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Defect-enhanced solid-state reaction behavior of shock-modified Ti+C powder mixture compacts
Journal of Materials Processing Technology 85 (1999) 79 – 82
Defect-enhanced solid-state reaction behavior of shock-modified Ti +C powder mixture compacts Jong-Heon Lee 1, Naresh N. Thadhani * School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332 -0245, USA
Abstract In this study, the reaction behavior of shock-densified Ti and C elemental powder mixtures was investigated. The recovered shock-modified compacts ( 88–90% TMD) were reaction-sintered in an induction-heated hot press at temperatures of 1100–1400°C, with holding times between 30 and 180 min. The reaction mechanism of shock-modified Ti and C powder mixtures involved defect-enhanced solid-state diffusion, with an activation energy four to six times lower than that of diffusion couples. The resulting reacted compacts had 85% density and highly refined microstructures with 5 mm grain size. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Shock modification; TiC synthesis; Reaction kinetics; Solid-state diffusion mechanism
1. Introduction
1.1. Experimental procedure
Shock-compression produces a highly activated state of powder mixtures by intimately dispersing the reactants, generating defects and introducing significant grain size reduction via fracturing, or formation of sub-grain structures [1]. These effects result in significantly increased mass transport rates and enhanced chemical reactivity of powders, creating new paths for the motion of point defects along grain boundaries [2]. In particular, solid – solid diffusivities of the powder mixture components can be accelerated by the presence of dislocations [3,4], shorter diffusion distances [5], more intimate cleansed surface contacts and higher packing densities [6], which result in favoring solid-state diffusion reactions during post-shock thermal treatments. In the present work, mixtures of titanium and carbon (graphite) elemental powders were shockmodified using a cylindrical implosion system, and the enhanced-solid state reaction mechanism was investigated based on the reaction behavior and kinetics of shock-modified compacts during subsequent reaction sintering.
Titanium (99.9% pure; 20 mm average particle size) and graphite (99.99% pure; 2 mm average particle size) powders were mixed in a 1Ti:0.95C mole ratio under an argon atmosphere in a slow-speed V-shaped mechanical blender. Shock modification experiments were conducted to prepare shock-modified Ti+ 0.95 graphite powder compacts ( 88–90% of theoretical maximum density), using a cylindrical single tube implosion system [7]. Subsequent reaction synthesis experiments were performed on shock-modified compacts placed in a low pressure induction-heated hot press [8], under the following operating cycle: temperatures between 1100 and 1400oC with a constant heating rate of 20oC min − 1, holding time of 30–60 min and an applied pressure of 15.5 MPa (2250 psi) during heating and 34.5 MPa (5000 psi) during holding and air-cooling. The purpose of using the temperature range 300oC below the melting temperature of Ti, was to avoid initiation of the combustion process and to allow the reaction to occur via enhanced solid-state diffusion. Optical microscopy analysis was carried to determine the area fraction of Ti present in as-shocked Ti+0.95C compacts (estimated to be 43%) using the ASTM standard point-counting analysis. The same method was used to evaluate the extent of reaction, based on
* Corresponding author. Fax: +1 404 8949140; e-mail: [email protected] 1 Present address: High Energy Rate Laboratory, Kumamoto University, Kumamoto 860, Japan. Tel.: + 81 96 3423050; fax: + 81 96 3423729; e-mail: [email protected]
0924-0136/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S0924-0136(98)00265-9
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J.-H. Lee, N.N. Thadhani / Journal of Materials Processing Technology 85 (1999) 79–82
Fig. 1. Optical micrographs showing an increase of reaction products with increasing temperatures from 1000 to 1300oC.
the measured area fraction of the unreacted titanium normalized by the area fraction of Ti ( 43%) in as-shocked Ti + C compacts. The normalized fraction was then subtracted from 1 to provide an estimate of the extent of reacted Ti in reaction-synthesized compacts.
2. Results and discussion
2.1. Characteristics of reaction-synthesized compacts Typical optical micrographs of synthesized compacts at varying temperatures between 1000 and 1300oC at a heating rate of 20oC min − 1 and a 60 min holding time, are shown in Fig. 1(a) – (c). It can be observed that a reaction layer forms at temperature as low as 1000oC and that the thickness of the titanium carbide product increases with temperature increase. The reactions initiate at interparticle boundaries of Ti and graphite powders and subsequently proceed into the Ti particles, as shown in Fig. 1(a) and (b). The Ti particle was almost fully consumed by the reacted layer and only a very small portion of remaining Ti was isolated by reaction product at 1200oC (Fig. 1(b)). Ti was observed to be completely depleted at T \1300oC and a 60 min holding time (Fig. 1(c)). In addition pores were formed, primarily due to volume change in the reacted product, while large dark areas corresponding to unreacted graphite still remained. Therefore, the reaction product formed is a non-stoichiometric TiCx compound. The fully reacted (with complete depletion of Ti) compacts had a density of 4.19 g cm − 3 (which is 85% of TMD of stoichiometric TiC) and highly refined microstructures with 5 mm size of equiaxed grains.
2.2. Reaction kinetics and mechanisms The reaction mechanism was evaluated by the apparent activation energy determined, based on Arrhenius kinetics. Fig. 2(a) shows the area fraction of reacted Ti in the shock-modified compacts as a function of reaction temperature for holding times varying from 30 to 180 min. The data replotted as a temperature–time plot for fixed reaction fractions between 0.3 and 0.8, was shown in Fig. 2(b). The apparent activation energy was determined from the slope of (Log10time) vs. (1/T), where T denotes temperature, to be in the range of 29–47 kcal mol − 1 (120–200 kJ mol − 1). These values are at least two to three times lower than the activation energy for diffusion of C into TiCx ( 100 kcal mol − 1) and about four to six times lower than that for the diffusion of Ti into TiCx ( 176 kcal mol − 1) obtained from diffusion-couple experiments [9]. The lowering of the activation energy is a manifestation of defect-enhanced solid-state diffusion. To understand the reaction between Ti and C in the shock-modified compacts, the reaction mechanism of Ti/C diffusion-couples can be considered first. Based on the activation energy values for diffusion of constituents [9], a solid-state reaction in Ti/C may occur by: (1) rapid diffusion of C into Ti to form stable TiCx layer; and then (2) very slow diffusion of C through the interfacial TiCx layer to react with Ti, as shown schematically in Fig. 3(a). The diffusivity of Ti through the TiCx layer is even slower, by a factor of 104. Thus, once a TiCx interfacial layer is formed, further reaction becomes very sluggish. The reaction mechanism of Ti and C (graphite) powders in shock-modified compacts is dominated by a solid-state diffusion process, as shown in Fig. 3(b), since the reaction occurs at temperatures sufficiently
J.-H. Lee, N.N. Thadhani / Journal of Materials Processing Technology 85 (1999) 79–82
below the titanium melting temperature and the microstructure of TiCx reaction product exhibits highly refined and equiaxed grains, with no evidence of a solidification process similar to a combustion synthesis reaction. The reaction mechanism in the shockmodified Ti +C powder mixture compacts can therefore be considered to involve: (1) an initial rapid diffusion of C into Ti, forming a TiCx interfacial reaction layer which may be a highly disordered hypostoichiometric phase produced by the rapid defect-enhanced diffusion; (2) subsequent diffusion of Ti and C into the disordered interfacial layer to be greatly accelerated with Ti being consumed; (3) continuous solid-state reaction product formation under the
Fig. 2. (a) The fraction of reacted Ti varying with reaction synthesis temperature; and (b) the plots of (1/T) vs. (Log10time) for activation evaluation.
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Fig. 3. Solid-state reaction mechanisms in Ti and C powder mixtures in (a) a diffusion couple; and (b) a shock-modified state.
condition of the heat dissipation rate higher than the heat release rate; and finally (4), formation of high density titanium carbide product ( 85% dense) via defect-enhanced solid-state reaction. Throughout the diffusion process, the reaction between Ti and C is also accompanied by the release of heat and a concomitant increase in temperature, due to the exothermic nature of the reaction between Ti and C to form titanium carbide product. The amount of heat released is a function of the fraction of material reacted and the reaction temperature itself. Hence, if reaction initiates at low temperature, then the entire reaction can occur completely in the solid-state, since the increase in temperature due to reaction would be smaller. Under such conditions, the rate of heat dissipation may exceed the rate of heat release, avoiding localization of heat and subsequent combustion reaction, which results in large volume changes in products ( 30–40% porosity) during solidification. The reaction-synthesized shock-modified compacts yield reaction products with 85% density at temperatures between 1200 and 1300oC, as shown in Fig. 1. Therefore, since significant reaction has occurred in the solid-state at temperatures up to 1200oC ( 97% Ti consumed), the amount of reaction heat available to be released beyond this temperature is insufficient to trigger a combustion reaction. Hence, the reaction products in the shock-modified compacts are formed by a process typical of that involving rapid solid-state diffusion. These results demonstrate that shock modification of Ti + C (graphite) powder mixtures creates a densepacked highly-activated state of reactants which undergo a defect-enhanced solid-state diffusion reaction. Furthermore, the heat release rate of reaction does not result in build-up and localization due to the high heat dissipation rate, subsequently a combustion reaction is inhibited. The 15% porosity retained in these compacts is partly due to the initial porosity (10%) present in the green compact prior to reaction and the balance due to the volume change.
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3. Conclusions
References
Shock modification can be used to make Ti +0.95 graphite green compacts ( 88 – 90% dense), which can be fully reacted at temperatures up to 1300oC with a 60 min holding time, resulting in the formation of TiCx. The apparent activation energy determined from Arrhenius kinetics is observed to be reduced by two to three times the activation energy for C diffusion into TiCx and by four to six times that for Ti diffusion into TiCx. The high density ( 85%) and highly refined microstructures ( 5 mm grain size) retained in the reaction-synthesized compacts can be attributed to the reaction being continuously dominated by a solid-state diffusion process, without the reaction being taken over by the combustion phenomenon. In shock-modified compacts, solid-state diffusion is significantly enhanced, such that the rate of heat dissipation exceeds that of heat generation; thermal localization is then inhibited and reaction continues to be dominated via the defectenhanced solid-state diffusion process.
[1] R.A. Graham, N.N. Thadhani, in: A.B. Sawaoka (Ed.), Shock Waves in Materials Science, Springer, Berlin, 1993, pp. 35–65. [2] E.K. Beauchamp, in: R.A. Graham, A.B. Sawaoka (Eds.), HighPressure Explosives Processing of Ceramics, Trans. Tech. Publ., Zu¨rich, 1987, p. 139. [3] W.F. Hammetter, R.A. Graham, B. Morosin, Y. Horie, in: S.C. Schmidt, N.C. Holmes (Eds.), Shock Waves in Condensed Matter, Elsevier, Amsterdam, 1988, p. 431. [4] E. Dunbar, N.N. Thadhani, R.A. Graham, J. Mater. Sci. 28 (1993) 2903. [5] G.T. Hida, I.J. Lin, in: J.B. Holt, Z. A. Munir (Eds.), Combustion and Plasma Synthesis of High Temperature Materials, VCH, Weinheim, 1990, p. 246. [6] S.C. Deevi, J. Mater. Sci. 26 (10) (1991) 2662; 26 (10) (1991) 3343. [7] J.-H. Lee, N.N. Thadhani, H.A. Grebe, Metall. Mater. Trans. A 27A (1996) 1749. [8] J.-H. Lee, N.N. Thadhani, in: L.E. Murr, K.P. Staudhammer, M.A. Meyers (Eds.), Metallurgical and Materials Applications of Shock-Wave and High Strain Rate Phenomena, Elsevier, New York, 1995, p. 637. [9] S. Sarian, J. Appl. Phys. 39 (7) (1968) 3305; 40 (9) (1969) 3515.
.
Defect-enhanced solid-state reaction behavior of shock-modified Ti +C powder mixture compacts Jong-Heon Lee 1, Naresh N. Thadhani * School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332 -0245, USA
Abstract In this study, the reaction behavior of shock-densified Ti and C elemental powder mixtures was investigated. The recovered shock-modified compacts ( 88–90% TMD) were reaction-sintered in an induction-heated hot press at temperatures of 1100–1400°C, with holding times between 30 and 180 min. The reaction mechanism of shock-modified Ti and C powder mixtures involved defect-enhanced solid-state diffusion, with an activation energy four to six times lower than that of diffusion couples. The resulting reacted compacts had 85% density and highly refined microstructures with 5 mm grain size. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Shock modification; TiC synthesis; Reaction kinetics; Solid-state diffusion mechanism
1. Introduction
1.1. Experimental procedure
Shock-compression produces a highly activated state of powder mixtures by intimately dispersing the reactants, generating defects and introducing significant grain size reduction via fracturing, or formation of sub-grain structures [1]. These effects result in significantly increased mass transport rates and enhanced chemical reactivity of powders, creating new paths for the motion of point defects along grain boundaries [2]. In particular, solid – solid diffusivities of the powder mixture components can be accelerated by the presence of dislocations [3,4], shorter diffusion distances [5], more intimate cleansed surface contacts and higher packing densities [6], which result in favoring solid-state diffusion reactions during post-shock thermal treatments. In the present work, mixtures of titanium and carbon (graphite) elemental powders were shockmodified using a cylindrical implosion system, and the enhanced-solid state reaction mechanism was investigated based on the reaction behavior and kinetics of shock-modified compacts during subsequent reaction sintering.
Titanium (99.9% pure; 20 mm average particle size) and graphite (99.99% pure; 2 mm average particle size) powders were mixed in a 1Ti:0.95C mole ratio under an argon atmosphere in a slow-speed V-shaped mechanical blender. Shock modification experiments were conducted to prepare shock-modified Ti+ 0.95 graphite powder compacts ( 88–90% of theoretical maximum density), using a cylindrical single tube implosion system [7]. Subsequent reaction synthesis experiments were performed on shock-modified compacts placed in a low pressure induction-heated hot press [8], under the following operating cycle: temperatures between 1100 and 1400oC with a constant heating rate of 20oC min − 1, holding time of 30–60 min and an applied pressure of 15.5 MPa (2250 psi) during heating and 34.5 MPa (5000 psi) during holding and air-cooling. The purpose of using the temperature range 300oC below the melting temperature of Ti, was to avoid initiation of the combustion process and to allow the reaction to occur via enhanced solid-state diffusion. Optical microscopy analysis was carried to determine the area fraction of Ti present in as-shocked Ti+0.95C compacts (estimated to be 43%) using the ASTM standard point-counting analysis. The same method was used to evaluate the extent of reaction, based on
* Corresponding author. Fax: +1 404 8949140; e-mail: [email protected] 1 Present address: High Energy Rate Laboratory, Kumamoto University, Kumamoto 860, Japan. Tel.: + 81 96 3423050; fax: + 81 96 3423729; e-mail: [email protected]
0924-0136/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII S0924-0136(98)00265-9
80
J.-H. Lee, N.N. Thadhani / Journal of Materials Processing Technology 85 (1999) 79–82
Fig. 1. Optical micrographs showing an increase of reaction products with increasing temperatures from 1000 to 1300oC.
the measured area fraction of the unreacted titanium normalized by the area fraction of Ti ( 43%) in as-shocked Ti + C compacts. The normalized fraction was then subtracted from 1 to provide an estimate of the extent of reacted Ti in reaction-synthesized compacts.
2. Results and discussion
2.1. Characteristics of reaction-synthesized compacts Typical optical micrographs of synthesized compacts at varying temperatures between 1000 and 1300oC at a heating rate of 20oC min − 1 and a 60 min holding time, are shown in Fig. 1(a) – (c). It can be observed that a reaction layer forms at temperature as low as 1000oC and that the thickness of the titanium carbide product increases with temperature increase. The reactions initiate at interparticle boundaries of Ti and graphite powders and subsequently proceed into the Ti particles, as shown in Fig. 1(a) and (b). The Ti particle was almost fully consumed by the reacted layer and only a very small portion of remaining Ti was isolated by reaction product at 1200oC (Fig. 1(b)). Ti was observed to be completely depleted at T \1300oC and a 60 min holding time (Fig. 1(c)). In addition pores were formed, primarily due to volume change in the reacted product, while large dark areas corresponding to unreacted graphite still remained. Therefore, the reaction product formed is a non-stoichiometric TiCx compound. The fully reacted (with complete depletion of Ti) compacts had a density of 4.19 g cm − 3 (which is 85% of TMD of stoichiometric TiC) and highly refined microstructures with 5 mm size of equiaxed grains.
2.2. Reaction kinetics and mechanisms The reaction mechanism was evaluated by the apparent activation energy determined, based on Arrhenius kinetics. Fig. 2(a) shows the area fraction of reacted Ti in the shock-modified compacts as a function of reaction temperature for holding times varying from 30 to 180 min. The data replotted as a temperature–time plot for fixed reaction fractions between 0.3 and 0.8, was shown in Fig. 2(b). The apparent activation energy was determined from the slope of (Log10time) vs. (1/T), where T denotes temperature, to be in the range of 29–47 kcal mol − 1 (120–200 kJ mol − 1). These values are at least two to three times lower than the activation energy for diffusion of C into TiCx ( 100 kcal mol − 1) and about four to six times lower than that for the diffusion of Ti into TiCx ( 176 kcal mol − 1) obtained from diffusion-couple experiments [9]. The lowering of the activation energy is a manifestation of defect-enhanced solid-state diffusion. To understand the reaction between Ti and C in the shock-modified compacts, the reaction mechanism of Ti/C diffusion-couples can be considered first. Based on the activation energy values for diffusion of constituents [9], a solid-state reaction in Ti/C may occur by: (1) rapid diffusion of C into Ti to form stable TiCx layer; and then (2) very slow diffusion of C through the interfacial TiCx layer to react with Ti, as shown schematically in Fig. 3(a). The diffusivity of Ti through the TiCx layer is even slower, by a factor of 104. Thus, once a TiCx interfacial layer is formed, further reaction becomes very sluggish. The reaction mechanism of Ti and C (graphite) powders in shock-modified compacts is dominated by a solid-state diffusion process, as shown in Fig. 3(b), since the reaction occurs at temperatures sufficiently
J.-H. Lee, N.N. Thadhani / Journal of Materials Processing Technology 85 (1999) 79–82
below the titanium melting temperature and the microstructure of TiCx reaction product exhibits highly refined and equiaxed grains, with no evidence of a solidification process similar to a combustion synthesis reaction. The reaction mechanism in the shockmodified Ti +C powder mixture compacts can therefore be considered to involve: (1) an initial rapid diffusion of C into Ti, forming a TiCx interfacial reaction layer which may be a highly disordered hypostoichiometric phase produced by the rapid defect-enhanced diffusion; (2) subsequent diffusion of Ti and C into the disordered interfacial layer to be greatly accelerated with Ti being consumed; (3) continuous solid-state reaction product formation under the
Fig. 2. (a) The fraction of reacted Ti varying with reaction synthesis temperature; and (b) the plots of (1/T) vs. (Log10time) for activation evaluation.
81
Fig. 3. Solid-state reaction mechanisms in Ti and C powder mixtures in (a) a diffusion couple; and (b) a shock-modified state.
condition of the heat dissipation rate higher than the heat release rate; and finally (4), formation of high density titanium carbide product ( 85% dense) via defect-enhanced solid-state reaction. Throughout the diffusion process, the reaction between Ti and C is also accompanied by the release of heat and a concomitant increase in temperature, due to the exothermic nature of the reaction between Ti and C to form titanium carbide product. The amount of heat released is a function of the fraction of material reacted and the reaction temperature itself. Hence, if reaction initiates at low temperature, then the entire reaction can occur completely in the solid-state, since the increase in temperature due to reaction would be smaller. Under such conditions, the rate of heat dissipation may exceed the rate of heat release, avoiding localization of heat and subsequent combustion reaction, which results in large volume changes in products ( 30–40% porosity) during solidification. The reaction-synthesized shock-modified compacts yield reaction products with 85% density at temperatures between 1200 and 1300oC, as shown in Fig. 1. Therefore, since significant reaction has occurred in the solid-state at temperatures up to 1200oC ( 97% Ti consumed), the amount of reaction heat available to be released beyond this temperature is insufficient to trigger a combustion reaction. Hence, the reaction products in the shock-modified compacts are formed by a process typical of that involving rapid solid-state diffusion. These results demonstrate that shock modification of Ti + C (graphite) powder mixtures creates a densepacked highly-activated state of reactants which undergo a defect-enhanced solid-state diffusion reaction. Furthermore, the heat release rate of reaction does not result in build-up and localization due to the high heat dissipation rate, subsequently a combustion reaction is inhibited. The 15% porosity retained in these compacts is partly due to the initial porosity (10%) present in the green compact prior to reaction and the balance due to the volume change.
82
J.-H. Lee, N.N. Thadhani / Journal of Materials Processing Technology 85 (1999) 79–82
3. Conclusions
References
Shock modification can be used to make Ti +0.95 graphite green compacts ( 88 – 90% dense), which can be fully reacted at temperatures up to 1300oC with a 60 min holding time, resulting in the formation of TiCx. The apparent activation energy determined from Arrhenius kinetics is observed to be reduced by two to three times the activation energy for C diffusion into TiCx and by four to six times that for Ti diffusion into TiCx. The high density ( 85%) and highly refined microstructures ( 5 mm grain size) retained in the reaction-synthesized compacts can be attributed to the reaction being continuously dominated by a solid-state diffusion process, without the reaction being taken over by the combustion phenomenon. In shock-modified compacts, solid-state diffusion is significantly enhanced, such that the rate of heat dissipation exceeds that of heat generation; thermal localization is then inhibited and reaction continues to be dominated via the defectenhanced solid-state diffusion process.
[1] R.A. Graham, N.N. Thadhani, in: A.B. Sawaoka (Ed.), Shock Waves in Materials Science, Springer, Berlin, 1993, pp. 35–65. [2] E.K. Beauchamp, in: R.A. Graham, A.B. Sawaoka (Eds.), HighPressure Explosives Processing of Ceramics, Trans. Tech. Publ., Zu¨rich, 1987, p. 139. [3] W.F. Hammetter, R.A. Graham, B. Morosin, Y. Horie, in: S.C. Schmidt, N.C. Holmes (Eds.), Shock Waves in Condensed Matter, Elsevier, Amsterdam, 1988, p. 431. [4] E. Dunbar, N.N. Thadhani, R.A. Graham, J. Mater. Sci. 28 (1993) 2903. [5] G.T. Hida, I.J. Lin, in: J.B. Holt, Z. A. Munir (Eds.), Combustion and Plasma Synthesis of High Temperature Materials, VCH, Weinheim, 1990, p. 246. [6] S.C. Deevi, J. Mater. Sci. 26 (10) (1991) 2662; 26 (10) (1991) 3343. [7] J.-H. Lee, N.N. Thadhani, H.A. Grebe, Metall. Mater. Trans. A 27A (1996) 1749. [8] J.-H. Lee, N.N. Thadhani, in: L.E. Murr, K.P. Staudhammer, M.A. Meyers (Eds.), Metallurgical and Materials Applications of Shock-Wave and High Strain Rate Phenomena, Elsevier, New York, 1995, p. 637. [9] S. Sarian, J. Appl. Phys. 39 (7) (1968) 3305; 40 (9) (1969) 3515.
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