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Magnesium nanocomposite via mechanochemical milling
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 64 (2004) 2009–2014 www.elsevier.com/locate/compscitech
Magnesium nanocomposite via mechanochemical milling L. L€ u a
a,*
, M.O. Lai a, W. Liang
b
Department of Mechanical Engineering, The National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore b Taiyuan University of Technology, China Received 23 August 2003; received in revised form 12 January 2004; accepted 20 February 2004 Available online 10 May 2004
Abstract Nanocomposite Mg5wt%Al–10.3%Ti was synthesized via mechanochemical milling of elemental powders of Mg, Al and Ti with polyethylene–glycol. Formation of TiH2 was observed after milling and the concentration of TiH2 was found to further increase after sintering. The nanocomposite shows an improvement in yield strength and ductility compared to its counterpart fabricated using a conventional powder metallurgy (P/M) process. The increase in mechanical properties is associated with the ultra fine grain size and the presence of nano-dispersoids in the matrix. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Nanostructured alloy
1. Introduction Light-weight magnesium alloys have recently received much attention due to their attractive properties. Although Mg alloys are generally believed to possess relatively low yield strength and poor ductility [1], they can be strengthened by incorporating either intermetallics or ceramic particulates [2–4]. Currently, most Mg-based composites have been fabricated through casting routes for the reason of cost effectiveness in comparison to other process techniques [5–7]. However, microstructures obtained from traditional cast generally reveal coarse structures. In addition, reactions between the Mg matrix and the reinforcements cause degradation of the composites [8]. Another difficulty encountered in the processing of Mg composites by casting is the incorporation of fine ceramic particulates. In most cases, only large ceramic particulates with size in the range of a few micrometer to tenth micrometer particulates are practically applicable. Some attempts in using mechanical or mechanochemical millings have been introduced in the fabrication of extremely fine and/or nanostructured Mgbased composites [9–11]. It has been found that strength
*
Corresponding author. Tel.: +65-6874-2236; fax: +65-6779-1459. E-mail address: [email protected] (L. L€ u).
0266-3538/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2004.02.018
of nanostructured Mg-based composites can be increased but on the cost of ductility [12,13]. The cause of ductility decrease is mainly due to selection of reinforcements. The present research focuses on the formation of nanocomposites with ultra-fine in situ TiH2 . Microstructure and mechanical properties of the composites are studied. Mechanisms of the formation of TiH2 will also be discussed. 2. Experimental procedures Mg alloys of nominal composition Mg5%Al–10.3%Ti were prepared via powder metallurgy and mechanochemical milling routes. Elemental powders of Mg of purity >98.5%, Al of purity 99.5% and Ti of purity >98% were used in the processes. For the alloy prepared by powder metallurgy method, the powders were mixed in a V-blender at 45 rpm for 2 h. For the composite prepared by mechanochemical milling process, the Mg, 5%Al and 10.3%Ti powder mixture was milled using a Fritsch planetary ball mill operating at 250 rpm. Forty 15 mm diameter balls were employed. A ball-to-powder weight ratio of about 20:1 was maintained. Polyethylene–glycol (H(OCH2 -CH2 )n OH) was added to the
L. L€u et al. / Composites Science and Technology 64 (2004) 2009–2014
3. Results and discussion 3.1. Structures Fig. 1(a) shows the spectrum of the as-mechanochemically milled power mixture prior to any heat treatment process. Several features of the diffraction can be observed. Firstly, all the Mg and Ti diffraction peaks can be observed to be broadened indicating a reduction in grain size. Due to the broadening, (1 0 0) Ti peak completely overlaps with (0 0 2) Mg peak after the milling. In spite of the overlap, the rest of the Ti diffraction peaks, like (0 0 2), (1 0 1) and (1 0 2), can still be clearly identified. In addition, it is clear that some new structures have been formed after the milling as indicated by the presence of the new peaks at 40.80° and 59.00°. It is generally understood that the solubility of Mg in Ti or vice versa is very small and negligible. The solubility has been found to decrease sharply with addition of Al in Mg [12]. It has been reported that solubility of Mg in Ti can be increased by mechanical alloying. However, the increase in alloy addition would lead to a shift in diffraction peaks due to the change in the lattice parameters. Therefore, the formation of the new peaks must have been originated from the forma-
Mg5Al10.3Ti Mg Ti TiH2 Al3Ti
(100) (200)
(100)
(004)
(110) (220)
(102)
(101)
(a) Milledpowder
(111)
(b) Milled + extruded (311) (222)
(220) (213)
(211)
(200) (004)
powder mixture with the purposes of prevention of agglomeration and excessive cold welding of the powders, and decomposition of it. To gain enough hydrogen, 6 wt% of polyethylene–glycol was used. Prior to mechanical alloying, powder mixtures were sealed with 99.9% pure argon gas in milling vials after evacuation with a vacuum pump. After mixing and mechanochemical milling, the powders were isostatically cold-pressed at 400 MPa to cylinders of about 40 mm diameter and 50 mm length. The compacts were then sintered at 450 °C for 2 h in a vacuum furnace. Extrusion of the compacts with an extrusion ration of 25:1 was carried out at 400 °C using graphite as a lubricant. The extruded rods were subjected to post heat treatments at three different temperatures of 350, 400 and 450 °C for 5 and 8 h. Tensile specimens of 5 mm diameter and 25 mm gauge length which cut from middle of the extrude rods were machined in accordance to ASTM E8M-96. Tensile testing was carried out at strain rate of 1.0 mm/min using an automated servo hydraulic INSTRON machine. A clip-on extensometer with 25 mm gauge length was used to record the displacement. X-ray diffraction (XRD) analysis was carried out using a Shimadzu Lab XRD-6000 X-ray diffractometer radiation. The crystalline with Cu Ka k ¼ 1:54056 A size was evaluated by the Hall–Williamson equation and transmission electron microscope (TEM).
Intensity (A.U.)
2010
(c) As-extruded
30
40
50
60
70
80
Diffraction angle (2θ˚) . Fig. 1. X-ray spectra of mechanochemically milled powder mixture and extruded rods: (a) as-milled powder; (b) extruded rod after 20 h of milling; (c) as-extruded rod directly from powder metallurgically sintered compact.
tion of new phases. Comparing the XRD spectrum of the milled specimens shown in Fig. 1(a) to that of the sintered and extruded specimen shown in Fig. 1(c), it can be seen that the diffraction peaks at 40.80° and 59.00° in the as-sintered specimen have become undetectable. The observation implies that the formation of the new phase is directly associated with the milling. Since polyethylene–glycol (H(OCH2 CH2 )n OH) consists mainly of hydrogen and Ti is very sensitive to it, Ti hydride could directly be formed during mechanochmical alloying via the decomposition of H(OCH2 CH2 )n OH through: ð1 þ 2nÞTi þ HðOCH2 CH2 Þn OH ! ð1 þ 2nÞTiH2 þ ð1 þ nÞCO þ ðn 1ÞC If it is assumed that n ¼ 1, it follows that 3Ti þ HðOCH2 CH2 ÞOH ! 3TiH2 þ 2CO The carbon monoxide evaporates as gas. From the above analysis it is clear that the two new diffraction peaks at 40.80° and 59.00° are TiH2 (2 0 0) and (2 2 0) at standard diffraction of 40.798° and 59.177°, respectively. From thermodynamic point of view, formation of TiH2 is possible since the excess standard Gibbs free energy of formation of TiH2 is )14.96 kJ/mol. It is interesting to note from Fig. 1(b) that Ti (0 0 2), (1 0 1) and (1 0 2) peaks completely disappear from the extruded rod that has been sintered after 20 h of milling.
L. L€u et al. / Composites Science and Technology 64 (2004) 2009–2014
Five TiH2 (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) diffraction peaks are clearly revealed after the extrusion. In addition to the TiH2 diffractions, two other peaks at 42.52° and 54.4° close to Al3 Ti (0 0 4) and (2 1 1) diffraction are also detected. Fig. 1(c) shows the diffraction spectrum of the asextruded specimen without milling. Very distinguished Mg diffractions can be observed. In addition, the diffraction peak from Ti can also be discovered even though (1 0 0) Ti peak overlaps with (0 0 2) Mg diffraction peak. Some traces of Al12 Mg17 diffraction are evident but they are not clear from the as-extruded specimens. The formation of in situ TiH2 can be divided into two stages. In the first stage, H(OCH2 CH2 )n OH is partially decomposed since it is not as stable as other types of processing control agent such as stearic acid [13]. Due to the small particles and grain sizes of both Mg and Ti, and the large amount of fresh surfaces created after some time of milling, hydrogen atoms may partially react with the Ti particles forming TiH2 . At the same time, free hydrogen atoms may also diffuse into the Mg matrix forming solid solution since the solubility of hydrogen in Mg is high even at room temperature. In the second stage, hydrogen atoms solutionized in Mg is released during sintering and these reacts with the remaining Ti form TiH2 . It should be noted that although Mg and Ti are immiscible below the melting of Mg in conventional processes, it has been found that solubility of Ti in Mg can be extended to about 6% by mechanochemical milling as a result of the formation of nanocrystalline structure [14–16]. Therefore, a part of the 10.3 wt% elemental Ti could be solid-soluted into Mg with the formation of nanostructured Mg during high energy mechanical alloying. Since the mechanochemically alloyed Mg is supersaturated with Ti, Ti will precipitate in Mg grains upon subsequent sintering. At the same time, hydrogen atoms are also released from the Mg lattice at high temperature. The Ti precipitates and the hydrogen released finally react with each other forming ultra-fine TiH2 particles. 3.2. Microstructure Fig. 2 shows TEM image of the mechanochemically milled and extruded specimen. The average grain size of the extruded specimen is about 30 nm although a few of the grains are in the range of about 90 nm. Fig. 2(b) is the SAD pattern of the nanograins. The diffraction is identified to be from Mg with index (1 0 0), (0 0 2), (0 1 1), ( 1 1 2) and (0 1 3) counting from the inner ring. Besides the fine grain size, other features can also be observed from the TEM image. It is noted that there exists a large amount of fine particles with size of a few nanometers. These nanoparticles are in principle ho-
2011
Fig. 2. TEM image of mechanochemically milled and extruded rod: (a) bright image showing nanograins with ultra fine dispersoids; (b) SAD pattern showing Mg diffraction.
mogeneously distributed in Mg matrix. Since the nanoparticles are too small to be identified, it is speculated that they may be MgO and TiH2 . The nano MgO particles may have originated from the fracture of oxide surfaces of the powder particles during milling and also from possible oxidation during milling while the nano TiH2 particles could have been formed during the sintering process when supersaturated Ti precipitated from Mg reacted with the free hydrogen atoms released from Mg. Besides the presence of nano-particles, some long ‘‘rod-like’’ features are also observed in microstructures and shown in Fig. 3. The width of the rods ranges from 3 to 7 nm. Fig. 4 shows the TEM image of the P/M processed specimen. It is clear that the grain size is much large than that of its milled counter part. Neither nano-particles nor nano-rods could be observed in the grains. Grain sizes of the specimens measured from the broadening of XRD diffraction peaks are summarized in Table 1. The crystalline size of the unmilled specimen is about 92 nm after extrusion but it starts to increase upon heating, reaching 325 and 434 nm after annealing at 400 and 450 °C, respectively. Dramatic grain growth in the mechanically alloyed specimen can be seen from
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L. L€u et al. / Composites Science and Technology 64 (2004) 2009–2014
sizes to 105 nm and 120 nm, respectively. These are much smaller than those processed by conventional powder metallurgy process. The smaller grain size is associated to the pinning effect of nano-particles in the Mg grain which resist grain growth during heating. Grain size measured using XRD spectra can be seen to be very close to that from TEM measurement. 3.3. Mechanical properties
Fig. 3. TEM image showing nano-rods embedded in the Mg matrix.
Fig. 4. TEM image of P/M processed Mg alloy.
Table 1 Grain size based on broadening of XRD diffraction peaks Mechanically alloyed specimen
Unmilled specimen Annealing temperature (°C)
Crystalline size (nm)
Annealing temperature (°C)
Crystalline size (nm)
As-extruded 350 400 450
129 170 325 434
As-milled As-extruded 350 400 450
24 77 78 105 120
the as-milled to the as-extruded specimens. The grain size of the as-milled powder is about 24 nm. Since finegrain powder consists of large surfaces of grain boundaries, grain growth is predominant during sintering. The grain size grows to about 77 nm after sintering and extrusion. However, there is almost no grain growth detected after annealing at 350 °C for 5 h. Further annealing at 400 and 450 °C only slightly increases grain
Table 2 lists the mechanical properties of P/M processed and mechanochemically milled specimens. Yield stresses of the mechanochemically milled specimen is 273 MPa which is about 15% higher than that for P/M processed specimen. The increase in yield stress is associated to two factors. Firstly, grain size of the milled specimen (about 77 nm) is much smaller than that for P/M processed counterpart. Secondly, there exists a large amount of dispersoids in the form of TiH2 caused by the reaction between polyethylene– glycol and Ti described in Section 3.1 and of MgO originated from oxide layer in the original Mg powder. Both smaller grain size and presence of dispersoids may have effectively opposed the movement of dislocations and twinning. Annealing at 350, 400 and 450 °C has led to an increase in yield stresses for all unmilled the specimens (Table 2). Increase in the YS has also been observed from mechanically alloyed specimens after annealing at 350 and 400 °C. A slight decrease was found for the specimen annealed at 450 °C. The increase in the YS value is believed to be associated with crack annihilation during post annealing. During the extrusion processing, some microcracks may have formed and these may cause crack initiation during tensile test. By post annealing in a vacuum furnace, the cracks formed during extrusion may be removed. It is noted that the elongation obtained from the mechanochemically milled specimen is lower than that of the P/M processed counterpart. However, the elongation of the former increases from the original 3.3% to 8.4% after annealing at 350 °C. The increase in ductility of mechanochemically milled specimen is probably due to release of internal stress and crack annihilation caused in extrusion when it was annealed at low temperature. A slight decrease to 7.5% was seen after annealing at 400 °C and a dramatic decrease from 7.5% to 5.5% after annealing at 450 °C. It is well known that Mg with its hcp structure has only three slip systems so that twinning becomes an important mechanism in contributing to the ductility. However, the ductility contributed by twinning may be restricted by the extremely fine grain size and the presence of nano-particles in the grains. It is also noted that it is difficult to observe dislocations in the nanograined Mg alloy (Fig. 3) since dislocations are
L. L€u et al. / Composites Science and Technology 64 (2004) 2009–2014
2013
Table 2 Yield (YS) and ultimate tensile strengths (UTS) of milled and unmilled specimens Annealing temperature (°C)
Unmilled specimen 0.2 YS (MPa)
UTS (MPa)
Elongation (%)
0.2 YS (MPa)
UTS (MPa)
Elongation (%)
As-extruded 350 400 450
235 238 242 255
330 335 334 347
5.8 4.3 4.1 4.4
265 273 284 270
289 301 307 299
3.3 8.4 7.5 5.5
Mechanically alloyed specimen
unstable in the ultra fine structure. Therefore, ductility of the mechanochemically milled specimen is believed to be mainly associated with grain boundary sliding. 3.4. Fractography Fractograph of the as-extruded unmilled specimen is given in Fig. 5. Large cracks with smooth surfaces, sharp edges, and cracked and fragmented Ti particles can be observed. Widespread transverse cracking and transgranular fracture can also been seen. Although some dimples are observed in the matrix, the fraction is generally shown to be brittle in nature. For the as-extruded mechanically alloyed specimen, large cracks can also be observed (Fig. 6) but no large Ti particles can be identified from the fracture surfaces. Cracked surfaces are basically rough and uneven. Presence of layered fracture surfaces and fine dimples suggest relatively ductile. The layered fracture surfaces might be an indication of a number of activated slip planes caused by the presence of nanoparticles within the grains. Very different fracture surface can be seen from the annealed mechanically alloyed specimen as shown in Fig. 7. The surface shows fine and uneven features. Many shallow dimples can be observed. Since rough and uneven surfaces occupy over a large area, more energy may be absorbed in fracturing and hence higher ductility.
Fig. 6. Fractography of mechanically alloyed specimen.
Fig. 7. Fractography of mechanically alloyed specimen after annealing at 350 °C.
4. Conclusions
Fig. 5. Fractography of the unmilled specimen.
1. Mg nanocomposite has been successfully synthesized through mechanochemical milling. Nanoparticles embedded in the Mg matrix have been observed. The grain size of the milled specimen is about 77 nm. 2. Formation of TiH2 was observed after the milling. Its amount increases after sintering. The mechanisms of
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L. L€u et al. / Composites Science and Technology 64 (2004) 2009–2014
formation of TiH2 is associated to the decomposition of polyethylene–glycol. 3. The mechanochemically milled specimens manifested higher yield stress and ductility. The increase in yield stress is associated to ultra fine grain size and dispersoids in the grains while the improved ductility is attributed to grain sliding.
References [1] Beer S, Frommeyer G, Schmid E. In: Mordike BL, Hehmann F, editors. Magnesium alloys and their applications, Germany; 1992. p. 317. [2] Li GH, Gill HS, Varin RA. Magnesium silicide intermetallic alloys. Metall Trans A 1993;24A:2383–7. [3] Roos U, Kainer KU, Mordike BL. Influence of particle shape on the mechanical properties of P/M produced QE22/SiCp-composites with various particle volume fractions. In: Proceedings of the Powder Metallurgy World Congress PM’94, vol. 3. Societe Francßaise de Metallurgie et de Materiaux, Paris, France; 1994. p. 2249–52. [4] Kainer KU, B€ ohm E. Optimizing of properties of magnesium alloys by squeeze casting. In: Croydon GB, Roller D, editors. Proceedings of the 29th International Symposium on Automotive Technology and Automation, Automotive Automation Ltd.; 1996. p. 653–60. [5] Greenfield P. Magnesium. London: Mills & Boon Ltd.; 1972. p. 18–22.
[6] Lo JSH, Carpenter GJC. Fabrication of SiC-reinforced AZ91D magnesium-based composites. In: Scotte ML, editor. Proceedings of the ICCM11, Gold Coast, Australia, 14–17 July 1997, Publ. Australian Composite Structures Society; 1997. p. 688–97. [7] Lloyd DJ. Particle reinforced aluminium and magnesium matrix composites. Int Mater Rev 1994;39(1):1–123. [8] Lu L, Lai MO, Gupta M, Chua BH, Osman M. Improvement of microstructure and mechanial properties of AZ91/SiC composite by mechanical alloying. J Mater Sci 2000;35:5553–61. [9] Lu L, Lai MO, Toh YH, Froyen L. Structure and properties of Mg5%Al10.3%Ti4.7%B synthesized via mechanical milling. Mater Sci Eng A 2001;334A:163–72. [10] Lu L, Froyen L. Mechanically alloyed high strength Mg5wt%Al10.3wt%Ti4.7wt%B alloy. Scripta Mater 1999;40: 1117–22. [11] Hwang S, Nishimura C, McCormick PG. Compressive mechanical properties of Mg–Ti–C nanocomposite synthesised by mechanical milling. Scripta Mater 2001;44:2457. [12] Petzow G, Effenberg G, editors. Ternary alloys, vol. 7. ASM International Edition, VCH Verlagsgesellschaft mbH; 1992. p. 42. [13] Lu L, Lai MO. Mechanical alloying. Dordrecht: Kluwer Academic Publishers; 1997. [14] Hida M, Asai K, Takemoto Y, Sakakibaba A. Solid solubility and transformation in mechanically alloyed Ti–Mg powders. Mater Trans Jpn Inst Metal 1996;37:1679–85. [15] Hida M, Asai K, Takemoto Y, Sakakibaba A. Mg-rich light alloys synthesised by mechanical alloying. Mater Sci Forum 1997;235–238:433–8. [16] Suryanarayana C, Froes FH. Titanium–magnesium alloys through mechanical alloying. J Mater Res 1990;5:1880–6.
Magnesium nanocomposite via mechanochemical milling L. L€ u a
a,*
, M.O. Lai a, W. Liang
b
Department of Mechanical Engineering, The National University of Singapore, 9 Engineering Drive 1, Singapore 117576, Singapore b Taiyuan University of Technology, China Received 23 August 2003; received in revised form 12 January 2004; accepted 20 February 2004 Available online 10 May 2004
Abstract Nanocomposite Mg5wt%Al–10.3%Ti was synthesized via mechanochemical milling of elemental powders of Mg, Al and Ti with polyethylene–glycol. Formation of TiH2 was observed after milling and the concentration of TiH2 was found to further increase after sintering. The nanocomposite shows an improvement in yield strength and ductility compared to its counterpart fabricated using a conventional powder metallurgy (P/M) process. The increase in mechanical properties is associated with the ultra fine grain size and the presence of nano-dispersoids in the matrix. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Nanostructured alloy
1. Introduction Light-weight magnesium alloys have recently received much attention due to their attractive properties. Although Mg alloys are generally believed to possess relatively low yield strength and poor ductility [1], they can be strengthened by incorporating either intermetallics or ceramic particulates [2–4]. Currently, most Mg-based composites have been fabricated through casting routes for the reason of cost effectiveness in comparison to other process techniques [5–7]. However, microstructures obtained from traditional cast generally reveal coarse structures. In addition, reactions between the Mg matrix and the reinforcements cause degradation of the composites [8]. Another difficulty encountered in the processing of Mg composites by casting is the incorporation of fine ceramic particulates. In most cases, only large ceramic particulates with size in the range of a few micrometer to tenth micrometer particulates are practically applicable. Some attempts in using mechanical or mechanochemical millings have been introduced in the fabrication of extremely fine and/or nanostructured Mgbased composites [9–11]. It has been found that strength
*
Corresponding author. Tel.: +65-6874-2236; fax: +65-6779-1459. E-mail address: [email protected] (L. L€ u).
0266-3538/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2004.02.018
of nanostructured Mg-based composites can be increased but on the cost of ductility [12,13]. The cause of ductility decrease is mainly due to selection of reinforcements. The present research focuses on the formation of nanocomposites with ultra-fine in situ TiH2 . Microstructure and mechanical properties of the composites are studied. Mechanisms of the formation of TiH2 will also be discussed. 2. Experimental procedures Mg alloys of nominal composition Mg5%Al–10.3%Ti were prepared via powder metallurgy and mechanochemical milling routes. Elemental powders of Mg of purity >98.5%, Al of purity 99.5% and Ti of purity >98% were used in the processes. For the alloy prepared by powder metallurgy method, the powders were mixed in a V-blender at 45 rpm for 2 h. For the composite prepared by mechanochemical milling process, the Mg, 5%Al and 10.3%Ti powder mixture was milled using a Fritsch planetary ball mill operating at 250 rpm. Forty 15 mm diameter balls were employed. A ball-to-powder weight ratio of about 20:1 was maintained. Polyethylene–glycol (H(OCH2 -CH2 )n OH) was added to the
L. L€u et al. / Composites Science and Technology 64 (2004) 2009–2014
3. Results and discussion 3.1. Structures Fig. 1(a) shows the spectrum of the as-mechanochemically milled power mixture prior to any heat treatment process. Several features of the diffraction can be observed. Firstly, all the Mg and Ti diffraction peaks can be observed to be broadened indicating a reduction in grain size. Due to the broadening, (1 0 0) Ti peak completely overlaps with (0 0 2) Mg peak after the milling. In spite of the overlap, the rest of the Ti diffraction peaks, like (0 0 2), (1 0 1) and (1 0 2), can still be clearly identified. In addition, it is clear that some new structures have been formed after the milling as indicated by the presence of the new peaks at 40.80° and 59.00°. It is generally understood that the solubility of Mg in Ti or vice versa is very small and negligible. The solubility has been found to decrease sharply with addition of Al in Mg [12]. It has been reported that solubility of Mg in Ti can be increased by mechanical alloying. However, the increase in alloy addition would lead to a shift in diffraction peaks due to the change in the lattice parameters. Therefore, the formation of the new peaks must have been originated from the forma-
Mg5Al10.3Ti Mg Ti TiH2 Al3Ti
(100) (200)
(100)
(004)
(110) (220)
(102)
(101)
(a) Milledpowder
(111)
(b) Milled + extruded (311) (222)
(220) (213)
(211)
(200) (004)
powder mixture with the purposes of prevention of agglomeration and excessive cold welding of the powders, and decomposition of it. To gain enough hydrogen, 6 wt% of polyethylene–glycol was used. Prior to mechanical alloying, powder mixtures were sealed with 99.9% pure argon gas in milling vials after evacuation with a vacuum pump. After mixing and mechanochemical milling, the powders were isostatically cold-pressed at 400 MPa to cylinders of about 40 mm diameter and 50 mm length. The compacts were then sintered at 450 °C for 2 h in a vacuum furnace. Extrusion of the compacts with an extrusion ration of 25:1 was carried out at 400 °C using graphite as a lubricant. The extruded rods were subjected to post heat treatments at three different temperatures of 350, 400 and 450 °C for 5 and 8 h. Tensile specimens of 5 mm diameter and 25 mm gauge length which cut from middle of the extrude rods were machined in accordance to ASTM E8M-96. Tensile testing was carried out at strain rate of 1.0 mm/min using an automated servo hydraulic INSTRON machine. A clip-on extensometer with 25 mm gauge length was used to record the displacement. X-ray diffraction (XRD) analysis was carried out using a Shimadzu Lab XRD-6000 X-ray diffractometer radiation. The crystalline with Cu Ka k ¼ 1:54056 A size was evaluated by the Hall–Williamson equation and transmission electron microscope (TEM).
Intensity (A.U.)
2010
(c) As-extruded
30
40
50
60
70
80
Diffraction angle (2θ˚) . Fig. 1. X-ray spectra of mechanochemically milled powder mixture and extruded rods: (a) as-milled powder; (b) extruded rod after 20 h of milling; (c) as-extruded rod directly from powder metallurgically sintered compact.
tion of new phases. Comparing the XRD spectrum of the milled specimens shown in Fig. 1(a) to that of the sintered and extruded specimen shown in Fig. 1(c), it can be seen that the diffraction peaks at 40.80° and 59.00° in the as-sintered specimen have become undetectable. The observation implies that the formation of the new phase is directly associated with the milling. Since polyethylene–glycol (H(OCH2 CH2 )n OH) consists mainly of hydrogen and Ti is very sensitive to it, Ti hydride could directly be formed during mechanochmical alloying via the decomposition of H(OCH2 CH2 )n OH through: ð1 þ 2nÞTi þ HðOCH2 CH2 Þn OH ! ð1 þ 2nÞTiH2 þ ð1 þ nÞCO þ ðn 1ÞC If it is assumed that n ¼ 1, it follows that 3Ti þ HðOCH2 CH2 ÞOH ! 3TiH2 þ 2CO The carbon monoxide evaporates as gas. From the above analysis it is clear that the two new diffraction peaks at 40.80° and 59.00° are TiH2 (2 0 0) and (2 2 0) at standard diffraction of 40.798° and 59.177°, respectively. From thermodynamic point of view, formation of TiH2 is possible since the excess standard Gibbs free energy of formation of TiH2 is )14.96 kJ/mol. It is interesting to note from Fig. 1(b) that Ti (0 0 2), (1 0 1) and (1 0 2) peaks completely disappear from the extruded rod that has been sintered after 20 h of milling.
L. L€u et al. / Composites Science and Technology 64 (2004) 2009–2014
Five TiH2 (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) diffraction peaks are clearly revealed after the extrusion. In addition to the TiH2 diffractions, two other peaks at 42.52° and 54.4° close to Al3 Ti (0 0 4) and (2 1 1) diffraction are also detected. Fig. 1(c) shows the diffraction spectrum of the asextruded specimen without milling. Very distinguished Mg diffractions can be observed. In addition, the diffraction peak from Ti can also be discovered even though (1 0 0) Ti peak overlaps with (0 0 2) Mg diffraction peak. Some traces of Al12 Mg17 diffraction are evident but they are not clear from the as-extruded specimens. The formation of in situ TiH2 can be divided into two stages. In the first stage, H(OCH2 CH2 )n OH is partially decomposed since it is not as stable as other types of processing control agent such as stearic acid [13]. Due to the small particles and grain sizes of both Mg and Ti, and the large amount of fresh surfaces created after some time of milling, hydrogen atoms may partially react with the Ti particles forming TiH2 . At the same time, free hydrogen atoms may also diffuse into the Mg matrix forming solid solution since the solubility of hydrogen in Mg is high even at room temperature. In the second stage, hydrogen atoms solutionized in Mg is released during sintering and these reacts with the remaining Ti form TiH2 . It should be noted that although Mg and Ti are immiscible below the melting of Mg in conventional processes, it has been found that solubility of Ti in Mg can be extended to about 6% by mechanochemical milling as a result of the formation of nanocrystalline structure [14–16]. Therefore, a part of the 10.3 wt% elemental Ti could be solid-soluted into Mg with the formation of nanostructured Mg during high energy mechanical alloying. Since the mechanochemically alloyed Mg is supersaturated with Ti, Ti will precipitate in Mg grains upon subsequent sintering. At the same time, hydrogen atoms are also released from the Mg lattice at high temperature. The Ti precipitates and the hydrogen released finally react with each other forming ultra-fine TiH2 particles. 3.2. Microstructure Fig. 2 shows TEM image of the mechanochemically milled and extruded specimen. The average grain size of the extruded specimen is about 30 nm although a few of the grains are in the range of about 90 nm. Fig. 2(b) is the SAD pattern of the nanograins. The diffraction is identified to be from Mg with index (1 0 0), (0 0 2), (0 1 1), ( 1 1 2) and (0 1 3) counting from the inner ring. Besides the fine grain size, other features can also be observed from the TEM image. It is noted that there exists a large amount of fine particles with size of a few nanometers. These nanoparticles are in principle ho-
2011
Fig. 2. TEM image of mechanochemically milled and extruded rod: (a) bright image showing nanograins with ultra fine dispersoids; (b) SAD pattern showing Mg diffraction.
mogeneously distributed in Mg matrix. Since the nanoparticles are too small to be identified, it is speculated that they may be MgO and TiH2 . The nano MgO particles may have originated from the fracture of oxide surfaces of the powder particles during milling and also from possible oxidation during milling while the nano TiH2 particles could have been formed during the sintering process when supersaturated Ti precipitated from Mg reacted with the free hydrogen atoms released from Mg. Besides the presence of nano-particles, some long ‘‘rod-like’’ features are also observed in microstructures and shown in Fig. 3. The width of the rods ranges from 3 to 7 nm. Fig. 4 shows the TEM image of the P/M processed specimen. It is clear that the grain size is much large than that of its milled counter part. Neither nano-particles nor nano-rods could be observed in the grains. Grain sizes of the specimens measured from the broadening of XRD diffraction peaks are summarized in Table 1. The crystalline size of the unmilled specimen is about 92 nm after extrusion but it starts to increase upon heating, reaching 325 and 434 nm after annealing at 400 and 450 °C, respectively. Dramatic grain growth in the mechanically alloyed specimen can be seen from
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sizes to 105 nm and 120 nm, respectively. These are much smaller than those processed by conventional powder metallurgy process. The smaller grain size is associated to the pinning effect of nano-particles in the Mg grain which resist grain growth during heating. Grain size measured using XRD spectra can be seen to be very close to that from TEM measurement. 3.3. Mechanical properties
Fig. 3. TEM image showing nano-rods embedded in the Mg matrix.
Fig. 4. TEM image of P/M processed Mg alloy.
Table 1 Grain size based on broadening of XRD diffraction peaks Mechanically alloyed specimen
Unmilled specimen Annealing temperature (°C)
Crystalline size (nm)
Annealing temperature (°C)
Crystalline size (nm)
As-extruded 350 400 450
129 170 325 434
As-milled As-extruded 350 400 450
24 77 78 105 120
the as-milled to the as-extruded specimens. The grain size of the as-milled powder is about 24 nm. Since finegrain powder consists of large surfaces of grain boundaries, grain growth is predominant during sintering. The grain size grows to about 77 nm after sintering and extrusion. However, there is almost no grain growth detected after annealing at 350 °C for 5 h. Further annealing at 400 and 450 °C only slightly increases grain
Table 2 lists the mechanical properties of P/M processed and mechanochemically milled specimens. Yield stresses of the mechanochemically milled specimen is 273 MPa which is about 15% higher than that for P/M processed specimen. The increase in yield stress is associated to two factors. Firstly, grain size of the milled specimen (about 77 nm) is much smaller than that for P/M processed counterpart. Secondly, there exists a large amount of dispersoids in the form of TiH2 caused by the reaction between polyethylene– glycol and Ti described in Section 3.1 and of MgO originated from oxide layer in the original Mg powder. Both smaller grain size and presence of dispersoids may have effectively opposed the movement of dislocations and twinning. Annealing at 350, 400 and 450 °C has led to an increase in yield stresses for all unmilled the specimens (Table 2). Increase in the YS has also been observed from mechanically alloyed specimens after annealing at 350 and 400 °C. A slight decrease was found for the specimen annealed at 450 °C. The increase in the YS value is believed to be associated with crack annihilation during post annealing. During the extrusion processing, some microcracks may have formed and these may cause crack initiation during tensile test. By post annealing in a vacuum furnace, the cracks formed during extrusion may be removed. It is noted that the elongation obtained from the mechanochemically milled specimen is lower than that of the P/M processed counterpart. However, the elongation of the former increases from the original 3.3% to 8.4% after annealing at 350 °C. The increase in ductility of mechanochemically milled specimen is probably due to release of internal stress and crack annihilation caused in extrusion when it was annealed at low temperature. A slight decrease to 7.5% was seen after annealing at 400 °C and a dramatic decrease from 7.5% to 5.5% after annealing at 450 °C. It is well known that Mg with its hcp structure has only three slip systems so that twinning becomes an important mechanism in contributing to the ductility. However, the ductility contributed by twinning may be restricted by the extremely fine grain size and the presence of nano-particles in the grains. It is also noted that it is difficult to observe dislocations in the nanograined Mg alloy (Fig. 3) since dislocations are
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2013
Table 2 Yield (YS) and ultimate tensile strengths (UTS) of milled and unmilled specimens Annealing temperature (°C)
Unmilled specimen 0.2 YS (MPa)
UTS (MPa)
Elongation (%)
0.2 YS (MPa)
UTS (MPa)
Elongation (%)
As-extruded 350 400 450
235 238 242 255
330 335 334 347
5.8 4.3 4.1 4.4
265 273 284 270
289 301 307 299
3.3 8.4 7.5 5.5
Mechanically alloyed specimen
unstable in the ultra fine structure. Therefore, ductility of the mechanochemically milled specimen is believed to be mainly associated with grain boundary sliding. 3.4. Fractography Fractograph of the as-extruded unmilled specimen is given in Fig. 5. Large cracks with smooth surfaces, sharp edges, and cracked and fragmented Ti particles can be observed. Widespread transverse cracking and transgranular fracture can also been seen. Although some dimples are observed in the matrix, the fraction is generally shown to be brittle in nature. For the as-extruded mechanically alloyed specimen, large cracks can also be observed (Fig. 6) but no large Ti particles can be identified from the fracture surfaces. Cracked surfaces are basically rough and uneven. Presence of layered fracture surfaces and fine dimples suggest relatively ductile. The layered fracture surfaces might be an indication of a number of activated slip planes caused by the presence of nanoparticles within the grains. Very different fracture surface can be seen from the annealed mechanically alloyed specimen as shown in Fig. 7. The surface shows fine and uneven features. Many shallow dimples can be observed. Since rough and uneven surfaces occupy over a large area, more energy may be absorbed in fracturing and hence higher ductility.
Fig. 6. Fractography of mechanically alloyed specimen.
Fig. 7. Fractography of mechanically alloyed specimen after annealing at 350 °C.
4. Conclusions
Fig. 5. Fractography of the unmilled specimen.
1. Mg nanocomposite has been successfully synthesized through mechanochemical milling. Nanoparticles embedded in the Mg matrix have been observed. The grain size of the milled specimen is about 77 nm. 2. Formation of TiH2 was observed after the milling. Its amount increases after sintering. The mechanisms of
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formation of TiH2 is associated to the decomposition of polyethylene–glycol. 3. The mechanochemically milled specimens manifested higher yield stress and ductility. The increase in yield stress is associated to ultra fine grain size and dispersoids in the grains while the improved ductility is attributed to grain sliding.
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