材料科学与工程导论材料科学与工程导论

Chapter 5 Diffusion

Photograph of a steel gear has been “case hardened”( 表面硬化 ). The outer surface layer was selectively hardened by a high temperature heat treatment during which carbon from the surrounding atmosphere diffused into the surface. The “case” appears as the dark outer rim （边缘） of that segment （ piece, 段） of the gear that has been sectioned （剖开） .

Why study diffusion?

Atoms or molecules can travel in the liquid as we have learned in the Chemistry class. However, do you ever think whether atoms or molecules can also travel in the solid?

Heat treatment can soften or harden metals when they are in solid. Then, what is the mechanisms of such changing for the properties of metals?

All of these questions are associated with one concept--- diffusion, the phenomenon of material transport by atomic motion.

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; 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 resistance to failure by fatigue have been enhanced by diffusing excess carbon or nitrogen

into the outer surface layer.

What should you be able to do after studying this chapter?

Grasp the the definition of diffusion and have basic understanding of how diffusion takes place.

Explain some phenomena occurring during a heat treatment process.

Words and phrases Fick’s first and second laws 菲克第一第二定

律 activation energy 活化能 concentration 浓度 concentration gradient 浓度梯度 concentration profile 浓度曲线 decarburizing 脱碳 diffusion coefficient 扩散系数 diffusion flux 扩散通量 driving force 驱动力

permeability 渗透性 interstitial diffusion 间隙扩散 steady-state diffusion 稳态扩散 non-steady-state diffusion 非稳态扩散 vacancy diffusion 空位扩散 self-diffusion 自扩散 Interdiffusion 互扩散 impurity diffusion 杂质扩散 vibrational 振动的 vicinity 邻近，附近 Discern v. notice/observe/detect 观察 subject (to) adj. prone/susceptible

Many 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 by atomic motion.

5.1 Introduction• Many 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 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.

diffusion couple( 扩散偶 ) that is formed by joining bars of two different metals together so that there 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 temperature of both metals), and cooled to room temperature.

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.

Concentrations of both metals vary with position as shown in Figure 6.2c. This result indicates that copper atoms have migrated or diffused into the nickel, and that nickel has diffused into copper.

Fig.6.2 Fig.6.2 (a) A copper-nickel diffusion (a) A copper-nickel diffusion couple after a high-temperature couple after a high-temperature heat treatment, showing the heat treatment, showing the alloyed diffusion zone.alloyed diffusion zone.(b) Schematic representations of (b) Schematic representations of Cu (colored circles) and Ni (gray Cu (colored circles) and Ni (gray circles) atom locations within the circles) atom locations within the couple.couple.(c) Concentrations of copper and (c) Concentrations of copper and nickel as a function of position nickel as a function of position across the couple.across the couple.

InterdifusionInterdifusion Interdifusion (impurity diffusion) --- 互扩散 The process that in a diffusion couple where atoms of o

ne metal diffuse into the other.

Interdiffusion may be discerned (observe/detect/ discover 观察 ) from a macroscopic perspective by changes in concentration which occur over time, as in the example for the Cu–Ni diffusion couple.

There is a net drift (flow/ movement) or transport of atoms from high to low concentration regions.

Self-diffusionSelf-diffusion

• Diffusion also occurs for pure metals, but all atoms exchanging 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.

5.2 ． Diffusion mechanisms( 扩散机理 )

From an atomic perspective, diffusion is just the stepwise migration( 移动 ) of atoms from lattice 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 then cause some lattice distortion during the displacement.

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 increases with rising temperature. Several different models for this atomic motion ha

ve been proposed; of these possibilities, two dominate （ important / noticeable 主要的） for metallic diffusion.

1. Vacancy diffusion （空位扩散） One mechanism involves the interchange of an atom from a

normal lattice position to an adjacent vacant lattice site or vacancy, as represented schematically in Figure 6.3a. This mechanism is aptly ( 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） the number of these defects that are present; significant concentrations of vacancies may exist in metals at elevated temperatures

Since diffusing atoms and vacancies exchange positions, the diffusion of atoms in one direction corresponds to （相当） the motion of vacancies in the opposite direction.

Both self-diffusion and interdiffusion occur by this mechanism; for the latter, the impurity atoms must substitute for （ replace 代替） host atoms.

2.Interstitial diffusion( 间隙扩散 )

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 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 diffusion (Figure 6.3b).

Question?Question?

1. In most metal alloys, interstitial diffusion occurs much more rapidly than diffusion by the vacancy mode. Why?

Important conclusionsImportant conclusions

Solid –state diffusions: a means of mass transport within solid materials by stepwise atomic motion.

The term “self-diffusion” refers to the 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.

For steady-state diffusion, the concentration profile ( 浓度曲线） of the diffusing species is time independent, and the flux or rate is proportional to the negative of the concentration gradient （浓度梯度） according to Fick’s first law.

The mathematics for non-steady state are described by Fick’s second law, a partial differential equation.

(Pls. translate following (Pls. translate following sentence into Chinese)sentence into Chinese)

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.

Skill Training 2: Translation Between English and Chinese

Part one: How to Translate English into Chinese

Example study1. A number of the important mechanical properties of materials have been discussed in this chapter. Concepts of stress and strain were first introduced. Stress is a measure of an applied mechanical load or force, normalized to take into account cross-sectional area.Steps:

1. Read through the paragraph

2. Find the subject and the verb of each sentence and under stand the general meaning.

3. Then add the attributive part on right place.

4. Re-organize your sentence according to Chinese grammar law

2. Two different stress parameters were defined---engineering stress and true stress. Strain represents the amount of deformation induced by a stress; both engineering and true strains are used.

Tips:

o The sentence with passive voice usually needs reverse the order of the sentence when translated into Chinese.

o Back ordered attributive clause or phrases usually put before the part it modified unless this part is very long .

3. For metals, the phenomenon of yielding occurs at the onset of plastic or permanent deformation; yield strength is determined by a strain offset method from the stress-strain behavior, 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.

4. Tensile strength corresponds to the maximum tensile stress that may be sustained by a specimen, whereas percents elongation and reduction in area are measures of ductility—the amount of plastic deformation that has occurred at fracture.

Tips:• Definite attributive sentence • How to translate the dash”-”• Change the word meaning 

感谢您使用 weleve 产品！1 、本课件发布于 2011 年 11 月 08 日，由全国专业人才教育网制作，未标明原创的课件均从网络收集或是会员上传，版权归原作者所有，请大家尊重作者的劳动成果，并积极上传自己的作品与大家一起分享交流。

2 、在使用本产品过程中，如有问题，欢迎联系专业人才教育网客服：Email ： harb2006@163.com QQ ： 1330691858 亢龙有悔全国专业人才教育网： www.weleve.com