- Email: [email protected]
Comment on “reaction layers around SiC particles in Ti: an electron microscopy study”
Scripta mater. 43 (2000) 285–286 www.elsevier.com/locate/scriptamat
COMMENT ON “REACTION LAYERS AROUND SiC PARTICLES IN Ti: AN ELECTRON MICROSCOPY STUDY” M.W. Barsoum Department of Materials Engineering, Drexel University, Philadelphia PA 19104 (Received February 8, 2000) (Accepted February 24, 2000) Keywords: Laser treatment; Stacking fault; Titanium; Carbides; Topotactic reaction
Introduction In a recent paper by Kooi et al. [1] a large number of stacking faults in the TiCx layers that form as a result of the reaction of Ti with SiC were observed. As the authors noted, this was somewhat surprising because the stacking fault energy of TiC is quite high. Based on a very careful TEM study, the authors proposed that either the presence of impurities, or the segregation of Si at the stacking faults was responsible for lowering their energy sufficiently for them to form in large numbers. The Si is believed to have most likely dissolved in the TiCx at higher temperatures. There is a simpler alternate explanation. We have recently shown that when Ti3SiC2 is placed in an environment in which the Si is induced to migrate outwards, such a cryolite [2] or molten Al [3] a topotactic reaction occurs in which the Ti3SiC2 is converted to a cubic phase of approximate chemistry Ti(C0.67,Si0.06). This phase forms in domains and is partially ordered. To understand the transformation refer to Fig. 1. To convert Ti3SiC2 (Fig. 1a) to TiC0.67, first remove the Si atoms, which results in a highly twinned rock salt structure (Fig. 1b). This structure in untenable, however, because the Ti atoms on either side of the vacant row of atoms are in a simple cubic arrangement. Interestingly enough the structure does not simply collapse, but rather every other Ti3C2 block of layers de-twins (by rotation around an axis perpendicular to c-axis). The two variants or domains are shown schematically in Fig. 1c. The removal of the Si atoms in essence allows a de-twinning of the Ti3C2 blocks into domains in which the orientations of the (111) planes are mirror images of each other. The calculated angle between the (111) planes, 38° 56⬘, is in excellent agreement with the measured value of ⬇ 39 °. The transformation is associated with a 15 vol. % shrinkage. It is also important in this context to point out that Ti3SiC2 does not melt but rather decomposes peritectically into TiCx and a liquid [4]. The decomposition temperature is a function of purity and environment. For example, small amounts of Fe can reduce the decomposition temperature from over 2000 °C to under 1600 °C [5]. If pure, however, the highest decomposition temperature reported is 2200 °C [6]. The mechanism that we propose would not only explain the presence of the stacking faults observed by Kooi et al. [1], but also the fact that their TiCx was Si-saturated. It also explains why some of the stacking faults they observed were unfaulted. The similarities between our results and those of Kooi et al. strongly suggest that what they observed was nothing but the decomposition of Ti3SiC2 that had presumably formed earlier in the reaction sequence. Consistent with this picture is the fact that the faulted TiCx was the one adjacent to the SiC particles. The subsequent outward diffusion of Si would 1359-6462/00/$–see front matter. © 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(00)00404-8
286
COMMENT ON REACTION LAYER AROUND SiC IN Ti
Vol. 43, No. 3
Figure 1. a) (12 10) plane in Ti3SiC2. Note that adjacent blocks of Ti3C2 are twins of each other. b) Same as a after loss of Si. c) Same as b, but after transformation that results in domains that are twins of each other. This structure is highly idealized in the sense that complete order is assumed and no Si remains.
then lead to the TiCx observed. As we noted in our paper, it would be very difficult indeed to create the structures we observed, especially the partial ordering, by the diffusion of Si into TiCx. The same, I believe, holds true for the work of Kooi et al. Finally it is worth noting that this topotactic reaction of Ti3SiC2 is quite common. For example, reacting Ti3SiC2 with graphite at temperatures in excess of 1450 °C, results in its topotactic carburization of Ti3SiC2 and its conversion into TiCx (x ⬎ 0.8) by the inwards diffusion of C and the escape of Si from the compact as a vapor phase [7]. A similar reaction occurs when Ti3SiC2 is reacted with molten Al [3]. References 1. 2. 3. 4. 5. 6. 7.
B. J. Kooi, M. Kabel, A. B. Kloosterman, and J. Th. De Hosson, Acta Mater. 47, 3105 (1999). M. W. Barsoum, T. El-Raghy, L. Farber, M. Amer, R. Christini, and A. Adams, J. Electrochem. Soc. 146, 3919 (1999). T. El-Raghy, M. W. Barsoum, and M. Sika, Submitted for publication. H. Nowotny and S. Windisch, in Annual Review of Materials Science, ed. R. Huggins, R. Bube, and R. Roberts, vol. 3, p. 171, Annual Review Inc., Palo Alto, CA (1973). N. Tzenov, M. W. Barsoum, and T. El-Raghy, J. Eur. Cer. Soc. in press. Y. Du, J. C. Schuster, H. Seifert, and F. Aldinger, J. Am. Ceram. Soc. in press. T. El-Raghy and M. W. Barsoum, J. Appl. Phys. 83, 112 (1998).
COMMENT ON “REACTION LAYERS AROUND SiC PARTICLES IN Ti: AN ELECTRON MICROSCOPY STUDY” M.W. Barsoum Department of Materials Engineering, Drexel University, Philadelphia PA 19104 (Received February 8, 2000) (Accepted February 24, 2000) Keywords: Laser treatment; Stacking fault; Titanium; Carbides; Topotactic reaction
Introduction In a recent paper by Kooi et al. [1] a large number of stacking faults in the TiCx layers that form as a result of the reaction of Ti with SiC were observed. As the authors noted, this was somewhat surprising because the stacking fault energy of TiC is quite high. Based on a very careful TEM study, the authors proposed that either the presence of impurities, or the segregation of Si at the stacking faults was responsible for lowering their energy sufficiently for them to form in large numbers. The Si is believed to have most likely dissolved in the TiCx at higher temperatures. There is a simpler alternate explanation. We have recently shown that when Ti3SiC2 is placed in an environment in which the Si is induced to migrate outwards, such a cryolite [2] or molten Al [3] a topotactic reaction occurs in which the Ti3SiC2 is converted to a cubic phase of approximate chemistry Ti(C0.67,Si0.06). This phase forms in domains and is partially ordered. To understand the transformation refer to Fig. 1. To convert Ti3SiC2 (Fig. 1a) to TiC0.67, first remove the Si atoms, which results in a highly twinned rock salt structure (Fig. 1b). This structure in untenable, however, because the Ti atoms on either side of the vacant row of atoms are in a simple cubic arrangement. Interestingly enough the structure does not simply collapse, but rather every other Ti3C2 block of layers de-twins (by rotation around an axis perpendicular to c-axis). The two variants or domains are shown schematically in Fig. 1c. The removal of the Si atoms in essence allows a de-twinning of the Ti3C2 blocks into domains in which the orientations of the (111) planes are mirror images of each other. The calculated angle between the (111) planes, 38° 56⬘, is in excellent agreement with the measured value of ⬇ 39 °. The transformation is associated with a 15 vol. % shrinkage. It is also important in this context to point out that Ti3SiC2 does not melt but rather decomposes peritectically into TiCx and a liquid [4]. The decomposition temperature is a function of purity and environment. For example, small amounts of Fe can reduce the decomposition temperature from over 2000 °C to under 1600 °C [5]. If pure, however, the highest decomposition temperature reported is 2200 °C [6]. The mechanism that we propose would not only explain the presence of the stacking faults observed by Kooi et al. [1], but also the fact that their TiCx was Si-saturated. It also explains why some of the stacking faults they observed were unfaulted. The similarities between our results and those of Kooi et al. strongly suggest that what they observed was nothing but the decomposition of Ti3SiC2 that had presumably formed earlier in the reaction sequence. Consistent with this picture is the fact that the faulted TiCx was the one adjacent to the SiC particles. The subsequent outward diffusion of Si would 1359-6462/00/$–see front matter. © 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(00)00404-8
286
COMMENT ON REACTION LAYER AROUND SiC IN Ti
Vol. 43, No. 3
Figure 1. a) (12 10) plane in Ti3SiC2. Note that adjacent blocks of Ti3C2 are twins of each other. b) Same as a after loss of Si. c) Same as b, but after transformation that results in domains that are twins of each other. This structure is highly idealized in the sense that complete order is assumed and no Si remains.
then lead to the TiCx observed. As we noted in our paper, it would be very difficult indeed to create the structures we observed, especially the partial ordering, by the diffusion of Si into TiCx. The same, I believe, holds true for the work of Kooi et al. Finally it is worth noting that this topotactic reaction of Ti3SiC2 is quite common. For example, reacting Ti3SiC2 with graphite at temperatures in excess of 1450 °C, results in its topotactic carburization of Ti3SiC2 and its conversion into TiCx (x ⬎ 0.8) by the inwards diffusion of C and the escape of Si from the compact as a vapor phase [7]. A similar reaction occurs when Ti3SiC2 is reacted with molten Al [3]. References 1. 2. 3. 4. 5. 6. 7.
B. J. Kooi, M. Kabel, A. B. Kloosterman, and J. Th. De Hosson, Acta Mater. 47, 3105 (1999). M. W. Barsoum, T. El-Raghy, L. Farber, M. Amer, R. Christini, and A. Adams, J. Electrochem. Soc. 146, 3919 (1999). T. El-Raghy, M. W. Barsoum, and M. Sika, Submitted for publication. H. Nowotny and S. Windisch, in Annual Review of Materials Science, ed. R. Huggins, R. Bube, and R. Roberts, vol. 3, p. 171, Annual Review Inc., Palo Alto, CA (1973). N. Tzenov, M. W. Barsoum, and T. El-Raghy, J. Eur. Cer. Soc. in press. Y. Du, J. C. Schuster, H. Seifert, and F. Aldinger, J. Am. Ceram. Soc. in press. T. El-Raghy and M. W. Barsoum, J. Appl. Phys. 83, 112 (1998).