MICROSTRUCTURAL DEVELOPMENT AND SINTERING KINETICS IN CERAMIC REINFORCED HIGH-SPEED STEEL METAL-MATRIX COMPOSITES

Studies were made of sintering kinetics and microstructural developmen t when additions of either TiC or NbC were made to a sintered M3/2 hig h speed steel. Sintering occurred by a supersolidus liquid phase mecha nism with diffusion controlled solution-reprecipitation as the predomi nant sintering mechanism, except in the case of TiC based composites w here a much slower, unidentified, liquid phase sintering reaction occu rred. Both ceramic carbide found to raise the temperature sinter to fu ll density but they also increased the width of the 'sintering window' and restricted the formation of the coarse carbide grain boundary fil ms and eutectics normally produced by oversintering in M3/2 high speed steel. These effects were related to changes in the alloy composition and phase equilibria caused by interaction between the steel and carb ide additive. In the case of the TiC composite, some TIC particles wer e retained but their solution into the steel caused the M6C matrix car bides to be replaced by a titanium/vanadium containing MC carbide. Con version of NbC into a more complex alloyed MC carbide containing both vanadium and molybdenum took place in the NbC based composite.