Directed melt oxidation and nitridation in aluminium alloys: B. S.s. Daniel and V. s R Murthy equilibrium thermodynamics, it is established that an oxygen partial pressure of -10 Pa is required for AIN formation. This is never achieved, but fortunately in the presence of local gettering agents such as Mg, Li or Na introduced as alloying additives, kinetics overrules Addition of H, to N,(forming gas) hel oxygen activity low and further improves the kinetics of the process,. Better wetting of the preform surface by the liquid metal is anticipated in a nitrogen atmosphere Composite growth Nucleation and growth mechanisms during high temperature oxidation of liquid aluminium alloys have been studied, 4: 20.27. In the early stages, when the alloy is held at above the liquidous temperature a duplex oxide(MgO MgAl O4) layer forms on the surface These oxide layers are porous and contain intercon nected microdiscontinuities. The liquid metal from the underneath reservoir wicks through these channels and emerges as small nodules on the surface. Many such Aloy Reservo nodules coalesce to form cauliflower, type of colony on the surface. Finally, several such colonies form planar oxidation front. At each stage, nodules are covered with a duplex layer and magnesium (or any Figure 2 Cross-sectional volatile species) in the alloy reservoir depletes with time When the sium content in the alloy drops belot a threshold value (i.e. 0.3 wt% Al2O3)columnar crystals Temperature and time nucleate and grow to several tens of microns uniter In addition to chemistry of the melt, control of the ruptedly (Figure 2). ALO, develops mainly as a with tion kinetics is established by varying temperature growth direction maintained parallel to the c-axis of time to arrive at the desired microstructure. At temper Al,o, " 1. These Al,O, crystals grow in an interconnected atures below 950@C selective conversion of magnesium manner, but appear isolated in two-dimensional to MgO-MgAl2O, takes place. On the other hand, sections. Finally, when there is a bulk growth temperatures above [400c lead to formation of a ALOyAl composite, an oxygen gradient builds porous material. Hence, process temperatures are across the layers and Mgo and MgA1 O in the layers confined between 1000oC and 1300%C .In contrast. underneath dissociate to give Mg", Al*and O-ions conversion of Al to aIN can be achieved at relatively Mg*ions diffuse to the surface and maintain the non lower temperatures(2750C)when compared to oxida- protective Mgo layer, whereas AP*and 02-contribute tion. Further, in nitridation, temperature has a more to growth of Al,03 3, 4.28 pronounced effect on structure; at lower temperatures, The number of steps that are involved in aIN/Al predominantly metal-matrix composites are obtained, composite growth are smaller compared to oxidation whereas at higher temperatures, metal-dispersed Here no complex surface oxides are formed. However, eramic matrix are generated. Such flexibility is not the basic processes such as nodule formation by wicking observed in oxide systems, where the final microstruc- and the conversion of these nodules to composite struc ture is ceramic dominant. To some extent, temperature ture ture are seen(Figure 3). The specific volume ratio of so dependent on the the nitride layer(VAIN/VA=1.26)is greater than unity composition of the alloy and gaseous species and is expected to form a protective nitride surface layer. Magnesium present in the alloy is reported to Atmosp have a catalytic effect on nitride formation such that Both reaction processes are sensitive to partial pressure helps to transfer the surface reaction into a volume of oxygen.4. Nagelbergreported a Po4 dependency reaction". Based on the kinetics of composite forma- for the oxidation of Al-10Si-3Mg alloy at a weight gain tion, Aluminium to aIn conversion is broadly classified of 0. 2 g/cm, whereas no significant influcnce of oxygen into four reaction domains. They are: (1)passivating partial pressure was noticed in a complex alloy". Thus surface nitridation(observed in many binary alloys), oxidation reaction kinetics appears to be largely depen-(2) n-controlled volume nitridation, ( 3)volume dent on alloy composition. On the other hand, for nitric involving outward growth of AIN-AI ture-free nitrogenous atmosphere is essential. From AIN-Al microstructures in domains(3)andya% effective conversion of Al to AIN, an oxygen and mois- and (4) break-away nitridation. The morphologies Materials Design Volume 16 Number 3 1995 157Figure 2 Cross-sectional microstructure depicting various phases of directed melt oxidation. Inset shows thr~-dimensional view of A&O, crystals Temperature and time In addition to chemistry of the melt, control of the reac￾tion kinetics is established by varying temperature and time to arrive at the desired microst~cture. At temper￾atures below 950°C selective conversion of magnesium to Mg~MgAl~~~ takes place. On the other hand, temperatures above 1400°C lead to formation of a porous material. Hence, process temperatures are confined between 1000°C and 1300°C. In contrast, conversion of Ai to AlN can be achieved at relatively lower temperatures (2750°C) when compared to oxida￾tion”. Further, in nitridation, temperature has a more pronounced effect on structure; at Iower temperatures, predominantly metal-matrix composites are obtained, whereas at higher temperatures, metal-dispersed ceramic matrix are generated. Such flexibility is not observed in oxide systems, where the final microstruc￾ture is ceramic dominant. To some extent, temperature and time combinations are also dependent on the composition of the alloy and gaseous species. Atmosphere Both reaction processes are sensitive to partial pressure of oxygen14~24. Nagelberg” reported a pg; dependency for the oxidation of Al-IOSi-3Mg alloy at a weight gain of 0.2 g/cm*, whereas no significant influence of oxygen partial pressure was noticed in a complex alloy”. Thus oxidation reaction kinetics appears to be largely depen￾dent on alloy composition. On the other hand, for effective conversion of Al to AIN, an oxygen and mois￾ture-free nitrogenous atmosphere is essential. From equilibrium the~odynamics, it is es~blished that an oxygen partial pressure of -lOmBPa is required for AIN fo~ation, This is never achieved, but fortunately in the presence of local gettering agents such as Mg, Li or Na introduced as alloying additives, kinetics overrules. Addition of H2 to N2 (forming gas) helps to keep the oxygen activity low and further improves the kinetics of the process’. Better wetting of the preform surface by the liquid metal is anticipated in a nitrogen atmosphere2? Composite growth Nucleation and growth mechanisms during high temperature oxidation of liquid aluminium alloys have been studied*3,~~26,“. In the early stages, when the alloy is held at above the liquidous temperature, a duplex oxide (MgO + MgA1,03 layer forms on the surface. These oxide layers are porous and contain intercon￾nected microdiscontinuities. The liquid metal from the underneath reservoir wicks through these channels and emerges as small nodules on the surface. Many such nodules coalesce to form a ‘cauliflower’ type of colony on the surface. Finally, several such colonies form a planar oxidation front. At each stage, nodules are covered with a duplex layer and magnesium (or any volatile species) in the alloy reservoir depletes with time. When the magnesium content in the alloy drops below a threshold value (i.e. 0.3 wt% Al,OJ columnar crystals nucleate and grow to several tens of microns uninter￾ruptedly (Figure 2). A&O, develops mainly as OL with growth direction maintained parallel to the c-axis of A1203”“. These A&O, crystals grow in an interconnected manner, but appear isolated in two-dimensional sections. Finally, when there is a bulk growth of AI,O,/Al composite, an oxygen gradient builds up across the layers and MgO and MgA1204 in the layers underneath dissociate to give Mg2’, A13+ and 0” ions. Mg” ions diffuse to the surface and maintain the non￾protective MgO layer, whereas AP’ and 0”” contribute to growth of Al20313,24? The number of steps that are involved in AlN/Al composite growth are smaller compared to oxidation. Here no complex surface oxides are formed. However, the basic processes such as nodule formation by wicking and the conversion of these nodules to composite struc￾ture are seen (Figure 3)29. The specific volume ratio of the nitride layer ( VxlN/VA, = 1.26) is greater than unity and is expected to form a protective nitride surface layer. Magnesium present in the alloy is reported to have a catalytic effect on nitride formation such that it helps to transfer the surface reaction into a volume reaction2’. Based on the kinetics of composite forma￾tion, Aluminium to AlN conversion is broadly classified into four reaction domains. They are: (1) passivating surface nitridation (observed in many binary alloys), (2) diffusion-controlled volume nitridation, (3) volume nitridation involving outward growth of AlN-Al and (4) break-away nitridation. The morphologies of AIN-Al mi~rostructures in domains (3) and (4) are Materials & Design Volume 16 Number 3 1995 157