机械工程材料双语讲义

Shandong Jiaotong University Bilingual Teaching MaterialsCourse: Engineering MaterialsDepartment: Mechanical EngineeringEditor: LI Wei2007-12-25ContentIntroduction --------------------------------------------------------------------------------------1 Chapter 1Structure of Mechanical Engineering Materials --------------------------------4 Chapter 2 Phase Diagrams ---------------------------------------------------------------------8 Chapter 3 Material’s Mechanical Behavior, Plastic Deformation and Recrystallization ---------------------------------------------------------------------------------------13 Chapter 4 Strengthening and Toughening of Mechanical Engineering Materials------18 §4.1 Strengthening ways and mechanisms of steels---------------------------------18 §4.2 Heat treatment process of steels--------------------------------------------------20 Chapter 5Common Metallic Materials------------------------------------------------------33 §5.1 Introduction--------------------------------------------------------------------------33 §5.2 Engineering structural steels------------------------------------------------------35 §5.3 Steels used for mechanical structure---------------------------------------------36 §5.4 Tool steels----------------------------------------------------------------------------39 §5.5 Stainless steels ----------------------------------------------------------------------43 §5.6 cast irons-----------------------------------------------------------------------------43 References --------------------------------------------------------------------------------------- Appendix 1--------------------------------------------------------------------------------------- Appendix 2--------------------------------------------------------------------------------------- Appendix 3--------------------------------------------------------------------------------------- Appendix 4--------------------------------------------------------------------------------------- Appendix 5---------------------------------------------------------------------------------------Introduction1. What is materials science and the significance of materials science study?① Materials and materials scienceMaterials, according to t he Webster’s dictionary, may be defined as substances of which something is composed or made.-------材料Materials science is primarily concerned with the basic knowledge about the internal structures, properties and processing of materials.-------材料学② Development of materialsHumankind and materials have evolved over the passage of time and are continuing to do so. All of us live in a world of dynamic change, and materials are no exception. The advancement of civilization has historically depended on the improvement of materials to work with. Prehistoric humans were restricted to naturally accessible materials such as stones, wood, bones and fur. Over time, they moved from the materials Stone Age into the newer Copper (Bronze) and Iron Ages.Nowadays we have many kinds of new materials which are produced into finished goods used in all kinds of fields. And the research on new materials and new technology is still going on.③ The significance of materials science/metal materials---------材料研究意义及学习目的The production and processing of materials into finished goods constitutes a large part of our present economy. Engineers design most manufactured products and the processing systems required for their production. Since products require materials, engineers should be knowledgeable about the internal structure and properties of materials so that they can choose the most suitable ones for each application and develop the best processing methods.Their properties can be varied by variations in processing during manufacture. Engineers are often required to decide what properties are required and which materials satisfy these requirements. For example, what properties would be required for the materials to be used for the manufacture of body amour (fig.0.1). It then needsto be decided how the material can be shaped and processed to achieve suitable service performance.Metal materials are the most widely used amongall materials. So it’s very necessary for us to learnthe structure, properties and processing of metalmaterials with the aim of providing an engineeringbasis for materials application and selection.The structure of a material will influence the properties and hence their performance in service. The properties will also influence how the materials can be processed. Processing will alter the structure of materials and hence properties. So there is a complex relationship between structure, properties and processing (fig.0.2).2. Classification of materials--------材料的分类For convenience, most engineering materials are divided into four main classes by the chemical composition of materials or the bond type:Metallic materials, ceramic materials, polymeric materials, composite materials Composite materials are made for specific purpose and consist of various combinations of the other classes, such as polymer-ceramic, ceramic-metals etc.By behavior in service, materials can be classified into structural materials and functional materials.3. Metallic materials<Composition> These materials are inorganic substances that are composed of one or more metallic elements and may also contain some nonmetallic elements. Examples of metallic elements are iron, copper, aluminum, nickel and titanium. Nonmetallic elements such as carbon, nitrogen and oxygen may also be contained in metallic materials.<Structure> Metals have a crystalline structure in which the atoms are arranged in an orderly manner.<Properties> Metals in general are good thermal and electrical conductors. Many metals are relatively strong and ductile at room temperature, and many maintain good strength even at high temperatures.<Types> Metals and alloys are commonly divided into two classes: ferrous metals and alloys, which contain a large percentage of iron such as steels and cast irons; nonferrous metals and alloys, which do not contain iron or contain only a relatively small amount of iron. Examples of nonferrous metals are aluminum, copper, zinc, titanium and nickel. The distinction between ferrous and nonferrous alloys is made because of the significantly higher usage and production of steels and cast irons when compared to other alloys.<Application> Metals in their alloyed and pure forms are used in numerous industries including aerospace, biomedical, semiconductor, electronic, energy, civil structural and transport. Scientists and engineers are constantly attempting to improve the properties of existing alloys and to design and produce new alloys with improved strength, high temperature strength and fatigue properties. The existing alloys may be improved by better chemistry, composition control and processing techniques.Many metal alloys such as titanium alloys, stainless steel, cobalt-base alloys（钴基合金）are also used in biomedical applications including orthopedic implants(矫形外科所用的植入物)，heart valves(心脏瓣膜)，fixation devices and screws. These materials offer high strength, stiffness and biocompatibility. Biocompatibility isimportant because the environment inside the human body is extremely corrosive and therefore materials used for such applications must be effectively impervious to this environment.Chapter 1 Structure of Mechanical Engineering Materials 1. Crystal structure of metals(金属的晶体结构)<1> crystalline and amorphous solids （晶体与非晶体）Solid may be categorized into crystalline and amorphous solids. The physical structure of solid materials depends mainly on the arrangements of the atoms, ions or molecules that make up the solid and the bonding forces between them.If the atoms or ions are arranged in order-------long-range order (LRO)-------the solid of material is called a crystalline solid or crystalline material. Examples are metals, alloys and some ceramic materials. Crystalline solids have fixed melting points.In contrast, there are some materials whose atoms and ions are not arranged in a long-range manner and possess only short-range order (SRO). This means that order exists only in the immediate neighborhood of an atom or a molecule. Such materials are classified as noncrystalline/amorphous materials. For example, most polymers, glasses and some metals are members of the amorphous class of materials. They don’t have fixed melting point.<2> the space lattice and unit cells (空间晶格与晶胞)（空间晶格）Atomic arrangements in crystalline solids can be described by referring the atoms to the points of intersection of a network of lines in three dimensions. Such a network is called a space lattice(Fig.1.1).(晶胞)In a space lattice, there are repeating units which can describe the characteristic of the whole lattice, we call the units-------unit cells(Fig.1.1).（晶格常数）The size and shape of the unit cell can be described by three lattice vectors(矢量、向量) a, b, c, originating from one corner of the unit cell. The axial lengths a, b and c and the interaxial angles α、β and γ are called the lattice constants of the unit cell.Fig.1.1 Space lattice and unit cell<3> Principal Metallic Crystal StructuresMost elementary metals (about 90%) crystallize upon solidification into three densely packed crystal structures: body-centered cubic (bcc), face-centered cubic (fcc) and hexagonal close-packed cubic (hcp), as shown in fig.1.2.Fig.1.2 Three metallic crystal structuresMost metals crystallized in these dense-packed structures because energy is released as the atoms come closer together and bond more tightly with each other. Thus, these structures are in lower and more stable energy arrangements.Next let’s discuss the three classical crys tal structures in detail.BCC: In this unit cell, there is one lattice point (atom) at each corner of the cube and one at the center of the cube. Each atom at the corner is shared by eight neighbor unit cells.FCC: one atom at each corner and one at the center of each cube face. The atom at each corner is also shared by eight neighbor unit cells and the atom at the center of each cube face is shared by two neighbor unit cells.HCP: In bcc and fcc, a=b=c and α=β=γ=90°; while in hcp, the bottom and top face areboth hexagon (六边形) and the constant “a” is the basal side length. The height of the hexagon prism is c. The ratio of c to a is called the axial ratio. For an ideal hcp crystal structure, the ratio is 1.633.2.Crystal structure of real crystals(实际晶体的结构)----------crystalline imperfections(晶体缺陷)In reality, crystals are never perfect and contain various types of imperfections and defects that affect many of their physical and mechanical properties, which in turn affect many important engineering properties of materials such as the cold formability of alloys, the electronic conductivity of semiconductors, the rate of migration of atoms in alloys, and the corrosion of metals.Crystal lattice imperfections are classified according to their geometry and shape. The three main divisions are (1) zero-dimensional or point defects; (2) one-dimensional or line defects (dislocation); (3) two-dimensional defects, that include external surfaces, grain boundaries, twins, low-angle boundaries, high-angle boundaries, twists, stacking faults, voids and precipitates. Three-dimensional macroscopic or bulk defects could also be included. Examples of these defects are pores, cracks and foreign inclusions.(1) Point DefectsThe simplest point defect is the vacancy, an atom site from which an atom is missing. Vacancies may be produced during solidification as a result of local disturbances during the growth of crystals, or they may be created by atomic rearrangements in an existing crystal due to atomic mobility. In metals the equilibrium concentration of vacancies rarely exceeds about 1 in 10000 atoms. Vacancies are equilibrium defects in metals, and their energy of formation is about 1 eV.Additional vacancies in metals can be introduced by plastic deformation, rapid cooling from higher temperatures to lower ones to entrap the vacancies, and by bombardment with energetic particles such as neutrons. Nonequlibrium vacancieshave a tendency to cluster, causing divacancies or trivacancies to form. Vacancies can move by exchanging positions with their neighbors. This process is important in the migration or diffusion of atoms in the solid state, particularly at elevated temperatures where atomic mobility is greater.Sometimes an atom in a crystal can occupy an interstitial site between surrounding atoms in normal atom sites. This type of point defect is called a self-interstitial, or interstitialcy(结点间). These defects do not generally occur naturally because of the structural distortion they cause, but they can be introduced into a structure by irradiation.In ionic crystals point defects are more complex due to the necessity to maintain electrical neutrality. When two oppositely charged ions are missing from an ionic crystal, a cation-anion divacancy is created that is known as a Schottky imperfection. If a positive cation moves into an interstitial site in an ionic crystal, a cation vacancy is created in the normal ion site. This vacancy-interstitialcy pair is called a Frenkel imperfection. The presence of these defects ionic crystals increases their electrical conductivity.Impurity atoms of the substitutional or interstitial type are also point defects and may be present in metallic or covalently bonded crystals. For example, very small amount of substitutional impurity atoms in pure silicon can greatly affect its electrical conductivity for use in electronic devices. Impurity ions are also point defects ionic crystals.(2) Line Defects (Dislocations)Line Defects, or dislocations, in crystalline solids are defects that cause lattice distortion centered around a line. Dislocations are created during the solidification of crystalline solids. They are also formed by the permanent or plastic deformation of crystalline solids and by vacancy condensation and by atomic mismatch in solid solutions.The two main types of dislocations are the edge and screw types. A combinationof the two gives mixed dislocations, which have edge and screw components. An edge dislocation is created in a crystal by the insertion of an extra half plane of atoms, just above the symbol ┴. The inverted “tee”, ┴indicates a positive edge dislocation, whereas the upright “tee”, ┬, indicates a negative edge dislocation.The displacement distance of the atoms around the dislocation is called the slip or Burgers vector b and is perpendicular to the edge-dislocation line. Dislocations are Nonequlibrium defects, and they store energy in the distorted region of the crystal lattice around the dislocation. The edge dislocation has a region of compressive strain where the extra half plane is and a region of tensile strain below the extra half plane of atoms.The screw dislocation can be formed in a perfect crystal by applying upward and downward shear stresses to regions of a perfect crystal that have been separated by a cutting plane. These shear stresses introduce a region of distorted crystal lattice in the form of a spiral ramp of distorted atoms of screw dislocation. The region of distorted crystal is not well defined and is at least several atoms in diameter. A region of shear strain is created around the screw dislocation in which energy is stored. The slip or Burgers vector of the screw dislocation is parallel to the dislocation line.Most dislocations in crystals are of the mixed type, having edge and screw components.(3) Planar DefectsPlanar defects include external surfaces, grain boundaries, twins, low-angle boundaries, high-angle boundaries, twists and stacking faults. The free or external surfaces are considered defects because the atoms on the surface are bonded to other atoms only on one side. Therefore, the surface atoms have a lower number of neighbors. As a result these atoms have a higher state of energy when compared to the atoms positions inside the crystal with an optimal number of neighbors. The higher energy associated with the atoms on the surface of a material makes the surface susceptible to erosion and reaction with elements in the environment. This pointfurther illustrates the importance of defects in the behavior of materials.Grain boundaries are surface imperfections in polycrystalline materials that separate grains (crystals) of different orientations. In metals grain boundaries are created during solidification when crystals formed from different nuclei grow simultaneously and meet each other. The shape of the grain boundaries is determined by the restrictions imposed by the growth of neighboring grains.The grain boundary itself is a narrow region between two grains of about two to five atomic diameters in width and is a region of atomic mismatch between adjacent grains. The atomic packing in grain boundaries also have some atoms in strained positions that raise the energy of the grain-boundary region.The higher energy of the grain boundaries and their more open structure make them a more favorable region for the nucleation and growth of precipitates. The lower atomic packing of the grain boundaries also allows for more rapid diffusion of atoms in the grain boundary region. At ordinary temperatures grain boundaries also restrict plastic flow by making it difficult for the movement of dislocations in the grain boundary region.3. Substitutional Solid Solutions and Interstitial Solid Solutions(1) Substitutional Solid Solutions ----置换固溶体In substitutional solid solutions formed by two elements, solute atoms can substitute for parent solvent atoms in a crystal lattice. The crystal structure of the parent element or solvent is unchanged, but the lattice may be distorted by the presence of the solute atoms, particularly if there is a significant difference in atomic diameters of the solute and solvent atoms.The following conditions are favorable for extensive solid solubility of one element in another:The diameters of the atoms of the elements must not differ by more than about 15 percent.If the atomic diameters of the two elements that form a solid solution differ, therewill be a distortion of the crystal lattice. Since the atomic lattice can only sustain a limited amount of contraction or expansion, there is a limit in the difference in atomic diameters that atoms can have and still maintain a solid solution with the same kind of crystal structure.◆The crystal structures of the two elements must be the same.◆There should be no appreciable difference in the electronegativities(电负性) of the two elements so that compounds will not form. Or else, the highly electropositive（带正电的，阳性的）element will lose electrons, the highly electronegative(带负电的，阴性的) element will acquire electrons, and compound formation will result.◆The two elements should have the same valence(化合价，原子价).If there is a shortage of electrons, the binding between them will be upset, resulting in conditions unfavorable for solid solubility.(2) Interstitial Solid Solutions---间隙固溶体In interstitial solutions the solute atoms fit into the spaces between the solvent or parent atoms. These spaces or voids are called interstices(间隙，空隙). Interstitial Solid Solutions can form when one atom is much larger than another. Examples of atoms that can form interstitial solid solutions due to their small size are hydrogen, carbon, nitrogen and oxygen.An important example of an interstitial solid solution is that formed by carbon in FCC γiron that is stable between 912 and 1394℃. The atomic radius of γiron is 0.129nm and that of carbon is 0.075nm, and so there is an atomic radius difference of 42 percent. However, in spite of this difference, a maximum of 2.11 percent(E in the Fe--Fe3C phase diagram) of the carbon can dissolve interstitially in iron at 1148℃. 4. Allotropy or polymorphism of pure irons(纯铁的同素异晶转变)Many elements and compounds exist in more than one crystalline form under different conditions of temperature and pressure. This phenomenon is termed allotropy or polymorphism. For example, iron, titanium and cobalt.The pure iron cooling curve shows a freezing temperature of 1538℃at whichpoint a high-temperature solid of BCC structure is formed called δiron. Upon additional cooling, at a temperature of approximately 1394℃, a solid-solid phase transformation of BCC δiron to an FCC solid called γiron takes place. With further cooling a second solid-solid phase transformation takes place at 912℃. In this transformation the FCC γiron reverts back to a BCC iron structure called αiron (Fig.1.3).In the phase diagram, line GS is called theallotropy transformation line of pure iron,indicating the transformation from γiron toαiron when cooling.Fig.1.3 Cooling curve for pure ironChapter 2 Phase DiagramsA phase in a material is a region that differs in its microstructure and /or composition from another region. Phase diagrams are graphical representations(代表图形) of what phases are present in a materials system at various temperatures, pressures and composition. Most phase diagrams are constructed by using equilibrium conditions and are used to understand and predict many aspects of the behavior of materials. --------相与相图Equilibrium conditions: cooling or heating very slowly. In most cases equilibrium is approached but never fully attained.Before discussing the phase diagram, we need to introduce crystallization and cooling curve of metals.1. Crystallization and cooling curveAs we learned in previous classed, solids may be categorized into crystalline and amorphous solids. If a metal transforms from a melted liquid into a crystalline solid,we call this process crystallization. -------结晶This process can be expressed by cooling curve. Cooling curve can be used to determine phase transition temperatures for both pure metals and alloys. A cooling curve is obtained by recording the temperature of a material versus time as it cools from a temperature higher than melting point to room temperature.The cooling curve for a pure metal is shown in Fig.2.1. If the metal cools under equilibrium condition, its temperature drops continuously along the line AB of the curve. At the melting point solidification begins and the cooling curve becomes and remains flat until solidification completes. In region BC, the metal is in the form of a mixture of solid and liquid phases. As point C is approached, solidification is complete. During the course, the temperature remains constant because there is a balance between the lost heat by the metal and the latent heat supplied by the solidifying metal. After C, the cooling curve will again show a drop in temperature with time(segment CD of the curve) .--------纯金属冷却曲线分析Fig.2.1 the cooling curve of a pure metalWe must note that the above discuss is based on cooling under equilibrium conditions, it’s a theoretic cooling process. In fact, crystallization of real metals need a degree of undercooling (过冷度), that is, metals cool below the freezing temperature.The cooling curves for alloys are similar to that of pure metals, but normally there are no flats in these curves. From cooling curves of metals and alloys, equilibrium diagrams of alloys are constructed. Take Cu-Ni phase diagram for example. The ordinate indicates temperature and the abscissa indicates chemical composition in weight percent.Fig.2.2 Construction of Cu-Ni phase diagram2. Types of phase diagramThe most simplest and classical binary alloy phase diagrams are isomorphous system and eutectic system.If the two components of the alloy are completely soluble in each other in both the liquid and solid states, only a single type of crystal structure exists for all compositions of the components, and therefore they are called isomorphous system. An important example is Cu-Ni system. (Fig 2.2 b)Many binary alloy systems have components that have limited solid solubility in each other as, for example, in the lead-tin (Pb-Sn) system (Fig. 2.3).In simple binary eutectic systems, there is a specific alloy composition known as the eutectic composition that freezes at a lower temperature than all other compositions. The temperature is called the eutectic temperature. In the phase diagram, the corresponding point (C in Fig.2.3) is called the eutectic point. Compositions to the left of the eutectic point are called hypoeutectic. Conversely, compositions to the right of the eutectic point are called hypereutectic.When liquid of eutectic composition is slowly cooled to the eutectic temperature, the single liquid phase transforms simultaneously into two solid forms (solid solution α and β). This transformation is known as the eutectic reaction and is written as:3. Iron-carbon phase diagram<1>IntroductionPlain carbon steels and cast irons contain not only carbon and iron elements, but also minor amounts of other elements such as silicon, phosphorus and sulfur, etc. However, in this course, they are treated as iron-carbon binary alloys. The effect of other elements in steels will be dealt with in later sections.Iron-carbon alloys containing over 6.69% carbon are too brittle to use in industry, so we only discuss part of the iron-carbon phase diagram-----the iron-iron carbide (Fe-Fe3C) phase diagram. Fe3C, is also called cementite, containing 6.69% carbon. The iron-iron carbide phase diagram is shown in Fig.2.4. All the room temperature phases obtained under different conditions are filled in the diagram.Fig.2.4 The iron-iron carbide phase diagram<2> Solid phases in the Fe-Fe3C phase diagram①Ferrite: this phase is an interstitial solid solution of carbon in the BCC α-iron crystal lattice.②Austenite: the interstitial solid solution of carbon in γ-iron.The solubility of carbon in α-iron and γ-iron is very small and their properties areclose to pure irons.③Cementite(Fe3C): a hard and brittle intermetallic<3> Invariant reactions in the Fe-Fe3C phase diagram(铁碳合金相图中的平衡反应)①eutectic reaction:In the diagram, there is a eutectic reaction which occurs at 1148℃. It can be written asAt the eutectic reaction point, liquid of 4.3% forms austenite (containing 2.11% C) and the compound Fe3C (containing 6.69% C). Line ECF is called eutectic line.Iron-carbon alloys containing 2.11～6.69% C are called cast irons. Eutectic reactions only occur in this part.②eutectoid reaction: In the lower part on the left of the diagram, there is a eutectoid line PSK on which the eutectoid reaction occurs. The temperature is 727℃. At the eutectoid reaction point S, solid austenite of 0.77%C produces ferrite（containing 0.0218%C）and cementite (containing 6.69%C). The eutectoid reaction takes place completely in the solid state. It can be written as follows.Iron-carbon alloys containing less than 2.11% C are called steels. Eutectoid reaction s only occur in this part. Steels containing 0.77%C are called eutectoid steels(共析钢). Steels containing less than 0.77%C are termed hypoeutectoid steels（亚共析钢）, and that of more than 0.77%C are designated hypereutectoid steels(过共析钢).<4> Characteristic lines and areas in the Fe-Fe3C phase diagramLine ACD is called liquidus and AECF solidus (液相线与固相线).The region above the liquidus is liquid area and the region below the solidus is called solid phase region. The region between the liquidus and solidus represents a two-phase region where both the liquid and solid phases coexist.<5>Properties of steels with the increasing of the carbon percentFrom left to right of the composition abscissa, the carbon percent increases. The very-low-carbon plain carbon steels(＜0.3%C) have relatively low strength but very。