Influences of mechanical activation and heating rate on reaction processes in combustion synthesis of NiAl-Al2O3 composites

Abstract

This paper investigates the combustion synthesis of NiAl-Al2O3 composites by the heating of Ni, NiO and Al powder mixture from 20 °C to 1300 °C. The influence of mechanical activation (without and with 1 h) and heating rate (20 °C/min and 40 °C/min) on thermal events in combustion synthesis process are investigated. Thermal events are assessed by Differential Thermal Analysis (DTA), X-Ray Diffractometry (XRD) and Scanning Electron Microscopy (SEM). By heating the sample without mechanical activation, exothermic reaction of NiO reduction by Al and Ni-Al intermetallic phases (Al3Ni, Al3Ni2, AlNi and AlNi3) production happened in the presence of molten Al at 870 °C as one DTA peak. Mechanical activation causes occurrence of these reactions more separately at lower temperatures (590 °C and 630 °C). By decrement in the heating rate, these reactions happened in a more dispersed way at lower temperatures (590 °C and 630 °C). The reaction temperature for nickel aluminides formation in the presence of eutectic melt of AlNi3-Al is constant regardless of heating rate in mechanically activated powder. A seven-stage mechanism for reactions in a mechanically activated sample at a heating rate of 40 °C/min is proposed. Initially, by heating of the powder, an exothermic reaction between nickel oxide and aluminum and also nickel and aluminum occurs at 590 °C in solid state. NiO is consumed totally and Al2O3 is produced from the nickel oxide reduction by aluminum. Nickel aluminide phases are formed from reactions between aluminum and primary nickel and also from aluminum and nickel formed by aluminothermic reduction of nickel oxide. With continuation of heating process, Al-Al3Ni eutectic transition happens at 630 °C and results in the liquid phase formation. Presence of the molten phase accelerates the exothermic nickel aluminides formation reactions. Aluminum is consumed totally in this stage and more intermetallic phases are developed. By increasing the temperature, diffusion is enhanced. The remaining Al3Ni melts at 856 °C. This small amount of molten phase slightly enhances exothermic reactions between nickel aluminides and leads the system towards the equilibrium phase. After this event, reactions progresses gradually. The nickel core shrinks by the diffusion progressively and the whole nickel aluminides system undergoes a gradual evolution towards the NiAl equilibrium phase.