Densification kinetics of W-Fe-Sn pseudoalloys in liquid phase sintering

Abstract

V. I. Nizhenko, V. Ya. Petrishchev, and V. V. Skorokhod UDC 621.762 The densification kinetics in liquid phase sintering of W-Fe-Sn powder composites containing 90 wt.% of a refractory component and 10 wt.% of a low-melting component is strongly dependent on the iron content in the melt. As the iron content in composites increases, the concentration dependence of their densification shows a maximum. Samples containing more than 1.5 wt.% Fe grow intensively because of the formation of W 6 Fe 7 intermetallide whose decomposition temperature is higher than that of liquid phase sintering.Great attention has been recently focused on the effect of interactions in complex composites on the kinetics of their densification in liquid phase sintering. The interacting systems are those whose components react with each other to form a liquid (solid) solution or new compounds. An important task of materials science is to identify basic patterns in forming powder metallurgy materials of interacting mixtures of fine powders.Experimental data on sintering of interacting disperse systems even now permit identifying a number of factors that may underlie the formation of materials with a certain porous structure and needed properties. An important factor is the liquid phase. It serves as a kind of activator in a disperse system of interacting metals since, along with bulk diffusion, the intermetallide formed in the system enables rapid diffusion transport of components through the melt, in which the diffusion coefficient is four to five orders of magnitude higher than in the solid phase [1].Of special interest are the papers [2-8] presenting a systematic experimental study of reactive liquid phase sintering accompanied by the synthesis of TiFe and TiNi intermetallides. The former intermetallide is capable of reversely sorbing hydrogen (hydrogen accumulator) and holds much promise for hydrogen energy [9] and the latter is a shape memory material.When the liquid phase forms, the intermetallide originates from contact melting at the interface of dissimilar particles; a substantial amount of heat is released in the process. The rate of nonisothermal heating of a billet is a factor that permits maintaining the balance between heat release and dissipation in reactive sintering. A heating rate of no greater than 15 deg/min should be kept in synthesizing titanium nickelide in controlled reactive sintering [2]. However, higher heating temperatures are permitted in synthesizing equiatomic TiFe intermetallide, whose heat of formation is almost half that of TiNi [3].Moreover, reactive sintering may be controlled by activating and deactivating additions. For example, 1 to 3 at.% Fe added to the Ti-Ni system decreases the formation rate of titanium nickelide and acts as an inhibitor (retarder) of reactive diffusion [4]. If nickel is introduced into the Ti-Fe system, the intermetallide is formed at a greater rate primarily owing to a decrease in the eutectic temperature from 1085 to 1040°C [3].