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 cheJoumrl 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 ini￾tiate 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 charac￾terized using scanning electron microscopy, trans￾mission 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 kfg￾doped 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 vapour￾phase 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 conjunc￾tion 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 possi￾bility of using Li to initiate directed melt oxida￾tion growth in the A1203/Al system. Li was chosen 929