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Reactive synthesis of titanium matrix composite powders
October 2002
Materials Letters 56 (2002) 322 – 328 www.elsevier.com/locate/matlet
Reactive synthesis of titanium matrix composite powders P.B. Joshi a, G.R. Marathe a, N.S.S. Murti a, V.K. Kaushik b,*, P. Ramakrishnan c a
Department of Metallurgical Engineering, M.S. University of Baroda, Baroda 390001, India b Research Centre, Indian Petrochemicals Corporation Limited, Baroda 391346, India c Department of Metallurgical Engineering and Material Science, Indian Institute of Technology, Mumbai 400076, India Received 25 June 2001; received in revised form 16 December 2001; accepted 18 December 2001
Abstract Mechanical activation of materials has emerged as a promising alternative to conventional thermally activated processes. The present work deals with the attrition of titanium powders to produce Ti – TiO2 composite powders by reactive milling (RM). The phase transformation/new phases formed during the course of milling have been characterized by sophisticated techniques such as X-ray diffraction (XRD), electron spectroscopy for chemical analysis (ESCA) and differential thermal analysis (DTA). The microhardness measurements showed progressive increase in hardness with increasing milling time. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Mechanical Alloying; Reactive milling; Titanium composites; Ti – TiO2 composites
1. Introduction Mechanical alloying (MA) was developed by J.S. Benjamin at INCO Alloys International in the late 1960s [1] and has traditionally been used for production of oxide-dispersion strengthened (ODS) super alloys for high-temperature applications, for example, ODS Fe, Ni, Al, Ti, etc. [2 – 4]. The scope and applications of MA have been considerably widened in last three decades. Mechanical alloying is now regarded as not being simply a method for production of ODS alloys but as a solid-state process that can be used to create new materials in terms of both composition and microstructure [5]. As of late mechanical alloying has been looked upon as one of the new methods of material synthesis and has gained much attention. As a result of this, MA can *
Corresponding author. Fax: +91-265-272098. E-mail address: [email protected] (V.K. Kaushik).
be used to initiate oxidation/reduction type of reactions, virtually turning a ball mill or an attritor into a chemical reactor [6]. This application of mechanical alloying is popularly termed as mechano-chemical synthesis or reactive milling (RM). Typical examples of reactive milling are reduction of cupric oxide to copper by calcium, co-reduction of CuO and ZnO by Ca to form brass, reduction of rare-earth chlorides or oxides by reducing metals like Ca, Mg, Mn, Ni, etc. [7– 10]. Reactive milling can also lead to formation of compounds such as oxides, carbides or nitrides in respective atmosphere [11]. During MA/RM, repeated fracturing and rewelding of powder particles occurs due to ball-powder-ball and ball-powder-container collisions. This process leads to the following major effects: (i) decrease in powder particle size and consequent increase in their reaction surface area. (ii) generation of atomically clean surfaces.
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 4 7 6 - 7
P.B. Joshi et al. / Materials Letters 56 (2002) 322–328
(iii) increase in defect density level of powders providing short-circuit diffusion paths. (iv) increase in temperature of the powder at certain location due to large amount of plastic deformation of the particles. All these effects account for high chemical reactivity of powder under attrition, leading to chemical reaction between the neighbouring particles of different composition or reaction of the powder with the atmosphere of the vial. Conventional methods of producing fine dispersion of a dispersoid in a metal/alloy matrix include milling, internal oxidation, selective reduction of mixed oxides, etc. [12,13]. However, quite often such methods do not offer ultrafine uniform dispersion of dispersoids in the matrix and hence give rise to the inferior properties of the composite. Reactive milling is supposed to overcome these limitations. The present work deals with the development of Ti– TiO2 composites by RM.
2. Experimental work Titanium powder of 99.9% purity and <20 Am particle size was subjected to high-energy attrition milling using a stainless steel vial of 200 cm3 capacity and 6.3 mm diameter hardened steel balls of AISI 52100 steel. The milling was carried out at a speed of 400 rpm in normal atmosphere of air and the ball-tocharge ratio was kept at 15:1. Powder samples were drawn after 24, 36 and 48 h of milling and were subjected to X-ray diffraction (XRD) to monitor the progress of formation of any new phase(s) during the course of milling. XRD of powder samples was done on Rigaku Gieger Flex D-max model of X-ray diffractometer between the 2h range of 30j and 60j at a scan speed of 3j per minute using Cu target and Cu˚ wavelength at a power Ka radiation of 1.5406 A rating of 40 kV and 20 mA. Fig. 1 shows XRD profiles for powder samples drawn after various milling times. Highly surface-sensitive analytical technique electron spectroscopy for chemical analysis (ESCA) has been used to see the changes occurring on surface of the powder particles during the course of reactive milling.
323
A Vacuum Generators ESCALAB MKII Spectrometer equipped with twin (aluminum and magnesium) X-ray source was used for recording of X-ray photoelectron spectroscopy (XPS) spectra. The anode of Mg source used in the present study was operated at 10 kV10 mA and the vacuum in the analysis chamber was maintained better than 510 8 mbar during analysis. The spectrometer was calibrated using Ag (3d5/2) photoelectron line [14] at 368.3 eV. Powder samples were placed onto double side adhesive tape and mounted on the sample holder. Samples were kept overnight in the preparation chamber before being transferred to analytical chamber for recording spectra. Data were collected and analyzed on DELL computer interfacing the spectrometer. Figs. 2 and 3 report results of ESCA studies. Powder samples were also examined for their thermal behavior using Seiko Instruments model SSC 5100 TG/DTA 32 Thermal Analysis System. The powder samples were subjected to differential thermal analysis (DTA) and thermogravimetric (TG) studies in air in the temperature range of 293– 1373 K at a heating rate of 288 K per minute. High-purity alpha alumina powder was used as the reference. Figs. 4 and 5 show the multiple profiles for DTA and TG studies, respectively. Finally, the microhardness measurements on milled samples were carried out using the microhardness attachment of the EPITYPE-2 microscope of Carl Zeiss make at 20 g load. At least three indentations were taken on each sample and the average hardness was calculated. Fig. 6 gives the variation of microhardness with respect to milling time.
3. Results and discussion Fig. 1 shows the XRD profiles for powder samples of starting material and milled powders drawn at different time intervals, with observed d values as per JCPDS file [15] nos. 5-0682 and 16-617 for corresponding phases (shown in parentheses) marked over the peaks. As per this figure, the diffraction profile for starting material, that is, pure Ti indicates diffraction peaks for Ti alone and not for any other phase. The profiles for 24, 36, and 48 h MA samples indicate presence of TiO2 peaks also, having d values ˚ which match well with the equal to 2.332 and 2.238 A
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P.B. Joshi et al. / Materials Letters 56 (2002) 322–328
Fig. 1. XRD profiles for powder samples drawn at different time intervals.
standard. This clearly confirms formation of TiO2 from Ti by oxidation during the course of reactive milling. With progressive milling, there is pronounced broadening of diffraction peaks, attributed to reasons like decrease in particle size and phase transformation from Ti to TiO2. The XPS survey scans for powder samples milled for 24, 36 and 48 h are given in Fig. 2. Intensities of Ti(2p) and O(1s) photoelectron lines were observed and monitored for powders during the course of
milling. These spectra clearly indicate improvement in titanium dispersion along with the increase in oxygen concentration resulting into formation of finer titanium powder particles as TiO2 phase during the course of reactive milling. Multiscan data collected in Ti (2p3/2; 2p1/2) binding energy range (Fig. 3) and precise measurements of binding energy and Auger parameter confirm [16] to the formation of TiO2 phase on powder surfaces during the course of milling. As per above discussion, XRD and ESCA studies clearly show the formation of TiO2 phase during the
P.B. Joshi et al. / Materials Letters 56 (2002) 322–328
325
Fig. 2. XPS scans for samples subjected to 24, 36 and 48 h milling times.
course of milling. The oxidation behavior of titanium was further confirmed by thermal analysis. According to Hauffe [17], the oxidation rate of titanium is initially cubic when the metal is pure. After a brief initial cubic period, the metal oxidises at parabolic rate when it already contains several percentage of oxygen. As per DTA plots given in Fig. 4, the first stage of oxidation is found to begin at 635 jC (908 K) and the second stage at about 822 jC (1095 K).
The mechanism of oxidation of titanium may be briefly summarized in following steps: (i) the passage of oxygen to the metal-oxide interface through an essentially non-protective scale. (ii) the solution of oxygen in the metal phase leading to nucleation and growth of oxide on and just beneath the surface.
Fig. 3. Ti (2p3/2; 2p1/2) photoelectron lines of TiO2 in 36 h milled sample.
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P.B. Joshi et al. / Materials Letters 56 (2002) 322–328
Fig. 4. Multiple DTA plots for pure Ti and milled samples.
(iii) the intermittent exfoliation of thin sections of the metal surface under the influence of stresses developed by the nucleation and solution processes. The exfoliation of oxide film generates fresh titanium surfaces. These freshly exposed titanium surfaces are already in activated state because of plastic deformation due to milling and hence the further oxidation gets preponed. This is confirmed by the thermal analysis data given in Fig. 4. As per these data, the oxidation of pure Ti commences at 635 jC (908 K), for 24 h MA sample at 612.8 jC (885.8 K), for 36 h MA sample at 593.1 jC (866.1 K) and for 48 h MA sample at 612.8 jC (885.8 K). The energy released due to oxidation (expressed in terms of area under the exothermic peak) is also found to decrease with increasing milling time ( 477.8 AV s/mg for pure Ti, 2837 AV s/mg for 36 h MA sample as per Fig. 4). Oxidation on progressive milling with less energy may also be explained in terms of the presence of relatively porous pre-existing oxide film on milled powder particle surfaces. Marginally higher amount of energy released on 48 h of milling ( 2860
AV s/mg) is due to the oxidation of new surfaces created because of particle fracturing as per Benjamin’s theory of MA. The TG plots for pure Ti as well as milled powders are given in Fig. 5. The shape of the curve for pure titanium in this figure clearly displays the two-stage oxidation process for pure titanium (a faster rate of oxidation from a temperature of 598 jC (871 K) and a relatively slower oxidation process from 817 jC (1090 K) onwards. As compared to this, the oxidation in the case of 24, 36 and 48 h MA samples occurred in a continuous manner, represented by typical S curves. The multiple TG plots (Fig. 5) also indicate that the percentage oxide formed decreases with increasing milling time (66.1% for pure titanium, 58.3% for 24 h MA sample and 54.4% for 36 h MA sample). The amount of oxide formed after 48 h of milling is slightly higher (56.9%). This is in view of oxidation of fresh surfaces created during MA. The creation of fresh particle surfaces is due to two reasons. Firstly, as a result of fracturing of powder particles on prolonged milling during MA and secondly due to increased brittleness of the oxidized powder particles.
P.B. Joshi et al. / Materials Letters 56 (2002) 322–328
327
Fig. 5. Multiple TG plots for pure Ti and milled powder samples.
The microhardness measurement on powder samples show rise in hardness value with increasing milling time as shown in Fig. 6. Hardness increases
due to cold working of powder particles by milling and formation of a harder phase, TiO2, by reactive milling.
Fig. 6. Variation of microhardness of Ti with respect to milling time.
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P.B. Joshi et al. / Materials Letters 56 (2002) 322–328
4. Conclusion From the above studies, it can be said that attrition of pure titanium powder in normal atmosphere of air can lead to formation of TiO2 as a uniformly dispersed phase on titanium particle surfaces by a process called reactive milling (RM). These reaction-milled Ti–TiO2 composite powders on consolidation and sintering are expected to offer superior high-temperature highstrength properties to the system.
Acknowledgements The authors thank the Research Centre of Indian Petrochemicals Corporation Limited (IPCL), Baroda and M/S TCR Advanced Engineering Pvt. Ltd., Baroda for providing the necessary facilities for materials characterization.
References [1] J.S. Benjamin, Mechanical alloying, Sci. Am. 234 (1976) 40. [2] J.S. Benjamin, M.J. Bomford, Dispersion strengthened aluminium made by mechanical alloying, Metall. Trans. 8A (1977) 1301, Aug. [3] E.S. Bhagiradha Rao, Mechanical alloying of thoria-dispersed nichrome, Trans. IIM 31 (4) (1978) 261, Aug.
[4] J. Naser, W. Richmann, H. Ferkel, Dispersion hardening of metals by nanoscale ceramic powders, Mater. Sci. Eng. A234 – 236 (1997) 467. [5] L. Lu, M.O. Lai, Formation of new materials in the solid state by mechanical alloying, Mater. Design 16 (1) (1995) 33. [6] G.B. Schaffer, P.G. McCormick, Appl. Phys. Lett. 55 (1989) 45. [7] G.B. Schaffer, P.G. McCormick, Review on mechanical alloying, Mater. Forum 16 (1992) 91. [8] K. Tokumitsu, Reduction of metal oxides by mechanical alloying method, Solid State Ionics 101 – 103 (1997) 103. [9] G.B. Schaffer, P.G. McCormick, Displacement reactions during mechanical alloying, Metall. Trans. 21A (1990) 2789, Oct. [10] G.B. Schaffer, P.G. McCormick, Reduction of metal oxides by mechanical alloying, Appl. Phys. Lett. 55 (1) (1989) 45, July. [11] S. Quintana-Molina, J.G. Cabanas-Moreno, R. Martinez-Sanchez, Proc. of World Congress on Powder Metallurgy and Particulate Materials, ASM International, Washington, DC, (1996). [12] V.A. Tracy, D.K. Worn, Powder Metall. (10) (1962) 34. [13] G.B. Alexander et al., US Patent No. 2972,529, Feb. 21 (1961). [14] V.K. Kaushik, XPS core level spectra and Auger parameters for some silver compounds, J. Electron Spectrosc. Relat. Phenom. 56 (1991) 273. [15] Powder Diffraction Files. Joint Committee on Powder Diffraction Standards (JCPDS), Pennsylvania (1967). [16] C.D. Wagner, L.H. Gale, R.H. Raymond, Two dimensional chemical plots: a standardized data set for use in identifying chemical states by X-ray photoelectron spectroscopy, Anal. Chem. 51 (1979) 466. [17] K. Hauffe, Z. Elektrochem. 63 (1959) 819.
Materials Letters 56 (2002) 322 – 328 www.elsevier.com/locate/matlet
Reactive synthesis of titanium matrix composite powders P.B. Joshi a, G.R. Marathe a, N.S.S. Murti a, V.K. Kaushik b,*, P. Ramakrishnan c a
Department of Metallurgical Engineering, M.S. University of Baroda, Baroda 390001, India b Research Centre, Indian Petrochemicals Corporation Limited, Baroda 391346, India c Department of Metallurgical Engineering and Material Science, Indian Institute of Technology, Mumbai 400076, India Received 25 June 2001; received in revised form 16 December 2001; accepted 18 December 2001
Abstract Mechanical activation of materials has emerged as a promising alternative to conventional thermally activated processes. The present work deals with the attrition of titanium powders to produce Ti – TiO2 composite powders by reactive milling (RM). The phase transformation/new phases formed during the course of milling have been characterized by sophisticated techniques such as X-ray diffraction (XRD), electron spectroscopy for chemical analysis (ESCA) and differential thermal analysis (DTA). The microhardness measurements showed progressive increase in hardness with increasing milling time. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Mechanical Alloying; Reactive milling; Titanium composites; Ti – TiO2 composites
1. Introduction Mechanical alloying (MA) was developed by J.S. Benjamin at INCO Alloys International in the late 1960s [1] and has traditionally been used for production of oxide-dispersion strengthened (ODS) super alloys for high-temperature applications, for example, ODS Fe, Ni, Al, Ti, etc. [2 – 4]. The scope and applications of MA have been considerably widened in last three decades. Mechanical alloying is now regarded as not being simply a method for production of ODS alloys but as a solid-state process that can be used to create new materials in terms of both composition and microstructure [5]. As of late mechanical alloying has been looked upon as one of the new methods of material synthesis and has gained much attention. As a result of this, MA can *
Corresponding author. Fax: +91-265-272098. E-mail address: [email protected] (V.K. Kaushik).
be used to initiate oxidation/reduction type of reactions, virtually turning a ball mill or an attritor into a chemical reactor [6]. This application of mechanical alloying is popularly termed as mechano-chemical synthesis or reactive milling (RM). Typical examples of reactive milling are reduction of cupric oxide to copper by calcium, co-reduction of CuO and ZnO by Ca to form brass, reduction of rare-earth chlorides or oxides by reducing metals like Ca, Mg, Mn, Ni, etc. [7– 10]. Reactive milling can also lead to formation of compounds such as oxides, carbides or nitrides in respective atmosphere [11]. During MA/RM, repeated fracturing and rewelding of powder particles occurs due to ball-powder-ball and ball-powder-container collisions. This process leads to the following major effects: (i) decrease in powder particle size and consequent increase in their reaction surface area. (ii) generation of atomically clean surfaces.
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 4 7 6 - 7
P.B. Joshi et al. / Materials Letters 56 (2002) 322–328
(iii) increase in defect density level of powders providing short-circuit diffusion paths. (iv) increase in temperature of the powder at certain location due to large amount of plastic deformation of the particles. All these effects account for high chemical reactivity of powder under attrition, leading to chemical reaction between the neighbouring particles of different composition or reaction of the powder with the atmosphere of the vial. Conventional methods of producing fine dispersion of a dispersoid in a metal/alloy matrix include milling, internal oxidation, selective reduction of mixed oxides, etc. [12,13]. However, quite often such methods do not offer ultrafine uniform dispersion of dispersoids in the matrix and hence give rise to the inferior properties of the composite. Reactive milling is supposed to overcome these limitations. The present work deals with the development of Ti– TiO2 composites by RM.
2. Experimental work Titanium powder of 99.9% purity and <20 Am particle size was subjected to high-energy attrition milling using a stainless steel vial of 200 cm3 capacity and 6.3 mm diameter hardened steel balls of AISI 52100 steel. The milling was carried out at a speed of 400 rpm in normal atmosphere of air and the ball-tocharge ratio was kept at 15:1. Powder samples were drawn after 24, 36 and 48 h of milling and were subjected to X-ray diffraction (XRD) to monitor the progress of formation of any new phase(s) during the course of milling. XRD of powder samples was done on Rigaku Gieger Flex D-max model of X-ray diffractometer between the 2h range of 30j and 60j at a scan speed of 3j per minute using Cu target and Cu˚ wavelength at a power Ka radiation of 1.5406 A rating of 40 kV and 20 mA. Fig. 1 shows XRD profiles for powder samples drawn after various milling times. Highly surface-sensitive analytical technique electron spectroscopy for chemical analysis (ESCA) has been used to see the changes occurring on surface of the powder particles during the course of reactive milling.
323
A Vacuum Generators ESCALAB MKII Spectrometer equipped with twin (aluminum and magnesium) X-ray source was used for recording of X-ray photoelectron spectroscopy (XPS) spectra. The anode of Mg source used in the present study was operated at 10 kV10 mA and the vacuum in the analysis chamber was maintained better than 510 8 mbar during analysis. The spectrometer was calibrated using Ag (3d5/2) photoelectron line [14] at 368.3 eV. Powder samples were placed onto double side adhesive tape and mounted on the sample holder. Samples were kept overnight in the preparation chamber before being transferred to analytical chamber for recording spectra. Data were collected and analyzed on DELL computer interfacing the spectrometer. Figs. 2 and 3 report results of ESCA studies. Powder samples were also examined for their thermal behavior using Seiko Instruments model SSC 5100 TG/DTA 32 Thermal Analysis System. The powder samples were subjected to differential thermal analysis (DTA) and thermogravimetric (TG) studies in air in the temperature range of 293– 1373 K at a heating rate of 288 K per minute. High-purity alpha alumina powder was used as the reference. Figs. 4 and 5 show the multiple profiles for DTA and TG studies, respectively. Finally, the microhardness measurements on milled samples were carried out using the microhardness attachment of the EPITYPE-2 microscope of Carl Zeiss make at 20 g load. At least three indentations were taken on each sample and the average hardness was calculated. Fig. 6 gives the variation of microhardness with respect to milling time.
3. Results and discussion Fig. 1 shows the XRD profiles for powder samples of starting material and milled powders drawn at different time intervals, with observed d values as per JCPDS file [15] nos. 5-0682 and 16-617 for corresponding phases (shown in parentheses) marked over the peaks. As per this figure, the diffraction profile for starting material, that is, pure Ti indicates diffraction peaks for Ti alone and not for any other phase. The profiles for 24, 36, and 48 h MA samples indicate presence of TiO2 peaks also, having d values ˚ which match well with the equal to 2.332 and 2.238 A
324
P.B. Joshi et al. / Materials Letters 56 (2002) 322–328
Fig. 1. XRD profiles for powder samples drawn at different time intervals.
standard. This clearly confirms formation of TiO2 from Ti by oxidation during the course of reactive milling. With progressive milling, there is pronounced broadening of diffraction peaks, attributed to reasons like decrease in particle size and phase transformation from Ti to TiO2. The XPS survey scans for powder samples milled for 24, 36 and 48 h are given in Fig. 2. Intensities of Ti(2p) and O(1s) photoelectron lines were observed and monitored for powders during the course of
milling. These spectra clearly indicate improvement in titanium dispersion along with the increase in oxygen concentration resulting into formation of finer titanium powder particles as TiO2 phase during the course of reactive milling. Multiscan data collected in Ti (2p3/2; 2p1/2) binding energy range (Fig. 3) and precise measurements of binding energy and Auger parameter confirm [16] to the formation of TiO2 phase on powder surfaces during the course of milling. As per above discussion, XRD and ESCA studies clearly show the formation of TiO2 phase during the
P.B. Joshi et al. / Materials Letters 56 (2002) 322–328
325
Fig. 2. XPS scans for samples subjected to 24, 36 and 48 h milling times.
course of milling. The oxidation behavior of titanium was further confirmed by thermal analysis. According to Hauffe [17], the oxidation rate of titanium is initially cubic when the metal is pure. After a brief initial cubic period, the metal oxidises at parabolic rate when it already contains several percentage of oxygen. As per DTA plots given in Fig. 4, the first stage of oxidation is found to begin at 635 jC (908 K) and the second stage at about 822 jC (1095 K).
The mechanism of oxidation of titanium may be briefly summarized in following steps: (i) the passage of oxygen to the metal-oxide interface through an essentially non-protective scale. (ii) the solution of oxygen in the metal phase leading to nucleation and growth of oxide on and just beneath the surface.
Fig. 3. Ti (2p3/2; 2p1/2) photoelectron lines of TiO2 in 36 h milled sample.
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P.B. Joshi et al. / Materials Letters 56 (2002) 322–328
Fig. 4. Multiple DTA plots for pure Ti and milled samples.
(iii) the intermittent exfoliation of thin sections of the metal surface under the influence of stresses developed by the nucleation and solution processes. The exfoliation of oxide film generates fresh titanium surfaces. These freshly exposed titanium surfaces are already in activated state because of plastic deformation due to milling and hence the further oxidation gets preponed. This is confirmed by the thermal analysis data given in Fig. 4. As per these data, the oxidation of pure Ti commences at 635 jC (908 K), for 24 h MA sample at 612.8 jC (885.8 K), for 36 h MA sample at 593.1 jC (866.1 K) and for 48 h MA sample at 612.8 jC (885.8 K). The energy released due to oxidation (expressed in terms of area under the exothermic peak) is also found to decrease with increasing milling time ( 477.8 AV s/mg for pure Ti, 2837 AV s/mg for 36 h MA sample as per Fig. 4). Oxidation on progressive milling with less energy may also be explained in terms of the presence of relatively porous pre-existing oxide film on milled powder particle surfaces. Marginally higher amount of energy released on 48 h of milling ( 2860
AV s/mg) is due to the oxidation of new surfaces created because of particle fracturing as per Benjamin’s theory of MA. The TG plots for pure Ti as well as milled powders are given in Fig. 5. The shape of the curve for pure titanium in this figure clearly displays the two-stage oxidation process for pure titanium (a faster rate of oxidation from a temperature of 598 jC (871 K) and a relatively slower oxidation process from 817 jC (1090 K) onwards. As compared to this, the oxidation in the case of 24, 36 and 48 h MA samples occurred in a continuous manner, represented by typical S curves. The multiple TG plots (Fig. 5) also indicate that the percentage oxide formed decreases with increasing milling time (66.1% for pure titanium, 58.3% for 24 h MA sample and 54.4% for 36 h MA sample). The amount of oxide formed after 48 h of milling is slightly higher (56.9%). This is in view of oxidation of fresh surfaces created during MA. The creation of fresh particle surfaces is due to two reasons. Firstly, as a result of fracturing of powder particles on prolonged milling during MA and secondly due to increased brittleness of the oxidized powder particles.
P.B. Joshi et al. / Materials Letters 56 (2002) 322–328
327
Fig. 5. Multiple TG plots for pure Ti and milled powder samples.
The microhardness measurement on powder samples show rise in hardness value with increasing milling time as shown in Fig. 6. Hardness increases
due to cold working of powder particles by milling and formation of a harder phase, TiO2, by reactive milling.
Fig. 6. Variation of microhardness of Ti with respect to milling time.
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P.B. Joshi et al. / Materials Letters 56 (2002) 322–328
4. Conclusion From the above studies, it can be said that attrition of pure titanium powder in normal atmosphere of air can lead to formation of TiO2 as a uniformly dispersed phase on titanium particle surfaces by a process called reactive milling (RM). These reaction-milled Ti–TiO2 composite powders on consolidation and sintering are expected to offer superior high-temperature highstrength properties to the system.
Acknowledgements The authors thank the Research Centre of Indian Petrochemicals Corporation Limited (IPCL), Baroda and M/S TCR Advanced Engineering Pvt. Ltd., Baroda for providing the necessary facilities for materials characterization.
References [1] J.S. Benjamin, Mechanical alloying, Sci. Am. 234 (1976) 40. [2] J.S. Benjamin, M.J. Bomford, Dispersion strengthened aluminium made by mechanical alloying, Metall. Trans. 8A (1977) 1301, Aug. [3] E.S. Bhagiradha Rao, Mechanical alloying of thoria-dispersed nichrome, Trans. IIM 31 (4) (1978) 261, Aug.
[4] J. Naser, W. Richmann, H. Ferkel, Dispersion hardening of metals by nanoscale ceramic powders, Mater. Sci. Eng. A234 – 236 (1997) 467. [5] L. Lu, M.O. Lai, Formation of new materials in the solid state by mechanical alloying, Mater. Design 16 (1) (1995) 33. [6] G.B. Schaffer, P.G. McCormick, Appl. Phys. Lett. 55 (1989) 45. [7] G.B. Schaffer, P.G. McCormick, Review on mechanical alloying, Mater. Forum 16 (1992) 91. [8] K. Tokumitsu, Reduction of metal oxides by mechanical alloying method, Solid State Ionics 101 – 103 (1997) 103. [9] G.B. Schaffer, P.G. McCormick, Displacement reactions during mechanical alloying, Metall. Trans. 21A (1990) 2789, Oct. [10] G.B. Schaffer, P.G. McCormick, Reduction of metal oxides by mechanical alloying, Appl. Phys. Lett. 55 (1) (1989) 45, July. [11] S. Quintana-Molina, J.G. Cabanas-Moreno, R. Martinez-Sanchez, Proc. of World Congress on Powder Metallurgy and Particulate Materials, ASM International, Washington, DC, (1996). [12] V.A. Tracy, D.K. Worn, Powder Metall. (10) (1962) 34. [13] G.B. Alexander et al., US Patent No. 2972,529, Feb. 21 (1961). [14] V.K. Kaushik, XPS core level spectra and Auger parameters for some silver compounds, J. Electron Spectrosc. Relat. Phenom. 56 (1991) 273. [15] Powder Diffraction Files. Joint Committee on Powder Diffraction Standards (JCPDS), Pennsylvania (1967). [16] C.D. Wagner, L.H. Gale, R.H. Raymond, Two dimensional chemical plots: a standardized data set for use in identifying chemical states by X-ray photoelectron spectroscopy, Anal. Chem. 51 (1979) 466. [17] K. Hauffe, Z. Elektrochem. 63 (1959) 819.