Microwave processing and characterization of nickel powder based metal matrix composite castings

Microwave heating is successfully employed for the in situ melting and casting of EWAC (nickel based) metallic powder to obtain bulk metal matrix composites reinforced with silicon carbide particles. Microwave processing of composites was carried out at 2.45 GHz frequency and at 900 W. Microwave susceptor/hybrid heating was utilized to melt nickel based metallic powders with varying volume fraction of SiC reinforcement. Microwave processed MMC's were characterized by studying the microstructures, intermetallic phases developed and mechanical properties (microhardness, tensile strength and percentage elongation). XRD study reported the formation of hard nickel silicides and chromium carbide phases. The SEM results provided insights on the formation of fine equiaxed grains. This may be due to the volumetric heating associated with the microwaves. The addition of SiC particles hindered the grain growth of matrix and resulted in lower average grain diameters. The microhardness was increased with an increase in the volume fraction of SiC reinforcement (from 5% to 10%) in the MMC castings and this may be the result of the formation of some intermetallic carbides and silicides, grain strengthening mechanism and uniform dispersion of SiC particles. Tensile strength of developed castings increased with increasing the reinforcement volume fraction but, with a significant decrease in the elongation. In comparison to the pure nickel-based casting, 10% SiC reinforced MMC revealed 2.4 times higher microhardness (980 +/- 208 HV0.1), 42.58% (442 +/- 20 MPa) higher tensile strength and 2.54 times reduction in ductility. The fracture specimen revealed that on adding SiC reinforcement, the fracture mode shifts from ductile to mixed (brittle + ductile) mode.