Thermodynamic Regularities of Nuclei Growth during Crystallization of Metastable Alloys

The process of crystal growth in a metastable multicomponent melt has a high speed of the solidification front, which captures atoms of some other components. As a result of such a growth, at the surface of the growing crystal the effect of “impurity capture” is observed, and the concentrations of components significantly deviate from the local equilibrium. Under such conditions, the conventional physico-chemical methods for description of processes at the interfacial surface become inapplicable. Therefore, a new variational approach was applied for an integrated description of diffusion and thermal processes at the phase interface. The growth rate of crystal nucleus in a metastable melt was obtained, using the methods of non-equilibrium thermodynamics. The developed approach allows estimation of the degree of metastable effects influence on a crystal growth rate.

Mechanical Alloying of Ti-Based Materials

Mechanical alloying (MA) is a simple and versatile dry powder processing technique that has been used for the manufacture of both equilibrium and metastable phases of commercially useful and scientifically interesting materials. It owes its origin to an industry need to develop a nickel-based super alloy for gas turbine applications that had both oxide dispersion strengthening and precipitation hardening. This far-from equilibrium powder metallurgy processing technique involves fracturing, welding and re-welding of powder particles in a High Energy Ball Mill (HEBM). MA is an economically viable process with important technical advantages. Its utmost advantage is in the synthesis of novel alloys, e.g., alloying of ordinarily immiscible elements, that is not possible by any other technique. As MA is a completely solid-state processing technique, the limitations imposed by phase diagrams do not apply to it. The MA process is capable of producing different types of metastable effects in a variety of alloy systems. Some of the metastable effects achieved by MA are solid solution formation and amorphisation. MA has the possibility of producing superior and enhanced materials than those produces by conventional methods. In this work a review of MA and its present and potential applications for Ti-based materials are presented.