Directed melt oxidation and nitridation of aluminium alloys: a comparison B S.S. Daniel and V.s. R Murthy Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur 208016, India Received 4 September 1995; accepted 21 September 1995 Directed melt oxidation and nitridation are recently developed in-situ methods for the formation of metal-ceramic composites. Both processes are based on liquid-gas reaction, wherein a nolten alloy is reacted with a gaseous species, i.e. oxygen or nitrogen, to form A2O,/Al or AIN/A composite microstructures Composite formation is controlled by the alloy composition partial between these two processes, there are differences in the microstructural development. Furthe pressure of gaseous species, processing tomperature and time. Although there are similarities the growth rates of these composites are accelerated using filler materials which provide sites for secondary nucleation and in turn compete with the primary growth of the ceramic phase. the mechanical properties of Al,O Al and AIN/Al are comparable, but AIN/Al exhibits higher thermal conductivity and sensitivity to moisture. Finally, the applications and limitations of these composites are presented Keywords: oxidation; nitridation; in-situ composite Introduction reactions, there are differences between the two. First To meet the demand of recent high-technology applica- in the former method the reacting gas is injected into tions, a large number of metal-matrix composites the molten pool and the reaction time is relatively short (MMCs)were developed adopting suitable processing to control the process effectively. In the latter the reac- methodologies-3. These conventional processing tion is a continuous process over an extended period of methods, however, have some limitations, i.e. residual time. Second, by modifying both alloy and gas compo- microporosity, uneven distribution of reinforcing mate- sition, a large number of reinforcing constituents can be rials(inhomogeneity), non-wetting of reinforcements, formed On the other hand, in direct melt reactions a matrix-rcinforccment interfacc microstructure control singular reinforcing phase is possible. Third, by blowing and its cleanliness. etc. To overcome these barriers. gases, fine(10-20 um) and isolated particles are formed several innovative processing methods have been devel- and, beyond a certain percentage, higher-volume frac oped, wherein the reinforcements are formed by in-sitt tions of dis ssible, whereas, in the reactions. These processes are broadly classified based DIMOX/PRIMEX processes the ceramic phases that on their reactant phases, i.e. liquid-gas, liquid-solid and re formed are large in percentage terms and moreover solid-solid reactions. In liquid-gas reaction processing the metal and ceramic phases are interconnected there one method of forming a reinforcing phase is by injec- is also a greater fiexibility in the volume fraction of rein tion of a reactive gas into a liquid alloy reservoir. forcing phase, i.e. both metal and ceramic matrix Depending on the alloy and gas composition, one or composites are feasible by controlling process variables more phases are formed+6. Another technique which en recently developed is again based on The basic principle liquid-gas reaction, but uses a different approach. In In directed melt composite reactions a molten alloy is this process(developed by Lanxide Corporation, USA) reacted with a gaseous species (sometimes a gaseous the liquid alloy is treated in a static/flowing gas en- mixture)to develop a metal-ceramic composite AIN/Al composites are formed as the reaction product. nitriding, ALO,Al or AIN/Al composites are obtain.or vironment. Depending on the atmosphere, AlOyAl or Depending on the atmosphere, whether oxidizing or hese processes are now commonly referred to as respectively(Figure 1). Under suitable conditions a directed melt oxidation(DIMOX), and direct melt reaction product initially forms on the surface of th Although both processes are based on liquid-gas uous wich y and the product grows outward by contin- nitridation(PRIMEX) respectively molten all kingof liquid alloys through microchannels that are present within the reaction product Thus the Correspondence to v.S.R. Murth final composite contains an interconnected and inter. penetrating network of metal and ceramic. Further 261-306995/03015507 Materials Desian Volume 16 Number 3 1995 155Directed melt oxidation and nitri~ation of a~uminium alloys: a comparison B. S. S. Daniet and V. S. R. gushy Department of Materials and Metalfurgical Engineering, Indian Institute of Tec~ffology, Ka~~ur ZU8U76, vodka Received 4 September 7995; accepted 21 September 1995 Directed melt oxidation and nitridation are recently developed in-situ methods for the formation of metal-ceramic composites. Both processes are based on liquid-gas reaction, wherein a molten alloy is reacted with a gaseous species, i.e. oxygen or nitrogen, to form AI&/AI or AINfAl composite microstructures. Composite formation is controlled by the alloy composition, partial pressure of gaseous species, processing temperature and time. Although there are similarities between these two processes, there are differences in the microstructural development. Further, the growth rates of these composites are accelerated using filler materials which provide sites for secondary nucleation and in turn compete with the primary growth of the ceramic phase. The mechanical properties of AI,OdAI and AIN/AI are comparable, but AIN/AI exhibits higher thermal conductivity and sensitivity to moisture. Finally, the applications and limitations of these composites are presented. Keywords; oxidation; nitridation; in-situ composite Introduction To meet the demand of recent high~technology applica￾tions, a large number of metal-matrix composites (MMCs) were developed adopting suitable processing methodologies’“. These conventional processing methods, however, have some limitations, i.e. residual microporosity, uneven distribution of reinforcing mate￾rials (inhomogeneity), non-wetting of reinforcements, matrix-reinfor~ment interface mi~rost~cture control and its cleanliness, etc. To overcome these barriers, several innovative processing methods have been devel￾oped, wherein the reinforcements are formed by in-situ reactions. These processes are broadly classified based on their reactant phases, i.e. liquid-gas, liquid-solid and solid-solid reactions4. In liquid-gas reaction processing, one method of forming a reinforcing phase is by injec￾tion of a reactive gas into a liquid alloy reservoir. Depending on the alloy and gas composition, one or more phases are formed4”. Another technique which has been recently developed is again based on liquid-gas reaction, but uses a different approach. In this process (developed by Lanxide Corporation, USA) the liquid alloy is treated in a static/flowing gas en￾vironment. Depending on the atmosphere, AI,O,/Al or AIN/AI composites are formed as the reaction product. These processes are now commonly referred to as directed melt oxidation (DIMOX)7,S and direct melt nitridation (PRIMEX)‘,“, respectively. Although both processes are based on liquid-gas Correspondence to V. S. R. Murthy 026%3069/95/030155-07 reactions, there are differences between the two. First, in the former method the reacting gas is injected into the molten pool and the reaction time is relatively short to control the process effectively. In the latter the reac￾tion is a continuous process over an extended period of time. Second, by modifying both alloy and gas compo￾sition, a large number of reinforcing constituents can be formed. On the other hand, in direct melt reactions a singular reinforming phase is possible. Third, by blowing gases, fine (lo-20 pm) and isolated particles are formed and, beyond a certain percentage, higher-volume frac￾tions of dispersoids are not possible, whereas, in the DIM~~RIMEX processes the ceramic phases that are formed are large in percentage terms and, moreover, the metal and ceramic phases are interconnected. There is also a greater flexibility in the volume fraction of rein￾forcing phase, i.e. both metal and ceramic matrix composites are feasible by controlling process variables. The basic principle In directed melt composite reactions a molten alloy is reacted with a gaseous species (sometimes a gaseous mixture) to develop a metal-ceramic composite. Depending on the atmosphere, whether oxidizing or nitriding, AI,OJAl or AlN/Al composites are obtained respectively (~~g~~~ I). Under suitable conditions, a reaction product initially forms on the surface of the molten alloy and the product grows outward by contin￾uous ‘wicking’ of liquid alloys through microchannels that are present within the reaction product. Thus the final composite contains an interconnected and inter￾penetrating network of metal and ceramic. Further, Materials St Desian Volume 76 Number 3 1995 155