Abstract

This paper presents a novel in situ production method of intermetallic matrix composites by reactive RF plasma spraying with premixed elemental powders. RF plasma spraying, which is characterized by high temperature and low velocity of the plasma flame, is suitable to synthesize in situ composites without preparing any ceramic fibers or particles. The flight velocity of molten titanium (Ti) droplets in the plasma flame is 39-47 m/s; it is approximately 1/3 of that in DC plasma flame at low pressure. Typical splat morphology of impinged Ti droplets on a stainless steel substrate is a disk-type with an outer peripheral fringe. If nitrogen (N2) partial pressure in the plasma gas increases or supplied Ti powder size becomes finer, the splats containing prominent dots with smaller flattening ratio appear along with the plain disk-type. An increase in N content is recognized in all the splats sprayed with higher N2 partial pressure or finer Ti powder, especially the splats containing prominent dots, which correspond to TiN, are twice as high N content as that in the plain disk-type. Aluminum (Al) splats are classified into two categories: a disk-type with an irregular outer periphery and a semi- massive-type. Oxygen exists on the splat surfaces, on which there are N concentrated areas corresponding to A1N. Consequently, the nitriding reaction proceeds on Ti and Al droplets during the flight and on a substrate. If the substrate temperature is higher than 873 K, with premixed Ti/Al powder, the formation of TiAl and Ti2AlN proceeds definitely on the substrate, because of negligible mutual collisions during the flight. Titanium aluminide matrix in situ composites sprayed with premixed Ti/AI powder contain more nitrides than those with TiAl compound powder, because of the higher N absorption by Ti and Al droplets that result in the exothermic reaction.

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