Directed melt oxidation and nitridation in aluminium alloys: B S S. Daniel and V.S. R Murthy A B AIN/AI Allo 100 C D 20 um 10m Figure 3 SEM micrographs showing various domains of nitride formation. (A)Surface nitridation and globule formation in the early stages of nitridation.(B)AIN/Al composite microstructure. (C)Deep etched sample showing morphology of interconnected AIN. (D) Ain crystal growth Table 1 Properties of Al2O, and AIN AIN Corundum Wurtzite 4.76;c1299a3.l1;c498 N】 RIDAIJON 140-170 TCE(25500C)(×10◆C Dielectric constant at 1 MHz Flexural strength(MPa) 5 Fracture toughness(MPa vm)2-3 Elastic modulus(GPa) 320-370 300-310 (15 N scale) 95.5 94.5 computed for different alloys during oxidation varies from 89 to 400 kJ/mol 1, 4 263. A typical growth for the Figure 4 Schematic diagram showing weight gain rates during DiMOX process can be up to 5-8 mm/day, but it can be further improved to 30 mm/day when preforms are shown in Figure 3. The AIN formed in the composite used32. The activation energy for AIN formation is 100 has a wurtzite structure with the growth direction kJ/mol and the growth rates are at least three orders oriented along the [0001] direction". The liquid metal higher compared to oxidation(Table 1) exhibits good wettability with aiN, and finds it easy to wick through the AIN crystals during growth. In oxida- Composites using filler materials tion, wicking is due to capillary action aided by magne- In the direct melt infiltration of composites, the growth in an oxide of reaction product(Al, O, /Al or AIN/AD)is restricted or system is expected to be slower channelled within the cavities of a filler material. The Both in nitridation and oxidation, different stages in filler material can be loose powder (or fibres )or owth are distinctly visible in themogravimetric analy- sintered porous ceramic body, which is usually in the sis (TGa)(Figure 4). The activation energy values shape of the final product. During infiltration, limited 158 Materials Design Volume 16 Number 3 1995Directed melt oxidation and nitfidation in a/u~i~iu~ a/fop: B. S. S. Daniel and V. S. R. hubby Figure 3 SEM micrographs showing various domains of nitride formation. (A) Surface nitridation and globule formation in the early stages nitrida .tion. (B) AlN/Al composite microstructure. (C) Deep etched sample showing morphology of inter~nnected AIN. (D) AIN crystal grov during break-away nitridatiin MgA$O~ Formation Tim4 - Figure 4 Schematic diagram showing weight gain rates during directed melt oxidation and nitr~dation shown in Figure 3. The AlN formed in the composite has a wurtzite structure with the growth direction oriented along the [OOOI] direction”. The liquid metal exhibits good wettability with A1N3’, and finds it easy to wick through the AlN crystals during growth. In oxida￾tion, wicking is due to capillary action aided by magne￾sium-induced wetting. Hence, liquid rise in an oxide system is expected to be slower. Both in nitridation and oxidation, different stages in growth are distinctly visible in themogravimetric analy￾sis (TGA) (Figure 4). The activation energy values of vth Table 1 Properties of A&O, and AIN Property AR AIN Crystal structure Corundum Wurtzite Lattice parameter (A) a 4.76; c 12.99 a 3.11; c4.98 Theoretical density &m/cm’) 3.98 3.26 Thermal conductivity (Wm.‘K-l) 26 14cL170 TCE (25-SOO’C) (x lO?‘C) 7.1 4.19 Dielectric constant at 1 MHz 9.5 Resistivity at RT (&rn) >10J4 ;Oli Flexural strength (MPa) 400 280-320 Fracture toughness (MPa t’;;) 2-3 Elastic modulus (GPa) 320-370 300-3 10 Rockwell hardness (I.5 N scale) 95.5 94.5 computed for different alloys during oxidation varies from 89 to 400 kJ/mo111~t4~~~31. A typical growth for the DIMOX process can be up to 5-8 mm/day, but it can be further improved to 30 mm/day when preforms are used32. The activation energy for AIN formation is -100 kJ/moI and the growth rates are at least three orders higher compared to oxidation (Table 1)9,20. Composites using filler materials In the direct melt in~ltration of composites, the growth of reaction product (Al,O,/Al or AlN/AI) is restricted or channelled within the cavities of a filler material. The filler material can be loose powder (or fibres) or a sintered porous ceramic body, which is usually in the shape of the final product. During infiltration, limited 158 Materials & Design Volume 16 Number 3 1995