NANO-STRUCTURED MATERIALS DEVELOPED BY CONTROLLED PHASE TRANSFORMATIONS AND SYNTHESIS PROCESSES

Guillermo Solórzano
Department of Materials Science and Metallurgy, PUC-Rio de Janeiro, Brazil
Caixa Postal: 38090 – Gávea CEP: 22451-970 Rio de Janeiro, Brazil
guilsol@dcmm.puc-rio.br

    One of the major goals of Materials Science is to design alloys with pre-specified, desirable, technological properties. To reach this goal it is necessary to have a thorough understanding of the fundamental mechanism underline materials behavior. Of central importance is the understanding the effects on materials properties as response of changes in composition and how combination of processing parameters, such as temperature, time, and applied deformation affect the microstructure. In addition to the equilibrium phase to the information available in phase diagrams, non-equilibrium dynamic processes and meta-stable phases are known to be crucial in determining materials properties. Diffusion-controlled nucleation and growth processes affect long-term phases and micro structural stability. A comprehensive understanding of the basic, underlying phenomena is indispensable in controlling these factors.
    Given the above mentioned perspective, this contribution will address a variety of phase transformations taking place in the solid state, as a mechanism for tailoring materials structure at the nano and micro scales. For the purpose of a phenomenological treatment, it is common to consider two major categories of diffusional phase transformations: transformations controlled by interphase diffusion and those controlled by lattice diffusion. The former correspond to classical heterogeneous and segregation phenomena at stationery grain boundaries and also to discontinuous precipitation, coarsening and dissolution processes in polycrystalline alloys. The latter is related to a number of homogeneous transformations such as meta-stable precipitation, spinodal decomposition and ordering reactions. After presenting the fundamentals underline the above mentioned phenomena, a number of examples will be presented as occurring in metallic and ceramic systems of a technological importance. It will be shown, by means of well-documented examples that the application of analytical electron microscopy (AEM) and high-resolution of electron microscopy (HREM) allows one to obtain, respectively, nano-scale compositional data together with structural information, down to the atomic level. The synthesis, processing and characterization procedures apply to these materials will, thereby, be presented based on worked examples aiming at illustrating the control of phase transformations at the nano-scale level together with the physical and mechanical properties of structural materials and devices. Several examples of research on these guide lines, currently in progress in our laboratory, will be presented.