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FeAl-B composites with nanocrystalline matrix produced by consolidation of mechanically alloyed powders
Journal of Alloys and Compounds 791 (2019) 75e80
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
FeAl-B composites with nanocrystalline matrix produced by consolidation of mechanically alloyed powders Marek Krasnowski a, *, Stanislaw Gierlotka b, Tadeusz Kulik a a b
Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142, Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2018 Received in revised form 28 January 2019 Accepted 24 March 2019 Available online 25 March 2019
Mechanically alloyed composite powders with equiatomic FeeAl stoichiometry and with the addition of 5, 10, 20 and 30 vol% of B were subjected to consolidation. For this purpose, hot-pressing at 800 C under the pressure of 7.7 GPa for 180 s was applied. A reference FeAl sample was prepared using the same procedure as these from the composite powders. The produced bulk materials were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy as well as characterised by hardness, density and open porosity measurements. For the compositions containing 5, 10 and 20% of B, it was possible to obtain composites with the nanocrystalline FeAl intermetallic matrix reinforced with homogenously distributed fine boron particles. For the material containing 30% of B, the boron particles were distributed in a two-phase matrix consisting of the dominant nanocrystalline FeAl intermetallic and minor amorphous phases. The density of the composite samples is in the range of 5.31 ÷ 4.57 g/cm3, which is lower than that of the FeAl reference sample, and decreases with the increase of B content. The hardness of the composite samples is in the range of 10.75 ÷ 12.1 GPa, is higher than that of the intermetallic FeAl reference sample (9.84 GPa), and increases with the increase of B content. To the best of our knowledge, the FeAl-B composites with nanocrystalline intermetallic matrix were produced for the first time. © 2019 Elsevier B.V. All rights reserved.
Keywords: A. nanostructured materials A. metal matrix composites A. intermetallics B. mechanical alloying B. powder metallurgy
1. Introduction Over the last few years, low-density materials with relatively high hardness and strength have been given significant attention from researchers. FeAl intermetallic has advantageous strength properties, moderate density and good heat resistance. Increasing polycrystalline alloys strength and hardness can be achieved, among others, by decreasing the grains size down to the nanometer range. Nanocrystalline alloys attract considerable attention due to their properties being usually better than those of microcrystalline materials. This concerns primarily strength and hardness [1e5]. One of the methods of nanocrystalline alloys preparation is mechanical alloying (MA). The products of this process are however powders, while most applications require a bulk form. Consolidation of mechanically alloyed powders into bulk full-density material with a nanocrystalline structure preserved is a task difficult to perform. The pressing ability of the MA milled powders is relatively
* Corresponding author. E-mail address: [email protected] (M. Krasnowski). https://doi.org/10.1016/j.jallcom.2019.03.334 0925-8388/© 2019 Elsevier B.V. All rights reserved.
low because of work hardening and the consolidation temperature must be kept low in order to prevent grain growth. Strength and hardness of an alloy can be increased by creating a composite structure containing reinforcing particles. There is a number of works devoted to bulk nanocrystalline FeAl intermetallic reinforced with hard compounds such as TiN, TiC, Al2O3 [6e8]. Using of reinforcing particles of low density should result in decrease of the density of the prepared material. Boron, being a material with very high hardness and low density, is well-suited for such applications. Its presence in a composite material should lead to the improvement of the specific strength. Addition of 30% vol. boron to an FeAl intermetallic phase is expected to decrease the composites density by 17%, relative to the pure intermetallic. Fe-Al alloys with Al content up to 40 at.% and with addition of B up to 20 at% have been produced by MA [9,10]. In those works, however, no composite structure with B inclusions embedded in powder particles was reported. Powders with composite structures comprised of B particles embedded in Al85Fe15 alloy [11] or in Al60Fe15Ti15(Co/Mg/Zr)5 alloys [12] have been synthesised by MA. Recently, we applied the MA process to obtain FeeAleB composite powders with equiatomic FeeAl stoichiometry and with the
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content of 5, 10, 20 and 30 vol% of B [13]. During milling B particles were ‘built into’ the matrix being formed in situ. Therefore, the resulting powder composites should have clear, contamination-free interfaces between the matrix and the reinforcing particles, which will result in good adhesion. The application of milling enabled attaining a uniform distribution of B particles in the matrix [13]. It has been also found that: (i) a nanocrystalline Fe(Al) solid solution was formed at the early stage of the processes, (ii) the increase of B content in the powder mixture caused a more intense decrease of the Fe(Al) grain size during MA, (iii) larger concentration of B in the mixture promoted the formation of an amorphous phase and for the powders containing 20 and 30% of B, an amorphous phase was also present in the MA products and for the latter it was the prevalent phase, (iv) depending on B content, the matrix of the composite powders consisted of nanocrystalline Fe(Al) or mixture of amorphous and nanocrystalline phases, (v) during heating up to 720 C of the final MA products containing 5 and 10% of B, the Fe(Al) transformed into the ordered FeAl intermetallic phase, while upon heating of the final MA products containing 20 and 30% of B, the amorphous phase crystallised and a phase isomorphic with AlFe2B2 was formed [13]. In the current work, we produced bulk nanocrystalline FeAl-B composites by hot-pressing of mechanically alloyed Fe-Al-B powders. The reference bulk FeAl sample was also produced (for this sample the sequence of the phase transformations as a function of the milling time was the same as for the previously described Fe50%Al powder [14]). The consolidated materials were characterised by structural investigations as well as by hardness, density and porosity measurements. 2. Experimental An elemental powder mixtures of Fe-50at.%Al with addition of 5, 10, 20 and 30 vol% of B (abbreviated in the text as B5, B10, B20 and B30 respectively) were mechanically alloyed in a SPEX 8000D highenergy ball mill. The quantity of added B was calculated assuming the measured densities of the B, Fe and Al powders. As a reference sample (B0), equiatomic Fe-Al powder mixture without B addition was mechanically alloyed under the same conditions. The total milling time was 30 h. After completion of the mechanical alloying, the powder particles had spherical shape and the diameter of 4 ÷ 40 mm. Boron inclusions of a size of 0.4 ÷ 3 mm were distributed inside the powder particles. The details can be found in Ref. [13]. Consolidation of the final MA products was carried out using a press equipped with a high-pressure toroid-type chamber. The shape of the chamber and the material of the gasket ensured that the pressure conditions were close to isostatic. The compaction processes were performed under the pressure of 7.7 GPa at the temperature of 1000, 900, 850 and 800 C for 3 min. The cell was pressurised at the rate of 0.5 GPa/min, prior to heating and the heating and cooling rates were 1000 C/min. The structural investigations of the consolidated samples were carried out by X-ray diffraction (XRD) using a Rigaku MiniFlex II diffractometer operating with CuKa radiation. The DHN-PDS software was employed for Ka2 component stripping from the XRD patterns and calculation and refinement of unit-cell parameters. The mean crystallite size and the mean microstrains, the latter also for the powders, were estimated by the Williamson-Hall method [15]. The instrumental broadening, determined using an Si standard, was subtracted from the experimental breadth to obtain the physical broadening of each diffraction line for the Williamson-Hall calculations. Hitachi S-3500 N scanning electron microscope (SEM) was used for observations of the consolidated materials. The SEM was equipped with energy dispersive X-ray spectroscopy (EDS) system.
Samples for SEM examinations were prepared using standard polishing technique. Hardness measurements of the bulks were conducted by the Vickers method using a ZWICK hardness testing machine under a load of 1 kg (HV1) imposed for 15 s. Vickers hardness value was the average of at least 20 indentations. The density of the bulks was determined using a Gibertini E154 balance equipped with a device for measuring the density of solids (Archimedes method). The mass measurements performed during density determination allowed to calculate the open porosity of the consolidated samples. 3. Results and discussion The prepared Fe-Al-B and Fe-Al powders were consolidated by high-pressure hot pressing at elevated temperature. Application of high temperature, in addition to ensuring high-quality consolidation, can introduce phase transformations in the compacted powders. Over the last few years we have consolidated mechanically alloyed nanocrystalline powders at high pressure and temperature maintaining nanocrystallinity and achieving expected phase changes, i.e. transformation of solid solution into ordered intermetallic compound [6,7,16]. Calorimetric investigations have shown that in the B30 powder heated without pressure to 630 C FeAl intermetallic phase appears [17], while in both the B20 and the B30 powders heated to 720 C a phase isomorphic with the AlFe2B2 forms [13]. High pressure is known to affect phase transformations, including amorphous phase crystallisation [11,12,16e19] and so the temperature of the phase transformation during high-pressure consolidation can be different from those observed in the calorimeter. For example, ordering of Fe(Al) solid solution in the FeAl50% alloy in the calorimeter occurred below 450 C [14], while it did not occur at 750 C during consolidation under 8 GPa [19]. So, the employed temperature of consolidation should be high, but not as high as to cause undesired transformations. To select the optimum processing temperature, the B30 powder was consolidated at 1000, 900, 850 and 800 C for 180 s. Fig. 1 gives XRD patterns of the B30 sample before [13] and after hot-pressing at various temperatures. The absence of boron diffraction lines in the patterns was discussed in the case of the produced powders [13]. The XRD patterns show that FeAl intermetallic phase was formed during hotpressing at 800 C, whereas use of higher consolidation temperatures caused creation of some amount of AlFe2B2-type phase and other, unidentified phase/phases. Based on the XRD results we cannot confirm the presence of small amount of an amorphous phase in the sample hot-pressed at 800 C which was revealed in €ssbauer spectroscopy [17]. this sample by Mo Recently we have reported the differences between the phase transformation in the mechanically alloyed B30 powder consolidated at 800 C under 7.7 GPa and the one heated up to 720 C in the calorimeter under atmospheric pressure [17]. In this study we observed another example of pressure effect on phase transformation. In the XRD pattern of the B30 powder consolidated at 1000 C under 7.7 GPa, broad, overlapping peaks of FeAl, AlFe2B2type and other, unidentified phase/phases are present while welldefined peaks of AlFe2B2 and Al13Fe4 phases were observed in the XRD pattern of the B30 powder heated up to 1000 C in the calorimeter under atmospheric pressure [17]. Considering phase transformations as diffusional processes, the effect of pressure can be explained by the fact that the diffusion coefficient decreases with increasing pressure, typically by a factor ranging from 2 to 10 for a pressure of 1 GPa [20]. Hence, the application of high pressure can reduce the mobility of atoms required for phase transformations at a given temperature. Since our goal was to obtain matrix containing FeAl intermetallic, the temperature of 800 C was selected for the consolidation
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Fig. 1. XRD patterns of the B30 powder: a) before consolidation [13], b) after consolidation at 800 C, c) after consolidation at 850 C, d) after consolidation at 900 C, e) after consolidation at 1000 C.
of the other powders. Fig. 2 shows the XRD patterns of the B0, B5, B10 and B20 powders before [13] and after consolidation. Comparing the patterns of bulk samples with those of the powders, one can see that for all compositions the (100) and (111) superlattice reflections of the ordered B2 structure appear. This evidences transformation of the Fe(Al) solid solution into an ordered FeAl intermetallic phase during consolidation. The values of the lattice parameter of the FeAl phase in the consolidated samples, together with the ones observed in the powders after heating up to 720 C in the calorimeter and for the Fe(Al) solid solution in the mechanically alloyed powders [13], are presented in Fig. 3. For the bulks, the FeAl lattice parameter is smaller than that of the Fe(Al) present in the milled samples, and similar to that of the FeAl observed in the B0, B5 and B10 powders heated in the calorimeter [13]. The decrease of the lattice parameter accompanying the Fe(Al) / FeAl transformation agrees with the literature data [14,21e23]. The FeAl lattice parameter in the bulk B20 and B30 samples is smaller than in the other bulks and decreases with the increase of boron content. The decrease of Fe(Al) lattice parameter with the increase of boron concentration in mechanically alloyed Fe-Al-B powders was reported by Rico et al. [10]. They claimed that reduction of lattice parameter is due to more boron atoms, which are smaller than Al and Fe, being dissolved in the bcc phase. The widths of XRD peaks of the bulk samples are smaller than those of the powders (Fig. 2), which indicates some growth of
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crystallites and the reduction of microstrains during consolidation. For the bulk B30 sample, similarly to the milled B20 and B30 powders, most of the diffraction profiles are so complicated that reliable fitting and deconvolution for accurate width determination were impossible. However, breadth of peaks suggests nanometric crystallite size of the FeAl phase in this sample. As shown in Fig. 4, the estimated mean crystallite size of FeAl in the bulk samples is larger than that of Fe(Al) in the milled powders (for the same composition) and, likewise the crystallite size of Fe(Al), decreases with the increase of B content [13]. The decrease of Fe(Al) crystallite size with the increase of B content was reported by Rico et al. for the mechanically alloyed Fe-Al-B powders [9,10] and by Izadi et al. for the samples produced by sintering of mechanically alloyed Fe-Al-B powders [24]. Fig. 4 shows also that for the same composition, the crystallite size of FeAl in the bulks hot-pressed at 800 C for 3 min is smaller than that in the powders heated under atmospheric pressure in the calorimeter at a rate 40 C/min up to 720 C followed by immediate cooling (200 C/min) [13]. Hence, in this case the effect of pressure has an impact on grain growth. As it was mentioned, the diffusion coefficient decreases with increasing pressure [20]. Since diffusion is involved in grain growth, the application of high pressure can reduce the mobility of grain boundaries at elevated temperature. We have already observed the effect of high pressure on grain growth hindering [6,7,21]. During consolidation the microstrain in the composites matrix gets reduced about two times: the mean values are in the range of 1.3 ÷ 1.5% for the B0, B5 and B10 powders and in the range of 0.6 ÷ 0.8% for the B0, B5, B10 and B20 bulk samples. The produced bulks were investigated in SEM. Fig. 5 presents images of the B5 and B30 samples as an example. The surface of the polished bulks is free of pores or cracks, which evidences good quality of consolidation. As in the mechanically alloyed powders, boron inclusions are present in the bulk samples. The EDS maps of the bulk B30 sample are shown in Fig. 6 as an example. Thus, the composite structure with B particles homogeneously distributed in the nanocrystalline FeAl or in the two-phase nanocrystalline FeAl and amorphous matrix is present in the produced bulk materials. The bulk samples were characterised by hardness, density and open porosity measurements. Fig. 7 shows values of bulk samples density as a function of B content. The measured density values error does not exceed ±0.014 g/cm3. As expected, the density of the bulk composites (5.31 ÷ 4.57 g/cm3) is lower than that of the reference FeAl sample and decreases with the increase of B content. The density of all the produced bulk materials follows the rule of mixture. The density of the B30 bulk sample represents 84.5% of that of the FeAl sample. The open porosity of the bulks is negligible. The hardness of the produced composites is in the range of 1096 ÷ 1233 HV1 (10.75 ÷ 12.1 GPa), thus higher than that of the FeAl intermetallic (1003 HV1, 9.84 GPa), and increases with the increase of B content (Fig. 8). However, while the hardness of the composites follows the rule of mixture, the hardness of the FeAl intermetallic falls below this trend. The microhardness of the nanocrystalline FeAl obtained in the same route as in this work, but hot-pressed at 1000 C was 1235 HV0.2 [21]. It should be mentioned that microhardness value depends on the applied load, and usually is greater than hardness value of the same material. For nanocrystalline FeAl intermetallic the microhardness of 680 HV0.3 [25], 683 HV0.5 [26] and hardness of 562 HV20 [27] were reported. For the nanocrystalline FeAl alloy containing 5 at.% of B obtained by sintering at 800 C of mechanically alloyed Fe-Al-B powders the microhardness of 490 HV0.1 was reported [24]. In this case, however, a composite structure with B particles embedded in the FeAl matrix was not reported. No other data on hardness of FeAl intermetallics containing B in an amount greater than the well described small addition which improves
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Fig. 2. XRD patterns of the: a) B0, b) B5, c) B10 and d) B20 powders before [13] and after consolidation at 800 C.
Fig. 3. Lattice parameter values of FeAl in bulks, together with the ones of Fe(Al) in the milled powders [13] and of FeAl in the milled powders after DSC examination [13], as a function of B concentration.
Fig. 4. Mean crystallites size values of FeAl in bulks, together with the ones of Fe(Al) in the milled powders [13] and of FeAl in the milled powders after DSC examination [13], as a function of B concentration. The values of R2 coefficient calculated from Williamson-Hall plot for each crystallites size are quoted.
mechanical properties due to B segregation to the FeAl grain boundaries [28,29] was found in the literature. The bulk nanocrystalline FeAl matrix composites, reinforced with particles other than B, were described in the literature. The microhardness of such composites containing 10 ÷ 50 at.% of TiC and TiN was in the range of 1363 ÷ 1608 HV0.2 [6] and 1305 ÷ 1497 HV0.2 [7] respectively. However, as mentioned above, the microhardness values are usually greater than those of hardness and the density of these composites is higher than that FeAl-B ones. For the nanocrystalline FeAl-Al2O3 composite the hardness of 600 HV20 was reported [8]. The microhardness of the microcrystalline FeAl-20 vol% Al2O3 composite was 407 HV0.5 [30]. For the FeAl-30 wt% TiC composite the microhardness of 840 HV0.3 was found [31].
4. Conclusions Mechanically alloyed composite powders with equiatomic FeeAl stoichiometry and with the content of 5, 10, 20 and 30 vol% of B were successfully consolidated by hot-pressing at 800 C under the pressure of 7.7 GPa. As a result of this process, for the compositions containing 5, 10 and 20% of B, bulk composites with the nanocrystalline FeAl intermetallic matrix reinforced with homogenously distributed fine boron particles were produced. In the case of the material containing 30% of B, the boron particles were distributed in the two-phase matrix consisting of the dominant nanocrystalline FeAl intermetallic and minor amorphous phases. Hence, during applied consolidation process, ordering of Fe(Al)
M. Krasnowski et al. / Journal of Alloys and Compounds 791 (2019) 75e80
Fig. 5. SEM images of the polished surface of the bulk composites: a) B5, b) B30.
Fig. 6. SEM image a) and EDS maps of the bulk B30 composite: b) Fe signal, c) Al signal, d) B signal.
Fig. 7. Density of the bulk samples as a function of B concentration.
Fig. 8. Vickers hardness of the bulk samples as a function of B concentration.
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solid solution into FeAl intermetallic phase occurred and the nanocrystalline structure of the matrix was maintained. Application of consolidation temperatures higher than 800 C resulted in undesirable phase transformations in the material containing 30% of B. The density of the composites (5.31 ÷ 4.57 g/cm3) is lower than that of the FeAl intermetallic reference sample and decreases with the increase of B content. The density of the bulk sample containing 30% of B represents 84.5% of that of the FeAl sample. The hardness of the composites (10.75 ÷ 12.1 GPa) is higher than that of the FeAl reference (9.84 GPa) and increases with the increase of B content. The hardness of the nanocrystalline matrix composites and of the nanocrystalline FeAl is relatively high. According to our best knowledge, the bulk FeAl-B composites with nanocrystalline intermetallic matrix were produced for the first time. The addition of boron to FeAl intermetallic is advantageous in the context of density decrease and hardness increase. Acknowledgements This research was financially supported by the National Science Centre, Poland, grant no. 2014/13/B/ST8/04289. References [1] K.S. Kumar, H. Van Swygenhoven, S. Suresh, Mechanical behavior of nanocrystalline metals and alloys, Acta Mater. 51 (2003) 5743e5774. [2] M.A. Meyers, A. Mishra, D.J. Benson, Mechanical properties of nanocrystalline materials, Prog. Mater. Sci. 51 (2006) 427e556. [3] C.C. Koch, Structural nanocrystalline materials: an overview, J. Mater. Sci. 42 (2007) 1403e1414. [4] H. Gleiter, Materials with ultrafine microstructures: retrospectives and perspectives, Nanostruct. Mater. 1 (1992) 1e19. [5] E. Hahn, M. Meyers, Grain-size dependent mechanical behavior of nanocrystalline metals, Mater. Sci. Eng. A 646 (2015) 101e134. [6] M. Krasnowski, T. Kulik, Nanocrystalline FeAl matrix composites reinforced with TiC obtained by hot-pressing consolidation of mechanically alloyed powders, Intermetallics 15 (2007) 1377e1383. [7] M. Krasnowski, T. Kulik, Nanocrystalline FeAl-TiN composites obtained by hotpressing consolidation of reactively milled powders, Scripta Mater. 57 (2007) 553e556. [8] I.J. Shon, S.H. Jo, J.M. Doh, J.K. Yoon, B.J. Park, Mechanical synthesis and rapid consolidation of nanostructured FeAleAl2O3 composites by high-frequency induction heated sintering, Ceram. Int. 38 (2012) 6035e6039. rez Alca zar, Effect of boron on structural and [9] M.M. Rico, J.M. Greneche, G.A. Pe magnetic properties of the Fe60Al40 system prepared by mechanical alloying, J. Alloys Compd. 398 (2005) 26e32. rez Alca zar, J.M. Greneche, Effect of boron in Fe70Al30 [10] M.M. Rico, G.A. Pe nanostructured alloys produced by mechanical alloying, Hyperfine Interact. 224 (2014) 313e321.
[11] M. Krasnowski, A. Antolak-Dudka, T. Kulik, Bulk amorphous Al85Fe15 alloy and Al85Fe15-B composites with amorphous or nanocrystalline-matrix produced by consolidation of mechanically alloyed powders, Intermetallics 19 (2011) 1243e1249. [12] M. Krasnowski, T. Kulik, Nanocrystalline or amorphous matrix Al60Fe15Ti15(Co/Mg/Zr)5-5%B composites produced by consolidation of mechanically alloyed powders e lightweight materials with high hardness, Intermetallics 28 (2012) 120e127. [13] M. Krasnowski, Phase transformations during mechanical alloying and subsequent heating of Fe-Al-B powders, J. Alloys Compd. 706 (2017) 110e115. [14] M. Krasnowski, A. Grabias, T. Kulik, Phase transformations during mechanical alloying of Fe-50%Al and subsequent heating of the milling product, J. Alloys Compd. 424 (2006) 119e127. [15] G.K. Williamson, W.H. Hall, X-ray line broadening from filed aluminium and wolfram, Acta Metall. 1 (1953) 22e31. [16] M. Krasnowski, T. Kulik, Nanocrystalline Al-Fe intermetallics e light weight alloys with high hardness, Intermetallics 18 (2010) 47e50. [17] M. Krasnowski, A. Grabias, J. Ferenc, T. Kulik, Structure, thermal stability and magnetic properties of mechanically alloyed (Fe-Al)-30vol.%B powders, J. Alloys Compd. 776 (2019) 215e223. [18] X.J. Gu, F. Ye, F. Zhou, K. Lu, Pressure effect on crystallization of mechanically alloyed amorphous Al85Fe15 alloy, Mater. Sci. Eng. A 278 (2000) 61e65. [19] M. Krasnowski, T. Kulik, FeAl-TiN Nanocomposite produced by reactive ball milling and hot-pressing consolidation, Scripta Mater. 48 (2003) 1489e1494. [20] H. Mehrer, Diffusion: introduction and case studies in metals and binary alloys, in: P. Heitjans, J. Karger (Eds.), Diffusion in Condensed Matter, Springer, Berlin, 2005, pp. 3e63. [21] M. Krasnowski, T. Kulik, Nanocrystalline FeAl intermetallic produced by mechanical alloying followed by hot-pressing consolidation, Intermetallics 15 (2007) 201e205. [22] R.A. Varin, J. Bystrzycki, A. Calka, Characterization of nanocrystalline Fe-45 at% Al intermetallic powders obtained by controlled ball milling and the influence of annealing, Intermetallics 7 (1999) 917e930. s, S. Surin ~ ach, J.S. Mun ~ oz, L. Lutterotti, S. Gialanella, [23] X. Amils, J. Nogue , Structural, mechanical and magnetic properties of nanostructured M.D. Baro FeAl alloys during disordering and thermal recovery, Nanostruct. Mater. 11 (1999) 689e695. [24] S. Izadi, G.H. Akbaria, K. Janghorban, Sintering and mechanical properties of mechanically alloyed FeeAle(B) nanostructures, J. Alloys Compd. 496 (2010) 699e702. ~ oz, A. Dodge, D.G. Morris, Structure, strength and toughness [25] M.A. Morris-Mun of nanocrystalline FeAl, Nanostruct. Mater. 11 (1999) 873e885. [26] R.A. Varin, T. Czujko, J. Bystrzycki, A. Calka, Cold-work induced phenomena in B2 FeAl intermetallics, Mater. Sci. Eng. A 329e331 (2002) 213e221. [27] I.Y. Ko, S.H. Jo, J.M. Doh, J.K. Yoon, I.J. Shon, Rapid consolidation of nanostructured FeAl compound by high frequency induction heating and its mechanical properties, J. Alloys Compd. 496 (2010) L1eL3. [28] M.A. Crimp, K. Vedula, Effect of boron on the tensile properties of B2 FeAl, Mater. Sci. Eng. A 78 (1986) 193e200. [29] C.T. Liu, E.P. George, Environmental embrittlement in boron-free and borondoped FeAl (40 at. % Al) alloys, Scripta Metall. Mater. 24 (1990) 1285e1290. [30] R. Subramanian, C.G. McKamey, J.H. Schneibel, L.R. Buck, P.A. Menchhofer, Iron aluminideeAl2O3 composites by in situ displacement reactions: processing and mechanical properties, Mater. Sci. Eng. A 254 (1998) 119e128. [31] B. Song, S. Dong, H. Liao, C. Coddet, Characterisations of TiC particle reinforced FeAl composite part fabricated by selective laser melting, Mater. Res. Innovat. 18 (2014) 50e56.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
FeAl-B composites with nanocrystalline matrix produced by consolidation of mechanically alloyed powders Marek Krasnowski a, *, Stanislaw Gierlotka b, Tadeusz Kulik a a b
Faculty of Materials Science and Engineering, Warsaw University of Technology, Woloska 141, 02-507 Warsaw, Poland Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142, Warsaw, Poland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2018 Received in revised form 28 January 2019 Accepted 24 March 2019 Available online 25 March 2019
Mechanically alloyed composite powders with equiatomic FeeAl stoichiometry and with the addition of 5, 10, 20 and 30 vol% of B were subjected to consolidation. For this purpose, hot-pressing at 800 C under the pressure of 7.7 GPa for 180 s was applied. A reference FeAl sample was prepared using the same procedure as these from the composite powders. The produced bulk materials were investigated by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy as well as characterised by hardness, density and open porosity measurements. For the compositions containing 5, 10 and 20% of B, it was possible to obtain composites with the nanocrystalline FeAl intermetallic matrix reinforced with homogenously distributed fine boron particles. For the material containing 30% of B, the boron particles were distributed in a two-phase matrix consisting of the dominant nanocrystalline FeAl intermetallic and minor amorphous phases. The density of the composite samples is in the range of 5.31 ÷ 4.57 g/cm3, which is lower than that of the FeAl reference sample, and decreases with the increase of B content. The hardness of the composite samples is in the range of 10.75 ÷ 12.1 GPa, is higher than that of the intermetallic FeAl reference sample (9.84 GPa), and increases with the increase of B content. To the best of our knowledge, the FeAl-B composites with nanocrystalline intermetallic matrix were produced for the first time. © 2019 Elsevier B.V. All rights reserved.
Keywords: A. nanostructured materials A. metal matrix composites A. intermetallics B. mechanical alloying B. powder metallurgy
1. Introduction Over the last few years, low-density materials with relatively high hardness and strength have been given significant attention from researchers. FeAl intermetallic has advantageous strength properties, moderate density and good heat resistance. Increasing polycrystalline alloys strength and hardness can be achieved, among others, by decreasing the grains size down to the nanometer range. Nanocrystalline alloys attract considerable attention due to their properties being usually better than those of microcrystalline materials. This concerns primarily strength and hardness [1e5]. One of the methods of nanocrystalline alloys preparation is mechanical alloying (MA). The products of this process are however powders, while most applications require a bulk form. Consolidation of mechanically alloyed powders into bulk full-density material with a nanocrystalline structure preserved is a task difficult to perform. The pressing ability of the MA milled powders is relatively
* Corresponding author. E-mail address: [email protected] (M. Krasnowski). https://doi.org/10.1016/j.jallcom.2019.03.334 0925-8388/© 2019 Elsevier B.V. All rights reserved.
low because of work hardening and the consolidation temperature must be kept low in order to prevent grain growth. Strength and hardness of an alloy can be increased by creating a composite structure containing reinforcing particles. There is a number of works devoted to bulk nanocrystalline FeAl intermetallic reinforced with hard compounds such as TiN, TiC, Al2O3 [6e8]. Using of reinforcing particles of low density should result in decrease of the density of the prepared material. Boron, being a material with very high hardness and low density, is well-suited for such applications. Its presence in a composite material should lead to the improvement of the specific strength. Addition of 30% vol. boron to an FeAl intermetallic phase is expected to decrease the composites density by 17%, relative to the pure intermetallic. Fe-Al alloys with Al content up to 40 at.% and with addition of B up to 20 at% have been produced by MA [9,10]. In those works, however, no composite structure with B inclusions embedded in powder particles was reported. Powders with composite structures comprised of B particles embedded in Al85Fe15 alloy [11] or in Al60Fe15Ti15(Co/Mg/Zr)5 alloys [12] have been synthesised by MA. Recently, we applied the MA process to obtain FeeAleB composite powders with equiatomic FeeAl stoichiometry and with the
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content of 5, 10, 20 and 30 vol% of B [13]. During milling B particles were ‘built into’ the matrix being formed in situ. Therefore, the resulting powder composites should have clear, contamination-free interfaces between the matrix and the reinforcing particles, which will result in good adhesion. The application of milling enabled attaining a uniform distribution of B particles in the matrix [13]. It has been also found that: (i) a nanocrystalline Fe(Al) solid solution was formed at the early stage of the processes, (ii) the increase of B content in the powder mixture caused a more intense decrease of the Fe(Al) grain size during MA, (iii) larger concentration of B in the mixture promoted the formation of an amorphous phase and for the powders containing 20 and 30% of B, an amorphous phase was also present in the MA products and for the latter it was the prevalent phase, (iv) depending on B content, the matrix of the composite powders consisted of nanocrystalline Fe(Al) or mixture of amorphous and nanocrystalline phases, (v) during heating up to 720 C of the final MA products containing 5 and 10% of B, the Fe(Al) transformed into the ordered FeAl intermetallic phase, while upon heating of the final MA products containing 20 and 30% of B, the amorphous phase crystallised and a phase isomorphic with AlFe2B2 was formed [13]. In the current work, we produced bulk nanocrystalline FeAl-B composites by hot-pressing of mechanically alloyed Fe-Al-B powders. The reference bulk FeAl sample was also produced (for this sample the sequence of the phase transformations as a function of the milling time was the same as for the previously described Fe50%Al powder [14]). The consolidated materials were characterised by structural investigations as well as by hardness, density and porosity measurements. 2. Experimental An elemental powder mixtures of Fe-50at.%Al with addition of 5, 10, 20 and 30 vol% of B (abbreviated in the text as B5, B10, B20 and B30 respectively) were mechanically alloyed in a SPEX 8000D highenergy ball mill. The quantity of added B was calculated assuming the measured densities of the B, Fe and Al powders. As a reference sample (B0), equiatomic Fe-Al powder mixture without B addition was mechanically alloyed under the same conditions. The total milling time was 30 h. After completion of the mechanical alloying, the powder particles had spherical shape and the diameter of 4 ÷ 40 mm. Boron inclusions of a size of 0.4 ÷ 3 mm were distributed inside the powder particles. The details can be found in Ref. [13]. Consolidation of the final MA products was carried out using a press equipped with a high-pressure toroid-type chamber. The shape of the chamber and the material of the gasket ensured that the pressure conditions were close to isostatic. The compaction processes were performed under the pressure of 7.7 GPa at the temperature of 1000, 900, 850 and 800 C for 3 min. The cell was pressurised at the rate of 0.5 GPa/min, prior to heating and the heating and cooling rates were 1000 C/min. The structural investigations of the consolidated samples were carried out by X-ray diffraction (XRD) using a Rigaku MiniFlex II diffractometer operating with CuKa radiation. The DHN-PDS software was employed for Ka2 component stripping from the XRD patterns and calculation and refinement of unit-cell parameters. The mean crystallite size and the mean microstrains, the latter also for the powders, were estimated by the Williamson-Hall method [15]. The instrumental broadening, determined using an Si standard, was subtracted from the experimental breadth to obtain the physical broadening of each diffraction line for the Williamson-Hall calculations. Hitachi S-3500 N scanning electron microscope (SEM) was used for observations of the consolidated materials. The SEM was equipped with energy dispersive X-ray spectroscopy (EDS) system.
Samples for SEM examinations were prepared using standard polishing technique. Hardness measurements of the bulks were conducted by the Vickers method using a ZWICK hardness testing machine under a load of 1 kg (HV1) imposed for 15 s. Vickers hardness value was the average of at least 20 indentations. The density of the bulks was determined using a Gibertini E154 balance equipped with a device for measuring the density of solids (Archimedes method). The mass measurements performed during density determination allowed to calculate the open porosity of the consolidated samples. 3. Results and discussion The prepared Fe-Al-B and Fe-Al powders were consolidated by high-pressure hot pressing at elevated temperature. Application of high temperature, in addition to ensuring high-quality consolidation, can introduce phase transformations in the compacted powders. Over the last few years we have consolidated mechanically alloyed nanocrystalline powders at high pressure and temperature maintaining nanocrystallinity and achieving expected phase changes, i.e. transformation of solid solution into ordered intermetallic compound [6,7,16]. Calorimetric investigations have shown that in the B30 powder heated without pressure to 630 C FeAl intermetallic phase appears [17], while in both the B20 and the B30 powders heated to 720 C a phase isomorphic with the AlFe2B2 forms [13]. High pressure is known to affect phase transformations, including amorphous phase crystallisation [11,12,16e19] and so the temperature of the phase transformation during high-pressure consolidation can be different from those observed in the calorimeter. For example, ordering of Fe(Al) solid solution in the FeAl50% alloy in the calorimeter occurred below 450 C [14], while it did not occur at 750 C during consolidation under 8 GPa [19]. So, the employed temperature of consolidation should be high, but not as high as to cause undesired transformations. To select the optimum processing temperature, the B30 powder was consolidated at 1000, 900, 850 and 800 C for 180 s. Fig. 1 gives XRD patterns of the B30 sample before [13] and after hot-pressing at various temperatures. The absence of boron diffraction lines in the patterns was discussed in the case of the produced powders [13]. The XRD patterns show that FeAl intermetallic phase was formed during hotpressing at 800 C, whereas use of higher consolidation temperatures caused creation of some amount of AlFe2B2-type phase and other, unidentified phase/phases. Based on the XRD results we cannot confirm the presence of small amount of an amorphous phase in the sample hot-pressed at 800 C which was revealed in €ssbauer spectroscopy [17]. this sample by Mo Recently we have reported the differences between the phase transformation in the mechanically alloyed B30 powder consolidated at 800 C under 7.7 GPa and the one heated up to 720 C in the calorimeter under atmospheric pressure [17]. In this study we observed another example of pressure effect on phase transformation. In the XRD pattern of the B30 powder consolidated at 1000 C under 7.7 GPa, broad, overlapping peaks of FeAl, AlFe2B2type and other, unidentified phase/phases are present while welldefined peaks of AlFe2B2 and Al13Fe4 phases were observed in the XRD pattern of the B30 powder heated up to 1000 C in the calorimeter under atmospheric pressure [17]. Considering phase transformations as diffusional processes, the effect of pressure can be explained by the fact that the diffusion coefficient decreases with increasing pressure, typically by a factor ranging from 2 to 10 for a pressure of 1 GPa [20]. Hence, the application of high pressure can reduce the mobility of atoms required for phase transformations at a given temperature. Since our goal was to obtain matrix containing FeAl intermetallic, the temperature of 800 C was selected for the consolidation
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Fig. 1. XRD patterns of the B30 powder: a) before consolidation [13], b) after consolidation at 800 C, c) after consolidation at 850 C, d) after consolidation at 900 C, e) after consolidation at 1000 C.
of the other powders. Fig. 2 shows the XRD patterns of the B0, B5, B10 and B20 powders before [13] and after consolidation. Comparing the patterns of bulk samples with those of the powders, one can see that for all compositions the (100) and (111) superlattice reflections of the ordered B2 structure appear. This evidences transformation of the Fe(Al) solid solution into an ordered FeAl intermetallic phase during consolidation. The values of the lattice parameter of the FeAl phase in the consolidated samples, together with the ones observed in the powders after heating up to 720 C in the calorimeter and for the Fe(Al) solid solution in the mechanically alloyed powders [13], are presented in Fig. 3. For the bulks, the FeAl lattice parameter is smaller than that of the Fe(Al) present in the milled samples, and similar to that of the FeAl observed in the B0, B5 and B10 powders heated in the calorimeter [13]. The decrease of the lattice parameter accompanying the Fe(Al) / FeAl transformation agrees with the literature data [14,21e23]. The FeAl lattice parameter in the bulk B20 and B30 samples is smaller than in the other bulks and decreases with the increase of boron content. The decrease of Fe(Al) lattice parameter with the increase of boron concentration in mechanically alloyed Fe-Al-B powders was reported by Rico et al. [10]. They claimed that reduction of lattice parameter is due to more boron atoms, which are smaller than Al and Fe, being dissolved in the bcc phase. The widths of XRD peaks of the bulk samples are smaller than those of the powders (Fig. 2), which indicates some growth of
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crystallites and the reduction of microstrains during consolidation. For the bulk B30 sample, similarly to the milled B20 and B30 powders, most of the diffraction profiles are so complicated that reliable fitting and deconvolution for accurate width determination were impossible. However, breadth of peaks suggests nanometric crystallite size of the FeAl phase in this sample. As shown in Fig. 4, the estimated mean crystallite size of FeAl in the bulk samples is larger than that of Fe(Al) in the milled powders (for the same composition) and, likewise the crystallite size of Fe(Al), decreases with the increase of B content [13]. The decrease of Fe(Al) crystallite size with the increase of B content was reported by Rico et al. for the mechanically alloyed Fe-Al-B powders [9,10] and by Izadi et al. for the samples produced by sintering of mechanically alloyed Fe-Al-B powders [24]. Fig. 4 shows also that for the same composition, the crystallite size of FeAl in the bulks hot-pressed at 800 C for 3 min is smaller than that in the powders heated under atmospheric pressure in the calorimeter at a rate 40 C/min up to 720 C followed by immediate cooling (200 C/min) [13]. Hence, in this case the effect of pressure has an impact on grain growth. As it was mentioned, the diffusion coefficient decreases with increasing pressure [20]. Since diffusion is involved in grain growth, the application of high pressure can reduce the mobility of grain boundaries at elevated temperature. We have already observed the effect of high pressure on grain growth hindering [6,7,21]. During consolidation the microstrain in the composites matrix gets reduced about two times: the mean values are in the range of 1.3 ÷ 1.5% for the B0, B5 and B10 powders and in the range of 0.6 ÷ 0.8% for the B0, B5, B10 and B20 bulk samples. The produced bulks were investigated in SEM. Fig. 5 presents images of the B5 and B30 samples as an example. The surface of the polished bulks is free of pores or cracks, which evidences good quality of consolidation. As in the mechanically alloyed powders, boron inclusions are present in the bulk samples. The EDS maps of the bulk B30 sample are shown in Fig. 6 as an example. Thus, the composite structure with B particles homogeneously distributed in the nanocrystalline FeAl or in the two-phase nanocrystalline FeAl and amorphous matrix is present in the produced bulk materials. The bulk samples were characterised by hardness, density and open porosity measurements. Fig. 7 shows values of bulk samples density as a function of B content. The measured density values error does not exceed ±0.014 g/cm3. As expected, the density of the bulk composites (5.31 ÷ 4.57 g/cm3) is lower than that of the reference FeAl sample and decreases with the increase of B content. The density of all the produced bulk materials follows the rule of mixture. The density of the B30 bulk sample represents 84.5% of that of the FeAl sample. The open porosity of the bulks is negligible. The hardness of the produced composites is in the range of 1096 ÷ 1233 HV1 (10.75 ÷ 12.1 GPa), thus higher than that of the FeAl intermetallic (1003 HV1, 9.84 GPa), and increases with the increase of B content (Fig. 8). However, while the hardness of the composites follows the rule of mixture, the hardness of the FeAl intermetallic falls below this trend. The microhardness of the nanocrystalline FeAl obtained in the same route as in this work, but hot-pressed at 1000 C was 1235 HV0.2 [21]. It should be mentioned that microhardness value depends on the applied load, and usually is greater than hardness value of the same material. For nanocrystalline FeAl intermetallic the microhardness of 680 HV0.3 [25], 683 HV0.5 [26] and hardness of 562 HV20 [27] were reported. For the nanocrystalline FeAl alloy containing 5 at.% of B obtained by sintering at 800 C of mechanically alloyed Fe-Al-B powders the microhardness of 490 HV0.1 was reported [24]. In this case, however, a composite structure with B particles embedded in the FeAl matrix was not reported. No other data on hardness of FeAl intermetallics containing B in an amount greater than the well described small addition which improves
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Fig. 2. XRD patterns of the: a) B0, b) B5, c) B10 and d) B20 powders before [13] and after consolidation at 800 C.
Fig. 3. Lattice parameter values of FeAl in bulks, together with the ones of Fe(Al) in the milled powders [13] and of FeAl in the milled powders after DSC examination [13], as a function of B concentration.
Fig. 4. Mean crystallites size values of FeAl in bulks, together with the ones of Fe(Al) in the milled powders [13] and of FeAl in the milled powders after DSC examination [13], as a function of B concentration. The values of R2 coefficient calculated from Williamson-Hall plot for each crystallites size are quoted.
mechanical properties due to B segregation to the FeAl grain boundaries [28,29] was found in the literature. The bulk nanocrystalline FeAl matrix composites, reinforced with particles other than B, were described in the literature. The microhardness of such composites containing 10 ÷ 50 at.% of TiC and TiN was in the range of 1363 ÷ 1608 HV0.2 [6] and 1305 ÷ 1497 HV0.2 [7] respectively. However, as mentioned above, the microhardness values are usually greater than those of hardness and the density of these composites is higher than that FeAl-B ones. For the nanocrystalline FeAl-Al2O3 composite the hardness of 600 HV20 was reported [8]. The microhardness of the microcrystalline FeAl-20 vol% Al2O3 composite was 407 HV0.5 [30]. For the FeAl-30 wt% TiC composite the microhardness of 840 HV0.3 was found [31].
4. Conclusions Mechanically alloyed composite powders with equiatomic FeeAl stoichiometry and with the content of 5, 10, 20 and 30 vol% of B were successfully consolidated by hot-pressing at 800 C under the pressure of 7.7 GPa. As a result of this process, for the compositions containing 5, 10 and 20% of B, bulk composites with the nanocrystalline FeAl intermetallic matrix reinforced with homogenously distributed fine boron particles were produced. In the case of the material containing 30% of B, the boron particles were distributed in the two-phase matrix consisting of the dominant nanocrystalline FeAl intermetallic and minor amorphous phases. Hence, during applied consolidation process, ordering of Fe(Al)
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Fig. 5. SEM images of the polished surface of the bulk composites: a) B5, b) B30.
Fig. 6. SEM image a) and EDS maps of the bulk B30 composite: b) Fe signal, c) Al signal, d) B signal.
Fig. 7. Density of the bulk samples as a function of B concentration.
Fig. 8. Vickers hardness of the bulk samples as a function of B concentration.
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solid solution into FeAl intermetallic phase occurred and the nanocrystalline structure of the matrix was maintained. Application of consolidation temperatures higher than 800 C resulted in undesirable phase transformations in the material containing 30% of B. The density of the composites (5.31 ÷ 4.57 g/cm3) is lower than that of the FeAl intermetallic reference sample and decreases with the increase of B content. The density of the bulk sample containing 30% of B represents 84.5% of that of the FeAl sample. The hardness of the composites (10.75 ÷ 12.1 GPa) is higher than that of the FeAl reference (9.84 GPa) and increases with the increase of B content. The hardness of the nanocrystalline matrix composites and of the nanocrystalline FeAl is relatively high. According to our best knowledge, the bulk FeAl-B composites with nanocrystalline intermetallic matrix were produced for the first time. The addition of boron to FeAl intermetallic is advantageous in the context of density decrease and hardness increase. Acknowledgements This research was financially supported by the National Science Centre, Poland, grant no. 2014/13/B/ST8/04289. References [1] K.S. Kumar, H. Van Swygenhoven, S. Suresh, Mechanical behavior of nanocrystalline metals and alloys, Acta Mater. 51 (2003) 5743e5774. [2] M.A. Meyers, A. Mishra, D.J. Benson, Mechanical properties of nanocrystalline materials, Prog. Mater. Sci. 51 (2006) 427e556. [3] C.C. Koch, Structural nanocrystalline materials: an overview, J. Mater. Sci. 42 (2007) 1403e1414. [4] H. Gleiter, Materials with ultrafine microstructures: retrospectives and perspectives, Nanostruct. Mater. 1 (1992) 1e19. [5] E. Hahn, M. Meyers, Grain-size dependent mechanical behavior of nanocrystalline metals, Mater. Sci. Eng. A 646 (2015) 101e134. [6] M. Krasnowski, T. Kulik, Nanocrystalline FeAl matrix composites reinforced with TiC obtained by hot-pressing consolidation of mechanically alloyed powders, Intermetallics 15 (2007) 1377e1383. [7] M. Krasnowski, T. Kulik, Nanocrystalline FeAl-TiN composites obtained by hotpressing consolidation of reactively milled powders, Scripta Mater. 57 (2007) 553e556. [8] I.J. Shon, S.H. Jo, J.M. Doh, J.K. Yoon, B.J. Park, Mechanical synthesis and rapid consolidation of nanostructured FeAleAl2O3 composites by high-frequency induction heated sintering, Ceram. Int. 38 (2012) 6035e6039. rez Alca zar, Effect of boron on structural and [9] M.M. Rico, J.M. Greneche, G.A. Pe magnetic properties of the Fe60Al40 system prepared by mechanical alloying, J. Alloys Compd. 398 (2005) 26e32. rez Alca zar, J.M. Greneche, Effect of boron in Fe70Al30 [10] M.M. Rico, G.A. Pe nanostructured alloys produced by mechanical alloying, Hyperfine Interact. 224 (2014) 313e321.
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