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1、Whystudydiffusion?AtomsormoleculescantravelintheliquidaswehavelearnedintheChemistryclass.However,doyoueverthinkwhetheratomsormoleculescanalsotravelinthesolid?Heattreatmentcansoftenorhardenmetalswhentheyareinsolid.Then,whatisthemechanismsofsuchchangingforthepropertiesofmetals?Allofthesequestionsareas
2、sociatedwithoneconcept-diffusion,thephenomenonofmaterialtransportbyatomicmotion.第1页/共29页Materials of all types are often heat treated to improve their properties.The phenomena that occur during a heat treatment almost always involve atomic diffusion.Often an enhancement of diffusion rate is desired;
3、on occasion measures are taken to reduce it.Heat-treating temperatures and times,and/or cooling rates are often predictable using the mathematics of diffusion and appropriate diffusion constants.The steel gear shown on this page has been case hardened(Section 9.14);that is,its hardness and resistanc
4、e to failure by fatigue have been enhanced by diffusing excess carbon or nitrogen into the outer surface layer.第2页/共29页Whatshouldyoubeabletodoafterstudyingthischapter?Graspthethedefinitionofdiffusionandhavebasicunderstandingofhowdiffusiontakesplace.Explainsomephenomenaoccurringduringaheattreatmentpr
5、ocess.第3页/共29页Words and phrasesFicks first and second laws 菲克第一菲克第一第二定律第二定律activation energy 活化能活化能concentration 浓度浓度concentration gradient 浓度梯度浓度梯度concentration profile 浓度曲线浓度曲线decarburizing 脱碳脱碳diffusion coefficient扩散系数扩散系数diffusion flux 扩散通量扩散通量driving force 驱动力驱动力第4页/共29页permeability 渗透性渗透性inter
6、stitial diffusion 间隙扩散间隙扩散steady-state diffusion 稳态扩散稳态扩散non-steady-state diffusion 非稳态扩散非稳态扩散vacancy diffusion 空位扩散空位扩散self-diffusion 自扩散自扩散Interdiffusion 互扩散互扩散impurity diffusion 杂质扩散杂质扩散vibrational 振动的振动的vicinity 邻近，附近邻近，附近Discern v.notice/observe/detect 观察subject(to)adj.prone/susceptible 第5页/共29
7、页Manyreactionsandprocessesthatareimportantinthetreatmentofmaterialsrelyonthetransferofmasseitherwithinaspecificsolid(ordinarilyonamicroscopiclevel)orfromaliquid,agas,oranothersolidphase.Thisisnecessarilyaccomplishedbydiffusion,thephenomenonofmaterialtransportbyatomicmotion.第6页/共29页5.1 Introduction M
8、any reactions and processes that are important in the treatment of materials rely on the transfer of mass either within a specific solid(ordinarily on a microscopic level)or from a liquid,a gas,or another solid phase.This is necessarily accomplished by diffusion,the phenomenon of material transport
9、by atomic motion.This chapter discusses the atomic mechanisms by which diffusion occurs,the mathematics of diffusion,and the influence of temperature and diffusing species on the rate of diffusion.第7页/共29页Diffusion couple(扩散偶)that is formed by joining bars of two different metals together so that th
10、ere is intimate contact between the two faces.It is illustrated for copper and nickel in Figure 6.1,which includes schematic representations of atom positions and composition across the interface.This couple is heated for an extended period at an elevated temperature(but below the melting temperatur
11、e of both metals),and cooled to room temperature.第8页/共29页Chemical analysis will reveal a condition similar/identical to that represented in Figure 6.2,namely,pure copper and nickel at the two extremities of the couple,separated by an alloyed region.c.This result indicates that copper atoms have migr
12、ated or diffused into the nickel,and that nickel has diffused into copper.第9页/共29页Fig.6.2(a)A copper-nickel diffusion couple after a high-temperature heat treatment,showing the alloyed diffusion zone.(b)Schematic representations of Cu(colored circles)and Ni(gray circles)atom locations within the cou
13、ple.(c)Concentrations of copper and nickel as a function of position across the couple.第11页/共29页InterdifusionInterdifusion (impurity diffusion)-互扩散 The process that in a diffusion couple where atoms of one metal diffuse into the other.Interdiffusion may be discerned(observe/detect/discover观察)from a
14、macroscopic perspective by changes in concentration which occur over time,as in the example for the CuNi diffusion couple.There is a net drift(flow/movement)or transport of atoms from high to low concentration regions.第12页/共29页Self-diffusionDiffusion also occurs for pure metals,but all atoms exchang
15、ing positions are of the same type;this is termed self-diffusion(自扩散).Self-diffusion is not normally subject(prone/suscepitible)to observation by noting compositional changes.第13页/共29页Diffusion mechanisms(扩散机理)From an atomic perspective,diffusion is just the stepwise migration(移动)of atoms from latti
16、ce site to lattice site.In fact,the atoms in solid materials are in constant motion,rapidly changing positions.For an atom to make such a move,two conditions must be met:(1)there must be an empty adjacent site,and (2)the atom must have sufficient energy to break bonds with its neighbor atoms and the
17、n cause some lattice distortion during the displacement.第14页/共29页This energy is vibrational in nature(Section 5.10).At a specific temperature some small fraction of the total number of atoms is capable of diffusive motion,by virtue of the magnitudes of their vibrational energies.This fraction increa
18、ses with rising temperature.Several different models for this atomic motion have been proposed;of these possibilities,two dominate（important/noticeable 主要的）for metallic diffusion.第15页/共29页1.Vacancy diffusion（空位扩散）One mechanism involves the interchange of an atom from a normal lattice position to an
19、adjacent vacant lattice site or vacancy,a.This mechanism is aptly(适宜地适宜地)termed vacancy diffusion.Of course,this process necessitates(成为必要)the presence of vacancies,and the extent to which vacancy diffusion can occur is a function of（的函数）（的函数）the number of these defects that are present;significant
20、concentrations of vacancies may exist in metals at elevated temperatures 第16页/共29页Since diffusing atoms and vacancies exchange positions,the diffusion of atoms in one direction corresponds to（相当）（相当）the motion of vacancies in the opposite direction.Both self-diffusion and interdiffusion occur by thi
21、s mechanism;for the latter,the impurity atoms must substitute for（replace代替）代替）host atoms.第17页/共29页2.Interstitial diffusion(间隙扩间隙扩散散)The second type of diffusion involves atoms that migrate form an interstitial position to a neighboring one that is empty.This mechanism is found for interdiffusion of
22、 impurities such as hydrogen,carbon,nitrogen,and oxygen,which have atoms that are small enough to fit into the interstitial positions.Host or substitutional impurity atoms rarely form interstitials and do not normally diffuse via this mechanism.This phenomenon is appropriately termed interstitial di
23、ffusion b).第19页/共29页Question?In most metal alloys,interstitial diffusion occurs much more rapidly than diffusion by the vacancy mode.Why?第20页/共29页Important conclusionsSolid state diffusions:a means of mass transport within solid materials by stepwise atomic motion.The term“self-diffusion”refers to t
24、he migration of host atoms,for impurity atoms,the term“interdiffusion”is used.Two mechanisms are possible:vacant and interstitial.For a given host metal,interstitial atomic species generally diffuse more rapidly.第21页/共29页For steady-state diffusion,the concentration profile (浓度曲线）of the diffusing spe
25、cies is time independent,and the flux or rate is proportional to the negative of the concentration gradient（浓度梯度）according to Ficks first law.The mathematics for non-steady state are described by Ficks second law,a partial differential equation.第22页/共29页(Pls.translate following sentence into Chinese
26、)The magnitude of the diffusion coefficient is indicative (symptomatic)of the rate of atomic motion.Being strongly dependent on and increasing exponentially with increasing temperature.第23页/共29页SkillTraining2:TranslationBetweenEnglishandChinesePartone:HowtoTranslateEnglishintoChinese第24页/共29页Example
27、study1.Anumberoftheimportantmechanicalpropertiesofmaterialshave been discussed inthischapter.Conceptsofstressandstrainwerefirstintroduced.Stressisameasureofanappliedmechanicalloadorforce,normalizedtotakeintoaccountcross-sectionalarea.Steps:ReadthroughtheparagraphFindthesubjectandtheverbofeachsentenc
28、eandunderstandthegeneralmeaning.Thenaddtheattributivepartonrightplace.1.Re-organizeyoursentenceaccordingtoChinesegrammarlaw第25页/共29页2.Twodifferentstressparameters were defined-engineeringstressandtruestress.Strainrepresentstheamountofdeformationinducedbyastress;bothengineeringandtruestrainsareused.T
29、ips:oThe sentence with passive voice usually needs reverse the order of the sentence when translated into Chinese.oBack ordered attributive clause or phrases usually put before the part it modified unless this part is very long.第26页/共29页3.Formetals,thephenomenonofyieldingoccursattheonsetofplasticorp
30、ermanentdeformation;yieldstrengthisdeterminedbyastrainoffsetmethodfromthestress-strainbehavior,which is indicative of the stress at which plastic deformation begins.Tips:Indefinite attributive clause is not usually put before the part it modified,especially when it is quite long.第27页/共29页4.Tensile s
31、trength corresponds to the maximum tensile stress that may be sustained by a specimen,whereaspercentselongationandreductioninareaaremeasuresofductilitytheamountofplasticdeformationthat has occurred at fracture.Tips:DefiniteattributivesentenceHowtotranslatethedash”-”Changethewordmeaning第28页/共29页感谢您的观看！第29页/共29页