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Synthesis of NiAl–TiC nanocomposite by mechanical alloying elemental powders
Materials Science and Engineering A249 (1998) 103 – 108
Synthesis of NiAl–TiC nanocomposite by mechanical alloying elemental powders L.Z. Zhou a,*, J.T. Guo a, G.J. Fan b a
Department of Superalloy and Intermetallics, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110015, China b National Key Laboratory for RSA, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110015, China Received 29 October 1997; received in revised form 7 February 1998
Abstract A NiAl–TiC nanocomposite has been synthesized by mechanical alloying from Ni, Al, Ti, and C powders. During milling, an abrupt reaction occurred, resulting in simultaneous formation of NiAl and TiC phases. It is suggested that two separate exothermic explosive reactions, i.e. Ni + AlNiAl and Ti + C TiC, were involved. However, the reactions were incomplete with the existence of a small amount of elemental powders. Prolonged milling led to a gradual formation of NiAl and TiC as well as grain refinement. The final grain size for TiC was 3.5 times as large as that for NiAl. The formation mechanism of the NiAl–TiC nanocomposite during mechanical alloying was also discussed. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Mechanical alloying; Nanocomposite; Intermetallic; Explosive reaction
1. Introduction NiAl, as a promising intermetallics, can be used as a high-temperature material due to its higher melting point, lower density, specific modulus, excellent thermal conductivity, and good oxidation resistance. However, cast polycrystalline NiAl suffers from low ambient temperature ductility and poor creep resistance at service temperature [1–4]. In an effort to overcome these shortcomings, to develop a nanocrystalline NiAl strengthened by a second phase may be a promising way, since the ductility can be improved by the refined microstructure [5,6] and the creep resistance can be enhanced by the reinforced particulates [7,8]. Explosive reaction (often termed SHS, combustion synthesis or self-sustained reaction) is a process in which initial reagents explosively transform into products after ignition due to a large exothermic heat of the reaction. Atzmon first suggested that the explosive reaction was a possible mechanism for the mechanical alloying of elements with large heat of formation [9]. Indeed, the explosive reaction has been found to be an * Corresponding author. Fax: + 86 24 3891320; e-mail: [email protected] 0921-5093/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093(98)00576-0
important mechanism in ball milling systems with large heat of formation, such as Mo–Si [10,11], Ti–B [12], Nb–Si [13], CuO–Ti [14], PbO–Si [15] and Ti–C [16] systems. In this study, four elemental powders (Ni, Al, Ti and C) have been mixed and milled in an effort to form NiAl nanocrystalline composites. As a result, after milling for a certain time, an abrupt reaction occurred and a large amount of NiAl and TiC phases formed. The structural evolution during mechanical alloying has been investigated.
2. Experimental details The Ni, Al, Ti and amorphous black carbon powders, with nominal purities of 98, 98, 99, and 99.9%, and average particle sizes of 10, 13, 75, and 10 mm, respectively, mixed at a composition of 43Ni–43Al– 7Ti–7C (atm.%), were milled in a high-energy ball mill. A mill container (60 mm in inner diameter and 70 mm in inner depth) made of stainless steel was used. The mill container was vibrated at a frequency of 20 Hz with a small gyratory motion with a radius of gyration of 5 cm. For each milling run, 10 g of powder mixture
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was loaded with ten 12-g stainless steel balls in the container in an Ar atmosphere. The ball-to-powder weight ratio was 12:1. A fan was used to keep the vial near room temperature during milling. The vial temperature was monitored with a chromel – alumel thermocouple inserted into a hole in the vial which was drilled to a distance of 2 mm from the inner wall of the vial. To identify the milling product, a trace of milled powder was taken out periodically for X-ray diffraction (XRD) analysis in a Rigaku D/max-rA X-ray diffractometer with Cu Ka radiation. The grain size was determined by using the conventional Scherrer equation. The morphology observations of selected samples were carried out in a Cambridge S-360 scanning electron microscope (SEM). Elemental distribution was conducted on a Shimadzu EPM-810Q electron microprobe.
3. Results Fig. 1 is the X-ray diffraction patterns for the elemental mixed powders of Ni, Al, Ti, and C with an atomic ratio of 43:43:7:7 after various periods of milling time. As shown in Fig. 1(a), the Bragg diffrac-
Fig. 1. X-ray diffraction pattern for the mixed powders. (a) as-mixed. (b) milled for 100 min. (c) just after the explosive reaction. milled for 2 h (d), 10 h (e), and 30 h (f). ( ) Ni. () Al, () Ti, ( ) NiAl, () TiC.
Table 1 Lattice parameter a for the Ni, Ti, and Al during initial state of milling as determined from the following elemental diffraction peaks Milling time (min)
Ni(200)
Ti(010)
Al(220)
As-mixed 60 100
3.5204 3.5294 3.5128
2.9464 2.9545 2.9319
4.0480 4.0458 4.0336
tion peaks corresponding to the Ni, Al, and Ti elements were present and no diffraction peak for carbon was observed, which can be attributed to its low content in the mixed powders and X-ray absorption effect. After 100 min of milling, no additional XRD peak was observed, and this stage of mechanical alloying only causes lowing of the X-ray intensity and broadening of the XRD patterns, as shown in Fig. 1(b). The Scherrer formula was applied to estimate the grain size by using Ni(200), Al(220), and Ti(011), by which the average grain sizes were calculated to be 16.4, 15.0, and 18.7 nm, respectively. Table 1 summarizes the variation of the lattice parameters for the Ni, Al, and Ti components during this initial stage of milling. It was noticed that the lattice parameter for Ni and Ti increased after 1 h of milling and then decreased with increasing milling time, and the lattice parameter for Al decreased steadily with milling time. This may be attributed to the solution of carbon atoms in Ni and Ti, and that subsequently increases the lattice parameters. After milling for a certain time, a small amount of TiC can form causing decrease of carbon content in Ni and Ti matrixes as well as their lattice parameters. In addition, the lattice parameters after milling for 100 min were even smaller than those at as-milled state, it means that defects originated from mechanical deformation must play an important role in the reduction of these lattice parameters. During mechanical alloying, a thermocouple was attached to the outer wall of the vial to detect the vial temperature. Fig. 2 shows the thermal history of the outer vial during mechanical alloying. It was found that the initial 105 min of milling causes a small temperature increase of the outer vial, which can be attributed to the frequent mechanical impacts occurring in the vial. After 105 min, the temperature of the outer vial rose abruptly, indicating that an exothermic reaction has occurred. A small amount of the sample after the abrupt exothermic reaction was collected and examined by XRD. The result was presented in Fig. 1(c), which indicates that a large amount of NiAl and TiC has formed with grain sizes of about 18 and 34 nm, respectively. Fig. 3(a, b) show the SEM images for the powders before and after the onset of the exothermic reaction, respectively. Fig. 3(a) shows a particle size of about 40 mm with rough surface. After the exothermic
L.Z. Zhou et al. / Materials Science and Engineering A249 (1998) 103–108
Fig. 2. Thermal history of the outer vial during ball milling the mixed powders.
chemical reaction, it was noticed from Fig. 3(b) that the particle size was reduced to about 10 mm, and the particle surface became smooth. The cross-sections and elemental distribution of milled powder particles before and after the exothermic reaction are displayed in Fig. 4(a, b). It was found that, before the reaction, laminar structure was clearly visible, Ni and Al particles were mixed but Ti flakes with thickness of about 1–5 mm inhomogeneously distributed in the powder particles (Fig. 4(a)). After the reaction (Fig. 4(b)), laminar structure disappeared, and large particles were composed of many small particles. Ti was still inhomogeneously distributed. From Fig. 1(c), it was also noticed that a small amount of elemental powders still existed in the sample after the exothermic reaction. With increasing milling
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time, the diffraction peaks corresponding to the starting elements gradually disappeared, indicating a gradual formation of NiAl and TiC compounds. After 10 h milling, XRD patterns show the presence of NiAl and TiC compounds and absence of starting elements. During the later stage of milling, the XRD patterns for the NiAl and TiC have been significantly broadened, which can be attributed to strain and the refinement of the grain sizes. The contribution of strain to Bragg-peak width is about 0.1°. The variation of the grain sizes for NiAl and TiC compounds with milling time, measured by using NiAl(100) and TiC(220) diffraction peaks, is showed in Fig. 5. Clearly, the mechanical alloying has caused a significant refinement of the grain sizes for NiAl and TiC compounds. After certain milling time, the grain sizes for the NiAl and TiC compounds both reach a constant value of 4 and 14 nm, respectively. Mechanical alloying of Ni50Al50 and Ti50C50 powders was also carried out on the above mentioned condition. It was found that after an incubation milling time (95 min for NiAl and 182 min for TiC), an abrupt exothermic reaction was observed (Fig. 6(a, b)).
4. Discussion Mechanical alloying has been considered to be a solid-state reaction, during which the alloying is accompanied by incorporation of dissimilar atoms into the matrix via atomic interdiffusion. Since the atomic diffusion is time dependent, sufficient milling time is required to obtain the final products. It is well known that the mechanical alloying is a versatile tool to synthesize various kinds of metastable phases, such as amorphous phases, supersaturated solid solutions, etc. However, the precise thermodynamic data and the information of the kinetic constraints during such a complex, nonequilibrium process are still lacking, and, therefore, the determination of the final products in a given alloy system becomes a major challenge.
Fig. 3. SEM images for the mixed powders before (a) and after (b) onset of the exothermic reaction.
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Fig. 4. SEM images as well as elemental distribution of cross-section of the milled particles before (a) and after (b) the exothermic reaction.
The observation of the explosive formation of the intermetallic compounds by mechanical alloying of the elemental powders blends was first reported by Atzmon [9]. A large exothermic heat release can even raise the temperature of the reactant materials to the melting temperature. Such a reaction cannot be interpreted by using conventional mechanism of interfacial diffusional
reaction. It was suggested that a large exothermic heat release should be involved in the system, which is a prerequisite for the explosive reaction. Such kind of reaction, once initiated, will be self-sustaining and the reaction proceeds by the propagation of a combustion front. In the present experiment, the early stage of milling before the abrupt exothermic reaction serves as
L.Z. Zhou et al. / Materials Science and Engineering A249 (1998) 103–108
107
an incubation period for this reaction. During this stage of mechanical alloying, the microstructure has been significantly refined. The broadening of XRD patterns indicates that the grain sizes of the components have been reduced to nanometer dimension. It has been well known that mechanical deformation during high-energy ball milling can form a laminar structure, which can increase the interfacial area. The refined microstructures during the incubation milling time can apparently reduce the ignition temperature for the explosive reaction. It has been reported that the propagation of SHS was easy when the particle size was small, but became difficult and eventually impossible as the particle size increased [17]. This suggests the existence of a critical particle size for SHS. When a critical particle size was attained by ball milling for 105 min, the heavy collisions of milling balls that can generate excess heat will act as ignition of the exothermic reaction. As long as the rate of heat generation far exceeds that of dissipation, the excess heat can cause the reaction in neighboring regions, and the reaction becomes self-propagating. From the results presented above, it is apparent that two separate chemical reactions were involved during mechanical alloying of the Ni, Al, Ti, and C elemental powders, i.e. the formation of the NiAl intermetallic compound between elements Ni and Al, and the formation of TiC between elements Ti and C. Such reactions are possible since separate mechanical alloying of Ni50Al50 and Ti50C50 powders causes the abrupt formation of NiAl and TiC compounds, which were also reported previously by other authors [9,16]. The incu-
Fig. 6. X-ray diffraction pattern for the Ni50Al50 powder (a) and Ti50C50 powder (b) before and after explosive reaction.
Fig. 5. Variation of the grain sizes of NiAl and TiC compounds with milling time.
bation times for the reaction in the Ni50Al50 and Ti50C50 powders are 95 min and 182 min, respectively. In this case, one may expect that the additions of Ti and C to the Ni–Al system will delay the process of chemical reaction between Ni and Al. Thus, it is understandable that the incubation time for the overall reaction in the 43Ni–43Al–7Ti–7C powders is 105 min, which is longer than that in the Ni50Al50 powders. The overall reaction has been slowed by the additions of Ti and C. Only one exothermic heat release was observed during the overall chemical reaction, suggesting that these two separated chemical reactions mentioned above occur almost simultaneously, i.e. the chemical reaction between Ti and C has been significantly accelerated in the
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overall reaction than that during mechanical alloying of the Ti–C binary system. This can be attributed to the large exothermic heat release during explosive reaction between Ni and Al, which can simultaneously heat neighboring unreacted Ti and C to its ignition temperature. After the chemical reaction, some unreacted components still exist, possibly due to inefficient contact between the particles [11]. During later stages of mechanical alloying, the reactions occur in a rather weak way. The gradual reaction should be based on explosions in individual grains, as suggested by Atzmon [9]. After the combustion reaction, the NiAl–TiC nanocomposite has formed. It is expected that prolonged mechanical alloying can refine the microstructure of the nanocomposite. As shown in Fig. 5, the grain sizes for NiAl and TiC have been significantly decreased and saturated to 4 and 14 nm after 30 h of milling. It is interesting to note that the final grain size for TiC is larger than that for NiAl. The melting point for TiC is 2776°C which is much higher than that of 1638°C for NiAl. It has been well known that the grain size achievable during high-energy ball milling of ductile materials is inversely proportional to the melting point of materials [18]. However, a different deformation mechanism is applied for such nanocomposite compared with ductile materials. Since TiC is embedded in the relatively soft NiAl matrix, the mechanical force will act on the NiAl matrix initially when a nanocomposite particle is trapped between milling balls. Due to relative softness of NiAl, the mechanical force can be reduced when it acts on the TiC phase. So the NiAl matrix undergoes much severe deformation. Besides, the hard TiC particles can act as micro-ball to mill NiAl matrix further. Therefore, it is understandable that the TiC phase exhibits a relatively larger grain size after prolonged mechanical alloying. It is well known that in situ production of a reinforcing phase offers significant advantages from technical and economic standpoints, yielding better tailorability of the composite properties. In the present nanocomposite, the TiC reinforcement is in situ formed instead of being added to the matrix. It can be expected that in situ reaction formation of nanocomposites can provide better properties over these produced by conventional methods [7].
.
5. Conclusion NiAl–TiC nanocomposite can be produced by mechanical alloying of elemental powders. The formation mechanism is suggested to be two separate reactions which occur almost simultaneously, i.e. the explosive reaction of NiAl causes high heat release and ignites the explosive reaction of TiC. Prolonged milling leads to the refinement of grain sizes. The final grain size for TiC is 3.5 times as large as that for NiAl, though TiC has a much higher melting point compared with NiAl. Acknowledgements The authors would like to acknowledge Professor Z.Q. Hu for the revision of the manuscript. This work was partially supported by the National Advanced Materials Committee of China and the National Natural Science Foundation of China. References [1] R.D. Noebe, R.R. Bowrnan, M.V. Nathal, Int. Mater. Rev. 38 (1993) 193. [2] D.B. Miracle, Acta. Metall. Mater. 41 (1993) 649. [3] R. Darolia, J. Mater. Sci. Technol. 10 (1994) 157. [4] E.P. George, M. Yamagochi, K.S. Kumar, C.T. Liu, Annu. Rev. Mater. Sci. 24 (1994) 409. [5] S. Dymek, M. Dollar, S.J. Hwang, P. Nash, Mater. Sci. Eng. A 152 (1992) 160. [6] H. Gleiter, Prog. Mater. Sci. 33 (1990) 223. [7] J.D. Whittenberger, E. Arzt, M.J. Luton, J. Mater. Res. 5 (1990) 271. [8] L. Wang, N. Beck, R.J. Arsenaut, Mater. Sci. Eng. A 177 (1994) 83. [9] M. Atzrnon, Phys. Rev. Lett. 64 (1990) 487. [10] S.N. Patankar, S.-Q. Xiao, J.J. Lewandowski, A.H. Heuer, J. Mater. Res. 8 (1993) 1311. [11] E. Ma, J. Pagan, G. Cranford, M. Atzmon, J. Mater. Res. 8 (1993) 1836. [12] Y.H. Park, H. Hashimoto, T. Abe, R. Watanabe, Mater. Sci. Eng. A 181/182 (1994) 1291. [13] T.P. Lou, G.J. Fan, B.Z. Ding, Z.Q. Hu, J. Mater. Res. 12 (1997) 1172. [14] G.B. Schaffer, P.G. McCormick, Metall. Trans. 21A (1990) 2789. [15] G.J. Fan, X.P. Song, M.X. Quan, Z.Q. Hu, Sci. Mater. 35 (1996) 1065. [16] Z.G. Liu, J.T. Guo, L.L. Ye, G.S. Li, Z.Q. Hu, Appl. Phys. Lett. 65 (1994) 2666. [17] S.C. Deevi, J. Mater. Sci. 26 (1991) 3343. [18] C.C. Koch, Nanostruct. Mater. 2 (1993) 109.
Synthesis of NiAl–TiC nanocomposite by mechanical alloying elemental powders L.Z. Zhou a,*, J.T. Guo a, G.J. Fan b a
Department of Superalloy and Intermetallics, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110015, China b National Key Laboratory for RSA, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110015, China Received 29 October 1997; received in revised form 7 February 1998
Abstract A NiAl–TiC nanocomposite has been synthesized by mechanical alloying from Ni, Al, Ti, and C powders. During milling, an abrupt reaction occurred, resulting in simultaneous formation of NiAl and TiC phases. It is suggested that two separate exothermic explosive reactions, i.e. Ni + AlNiAl and Ti + C TiC, were involved. However, the reactions were incomplete with the existence of a small amount of elemental powders. Prolonged milling led to a gradual formation of NiAl and TiC as well as grain refinement. The final grain size for TiC was 3.5 times as large as that for NiAl. The formation mechanism of the NiAl–TiC nanocomposite during mechanical alloying was also discussed. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Mechanical alloying; Nanocomposite; Intermetallic; Explosive reaction
1. Introduction NiAl, as a promising intermetallics, can be used as a high-temperature material due to its higher melting point, lower density, specific modulus, excellent thermal conductivity, and good oxidation resistance. However, cast polycrystalline NiAl suffers from low ambient temperature ductility and poor creep resistance at service temperature [1–4]. In an effort to overcome these shortcomings, to develop a nanocrystalline NiAl strengthened by a second phase may be a promising way, since the ductility can be improved by the refined microstructure [5,6] and the creep resistance can be enhanced by the reinforced particulates [7,8]. Explosive reaction (often termed SHS, combustion synthesis or self-sustained reaction) is a process in which initial reagents explosively transform into products after ignition due to a large exothermic heat of the reaction. Atzmon first suggested that the explosive reaction was a possible mechanism for the mechanical alloying of elements with large heat of formation [9]. Indeed, the explosive reaction has been found to be an * Corresponding author. Fax: + 86 24 3891320; e-mail: [email protected] 0921-5093/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093(98)00576-0
important mechanism in ball milling systems with large heat of formation, such as Mo–Si [10,11], Ti–B [12], Nb–Si [13], CuO–Ti [14], PbO–Si [15] and Ti–C [16] systems. In this study, four elemental powders (Ni, Al, Ti and C) have been mixed and milled in an effort to form NiAl nanocrystalline composites. As a result, after milling for a certain time, an abrupt reaction occurred and a large amount of NiAl and TiC phases formed. The structural evolution during mechanical alloying has been investigated.
2. Experimental details The Ni, Al, Ti and amorphous black carbon powders, with nominal purities of 98, 98, 99, and 99.9%, and average particle sizes of 10, 13, 75, and 10 mm, respectively, mixed at a composition of 43Ni–43Al– 7Ti–7C (atm.%), were milled in a high-energy ball mill. A mill container (60 mm in inner diameter and 70 mm in inner depth) made of stainless steel was used. The mill container was vibrated at a frequency of 20 Hz with a small gyratory motion with a radius of gyration of 5 cm. For each milling run, 10 g of powder mixture
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was loaded with ten 12-g stainless steel balls in the container in an Ar atmosphere. The ball-to-powder weight ratio was 12:1. A fan was used to keep the vial near room temperature during milling. The vial temperature was monitored with a chromel – alumel thermocouple inserted into a hole in the vial which was drilled to a distance of 2 mm from the inner wall of the vial. To identify the milling product, a trace of milled powder was taken out periodically for X-ray diffraction (XRD) analysis in a Rigaku D/max-rA X-ray diffractometer with Cu Ka radiation. The grain size was determined by using the conventional Scherrer equation. The morphology observations of selected samples were carried out in a Cambridge S-360 scanning electron microscope (SEM). Elemental distribution was conducted on a Shimadzu EPM-810Q electron microprobe.
3. Results Fig. 1 is the X-ray diffraction patterns for the elemental mixed powders of Ni, Al, Ti, and C with an atomic ratio of 43:43:7:7 after various periods of milling time. As shown in Fig. 1(a), the Bragg diffrac-
Fig. 1. X-ray diffraction pattern for the mixed powders. (a) as-mixed. (b) milled for 100 min. (c) just after the explosive reaction. milled for 2 h (d), 10 h (e), and 30 h (f). ( ) Ni. () Al, () Ti, ( ) NiAl, () TiC.
Table 1 Lattice parameter a for the Ni, Ti, and Al during initial state of milling as determined from the following elemental diffraction peaks Milling time (min)
Ni(200)
Ti(010)
Al(220)
As-mixed 60 100
3.5204 3.5294 3.5128
2.9464 2.9545 2.9319
4.0480 4.0458 4.0336
tion peaks corresponding to the Ni, Al, and Ti elements were present and no diffraction peak for carbon was observed, which can be attributed to its low content in the mixed powders and X-ray absorption effect. After 100 min of milling, no additional XRD peak was observed, and this stage of mechanical alloying only causes lowing of the X-ray intensity and broadening of the XRD patterns, as shown in Fig. 1(b). The Scherrer formula was applied to estimate the grain size by using Ni(200), Al(220), and Ti(011), by which the average grain sizes were calculated to be 16.4, 15.0, and 18.7 nm, respectively. Table 1 summarizes the variation of the lattice parameters for the Ni, Al, and Ti components during this initial stage of milling. It was noticed that the lattice parameter for Ni and Ti increased after 1 h of milling and then decreased with increasing milling time, and the lattice parameter for Al decreased steadily with milling time. This may be attributed to the solution of carbon atoms in Ni and Ti, and that subsequently increases the lattice parameters. After milling for a certain time, a small amount of TiC can form causing decrease of carbon content in Ni and Ti matrixes as well as their lattice parameters. In addition, the lattice parameters after milling for 100 min were even smaller than those at as-milled state, it means that defects originated from mechanical deformation must play an important role in the reduction of these lattice parameters. During mechanical alloying, a thermocouple was attached to the outer wall of the vial to detect the vial temperature. Fig. 2 shows the thermal history of the outer vial during mechanical alloying. It was found that the initial 105 min of milling causes a small temperature increase of the outer vial, which can be attributed to the frequent mechanical impacts occurring in the vial. After 105 min, the temperature of the outer vial rose abruptly, indicating that an exothermic reaction has occurred. A small amount of the sample after the abrupt exothermic reaction was collected and examined by XRD. The result was presented in Fig. 1(c), which indicates that a large amount of NiAl and TiC has formed with grain sizes of about 18 and 34 nm, respectively. Fig. 3(a, b) show the SEM images for the powders before and after the onset of the exothermic reaction, respectively. Fig. 3(a) shows a particle size of about 40 mm with rough surface. After the exothermic
L.Z. Zhou et al. / Materials Science and Engineering A249 (1998) 103–108
Fig. 2. Thermal history of the outer vial during ball milling the mixed powders.
chemical reaction, it was noticed from Fig. 3(b) that the particle size was reduced to about 10 mm, and the particle surface became smooth. The cross-sections and elemental distribution of milled powder particles before and after the exothermic reaction are displayed in Fig. 4(a, b). It was found that, before the reaction, laminar structure was clearly visible, Ni and Al particles were mixed but Ti flakes with thickness of about 1–5 mm inhomogeneously distributed in the powder particles (Fig. 4(a)). After the reaction (Fig. 4(b)), laminar structure disappeared, and large particles were composed of many small particles. Ti was still inhomogeneously distributed. From Fig. 1(c), it was also noticed that a small amount of elemental powders still existed in the sample after the exothermic reaction. With increasing milling
105
time, the diffraction peaks corresponding to the starting elements gradually disappeared, indicating a gradual formation of NiAl and TiC compounds. After 10 h milling, XRD patterns show the presence of NiAl and TiC compounds and absence of starting elements. During the later stage of milling, the XRD patterns for the NiAl and TiC have been significantly broadened, which can be attributed to strain and the refinement of the grain sizes. The contribution of strain to Bragg-peak width is about 0.1°. The variation of the grain sizes for NiAl and TiC compounds with milling time, measured by using NiAl(100) and TiC(220) diffraction peaks, is showed in Fig. 5. Clearly, the mechanical alloying has caused a significant refinement of the grain sizes for NiAl and TiC compounds. After certain milling time, the grain sizes for the NiAl and TiC compounds both reach a constant value of 4 and 14 nm, respectively. Mechanical alloying of Ni50Al50 and Ti50C50 powders was also carried out on the above mentioned condition. It was found that after an incubation milling time (95 min for NiAl and 182 min for TiC), an abrupt exothermic reaction was observed (Fig. 6(a, b)).
4. Discussion Mechanical alloying has been considered to be a solid-state reaction, during which the alloying is accompanied by incorporation of dissimilar atoms into the matrix via atomic interdiffusion. Since the atomic diffusion is time dependent, sufficient milling time is required to obtain the final products. It is well known that the mechanical alloying is a versatile tool to synthesize various kinds of metastable phases, such as amorphous phases, supersaturated solid solutions, etc. However, the precise thermodynamic data and the information of the kinetic constraints during such a complex, nonequilibrium process are still lacking, and, therefore, the determination of the final products in a given alloy system becomes a major challenge.
Fig. 3. SEM images for the mixed powders before (a) and after (b) onset of the exothermic reaction.
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Fig. 4. SEM images as well as elemental distribution of cross-section of the milled particles before (a) and after (b) the exothermic reaction.
The observation of the explosive formation of the intermetallic compounds by mechanical alloying of the elemental powders blends was first reported by Atzmon [9]. A large exothermic heat release can even raise the temperature of the reactant materials to the melting temperature. Such a reaction cannot be interpreted by using conventional mechanism of interfacial diffusional
reaction. It was suggested that a large exothermic heat release should be involved in the system, which is a prerequisite for the explosive reaction. Such kind of reaction, once initiated, will be self-sustaining and the reaction proceeds by the propagation of a combustion front. In the present experiment, the early stage of milling before the abrupt exothermic reaction serves as
L.Z. Zhou et al. / Materials Science and Engineering A249 (1998) 103–108
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an incubation period for this reaction. During this stage of mechanical alloying, the microstructure has been significantly refined. The broadening of XRD patterns indicates that the grain sizes of the components have been reduced to nanometer dimension. It has been well known that mechanical deformation during high-energy ball milling can form a laminar structure, which can increase the interfacial area. The refined microstructures during the incubation milling time can apparently reduce the ignition temperature for the explosive reaction. It has been reported that the propagation of SHS was easy when the particle size was small, but became difficult and eventually impossible as the particle size increased [17]. This suggests the existence of a critical particle size for SHS. When a critical particle size was attained by ball milling for 105 min, the heavy collisions of milling balls that can generate excess heat will act as ignition of the exothermic reaction. As long as the rate of heat generation far exceeds that of dissipation, the excess heat can cause the reaction in neighboring regions, and the reaction becomes self-propagating. From the results presented above, it is apparent that two separate chemical reactions were involved during mechanical alloying of the Ni, Al, Ti, and C elemental powders, i.e. the formation of the NiAl intermetallic compound between elements Ni and Al, and the formation of TiC between elements Ti and C. Such reactions are possible since separate mechanical alloying of Ni50Al50 and Ti50C50 powders causes the abrupt formation of NiAl and TiC compounds, which were also reported previously by other authors [9,16]. The incu-
Fig. 6. X-ray diffraction pattern for the Ni50Al50 powder (a) and Ti50C50 powder (b) before and after explosive reaction.
Fig. 5. Variation of the grain sizes of NiAl and TiC compounds with milling time.
bation times for the reaction in the Ni50Al50 and Ti50C50 powders are 95 min and 182 min, respectively. In this case, one may expect that the additions of Ti and C to the Ni–Al system will delay the process of chemical reaction between Ni and Al. Thus, it is understandable that the incubation time for the overall reaction in the 43Ni–43Al–7Ti–7C powders is 105 min, which is longer than that in the Ni50Al50 powders. The overall reaction has been slowed by the additions of Ti and C. Only one exothermic heat release was observed during the overall chemical reaction, suggesting that these two separated chemical reactions mentioned above occur almost simultaneously, i.e. the chemical reaction between Ti and C has been significantly accelerated in the
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overall reaction than that during mechanical alloying of the Ti–C binary system. This can be attributed to the large exothermic heat release during explosive reaction between Ni and Al, which can simultaneously heat neighboring unreacted Ti and C to its ignition temperature. After the chemical reaction, some unreacted components still exist, possibly due to inefficient contact between the particles [11]. During later stages of mechanical alloying, the reactions occur in a rather weak way. The gradual reaction should be based on explosions in individual grains, as suggested by Atzmon [9]. After the combustion reaction, the NiAl–TiC nanocomposite has formed. It is expected that prolonged mechanical alloying can refine the microstructure of the nanocomposite. As shown in Fig. 5, the grain sizes for NiAl and TiC have been significantly decreased and saturated to 4 and 14 nm after 30 h of milling. It is interesting to note that the final grain size for TiC is larger than that for NiAl. The melting point for TiC is 2776°C which is much higher than that of 1638°C for NiAl. It has been well known that the grain size achievable during high-energy ball milling of ductile materials is inversely proportional to the melting point of materials [18]. However, a different deformation mechanism is applied for such nanocomposite compared with ductile materials. Since TiC is embedded in the relatively soft NiAl matrix, the mechanical force will act on the NiAl matrix initially when a nanocomposite particle is trapped between milling balls. Due to relative softness of NiAl, the mechanical force can be reduced when it acts on the TiC phase. So the NiAl matrix undergoes much severe deformation. Besides, the hard TiC particles can act as micro-ball to mill NiAl matrix further. Therefore, it is understandable that the TiC phase exhibits a relatively larger grain size after prolonged mechanical alloying. It is well known that in situ production of a reinforcing phase offers significant advantages from technical and economic standpoints, yielding better tailorability of the composite properties. In the present nanocomposite, the TiC reinforcement is in situ formed instead of being added to the matrix. It can be expected that in situ reaction formation of nanocomposites can provide better properties over these produced by conventional methods [7].
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5. Conclusion NiAl–TiC nanocomposite can be produced by mechanical alloying of elemental powders. The formation mechanism is suggested to be two separate reactions which occur almost simultaneously, i.e. the explosive reaction of NiAl causes high heat release and ignites the explosive reaction of TiC. Prolonged milling leads to the refinement of grain sizes. The final grain size for TiC is 3.5 times as large as that for NiAl, though TiC has a much higher melting point compared with NiAl. Acknowledgements The authors would like to acknowledge Professor Z.Q. Hu for the revision of the manuscript. This work was partially supported by the National Advanced Materials Committee of China and the National Natural Science Foundation of China. References [1] R.D. Noebe, R.R. Bowrnan, M.V. Nathal, Int. Mater. Rev. 38 (1993) 193. [2] D.B. Miracle, Acta. Metall. Mater. 41 (1993) 649. [3] R. Darolia, J. Mater. Sci. Technol. 10 (1994) 157. [4] E.P. George, M. Yamagochi, K.S. Kumar, C.T. Liu, Annu. Rev. Mater. Sci. 24 (1994) 409. [5] S. Dymek, M. Dollar, S.J. Hwang, P. Nash, Mater. Sci. Eng. A 152 (1992) 160. [6] H. Gleiter, Prog. Mater. Sci. 33 (1990) 223. [7] J.D. Whittenberger, E. Arzt, M.J. Luton, J. Mater. Res. 5 (1990) 271. [8] L. Wang, N. Beck, R.J. Arsenaut, Mater. Sci. Eng. A 177 (1994) 83. [9] M. Atzrnon, Phys. Rev. Lett. 64 (1990) 487. [10] S.N. Patankar, S.-Q. Xiao, J.J. Lewandowski, A.H. Heuer, J. Mater. Res. 8 (1993) 1311. [11] E. Ma, J. Pagan, G. Cranford, M. Atzmon, J. Mater. Res. 8 (1993) 1836. [12] Y.H. Park, H. Hashimoto, T. Abe, R. Watanabe, Mater. Sci. Eng. A 181/182 (1994) 1291. [13] T.P. Lou, G.J. Fan, B.Z. Ding, Z.Q. Hu, J. Mater. Res. 12 (1997) 1172. [14] G.B. Schaffer, P.G. McCormick, Metall. Trans. 21A (1990) 2789. [15] G.J. Fan, X.P. Song, M.X. Quan, Z.Q. Hu, Sci. Mater. 35 (1996) 1065. [16] Z.G. Liu, J.T. Guo, L.L. Ye, G.S. Li, Z.Q. Hu, Appl. Phys. Lett. 65 (1994) 2666. [17] S.C. Deevi, J. Mater. Sci. 26 (1991) 3343. [18] C.C. Koch, Nanostruct. Mater. 2 (1993) 109.