Diffusion in Solids DIFFUSION Atom movement from a region of high concentration to a region of low concentration Driving Force = Concentration Gradient Importance of Diffusion Many reactions and processes rely on Diffusion: • nucleation and growth • recrystallization • phase transformations • creep • carburization and nitridation Case Study 1: Carburizing • Case Hardening: --Diffuse carbon atoms into the host iron atoms at the surface. --Example of interstitial diffusion is a case hardened gear. Fig. 5.0, Callister 6e. (Fig. 5.0 is courtesy of Surface Division, MidlandRoss.) 8 Case Study 2: Doping • Doping Silicon with P for n-type semiconductors: • Process: 1. Deposit P rich layers on surface. silicon 2. Heat it. 3. Result: Doped semiconductor regions. Fig. 18.0, Callister 6e. silicon 9 Types of Diffusion Self-diffusion - atoms of same material moving within lattice Interdiffusion or Impurity Diffusion - one type of material diffuses into another type of material Prerequisites of Diffusion For atoms to diffuse: a) There must be an empty adjacent site b) The atom must have sufficient energy to break bonds with its neighbor and then cause lattice distortion during displacement Diffusion Mechanisms Vacancy Diffusion occurs at high temperature since more vacancies are formed Qv N v N exp kT Diffusion Mechanisms Interstitial Diffusion • involves interdiffusion of impurities such as H, C, N or O Mathematical Models of Diffusion Two Models Steady-State Diffusion Non-Steady State Diffusion Diffusion Flux, J = quantitative measure of diffusion Units = kg/m2-s Diffusion Case I Steel 0.2% carbon Con’c changes linearly over distance Con’c Profile doesn’t change over time Distance, x Steady-State Diffusion • state of diffusion where diffusion flux (mass/area-time) does not change with time and position dC J D dx J = diffusion flux (kg/m2-s) D = diffusion coefficient (m2/s) C = concentration (kg/m3) x = distance (m) Fick’s First Law • not observed in practical applications Diffusion Case II Steel 0.2% carbon Con’c gradient changes over time and position Distance, x Nonsteady-State Diffusion • occurs when diffusion flux and concentration gradient vary with time • most practical diffusion situations are nonsteady ones • governed by Fick’s Second Law: C C D 2 t x 2 Nonsteady-State Diffusion Solution of this differential equation depends on the initial boundary conditions. One solution maybe: C x Co x 1 erf Cs Co 2 Dt Cx = concentration at depth x Cs = concentration at the surface Co = initial concentration X` erf(x) erfc(x) x erf(x) erfc(x) 0.00 0.0000000 1.0000000 1.30 0.9340079 0.0659921 0.05 0.0563720 0.9436280 1.40 0.9522851 0.0477149 0.10 0.1124629 0.8875371 1.50 0.9661051 0.0338949 0.15 0.1679960 0.8320040 1.60 0.9763484 0.0236516 0.20 0.2227026 0.7772974 1.70 0.9837905 0.0162095 0.25 0.2763264 0.7236736 1.80 0.9890905 0.0109095 0.30 0.3286268 0.6713732 1.90 0.9927904 0.0072096 0.35 0.3793821 0.6206179 2.00 0.9953223 0.0046777 0.40 0.4283924 0.5716076 2.10 0.9970205 0.0029795 0.45 0.4754817 0.5245183 2.20 0.9981372 0.0018628 0.50 0.5204999 0.4795001 2.30 0.9988568 0.0011432 0.55 0.5633234 0.4366766 2.40 0.9993115 0.0006885 0.60 0.6038561 0.3961439 2.50 0.9995930 0.0004070 0.65 0.6420293 0.3579707 2.60 0.9997640 0.0002360 0.70 0.6778012 0.3221988 2.70 0.9998657 0.0001343 0.75 0.7111556 0.2888444 2.80 0.9999250 0.0000750 0.80 0.7421010 0.2578990 2.90 0.9999589 0.0000411 0.85 0.7706681 0.2293319 3.00 0.9999779 0.0000221 0.90 0.7969082 0.2030918 3.10 0.9999884 0.0000116 0.95 0.8208908 0.1791092 3.20 0.9999940 0.0000060 1.00 0.8427008 0.1572992 3.30 0.9999969 0.0000031 1.10 0.8802051 0.1197949 3.40 0.9999985 0.0000015 1.20 0.9103140 0.0896860 3.50 0.9999993 0.0000007 Factors Affecting Diffusion Solute Solvent D(500 C) D(1000 C) Carbon FCC Iron 5 x 10-15 3 x 10-11 Carbon BCC Iron 10-12 2 x 10-9 Iron FCCAPF Iron Iron BCC Iron 10-20 3 x 10-14 Carbon HCP Ti 3 x 10-16 2 x 10-11 Silver Silver(crystal) 10-17 10-12 Silver Silver(grain boundary) 10-11 - TEMPERATURE 2 x 10-23 2 x 10-16 Factors Affecting Diffusion Solute Solvent D(500 C) D(1000 C) Carbon FCC Iron 5 x 10-15 3 x 10-11 Carbon BCC Iron 10-12 2 x 10-9 Iron FCC Iron 2 x 10-23 2 x 10-16 Iron BCC Iron 10-20 3 x 10-14 Carbon Size Ti of HCP 3 x 10-16 2 x 10-11 10-17 10-12 10-11 - Silver Silver Diffusing Silver( crystal) Specie Silver(grain boundary) Factors Affecting Diffusion Solute Solvent D(500 C) D(1000 C) Carbon FCC Iron 5 x 10-15 3 x 10-11 Carbon BCC Iron 10-12 Iron FCC Iron -9 2 x 10 Bond 2 x 10-23 Strength 2 x 10-16 of Host Iron BCC Iron 10-20 3 x 10-14 Carbon HCP Ti 3 x 10-16 2 x 10-11 Silver Silver(crystal) 10-17 10-12 Silver Silver(grain boundary) 10-11 - Factors Affecting Diffusion Solute Solvent D(500 C) D(1000 C) Carbon FCC Iron 5 x 10-15 3 x 10-11 Carbon BCC Iron 10-12 2 x 10-9 Iron FCC Iron 2 x 10-23 2 x 10-16 Iron BCC Iron 10-20 3 x 10-14 Carbon HCP Ti 3 x 10-16 2 x 10-11 Silver Silver(crystal) 10-17 10-12 Silver(grain boundary) 10-11 - Silver Defects Factors Affecting Diffusion 1. Temperature - increasing temperature increases diffusion rates Qd D Do exp RT Do = a temp-independent pre-exponential Qd = activation energy R = gas constant Factors Affecting Diffusion 2. Diffusing Species - the smaller the diffusing atom, the faster diffusion is 3. Atomic Packing Factor - the lower the APF, the faster diffusion is 4. Bonds of Structure - the weaker the bond, the faster diffusion is 5. Presence of other diffusion paths - dislocations and grain boundaries hastens diffusion Applications of Diffusion Carburizing • technique of case hardening of steel by increasing carbon content of the surface • involves diffusing carbon into the steel from a gas, liquid or solid source Nitriding • involves diffusing nitrogen into steel for added strength CARBURIZING KINKERDALL VOIDS QUIZ In which case will diffusion occur faster? Con’c B A Distance, x QUIZ a. b. c. d. Diffusion is inversely proportional to the following except: Temperature Size of diffusing specie Bond strength APF QUIZ In which metal, fine-grain or coarsegrain metal will diffusion occur faster? Why? QUIZ 1. The diffusion coefficients for nickel in iron are given at two different temperatures: T(K) D(m2/s) 1473 2.2 x 10-15 1673 4.8 x 10-14 Determine the values of Do and the activation energy Qd. (R = 8.31 J/mol-K) 2. If a condition of steady state is achieved during the carburization of an iron plate done at 700C, calculate the diffusion flux of carbon through the plate if the concentrations of carbon at positions of 5 and 10 mm beneath the carburizing surface are 1.2 and 0.8 kg/m3, respectively. Assume a diffusion coefficient of 3 x 10-11 m2/s at this temperature. QUIZ In which alloy, Steel A = 0.8% Carbon or Steel B = 0.2% Carbon will diffusion of carbon occur faster? Why? In which metal, fine-grain or coarsegrain metal will diffusion occur faster? Why? Thanks for Listening! 1. The diffusion coefficients for nickel in iron are given at two different temperatures: T(K) D(m2/s) 1473 2.2 x 10-15 1673 4.8 x 10-14 Determine the values of Do and the activation energy Qd. (R = 8.31 J/mol-K) 2. If a condition of steady state is achieved during the carburization of an iron plate done at 700C, calculate the diffusion flux of carbon through the plate if the concentrations of carbon at positions of 5 and 10 mm beneath the carburizing surface are 1.2 and 0.8 kg/m3, respectively. Assume a diffusion coefficient of 3 x 10-11 m2/s at this temperature.