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Consolidation of nanocrystalline Al–5 at.% Ti alloy powders by ultra high-pressure hot pressing
Materials Science and Engineering A323 (2002) 293– 300 www.elsevier.com/locate/msea
Consolidation of nanocrystalline Al–5 at.% Ti alloy powders by ultra high-pressure hot pressing Kyoung Il Moon *, Hee Sub Park, Kyung Sub Lee Department of Metallurgical Engineering, Hanyang Uni6ersity, Seoul 133 -791, South Korea Received 11 August 2000; received in revised form 28 March 2001
Abstract Consolidation behavior of nanocrystalline Al–5 at.% Ti powders has been investigated by using ultra high-pressure hot pressing method. Nanocrystalline Al–5 at.% Ti compacts with full density were successfully processed by hot pressing for 250 s at 120°C under 4.8 GPa with a grain size less than 50 nm. It is considered that each grain in as-compacted materials maintained random orientation with respect to its neighboring grains. The consolidation temperature, 120°C, is about 300– 400°C lower than conventional one. Abnormal grain growth was observed in specimens prepared at temperature over 300°C, which was over one half of the absolute melting temperature of Al. Some grains grew up over 500 nm in these specimens. Rockwell hardness and Vickers micro-hardness values of the specimen prepared by the proper conditions were 105.2HRB and 243.7HV, respectively. This hardness value was one of the highest one ever obtained in Al– 5 at.% Ti alloys. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanocrystalline Al–5 at.% Ti; Ultra high-pressure consolidation; Abnormal grain growth; low temperature consolidation.
1. Introduction There have been many efforts to improve the low ductility of Al –Ti alloys at room temperature by alloying additions, grain refinement and heat treatment [1,2]. Special attention has recently been paid to the development of the resulting properties in metals and alloys with nano-sized grains. The desirable grain size for nanocrystalline material is generally below 100 nm, recently below 10 – 20 nm, since this value is in the range that various properties of conventional materials begin to change significantly due to the confinement effects [3–7]. Among many available techniques including the gas condensation method which was most widely used before and through 1990s, mechanical alloying (MA) attracts attention as a versatile method of producing nanocrystalline materials with a broad range of chemical composition and atomic structure [3,8,9]. MA is also considered as a mass production process. In our previous reports [10,11], it was shown that nanocom* Corresponding author. E-mail address: [email protected] (K.I. Moon).
posite Al –Ti alloys were successfully synthesized by reactive ball milling (RBM) in H2 and N2 atmospheres. Systematic studies on initial RBM stage were also performed to investigate the nanocrystallization process [12]. Most synthesizing methods produce nanocrystalline materials as a powder form. Consolidation of powder is required by various techniques such as hot pressing, hot extrusion, hot isostactic pressing (HIP). However, grain growth occurs readily during high temperature consolidation of nanocrystalline materials [13], because nanocrystalline materials are thermally unstable. Thus, the full benefit of nanocrystalline materials may be preserved only if the consolidated specimen would maintain its nano-sized grain. These days, two new consolidation methods are introduced to maintain the fine microstructure: one is to apply high pressure at low temperature and the other is to minimize high temperature exposure time to a few minutes and a few seconds such as plasma activation sintering (PAS) [14] and electro-discharging compaction [15]. Pressure application during sintering is known to achieve full densification of some nanocrystalline powders with minimal grain growth [14]. Many studies have
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demonstrated the effect of applied compressive pressures, especially under ultra high pressure higher than 1 GPa, on the final structure of nanophase materials during consolidation [16,17]. We also expect that severe deformation under ultra high compressive stress to generate ultra high pressure at a proper consolidation temperature may induce recrystallization of specimens and this will have positive effects on refinement of final specimen. In this study, the results on the consolidation behavior and microstructures evolution of nanocrystalline Al –Ti alloy powder, which was prepared by reactive ball milled in H2 [8,18], during hot pressing under ultra high-pressure of 4– 6 GPa are presented.
2. Experimental RBM of elemental powders of Al (− 325 mesh, 99.5% pure) and Ti (−325 mesh, 99.9% pure) was performed to yield Al– 5 at.% Ti composition in a large high energy ball mill (attritor) having a capacity of 7.8 l under flowing H2 gas. Milling time and milling speed were 30 h and 250 rpm, respectively. Total powder weight was 200 g and ball to powder weight ratio was 50:1.2 wt.% stearic acid (CH3(CH2)16COOH) was added as a process control agent. Before milling, the attritor was evacuated by rotary pump up to about 10 − 3 torr and then a pressure of 1.5 atm H2 gas was maintained during milling. As-milled powders after passing through a − 200 mesh sieve were examined by XRD, SEM and TEM. DSC analysis was performed at a heating rate of 10 K s − 1 in a flowing Ar atmosphere to confirm the consolidation temperature of as-milled powder. The consolidation temperature was determined as a temperature at which the complete decomposition of TiH2 and the formation of Al3Ti occurred. RBM process and the analyses on the as-milled powders are given elsewhere [10,12]. After RBM, particle size and grain size of as-milled powder were 20 mm and less than 10 nm, respectively. As-milled powders sieved into − 200 mesh were hot degassed for 20 min at 500°C in a vacuum furnace for the removal of hydrogen and formation of titanium trialuminide prior to consolidation. Since the reaction of hydrogen with titanium during RBM in H2 is reversible, hydrogen can be removed easily from the alloys by vacuum annealing. Hydrogen was used in this study as a temporary alloying element maximizing the refinement of both particle size and grain size of Al–Ti alloy. During the hot degassing process, the grain size and particle size of as-milled powders were maintained. The degassed RBM powders were subsequently consolidated by vacuum (10 − 3 torr) hot pressing under ultra high pressure. For this, 24 g of ball milled and hot degassed powders were compacted in cylindrical Ta can
28 mm in diameter and 20 mm in height. The hot pressing was performed uniaxially in a belt-type highpressure apparatus. The tool was heated by graphite heater. Compressive pressure was generated by hydraulic force and they maintained until full consolidation of the compact was achieved. The pressing pressures were 4.8 and 5.3 GPa and the pressing temperatures were 120, 240, 300 and 360°C. After hot pressing, the height of a specimen was reduced to 12 mm. The densities were measured using helium pycnometry. The microstructure of as-pressed specimens was examined by TEM and HREM. The grain size of consolidated samples was usually estimated by TEM. However, the grain sizes in the specimens having abnormal grain growth were examined by FE-SEM. For FE-SEM investigation, as-pressed specimen were etched with an electrolyte consisting of 50 v/o methanol, 30 v/o hydrochloric acid, 20 v/o nitric acid and one drop of hydrofluoric acid. Since there were big difference in size between small grain and large grain, both small and large grain sizes were represented in the specimens experiencing abnormal grain growth. Hardness was measured with Rockwell hardness tester on the B scale. The Vickers micro-hardness measurement was done at a load of 500 g with a Leitz micro-hardness tester. The load time was 15 s and at least ten measurements were carried out.
3. Results and discussion The early stage of nanocrystallization process of Al– 10 wt.% Ti alloy during RBM in H2 and microstructure changes of as-milled powder during heat treatment which is a degassing process to remove hydrogen in as-milled powder were studied and presented in another manuscript [12]. These results are shown in Fig. 1. TEM micrographs in Fig. 1 show the microstructure changes of Al–10 wt.% Ti with milling time and hot degassing process. After 1 h milling in H2, there were layered structures indicating preferred orientation of the grains inside powder as shown in Fig. 1(a), and grain size in the layered structures was about 20 nm from the observation of dark field image of Fig. 1(b). It is clear from Fig. 1(a) that the milling energy up to 1 h was not enough to form nanocrystalline structure with a random atomic arrangement. Further milling made grain boundary rotated each other and after 50 h milling, a nanocrystalline Al–10 wt.% Ti alloy powder characterized by its small grains less than 10 nm and random orientation in atomic arrangement was obtained as shown in Fig. 1(c). Random orientation of grains in Fig. 1(c) was confirmed by perfect ring patterns in selected area diffraction (SAD) patterns taken from the area of 1 mm2 and direct observation on atomic arrangement in grains by HREM also helped
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elucidate their structure and they were given elsewhere [10]. As thermally metastable materials, nanocrystalline materials lose their unique characteristic by grain growth during heat treatment. For this process, diffusion or mass flow is required for grain boundary migration. Since the high-angle boundary of nanocrystalline material may undergo grain boundary migration or sliding, it is thought that the mentioned process can easily occur at lower driving force. Nanocrystalline Al –10 wt.% Ti alloy powder ball milled in H2 maintained its small grain size well less than 20 nm after hot degassed at 500°C as shown in Fig. 1(d). Such temperature is over 80% of the absolute melting temperature of
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aluminum so that thermal activation energy for grain growth must be enough, but the grain growth did not occurred significantly during heat treatment without mechanical force inducing deformation and during short time heat treatment at high temperature. According to our previous thermal analyses with DSC, the decomposition of TiH2 and subsequent formation of Al3Ti occurred between 370 and 480°C. Thus, the hot degassed temperature of as-milled powder was decided as 500°C [10]. Crystalline materials densify during sintering by mass flow processes of grain boundary diffusion, volume diffusion and plastic flow. Densification occurs because the pores are annihilated at the interparticle
Fig. 1. TEM micrographs of Al –10 wt.% Ti powders: bright field image (a) and dark field image (b) of the powders ball milled in H2 for 1 h; (c) bright field image of the powder ball milled for 50 h in H2; (d) bright field image of the powder ball milled for 50 h in H2 and subsequently heat treated for 10 min at 500°C.
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Table 1 The properties of as-pressed Al–Ti alloys Specimen number
{1} {2} {3} {4} {5} {6} {7} {8}
Hot pressing conditions Pressure (GPa)
Temperature (°C)
Time (s)
4.5 4.8 4.8 4.8 4.8 4.8 5.3 5.3
240 300 240 300 360 120 240 360
50 50 250 250 250 250 250 250
Grain size (nm) (by TEM & FE-SEM)
HV
HRB
Abnormal grain growth
90 90/300a 120 150/300 150/500 50 140 160/600
176.7 156.3 166 144.3 140.5 243.7 149.3 124
91.7 83.8 87.3 74.2 70.5 105.2 85.1 66.8
N Y N Y Y N N Y
a Both small grain size and large grain size are represented in the specimens experiencing abnormal grain growth. All grain sizes are given in approximate value.
boundaries, dislocations, phase boundaries or other microstructural grain boundaries [19]. For most materials, grain boundary diffusion is active during sintering because of its lower activation energy in comparison with that of volume diffusion. This is one reason that nanocrystalline materials where grain boundaries consists of 10–50 vol.% of materials can be successively consolidated in a short period of time at lower temperature. In addition, applied pressure above the yield strength of compacting material increase the contact area between powders by plastic flow so that mass flow occurs more easily during the sintering process and thus aids densification [20]. Moreover, it is also reported in the stainless steel powder that the application of an external pressure in the GPa range makes plastic flow possible at low temperature and full density can be obtained at low temperatures [21]. In this study, eight specimens were prepared from various pressing conditions. The conditions and some properties of as-pressed specimens are presented in Table 1. Due to the fine grain size and enhanced plastic flow effects mentioned above, the full density over 99% of theoretical density was reached even at low temperatures between 120 and 360°C in all Al– 5 at.% Ti alloy compacts prepared by applying ultra high pressure between 4.8 and 5.3 GPa. The relative density of all hot pressed specimens were 99% and over. The most noticeable fact was that a specimen with a full density could be prepared at a temperature as low as 120°C within very short time of 250 s This temperature is about 300–400°C lower than conventional one. Full densification at this low temperature is facilitated not only by very high grain boundary diffusivity due to its nanocrystalline state but also by ultimate high external pressure accelerates densification. The grain boundarydiffusion-controlled shrinkage rate d(DL/L0)/dt is [22] d(DL/L0) 13.3lDBdPE = dt kTG 2
(1)
where T is the absolute temperature, s is Boltzmann’s constant, d is the atomic volume, l is the grain boundary width, DB is the grain boundary diffusivity, G is the grain size, and PE is the effective pressure. According to Eq. (1), if the shrinkage for the effective densification is constant, both a considerable decrease in the grain size and a considerable increase in the pressure can make sintering temperature decrease very much. Fig. 2 shows XRD patterns of Al–5 at.% Ti powders reactive ball milled for 50 h and degassed at 500°C, and
Fig. 2. XRD patterns of Al – 5 at.% Ti powders (a) ball milled for 50 h in H2 and (b) hot degassed at 500°C; XRD patterns of Al–5 at.% Ti specimens (c) hot pressed at 120°C (specimen no. {6}) and (d) 300°C (specimen no. {4}) under a pressure of 4.8 GPa.
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Al – 5 at.% Ti specimens hot pressed at 120 (specimen no. {6}) and 300°C (specimen no. {4}) under a pressure of 4.8 GPa. After 50 h RBM, as-milled powder was consisted of Al and TiH2 phases as seen in Fig. 2(a). The TiH2 phase had a beneficial effect on refinement of both grain size and particle size during RBM, and it decomposed into Ti and realloyed with Al to form Al3Ti during hot degassing at 500°C. However, some amount of Ti remained without transforming to Al3Ti after degassing at 500°C and Ti peak was still observed in XRD pattern of as-degassed powder in Fig. 2(b). Al3Ti was generally formed between 360 and 480°C in Al –Ti powder RBM’ed in H2 according to our previous experiment [18] and DSC analysis on the powder prepared in this experiment also showed a similar behavior compared with the previous result. Thus, the XRD result on the as-degassed powder failed to our expectation that all Ti elements must be transformed into Al3Ti after hot degassing process. This was considered to be related to the fact that the heat treatment was not enough to induce the expected phase transformation. However, XRD pattern of a specimen hot pressed at 120°C in Fig. 2(c) showed that Ti peak decreased with increasing of Al3Ti peak meaning that Ti was transformed into Al3Ti by reaction with Al at low temperature. Although 120°C was not enough temperature to form Al3Ti phase, additional heat could be produced by mechanical deformation during ultra highpressure hot pressing and this could offer sufficient energy to form Al3Ti. The Ti peak disappeared after hot pressing at 300°C resulting from complete transformation of Ti to Al3Ti, as shown in Fig. 2(d). Because of very fast consolidation at low temperature, specimen no. {6} maintained its grain size as 50 nm. The size is considered as one of the smallest one ever obtained in Al– Ti system by various consolidation methods. And thus, its hardness values were very high compared with nanocrystalline Al– Ti compacts by other methods such as hot extrusion and PAS process [18,23]. However, the grain size increased rapidly with increasing temperature under ultra high pressure. Especially, abnormal grain growth was observed in specimens prepared at temperature over 300°C, which was over one half of the absolute melting temperature of Al. Some grains grew up over 500 nm under ultra high pressure as indicated in Table 1. Since the abnormal grain growth occurred above 300°C, temperature is considered as a main factor inducing the abnormal grain growth. It is thought that the abnormal grain growth observed in the specimens prepared above 300°C is related with a dynamic recrystallization process. In this study, abnormal grain growths were observed in specimens with high-pressure application above 300°C. Thus, it is considered that a specific hydraulic force to induce high pressure is another important
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factor causing an abnormal grain growth during consolidation. An abnormal grain growth was not found in the hot extruded specimens although they had long time processes consisting of hot degassing and pre-heat treatment exposed at 500°C without applying pressure, but, under applying pressure of 1.5 GPa, its heating time is only 10 min during hot extrusion process [18]. Abnormal grain growth has been generally reported both in ceramic and metal during consolidation processes under high pressure [24,25]. Besson and Abouaf [24] reported that the grain size of hot isostatic pressed alumina specimens increased with applied HIP pressure. They also showed that abnormal grain growth was observed in specimen prepared under pressure while it was not observed during sintering without pressure even after consolidation for long time (100 h) at high temperature (1400°C). It is also well established that grain growth can be coupled with deformation in fine-grained materials [26]. According to Clark and Alden [26], grain boundary sliding and grain rotations occur as a deformation in fine-grained specimens. And if the misorientation of two grains is eliminated by grain boundary sliding, then the two original grains will have coalesced into one. After all, it is confirmed that the abnormal grain growth occurs in case that proper deformations are applied at proper temperature. Thus, it is believed that this abnormal grain growth may be a dynamic recrystallization process. Fig. 3 shows TEM micrographs of the hot pressed specimens. The grain size of specimen no. {6} observed from dark field image was less than 50 nm as shown in Fig. 3(a) and SAD patterns of 1 mm2 area were nearly perfect ring type meaning random misorientation of each grain in Fig. 3(b). TEM dark field image of Fig. 3(c) shows that specimen no. {4} consisted of small grains less than 100 nm and very large grain with sizes of 300–500 nm. SAD patterns of Fig. 3(d) were spot patterns not showing any trace of ring patterns seen in the as-milled powder. This means that the grains in this specimens lost random orientation properties which are considered as a characteristic of the nanocrystalline materials. In a large grained specimen in Fig. 3(e), highly deformed areas, which were considered as dislocation networks and indicated as 1–3, were present. Sub-grain boundaries were also shown in this specimen and they were hard to see in this figure but indicated as 4, 5. Such defects were not observed in grains which might be experienced complete grain growth, as seen Fig. 3(f). Besson and Abouaf [24] also reported that such defects were easily observed in the specimens suffered enhanced grain growth. More detailed subgrain microstructures were investigated in hot pressed specimens by HREM. Fig. 4 shows that four grains in the specimen no. {6} indicated as 1–4 had one direction in atomic arrangement. This could be another
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Fig. 3. TEM micrographs of specimens hot pressed under a pressure of 4.8 GPa: (a) dark field image and (b) SAD patterns of Al –5 at.% Ti specimen hot pressed at 120°C (specimen no. {6}); (c) dark field image, (d) SAD patterns, (e) bright field image and (f) bright field image of Al –5 at.% Ti specimen hot pressed at 300°C (specimen no. {4}).
evidence that the random orientation properties of nanocrystalline grains might be lost gradually during consolidation process.
Table 2 shows the results of the specimens obtained from various MA and consolidation processes by our research team. The final grain size was smaller in
K.I. Moon et al. / Materials Science and Engineering A323 (2002) 293–300
Fig. 4. HREM micrographs of Al –5 at.% Ti specimens hot pressed at 120°C (specimen no. {6}) under a pressure of 4.8 GPa.
specimens using the powders prepared by more effective MA process such as RBM and low temperature MA. The reason was that as-milled powders produced by RBM and low temperature MA had more refined grain and particle sizes and thus they could be consolidated to full density more effectively within a shorter time at lower temperature. The specimens prepared by plasma activation sintering (PAS) at 400°C had grain sizes less than 50 nm. However, full density could not be achieved in these specimens. Because a graphite mold was used in PAS process, a sintering pressure could not exceed 70 MPa. The force to generate the pressure of this value was considered as too small to induce plastic flow for sintering between powders, thus RBM powder could not be densified at 400°C. It was fully densified at 500°C and the grain size of such specimen was 50–70 nm. The powders prepared by low temperature MA
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had very small grain size less than 20 nm, thus they could be consolidated at lower temperature of 400°C by vacuum hot pressing (VHP) process. However, processing time was very long under 500 MPa, 3 h, in VHP process, an abnormal grain growth was observed in all the specimens. This indicated the possibility that abnormal grain growth could be occurred under low pressure below GPa level if heating time is enough long. In VHP specimen, small grain size was less than 50nm while large grain resulting from abnormal grain growth was a few mm. The specimens hot extruded from RBM powders had grain sizes between 50 and 100 nm because heating time under high pressure corresponding to 1.5 GPa was only 10 min in this process, although hot extrusion was long time and high temperature process compared with other processes. In hot extruded specimens, an abnormal grain growth was not observed. In this study using hot pressing under ultra high-pressure of 4.8–5.3 GPa, the smallest grain size was obtained at 120°C and 50 nm. Abnormal grain growth was not observed in this specimen, thus this specimen was considered as the best one maintaining the properties of nanocrystalline materials among all specimens prepared by various methods. During the ultra high-pressure hot pressing, various specimens with broad range of grain sizes were obtained. Mechanical tests of as-prepared specimens are in progress by compression test.
4. Conclusions Nanocrystalline Al–5 at.% Ti alloy was successively synthesized by RBM in H2 and hot pressing for 250 s at 120°C under 4.8 GPa and its grain size was less than 50 nm. This is considered as a proper consolidation condition in this study. It is also considered that as-compacted materials maintain the unique grain boundary properties of nanocrystalline material, that is, high-an-
Table 2 The comparison of grain sizes in various consolidated specimens Types of milling and consolidation
Grain size (by TEM) (nm)
Relative density
Ref.
RBM in H2 and hot pressing under ultra high pressure (4.8–5.4 GPa)
50–150/500 Abnormal GGa 50–100 30–70 Al: 300–500 Al3Ti: 100–150 20–50 nm/a few mm Abnormal GG
99% and over
This study
99% and over 80–99%b 99% and over
[18] [23] [27]
99% and over
[28]
RBM in H2 and hot extrusion RBM in H2 and PAS MA in Ar and hot extrusionc MA at low temperature of −85°C and hot pressing under pressures of 300 and 500 MPad
a
Grain growth. Full density could not be achieved in PAS specimens having grain size less than 50 nm. c Specimen composition is Al–8 wt.% Ti. d Because of long time heat treatment, abnormal grain growth is observed in all specimens. b
300
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gle grain boundary. The consolidation temperature of 120°C is about 300– 400°C lower than conventional one. Abnormal grain growth was observed in specimens prepared at temperature over 300°C, which was over one half of absolute melting temperature of Al. Some grains grew up over 500 nm in this specimens. Hardness and micro-hardness values of the specimen prepared by the proper conditions were 105.2HRB and 243.7HV, respectively. This hardness value was one of the highest one ever obtained in Al–5 at.% Ti alloys.
Acknowledgements The authors acknowledge gratefully the financial support of the Korea Science and Engineering Foundation [98-0300-03-01-5]. This work was also supported by the Brain Korea 21 project.
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[7] S.R. Agnew, B.R. Elliott, C.J. Youngdahl, K.J. Hemker, J.R. Weertman, Mater. Sci. Eng. A285 (2000) 391. [8] M. Sherif El-Eskandarany, J. Alloys Comp. 305 (2000) 225. [9] J.S. Benjamin, in: P.H. Shingu (Ed.), Journal of Material Science Forum, vols. 88 – 90, Trans Tech Publication Ltd, Zurich, 1992, p. 3. [10] K.I. Moon, K.S. Lee, J. Alloys Comp. 264 (1998) 258. [11] M.S. Oh, K.I. Moon, K.S. Lee, J. Alloys Comp. 279 (1998) 201. [12] K.I. Moon, K.S. Lee, in: J.J. Oakes, J.H. Reinshagen (Eds.), Advances in powder Metallurgy and Particulate Materials — 1998, vol. 2, MPIF, Princeton, NJ, 1998, p. 789. [13] H. Hahn, J. Logas, R.S. Averback, J. Mater. Res. 5 (1990) 609. [14] J.R. Groza, R.J. Dowding, Nanostructured Mater. 7 (1996) 749. [15] D.K. Kim, K. Okazaki, in: P.H. Shingu (Ed.), Journal of Material Science Forum, vols. 88 – 90, Trans Tech Publication Ltd, Zurich, 1992, p. 553. [16] J.-P. Ahn, J.-K. Park, M.-Y. Huh, J. Am. Ceram. Soc. 80 (1997) 2165. [17] M.S. Haji-Mahmood, L.S. Chumbley, Nanostructured Mater. 7 (1996) 95. [18] K.I. Moon, K.S. Lee, J. Alloys Comp. 291 (1999) 312. [19] R.M. German, Sintering Theory and Practice, Wiley, New York, 1996, p. 95. [20] N.A. Fleck, L.T. Kuhn, R.M. McMeeking, J. Mech. Phys. Solids 40 (1992) 1139. [21] E.Y. Gutmanas, Powder Met. Intern. 12 (1980) 178. [22] M.R. Notis, R.H. Smoak, V. Kirshnamachari, in: G.C. Kuczynski (Ed.), Sintering and Catalysis, Plenum Press, New York, 1975, pp. 493 – 507. [23] J.R. Ryu, K.I. Moon, K.S. Lee, J. Alloys Comp. 296 (2000) 157. [24] J. Besson, M. Abouaf, Acta Metall. 39 (1991) 2225. [25] T.G. Nieh, P. Luo, W. Nellis, D. Lesuer, D. Benson, Acta Metall. 44 (1996) 3781. [26] M.A. Clark, T.H. Alden, Acta Metall. 32 (1984) 1335. [27] B.J. Kim, K.I. Moon, K.S. Lee, J. Alloys Comp. 292 (1999) 174. [28] J.H. Choi, K.I. Moon, J.K. Kim, Y.M. Oh, J.H. Suh, S.J. Kim, J. Alloys Comp. 315 (2001) 178.
Consolidation of nanocrystalline Al–5 at.% Ti alloy powders by ultra high-pressure hot pressing Kyoung Il Moon *, Hee Sub Park, Kyung Sub Lee Department of Metallurgical Engineering, Hanyang Uni6ersity, Seoul 133 -791, South Korea Received 11 August 2000; received in revised form 28 March 2001
Abstract Consolidation behavior of nanocrystalline Al–5 at.% Ti powders has been investigated by using ultra high-pressure hot pressing method. Nanocrystalline Al–5 at.% Ti compacts with full density were successfully processed by hot pressing for 250 s at 120°C under 4.8 GPa with a grain size less than 50 nm. It is considered that each grain in as-compacted materials maintained random orientation with respect to its neighboring grains. The consolidation temperature, 120°C, is about 300– 400°C lower than conventional one. Abnormal grain growth was observed in specimens prepared at temperature over 300°C, which was over one half of the absolute melting temperature of Al. Some grains grew up over 500 nm in these specimens. Rockwell hardness and Vickers micro-hardness values of the specimen prepared by the proper conditions were 105.2HRB and 243.7HV, respectively. This hardness value was one of the highest one ever obtained in Al– 5 at.% Ti alloys. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Nanocrystalline Al–5 at.% Ti; Ultra high-pressure consolidation; Abnormal grain growth; low temperature consolidation.
1. Introduction There have been many efforts to improve the low ductility of Al –Ti alloys at room temperature by alloying additions, grain refinement and heat treatment [1,2]. Special attention has recently been paid to the development of the resulting properties in metals and alloys with nano-sized grains. The desirable grain size for nanocrystalline material is generally below 100 nm, recently below 10 – 20 nm, since this value is in the range that various properties of conventional materials begin to change significantly due to the confinement effects [3–7]. Among many available techniques including the gas condensation method which was most widely used before and through 1990s, mechanical alloying (MA) attracts attention as a versatile method of producing nanocrystalline materials with a broad range of chemical composition and atomic structure [3,8,9]. MA is also considered as a mass production process. In our previous reports [10,11], it was shown that nanocom* Corresponding author. E-mail address: [email protected] (K.I. Moon).
posite Al –Ti alloys were successfully synthesized by reactive ball milling (RBM) in H2 and N2 atmospheres. Systematic studies on initial RBM stage were also performed to investigate the nanocrystallization process [12]. Most synthesizing methods produce nanocrystalline materials as a powder form. Consolidation of powder is required by various techniques such as hot pressing, hot extrusion, hot isostactic pressing (HIP). However, grain growth occurs readily during high temperature consolidation of nanocrystalline materials [13], because nanocrystalline materials are thermally unstable. Thus, the full benefit of nanocrystalline materials may be preserved only if the consolidated specimen would maintain its nano-sized grain. These days, two new consolidation methods are introduced to maintain the fine microstructure: one is to apply high pressure at low temperature and the other is to minimize high temperature exposure time to a few minutes and a few seconds such as plasma activation sintering (PAS) [14] and electro-discharging compaction [15]. Pressure application during sintering is known to achieve full densification of some nanocrystalline powders with minimal grain growth [14]. Many studies have
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demonstrated the effect of applied compressive pressures, especially under ultra high pressure higher than 1 GPa, on the final structure of nanophase materials during consolidation [16,17]. We also expect that severe deformation under ultra high compressive stress to generate ultra high pressure at a proper consolidation temperature may induce recrystallization of specimens and this will have positive effects on refinement of final specimen. In this study, the results on the consolidation behavior and microstructures evolution of nanocrystalline Al –Ti alloy powder, which was prepared by reactive ball milled in H2 [8,18], during hot pressing under ultra high-pressure of 4– 6 GPa are presented.
2. Experimental RBM of elemental powders of Al (− 325 mesh, 99.5% pure) and Ti (−325 mesh, 99.9% pure) was performed to yield Al– 5 at.% Ti composition in a large high energy ball mill (attritor) having a capacity of 7.8 l under flowing H2 gas. Milling time and milling speed were 30 h and 250 rpm, respectively. Total powder weight was 200 g and ball to powder weight ratio was 50:1.2 wt.% stearic acid (CH3(CH2)16COOH) was added as a process control agent. Before milling, the attritor was evacuated by rotary pump up to about 10 − 3 torr and then a pressure of 1.5 atm H2 gas was maintained during milling. As-milled powders after passing through a − 200 mesh sieve were examined by XRD, SEM and TEM. DSC analysis was performed at a heating rate of 10 K s − 1 in a flowing Ar atmosphere to confirm the consolidation temperature of as-milled powder. The consolidation temperature was determined as a temperature at which the complete decomposition of TiH2 and the formation of Al3Ti occurred. RBM process and the analyses on the as-milled powders are given elsewhere [10,12]. After RBM, particle size and grain size of as-milled powder were 20 mm and less than 10 nm, respectively. As-milled powders sieved into − 200 mesh were hot degassed for 20 min at 500°C in a vacuum furnace for the removal of hydrogen and formation of titanium trialuminide prior to consolidation. Since the reaction of hydrogen with titanium during RBM in H2 is reversible, hydrogen can be removed easily from the alloys by vacuum annealing. Hydrogen was used in this study as a temporary alloying element maximizing the refinement of both particle size and grain size of Al–Ti alloy. During the hot degassing process, the grain size and particle size of as-milled powders were maintained. The degassed RBM powders were subsequently consolidated by vacuum (10 − 3 torr) hot pressing under ultra high pressure. For this, 24 g of ball milled and hot degassed powders were compacted in cylindrical Ta can
28 mm in diameter and 20 mm in height. The hot pressing was performed uniaxially in a belt-type highpressure apparatus. The tool was heated by graphite heater. Compressive pressure was generated by hydraulic force and they maintained until full consolidation of the compact was achieved. The pressing pressures were 4.8 and 5.3 GPa and the pressing temperatures were 120, 240, 300 and 360°C. After hot pressing, the height of a specimen was reduced to 12 mm. The densities were measured using helium pycnometry. The microstructure of as-pressed specimens was examined by TEM and HREM. The grain size of consolidated samples was usually estimated by TEM. However, the grain sizes in the specimens having abnormal grain growth were examined by FE-SEM. For FE-SEM investigation, as-pressed specimen were etched with an electrolyte consisting of 50 v/o methanol, 30 v/o hydrochloric acid, 20 v/o nitric acid and one drop of hydrofluoric acid. Since there were big difference in size between small grain and large grain, both small and large grain sizes were represented in the specimens experiencing abnormal grain growth. Hardness was measured with Rockwell hardness tester on the B scale. The Vickers micro-hardness measurement was done at a load of 500 g with a Leitz micro-hardness tester. The load time was 15 s and at least ten measurements were carried out.
3. Results and discussion The early stage of nanocrystallization process of Al– 10 wt.% Ti alloy during RBM in H2 and microstructure changes of as-milled powder during heat treatment which is a degassing process to remove hydrogen in as-milled powder were studied and presented in another manuscript [12]. These results are shown in Fig. 1. TEM micrographs in Fig. 1 show the microstructure changes of Al–10 wt.% Ti with milling time and hot degassing process. After 1 h milling in H2, there were layered structures indicating preferred orientation of the grains inside powder as shown in Fig. 1(a), and grain size in the layered structures was about 20 nm from the observation of dark field image of Fig. 1(b). It is clear from Fig. 1(a) that the milling energy up to 1 h was not enough to form nanocrystalline structure with a random atomic arrangement. Further milling made grain boundary rotated each other and after 50 h milling, a nanocrystalline Al–10 wt.% Ti alloy powder characterized by its small grains less than 10 nm and random orientation in atomic arrangement was obtained as shown in Fig. 1(c). Random orientation of grains in Fig. 1(c) was confirmed by perfect ring patterns in selected area diffraction (SAD) patterns taken from the area of 1 mm2 and direct observation on atomic arrangement in grains by HREM also helped
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elucidate their structure and they were given elsewhere [10]. As thermally metastable materials, nanocrystalline materials lose their unique characteristic by grain growth during heat treatment. For this process, diffusion or mass flow is required for grain boundary migration. Since the high-angle boundary of nanocrystalline material may undergo grain boundary migration or sliding, it is thought that the mentioned process can easily occur at lower driving force. Nanocrystalline Al –10 wt.% Ti alloy powder ball milled in H2 maintained its small grain size well less than 20 nm after hot degassed at 500°C as shown in Fig. 1(d). Such temperature is over 80% of the absolute melting temperature of
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aluminum so that thermal activation energy for grain growth must be enough, but the grain growth did not occurred significantly during heat treatment without mechanical force inducing deformation and during short time heat treatment at high temperature. According to our previous thermal analyses with DSC, the decomposition of TiH2 and subsequent formation of Al3Ti occurred between 370 and 480°C. Thus, the hot degassed temperature of as-milled powder was decided as 500°C [10]. Crystalline materials densify during sintering by mass flow processes of grain boundary diffusion, volume diffusion and plastic flow. Densification occurs because the pores are annihilated at the interparticle
Fig. 1. TEM micrographs of Al –10 wt.% Ti powders: bright field image (a) and dark field image (b) of the powders ball milled in H2 for 1 h; (c) bright field image of the powder ball milled for 50 h in H2; (d) bright field image of the powder ball milled for 50 h in H2 and subsequently heat treated for 10 min at 500°C.
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Table 1 The properties of as-pressed Al–Ti alloys Specimen number
{1} {2} {3} {4} {5} {6} {7} {8}
Hot pressing conditions Pressure (GPa)
Temperature (°C)
Time (s)
4.5 4.8 4.8 4.8 4.8 4.8 5.3 5.3
240 300 240 300 360 120 240 360
50 50 250 250 250 250 250 250
Grain size (nm) (by TEM & FE-SEM)
HV
HRB
Abnormal grain growth
90 90/300a 120 150/300 150/500 50 140 160/600
176.7 156.3 166 144.3 140.5 243.7 149.3 124
91.7 83.8 87.3 74.2 70.5 105.2 85.1 66.8
N Y N Y Y N N Y
a Both small grain size and large grain size are represented in the specimens experiencing abnormal grain growth. All grain sizes are given in approximate value.
boundaries, dislocations, phase boundaries or other microstructural grain boundaries [19]. For most materials, grain boundary diffusion is active during sintering because of its lower activation energy in comparison with that of volume diffusion. This is one reason that nanocrystalline materials where grain boundaries consists of 10–50 vol.% of materials can be successively consolidated in a short period of time at lower temperature. In addition, applied pressure above the yield strength of compacting material increase the contact area between powders by plastic flow so that mass flow occurs more easily during the sintering process and thus aids densification [20]. Moreover, it is also reported in the stainless steel powder that the application of an external pressure in the GPa range makes plastic flow possible at low temperature and full density can be obtained at low temperatures [21]. In this study, eight specimens were prepared from various pressing conditions. The conditions and some properties of as-pressed specimens are presented in Table 1. Due to the fine grain size and enhanced plastic flow effects mentioned above, the full density over 99% of theoretical density was reached even at low temperatures between 120 and 360°C in all Al– 5 at.% Ti alloy compacts prepared by applying ultra high pressure between 4.8 and 5.3 GPa. The relative density of all hot pressed specimens were 99% and over. The most noticeable fact was that a specimen with a full density could be prepared at a temperature as low as 120°C within very short time of 250 s This temperature is about 300–400°C lower than conventional one. Full densification at this low temperature is facilitated not only by very high grain boundary diffusivity due to its nanocrystalline state but also by ultimate high external pressure accelerates densification. The grain boundarydiffusion-controlled shrinkage rate d(DL/L0)/dt is [22] d(DL/L0) 13.3lDBdPE = dt kTG 2
(1)
where T is the absolute temperature, s is Boltzmann’s constant, d is the atomic volume, l is the grain boundary width, DB is the grain boundary diffusivity, G is the grain size, and PE is the effective pressure. According to Eq. (1), if the shrinkage for the effective densification is constant, both a considerable decrease in the grain size and a considerable increase in the pressure can make sintering temperature decrease very much. Fig. 2 shows XRD patterns of Al–5 at.% Ti powders reactive ball milled for 50 h and degassed at 500°C, and
Fig. 2. XRD patterns of Al – 5 at.% Ti powders (a) ball milled for 50 h in H2 and (b) hot degassed at 500°C; XRD patterns of Al–5 at.% Ti specimens (c) hot pressed at 120°C (specimen no. {6}) and (d) 300°C (specimen no. {4}) under a pressure of 4.8 GPa.
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Al – 5 at.% Ti specimens hot pressed at 120 (specimen no. {6}) and 300°C (specimen no. {4}) under a pressure of 4.8 GPa. After 50 h RBM, as-milled powder was consisted of Al and TiH2 phases as seen in Fig. 2(a). The TiH2 phase had a beneficial effect on refinement of both grain size and particle size during RBM, and it decomposed into Ti and realloyed with Al to form Al3Ti during hot degassing at 500°C. However, some amount of Ti remained without transforming to Al3Ti after degassing at 500°C and Ti peak was still observed in XRD pattern of as-degassed powder in Fig. 2(b). Al3Ti was generally formed between 360 and 480°C in Al –Ti powder RBM’ed in H2 according to our previous experiment [18] and DSC analysis on the powder prepared in this experiment also showed a similar behavior compared with the previous result. Thus, the XRD result on the as-degassed powder failed to our expectation that all Ti elements must be transformed into Al3Ti after hot degassing process. This was considered to be related to the fact that the heat treatment was not enough to induce the expected phase transformation. However, XRD pattern of a specimen hot pressed at 120°C in Fig. 2(c) showed that Ti peak decreased with increasing of Al3Ti peak meaning that Ti was transformed into Al3Ti by reaction with Al at low temperature. Although 120°C was not enough temperature to form Al3Ti phase, additional heat could be produced by mechanical deformation during ultra highpressure hot pressing and this could offer sufficient energy to form Al3Ti. The Ti peak disappeared after hot pressing at 300°C resulting from complete transformation of Ti to Al3Ti, as shown in Fig. 2(d). Because of very fast consolidation at low temperature, specimen no. {6} maintained its grain size as 50 nm. The size is considered as one of the smallest one ever obtained in Al– Ti system by various consolidation methods. And thus, its hardness values were very high compared with nanocrystalline Al– Ti compacts by other methods such as hot extrusion and PAS process [18,23]. However, the grain size increased rapidly with increasing temperature under ultra high pressure. Especially, abnormal grain growth was observed in specimens prepared at temperature over 300°C, which was over one half of the absolute melting temperature of Al. Some grains grew up over 500 nm under ultra high pressure as indicated in Table 1. Since the abnormal grain growth occurred above 300°C, temperature is considered as a main factor inducing the abnormal grain growth. It is thought that the abnormal grain growth observed in the specimens prepared above 300°C is related with a dynamic recrystallization process. In this study, abnormal grain growths were observed in specimens with high-pressure application above 300°C. Thus, it is considered that a specific hydraulic force to induce high pressure is another important
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factor causing an abnormal grain growth during consolidation. An abnormal grain growth was not found in the hot extruded specimens although they had long time processes consisting of hot degassing and pre-heat treatment exposed at 500°C without applying pressure, but, under applying pressure of 1.5 GPa, its heating time is only 10 min during hot extrusion process [18]. Abnormal grain growth has been generally reported both in ceramic and metal during consolidation processes under high pressure [24,25]. Besson and Abouaf [24] reported that the grain size of hot isostatic pressed alumina specimens increased with applied HIP pressure. They also showed that abnormal grain growth was observed in specimen prepared under pressure while it was not observed during sintering without pressure even after consolidation for long time (100 h) at high temperature (1400°C). It is also well established that grain growth can be coupled with deformation in fine-grained materials [26]. According to Clark and Alden [26], grain boundary sliding and grain rotations occur as a deformation in fine-grained specimens. And if the misorientation of two grains is eliminated by grain boundary sliding, then the two original grains will have coalesced into one. After all, it is confirmed that the abnormal grain growth occurs in case that proper deformations are applied at proper temperature. Thus, it is believed that this abnormal grain growth may be a dynamic recrystallization process. Fig. 3 shows TEM micrographs of the hot pressed specimens. The grain size of specimen no. {6} observed from dark field image was less than 50 nm as shown in Fig. 3(a) and SAD patterns of 1 mm2 area were nearly perfect ring type meaning random misorientation of each grain in Fig. 3(b). TEM dark field image of Fig. 3(c) shows that specimen no. {4} consisted of small grains less than 100 nm and very large grain with sizes of 300–500 nm. SAD patterns of Fig. 3(d) were spot patterns not showing any trace of ring patterns seen in the as-milled powder. This means that the grains in this specimens lost random orientation properties which are considered as a characteristic of the nanocrystalline materials. In a large grained specimen in Fig. 3(e), highly deformed areas, which were considered as dislocation networks and indicated as 1–3, were present. Sub-grain boundaries were also shown in this specimen and they were hard to see in this figure but indicated as 4, 5. Such defects were not observed in grains which might be experienced complete grain growth, as seen Fig. 3(f). Besson and Abouaf [24] also reported that such defects were easily observed in the specimens suffered enhanced grain growth. More detailed subgrain microstructures were investigated in hot pressed specimens by HREM. Fig. 4 shows that four grains in the specimen no. {6} indicated as 1–4 had one direction in atomic arrangement. This could be another
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Fig. 3. TEM micrographs of specimens hot pressed under a pressure of 4.8 GPa: (a) dark field image and (b) SAD patterns of Al –5 at.% Ti specimen hot pressed at 120°C (specimen no. {6}); (c) dark field image, (d) SAD patterns, (e) bright field image and (f) bright field image of Al –5 at.% Ti specimen hot pressed at 300°C (specimen no. {4}).
evidence that the random orientation properties of nanocrystalline grains might be lost gradually during consolidation process.
Table 2 shows the results of the specimens obtained from various MA and consolidation processes by our research team. The final grain size was smaller in
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Fig. 4. HREM micrographs of Al –5 at.% Ti specimens hot pressed at 120°C (specimen no. {6}) under a pressure of 4.8 GPa.
specimens using the powders prepared by more effective MA process such as RBM and low temperature MA. The reason was that as-milled powders produced by RBM and low temperature MA had more refined grain and particle sizes and thus they could be consolidated to full density more effectively within a shorter time at lower temperature. The specimens prepared by plasma activation sintering (PAS) at 400°C had grain sizes less than 50 nm. However, full density could not be achieved in these specimens. Because a graphite mold was used in PAS process, a sintering pressure could not exceed 70 MPa. The force to generate the pressure of this value was considered as too small to induce plastic flow for sintering between powders, thus RBM powder could not be densified at 400°C. It was fully densified at 500°C and the grain size of such specimen was 50–70 nm. The powders prepared by low temperature MA
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had very small grain size less than 20 nm, thus they could be consolidated at lower temperature of 400°C by vacuum hot pressing (VHP) process. However, processing time was very long under 500 MPa, 3 h, in VHP process, an abnormal grain growth was observed in all the specimens. This indicated the possibility that abnormal grain growth could be occurred under low pressure below GPa level if heating time is enough long. In VHP specimen, small grain size was less than 50nm while large grain resulting from abnormal grain growth was a few mm. The specimens hot extruded from RBM powders had grain sizes between 50 and 100 nm because heating time under high pressure corresponding to 1.5 GPa was only 10 min in this process, although hot extrusion was long time and high temperature process compared with other processes. In hot extruded specimens, an abnormal grain growth was not observed. In this study using hot pressing under ultra high-pressure of 4.8–5.3 GPa, the smallest grain size was obtained at 120°C and 50 nm. Abnormal grain growth was not observed in this specimen, thus this specimen was considered as the best one maintaining the properties of nanocrystalline materials among all specimens prepared by various methods. During the ultra high-pressure hot pressing, various specimens with broad range of grain sizes were obtained. Mechanical tests of as-prepared specimens are in progress by compression test.
4. Conclusions Nanocrystalline Al–5 at.% Ti alloy was successively synthesized by RBM in H2 and hot pressing for 250 s at 120°C under 4.8 GPa and its grain size was less than 50 nm. This is considered as a proper consolidation condition in this study. It is also considered that as-compacted materials maintain the unique grain boundary properties of nanocrystalline material, that is, high-an-
Table 2 The comparison of grain sizes in various consolidated specimens Types of milling and consolidation
Grain size (by TEM) (nm)
Relative density
Ref.
RBM in H2 and hot pressing under ultra high pressure (4.8–5.4 GPa)
50–150/500 Abnormal GGa 50–100 30–70 Al: 300–500 Al3Ti: 100–150 20–50 nm/a few mm Abnormal GG
99% and over
This study
99% and over 80–99%b 99% and over
[18] [23] [27]
99% and over
[28]
RBM in H2 and hot extrusion RBM in H2 and PAS MA in Ar and hot extrusionc MA at low temperature of −85°C and hot pressing under pressures of 300 and 500 MPad
a
Grain growth. Full density could not be achieved in PAS specimens having grain size less than 50 nm. c Specimen composition is Al–8 wt.% Ti. d Because of long time heat treatment, abnormal grain growth is observed in all specimens. b
300
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gle grain boundary. The consolidation temperature of 120°C is about 300– 400°C lower than conventional one. Abnormal grain growth was observed in specimens prepared at temperature over 300°C, which was over one half of absolute melting temperature of Al. Some grains grew up over 500 nm in this specimens. Hardness and micro-hardness values of the specimen prepared by the proper conditions were 105.2HRB and 243.7HV, respectively. This hardness value was one of the highest one ever obtained in Al–5 at.% Ti alloys.
Acknowledgements The authors acknowledge gratefully the financial support of the Korea Science and Engineering Foundation [98-0300-03-01-5]. This work was also supported by the Brain Korea 21 project.
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