THE INFLUENCE OF MECHANICAL ALLOYING ON THE SYNTHESIS OF NI-3(SI, TI)INTERMETALLICS

Reactive powder metallurgy offers an interesting alternative technolog y for the production of nickel silicide based intermetallic materials. The heat release during reaction and the microstructure of the produc t can be controlled by mechanical alloying of the reactants. In order to determine the optimal processing conditions for mechanical alloying , the relationships among the processing conditions, the microstructur e of the mechanically alloyed powder and the reaction mechanism have b een identified. This was done with the aid of a mathematical descripti on of the mechanical alloying process in a planetary ball mill, which allowed the prediction of the hardness and deformation of the material as a function of the milling conditions. Experimental verification un der a broad range of conditions shows a good agreement between the mea sured and predicted hardness values. The relation between microstructu re and milling conditions was quantitatively expressed as an inverse e xponential relation between the calculated strain and the crystallite size of the material. The mechanism of synthesis of the Ni-3(Si, Ti) p hase in the mechanically alloyed powder was studied. It is shown that the transition of a high-temperature self propagating reaction in the unmilled powder to a low temperature solid-state reaction depends on t he crystallite size of the milled powder. By using the relations among milling conditions, microstructural characteristics and reaction beha vior of the mechanically alloyed powder, milling maps with ''equivalen t milling conditions'' were calculated. These milling maps represent t he sufficient conditions to obtain the desired level of strain in the material. By using these maps, the experimental work for optimization of the mechanical alloying process can be greatly reduced. (C) 1998 Ac ta Metallurgica Inc.