Molecular Diffusion in Metal Alloys Aaron Morrison ME 447

 Why is this important?  Case Hardening  Doping  Three types of Diffusion within Metals  Interstitial Diffusion  Self-Diffusion  Diffusion in Subsitutional Alloys Background

 Diffusion occurs because of defects in the solids.  Diffusion commonly occurs at the grain boundaries, inner/outer surfaces and dislocations.  The diffusion along linear, planar and surface defects is generally faster than diffusion which occurs in the lattice, they are also termed high diffusivity or easy diffusion paths Background

 Diffusing Species  Temperature  Lattice Structure  Presence of Defects  Grain size  Porosity of the alloy. Factors that Influence Diffusion

 Diffusion occurs faster for  Open crystal structures  Lower melting temperature materials  Smaller diffusing atoms  Cations  Lower density materials Factors that Influence Diffusion

Interstitial Diffusion  Must assume interstitial openings for atoms.  Steady State Diffusion  D 0 is the frequency factor and Q ID is equivalent to the enthalpy of interstitial atom migration

Self-Diffusion  Requires adjacent vacancies.  Diffusion follows:  Q SD is the activation enthalpy for self-diffusion which includes both vacancy migration and formation of enthalpy terms

Self Diffusion

Subsitutional Alloys

 Carburization  Process in which carbon is diffused into low carbon steel.  Increases hardness of steel, fatigue/tensile strength and wear resistance. Example Model

 Assume:  No volume changes occur in lattice during diffusion.  Non-steady state (Interstitial concentration varies with time)  Diffusivity is independent of composition  Temperature between 1600°F and 180o°F  No Reactions Carburization

 Beginning with Fick’s 2 nd Law  With the assumption D B is not a function of concentration Carburization

 Final Solution Carburization

 Additional types of Carburization:  Two Step Carburizing  Variation of Surface Carbon Potential and Temp During Treatment  Vacuum Carburization Carburization

 Porter, D.A., and Easterling, K.E., Phase Transformations in Metals and Alloys, 2nd ed., Chapman & Hall, 1992  Johnson, D.D. CHAPTER 6: DIFFUSION IN SOLIDS. 1st ed. Illinois: MSE, Web. 14 May 2015  Christian, J.W., The theory of transformations in metals and alloys, 2nd ed., Pergamon, 1975  Shewmon, P.G., Diffusion in solids, 2nd ed., TMS, 1989 References