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Maps of Fe–Al phases formation kinetics parameters during isothermal sintering
Thermochimica Acta 545 (2012) 14–19
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Maps of Fe–Al phases formation kinetics parameters during isothermal sintering ´ Ewelina Pochec´ ∗ , Stanisław Józwiak, Krzysztof Karczewski, Zbigniew Bojar Department of Advanced Materials and Technology, Military University of Technology, Poland
a r t i c l e
i n f o
Article history: Received 4 April 2012 Received in revised form 14 June 2012 Accepted 15 June 2012 Available online 23 June 2012 Keywords: Intermetallics Self-propagating synthesis Sintering X-ray diffraction Electron microscopy
a b s t r a c t The influence of technological parameters (compaction pressure and sintering temperature) on Fe–Al phase formation was investigated. The kinetics of phase transformation preceding and during an SHS reaction was studied in isothermal conditions by DSC using the JMA (Johnson–Mehl–Avrami) model. This model allowed us to determine basic kinetic parameters, including the Avrami exponent, which characterises the rate and manner of particular phase nucleation. The activation energy (Ea ) of particular phase formation was determined by the Kissinger method. XRD analysis and SEM observations of sintered material showed that not only Fe2 Al5 phase and low-aluminium solid solution in iron but also aluminiumrich FeAl2 and FeAl3 phases are formed during the sintering of an FeAl50 elementary powder mixture in isothermal conditions with an SHS reaction. The above conclusions were confirmed by iron-based solid solution lattice parameter studies and microhardness measurements. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Self-propagating high-temperature synthesis (SHS) has been investigated for many years as a method that allows the production of composites, ceramics and intermetallic compounds [1–3]. Combustion synthesis may occur during the sintering of Fe–Al phase-based materials and is an alternative method which allows one to obtain phases of the Fe–Al equilibrium system, such as FeAl3 , Fe2 Al5 , FeAl2 and most importantly FeAl [4,5]. However, due to the high speed of the SHS reaction and its strongly exothermic nature, it is not possible to follow the process by classical methods, such as SEM (scanning electron microscopy) and XRD (X-ray diffraction). Therefore, Differential Scanning Calorimetry (DSC) and the JMA model are used to determine the basic kinetic parameters of the reaction, including the Avrami exponent, which characterises the speed and manner of particular phase formation [6–8]. The JMA model is widely used to describe the kinetics of phase transformations taking place in various materials. For example, crystallisation kinetics in Cu46 Zr45 Al7 Y2 bulk metallic glass and in amorphous materials were illustrated by the JMA model [9–11]. The JMA model was also used to model the hardening of tool steel [12]. In this work, the phenomena that occur around an SHS reaction during sintering of an elemental iron and aluminium powder
∗ Corresponding author at: Kaliskiego 2 St., 00-908 Warsaw, Poland. Tel.: +48 22 683 95 45; fax: +48 22 683 95 45. ´ E-mail address: [email protected] (E. Pochec). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.06.015
mixture were explained. These processes must be explored and understood because they may influence the final composition and properties of Fe–Al sinters. This study shows that, besides Fe2 Al5 and Fe(Al) solid solution (as assumed by other researchers) [13,14], aluminium-rich phases FeAl3 and FeAl2 arise during the sintering of Fe and Al powders, which confirms the results of our previous work [15].
2. Experimental details The investigated materials included elemental powders of 99.8% pure iron with an average particle size of 200 m and 99.6% pure aluminium with an average particle size of 70 m. The powders were preliminarily mixed at a stoichiometric ratio of 50% Fe and 50% Al in a Uniball 5 mill using any balls. The samples 10 mm in diameter and 5 mm high for SEM microstructure observations were obtained by uniaxial cold pressing at a pressure of 300, 700 and 1000 MPa and heated for 2 h in the temperature range of 350–580 ◦ C with a step of 50 ◦ C. Cylindrical DSC specimens having a mass of approximately 20 mg and a diameter of 3 mm were obtained by uniaxial cold pressing at a pressure of 300, 700 and 1000 MPa. The kinetics of Fe–Al phase formation were examined in a Setaram Labsys DSC/DTA/TG under isothermal conditions in a temperature range of 570–645 ◦ C with a step of 5 ◦ C. The preliminary heating rate was 50 ◦ C/min. The samples were heated to the desired temperature, then the isothermal annealing was conducted until the start and the end of the SHS reaction. Analysis of the microstructure and chemical composition of the sinters was performed by SEM (Philips XL30LaB6 ) with EDS
E. Poche´c et al. / Thermochimica Acta 545 (2012) 14–19
15
Fig. 2. XRD patterns for FeAl50 elementary powder mixture preliminarily coldcompacted at a pressure of 1000 MPa and isothermally sintered at 350 ◦ C and 570 ◦ C.
at pressures of 300, 700 and 1000 MPa, and heated for 2 h at a temperature range of 350–580 ◦ C. 3. Results and discussion
Fig. 1. Microstructure changes for FeAl50 elementary powder mixture preliminarily compacted at a pressure of 1000 MPa and isothermally sintered at 450 ◦ C (a), 570 ◦ C (b) or 580 ◦ C (c).
(EDAX-DX4). The XRD (Seifert 3003, Co radiation) method was used to study the phase composition of the samples. The iron-based solid solution lattice parameter was calculated based on the Bragg condition after annealing at a temperature range of 350–647 ◦ C. Measurements of microhardness were carried out on samples 10 mm in diameter and 5 mm high, which were cold compacted
Microscopic observations supported by EDS chemical microanalysis results show that processing temperature has a strong influence on the phase composition of the studied sinters. At 350 ◦ C, in the contact zones between Fe and Al particles, no diffusion effects are observed. When the sintering temperature increases to 400–500 ◦ C, the mass transport of iron and aluminium atoms results in single precipitates of the FeAl3 phase (Fig. 1a). When an SHS reaction occurs at a temperature range of 570–580 ◦ C (the exact temperature depends on the compaction pressure) the aluminium-rich phases of the Fe–Al equilibrium system (FeAl3 , Fe2 Al5 , and FeAl2 ) are formed (Fig. 1b and c). No precipitates of B2-ordered FeAl secondary solid solution are observed at this temperature range. SEM results were confirmed by XRD analysis. For samples sintered at 350 ◦ C, there are only A2-Fe␣ and Al–A1 diffraction peaks
Fig. 3. Microhardness results obtained for Fe (a) and Al (b) particles in FeAl50 raw powder mixture compacted at 700 MPa and annealed at 350–580 ◦ C for 2 h.
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E. Poche´c et al. / Thermochimica Acta 545 (2012) 14–19
Fig. 4. DSC “total” curves divided into individual peaks resulting from the formation of particular Fe–Al phases from FeAl50 powder mixture preliminarily compacted at a pressure of 300 MPa and isothermally sintered at given temperatures.
in the pattern (Fig. 2a). Diffraction peaks from FeAl3 and Fe2 Al5 phases are visible in samples that were sintered at 580 ◦ C (Fig. 2b). The above observations are consistent with the results of the microhardness measurements (Fig. 3). Up to a temperature of approximately 450 ◦ C, only recovery and re-crystallisation occur. The increase in the measured parameter in the temperature range of 500–580 ◦ C is caused by the formation of aluminium-rich phases. For further analysis of phenomena which occur at temperatures over 580 ◦ C, calorimetric studies were performed on the DSC specimens in a temperature range of 570–645 ◦ C. Isothermal annealing was conducted until the start and the end of the SHS reaction. The higher the isothermal heating temperature, the shorter the time of initiation and completion of SHS (Fig. 4). The calorimetric measurements indicated that the heating temperature had a strong influence on the structure of sinters. At a temperature of 600 ◦ C, only diffraction peaks from iron, aluminium and aluminium-rich phases FeAl3 and Fe2 Al5 are observed in the XRD pattern. The increase in sintering temperature causes further rebuilding of sinter structures. As a result, at a temperature of 647 ◦ C, the whole spectrum of Fe–Al phases is observed without
diffraction peaks from elemental powder of iron and aluminium (Fig. 5). The SHS reaction which leads to the appearance of the FeAl phase runs at such a high speed that it is impossible to analyse it using SEM and XRD. To determine the basic kinetics parameters of
Fig. 5. Examples of XRD patterns for FeAl50 samples compacted at 300 MPa and isothermally sintered at 600 ◦ C and 647 ◦ C.
Fig. 6. Changes in Avrami exponent values for samples preliminarily compacted at a pressure of 700 MPa and sintered in isothermal conditions.
E. Poche´c et al. / Thermochimica Acta 545 (2012) 14–19
Fig. 7. Changes in the iron-based solid solution lattice parameter as an effect of isothermal sintering of FeAl50 powder mixture compacted at 300 MPa.
17
Fig. 9. Map of enthalpy of phase formation (enthalpy of full DSC curve).
the Fe–Al phase formation reaction, DSC and JMA modelling were used. To describe a wide variety of isothermal solid-state transformations, the JMA equation is used [16,17]:
phase formed can be calculated by the Kissinger method from the following equation:
˛(t) = 1 − exp[−(Kt)n ],
where t(˛) is the transformation time, t0 is the time of transformation start and E(˛) is the activation energy of each phase formed. The activation energy value can be obtained from the slope of the line ln(t(˛)) versus 103 /T. Based on DSC curves and XRD analysis of samples sintered under different conditions, one may conclude that different phases appear at different temperature ranges. According to the Fe–Al phase equilibrium diagram [18], the sequence of phases appearing during sintering should be FeAl3 , Fe2 Al5 , FeAl2 , and FeAl. The experimental heat flow effect equal to the surface under the DSC curve can be treated as the sum of
(1)
where ˛(t) is the degree of transformation, and n is the Avrami exponent characterising the speed and manner of a particular phase transformation. Applying a double logarithm to each side of Eq. (1) leads to the following equation: ln[− ln(1 − ˛(t))] = n ln K + n ln t.
(2)
The Avrami exponent n can be easily determined from the slope of the curve [−ln(1 − ˛(t))] versus time. The activation energy of each
E(a)
t(˛) = t0 exp −
RT
,
Fig. 8. Activation energy maps for each phase formed during sintering in isothermal conditions: (a) FeAl3 , (b) Fe2 Al5 , (c) FeAl2 , and (d) FeAl.
(3)
18
E. Poche´c et al. / Thermochimica Acta 545 (2012) 14–19
Fig. 10. Changes in the Avrami exponent as a function of compaction pressure and sintering temperature for phases sintered in isothermal conditions: (a) FeAl3 , (b) Fe2 Al5 , (c) FeAl2 , and (d) FeA.
elemental heat flow effects (Fig. 4). Component peaks from particular phases were designated using the least squares method. The kinetics of phase transformation during isothermal sintering of a mixture of Fe and Al elemental powder particles were described using the JMA model. The Avrami exponent for each Fe–Al phase during nucleation and growth under isothermal conditions was designated (Fig. 6). Based on the values of the Avrami exponent, it can be concluded that the best conditions for each phase formation appear in a specific temperature range. In this case, it is the area of conditions where the calculated Avrami parameter is close to 4. This might be treated as confirmation that the phases result from the reaction running at increasing speeds of nucleation. Studies of the iron-based solid solution lattice parameter in the temperature range of 350–645 ◦ C show that the values of this parameter change only slightly (Fig. 7). In the temperature range of 350–600 ◦ C, it is practically constant, so there are no diffusion processes occurring. At the beginning of the SHS reaction, the analysed parameter rapidly increases to the temperature range of 600–640 ◦ C. This confirms the formation of solid solution, but the amount of aluminium atoms in the iron-based solid solution lattice is very small—at the level of hundredths of one percent. The correlation between the applied compaction pressure and temperature allowed the mapping of activation energy of particular phase formation (Fig. 8). The most frequent is the FeAl phase. The lowest activation energy allowing for formation of this phase is observed at a compaction pressure of 700 MPa in a temperature range of 605–615 ◦ C. The activation energies of FeAl3 and Fe2 Al5 phase formation are higher in this range of parameters, therefore an FeAl phase is formed. The map of activation energy correlates with the map of enthalpy of phase formation calculated as the enthalpy of the full DSC curve (Fig. 9). The enthalpy map shows that, at a pressure of 700 MPa, high values of activation energy for phase formation allow one to obtain the FeAl phase with the lowest total enthalpy of reaction. Additionally, an analysis of Avrami exponents given in such coordinate systems indicates that the process of phase formation is a reaction with an increasing rate of nucleation
when the Avrami exponent is close to 4. These areas are indicated with arrows in Fig. 10. Interestingly, these areas are observed in all phases at a pressure of 700 MPa and a temperature of 605–615 ◦ C. 4. Conclusions 1. Irrespective of the compaction pressure, after sintering of Fe and Al elemental powder mixtures, aluminium-rich phases of the Fe–Al system are observed. 2. The sequence of phase formation during isothermal sintering coupled with an SHS reaction is consistent with the Fe–Al phase diagram and follows this sequence: FeAl3 , Fe2 Al5 , FeAl2 , and FeAl. 3. The changes of the Fe(Al) solid solution lattice parameter show that temperatures above 580 ◦ C, the effectiveness of aluminium diffusion into the surface layer of iron particles increases dramatically, irrespective of the compaction pressure. 4. At a heating temperature of 605–615 ◦ C, an increase in the nucleation rate of newly formed phases during the transformation of a mixture of Fe and Al is observed. The increase is confirmed by obtaining values of Avrami exponents close to 4. 5. An analysis of the results obtained using the JMA model, combined with an analysis of the sinters’ structures, indicates that, at this stage of processing, the desired configuration of the phases is obtained after the consolidation of Fe–Al compacted at 700 MPa and sintered at 605–615 ◦ C. References [1] P. Novak, V. Knotek, M. Voderova, J. Kubasek, J. Serak, A. Michalcova, D. Vojtech, Intermediary phases formation in Fe–Al–Si alloys during reactive sintering, J. Alloys Compd. 497 (2010) 90–94. [2] B. Eftekhari Yekta, V.K. Margbussian, Sintering of .g.ss . and gahnite–glass–ceramic/silicon carbide composites, J. Eur. Ceram. Soc. 19 (1999) 2969–2973. [3] K. Morsi, Review: reaction synthesis processing of Ni–Al intermetallic materials, Mater. Sci. Eng. A299 (2001) 1–15.
E. Poche´c et al. / Thermochimica Acta 545 (2012) 14–19 ´ [4] S. Józwiak, K. Karczewski, Z. Bojar, Kinetics of reactions in FeAl synthesis studied by DTA technique and JMA model, Intermetallics 18 (2010) 1332–1337. ´ [5] K. Karczewski, S. Józwiak, M. Chojnacki, Z. Bojar, The influence of different additives on the kinetics of self-propagating high-temperature synthesis Turing the sintering process of Fe and Al elemental powders, Intermetallics 18 (2010) 1401–1404. [6] E. Illekova, F. Malizia, F. Ronconi, The complex DSC analysis of the first crystallization peak of Fe80 Si10 B10 metallic glass, Thermochim. Acta 282/283 (1996) 91–100. [7] J. Malek, The applicability of Johnson–Mehl–Avrami model in the thermal analysis of the crystallization kinetics of glasses, Thermochim. Acta 267 (1995) 61–73. [8] A.T. Lorenzo, M.L. Arnal, J. Albuerne, A.J. Muller, DSC isothermal polymer crystallization kinetics measurements and the use of the Avrami equation to fit the data: guidelines to avoid common problems, Polym. Test. 26 (2007) 222–231. [9] J. Malek, Kinetic analysis of crystallization processes in amorphous materials, Thermochim. Acta 355 (2000) 239–253. [10] Y. Ouyang, L. Wang, H. Chen, X. Cheng, X. Zhong, Y. Feng, The formation and crystallization of amorphous Al65 Fe20 Zr15 , J. Non-Cryst. Solids 354 (2008) 5555–5558.
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´ [11] A. Bokota, T. Domanski, Modelling and numerical analysis of hardening phenomena of tools steel elements, Arch. Metall. Mater. 54 (2009) 575–587. [12] A.A. Abu-Sehly, S.N. Alamri, A.A. Joraid, Measurements of DSC isothermal crystallization kinetics in amorphous selenium bulk samples, J. Alloys Compd. 476 (2009) 348–351. [13] S. Paris, E. Gaffet, D. Vrel, D. Thiaudiere, M. Gailhanou, F. Bernard, Time-resolved XRD experiments for a fine description of mechanisms induced during reactive sintering, Sci. Siniter. 37 (2005) 27–34. ˙ Z. Bojar, Influence of sintering process temperature [14] K. Karczewski, S. Józwiak, on the formation and final microstructure of FeAl based intermetallics obtained ˙ Mater. 3–4 (2007) 552–558. from the elementary Fe and Al powders, Inz. ´ ´ Z. Bojar, European Congress on Advanced [15] S. Józwiak, K. Karczewski, E. Pochec, Materials and Processes, Euromat 2009, Glasgow, United Kingdom, September 7–10, 2009, 2009. [16] F. Liu, C.L. Yang, G.C. Yang, J.S. Li, Deviations from the classical Johnson-MehlAvrami kinetics, J. Alloys Compd. 460 (2008) 326–330. [17] P. Supaphol, Application of the Avrami, Tolbin, Malkin and Clubanovici-Segal macrokinetic models to isothermal crystallization of syndiotactie polypropylene, Thermochim. Acta 370 (2001) 37–48. [18] T.B. Masalski, Binary Alloy Phase Diagrams, vol. 1, American Society for Metals, Metals Park, 1986, 112.
Contents lists available at SciVerse ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Maps of Fe–Al phases formation kinetics parameters during isothermal sintering ´ Ewelina Pochec´ ∗ , Stanisław Józwiak, Krzysztof Karczewski, Zbigniew Bojar Department of Advanced Materials and Technology, Military University of Technology, Poland
a r t i c l e
i n f o
Article history: Received 4 April 2012 Received in revised form 14 June 2012 Accepted 15 June 2012 Available online 23 June 2012 Keywords: Intermetallics Self-propagating synthesis Sintering X-ray diffraction Electron microscopy
a b s t r a c t The influence of technological parameters (compaction pressure and sintering temperature) on Fe–Al phase formation was investigated. The kinetics of phase transformation preceding and during an SHS reaction was studied in isothermal conditions by DSC using the JMA (Johnson–Mehl–Avrami) model. This model allowed us to determine basic kinetic parameters, including the Avrami exponent, which characterises the rate and manner of particular phase nucleation. The activation energy (Ea ) of particular phase formation was determined by the Kissinger method. XRD analysis and SEM observations of sintered material showed that not only Fe2 Al5 phase and low-aluminium solid solution in iron but also aluminiumrich FeAl2 and FeAl3 phases are formed during the sintering of an FeAl50 elementary powder mixture in isothermal conditions with an SHS reaction. The above conclusions were confirmed by iron-based solid solution lattice parameter studies and microhardness measurements. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Self-propagating high-temperature synthesis (SHS) has been investigated for many years as a method that allows the production of composites, ceramics and intermetallic compounds [1–3]. Combustion synthesis may occur during the sintering of Fe–Al phase-based materials and is an alternative method which allows one to obtain phases of the Fe–Al equilibrium system, such as FeAl3 , Fe2 Al5 , FeAl2 and most importantly FeAl [4,5]. However, due to the high speed of the SHS reaction and its strongly exothermic nature, it is not possible to follow the process by classical methods, such as SEM (scanning electron microscopy) and XRD (X-ray diffraction). Therefore, Differential Scanning Calorimetry (DSC) and the JMA model are used to determine the basic kinetic parameters of the reaction, including the Avrami exponent, which characterises the speed and manner of particular phase formation [6–8]. The JMA model is widely used to describe the kinetics of phase transformations taking place in various materials. For example, crystallisation kinetics in Cu46 Zr45 Al7 Y2 bulk metallic glass and in amorphous materials were illustrated by the JMA model [9–11]. The JMA model was also used to model the hardening of tool steel [12]. In this work, the phenomena that occur around an SHS reaction during sintering of an elemental iron and aluminium powder
∗ Corresponding author at: Kaliskiego 2 St., 00-908 Warsaw, Poland. Tel.: +48 22 683 95 45; fax: +48 22 683 95 45. ´ E-mail address: [email protected] (E. Pochec). 0040-6031/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tca.2012.06.015
mixture were explained. These processes must be explored and understood because they may influence the final composition and properties of Fe–Al sinters. This study shows that, besides Fe2 Al5 and Fe(Al) solid solution (as assumed by other researchers) [13,14], aluminium-rich phases FeAl3 and FeAl2 arise during the sintering of Fe and Al powders, which confirms the results of our previous work [15].
2. Experimental details The investigated materials included elemental powders of 99.8% pure iron with an average particle size of 200 m and 99.6% pure aluminium with an average particle size of 70 m. The powders were preliminarily mixed at a stoichiometric ratio of 50% Fe and 50% Al in a Uniball 5 mill using any balls. The samples 10 mm in diameter and 5 mm high for SEM microstructure observations were obtained by uniaxial cold pressing at a pressure of 300, 700 and 1000 MPa and heated for 2 h in the temperature range of 350–580 ◦ C with a step of 50 ◦ C. Cylindrical DSC specimens having a mass of approximately 20 mg and a diameter of 3 mm were obtained by uniaxial cold pressing at a pressure of 300, 700 and 1000 MPa. The kinetics of Fe–Al phase formation were examined in a Setaram Labsys DSC/DTA/TG under isothermal conditions in a temperature range of 570–645 ◦ C with a step of 5 ◦ C. The preliminary heating rate was 50 ◦ C/min. The samples were heated to the desired temperature, then the isothermal annealing was conducted until the start and the end of the SHS reaction. Analysis of the microstructure and chemical composition of the sinters was performed by SEM (Philips XL30LaB6 ) with EDS
E. Poche´c et al. / Thermochimica Acta 545 (2012) 14–19
15
Fig. 2. XRD patterns for FeAl50 elementary powder mixture preliminarily coldcompacted at a pressure of 1000 MPa and isothermally sintered at 350 ◦ C and 570 ◦ C.
at pressures of 300, 700 and 1000 MPa, and heated for 2 h at a temperature range of 350–580 ◦ C. 3. Results and discussion
Fig. 1. Microstructure changes for FeAl50 elementary powder mixture preliminarily compacted at a pressure of 1000 MPa and isothermally sintered at 450 ◦ C (a), 570 ◦ C (b) or 580 ◦ C (c).
(EDAX-DX4). The XRD (Seifert 3003, Co radiation) method was used to study the phase composition of the samples. The iron-based solid solution lattice parameter was calculated based on the Bragg condition after annealing at a temperature range of 350–647 ◦ C. Measurements of microhardness were carried out on samples 10 mm in diameter and 5 mm high, which were cold compacted
Microscopic observations supported by EDS chemical microanalysis results show that processing temperature has a strong influence on the phase composition of the studied sinters. At 350 ◦ C, in the contact zones between Fe and Al particles, no diffusion effects are observed. When the sintering temperature increases to 400–500 ◦ C, the mass transport of iron and aluminium atoms results in single precipitates of the FeAl3 phase (Fig. 1a). When an SHS reaction occurs at a temperature range of 570–580 ◦ C (the exact temperature depends on the compaction pressure) the aluminium-rich phases of the Fe–Al equilibrium system (FeAl3 , Fe2 Al5 , and FeAl2 ) are formed (Fig. 1b and c). No precipitates of B2-ordered FeAl secondary solid solution are observed at this temperature range. SEM results were confirmed by XRD analysis. For samples sintered at 350 ◦ C, there are only A2-Fe␣ and Al–A1 diffraction peaks
Fig. 3. Microhardness results obtained for Fe (a) and Al (b) particles in FeAl50 raw powder mixture compacted at 700 MPa and annealed at 350–580 ◦ C for 2 h.
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E. Poche´c et al. / Thermochimica Acta 545 (2012) 14–19
Fig. 4. DSC “total” curves divided into individual peaks resulting from the formation of particular Fe–Al phases from FeAl50 powder mixture preliminarily compacted at a pressure of 300 MPa and isothermally sintered at given temperatures.
in the pattern (Fig. 2a). Diffraction peaks from FeAl3 and Fe2 Al5 phases are visible in samples that were sintered at 580 ◦ C (Fig. 2b). The above observations are consistent with the results of the microhardness measurements (Fig. 3). Up to a temperature of approximately 450 ◦ C, only recovery and re-crystallisation occur. The increase in the measured parameter in the temperature range of 500–580 ◦ C is caused by the formation of aluminium-rich phases. For further analysis of phenomena which occur at temperatures over 580 ◦ C, calorimetric studies were performed on the DSC specimens in a temperature range of 570–645 ◦ C. Isothermal annealing was conducted until the start and the end of the SHS reaction. The higher the isothermal heating temperature, the shorter the time of initiation and completion of SHS (Fig. 4). The calorimetric measurements indicated that the heating temperature had a strong influence on the structure of sinters. At a temperature of 600 ◦ C, only diffraction peaks from iron, aluminium and aluminium-rich phases FeAl3 and Fe2 Al5 are observed in the XRD pattern. The increase in sintering temperature causes further rebuilding of sinter structures. As a result, at a temperature of 647 ◦ C, the whole spectrum of Fe–Al phases is observed without
diffraction peaks from elemental powder of iron and aluminium (Fig. 5). The SHS reaction which leads to the appearance of the FeAl phase runs at such a high speed that it is impossible to analyse it using SEM and XRD. To determine the basic kinetics parameters of
Fig. 5. Examples of XRD patterns for FeAl50 samples compacted at 300 MPa and isothermally sintered at 600 ◦ C and 647 ◦ C.
Fig. 6. Changes in Avrami exponent values for samples preliminarily compacted at a pressure of 700 MPa and sintered in isothermal conditions.
E. Poche´c et al. / Thermochimica Acta 545 (2012) 14–19
Fig. 7. Changes in the iron-based solid solution lattice parameter as an effect of isothermal sintering of FeAl50 powder mixture compacted at 300 MPa.
17
Fig. 9. Map of enthalpy of phase formation (enthalpy of full DSC curve).
the Fe–Al phase formation reaction, DSC and JMA modelling were used. To describe a wide variety of isothermal solid-state transformations, the JMA equation is used [16,17]:
phase formed can be calculated by the Kissinger method from the following equation:
˛(t) = 1 − exp[−(Kt)n ],
where t(˛) is the transformation time, t0 is the time of transformation start and E(˛) is the activation energy of each phase formed. The activation energy value can be obtained from the slope of the line ln(t(˛)) versus 103 /T. Based on DSC curves and XRD analysis of samples sintered under different conditions, one may conclude that different phases appear at different temperature ranges. According to the Fe–Al phase equilibrium diagram [18], the sequence of phases appearing during sintering should be FeAl3 , Fe2 Al5 , FeAl2 , and FeAl. The experimental heat flow effect equal to the surface under the DSC curve can be treated as the sum of
(1)
where ˛(t) is the degree of transformation, and n is the Avrami exponent characterising the speed and manner of a particular phase transformation. Applying a double logarithm to each side of Eq. (1) leads to the following equation: ln[− ln(1 − ˛(t))] = n ln K + n ln t.
(2)
The Avrami exponent n can be easily determined from the slope of the curve [−ln(1 − ˛(t))] versus time. The activation energy of each
E(a)
t(˛) = t0 exp −
RT
,
Fig. 8. Activation energy maps for each phase formed during sintering in isothermal conditions: (a) FeAl3 , (b) Fe2 Al5 , (c) FeAl2 , and (d) FeAl.
(3)
18
E. Poche´c et al. / Thermochimica Acta 545 (2012) 14–19
Fig. 10. Changes in the Avrami exponent as a function of compaction pressure and sintering temperature for phases sintered in isothermal conditions: (a) FeAl3 , (b) Fe2 Al5 , (c) FeAl2 , and (d) FeA.
elemental heat flow effects (Fig. 4). Component peaks from particular phases were designated using the least squares method. The kinetics of phase transformation during isothermal sintering of a mixture of Fe and Al elemental powder particles were described using the JMA model. The Avrami exponent for each Fe–Al phase during nucleation and growth under isothermal conditions was designated (Fig. 6). Based on the values of the Avrami exponent, it can be concluded that the best conditions for each phase formation appear in a specific temperature range. In this case, it is the area of conditions where the calculated Avrami parameter is close to 4. This might be treated as confirmation that the phases result from the reaction running at increasing speeds of nucleation. Studies of the iron-based solid solution lattice parameter in the temperature range of 350–645 ◦ C show that the values of this parameter change only slightly (Fig. 7). In the temperature range of 350–600 ◦ C, it is practically constant, so there are no diffusion processes occurring. At the beginning of the SHS reaction, the analysed parameter rapidly increases to the temperature range of 600–640 ◦ C. This confirms the formation of solid solution, but the amount of aluminium atoms in the iron-based solid solution lattice is very small—at the level of hundredths of one percent. The correlation between the applied compaction pressure and temperature allowed the mapping of activation energy of particular phase formation (Fig. 8). The most frequent is the FeAl phase. The lowest activation energy allowing for formation of this phase is observed at a compaction pressure of 700 MPa in a temperature range of 605–615 ◦ C. The activation energies of FeAl3 and Fe2 Al5 phase formation are higher in this range of parameters, therefore an FeAl phase is formed. The map of activation energy correlates with the map of enthalpy of phase formation calculated as the enthalpy of the full DSC curve (Fig. 9). The enthalpy map shows that, at a pressure of 700 MPa, high values of activation energy for phase formation allow one to obtain the FeAl phase with the lowest total enthalpy of reaction. Additionally, an analysis of Avrami exponents given in such coordinate systems indicates that the process of phase formation is a reaction with an increasing rate of nucleation
when the Avrami exponent is close to 4. These areas are indicated with arrows in Fig. 10. Interestingly, these areas are observed in all phases at a pressure of 700 MPa and a temperature of 605–615 ◦ C. 4. Conclusions 1. Irrespective of the compaction pressure, after sintering of Fe and Al elemental powder mixtures, aluminium-rich phases of the Fe–Al system are observed. 2. The sequence of phase formation during isothermal sintering coupled with an SHS reaction is consistent with the Fe–Al phase diagram and follows this sequence: FeAl3 , Fe2 Al5 , FeAl2 , and FeAl. 3. The changes of the Fe(Al) solid solution lattice parameter show that temperatures above 580 ◦ C, the effectiveness of aluminium diffusion into the surface layer of iron particles increases dramatically, irrespective of the compaction pressure. 4. At a heating temperature of 605–615 ◦ C, an increase in the nucleation rate of newly formed phases during the transformation of a mixture of Fe and Al is observed. The increase is confirmed by obtaining values of Avrami exponents close to 4. 5. An analysis of the results obtained using the JMA model, combined with an analysis of the sinters’ structures, indicates that, at this stage of processing, the desired configuration of the phases is obtained after the consolidation of Fe–Al compacted at 700 MPa and sintered at 605–615 ◦ C. References [1] P. Novak, V. Knotek, M. Voderova, J. Kubasek, J. Serak, A. Michalcova, D. Vojtech, Intermediary phases formation in Fe–Al–Si alloys during reactive sintering, J. Alloys Compd. 497 (2010) 90–94. [2] B. Eftekhari Yekta, V.K. Margbussian, Sintering of .g.ss . and gahnite–glass–ceramic/silicon carbide composites, J. Eur. Ceram. Soc. 19 (1999) 2969–2973. [3] K. Morsi, Review: reaction synthesis processing of Ni–Al intermetallic materials, Mater. Sci. Eng. A299 (2001) 1–15.
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