matls 1m03

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McMaster University
Materials Science and Engineering

METE 229 Materials Science and Engineering by Kadri Aydınol (D-103) any version of the book (W.D. Callister, Materials Science and Engineering an Introduction) down to the 4th edition will do. Chapter 1: Introduction Engineering properties are very much dependent on the structure of matter, although METE is not directly concerned with the structure a basic classification system for materials: ● metals and alloys: metallic elements or combinations. Large number of nonlocalized electrons so good heat-electricity conductors, opaque, strong but deformable ● ceramics: metal-nonmetal compounds, heat and electricity insulators, resistant to high temperature and harsh environments, hard but brittle ● polymers: C, H and other nonmetallic based organic compounds. Large molecular structure, low density, extremely flexible ● composites: consists more than one type, displays a combination of best characteristics ● semiconductors: electrical properties intermediate between conductors and insulators ● biomaterials: implanted in human body, must be nontoxic and compatible with body tissues Properties depend on structure: ex: hardness vs structure of steel. Examining the microstructures of steel with different cooling rates, we observe different clusters in the structure: higher cooling rate (“quenching”) yields greater hardness. So here we have the processing changing the structure. Electrical resistivity of copper, for example, can be increased by adding impurities or deforming. The Materials Selection Process 1. Pick Application, determine required Properties (mechanical, electrical, thermal, magnetic, optical, deteriorative) 2. From the Properties, identify candidate Material(s) with their structure and composition 3. Identify the required Processing to change structure and overall shape (casting, sintering, vapor deposition, doping, forming, joining, annealing) Thermal properties: thermal expansion, thermal conductivity and thermal shock resistance Thick glass cup with hot liquid poured into it will actually fracture more easily since the difference between the inner and outer surfaces is larger Magnetic properties: variety of applications: electric motors, medical imaging, data storage etc. Strength also depends on composition, adding 3 atomic % Si makes Fe a better recording medium! Optical properties: transmittance of aluminum oxide depends on material structure: higher porosity yields lower transparency Deteriorative properties: stress and saltwater causes cracks in metals, heat treatment slows crack speed in salt water Chapter 2: Bonding and Properties Issues to address: What promotes bonding, what types of bonds are there? What properties are inferred from bonding? Bohr atom model (discrete orbitals) vs Wave Mechanical Model (probabilistic orbitals) Electrons in the wave mechanical model tend towards empty orbitals with lowest energy n: 1, 2, 3, 4 - principal quantum number l: s, p, d, f - second quantum number m İ 1, 3, 5, 7 - number of energy states m s magnetic spin 2 2 6 2 2 Si (Z = 14) has the configuration 1s 2s 2p 3s 3p Stable electron configurations: noble gases have complete s and p subshells and tend to be unreactive. Most elements have unstable configurations. In the periodic table, elements in a column have similar valance structure. On the left we have the electropositive elements that readily give up electrons to become + ions, and vice versa. Energy State Diagrams may contain stable (global minimum), unstable and metastable (local minimum) states. The amount of activation energy needed to carry the system from one state to another determines the rate of reaction / direction of movement between states Atomic Bonding: atoms form bonds to lower their energy. for each pair of atoms we have a certain distance called the equilibrium interatomic spacing, at which the attractive and repulsive forces between the atoms balance each other. To be able to break the bond (ie pull the atoms infinitely far away) at least 0 amount of energy will be needed (bond energy). Ionic Bonding: occurs between + and - ions with large differences in electronegativity, requires electron transfer. Predominant bonding in ceramics. Between the ions themselves, there will be coloumbic attraction. As a result, we will have the ions with stable electron configuration. The specific arrangement of ions (alternating sequence of anions and cations) keeps the structure from collapsing and imploding. This structure also brings about the brittleness of ceramic materials. Covalent Bonding: Requires shared electrons. Electronegativities are comparable. Possible between nonmetals, as well as metals and nonmetals. A few ceramic materials (carbides and borides) form covalent bonds. Bond strength can vary (from bismuth to diamond). Metallic Bonding: Generally strong bonds arising from a sea of donated valance electrons. Primary bond for metals and their alloys. Secondary bonding: weak bonds arising from interaction between dipoles (fluctuating dipoles with asymmetric electron clouds or molecule-induced permanent dipoles), such as Van der Waals forces. Can occur in liquids or polymers, the weak bonds are why they have low softening temperatures. Properties from bonding - Melting Temperature ● stretching the bonds beyond the bonding distance decreases the bonding energy ● lower bonding energy gives lower melting temperature ● liquids and solids are distinguised according to fluidity/viscosity, concerning the movement of atoms/molecules. ● large for ceramics, moderate for metals, low for polymers ● for polymers we have another defined temperature called “glass transition temperature” where the material is neither liquid nor solid Properties from bonding - Elasticity ● elasticity is about the material resuming its original shape when force is removed ● lower elastic modulus means material is easier to deform ● ● elastic modulus is larger if the bond energy is larger or if the curvature of the energy curve at the bond length is less steep - to achieve the same amount of seperation between atoms, we have to provide more energy for a material with less steep energy curve. (E ~ curvature at0r ) ● large elastic modulus for ceramics, moderate for metals, small for polymers Properties from bonding - Thermal Expansion ● the thermal expansion coefficient alpha is larger0if E is smaller. ● ● alpha ~ symmetry at r0 ● small for ceramics, moderate for metals, large for polymers Chapter 3: Crystal Structures and Properties How do atoms assemble into solid structures? How does density depend on structure? When to material properties vary with sample/part orientation? Energy and Packing: dense, regularly packed structures tend to have lower energy. summing the energies of all pairs in a non-dense random packing would yield a large energy value for the system, which would be unstable. Materials and Packing: ● in crystalline materials, we have periodically packed atoms in 3D arrays, which is typical of metals, many ceramics and some polymers ● in noncrystalline (amorphous) materials we have no period packing, typical of complex structures and rapid cooling Crystal Structures To identify the crystal structure, look for repeating patterns (in terms of repeating surrounding atoms) by placing imaginary lattice points, then connecting these lattice points to create a 3D structure. There are six parameters (3 edge lengths, 3 planes) defining this shape. Depending on these parameters we have 7 different crystal systems. ● cubic - 3 edges same, angles orthogonal ● tetragonal - 2 edges same, angles orthogonal ● orthorombic - arbitrary edges, angles orthogonal ● triclinic - all arbitrary edges with arbitrary angles ● monoclinic ● hexagonal ● rhombohedral we don’t have, for example, pyramids as a crystal systems, because it lacks the property of filling space as we repeat it over the axes. Metallic Crystals ● tend to be densely packed, with simple crystal structures ● reason for dense packing: typically has only one element so all atomic radii is the same, nearest neighbor distances tend to be small, metal bonding isn’t directional Atomic Packing Factor (APF) Simple Cubic Structure (SC) - rare due to poor packing, close packed directions are cube edges. coordination number = 6 (# nearest neighbors), APF of 0.52 Body Centered Cubic Structure (BCC)- close packed directions are cube diagonals, coordination number=8, APF of 0.68 Face Centered Cubic Structure (FCC) - close packed directions are face diagonals, coordination number=12, APF of 0.74, best possible packing for equal-sized atoms, considered to be a “closely packed” structure Hexagonal Close-packed Structure (HCP) - coordination number=12, APF of 0.74 Theoretical Density, ro differs slightly from the actual value Metals have larger theoretical density due to their close packing (metallic bonding) and large atomic mass. Crystals as Building Blocks As the size of the structure grows, keeping all blocks in the same orientation becomes more difficult. During the transformation from liquid to solid, first a few atoms come together to form a nucleus or seed, then more and more atoms will join it to cause solidification. If several seeds form, they will almost certainly be at different orientations and thus will grow blocks (grains) with different orientation (polycrystal). Most engineering materials are polycrystals. In very few cases it is possible to grow the structure from a single seed with a homogenous arrangement (single-crystal). Both are advantageous in different scenarios. Crystal Directions Directions in the form of [uvw]: find the projections along the three axes in fractions of the unit distances on each axis & reduce them to smallest integers Crystal Planes Planes in the form of (hkl): find the intercepts on the three axes in fractions of the unit distances on each axis, take the reciprocals and reduce them to smallest integers. For planes parallel to one of the axes, one of the intercepts can be infinity, giving the number zero with its reciprocal. Single vs Polycrystals Sİngle Crystals: Properties vary with direction (anisotropic), for example in BCC iron the elastic modulus varies with crystal direction (crystal direction dependent). Polycrystals: Properties may or may not vary with direction. If grains are randomly oriented, properties are isotropic (since all grains are randomly oriented, the overa
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