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 sur￾face 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 unoxi￾dized 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 construc￾tion 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 process￾ing under an inert atmosphere. This removal of coherent film from the molten alloy also allows inti￾mate 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 oxygen￾containing 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 environ￾ments. In dry air the oxidation sequence with increas￾ing 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 protec￾tive 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 alumi￾nate 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)