can be seen by monitoring XRD peak

intensities of (Fe,Co,Ni)3B phase. For example, peak intensities of (112) and (121) peaks of (Fe,Co,Ni)3B phase which are located at 2 =37.67 and 41.61o, respectively, increase with annealing time significantly. However, after annealing for 150 min, it is found that volume fraction of (Fe,Co,Ni)3B phase decreases, which can be seen by examining the intesities of the XRD peaks mentioned above. At the same time, precipitation of (Fe,Co,Ni)2B (space group I4/mcm) phase begins. For this reason, it is concluded that at the annealing temperature of 825 K, for the annealing time of 150 min or longer, (Fe,Co,Ni)3B phase begins to transform into (Fe,Co,Ni)2B phase. After annealing at 900 K for 10 min, precipitation of FeCo, (Fe,Co,Ni)3B and fcc (space group Fm3m) phases are observed. As the annealing time is increased, volume fraction of FeCo and (Fe,Co,Ni)3B phases decrease whereas volume fraction of fcc phase increases. Also, precipitation of (Fe,Co,Ni)2B phase is observed for annealing times of 50 min and longer. After annealing for 100 min, FeCo phase completely dissolves and fcc phase and (Fe,Co,Ni)2B phase become the majority phases. After annealing at 950 K for 10 min, fcc, (Fe,Co,Ni)2B and (Fe,Co,Ni)3B phases precipitate. At this annealing temperature, volume fraction of (Fe,Co,Ni)3B phase is significantly lower than those precipitate at 825 and 900 K. As the annealing time is increased, volume fraction of (Fe,Co,Ni)3B phase decreases whereas volume fraction of (Fe,Co,Ni)2B phase increases. However, after annealing for 150 min, there is still some amount of (Fe,Co,Ni)3B phase in the structure. Microhardness of the as-cast sample is found to be 1042 Hv. After annealing at 825 K for 10 min, microhardness of the sample is measured as 1143 Hv. As the annealing time is increased, no significant change in microhardness is observed. Microhardness values are found to be between 1124 and 1143 Hv (Fig. 3). Microhardness of the sample annealed at 900 K for 10 min was determined to be 1129 Hv. For this annealing temperature, microhardness values decrease with annealing time. In fact, after annealing for 150 min, microhardness value is found to be 963 Hv. For the annealing temperature of 950 K, microhardness of the sample is found to be 972 Hv, after annealing for 10 min. As the annealing time is increased, a slight decrease in microhardness is observed. For the annealing time of 150 min, microhardness is measured as 895 Hv. After annealing at 825 K for 10 min, the structure consists of amorphous phase, FeCo phase and (Fe,Co,Ni)3B phase. Because microhardness of FeCo phase is insignificant compared to the microhardness of the other two phases, the main contribution to the microhardness come from the amorphous phase and (Fe,Co,Ni)3B phase. As the annealing time is increased, contribution of (Fe,Co,Ni)3B phase to microhardness becomes higher whereas contribution of amorpous phase to microhardness becomes lower. One might

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expect that as the annealing temperature is increased, microhardness becomes higher due to the fact that volume fraction of (Fe,Co,Ni)3B phase increases and it

has higher microhardness than amorphous phase. It is believed that

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Figure 2. XRD patterns of the as-cast alloy and samples annealed at 825, 900 and 950 K for 10, 50, 100 and 150 min.

Figure 3. Microhardness of the samples as a function of annealing time for annealing temperatures of 825, 900 and 950 K. this improvement is not observed because as the annealing time is increased average particle size of (Fe,Co,Ni)3B becomes larger. As a result, expected improvement due to the volume fraction incrase is not obtained. At the annealing temperature of 900 K, microhardness values decrease with annealing time even though volume fraction of (Fe,Co,Ni)2B increases. It is most likely that this degredation in microhardness results from the fact that average size of (Fe,Co,Ni)2B precipitates increases with annealing time. The other reason of this decrease is that volume fraction of the fcc phase, which is softer than the other phases precipitate in the alloy system, increases with annealing time. At the annealing temperature of 950 K, after annealing for 10 min, the structure mainly consists of (Fe,Co,Ni)2B and fcc phases. There is also some small amount of (Fe,Co,Ni)3B phase in the structure. Because microhardness of (Fe,Co,Ni)2B is quite high[3], and it has high volume fraction, higher microhardness value is expected. However, microhardness, 972 Hv, is even lower than the microhardness of the amorphous phase, 1042 Hv. It is very likely that although volume fraction of (Fe,Co,Ni)2B phase is high, its average particle size is also quite high even after annealing for 10 min. As a result, the microhardness is lower than expected. Microhardness values obtained for longer annealing times support this explanation. 4. Conclusion

i. After annealing the amorphous alloy at 825 K, FeCo and (Fe,Co,Ni)3B phases precipitate. Also small volume fraction of (Fe,Co,Ni)2B phase precipitates for longer annealing times. At this annealing temperature, microhardness values are found to be between 1124 and 1143 Hv.

ii. At the annealing temperature of 900 K, FeCo, (Fe,Co,Ni)3B and fcc phases precipitate for the

annealing times of 10 and 50 min. As the annealing time is increased, fcc and (Fe,Co,Ni)2B phases becomes the dominant phases. Microhardness values decrease from 1129 to 963 Hv.

iii. At the annealing temperature of 950, (Fe,Co,Ni)2B and fcc phases precipitate. There is also small volume fraction of (Fe,Co,Ni)3B phase in the structure. After annealing at 950 K, microhardness values are found to be between 895 and 972 Hv, which mainly results from (Fe,Co,Ni)2B phase.

Acknowledgement The authors would like to thank Mr. Serhat T k z for helping with SEM study at Afyon Kocatepe University. References [1] M. Iqbal, J. I. Akhter, H. F. Zhang and Z. Q. Hua, Journal of Non-Crystalline Solids, 354 (2008) 3284–3290. [2] J. Fornell, S. Gonza´lez, E. Rossinyol, S.

Surin˜ach, M. D. Baro´, D. V. Louzguine-Luzgin, J. H. Perepezko, J. Sort and A. Inoue, Acta Materialia, 58 (2010) 6256–6266. [3] A. Hitit, M. Geçgin and P. Öztürk, J. Mater. Sci. Technol., 31(2) (2015) 148-152. [4] A. Inoue, B.L. Shen and C.T. Chang,

Intermetallics, 14 (2006) 936–944. [5] A. Inoue, B. L. Shen, H. Koshiba, H. Kato and

A.R. Yavari, Acta Materialia, 52 (2004) 1631–1637. [6] A. Hitit, . Tala and R. Kara, Indian Journal of Engineering & Materials Sciences, 21 (2014) 111-115. [7] Z. O. Yazici, A. Hitit, Y. Yalcin and M. Ozgul,

Met. Mater. Int., 22 (2016) 50-57. [8] T. Qi, Y. Li , A. Takeuchi, G. Xie, H. Miao and

Wei Zhang, Intermetallics, 66 (2015) 8-12.