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T.C FATİH SULTAN MEHMET VAKIF ÜNV. Introduction to Materials Science (Malzeme Bilimi) BME 374 Prof. Dr. Fevzi YILMAZ NATURE and STRUCTURE of MATERIALS INTRODUCTION A "material" is defined as matter that has qualities which give it individuality and by which it may be categorized. Thus all physical objects are composed of matter. "Matter" is anything that has mass and occupies space. Matter exists in solid, liquid and gas states. (A "solid" is a sample of matter that has a fixed volume and fixed shape; a "liquid" is a sample of matter that has a fixed volume, but takes the shape of the container it occupies; and a "gas" is a sample of matter that has neither a fixed volume nor a fixed shape.) Engineering design and construction of safe, serviceable, and economic structures are greatly dependent on the proper selection and use of materials. NATURE and STRUCTURE of MATERIALS The internal structure of various materials is very useful in understanding how and why different materials behave differently under various conditions. NATURE and STRUCTURE of MATERIALS SUBSTANCES THAT FORM THE MATTER Atom Molecules Compound Matter NATURE and STRUCTURE of MATERIALS Atom : An "atom" is the smallest structural unit of all solids, liquids, and gases. In other words, it is the smallest particle of an element that possesses the chemical and physical properties of that element. Element: An "element" is a substance that cannot be broken down into substances of simpler composition any further by chemical reactions. It is composed entirely of like atoms having the same atomic number. Molecules and Compounds: The smallest particle which matter can be divided without destroying its identity and characteristic properties is called a "molecule". It is the smallest particle of any matter that can exist and still be that matter. NATURE and STRUCTURE of MATERIALS Atomic Number and Atomic Mass (Atomic Weight) Atomic Number -- The number of electrons which surround a neutral atom is termed "atomic number". Since the number of electrons in a neutral atom is equal to the number of protons in it, atomic number may also be defined as the number of protons in its nucleus. Atomic Mass (Atomic Weight) Since the mass of an electron is very small (about 0.0005 as much as the mass of a proton or a neutron) the mass of an atom is considered to be nearly equal to the total mass of protons and neutrons in the nucleus. In other words, the mass of an atom is concentrated in the nucleus while the electrons account for most of the volume. NATURE and STRUCTURE of MATERIALS NATURE and STRUCTURE of MATERIALS ATOMIC BONDING When the electron configuration it is seen that some elements such as Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe) and Radon (Rn) all have eight electrons at their outermost shells. These elements have a balanced electron configuration. Helium (He) also has a balanced electron configuration since its total number of two electrons are placed at the first shell. All of those above mentioned elements do not tend to take an electron from another element or lose an electron to another element. Therefore, all these gases are named as inert (chemically inactive) or noble gases. On the other hand, most elements, unlike the noble gases, need to achieve the highly stable configuration of having eight electrons available for their outermost shells. This process takes place through one of the following procedures: • Receiving extra electrons, • Releasing electrons, or • Sharing electrons. NATURE and STRUCTURE of MATERIALS The first two of the above procedures (receiving electrons or releasing electrons) produce ions with a negative charge or positive charge, respectively. So the negative or the positive ions show coulombic * attractions to other ions of unlike charges. The third procedure (sharing electrons) lead to intimate association between atoms. All of these processes produce strong bonds (also called primary bonds, or chemical bonds) between atoms. In addition to the strong bonds, there are also always present some weaker bonds (also called secondary bonds or van der Waals** bonds) among atoms. Thus the types of bonds that can form in materials can be divided into two major categories; I. The strong bonds (primary bonds), 2. The weak or secondary bonds (van der Waals bonds). NATURE and STRUCTURE of MATERIALS The Strong Bonds (Primary Bonds) The types of strong bonds (also called Primary Bonds) are grouped as ionic bonds, covalent bonds, and metallic bonds. Ionic Bond Ionic bond is the chemical bond involving electron transfer between atoms. NATURE and STRUCTURE of MATERIALS Atoms of elements such as sodium which has one atom in its outermost atom and calcium which has two atoms in its outermost shell easily release these electrons and turn into positively charged ions. Likewise, those elements such as chlorine and oxygen, with seven or six electrons in their outer shell, respectively, easily receive electrons until they have eight electrons in their outermost shell and become negatively charged ions. Since there is always an attraction between the positively and negatively charged materials, a strong bond is established. Fig.1.6 illustrates the ionization of the sodium and chlorine atoms and the ionic bond between these two atoms. NATURE and STRUCTURE of MATERIALS a negative charge possesses an attraction for all positively charged particles, and a positive charge possesses an attraction for all negatively charged particles. Consequently, sodium ions surround themselves with negative chlorine ions, and chlorine ions surround themselves with positive sodium ions, the attraction being equal in all directions. The major requirement in an ionically bonded material is that the number of negative charges equal to the number of positive charges. NATURE and STRUCTURE of MATERIALS Covalent Bond Covalent bond is the chemical bond involving electron sharing between atoms. the electronic structure of an atom is relatively stable if there are eight electrons in its outermost shell. (An exception is the first shell, which can be stable with two electrons.) Sometimes an atom may acquire the stable condition by sharing electrons with an adjacent atom. For example, hydrogen atom, H, has only one electron in its first shell and needs a total number of two electrons in its first shell to become stable. This condition is achieved by sharing an electron with another hydrogen atom as shown in Fig.1.7(a). Again, another example is the oxygen atom, 0, which has six electrons in its outermost shell and needs a total number of eight electrons to become stable. Such a condition is achieved by sharing four electrons with an adjacent oxygen atom (two electrons from each) as shown in Fig.l.7(b). NATURE and STRUCTURE of MATERIALS An atom can share one or more electrons with an atom of a different element too. For example, fluorine, F, has seven electrons in its outermost shell and shares one atom with the hydrogen atom, H, to achieve a stable condition as shown in Fig.l.7(c). Covalent bonds between two atoms result in the formation of diatomic molecules. NATURE and STRUCTURE of MATERIALS Polyatomic (poly means many) combinations by covalent bond are also common. For example, carbon, C, which has four electrons in its outermost shell shares four electrons with four hydrogen atoms, H, and thus methane, CH4, is formed as simply shown in Fig.1.8. Figure 1.8 Covalent bond of methane NATURE and STRUCTURE of MATERIALS Figure 1.9 Representation of ethane, C2H4, molecules: (a) conventional representation (b) electron pair representation NATURE and STRUCTURE of MATERIALS Metallic Bond Metallic bond is the chemical bond involving the nondirectional sharing of delocalized electrons. If there are only a few valence electrons (outermost shell electrons) within an atom, these may be removed relatively easily while the balance of the electrons are held firmly to the nucleus. Removal of the valence electrons forms a structure of free electrons and an ion core consisting of the nucleus and nonvalence electrons. Thus in a metal structure there results positive cores of atoms and the electrons removed from their atoms. These free electrons form an "electron "cloud. As a result, an attraction is developed between the positive ion cores and the free electrons of the atoms in metal structures. For example, an atom of sodium element has one electron at its outermost shell and this electron is easily removed leaving behind a positive ion core of sodium. When the other sodium atoms in the metal structure act the same way, there happens many positive ions of sodium and a cloud of free electrons. Thus results an attraction among the positive ion cores of sodium and the free electrons. This is simply illustrated in Fig.1.10. NATURE and STRUCTURE of MATERIALS Figure 1.10 Metallic bond in sodium metal The free electrons give the metal its characteristically high electrical conductivity, since they are free to move in an electric field. They can transfer thermal energy from a high to a low temperature level; thus they are associated with the high thermal conductivity behavior of metals. They absorb light energy so that they cause the metals to become opaque to transmitted light. NATURE and STRUCTURE of MATERIALS The Weak or Secondary Bonds (Van der Waals Bonds) Secondary or the van der Waals bonds are defined as the atomic bonds that exist without electron transfer or sharing. Actually, the mechanism of secondary bonding is somewhat similar to ionic bonding, that is, this kind of bonding results from the attraction of opposite charges. The main difference between the ionic bond and the secondary bond is that, in the first one there is electron transfer between atoms leading to a chemical bond whereas in the latter one there is no electron transfer and there exists physical bond. NATURE and STRUCTURE of MATERIALS Interatomic, Interionic and Intermolecular Forces Forces acting between atoms, ions, or molecules can be of two kinds: Forces of attraction and forces of repulsion. While the forces of attraction pull the atoms, ions, or molecules together, the forces of repulsion cause them to be a certain distance away from each other. When the attractive forces are equal to the repulsive forces, an equilibrium is established. The balance between attractive and repulsive forces is illustrated in Fig.1.14 by using the ionic bond. NATURE and STRUCTURE of MATERIALS Figure 1.14 Interatomic distances: (a) The equilibrium spacing r0 , is the distance at which the attractive forces equal the repulsive forces; (b) The lowest potential energy occurs when r0 is the interatomic distance represents the variation of potential energy with interatomic spacing. NATURE and STRUCTURE of MATERIALS Assuming the potential energy of atoms to be zero at an infinite distance of separation, the energy will decrease as the atoms approach each other until the potential energy reaches a minimum at an equilibrium distance r0 where the forces of attractions and repulsion are balanced. NATURE and STRUCTURE of MATERIALS Any attempt to bring two atoms or molecules closer together than the equilibrium distance (as in the case of applying compression to materials) greatly increases the forces of repulsion between them. As the repulsive forces get very big, the attractive forces that were providing cohesion between the atoms or molecules can no longer hold them together and breaking occurs. When the atoms or molecules are pulled away from each other (as in the case of applying tension to materials) the spacing between them increases. As a result, the potential energy increases leading to breaking of the material. NATURE and STRUCTURE of MATERIALS Generalization on the Relation of Some Material Properties to the Atomic Bonding Characteristics Since all materials are made of atoms, obviously the properties of all materials are related to their atomic structures. Thus the electron configuration, atomic number, atomic mass, valence, ionization, atomic radius, interatomic or intermolecular forces and the secondary forces are the controlling aspects on the development of atomic or molecular bonds and material properties. The relation of some engineering properties of materials to the atomic bonding characteristics may briefly be summarized as follows: NATURE and STRUCTURE of MATERIALS 1. Density: Density is controlled by atomic weight, atomic radius, and coordination number. Coordination number is a significant factor because it controls the atomic packing. 2. Melting and boiling temperatures: Melting and boiling temperatures can be correlated with the depth of the energy trough shown in Fig.1.14(b ). Atoms have minimum energy at a temperature of absolute zero, and this corresponds to the bottom of the energy trough. Increased temperatures raise the energy and cause the atoms to separate themselves. A greater depth in energy trough means that a higher energy is required to separate the atoms leading to a higher melting temperature for the material. NATURE and STRUCTURE of MATERIALS 3. Strength: Strength is correlatable with the height of the sum curve (shown with dotted line) in Fig.1.14(a). That force, when related to the cross-sectional area, gives the stress to separate atoms. Deeper energy trough occurs when the interatomic forces of attractions are higher. Since deeper energy trough also leads to higher melting points as explained above, it can be observed that materials with high melting points are often the stronger (harder) materials. In contrast, in materials with weaker bonds, generally there is a correlation between softness and low melting point. NATURE and STRUCTURE of MATERIALS 4. Modulus of elasticity: The modulus of elasticity can be calculated from the slope of the sum curve of Fig.1. 14(a), as shown in Fig.1.15. At the equilibrium distance r0 , the net force P is zero, and dP/dr relates to stress to strain. For deeper energy troughs (for stronger materials), the slope of this sum curve increases. Extreme compression or extreme tension raise or lower the modulus of elasticity, respectively. Figure 1.15 Slope of sum curve of interatomic forces NATURE and STRUCTURE of MATERIALS 5. Thermal expansion: Thermal expansion is related to the atomic packing factor and vary inversely with the melting temperatures. Higher melting point materials have deeper and therefore more symmetrical energy troughs. Their mean interatomic distance increase less with a given change in thermal energy. Lower-melting point materials have less deeper and less symmetrical energy troughs. Thus they have higher thermal expansion. NATURE and STRUCTURE of MATERIALS 6. Electrical conductivity: Electrical conductivity is dependent on the type of atomic bond. Ionically or covalently bonded materials are extremely poor conductors, because the electrons are not free to leave the atoms. On the other hand, in metallically bonded materials, the free electrons easily move and conduct electricity. 7. Thermal conductivity: As explained for electrical conductivity, because of the electrons not being able to move freely in ionically or covalently bonded materials, the thermal conductivity is poor too. Since the electrons are free to move in metallically bonded materials, thermal conductivity is higher. NATURE and STRUCTURE of MATERIALS 8. Optical properties: Metallically bonded materials become opaque to transmitted light since the freely moving electrons can absorb the light energy. 9. Chemical properties: In general, chemical properties are related to the valence electrons and formation or disruption of bonds. Among the various types of chemical reaction, the corrosion reaction is probably the most significant in engineering. In corrosion, the separation of a metallic ion from the metal involves the removal of valence electrons from the outermost shell of the atom. NATURE and STRUCTURE of MATERIALS ATOMIC ARRANGEMENTS IN MOLECULAR, AMORPHOUS, AND CRYSTAL STRUCTURES The way that the atoms are arranged in a material is a major factor that affects the formation of its structure and the properties it possesses. Atomic arrangements may be classified as follows: • Molecular structures, • Amorphous structures, and • Crystal structures. NATURE and STRUCTURE of MATERIALS Molecular Structures and Atomic Arrangements As previously defined, a molecule is the unit of matter that is formed between two or more atoms which are strongly bonded together. The atoms in a molecule are generally bonded to each other by covalent bonds. Thus intramolecular attractions are very strong. In other words, "a molecule is a discrete group of atoms where the atoms are held together by primary bonds". NATURE and STRUCTURE of MATERIALS Although the atoms that compose a molecule are held together by strong bonds, these discrete groups of atoms, the molecules, are attached to each other by the weak secondary bonds. Therefore, the molecular solids are soft because the molecules in such solids can slide past each other with small stress applications. In addition, molecular compounds have lower melting and lower boiling temperatures compared with other materials. Structure of some simple molecules have been already shown in Fig.1.16. Some molecules have large numbers of atoms (as many as several thousand). Whether the molecule is small like CH4 (methane), or much larger than that like pentatriaconte, the bonds between the atoms of all molecules are strong and the bonds between the molecules are weak secondary bonds. Figure 1.16 Molecule with over 100 atoms NATURE and STRUCTURE of MATERIALS A single unit molecule is called "monomer". A monomer is the building block of a long chain or network molecule. Fig.1.17(a) illustrates several unit molecules (monomers) of C2H4 (ethylene). Molecular structures that are made up of many repeating units or mers are called polymers. Polymerization reactions occur by two main mechanisms: addition polymerization and condensation polymerization. Fig.1.17(b) shows a polymer containing many C2H4 mers, or units. The word "poly (many)" as a prefix to the name of the particular molecule. Polymer molecule consisting of vinly chloride mers, C2H3Cl, is called polyvinylchloride, PVC. In construction of this polymer, the original double bond of ethylene monomer seen in Fig.1.17(a) is broken to form two single bonds and thus the adjacent mers are connected to each other. Thus a large molecule is formed. Figure 1.17 Addition polymerization of ethylene: (a) Monomers of C2H4 ethylene; (b) Ethylene polymer formed by connection of many ethylene mers NATURE and STRUCTURE of MATERIALS Isomers -- In molecules of the same composition, more than one atomic arrangements are usually possible. Variations in the structure of molecules with the same composition are called "isomers". Fig.1.18 illustrates an example of isomers of propyl alcohol, the normal propyl alcohol and isopropyl alcohol. Although the compositions of these two alcohols are same, their structures are different. Figure 1.18 Isomers of propanol: (a) Normal propyl alcohol; (b) Isopropyl alcohol NATURE and STRUCTURE of MATERIALS Amorphous Structures and Their Atomic Arrangements « Amorphous» means having no definite form. While the crystal structures have an ordered and three dimensional, geometric arrangement that repeats itself the amorphous materials lack the repetitive pattern of crystals. Fig.1.19 shows the general arrangements of atoms in a crystalline solid and in amorphous materials such as liquids and gases. Besides the gases and liquids, some solids such as glasses, tars and asphalts have amorphous structures too. Figure 1.19 Arrangements of Atoms in (a) A crystalline solid, (b) A liquid, and (c) A gas (a) (b) (c) NATURE and STRUCTURE of MATERIALS Gases -- There is no structure in a gas other than the structure of individual molecules that it contains. Each atom or molecule in the gas is quite far from other atoms or molecules. Therefore, they are free to move independently. Since the atoms or molecules in a gas are free to move independently, a gas which fills an available space exerts pressure on its surroundings. Liquids -- Like gases, liquids are fluids that do not have the long-range repetitive pattern of crystals. However, unlike gases, the atoms are not very far from each other and they are not independent. In a way, the arrangements of atoms show some similar characteristics to that of a crystal. The difference between the atomic arrangements in crystal structures and liquids is that, crystal structures have a crystalline pattern of long-range order and the liquids have a short-range structure in which the interatomic distances between first neighbors are fairly uniform. NATURE and STRUCTURE of MATERIALS Glasses -- Glasses are non crystalline solids. They have a short range structure in which the interatomic distances between first neighbors are fairly uniform. In other words, they lack long range order of their atoms. At sufficiently high temperatures they form true liquids and increase their volume. Volume changes and expansion characteristics of glasses are shown in Fig.1.20. The term "glass" applies to all materials which have the expansion characteristics shown in that figure. Glasses may be either inorganic or organic. Figure 1.20 Volume changes in glasses NATURE and STRUCTURE of MATERIALS As can be seen in Fig.1.20, a glass is in liquid form at a sufficiently high temperature. The atoms have freedom to move around and respond to shear stresses. When it is super cooled below the melting temperature Tm, thermal contraction takes place. This contraction is caused by atomic rearrangements which produce more efficient packing of atoms. Below a certain temperature called fictive temperature Tf, no further rearrangements of atoms takes place and only further contraction is caused by reduced thermal vibrations of atoms. NATURE and STRUCTURE of MATERIALS Crystal Structures and Their Atomic Arrangements Crystals are the structures that consist of regular arrangements of atoms that have repeating patterns in three dimensions. Most engineering materials have crystalline structure. The repeating three-dimensional pattern in crystals is due to atomic coordination within the material. This pattern sometimes controls the external shape of the crystals. The smallest volumetric unit that consists of regular arrangements of atoms is called a "unit cell". All crystals are made of repeating patterns of unit cells. NATURE and STRUCTURE of MATERIALS Crystal Systems and Crystal Lattices Atomic packing in all crystals may take one of the seven crystal patterns (systems) the names and geometry of which are shown in Table 1.4. Trigonal = Rhombohedral NATURE and STRUCTURE of MATERIALS Phases, Compositional Variations and Impure Phases The term "phase" is defined as "a structurally homogeneous part of a material system". Some metals, such as copper and zinc do not contain any foreign elements in their structure; they are pure. On the other hand, there are some metals that are not pure at all. In some cases, foreign elements are intentionally added to a material for the purpose of improving its properties. For example, inclusion of zinc element to copper produces brass. In such a case, the foreign element becomes a part of the crystal lattice of copper while the copper still maintains its body centered cubic structure. If an addition becomes an integral part of the solid phase, the resulting phase is called a solid solution. + = NATURE and STRUCTURE of MATERIALS 2 grains in a solid. Figure 1.32 Grain boundary The shape of a grain in a solid is usually controlled by the presence of surrounding grains. The atoms of any particular grain are arranged with one orientation and one pattern characterized by the unit cell. However, there exists a transition zone (grain boundary) between two adjacent grains and at this zone the atoms are not aligned with either grain. The grain boundary is considered to be two dimensional, although it actually has a finite thickness of 2 to 10 or more atomic distances. The mismatch of the orientation of adjacent grains produces a less efficient packing of the atoms along the boundary. Therefore, the atoms along the boundary have a higher energy than those within the grain. So those atoms at the grain boundaries can easily be separated from the grains. NATURE and STRUCTURE of MATERIALS Grain boundary samples MECHANICAL PROPERTIES Shape changing property is used as indicator of ductility, the ability of a material to be elongated in tension. Brittleness is having hardness and rigidity but little tensile strength; breaking readily with a comparatively smooth fracture, as glass. The ability of a metal to deform plastically and to absorb energy in the process before fracture is termed toughness. In materials science, fracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for many design applications. Hardness is a measure of how resistant solid matter is to various kinds of permanent shape change when a compressive force is applied. MECHANICAL PROPERTIES MECHANICAL PROPERTIES a.) Ductile b.) Brittle MECHANICAL PROPERTIES . PHYSICAL PROPERTIES The modulus of elasticity (also known as the elastic modulus, the tensile modulus, or Young's modulus) is a number that measures an object or substance's resistance to being deformed elastically (i.e., non-permanently) when a force is applied to it. In physics, thermal conductivity (often denoted k, λ, or κ) is the property of a material to conduct heat. Thermal expansion is the tendency of matter to change in shape, area, and volume in response to a change in temperature. Heat capacity or thermal capacity is a measurable physical quantity equal to the ratio of the heat added to (or removed from) an object to the resulting temperature change. Fatigue limit, endurance limit, and fatigue strength are all expressions used to describe a property of materials: the amplitude (or range) of cyclic stress that can be applied to the material without causing fatigue failure. FIGURE OUT - COMPARISON Yield Strength σys= 370MPa=37 kg/mm2 Elastic Modules E=216GPa=216.000.MPa= 216.000.106Pa= =216.000.106 N/m2= =21.600.106 kg/m2= =21.600 kg/mm2 Kıc-fracture toughness High Kıc crack(a) tolerant Kıc= σ √a= MPa.a1/2= MPa.m1/2 Specific heat (J/kg.K cal/g.0C) Air=0,2cal/g.0C Water=1 cal/g.0C Soil= 0,3-1,5 cal/g.0C metal<seramic<polymer Th. Conductivity (W/m.K) Polymer, elostomer= 10-3. metal Th.Expansion (10-6/0C, 10-6/K) P,E = 10 . metal Ceramic 0,5.10-6/0C Polymer 50.10-6/0C -400. 10-6/0C Invar 0,7. 10-6/0C MATERIALS Fiat: Naylon 8, Çelik 0.5, Alüminyum 4, Titanyum 26, Cam 0,8 Yoğunluk, Özgül Kütle: Cam2.5, Titanyum 5,Alümin. 2.7, çelik 7.8 Teknik Özellikler Elastik Modül, Direngenlik: Çelik 210,Alümin.75, PVC 3, Naylon3 % Uzama: Naylon 1000, PC 100, Çelik25, Cam 0 Kırılma Tokluğu: Cam 0.6, Çelik 60, Titany.80, Polimer komp. 10 Sertlik:PVC10,Çelik 400, Cam 460 Akma Mukavemeti: Cam 30, Çelik1000, PVC45 Servis: PVC-20-70,Çelik-70-360, Cam-250-250 Özgül ısı: Cam900,Çelik480,PVC1400, Termal iletkenlik: Cam1,Çelik50, Alümin.200, Polimer köpük0.05 Termal genleşme: Cam9.Çelik12, Alümin.20, Polimer köpük100 Ekolojik Özellikler Enerji miktarı: Cam22,Çelik65, Alümin.275, Polimer köpük170 Cam-yüksek,Çelik-yüksek, Alümin.-yüksek, Polimer köpük-düşük Estetik Özellikler Düş.-yük. titreş:Cam7-8,Çelik9, Alümin.8-9, Polimer köpük2-3 Sönümleme-çınlama:Cam8-9,Çelik6-7, Alümin.5-8, Polim-köp2 Cam7-8,Çelik9, Alümin.8-9, Polimer köpük2-3 Cam5-6,Çelik9, Alümin.9-10, Polimer köpük8-9 Parlaklık:Çelik-%59,Alüminyum-%89 Geçirgen-opak:PC-optik kalite, Alüminyum-opak Diğer polimerlere göre göreceli özellikler Soğuğa dayanım Korozyon direnci Sönümleme Yanma geciktiriciliği Ağır Sıcağa dayanım Kayganlık Sağlam References T. Y. ERDOGAN v. d., «Materials Science for Civil Engineering», ODTU Yayıncılık, 2010, Ankara M. ASHBY and K. JOHNSON, «Materials and Design», Elsevier Butterworth Heinemann, 2006, Amsterdam.