2. • Conocer la clasificación 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 a la 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 a la 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
21. The percent ionic character of a bond
between elements A and B (A being the
most electronegative) may be approximated
by the expression:
21
22. The remaining nonvalence electrons 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 an explanation 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 why hydrogen 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 atómicos y 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 may be 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 material is 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 cells for 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
Table 3.1 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
34. Two other important characteristics 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 is the fraction of solid sphere
volume in a unit cell, assuming the atomic
hard sphere model, or
35
37. Ejercicios
• Show that the 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
Because ceramics 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
50. Polimorfismo y Alotropía
Some metals, 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
One familiar 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
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
