University:
York University
Course:
CHEM 1100 | Chemistry and Materials Science for Engineers
Academic year:

2021
Views:

177

Pages:

3
Author:

Nick T.

How = mechanism Thermodynamics = if something is doable. Kinetic = how long will it take to happen. Diffusion is mass transport by atomic motion. In gases & liquids = random motion (Brownian) In solids = vacancy diffusion or interstitial diffusion Alloys? mix of 2 or more metals. Metallic bond < Alloys Types of Diffusion 1) Inter-diffusion | impurity Diffusion (Ni, Cu - Same structure) Initially Interface 100% Cu 100% Ni After some time Heat below melting temp Alloyed area 0% Cu 100% Ni After heating but below melting point of both Keep them in solid state. Cu start diffusing into Ni Concentration Profiles Not mixing cause its in room temp. Still In an alloy Atoms tend to migrate from regions of high concentration to regions of low concentration. 2) Self-Diffusion In an elemental solid, atoms also migrate. Label some atoms After some time Same element Label with isotopes of same element (only few) 4 in this example All the atoms that change positions are of the same metal (we do not observe compositional changes). Isotopes = equal number of protons but with different numbers of neutrons in their nuclei; hence differ in atomic mass but not chemical properties. Definition (what?) Types Mechanism (how it happens) Vacancy Diffusion Atoms are in constant motion Exchange with vacancies Applies to substitutional impurities atoms. Increasing elapsed time Rate depends on: Number of vacancies Activation energy during exchange The more vacancies, the more diffusion happens. Happens at high temp. 2) Interstitial diffusion Smaller atoms can diffuse between atoms. Position of interstitial atom before diffusionPosition of interstitial atom after diffusion Interstitial atoms are small. more rapid than vacancy diffusion Why? The smaller the size = faster travel (diffusion) More flexible Processing using diffusion Goal = increase hardness IRON (Fe) How to make it harder = carbonization or nitrization Adding carbon or nitrogen interstitially (radius so small) to make it harder. When we bring material back to room temp, it'll be harder. Fe Fe Fe Fe Carbon or nitrogen Inlet gas CH₄, NH₃, methane ammonia 700°C - 900°C Out let gas CH₄, H₂ Methane with hydrogen keep diffusing into Fe until you have 0.4%