Formation of crystalline and amorphous solid solutions of W–Ni–Fe powder during mechanical alloying

Abstract

The tungsten heavy alloy with the composition of 76.6W–17.3Ni–6.1Fe in atom percent was mechanically alloyed (MA) from the elemental powders of W, Ni and Fe. Nanocrystalline supersaturated solid solutions and amorphous phase were obtained during MA. Phase evolution, grain size and lattice distortion of these powders were determined and discussed. A thermodynamic model was developed based on semi-experimental theory of Miedema to calculate the driving force for phase evolution. The thermodynamic analysis showed that there is no chemical driving force to form the solid solution and the amorphous phase. The effect of the work of milling on the amorphization during MA was discussed and the model of multilayer amorphization during MA was applied to illustrate the feasibility of amorphization of powder with neither Δ H mix⪡0 nor D B⪢ D A. The driving force for amorphization is provided not by the negative heat of mixing or the stored energy in the grain boundaries but by the sharp concentration gradients in this system. Amorphization is mechanically driven and not by the negative heat of mixing. Crystallization is suppressed by sharp concentration gradients.