996 Elsevier Science Limi Printed in Great Brit s0955-2219(96)00019-2 The Use of lithium as a dopant in the directed Melt Oxidation of aluminium X Gu&r.j hand* Department of Engineering Materials, University of Sheffield, Sir Robert Hadfield Building Mappin Street, Sheffield SI 3DJ. UK (Received 12 October 1995: revised version received 21 December 1995; accepted 3 January 1996) Abstract distinguish the two cases by referring to bo produced by growth into free space as unreinfor A Li source has been used to initiate directed melt bodiesand to ones produced by growth into a oxidation of Al, the Li source used was Li, CO,. preform body as'reinforced bodies Growth both into free space, in which case Li,CO The growth of products by directed melt oxidation powder was placed on the metal surface, and into depends crucially on the presence of dopants, which form bodies FLSi culate a-A1,O, initiate and maintain the proc mixed with a doping amount of Li, CO3, has been may be introduced by alloying with the pure parent examined. In both cases ir is shoun that Li may metal or externally in the form of elemental or oxide tiate the directed oxidation reactions in the absence of powders any other dopant.s and that Li is therefore an effective Much of the published literature on this process dopant for the production of Al,OyAl by the directed has used alloys to introduce the doping elements melt oxidation process. The products were charac- For example, growth of Al,O3/Al composites from fericed using scanning electron microscopy, trans- Al alloys containing Mg either alone or in conjunc- mission electron microscopy and X-ray diffraction. tion with Si has been studied by several authors. -3 growth in the Li-doped svstem is postulated. This or MgO powders has received some attention, 5 A cyclic reaction sequence for Al,O,/Al composite More recently, the use of external doping with My process is initiated by the formation of LiAlsO It has been shown that Mg, either in elemental which aids the breakdown of the stable oxide film form or as part of a compound, can initiate directed that would normally form on aluminium in a similar melt oxidation reactions in the AlO,/Al system fashion to magnesium aluminium spinel in the Mg- For example, Xiao and Derby* have shown that Hoped sy stem. The process involves motion of Li from MgO may be used as an external dopant to initiate wvithin the growth to the reaction front, this can growth with pure Al, and that oxide growth occurs ccur because of the high vapour pressure of Li at in the temperature range 1100-1400C, with no the reaction temperature. The effects of the preform incubation period. Our previous work, in which Mg body on these cyclic reactions are also considered. powder was used as external dopant for directed c1996 Elsevier Science Limited oxidation of pure Al, also showed that only Mg is necessary to initiate and sustain the reaction growth Mg can initiate growth in the Al,O /Al system as it Introduction promotes the formation of a non-protective oxide layer at the interface between the growth oxide and The directed melt oxidation process involves the oxidant. This layer plays an important role in the directed growth of a composite product from a bulk subsequent directed oxidation cyclic reaction molten metal via oxidation of the melt by a vap sequence. By comparison Si seems only to accelerate phase oxidant (e.g air). This composite comprises an the reaction process, probably by modifying the interconnected ceramic reaction product and, usually, viscosity of the aluminium melt. Na and Sn ha several per cent of residual metal. The product also been examined as possible dopants: Na can may be shaped by growing the product either into initiate the process although it leads to low qualit a defined empty space or into a shaped region products and Sn apparently has similar effects to Si containing a preform comprising a loosely packed In the current work we have examined the possi- filler. ceramic fibres or whiskers. In this paper we bility of using Li to initiate directed melt oxida- *To whom correspondence should be addressed tion growth in the Al,O3/Al system. Li was che
Joumrl of r/w Europem Ceramic Society 16 (1996) 929-935 C 1996 Else&r Science Limited Printed in Great Britain. All rights reserved SO955-2219(96)OOOl9-2 0955-2219/961$15.00 The Use of Lithium as a Dopant in the Directed Melt Oxidation of Aluminium X. Gu & R. J. Hand* Department of Engineering Materials, University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield Sl 3DJ. UK (Received 12 October 1995; revised version received 21 December 1995; accepted 3 January 1996) Abstract A Li source has been used to initiate directed melt oxidation of AI: the Li source used was Li,COj. Grow?th both into free space, in which case Li,CO, poivder was placed on the metal surfhce, and into preform bodies comprising pure particulate cw-Al,O, mixed with a doping amount of Li,CO,, has been examined. In both cases it is sholt’n that Li may initiate the directed oxidation reactions in the absence of any other dopants and that Li is therefore an efective dopant for the production of Al,O,/AI by the directed melt oxidation process. The products were characterized using scanning electron microscopy, transmission electron microscopy and X-ray d@action. A cyclic reaction sequence for AI,O,/AI composite grott’th in the Li-doped sqsstem is postulated. This process is initiated by the ,formation of LiA150R which aids the breakdotlw of the stable oxide film that would normally jbrm on aluminium in a similar jbshion to magnesium aluminium spine1 in the kfgdoped system. The process involves motion of Li from lzlithin the grow*th to the reaction front,. this can occur because of the high vapour pressure of Li at the reaction temperature. The efSects of the preform body on these cyclic reactions are also considered. 0 1996 Elsevier Science Limited. Introduction The directed melt oxidation process involves the directed growth of a composite product from a bulk molten metal via oxidation of the melt by a vapourphase oxidant (e.g air).’ This composite comprises an interconnected ceramic reaction product and, usually, several per cent of residual metal. The product may be shaped by growing the product either into a defined empty space or into a shaped region containing a preform comprising a loosely packed filler, ceramic fibres or whiskers. In this paper we *To whom coriespondence should be addressed. distinguish the two cases by referring to bodies produced by growth into free space as ‘unreinforced bodies’ and to ones produced by growth into a preform body as ‘reinforced bodies’. The growth of products by directed melt oxidation depends crucially on the presence of dopants, which initiate and maintain the process. These dopants may be introduced by alloying with the pure parent metal or externally in the form of elemental or oxide powders. Much of the published literature on this process has used alloys to introduce the doping elements. For example, growth of A1203/Al composites from Al alloys containing Mg either alone or in conjunction with Si has been studied by several authors.‘-’ More recently, the use of external doping with Mg or MgO powders has received some attention.4.5 It has been shown that Mg, either in elemental form or as part of a compound, can initiate directed melt oxidation reactions in the Al,O,/Al system. For example, Xiao and Derby4 have shown that MgO may be used as an external dopant to initiate growth with pure Al, and that oxide growth occurs in the temperature range 1 lOO-14OO”C, with no incubation period. Our previous work,j in which Mg powder was used as external dopant for directed oxidation of pure Al, also showed that only Mg is necessary to initiate and sustain the reaction growth. Mg can initiate growth in the Al,OJAl system as it promotes the formation of a non-protective oxide layer at the interface between the growth oxide and oxidant. This layer plays an important role in the subsequent directed oxidation cyclic reaction sequence. By comparison Si seems only to accelerate the reaction process, probably by modifying the viscosity of the aluminium melt.2 Na and Sn .have also been examined as possible dopants; Na can initiate the process although it leads to low quality products and Sn apparently has similar effects to Si. In the current work we have examined the possibility of using Li to initiate directed melt oxidation growth in the A1203/Al system. Li was chosen 929
X Gu.r. ho as a potential dopant because examination of the The prepared systems were heated in air at 200C riodic table reveals that in general, on moving h in a muffle furnace to a soaking temperature of diagonally across the periodic table the elements 700, 900 or 1180 C Samples fired to 700 and 900C have certain similarities. This is because as one moves were held at these temperatures for 3 h, whereas across a period, the charge on the ions increases samples fired to 1180C were held at this temperature and the size decreases, causing the polarizing powder for 35 h. In all cases the samples were cooled to to decrease, whereas on moving down a group, the room temperature inside the furnace size increases and polarizing power increases. On Fired samples were sectioned parallel or perpen moving diagonally these two effects partly cancel dicular to the growth direction and the different each other, so that there is no marked change in regions analyscd by qualitative X-ray diffraction properties. The type and strength of bond formed These samples were subsequently mounted in epoxy and the properties of the coMpounds are therefore resin, ground and diamond polished to I un, before often similar, although the valency is different. The carbon coating for examination using scanning elec- similarities between diagonally related pairs of ele- tron microscopy (SEM; Jeol JSM 6400)and energy ments are usually weaker than those within a dispersive spectroscopy (EDS; Link Analytical group, but they are quite pronounced for the fol- 6276) For transmission electron microscopy (TEM), lowing of elements.6 Li and Mg; Be and Al; the samples were punched to 3 mm discs, mechani- B and Si. Therefore, there are some similarities in cally thinned to around 20 um, ion milled at 6.0 keV the properties of Mg and Li which were expected to until perforation, coated with carbon, and examined allow Li to be used as dopant in directed oxidation in a Philips EM420 at 100 keV of aluminium; in particular, both Mg and li have very high vapour pressures at high temperature and they both form spinel structures with Al,O3 Results In this paper the use of Li, CO, as an externally applied lithium source in the directed melt oxidation Growth into free space of aluminium has been examined. Both unreinforced Small, soft, black compacts were produced and ar-Al2O, particulate-reinforced bodies have been samples containing 2 47 or 7.41 wt% Li, cO produced In both cases, a detailed description of the fired to 700C for 3 h. XRD(Fig. 2)showed that microstructure is provided these compacts consisted of Al, Li, CO3, a-LiAIO2, Experimental Procedure a block of 99- 8% pure Al(Alcan) was placed in a cavity shaped in an alumina crucible using fine alumina powder. Between 1.25 and 7 50 wt% (based on the weight of Al) reagent grade Li, CO3 powder(Fisons)was either applied directly on the surface of the Al block for composites grown into free space, or mixed with fine pure a-Al,O3(Al7 Alcoa) for composites grown into a preform body. Fig. 2. XRD traces of samples fired for 3 h at 700C: (a) The respective experimental arrangements are Al-2 47 wt% Li, CO (b)A1-7-41 wt% Li, cO,(A. Al: LO given in Figs I(a)and l(b) Li,CO3, a, ae-LiAlO2: y, y-LiA1O2; LA, LiAlyOg) Vapour Phase Oxidant Vapour Phase Oxd 八人 Experimental arrangements used. (a) Unreinforced dopant directly placed on the Al block. (b)Rein- bodies: dopant mixed with filler and the mixture Fig. 3. XRD trace of Al-1. 23 wt% Li, CO, sample fired for placed above the al block 35 h at 1180%C(A, Al; AO, AL,O: LA. LiAlsos
930 x. Gu. R. J. Haid as a potential dopant because examination of the periodic table reveals that, in general, on moving diagonally across the periodic table the elements have certain similarities. This is because as one moves across a period, the charge on the ions increases and the size decreases, causing the polarizing powder to decrease, whereas on moving down a group, the size increases and polarizing power increases. On moving diagonally these two effects partly cancel each other, so that there is no marked change in properties. The type and strength of bond formed and the properties of the compounds are therefore often similar, although the valency is different. The similarities between diagonally related pairs of elements are usually weaker than those within a group, but they are quite pronounced for the following pairs of elements.6 Li and Mg; Be and Al; B and Si. Therefore, there are some similarities in the properties of Mg and Li which were expected to allow Li to be used as dopant in directed oxidation of aluminium; in particular, both Mg and Li have very high vapour pressures at high temperature and they both form spine1 structures with Al,O,. In this paper the use of Li,CO, as an externally applied lithium source in the directed melt oxidation of aluminium has been examined. Both unreinforced and a-A&O, particulate-reinforced bodies have been produced. In both cases, a detailed description of the microstructure is provided. Experimental Procedure A block of 99.8% pure Al (Alcan) was placed in a cavity shaped in an alumina crucible using fine alumina powder. Between 1.25 and 7.50 wt% (based on the weight of Al) reagent grade L&CO, powder (Fisons) was either applied directly on the surface of the Al block for composites grown into free space, or mixed with fine pure a-A&O3 (A17, Alcoa) for composites grown into a preform body. The respective experimental arrangements are given in Figs l(a) and l(b). Vapour Phase Oxidant (a) @I Fig. 1. Experimental arrangements used. (a) Unreinforced bodies: dopant directly placed on the Al block. (b) Reinforced bodies: dopant mixed with filler and the mixture placed above the Al block. The prepared systems were heated in air at 200°C h-i in a muffle furnace to a soaking temperature of 700, 900 or 1180°C. Samples fired to 700 and 900°C were held at these temperatures for 3 h, whereas samples fired to 1180°C were held at this temperature for 35 h. In all cases the samples were cooled to room temperature inside the furnace. Fired samples were sectioned parallel or perpendicular to the growth direction and the different regions analysed by qualitative X-ray diffraction. These samples were subsequently mounted in epoxy resin, ground and diamond polished to 1 pm, before carbon coating for examination using scanning electron microscopy (SEM; Jeol JSM 6400) and energy dispersive spectroscopy (EDS; Link Analytical 6276). For transmission electron microscopy (TEM), the samples were punched to 3 mm discs, mechanically thinned to around 20 pm, ion milled at 6.0 keV until perforation, coated with carbon, and examined in a Philips EM420 at 100 keV. Results Growth into free space Small, soft, black compacts were produced when samples containing 2.47 or 7.41 wt% Li,C03 were fired to 700°C for 3 h. XRD (Fig. 2) showed that these compacts consisted of Al, Li2C03, a-LiAIOZ, A A ‘i in 30 40 so 80 70 so 80 rm Degmes2-lheta Fig. 2. XRD traces of samples fired for 3 h at 700°C: (a) Al-2.47 wt% Li,CO,: (b) Al-7.41 wt% Li,CO, (A, Al; LC, Li,CO,; (Y, a-LiAIO,; y, y-LiAlO?; LA, LiAI,O,). A0 A0 50 ml 70 Degmes2-lheta Fig. 3. XRD trace of Al-l.23 wt% Li,CO, sample fired for 35 h at 1180°C (A, Al; AO, A1203; LA, LiAl,Os)
Use of Li in directed melt oxidation of A/ 931 y-LiA1O2 and LiAlsO, and that, within the limits well as a small amount of AIN, which was identified of detection, no Al2O3 had been produced. by EDS(Fig 8). Around the outermost surface of In samples doped with 1.23 wt% Li, CO, and these samples, a thin Li-containing layer was fired at 1180C, Al,O Al growth was obtained; the observed(Fig 9) d block was totally exhausted after a soaking time Although it is very difficult to identify the ceramic of 35 h Growth proceeded not only upwards from oxide as Al2O3 or LiAlsOs using EDS since lithium he top surface of the Al block but sideways into the barrier material. A section cut parallel to the growth was analysed by XRD which showed(Fig 3)that the product was mainly Al and a-Al2O3, in addition a small amount of Lialsog was detected. Cross- sections of the growth product were also analysed by SEM. At the top of the sample there was a thick layer comprising Al pockets within a ceramic matrix ( Fig. 4). Beneath this thick layer there was a series of thinner alternating dense and less dense layers (Fig. 5). The dense layers contained Al channels and the less dense ones contained no al. In the centre of this sample(Fig. 6), an interconnected matrix was obtained. At the bottom of the sample Fig. 6. SEM oh of the Al-1 23 wt% Li2CO, sample ( Fig. 7), further alternating dense and less dense fired to 11800 5 h. showing interconnected Al,O,/A layers were found. In addition, in the base of the x in the centre region product, the pr of Li-containing phases ca be inferred from the backscattered electron image, as 5u1636 ig. 7. SEM micrograph of the Al-1,23 wt% Li,CO, sample Fig 4. SEM micrograph of the Al-1-23 wt% Li2CO, sample fired to 1180C for 35 h, showing a thick matrix layer at the Fig. 8. Backscattered electron image of the Al-1. 23 wt% 5. SEM micrograph of the Al-1 23 wt% Li, CO, sample Li, CO, sample fired to 1180C for 35 h, show d to 1180C for 35 h, showing alternating dense and less Li-containing phase (dark feature A)and AIN dense layers feature-B)
Use of Li in directed melt oxidation of Al 931 y-LiAIOz and LiAl,Os, and that, within the limits of detection, no Al,O, had been produced. In samples doped with 1.23 wt% L&CO3 and fired at 118O”C, Al,O,/Al growth was obtained; the Al block was totally exhausted after a soaking time of 35 h. Growth proceeded not only upwards from the top surface of the Al block but sideways into the barrier material. A section cut parallel to the growth was analysed by XRD which showed (Fig. 3) that the product was mainly Al and a-AlzOj; in addition, a small amount of LiAi50s was detected. Crosssections of the growth product were also analysed by SEM. At the top of the sample there was a thick layer comprising Al pockets within a ceramic matrix (Fig. 4). Beneath this thick layer there was a series of thinner alternating dense and less dense layers (Fig. 5). The dense layers contained Al channels and the less dense ones contained no Al. In the centre of this sample (Fig. 6), an interconnected matrix was obtained. At the bottom of the sample (Fig. 7) further alternating dense and less dense layers were found. In addition, in the base of the product, the presence of Li-containing phases can be inferred from the backscattered electron image, as Fig. 4. SEM micrograph of ,the Al-l-23 wt% L&CO3 sample fired to 1180°C for 35 h, showing a thick matrix layer at the top. Fig. 5. SEM micrograph of the Al-l .23 wt% Li,CO, sample fired to 1180°C for 35 h, showing alternating dense and less dense layers. well as a small amount of AlN, which was identified by EDS (Fig. 8). Around the outermost surface of these samples, a thin Li-containing layer was observed (Fig. 9). Although it is very difficult to identify the ceramic oxide as Al,O, or LiAl,Os using EDS since lithium Fig. 6. SEM micrograph of the Al-l.23 wt’% L&CO, sample fired to 1180°C for 35 h, showing interconnected A120,/A1 matrix in the centre region. Fig. 7. SEM micrograph of the Al-l.23 wt% L&CO, sample fired to 1180°C for 35 h, showing alternating dense and less dense layers at the bottom of the product. Fig. 8. Backscattered electron image of the Al-l.23 wt% Li,C03 sample fired to 1180°C for 35 h, showing a Li-containing phase (dark feature - A) and AlN (light feature - B)
X. Gu, R.J. Hand is too light, TEM showed up differences between these two phases. High-angle LiAlSOk-LialsO grain boundaries were commonly observed(Fig. 10), which was similar to the high-angle MgAl,Oa MgAL,OA grain boundaries reported by Breval et al. on the Mg-doped system. It was also found that many Fig. 12. TEM micrograph of Al,O, feature showing low- angle Al,O3-AlLO3 grain boundary and high-angle AlO Al grain boundary. Diffraction pattern shows Al,O3 0 1 00 922日KU 2 8 m m Fig 9. SEM micrograph of the surface of a sample fired to 180C for 35 h showing a thin, Li- containing layer on the surface LiAIsON 250nm Fig. 13. TEM micrograph showing Al channel between LiAIsO crystals. Diffraction patterns show Al [0 T 3)(right) and LiAlsos [0 4 3](left) 150nm inclusions of unoxidized al remained within the LiAlsOg matrix(Fig. 11). Low-angle grain bound- fig. 10. TEM m aries were observed between Al2O3 grains(Fig 12 a high-angle LiAl O,O, grain boundary. Diffraction and Al pockets were found set in the Al2O3-Al2O3 pattern shows LiAlsOg [1923] grain boundaries with a high-angle AH-AlO, phase boundary. This was also in agreement with the fea- tures seen in the directed melt oxidation of Al-Mg alloys. .3.7 Some thin channels of aluminium were also found(Fig. 13), which separate neighbouring LiAl Og crystals rather than Al2O, crystals reported in the Mg-doped system by Newkirk et al LiAlsOB Apart from a small amount of surface oxidation of the aluminium blocks, no growth was found in samples containing either 2. 47 or 7-41 wt% Li, CO3 Chat had been healed to 900%C for 3 h, XRD showed however(Fig. 14), that after firing the filler powde 200nm mixture consisted of a-Al2O3, CY-LiAIO2, Y-LiAlO2 and DiAlog. The greater the initial Li,CO,.con Fig. 11. TEM micrograph showing inclusions of Al with tent, the more y-LiAIO, was obtained in the fired mixture
932 X. GM, R. J. Hand is too light, TEM showed up differences between these two phases. High-angle LiA&O,-LiAl,O, grain boundaries were commonly observed (Fig. lo), which was similar to the high-angle MgA1,04-MgA&O, grain boundaries reported by Breval et al. on the Mg-doped system. ’ It was also found that many Fig. 12. TEM micrograph of Al,O, feature showing lowangle Al@-A&O, grain boundary and high-angle AI&Al grain boundary. Diffraction pattern shows A1203 [006]. Fig. 9. SEM micrograph of the surface of a sample tired to I 180°C for 35 h, showing a thin, Li-containing layer on the surface. Fig. 10. TEM micrograph of a LiAl,O, feature showing a high-angle LiAl,Os-LiAI,Os grain boundary. Diffraction pattern shows LiAI,O, [1923]. Fig. 11. TEM micrograph showing inclusions of Al within LiAl,O* matrix. Fig. 13. TEM micrograph showing Al channel between LiAl,O, crystals. Diffraction patterns show Al [0 i 31 (right) and LiAI,O, [0 4 31 (left). inclusions of unoxidized Al remained within the LiAl,Os matrix (Fig. 11). Low-angle grain boundaries were observed between A&O, grains (Fig. 12) and Al pockets were found set in the A1,03-A1203 grain boundaries with a high-angle AI-AI,O, phase boundary. This was also in agreement with the features seen in the directed melt oxidation of Al-Mg alloys.‘,3,7 Some thin channels of aluminium were also found (Fig. 13), which separate neighbouring LiAl,Os crystals rather than A&O, crystals reported in the Mg-doped system by Newkirk et al.’ Growth into a preform body Apart ‘from a small amount of surface oxidation of the aluminium blocks, no growth was found in samples containing either 2.47 or 7.41 wt% Li,CO, that had been heated to 900°C for 3 h. XRD showed, however (Fig. 14), that after firing the filler powder mixture consisted of a-A&O,, a-LiAlO*, y-LiAIOz and LiA1,08. The greater the initial Li2C03. content, the more y-LiAlO* was obtained in the fired mixture
Al to the extent that traditional materials of construc- tion used in the melting and transfer of aluminium alloy are rendered inadequate. Furthermore, rapid reaction of lithium with oxygen and water from th ambient atmosphere renders the oxide crust that forms on the molten metal non-protective, leading to severe volatilization of lithium, necessitating process- ing undcr an incrt atmosphere. This rcmoval of film from the molten allo Inlate contact with particles and promotes wetting Fig. 14. XRD traces of AlO3 particle-reinforced samples fire and infiltration for 3 h at 900C:(a)Al-2 47 wt% Li,CO3:(b)Al-7 41 wt Butler and co-workers 0, showed that a binar Li, CO,(A. Al: c, a-LiAIO,: y, y-LiAIO: LA. LiAIsO, Al-3 wt% Li alloy developed surface films of spinel oxides such as LiAl5oOg and y-LIAIO, in oxygen- containing environments around 500oC. Under pure oxygen y-LiAlO, and a-Al,O3 developed at around 700C on alloys containing relatively low levels of Li X-ray analysis 2 of oxide films grown in air at 750oC on al-0-3 wt% Li and al-12 wt%o Li indicated that r-ALO, was present in addition to y-LiAlO Field and co-workers 1. 13 studied the oxidation of liquid Al-3 wt% Li alloy under different environ ments. In dry air the oxidation sequence with increas- ing temperature was Li,O- Li, cO r-LiAIO2 In wet air Li, O and Li, CO, were stable up to 500oC but above this temperature a mixture of cubic spinel LiAl, Os and LiAlO2 existed. The surface of molten Fig. 15. SEM micrograph of ALO, particle-reinforced Al-3 wt% Li appeared to behave chemically like pure Al-1. 23 wt/ Li-CO, sample fired to 1 180C for 35 h oxidation. Oxidation was not limited by Li diffusion In systems doped with between 1.24 and 7.41 wt% but controlled by the nucleation and growth of Li, CO, that had been heated to 1180 C for 24 or crystalline reaction products at the metal-oxide 35 h, irregular growths were obtained in the lower Interface part of the dopant/filler mixture and part of the n pure aluminium and an external aluminium block was consumed. On inspection of LiCo rather than an Al-Li alloy, have the cross-section, it was difficult to distinguish been used. XRD of Li,CO, fired at 600C, and DTA between alumina filler and growth. The section was traces of Li2 cO, heated from room temperature analysed by XRD which showed that although 1 180C, show that Li CO, is very stable even above LiAl O was present in addition to a-Al O, and 660C ( the melting point of aluminium)and that AL, no LiAlO, was present. The fired mixture the decomposition reaction occurs around 730oC above the growth product consisted of a-A1,O, Thus, in the directed melt oxidation process below filler) and LiAls Og. Unlike the unreinforced bod 730C, Li, CO, may react directly with the protec there were no alternating dense/less dense layers tive AL,O, oxide layers that will be present on the within the micro-structure of the reinforced body. a pure aluminium lithium aluminate phase was concentrated on the sur Li,CO3+ Al,O3(film)=2LiAlO,+ co, (1) an interconnected Al,O,Al matrix that containedor filler AlO, particles(0. 3 um diameter)and unoxi- dized al channels was formed(Fig. 15) Li, CO3+ 5Al2O3(film)=2LiAlsOg CO2.(2) Th id the breakdown of stable oxide film in a similar fashion to magnesium alumi- Discussion nate spinel, MgAl,O4, in the Mg-doped system On the other hand, above the melting point of Ginsberg and Datta claimed that lithium confers a aluminium. molten aluminium may also react greater reactivity to aluminium melt than any other directly with Li, CO, to form LiAl,Os or LiAlO2,i.e alloying element. Small amounts of lithium(3 wt%) dramatically alter the nature of the molten alloy 4Al+ 2Li,CO3+302F4LiAlO2 +2CO2(3)
Use of Li in directed melt oxidation of Al 933 A0 5 y Ah" A” h. .Ji I, h hi Fig. 14. XRD traces of A&O, particle-reinforced samples fired for 3 h at 900°C: (a) Al-2.47 wt% L&CO,; (b) Al-7.41 wt% L&CO, (A, Al; (Y, cu-LiAlO,; y, y-LiAlO,; LA, LiAI,O*). Fig. 15. SEM micrograph of AllO, particle-reinforced Al-l.23 wt% Li,CO, sample fired to 1180°C for 35 h. In systems doped with between 1.24 and 7.41 wt% Li2C0, that had been heated to 1180°C for 24 or 35 h, irregular growths were obtained in the lower part of the dopant/filler mixture and part of the aluminium block was consumed. On inspection of the cross-section, it was difficult to distinguish between alumina filler and growth. The section was analysed by XRD which showed that although LiAl,O, was present in addition to (~-A1~0~ and Al, no LiAlO, was present. The fired mixture above the growth product consisted of a-Al,O, (filler) and LiAl,O,. Unlike the unreinforced body, there were no alternating dense/less dense layers within the micro-structure of the reinforced body. A lithium aluminate phase was concentrated on the surface of the product body and, within the growth, an interconnected Al,O,/Al matrix that contained filler A&O3 particles (0.3 pm diameter) and unoxidized Al channels was formed (Fig. 15). Discussion Ginsberg and Datta’ claimed that lithium confers a greater reactivity to aluminium melt than any other alloying element. Small amounts of lithium (3 wt%) dramatically alter the nature of the molten alloy to the extent that traditional materials of construction used in the melting and transfer of aluminium alloy are rendered inadequate. Furthermore, rapid reaction of lithium with oxygen and water from the ambient atmosphere renders the oxide crust that forms on the molten metal non-protective, leading to severe volatilization of lithium, necessitating processing under an inert atmosphere. This removal of coherent film from the molten alloy also allows intimate contact with particles and promotes wetting and infiltration.’ Butler and co-workers “J’ showed that a binary Al-3 wt% Li alloy developed surface films of spine1 oxides such as LiAl,O, and y-LiAIOz in oxygencontaining environments around 500°C. Under pure oxygen y-LiAlO, and a-A&O, developed at around 700°C on alloys containing relatively low levels of Li. X-ray analysis I2 of oxide films g r o wn in air at 750°C on Al-O.3 wt% Li and Al-l .2 wtO/o Li indicated that 3hAL203 was present in addition to y-LiAlO*. Field and co-workers”,‘3 studied the oxidation of liquid Al-3 wt% Li alloy under different environments. In dry air the oxidation sequence with increasing temperature was L&O -+ Li,CO, + y-LiAIOz. In wet air, Li,O and L&CO, were stable up to 500°C but above this temperature a mixture of cubic spine1 LiAl,Os and LiAlO, existed. The surface of molten Al-3 wt% Li appeared to behave chemically like pure lithium and thus Al played only a minor role during oxidation. Oxidation was not limited by Li diffusion but controlled by the nucleation and growth of crystalline reaction products at the metal-oxide interface. In this work, pure aluminium and an external Li2C03 dopant, rather than an Al-Li #alloy, have been used. XRD of Li,CO, fired at 600°C and DTA traces of L&CO, heated from room temperature to 118O”C, show that Li2C03 is very stable even above 660°C (the melting point of aluminium) and that the decomposition reaction occurs around 730°C. Thus, in the directed melt oxidation process below 730°C Li2C03 may react directly with the protective A&O, oxide layers that will be present on the pure aluminium: Li,CO, + A1203 (film) + 2LiA102 + CO, (1) or Li,C03 + 5Al,O, (film) + 2LiAl,O, + CO,. (2) These reactions can aid the breakdown of stable oxide film in a similar fashion to magnesium aluminate spinel, MgA1204, in the Mg-doped system.’ On the other hand, above the melting point of aluminium, molten aluminium may also react directly with Li2C03 to form LiAl,O, or LiAlO,, i.e. 4Al + 2Li2C03 +30, +4LiAlO, +2CO, (3)