dont do 8.1, the rest should be able to be done without any mroe info Describe t
ID: 539180 • Letter: D
Question
dont do 8.1, the rest should be able to be done without any mroe info
Describe the microstructural changes that occur during recrystallization and grain growth and identify the thermodynamic driving forces for each of these two processes. distinguish between nucleation and growth in a phase transformation. define surface energy and distinguish it from volume energy. show that nucleation is based on a balance between volume free energy driving formation of a new phase and surface free energy hindering formation of that same phase. use Fick's First Law. given the boundary conditions and initial conditions characterizing some diffusion process, sketch the evolution of composition profiles with time use Fick's Second Law. describe the temperature dependence of the diffusivity in terms of Arrhenius behavior qualitatively compare the diffusivities of various materials describe the vacancy mechanism of solid-state diffusion.Explanation / Answer
8.2. Phase is definied as a physically distinct but essentially homogeneous part of a system separated from other parts by boundary surface.
Phase transformation is a phenomenon that involves a change in the microstructure and results formation of a new phase having distinctly different chemical/physical properties. It requires two processes, viz., nucleation and growth.
Nucleation: It is the first step of the phase transformtaion process that involves the formaton of very small particles or nuclei of the new phase. A nuclei is formed when Gibbs free energy of the system decreases. Nucleation is of two types, homogeneous and heterogeneous.
Growth: Growth is the phenomenon of increment of size of the nuclei at the expense of the surrounding parent phase.
8.3. The work (per unit area) necessary to create or extend one unit of surface area is called the surafce energy of the material. The surafce energy is tha summation of all intermolecular forces that are on the surface of the material.
The surafce energy dominates near the surface where the load is initilaly applied, while the volume energy the latter increase with time as the load is spread into the volume with time.
8.8. Diffusivity is a rate of diffusion which physically implies that the mass of substance diffuses through a unit surface in a unit time at a unit concentratio gradient.
The diffusivity of solids at different temperature is generally found to obey Arhenius equation:
D = Do exp (-Ediff/RT)
where D is diffusion coefficient, Do is maximum diffusivity, Ediff is the activation energy of diffusion, T is absolute temperature. This relation of diffusivity and temperature implies that diffusvity increases exponentially with temperature.
8.10. Diffusion of a material can occur via various mechanisms. The most energetically favorable process is the vacancy mechanism which involves an interchange of places by an atom and a neighboring vacancy. A vacancy is a missing atom in the lattice. This process requires not only the motion of vacancies, but also the presence of vacancies. The unit step in vacancy mechanism is an atom breaks its bonds and jumps into neighboring vacant site.
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.