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Unidad I Ing. de Materiales
This document is the first unit of a course on the structure, arrangements, and movements of atoms taught by Dr. Edgar García Hernández. The unit introduces materials science and engineering concepts. It discusses atomic structure, crystalline arrangements of metals and ceramics, imperfections in crystals like point defects and dislocations, and atomic movements in solids under mechanical treatments. The unit provides information on crystal structures, unit cells, coordination numbers, and calculating material properties based on structure.
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Unidad I Ing. de Materiales
- 1. Unidad I Estructura, arreglos y movimientos de los átomos Dr. Edgar García Hernández División de Estudios de Posgrado e Investigación/Departamento de Ingeniería Química y Bioquímica e-mail: eddgarcia@hotmail.com © 2013 ITZ 1
- 2. • Conocer laclasificación de los materiales, según su estructura atómica, así como también sus arreglos atómicos. • Que el alumno conozca los tipos de dislocaciones y su origen así como sus efectos en los materiales. • Conocer los movimientos atómicos que se tiene al aplicar a un material ciertos tratamientos y trabajos mecánicos que ocasionan la difusión.
- 3. Contenido 1.1. Introducción ala ciencia e ingeniería de los materiales 1.2. Estructura atómica 1.3. Arreglos atómicos y iónicos 1.4. Imperfecciones en los arreglos atómicos y iónicos 1.5. Movimientos de átomos y iones en los 3
- 4. 1.1. Introducción ala ciencia e ingeniería de materiales La ingeniería es la profesión en la que se aplica con criterio un conocimiento de las ciencias matemáticas y naturales, adquirido por el estudio, la experiencia y la práctica, con objeto de desarrollar formas de utilización económica de los materiales y las fuerzas de la naturaleza para el beneficio de la humanidad. 4
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- 6. Energía de ionización Afinidad electrónica 6
- 7. Radio atómico Electronegatividad 7
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- 21. The percent ioniccharacter of a bond between elements A and B (A being the most electronegative) may be approximated by the expression: 21
- 22. The remaining nonvalenceelectrons and atomic nuclei form what are called ion cores, which possess a net positive charge equal in magnitude to the total valence electron charge per atom. 22
- 23. Problemas 1. Offer anexplanation as to why covalently bonded materials are generally less dense than ionically or metallically bonded ones. 2. Compute the percents ionic character of the interatomic bonds for the following compounds: TiO2, ZnTe, CsCl, InSb, and MgCl 23
- 24. 1. Explain whyhydrogen fluoride (HF) has a higher boiling temperature than hydrogen chloride (HCl) (19.4 vs. -85 oC), even though HF has a lower molecular weight. 2. On the basis of the hydrogen bond, explain the anomalous behavior of water when it freezes. That is, why is there volume expansion upon solidification? 24
- 25. 1.3. Arreglos atómicosy iónicos This photograph shows a diffraction pattern produced for a single crystal of gallium arsenide using a transmission electron microscope. The brightest spot near the center is produced by the incident electron beam, which is parallel to a 110 crystallographic direction. Each of the other white spots results from an electron beam that is diffracted by a specific set of crystallographic planes. (Photograph courtesy of Dr. Raghaw S. Rai, Motorola, Inc., Austin, Texas.) 25
- 26. Solid materials maybe classified according to the regularity with which atoms or ions are arranged with respect to one another. 1. crystalline materials. 2. Semi crystalline materials. 3. Noncrystalline or amorphous materials 26
- 27. A crystalline materialis one in which the atoms are situated in a repeating or periodic array over large atomic distances; that is, long-range order exists, such that upon solidification, the atoms will position themselves in a repetitive three-dimensional pattern, in which each atom is bonded to its nearest-neighbor atoms. All metals, many ceramic materials, and certain polymers form crystalline structures under normal solidification conditions. 27
- 28. Celda Unitaria Unit cellsfor most crystal structures are parallelepipeds or prisms having three sets of parallel faces; one is drawn within the aggregate of spheres (Figure 3.1c), which in this case happens to be a cube. A unit cell is chosen to represent the symmetry of the crystal structure, wherein all the atom positions in the crystal may be generated by translations of the unit cell integral distances along each of its edges. 28
- 29. Estructuras cristalinas metálicas Table3.1 presents the atomic radii for a number of metals. Three relatively simple crystal structures are found for most of the common metals: face-centered cubic, body- centered cubic, and hexagonal close packed. 29
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- 31. FCC 31
- 32. BCC 32
- 33. HCP 33
- 34. Two other importantcharacteristics of a crystal structure are the coordination number and the atomic packing factor (APF). For metals, each atom has the same number of nearest-neighbor or touching atoms, which is the coordination number. For face-centered cubics, the coordination number is 12. 34
- 35. The APF isthe fraction of solid sphere volume in a unit cell, assuming the atomic hard sphere model, or 35
- 36. Calculando la densidadde metales 36
- 37. Ejercicios • Show thatthe atomic packing factor for BCC is 0.68. • Some hypothetical metal has the simple cubic crystal structure shown in Figure 3.40. If its atomic weight is 70.4 g/mol and the atomic radius is 0.126 nm, compute its density. 37
- 38. Materiales cristalinos cerámicos Becauseceramics are composed of at least two elements, and often more, their crystal structures are generally more complex than those for metals. The atomic bonding in these materials ranges from purely ionic to totally covalent; many ceramics exhibit a combination of these two bonding types, the degree of ionic character being dependent on the electronegativities of the atoms. 38
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- 40. Estabilidad de estructurascristalinas cerámicas 40
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- 49. Calculando la densidadde cerámicos 49
- 50. Polimorfismo y Alotropía Somemetals, as well as nonmetals, may have more than one crystal structure, a phenomenon known as polymorphism. When found in elemental solids, the condition is often termed allotropy. The prevailing crystal structure depends on both the temperature and the external pressure. 50
- 51. Polimorfismo y Alotropía Onefamiliar example is found in carbon as discussed in the previous section: graphite is the stable polymorph at ambient conditions, whereas diamond is formed at extremely high pressures. Also, pure iron has a BCC crystal structure at room temperature, which changes to FCC iron at 912C (1674F). Most often a modification of the density and other physical properties accompanies a polymorphic transformation. 51
- 52. Carbono 52
- 53. Carbono 53
- 54. Carbono 54
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- 69. Imperfecciones en cristales •Realmente no existen cristales perfectos sino que contienen varios tipos de imperfecciones y defectos, que afectan a muchas de sus propiedades físicas y mecánicas y también influyen en algunas propiedades de los materiales a nivel de aplicación ingenieril tal como la capacidad de formar aleaciones en frío, la conductividad eléctrica y la corrosión. 69
- 70. Imperfecciones en cristales •Las imperfecciones se clasifican según su geometría y forma así: – Defectos puntuales o de dimensión cero – Defectos lineales o de una dimensión llamados también dislocaciones – Defectos de dos dimensiones • También deben incluirse los defectos macroscópicos tales como fisuras, poros y las inclusiones extrañas. • 70
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- 81. Fin de laUnidad I 81