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Formation of fcc titanium during heating high-energy, ball-milled Al–Ti powders
August 2001
Materials Letters 50 Ž2001. 149–153 www.elsevier.comrlocatermatlet
Formation of fcc titanium during heating high-energy, ball-milled Al–Ti powders D.L. Zhang ) , D.Y. Ying Department of Materials and Process Engineering, The UniÕersity of Waikato, PriÕate Bag 3105, Hamilton, New Zealand Received 2 November 2000; accepted 12 November 2000
Abstract Phase transformation during high-energy milling Al–25 at.%Ti and Ti–25 at.%Al powders and during heating of ball-milled powders of the same compositions was studied. It was found that a small amount of Ti underwent an allotropic transformation from hcp Ti to fcc Ti during milling. This transformation also occurred during heating the properly milled Al–25 at.%Ti and Ti–25 at.%Al powders. The transformation was endothermic, and the onset temperature of this transformation was 3218C. It is likely that only thin Ti layers which have a nanometer-scale thickness and are embedded in Al matrix can undergo this transformation. q 2001 Elsevier Science B.V. All rights reserved. Keywords: fcc titanium; Endothermic transformation; High-energy ball milling; Al–Ti powders
1. Introduction It has been well established that under equilibrium conditions the crystal structure of solid titanium metal is either hexagonal close packed Žhcp. Ž a phase. or body centre cubic Žbcc. Žb phase. depending on temperature and the type and amount of solute atoms w1x. The a phase is a low-temperature phase, while b phase is a high-temperature phase. Recently, it has been reported that hcp Ti can be transformed into fcc Ti in multilayer Ti–Al, Ti–Ni, and Ti–Ag thin films during thinning the cross-section of the thin films using ion beams w2–5x. Fcc Ti was also observed in TirAl multilayer thin films with very small thickness Ž; 5 nm. w6x. On the other hand, Kado w7x
) Corresponding author. Tel.: q64-7-838-4783; fax: q64-7838-4835. E-mail address: [email protected] ŽD.L. Zhang..
reported that fcc Ti formed when a layer of Ti with a thickness of a few nanometers was deposited on a MgO single crystal substrate. This paper describes the results of a study, which demonstrates that hcp Ti can be transformed into fcc Ti during high-energy ball milling of Al–Ti powders and during subsequent heating of the mechanically milled powders, and that this transformation is endothermic.
2. Experimental technique A mixture of elemental powders of 99.97% pure Al Žaverage particle size: 50 mm. and 99.9% pure Ti Žaverage particle size: 50mm. was subjected to high-energy ball milling in a SPEX 8000 MixerrMill. The nominal composition of the powder mixture was controlled to be either Ti–25 at.%Al or Al–25 at.%Ti. To prepare for the milling, 3 g of Al–Ti
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 4 3 3 - X
150
D.L. Zhang, D.Y. Ying r Materials Letters 50 (2001) 149–153
powder mixture and four stainless steel balls with a diameter of 12.5 mm, were sealed in a hardened steel vial in a glove box filled with 99.9% pure argon. The composition of the milled powder was analysed by using inductive coupled plasma mass spectrometer, and it was confirmed that the powders contained less than 0.16 wt.%Fe and 0.02 wt.%Cr. The microstructure of the milled powder was examined by using X-ray diffraction ŽXRD. analysis, optical microscopy and scanning electron microscopy ŽSEM.. The XRD analysis was performed in a Philips X-Pert system diffractometer with Cu K a radiation and a copper single crystal monochromator. A Hitachi S4000 Scanning Electron Microscope operated at a voltage of 20 kV was utilised for the SEM examination. Samples of the ball-milled powders were analyzed using a TA Instruments DSC2920 differential scanning calorimeter ŽDSC. under flowing argon. The heating rate was 208Crmin.
3. Results and discussion Fig. 1 shows a typical SEM micrograph of the powder particle cross-section of an Al–25 at.%Ti powder after milling for 8 h. As shown in Fig. 1, after the Al–Ti powder mixture was milled for 8 h, most of the Ti phase was deformed into discontinuous thin layers, which were embedded in the Al
Fig. 1. SEM backscattered electron micrograph of the cross-section of the powder particles in an Al–25 at.%Ti powder after milling for 8 h. The dark phase is Al and the bright phase is Ti.
Fig. 2. DSC trace of the Al–25 at.%Ti powder after milling for 8 h.
matrix. The thickness of the Ti layers was in the range of 0.2–5 mm based on SEM examination. Fig. 2 shows a typical DSC trace obtained by heating the 8-h milled Al–25 at.%Ti powder. The DSC trace exhibits a clear endothermic peak with an onset temperature of 3218C and a peak temperature of 3298C, and a few exothermic peaks in the temperature range of 350–5508C. The exothermic peaks were caused by the reaction between Al and Ti, forming a disordered AlŽTi. solid solution, and the subsequent transformation from the solid solution into ordered Al 3Ti and Al 5Ti 2 intermetallic compounds. The details of the study on the exothermic peaks has been described and discussed in another paper w8x, so will not be discussed here. The observation of an endothermic peak on the DSC trace was a surprise. In order to find out the cause for the endothermic reaction, the 8-h milled Al–25 at.%Ti powder was heated in DSC from room temperature to 350 and 5508C, respectively, cooled to room temperature, and then analysed using XRD. Fig. 3Ža. – Žc. shows the XRD patterns of the as-milled powder and the heat-treated powders, respectively. The majority of the peaks in the XRD pattern of the as-milled powder can be attributed to fcc Al and hcp Ti. However, there is a small peak at 2 u s 59.68, which could not be attributed to fcc Al, hcp Ti, or any of the intermediate phases in the Al–Ti binary system. When the as-milled powder was heated to 3508C, the intensity of this peak increased dramatically, and in the mean time a new peak appeared at 2 u s 41.08 appeared. The ratio between the intensity of the hcp Ti 10.04 peak and
D.L. Zhang, D.Y. Ying r Materials Letters 50 (2001) 149–153
151
Table 1 The positions and relative intensities of the new peaks in Fig. 3, which can be attributed to a new fcc phase
Fig. 3. XRD patterns of the 8-h milled Al–25 at.%Ti powders under Ža. as-milled condition and and after being heated to Žb. 3508C and Žc. 5508C, respectively.
that of the hcp Ti 10.14 peak also increased significantly from 0.298 to 0.465. It was also noticed that each of the Al peaks in the pattern had a tail on the right. These tails are likely due to formation of a small amount of AlŽTi. solid solution at temperatures below 3508C. After the powder was heated to 5508C, most of the hcp Ti peaks Žincluding the strongest hcp Ti 10.14 peak. disappeared from the XRD pattern, but the peak at 2 u s 35.38, which was previously attributed to hcp Ti 10.04 peak was still in the pattern. Since the disappearance of the strongest peak of hcp Ti means that hcp Ti was consumed by the reactions which occur during heating to 5508C, it is not reasonable to attribute the peak at 2 u s 35.38 to hcp Ti 1004 peak any more. This suggests that this peak is also a new peak. In the mean time, new peaks at 2 u s 71.08 and 2 u s 74.78 were also evident in the pattern and the peaks at 2 u s 41.08 and 2 u s 59.68 were still present. There were other new peaks in the XRD pattern, but they could be attributed to Al 5Ti 2 compound, which was formed through transformation from the disordered AlŽTi. solid solution formed at lower temperatures w8x. The five new peaks which could not be attributed to Al 5Ti 2 or Al 3Ti or any other known Al xTi y intermediate phases are listed in Table 1.
Peak no.
2 u Ž8.
d Spacing Žnm.
Intensity
Corresponding plane of fcc phase
1 2 3 4 5
35.3 41 59.6 71 74.4
0.2538 0.2198 0.1552 0.1327 0.1269
100 30 30 35 25
1114 2004 2204 3114 2224
As shown in Table 1, the new peaks can be easily indexed as peaks of an fcc phase with a lattice parameter a s 0.4396 nm. It is now becomes clear that the endothermic peak on the DSC trace of the 8-h milled Al–25 at.%Ti powder was caused by a transformation to form the new fcc phase. The lattice parameter of this fcc phase is almost same as the lattice parameter of the fcc Ti observed by Jankowski and Wall w2x and Van Heerden et al. w3x Žthe difference is only 0.5%.. When the 8-h milled Al–35 at.%Ti powder was heated to 6508C, the peaks due to fcc Ti disappeared from XRD pattern. Fig. 4 shows a typical SEM backscattered electron micrograph of the powder particles in the Ti–25 at.%Al powder after milling for 4 h. The majority of the Al phase was deformed into thin layers which were embedded in the titanium matrix. The thickness of the Al layers was in the range of 0.2–2 mm.
Fig. 4. SEM backscattered electron micrograph of the cross-section of the powder particles in a Ti–25 at.%Al powder after milling for 4 h. The dark phase is Al and the bright phase is Ti.
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D.L. Zhang, D.Y. Ying r Materials Letters 50 (2001) 149–153
However, in some regions, thin titanium layers embedded inside Al were also observed Žas shown by the arrow in Fig. 4.. Fig. 5 shows a typical DSC trace of the 4-h milled Ti–25 at.%Al powder. As shown in Fig. 5, the DSC trace exhibited an endothermic peak with an onset temperature of 3218C and a peak temperature of 3298C, and an exothermic peak with an onset temperature of 4398C and a peak temperature of 4968C. The exothermic peak corresponded to reaction between Ti and Al, forming an hcp TiŽAl. solid solution w8x. To identify the reaction corresponding to the endothermic peak, the 4-h milled powder Ti–25 at.%Al powder was heated to 3608C in DSC, cooled to room temperature, and then analysed using XRD. Fig. 6Ža. and Žb. shows the XRD patterns of the as-milled and heat-treated Ti–25 at.%Al powder. Most of the peaks in the XRD pattern of the as-milled powder could be attributed to fcc Al and hcp Ti phases, but similar to the 8-h milled Al–25 at.%Ti powder, there was a weak but clear peak at 2 u s 59.68. After the powder was heated to 3608C, this peak became substantially stronger while the other peaks remained largely unchanged. In the mean time, a new peak also appeared at 2 u s 41.08. As discussed above, these new peaks could be attributed to the new fcc phase. The other peaks of the fcc phase overlap with hcp Ti, so they could not be unambiguously identified. The appearance of the new peaks indicates that the endothermic peak corresponded to formation of the new fcc phase. When the powder was heated to 5508C, the peaks of fcc Ti disappeared from the XRD pattern. It is certain that the new phase formed during milling and during subsequent heating of the mechanical milled powders has an fcc structure. It is
Fig. 5. DSC trace of the Ti–25 at.%Ti powder after milling for 4 h.
Fig. 6. XRD patterns of the 4-h milled Ti–25 at.%Al powders under Ža. as-milled condition and Žb. after being heated to 3608C.
very likely that the new phase is fcc Ti. One may argue that the new fcc phase may not necessarily be fcc Ti, and that the new phase might be titanium hydride compound which also has a cubic structure with a lattice parameter a s 0.4454 nm, close to the lattice parameter of the new phase Žthe difference is 1.3%.. However, both the reaction between titanium and hydrogen forming titanium hydride and the precipitation of titanium hydride from TiŽH. solid solution are exothermic, and the present study clearly shows that the reaction which leads to formation of the new fcc phase is endothermic, rather than exothermic. Therefore, this new phase cannot be titanium hydride. For the same reason, the possibility for this new phase to be a metastable titanium suboxides can also be discounted. Therefore, the new fcc phase formed through the endothermic transformation during heating the mechanically milled Al–Ti powders is fcc Ti. It appears that formation of Ti thin layer in the powder particle is an essential precondition for the formation of fcc Ti during milling or heating, as this transformation was not observed when the Al–25 at.%Ti powder was milled for only 4 h. This is in agreement with the previous observation that hcp Ti can be transformed into fcc Ti in Ti–Al multilayer thin films w2,3x. Apparently only a small fraction of the hcp Ti in the mechanically milled powders was transformed into fcc Ti. This fraction of Ti might be
D.L. Zhang, D.Y. Ying r Materials Letters 50 (2001) 149–153
those Ti layers with sufficiently small thickness. Further work is necessary to determine the critical thickness needed to facilitate this transformation. With prolonged milling, the fraction of hcp Ti layers with sufficiently small thickness will increase. However, the thin hcp Ti layers are also likely to react with Al at temperature below 3218C, so the increase of the volume fraction of very thin hcp Ti layers may not necessarily increase the amount of hcp Ti to be transformed into fcc Ti. It is interesting to note that once hcp Ti is transformed into fcc Ti, its reactivity with Al is reduced as evidenced by the earlier consumption of hcp Ti by the Al–Ti reaction than that of fcc Ti during heating. The fact that the endothermic transformation from hcp Ti to fcc Ti only occurs until the powder is heated to 3218C shows that this transformation needs to be activated. The temperature needed for this activation is quite low and can be reached in milling process, so the small amount of fcc Ti formed during milling may also be a result of the thermally activated transformation from hcp Ti to fcc Ti. From this, it is possible to postulate that the previous observation that the hcp ™ fcc transformation only occurs when the thin film was thinned using ion beam milling may be due to the need of activation of such transformation. In this case, ion bombardment exerts same activation effect as heating, i.e. to make the atoms more active.
4. Conclusions A small amount of Ti undergoes an allotropic transformation from hcp Ti to fcc Ti during high-en-
153
ergy milling Al–25 at.%Ti and Ti–25 at.%Al powders. This transformation also occurs during heating the properly milled Al–25 at.%Ti and Ti–25 at.%Al powders. The transformation is endothermic, and the onset temperature of this transformation is 3218C. It is likely that only the thin Ti layers, which have a nanometer scale thickness and are embedded in Al matrix can undergo this transformation.
Acknowledgements The authors would like to thank the School of Science and Technology, University of Waikato, for the financial support provided to the research on processing of powder materials.
References w1x I.J. Polmear, Light Alloys: Metallurgy of The Light Metals, 3rd edn., Arbold, London, 1995, 248. w2x A.F. Jankowski, M.A. Wall, J. Mater. Res. 9 Ž1994. 31. w3x D. van Heerden, D. Josell, D. Shechtman, Acta Mater. 44 Ž1996. 297. w4x D. Josell, D. Shechtman, D. van Heerden, Mater. Lett. 22 Ž1995. 275. w5x T. Tepper, D. Shechtman, D. van Heerden, D. Josell, Mater. Lett. 33 Ž1997. 181. w6x R. Banerjee, S.A. Dregia, H.L. Fraser, Acta Mater. 47 Ž1999. 4225. w7x T. Kado, Surf. Sci. 454–456 Ž2000. 783. w8x D.L. Zhang, D.Y. Ying, in: D.L. Zhang, K.L. Pickering, X.X. Xiong ŽEds.., Proc. of 1st International Conference on Advanced Materials Processing, Institute of Materials Engineering, Australasia, Melbourne, 2000, p. 23.
Materials Letters 50 Ž2001. 149–153 www.elsevier.comrlocatermatlet
Formation of fcc titanium during heating high-energy, ball-milled Al–Ti powders D.L. Zhang ) , D.Y. Ying Department of Materials and Process Engineering, The UniÕersity of Waikato, PriÕate Bag 3105, Hamilton, New Zealand Received 2 November 2000; accepted 12 November 2000
Abstract Phase transformation during high-energy milling Al–25 at.%Ti and Ti–25 at.%Al powders and during heating of ball-milled powders of the same compositions was studied. It was found that a small amount of Ti underwent an allotropic transformation from hcp Ti to fcc Ti during milling. This transformation also occurred during heating the properly milled Al–25 at.%Ti and Ti–25 at.%Al powders. The transformation was endothermic, and the onset temperature of this transformation was 3218C. It is likely that only thin Ti layers which have a nanometer-scale thickness and are embedded in Al matrix can undergo this transformation. q 2001 Elsevier Science B.V. All rights reserved. Keywords: fcc titanium; Endothermic transformation; High-energy ball milling; Al–Ti powders
1. Introduction It has been well established that under equilibrium conditions the crystal structure of solid titanium metal is either hexagonal close packed Žhcp. Ž a phase. or body centre cubic Žbcc. Žb phase. depending on temperature and the type and amount of solute atoms w1x. The a phase is a low-temperature phase, while b phase is a high-temperature phase. Recently, it has been reported that hcp Ti can be transformed into fcc Ti in multilayer Ti–Al, Ti–Ni, and Ti–Ag thin films during thinning the cross-section of the thin films using ion beams w2–5x. Fcc Ti was also observed in TirAl multilayer thin films with very small thickness Ž; 5 nm. w6x. On the other hand, Kado w7x
) Corresponding author. Tel.: q64-7-838-4783; fax: q64-7838-4835. E-mail address: [email protected] ŽD.L. Zhang..
reported that fcc Ti formed when a layer of Ti with a thickness of a few nanometers was deposited on a MgO single crystal substrate. This paper describes the results of a study, which demonstrates that hcp Ti can be transformed into fcc Ti during high-energy ball milling of Al–Ti powders and during subsequent heating of the mechanically milled powders, and that this transformation is endothermic.
2. Experimental technique A mixture of elemental powders of 99.97% pure Al Žaverage particle size: 50 mm. and 99.9% pure Ti Žaverage particle size: 50mm. was subjected to high-energy ball milling in a SPEX 8000 MixerrMill. The nominal composition of the powder mixture was controlled to be either Ti–25 at.%Al or Al–25 at.%Ti. To prepare for the milling, 3 g of Al–Ti
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 4 3 3 - X
150
D.L. Zhang, D.Y. Ying r Materials Letters 50 (2001) 149–153
powder mixture and four stainless steel balls with a diameter of 12.5 mm, were sealed in a hardened steel vial in a glove box filled with 99.9% pure argon. The composition of the milled powder was analysed by using inductive coupled plasma mass spectrometer, and it was confirmed that the powders contained less than 0.16 wt.%Fe and 0.02 wt.%Cr. The microstructure of the milled powder was examined by using X-ray diffraction ŽXRD. analysis, optical microscopy and scanning electron microscopy ŽSEM.. The XRD analysis was performed in a Philips X-Pert system diffractometer with Cu K a radiation and a copper single crystal monochromator. A Hitachi S4000 Scanning Electron Microscope operated at a voltage of 20 kV was utilised for the SEM examination. Samples of the ball-milled powders were analyzed using a TA Instruments DSC2920 differential scanning calorimeter ŽDSC. under flowing argon. The heating rate was 208Crmin.
3. Results and discussion Fig. 1 shows a typical SEM micrograph of the powder particle cross-section of an Al–25 at.%Ti powder after milling for 8 h. As shown in Fig. 1, after the Al–Ti powder mixture was milled for 8 h, most of the Ti phase was deformed into discontinuous thin layers, which were embedded in the Al
Fig. 1. SEM backscattered electron micrograph of the cross-section of the powder particles in an Al–25 at.%Ti powder after milling for 8 h. The dark phase is Al and the bright phase is Ti.
Fig. 2. DSC trace of the Al–25 at.%Ti powder after milling for 8 h.
matrix. The thickness of the Ti layers was in the range of 0.2–5 mm based on SEM examination. Fig. 2 shows a typical DSC trace obtained by heating the 8-h milled Al–25 at.%Ti powder. The DSC trace exhibits a clear endothermic peak with an onset temperature of 3218C and a peak temperature of 3298C, and a few exothermic peaks in the temperature range of 350–5508C. The exothermic peaks were caused by the reaction between Al and Ti, forming a disordered AlŽTi. solid solution, and the subsequent transformation from the solid solution into ordered Al 3Ti and Al 5Ti 2 intermetallic compounds. The details of the study on the exothermic peaks has been described and discussed in another paper w8x, so will not be discussed here. The observation of an endothermic peak on the DSC trace was a surprise. In order to find out the cause for the endothermic reaction, the 8-h milled Al–25 at.%Ti powder was heated in DSC from room temperature to 350 and 5508C, respectively, cooled to room temperature, and then analysed using XRD. Fig. 3Ža. – Žc. shows the XRD patterns of the as-milled powder and the heat-treated powders, respectively. The majority of the peaks in the XRD pattern of the as-milled powder can be attributed to fcc Al and hcp Ti. However, there is a small peak at 2 u s 59.68, which could not be attributed to fcc Al, hcp Ti, or any of the intermediate phases in the Al–Ti binary system. When the as-milled powder was heated to 3508C, the intensity of this peak increased dramatically, and in the mean time a new peak appeared at 2 u s 41.08 appeared. The ratio between the intensity of the hcp Ti 10.04 peak and
D.L. Zhang, D.Y. Ying r Materials Letters 50 (2001) 149–153
151
Table 1 The positions and relative intensities of the new peaks in Fig. 3, which can be attributed to a new fcc phase
Fig. 3. XRD patterns of the 8-h milled Al–25 at.%Ti powders under Ža. as-milled condition and and after being heated to Žb. 3508C and Žc. 5508C, respectively.
that of the hcp Ti 10.14 peak also increased significantly from 0.298 to 0.465. It was also noticed that each of the Al peaks in the pattern had a tail on the right. These tails are likely due to formation of a small amount of AlŽTi. solid solution at temperatures below 3508C. After the powder was heated to 5508C, most of the hcp Ti peaks Žincluding the strongest hcp Ti 10.14 peak. disappeared from the XRD pattern, but the peak at 2 u s 35.38, which was previously attributed to hcp Ti 10.04 peak was still in the pattern. Since the disappearance of the strongest peak of hcp Ti means that hcp Ti was consumed by the reactions which occur during heating to 5508C, it is not reasonable to attribute the peak at 2 u s 35.38 to hcp Ti 1004 peak any more. This suggests that this peak is also a new peak. In the mean time, new peaks at 2 u s 71.08 and 2 u s 74.78 were also evident in the pattern and the peaks at 2 u s 41.08 and 2 u s 59.68 were still present. There were other new peaks in the XRD pattern, but they could be attributed to Al 5Ti 2 compound, which was formed through transformation from the disordered AlŽTi. solid solution formed at lower temperatures w8x. The five new peaks which could not be attributed to Al 5Ti 2 or Al 3Ti or any other known Al xTi y intermediate phases are listed in Table 1.
Peak no.
2 u Ž8.
d Spacing Žnm.
Intensity
Corresponding plane of fcc phase
1 2 3 4 5
35.3 41 59.6 71 74.4
0.2538 0.2198 0.1552 0.1327 0.1269
100 30 30 35 25
1114 2004 2204 3114 2224
As shown in Table 1, the new peaks can be easily indexed as peaks of an fcc phase with a lattice parameter a s 0.4396 nm. It is now becomes clear that the endothermic peak on the DSC trace of the 8-h milled Al–25 at.%Ti powder was caused by a transformation to form the new fcc phase. The lattice parameter of this fcc phase is almost same as the lattice parameter of the fcc Ti observed by Jankowski and Wall w2x and Van Heerden et al. w3x Žthe difference is only 0.5%.. When the 8-h milled Al–35 at.%Ti powder was heated to 6508C, the peaks due to fcc Ti disappeared from XRD pattern. Fig. 4 shows a typical SEM backscattered electron micrograph of the powder particles in the Ti–25 at.%Al powder after milling for 4 h. The majority of the Al phase was deformed into thin layers which were embedded in the titanium matrix. The thickness of the Al layers was in the range of 0.2–2 mm.
Fig. 4. SEM backscattered electron micrograph of the cross-section of the powder particles in a Ti–25 at.%Al powder after milling for 4 h. The dark phase is Al and the bright phase is Ti.
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D.L. Zhang, D.Y. Ying r Materials Letters 50 (2001) 149–153
However, in some regions, thin titanium layers embedded inside Al were also observed Žas shown by the arrow in Fig. 4.. Fig. 5 shows a typical DSC trace of the 4-h milled Ti–25 at.%Al powder. As shown in Fig. 5, the DSC trace exhibited an endothermic peak with an onset temperature of 3218C and a peak temperature of 3298C, and an exothermic peak with an onset temperature of 4398C and a peak temperature of 4968C. The exothermic peak corresponded to reaction between Ti and Al, forming an hcp TiŽAl. solid solution w8x. To identify the reaction corresponding to the endothermic peak, the 4-h milled powder Ti–25 at.%Al powder was heated to 3608C in DSC, cooled to room temperature, and then analysed using XRD. Fig. 6Ža. and Žb. shows the XRD patterns of the as-milled and heat-treated Ti–25 at.%Al powder. Most of the peaks in the XRD pattern of the as-milled powder could be attributed to fcc Al and hcp Ti phases, but similar to the 8-h milled Al–25 at.%Ti powder, there was a weak but clear peak at 2 u s 59.68. After the powder was heated to 3608C, this peak became substantially stronger while the other peaks remained largely unchanged. In the mean time, a new peak also appeared at 2 u s 41.08. As discussed above, these new peaks could be attributed to the new fcc phase. The other peaks of the fcc phase overlap with hcp Ti, so they could not be unambiguously identified. The appearance of the new peaks indicates that the endothermic peak corresponded to formation of the new fcc phase. When the powder was heated to 5508C, the peaks of fcc Ti disappeared from the XRD pattern. It is certain that the new phase formed during milling and during subsequent heating of the mechanical milled powders has an fcc structure. It is
Fig. 5. DSC trace of the Ti–25 at.%Ti powder after milling for 4 h.
Fig. 6. XRD patterns of the 4-h milled Ti–25 at.%Al powders under Ža. as-milled condition and Žb. after being heated to 3608C.
very likely that the new phase is fcc Ti. One may argue that the new fcc phase may not necessarily be fcc Ti, and that the new phase might be titanium hydride compound which also has a cubic structure with a lattice parameter a s 0.4454 nm, close to the lattice parameter of the new phase Žthe difference is 1.3%.. However, both the reaction between titanium and hydrogen forming titanium hydride and the precipitation of titanium hydride from TiŽH. solid solution are exothermic, and the present study clearly shows that the reaction which leads to formation of the new fcc phase is endothermic, rather than exothermic. Therefore, this new phase cannot be titanium hydride. For the same reason, the possibility for this new phase to be a metastable titanium suboxides can also be discounted. Therefore, the new fcc phase formed through the endothermic transformation during heating the mechanically milled Al–Ti powders is fcc Ti. It appears that formation of Ti thin layer in the powder particle is an essential precondition for the formation of fcc Ti during milling or heating, as this transformation was not observed when the Al–25 at.%Ti powder was milled for only 4 h. This is in agreement with the previous observation that hcp Ti can be transformed into fcc Ti in Ti–Al multilayer thin films w2,3x. Apparently only a small fraction of the hcp Ti in the mechanically milled powders was transformed into fcc Ti. This fraction of Ti might be
D.L. Zhang, D.Y. Ying r Materials Letters 50 (2001) 149–153
those Ti layers with sufficiently small thickness. Further work is necessary to determine the critical thickness needed to facilitate this transformation. With prolonged milling, the fraction of hcp Ti layers with sufficiently small thickness will increase. However, the thin hcp Ti layers are also likely to react with Al at temperature below 3218C, so the increase of the volume fraction of very thin hcp Ti layers may not necessarily increase the amount of hcp Ti to be transformed into fcc Ti. It is interesting to note that once hcp Ti is transformed into fcc Ti, its reactivity with Al is reduced as evidenced by the earlier consumption of hcp Ti by the Al–Ti reaction than that of fcc Ti during heating. The fact that the endothermic transformation from hcp Ti to fcc Ti only occurs until the powder is heated to 3218C shows that this transformation needs to be activated. The temperature needed for this activation is quite low and can be reached in milling process, so the small amount of fcc Ti formed during milling may also be a result of the thermally activated transformation from hcp Ti to fcc Ti. From this, it is possible to postulate that the previous observation that the hcp ™ fcc transformation only occurs when the thin film was thinned using ion beam milling may be due to the need of activation of such transformation. In this case, ion bombardment exerts same activation effect as heating, i.e. to make the atoms more active.
4. Conclusions A small amount of Ti undergoes an allotropic transformation from hcp Ti to fcc Ti during high-en-
153
ergy milling Al–25 at.%Ti and Ti–25 at.%Al powders. This transformation also occurs during heating the properly milled Al–25 at.%Ti and Ti–25 at.%Al powders. The transformation is endothermic, and the onset temperature of this transformation is 3218C. It is likely that only the thin Ti layers, which have a nanometer scale thickness and are embedded in Al matrix can undergo this transformation.
Acknowledgements The authors would like to thank the School of Science and Technology, University of Waikato, for the financial support provided to the research on processing of powder materials.
References w1x I.J. Polmear, Light Alloys: Metallurgy of The Light Metals, 3rd edn., Arbold, London, 1995, 248. w2x A.F. Jankowski, M.A. Wall, J. Mater. Res. 9 Ž1994. 31. w3x D. van Heerden, D. Josell, D. Shechtman, Acta Mater. 44 Ž1996. 297. w4x D. Josell, D. Shechtman, D. van Heerden, Mater. Lett. 22 Ž1995. 275. w5x T. Tepper, D. Shechtman, D. van Heerden, D. Josell, Mater. Lett. 33 Ž1997. 181. w6x R. Banerjee, S.A. Dregia, H.L. Fraser, Acta Mater. 47 Ž1999. 4225. w7x T. Kado, Surf. Sci. 454–456 Ž2000. 783. w8x D.L. Zhang, D.Y. Ying, in: D.L. Zhang, K.L. Pickering, X.X. Xiong ŽEds.., Proc. of 1st International Conference on Advanced Materials Processing, Institute of Materials Engineering, Australasia, Melbourne, 2000, p. 23.