材料科學(xué)基礎(chǔ)英文版課件_(13)(編輯修改稿)
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【文章內(nèi)容簡(jiǎn)介】 )(e r f ??????? CBAo)(e r f)(e r f CCBA ??????0)(e r f)(e r f ????? CBAABC=0 A+BC=Co A+B+C=0 A=0 B=Co/2 C=Co/2 )2(e r f)2(e r fDthxCDthxBACx?????Mathematics of Diffusion (10) )2(e r f1osoDtxCCCC x ????When Cx reaches a certain value at a particular position at different temperatures c on s t a n toso ???CCCC xTherefore c o n s t a n t42?DtxFor example, if the same diffusion effect is obtained at two different temperatures T1 and T2, there is D1t1=D2t2 Mathematics of Diffusion (11) Factors Affecting Diffusion (1) (1) Diffusing species Diffusing species affect diffusion coefficient. Different diffusing species have different diffusion coefficients. . carbon diffusion in ?Fe, Carbon diffusion is much faster than Fe selfdiffusion. At 500oC, DC= m2/s (interstitial diffusion) and DFe= m2/s (vacancy diffusion), DC/DFe = (2) Temperature ?????? ??RTQDD do e x pwhere Do = preexponential constant Qd = activation energy for diffusion (J/mol) R = gas constant ( J/molK T = absolute temperature (K) (oC + ) Factors Affecting Diffusion (2) logD has a linear relationship with reciprocal temperature Factors Affecting Diffusion (3) ?103 (1/K) Diffusion coefficients are usually determined by measuring these straight lines RQs l op e d??)s l o p e( RQ ??)1(lnln do TRQDD ??)1( ogl og do TRQDD ??logDo is the intercept on the vertical axis Example Problem 1 For a steel, it has been determined that a carburizing heat treatment of 10 h duration will raise the carbon concentration to wt% at a point mm from the surface. Estimate the time required to achieve the same concentration at a position for the same steel and at the same carburizing temperature. )2(e r f1osoDtxCCCC x ????Solution Since both cases reach the same carbon concentration at the same temperatu
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