UNIVERSIDAD AUTÓNOMA DE ZACATECAS Programa de Ingeniería Mecánica Av. López Velarde No. 801 Col. Centro Zacatecas, Zacatecas, C.P. 98000 Tel. 01 (492) 923 94 07 ext. 1615 Notas del Programa Metalurgia Física Material Preparado por: Dr. Víctor Hugo Baltazar Hernández
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UNIVERSIDAD AUTÓNOMA DE ZACATECAS
Programa de Ingeniería Mecánica
Av. López Velarde No. 801
Col. Centro
Zacatecas, Zacatecas, C.P. 98000
Tel. 01 (492) 923 94 07 ext. 1615
Notas del Programa
Metalurgia Física
Material Preparado por:
Dr. Víctor Hugo Baltazar Hernández
Notas de la materia Metalurgia Física
Dr. Víctor Hugo Baltazar Hernández 2012 2
CONTENIDO
1 LAS HERRAMIENTAS DEL METALURGISTA .............................................................................. 3
1.1 MÉTODOS PARA LA CARACTERIZACIÓN DE LOS METALES Y SUS ALEACIONES ................................. 3
1.1.1 La metalografía .................................................................................................... 3
1.2 MICROSCOPÍA ÓPTICA, ELECTRÓNICA (SEM) Y DE TRANSMISIÓN (TEM) ....................................... 8
1.1 Métodos para la caracterización de los metales y sus aleaciones
1.1.1 La metalografía
La metalografía o microscopía estudia microscópicamente las características estructurales
de un metal o de una aleación. Sin duda, el microscopio es la herramienta más importante del
metalurgista tanto desde el punto de vista científica como desde el técnico.
Es posible determinar el tamaño de grano, y el tamaño, forma y distribución de varias fases
e inclusiones que tienen gran efecto sobre las propiedades mecánicas del metal. La microestructura
revelara el tratamiento mecánico y térmico del metal y, bajo un conjunto de condiciones dadas,
podrá predecirse su comportamiento esperado.
La experiencia ha demostrado que el éxito en el estudio microscopico depende en mucho
del cuidado que se tenga para preparar la muestra. El microscopio mas costoso no revelara la
estructura de una muestra que haya sido preparada en forma deficiente. El procedimiento que se
sigue en la preparación de una muestra es comparativamente sencillo y requiere de una técnica
desarrollada solo después de práctica constante. El último objetivo es obtener una superficie plana,
sin ralladuras, semejante a un espejo. Las etapas necesarias para preparar adecuadamente una
muestra metalográfica se explican en lo siguientes subíndices [1
1.1.1.1 Muestreo
].
La selección de una muestra para estudio micrasc6pico puede ser muy importante. Si lo que
se va a investigar es una falla, se debe escoger la muestra más próxima al área de la falla y
comparársele con una tomada de la sección normal o sana.
Si el material es suave, como metales o aleaciones no ferrosas y aceros no tratados
térmicamente, la sección puede obtenerse por corte manual con una segueta. Si el material es duro,
la sección puede obtenerse mediante un disco cortador abrasivo, el cual es un plato delgado
fabricado de un abrasivo de tipo adecuado, que gira a alta velocidad. La muestra debe mantenerse
fría durante la operación de corte [1].
1.1.1.2 Esmerilado burdo o tosco
Siempre que sea posible, la muestra debe ser de un tamaño fácil de manipular. Una muestra
blanda se puede aplanar si se mueve lentamente hacia arriba y hacia abajo a través de la superficie
Notas de la materia Metalurgia Física
Dr. Víctor Hugo Baltazar Hernández 2012 4
de una lima plana poco áspera. La muestra blanda o dura puede esmerilarse burdamente sobre una
lija de banda (rotatoria), manteniendo la muestra fría sumergiéndola frecuentemente en agua
durante la operación de esmerilado. En todas las operaciones de esmerilado y pulido, la muestra
debe moverse en sentido perpendicular a las ralladuras existentes. Esto facilitara darse cuenta del
momento en que las ralladuras mas profundas hayan sido sustituidas por las menos profundas,
características del abrasivo más fino. El esmerilado continúa hasta que la superficie quede plana y
libre de mellas, rebabas, etc., y todas las ralladuras debidas al corte manual o al disco cortador no
son visibles. La Figura 1.1 muestra la superficie después del esmerilado [1].
a) b)
c)
Figura 1.1 a) Superficie de la muestra después del esmerilado burdo, amplificación 100x. b) Superficie de la muestra después del pulido intermedio en papel 400, amplificaci6n 100x. c) Superficie de la muestra sin ralladuras después del pulido final, amplificación 50x. Los puntos negros son impurezas de óxido.
1.1.1.3 Montaje
Las muestras pequeñas o de forma incomoda deben montarse de alguna manera para
facilitar el pulido intermedio y final. Alambres, varillas pequeñas, muestras de hoja metálica,
secciones delgadas, etc., deben montarse en un material adecuado o sujetarse rígidamente en una
monta mecánica.
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Dr. Víctor Hugo Baltazar Hernández 2012 5
Los materiales plásticos sintéticos que se acoplan a la muestra en una prensa para montar
especial proporcionan las bases de un tamaño uniforme conveniente (generalmente de 2,5, 3, ó 4 cm.
de diámetro) para manipular las muestras en ulteriores operaciones de pulido. Estas bases, cuando
se han hecho en forma adecuada, son muy resistentes a la acción de los reactivos de ataque que se
emplean comúnmente. La resina termofijadora que mas se emplea para montar muestras es la
baquelita. Los polvos para moldear baquelita se fabrican en colores de este material, lo cual
simplifica la identificaci6n de las muestras montadas. La muestra y la cantidad correcta de polvo de
baquelita, o una preforma de baquelita, se colocan en el cilindro de la prensa de montar. La
temperatura aumenta gradualmente hasta 150°C y se aplica una presión de moldeo de unas 4 000
lbs/pulg2 simultáneamente. Una vez que la baquelita esta adherida y curada cuando se alcanza esta
temperatura, la base con la muestra puede extraerse del dado de moldeo mientras esta caliente.
La lucita es la resina termoplástica mas común; es completamente transparente cuando se
moldea en forma adecuada, como se ve en la Figura 1.2. Esta transparencia resulta útil cuando es
necesario observar la sección exacta que se pule o cuando por cualquier otra razón se desea ver por
completo la muestra en la base. Al contrario de los plásticos termofijados, las resinas termoplásticas
no sufren cura a la temperatura de moldeo, sino que adquieren estabilidad al enfriarse. La muestra y
la cantidad de polvo de lucita adecuadas se colocan en la prensa para montar y se someten a la
misma temperatura y presión que para la baquelita (150°C y 4 000 Ibs,/pulg2
) . Una vez alcanzada
esta temperatura, se quita la bobina de calentamiento y las aletas de enfriamiento se colocan
alrededor del cilindro para enfriar la base hasta 75 °C en unos 7 min. al tiempo que se mantiene la
presión de moldeo. Si se saca la base todavía caliente o si se deja enfriar lentamente en el cilindro
de moldeo a la temperatura ambiente sin sacarla, se opacara.
Figura 1.2 a) Muestra montada en baquelita, aumentada 2x, b) muestra montada en lucita, aumentada 2x, c) muestra sostenida en un dispositivo de sujeción de metal, aumentada 2x
Las muestras pequeñas pueden montarse en forma conveniente para prepararlas
métalográficamente en un dispositivo de sujeción hecho en el laboratorio, como el de la Figura 1.2c.
Las muestras laminares delgadas, cuando se montan en tal dispositivo de sujeción, suelen alternarse
Notas de la materia Metalurgia Física
Dr. Víctor Hugo Baltazar Hernández 2012 6
con hojas metálicas "rellenadoras" de metal que tienen aproximadamente la misma dureza que las
muestras. Si se usan hojas rellenadoras, se conservara la superficie libre de las irregularidades de la
muestra y se evitara, de alguna manera, que los bordes de la muestra se redondeen durante el pulido
[1].
1.1.1.4 Pulido intermedio
Después de montada, la muestra se pule sobre una serie de hojas de esmeril o lija con
abrasivos más finos, sucesivamente. El primer papel es generalmente No. 300, luego 400, 600, 800,
y finalmente 1200.
La Figura 1.1b muestra la superficie después del pulido intermedio con lija de 400. Por lo
general, las operaciones de pulido intermedio con lijas de esmeril se hacen en seco; sin embargo, en
ciertos casos, como el de preparación de materiales suaves, se puede usar un abrasivo de carburo de
silicio. Comparado con el papel esmeril, el carburo de silicio tiene mayor rapidez de remoción y,
como su acabado es a base de resina, se puede utilizar con un lubricante, el cual impide el sobre-
calentamiento de la muestra, minimiza el daño cuando los metales son blandos y también
proporciona una acción de enjuague para limpiar los productos removidos de la superficie de la
muestra, de modo que el papel no se ensucie [1].
1.1.1.5 Pulido fino
El tiempo utilizado y el éxito del pulido fino dependen en mucho del cuidado puesto
durante los pasos de pulido previo. La última aproximación a una superficie plana libre de
ralladuras se obtiene mediante una rueda giratoria húmeda cubierta con un paño especial cargado
con partículas abrasivas cuidadosamente seleccionadas en su tamaño. Existe gran disponibilidad de
abrasivos para efectuar el último pulido. En tanto que muchos harán un trabajo satisfactorio, parece
haber preferencia por la forma gamma del oxido de aluminio para pulir materiales ferrosos y de los
basados en cobre, y oxido de cerio para pulir aluminio, magnesio y sus aleaciones. Otros abrasivos
para pulido final que se emplean a menudo son la pasta de diamante, oxido de cromo y oxido de
magnesio.
La selección de un paño para pulir depende del material que vaya a pulirse y el propósito
del estudio metalográfico. Se pueden encontrar paños de lanilla o pelillo variable, desde aquellos
que no tienen pelillo (como la seda) hasta aquellos de pelillo intermedio (como paño ancho, paño de
billar y lonilla) además de aquellos de pelillo profundo (como el terciopelo). También se pueden
encontrar paños sintéticos para pulir con fines de pulido general, de los cuales el Gamal y el
Micropaño son los que se utilizan más ampliamente. Una muestra pulida en forma adecuada
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Dr. Víctor Hugo Baltazar Hernández 2012 7
mostrara únicamente las inclusiones no metálicas; además, estará libre de ralladuras (Figura 1.1c)
[1].
1.1.1.6 Ataque
El propósito del ataque químico es hacer visibles las características estructurales del metal o
aleación. El proceso debe ser tal que queden claramente diferenciadas las partes de la
microestructura. Esto se logra mediante un reactivo apropiado que somete a la superficie pulida a
una acción química.
a)
b) c)
Figura 1.3 a) Fotomicrografía de la diferencia en composición química de las fases. B) Fotomicrografía de hierro puro, c) Ilustración del aspecto microscópico de las fronteras de grano que aparecen como líneas obscuras.
En las aleaciones compuestas de dos o más fases, las componentes se revelan durante la
acción química, al atacar preferencialmente, el reactivo, a una o más de estas constituyentes debido
a la diferencia en composición química de las fases (Figura 1.3a). En las aleaciones uniformes de
una sola fase o metales puros, se obtiene contraste y las fronteras de grano se hacen visibles debido
a las diferencias en la rapidez a que los diversos granos son atacados por el reactivo (Figura 1.3b).
Esta diferencia en la rapidez de ataque esta asociada principalmente con el ángulo que guardan las
diferentes secciones de grano con el plano de la superficie pulida. Debido al ataque químico por el
Notas de la materia Metalurgia Física
Dr. Víctor Hugo Baltazar Hernández 2012 8
reactivo de ataque, las fronteras de grano aparecerían como valles en la superficie pulida. Al chocar
con la orilla de estos valles, la luz del microscopio se reflejara fuera del microscopio, haciendo que
las fronteras de grano aparezcan como líneas oscuras. Esto se muestra en la Figura 1.3c. La
selección del reactivo de ataque esta determinada por el metal o aleación y la estructura especifica
que se desea ver. Ver tabla 1.3 en [] en el cual se enumeran algunos de los reactivos de ataque
comunes [1].
1.2 Microscopía óptica, electrónica (SEM) y de transmisión (TEM)
1.2.1 Microscopía óptica (light microscopy)
1.2.1.1 Basic Principles
The light microscope provides two-dimensional representation of structure over a total
magnification range of roughly ×40 to ×1250. Interpretation of such images is a matter of skill and
experience and needs to allow for the three-dimensional nature of features observed. The main
components of a benchtype microscope are (1) an illumination system comprising a light source and
variable apertures, (2) an objective lens and an ocular lens (eyepiece) mounted at the ends of a
cylindrical body tube, and (3) a specimen stage (fixed or rotatable). Metallic specimens that are to
be examined at high magnifications are successively polished with 6, 1 and sometimes 0.25 μm
diamond grit. Examination in the as-polished condition, which is generally advisable, will reveal
structural features such as shrinkage or gas porosity, cracks and inclusions of foreign matter.
Etching with an appropriate chemical reagent is used to reveal the arrangement and size of grains,
phase morphology, compositional gradients (coring), orientation-related etch pits and the effects of
plastic deformation. Although actually only a few atomic diameters wide, grain boundaries are
preferentially and grossly attacked by many etchants. In bright-field illumination, light is reflected
back towards the objective from reflective surfaces, causing them to appear bright. Dark-field
illumination reverses this effect, causing grain boundaries to appear bright. The degree of chemical
attack is sensitive to crystal orientation and an etched polycrystalline aggregate will often display its
grain structure clearly (Figura 1.4a). Preparation techniques for ceramics are essentially similar to
those for metals and alloys. However, their porosity can cause two problems. First, there is a risk of
entrapping diamond particles during polishing, making ultrasonic cleaning advisable. Second, it
may be necessary to strengthen the structure by impregnating with liquid resin in vacuo, provided
that pores are interconnected. The objective, the most important and critical component in the
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Dr. Víctor Hugo Baltazar Hernández 2012 9
optical train of the light microscope, is made up of a number of glass lenses and, sometimes, fluorite
(CaF2) lenses also. Lenses are subject to spherical and chromatic aberrations. Minimization and
correction of these undesirable physical effects, greatly aided by modern computational techniques,
is possible and objectives are classified according to the degree of correction, i.e. achromats,
fluorites (semi-apochromats), apochromats. Lenses are usually coated in order to increase light
transmission. As magnification is increased, the depth of field of the objective becomes smaller,
typically falling from 250μm at ×15 to 0.08μm at ×1200, so that specimen flatness becomes more
critical. The focal length and the working distance (separating its front lens from the specimen) of
an objective differ. For instance, an f 2mm objective may have a working distance of 0.15 mm. [2
]
Figura 1.4 (a) Reflection of light from etched specimen. (b) Use of oil to improve numerical aperture of objective.
Resolution, rather than magnification, is usually the prime concern of the skilled
microscopist. It is the smallest separating distance (δ) that can be discerned between two lines in the
image. The unaided eye, at the least distance of comfortable vision (about 250 mm), can resolve 0.1
mm. Confusingly, the resolution value for a lens with a so-called high resolving power is small.
Resolution is determined by (1) the wavelength (λ) of the radiation and (2) the numerical aperture
(NA) of the objective, and is expressed by the Abbe formula
δ=λ/2NA.
The numerical aperture value, which is engraved upon the side of the objective, indicates
the light-gathering power of the compound lens system and is obtained from the relation NA=n sin
α, where n is the refractive index of the medium between the front lens face of the objective and the
specimen and α is the semi-apex angle of the light cone defined by the most oblique rays collected
by the lens. Numerical apertures range in typical value from 0.08 to 1.25. Despite focusing
difficulties and the need for costly lenses, efforts have been made to use short-wavelength
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Dr. Víctor Hugo Baltazar Hernández 2012 10
ultraviolet radiation: developments in electron microscopy have undermined the feasibility of this
approach. Oil-immersion objectives enable the refractive index termto be increased (Figura 1.4b).
Thus, by replacing air (n=1) with a layer of cedar wood oil (n=1.5) or monobromonaphthalene
(n=1.66), the number of rays of reflected light accepted by the front lens of the objective is
increased, and resolution and contrast are improved. The range of wavelengths for visible light is
approximately 400–700 nm; consequently, using the Abbe formula, it can readily be shown that the
resolution limit of the light microscope is of the order of 200 nm. The ‘useful’ range of
magnification is approximately 500–1000NA. The lower end of the range can be tiring to the eyes;
at the top end, oil-immersion objectives are useful [2].
Magnification is a subjective term; for instance, it varies with the distance of an image or
object from the eye. Hence, microscopists sometimes indicate this difficulty by using the more
readily defined term ‘scale of reproduction’, which is the lineal size ratio of an image (on a viewing
screen or photomicrograph) to the original object. Thus, strictly speaking, a statement such as ×500
beneath a photomicrograph gives the scale of reproduction, not the magnification [2].
The ocular magnifies the image formed by the objective: the finally observed image is
virtual. It can also correct for certain objective faults and, in photomicrography, be used to project a
real image. The ocular cannot improve the resolution of the system but, if inferior in quality, can
worsen it. The most widely used magnifications for oculars are ×8 and ×12.5 [2].
Two-dimensional features of a standard bench microscope, the mechanical tube length tm
and optical tube length to, are of special significance. The former is the fixed distance between the
top of the body tube, on which the ocular rests, and the shoulder of the rotatable nosepiece into
which several objectives are screwed. Objectives are designed for a certain tm value. A value of 160
mm is commonly used. (In Victorian times, it was 250 mm, giving a rather unwieldy instrument.)
The optical tube length to is the distance between the front focal point of the ocular and the rear
focal plane of the objective. Parfocalization, using matched parfocal objectives and oculars, enables
the specimen to remain in focus when objectives are step-changed by rotating the nosepiece. With
each change, to changes but the image produced by the objective always forms in the fixed focal
phase of the ocular. Thus, the distance between the specimen and the aerial image is kept constant.
Some manufacturers base their sequences of objective and ocular magnifications upon preferred
numbers1 rather than upon a decimal series. This device facilitates the selection of a basic set of
lenses that is comprehensive and ‘useful’ (exempt from ‘empty’ magnification). For example, the
Michel series of ×6.3, ×8, ×10, ×12.5, ×16, ×20, ×25, etc., a geometrical progression with a
common ratio of approximately 1.25, provides a basis for magnification values for objectives and
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Dr. Víctor Hugo Baltazar Hernández 2012 11
oculars. This rational approach is illustrated in Figura 1.5. Dashed lines represent oculars and thin
solid lines represent objectives. The bold lines outline a box within which objective/ocular
combinations give ‘useful’ magnifications. Thus, pairing of a ×12.5 ocular with a ×40 objective
(NA=0.65) gives a ‘useful’ magnification of ×500 [2].
Figura 1.5 Range of ‘useful’ magnification in light microscope (from Optical Systems for the Microscope, 1967, p. 15; by courtesy of Carl Zeiss, Germany).
1.2.1.2 Selected microscopical techniques
Phase-contrast microscopy Phase-contrast microscopy is a technique that enables special surface features to be studied
even when there is no color or reflectivity contrast. The light reflected from a small depression in a
metallographic specimen will be retarded in phase by a fraction of a light wavelength relative to that
reflected from the surrounding matrix and, whereas in ordinary microscopy a phase difference in the
light collected by the objective will not contribute to contrast in the final image, in phase-contrast
microscopy small differences in phases are transformed into differences in brightness which the eye
can detect [2].
General uses of the technique include the examination of multi-phased alloys after light
etching, the detection of the early stages of precipitation, and the study of cleavage faces, twins and
other deformation characteristics. The optimum range of differences in surface level is about 20–50
nm, although under favorable conditions these limits may be extended. A schematic diagram of the
basic arrangement for phase contrast in the metallurgical microscope is shown in Figura 1.6a. A
hollow cone of light produced by an annulus A is reflected by the specimen and brought to an
image in the back focal plane of the objective. A phase plate of suitable size should, strictly, be
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Dr. Víctor Hugo Baltazar Hernández 2012 12
positioned in this plane but, for the ease of interchangeability of phase plates, the position Q in front
of the eyepiece E is often preferred. This phase plate has an annulus, formed either by etching or
deposition, such that the light it transmits is either advanced or retarded by a quarter of a
wavelength relative to the light transmitted by the rest of the plate and, because the light reflected
from a surface feature is also advanced or retarded by approximately λ/4, the beam is either in phase
or approximately λ/2 or π out of phase with that diffracted by the surface features of the specimen.
Consequently, reinforcement or cancellation occurs, and the image intensity at any point depends
on the phase difference produced at the corresponding point on the specimen surface, and this in
turn depends upon the height of this point relative to the adjacent parts of the surface. When the
light passing through the annulus is advanced in phase, positive phase contrast results and areas of
the specimen which are proud of the matrix appear bright and depressions dark; when the phase is
retarded, negative contrast is produced and ‘pits’ appear bright and ‘hills’ dark [2].
Figura 1.6 Schematic arrangement of microscope system for phase-contrast (a) and polarized light (b) microscopy.
Polarized-light microscopy
The basic arrangement for the use of polarized light is shown in Figura 1.6b. The only
requirements of this technique are that the incident light on the specimen be plane polarized and that
the reflected light be analyzed by a polarizing unit in a crossed relation with respect to the polarizer,
i.e. the plane of polarization of the analyzer is perpendicular to that of the polarizer [2].
The application of the technique depends upon the fact that plane-polarized light striking
the surface of an optically isotropic metal is reflected unchanged if it strikes at normal incidence. If
the light is not at normal incidence the reflected beam may still be unchanged, but only if the angle
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Dr. Víctor Hugo Baltazar Hernández 2012 13
of incidence is in, or at right angles to, the plane of polarization, otherwise it will be elliptically
polarized. It follows that the unchanged reflected beam will be extinguished by an analyzer in the
crossed position, whereas an elliptically polarized one cannot be fully extinguished by an analyzer
in any position. When the specimen being examined is optically anisotropic, the light incident
normally is reflected with a rotation of the plane of polarization and as elliptically polarized light;
the amount of rotation and of elliptical polarization is a property of the metal and of the crystal
orientation. If correctly prepared, as-polished specimens of anisotropic metals will ‘respond’ to
polarized light and a grain-contrast effect is observed under crossed polars as a variation of
brightness with crystal orientation. Metals which have cubic structure, on the other hand, will
appear uniformly dark under crossed polars, unless etched to invoke artificial anisotropy, by
producing anisotropic surface films or well-defined pits. An etch pit will reflect the light at oblique
incidence and elliptically polarized light will be produced. However, because such a beam cannot
be fully extinguished by the analyzer in any position, it will produce a background illumination in
the image which tends to mask the grain-contrast effect [2].
Clearly, one of the main uses of polarized light is to distinguish between areas of varying
orientation, since these are revealed as differences of intensity under crossed polars. The technique
is therefore very useful for studying the effects of deformation, particularly the production of
preferred orientation, but information on cleavage faces, twin bands and sub-grain boundaries can
also be obtained. If a ‘sensitive tint’ plate is inserted between the vertical illuminator and the
analyzer, each grain of a sample may be identified by a characteristic color which changes as the
specimen is rotated on the stage. This application is useful in the assessment of the degree of
preferred orientation and in recrystallization studies. Other uses of polarized light include
distinguishing and identifying phases in multi-phase alloys [2].
Near-perfect extinction occurs when the polars of a transmission microscope are crossed. If
a thin section or slice of ceramic, mineral or rock is introduced and the stage slowly rotated,
optically anisotropic crystals will produce polarization colors, developing maximum brilliance at
45◦ to any of the four symmetrical positions of extinction. The color of a crystal depends upon its
birefringence, or capacity for double-refraction, and thickness. By standardizing the thickness of the
section at 30–50μm and using a Michel–Lévy color chart, it is possible to identify crystalline
species. In refractory materials, it is relatively easy to identify periclase (MgO), chromite (FeCrO4),
tridymite (SiO2) and zircon (ZrSiO4) by their characteristic form and color [2].
As birefringence occurs within the crystal, each incident ray forms ordinary and
extraordinary rays which are polarized in different planes and travel through the crystal at different
velocities. On leaving the analyzer, these out-of-phase ‘fast’ and ‘slow’ rays combine to produce the
Notas de la materia Metalurgia Física
Dr. Víctor Hugo Baltazar Hernández 2012 14
polarization color. This color is complementary to color cancelled by interference and follows
Newton’s sequence: yellow, orange, red, violet, blue, green. More delicate, higher-order colors are
produced as the phase difference between the emergent rays increases. Anisotropic crystals are
either uniaxial or biaxial, having one or two optic axes, respectively, along which birefringence
does not occur. (Optic axes do not necessarily correspond with crystallographic axes.) It is therefore
possible for quartz (uniaxial) and mica (biaxial) crystals to appear black because of their orientation
in the slice. Uniaxial (tetragonal and hexagonal systems) can be distinguished from biaxial crystals
(orthorhombic, triclinic and monoclinic systems) by introducing a Bertrand lens into the light train
of the microscope to give a convergent beam, rotating the stage and comparing their interference
figures: uniaxial crystals give a moving ‘ring and brush’ pattern, biaxial crystals give two static
‘eyes’. Cubic crystals are isotropic, being highly symmetrical. Glassy phases are isotropic and also
appear black between crossed polars; however, glass containing residual stresses from rapid cooling
produces fringe patterns and polarization colors. The stress-anisotropic properties of plastics are
utilized in photoelastic analyses of transparent models of engineering structures or components
made from standard sheets of constant thickness and stressoptic coefficient (e.g. clear Bakelite,
epoxy resin). The fringe patterns produced by monochromatic light and crossed polars in a
polariscope reveal the magnitude and direction of the principal stresses that are developed when
typical working loads are applied [2].
1.2.2 Microscopía Electrónica de Barrido (Scanning Electron Microscopy)
1.2.2.1 Introduction
In 1993, Charles Smithart was convicted of the murder of an 11-year-old girl in the town of
Glennallen, Alaska. Prosecutors suspected Smithart after he was spotted at the scene of the crime,
but they had no evidence directly linking him to the murder. That's where a scanning electron
microscope (SEM) came in.
Using the X-ray spectroscopy detector of an SEM, a forensic scientist analyzed bits of iron
found at the scene of the crime. He found that they had a globular shape that only welding or
grinding produces. As it turned out, Smithart had a welding rig in his shop and would sometimes
repair bicycles for the local children. Thanks to the tremendous capabilities of scanning electron
microscopes, prosecutors had the evidence they needed to link Smithart to the crime.
Why was an SEM, rather than a regular light, or optical, microscope from the local high
school, necessary to examine the evidence for Smithart's trial? For one thing, SEMs can magnify
objects at upward of 300,000 times the size of the object studied. Scientists refer to this number as
the magnification power and denote it, for example, as 300,000x. In contrast, run-of-the-mill
As with many things, an SEM is more than the sum of its parts. Read on to see how all of
these components work together to create astounding images of very, very tiny things [4
1.2.2.3 Effects of Electron Bombardment
].
Electron bombardment of a sample is unique to microprobe analysis and produces a large
number of effects from the target material (Figura 1.8). The incident electrons interact with
specimen atoms and are significantly scattered by them (rather than penetrating the sample in a
linear fashion). Most of the energy of an electron beam will eventually end up heating the sample
(phonon excitation of the atomic lattice); however, before the electrons come to rest, they undergo
two types of scattering: elastic and inelastic.
In elastic scattering, the electron trajectory changes, but its kinetic energy and velocity
remain essentially constant (due to large differences between the mass of the electron and nucleus).
This process is known as electron backscattering (although later we will confine the term
"backscattered electrons" to those scatter out of the sample).
Figura 1.8 Effects produced by electron bombardment of a material.
In inelastic scattering, the trajectory of the incident electron is only slightly perturbed, but
energy is lost through interactions with the orbital electrons of the atoms in the specimen. Inelastic
interactions produce diverse effect including:
phonon excitation (heating)
cathodoluminescence (visible light fluorescence)
continuum radiation (bremsstrahlung or “braking” radiation)
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Dr. Víctor Hugo Baltazar Hernández 2012 18
characteristic x-ray radiation
plasmon production (secondary electrons)
Auger electron production (ejection of outer shell electrons)
Two major factors control which effects can be detected from the interaction volume. First,
some effects are not produced from certain parts of the interaction volume (Figura 1.9). Beam
electrons lose energy as they traverse the sample due to interactions with it and if too much energy
is required to produce an effect, it will not be possible to produce it from deeper portions of the
volume. Second, the degree to which an effect, once produced, can be observed is controlled by
how strongly it is diminished by absorption and scattering in the sample.
For example, although secondary and Auger electrons are produced throughout the
interaction volume, they have very low energies and can only escape from a thin layer near the
sample's surface. Similarly, soft X-rays, which are absorbed more easily than hard X-rays, will
escape more readily from the upper portions of the interaction volume. Absorption is an important
phenomenon and is discussed in more detail below.
Figura 1.9 Generalized illustration of interaction volumes for various electron-specimen interactions. Auger electrons (not shown) emerge from a very thin region of the sample surface (maximum depth about 50 Å) than do secondary electrons (50-500 Å).
1.2.2.4 SEM imaging
Secondary Electrons
Figure 1.10 shows the microstructure of the specimen by means of secondary electrons
(SE) signal at a voltage level of 20kV and a spot size of 40 µm. Some features can be identified
from this SE signal based on the surface topography (due to prior etching). These features have
been labelled as “a” and “b” that possibly correspond to different phases (Figure 1.10), however the
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Dr. Víctor Hugo Baltazar Hernández 2012 19
information obtained from this SE signal is merely qualitatively and cannot provide further
information other than imaging. As secondary electrons are generated through inelastic interactions
and their interaction volume is in the order of a few hundreds of nanometers (50 to 500 Ǻ) into the
surface; only basic information of the topography is to be collected from this type of signal.
The interaction or excitation volume of electrons is hemispherical jug-shaped with neck of
jug at the specimens surface and is directly proportional on the accelerating voltage and inversely
proportional to material’s density. Electron penetration generally ranges from 1-5 µm with the
beam incident perpendicular to the sample. Thus for phases having high density the interaction
volume is lower. As the accelerating voltage increases, then the interaction volume (given by x and
y) increases according to the relations given by equations 1 and 2 [5
ρµ
5.11.0)( oEmx =
].
Equation 1
ρµ
5.1077.0)( oEmy = Equation 2
Where Eo is the accelerating voltage (keV) and ρ is the material’s density. Of course, the
interaction volume for SE signal has to be extracted from the total volume of interaction. For
example, bombarding a material with density of 2.7 g/cm3
(assuming an aluminium alloy) and
accelerating voltage of 20 kV (this experiment) the total penetration of electrons (in other words
volume of interaction) gives:
x = 3.3125 µm and y = 2.55 µm
The depth of electron penetration of an electron beam is also a function of its angle of
incidence, the magnitude of its current and the average atomic number.
Moreover, the main reason for coating a non-conductive specimen (such a conductive
sample mounted in bakelite) with a conductive material (such as gold) is to increase the number of
secondary electrons that will be emitted from the sample.
Figura 1.11 shows a SEM micrograph obtained by means of backscattered electrons (BS)
signal. Careful observations on this image clearly revealed three distinct regions (well contrasted)
identified as: dark, grey and white colored. Therefore, this metal resulted in three distinct phases as
identified by the difference on image contrast. The identified phases were labeled as “a”, “b” and “c”
as indicated in Figura 1.11. Black areas (indicated by the arrows) correspond apparently to gaps
within the material possibly due to the etching, in this work EDS analysis was not conducted in
those areas.
Backscattered electrons are produced by elastic interactions of beam electrons with nuclei
of atoms in the specimen. Many incident electrons undergo a series of such elastic event that cause
them to scattered back out the specimen. The fraction of the beam electrons backscattered in this
way varies strongly with the atomic number Z of the scattering atoms, but does not change much
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with changes in Eo. The backscattered coefficient, which is the number of produced backscattered
electrons, is given by equation 3 [6
pe
bs
ηηη =
]:
Equation 3
where ηpe is the number of incident electrons and ηbs
3724 103.81086.1016.00254.0 ZxZxZ −− +−+−=η
is the number of backscattered
electrons. For a pure element, the backscattered coefficient, depend on Z, can be calculated by:
Equation 4
For homogeneous mixtures ηmix
∑=i
iimix Cηη
is calculated from the weight fractions of elemental
components by:
In summary, dark phase correspond to the lowest atomic number (Z) phase in this alloy.
Grey and lighter regions correspond to higher Z numbers. Basically as all phases (elements) have
different size nuclei, as the size of the atom nuclei increases, the number of BSE increases.
Figura 1.11 Backscattered electrons (BS) image sampling
1.2.2.5 Volume of Excitation
Two factors limit the size and shape of the interaction volume: (1) energy loss through
inelastic interactions and (2) electron loss or backscattering through elastic interactions. The
resulting excitation volume is a hemispherical to jug-shaped region with the neck of jug at the
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Dr. Víctor Hugo Baltazar Hernández 2012 22
specimen surface. The analyst must remember that the interaction volume penetrates a significant
depth into the sample and avoid edges where it may penetrate overlapping materials. The depth of
electron penetration of an electron beam and the volume of sample with which it interacts are a
function of its angle of incidence, the magnitude of its current, the accelerating voltage, and the
average atomic number (Z) of the sample. Of these, accelerating voltage and density play the largest
roles in determining the depth of electron interaction (Figura 1.12).
Figura 1.12 Schematic depiction of the variation of interaction volume shape with average sample atomic number (Z) and electron beam accelerating voltage (Eo
). The actual shape of the interaction volume is not as long-necked since the electron beam in microprobe analysis has a diameter of about 1 µm (see Figure 2.1b).
Electron penetration generally ranges from 1-5 µm with the beam incident perpendicular to
the sample. The depth of electron penetration is approximately (Potts, 1987, p. 336):
For example, bombarding a material with a density of 2.5 g/cm3, about the minimum
density for silicate minerals, with Eo = 15 keV, gives x = 2.3 µm. The width of the excited volume
can be approximated by (Potts, 1987, p. 337):
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Both of these are empirical expressions. A theoretical expression for the "range" of an
electron, the straight line distance between where an electron enters and its final resting place, for a
given Eo
is (Kanaya & Okayama, 1972):
The volume of interaction can be modeled by Monte Carlo simulation. In such models, the
likelihood of incident electrons interacting with the sample and scattering and the angle of
deflection are determined probabilistically. X-ray generation depths depend strongly on density and
accelerating voltage (Figure 2.2b.). The results derived from Monte Carlo modeling yield a volume
of interaction that is very similar to that determined by etching experiments. The excited volume is
roughly spherical and truncated by the specimen surface. The depth of the center of the sphere
decreases with increasing atomic number of the target [7
].
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Figura 1.13 Comparison of electron paths (top) and sites of X-ray excitation (bottom) in targets of aluminum, copper, and gold at 20 keV, simulated in a Monte Carlo procedure.
1.2.3 Microscopía Electrónica de Transmisión (Transmission Electron Microscopy)
1.2.3.1 Introduction
A typical commercial transmission electron microscope (TEM) costs about $2 for each
electron volt of energy in the beam, and if you add on all the options, it can cost about $4-5 per eV.
As you'll see, we use beam energies in the range from 100,000-400,000 eV, so a TEM becomes an
extremely expensive piece of equipment. Consequently, there have to be very sound scientific
reasons for investing such a large amount of money in one microscope [8
Transmission electron microscopy (TEM) is the pre-eminent method for determining
dislocations and other crystallographic defects character and for performing chemical and
crystallographic analysis of micrometer and smaller precipitates and other microstructures. Use of
TEM in materials science/engineering can be introduced here in only a few additional pages and is
well worth the small increment of effort. Since most defect characterization requires an
understanding of diffraction contrast, this is an important constituent of this chapter. A TEM
equipment is shown in
].
Figura 1.14.
Figura 1.14 A transmission electron microscope Philips CM-12
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1.2.3.2 Depth of field
The depth of field of a microscope is a measure of how much of the object we are looking at
remains "in focus" at the same time. Like the resolution, this property is governed by the lenses in
the microscope. The best electron lens is not a very good one, as we've already mentioned, and has
been compared to using the bottom of a Coca-Cola bottle as a lens for light microscopy. To
minimize this problem we have to use very small limiting apertures in the lenses, narrowing the
beam down to a thin "pencil" of electrons which at most is a few micrometers across. These
apertures cut down the intensity of the electron beam, but also act to increase the depth of focus of
the images that we produce. Remember that "depth of field" refers to the specimen while "depth of
focus" refers to the image.
While this large depth of field is chiefly used in the SEM to produce 3D-like images of the
surfaces of specimens with large changes in topography, it is also critical in the TEM. It turns out
that in the TEM, all of the specimen is usually in focus at the same time, independent of the
specimen topography, as long as it's electron transparent!
Figura 1.15 shows a TEM image of some dislocations in a crystal. The dislocations appear
to start and finish in the specimen, but in fact they are threading their way through the specimen
from the top to the bottom, and they remain in sharp focus at all times. Furthermore, we can record
the final image at different positions below the final lens of the instrument and it will still be in
focus.
Figura 1.15 TEM image of dislocations in GaAs (Gallium arsenide). A band of dislocations threads through the thin specimen from the top to the bottom but remains in focus through the foil thickness.
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1.2.3.3 Diffraction
Thompson and Reid showed that electrons could be diffracted when passing through thin
crystals of nickel, and the possibility of combining electron diffraction into TEMs was realized by
Kossel and Móllenstedt (1939). Today, electron diffraction is an indispensable part of TEM and is
arguably the most useful aspect of TEM for materials scientists. Figura 1.16 shows a TEM
diffraction pattern which contains information on the crystal structure, lattice repeat distance, and
specimen shape, as well as being a most striking pattern. We'll see that the pattern can always be
related to the image of the area of the specimen from which it came, in this case shown in the inset.
You will also see in Part II that, in addition to the things we just listed, you can conduct a complete
crystallographic symmetry analysis of minuscule crystals, including such esoteric aspects as point-
group and space-group determination, and at all times the crystallography can be related to the
image of your specimen. There is no similar capability on a light microscope because of the
relatively large wavelength of visible light.
So an electron microscope can produce atomic level images, can generate a variety of
signals telling you about your sample chemistry and crystallography, and you can always produce
images that are in focus. There are many other good reasons why you should use electron
microscopes. We hope they will become evident as you read through this book. At the same time
there are many reasons why you should not always seek to solve your problems with the TEM, and
it is most important that you realize what the instrument cannot do, as well as knowing its
capabilities [8].
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Figura 1.16 TEM diffraction pattern from a thin foil of Al-Li-Cu containing various precipitate phases, shown in the inset image. The central spot (X) contains electrons that come directly through the foil and the other spots and lines are diffracted electrons which are scattered from different crystal planes.
1.2.3.4 Interpreting TEM images
Another problem is that the TEM presents us with 2D images of 3D specimens, viewed in
transmission. Our eyes and brain routinely understand reflected light images but are ill-equipped to
interpret TEM images, and so we must be cautious. Hayes (1980) illustrates this problem well by
showing a picture of two rhinos, side by side such that the head of one appears attached to the rear
of the other (see Figure 1.17). As Hayes puts it: "when we see this image we laugh" (because we
understand its true nature in 3D) "but when we see equivalent (but more misleading) images in the
TEM, we publish!" So beware of artifacts, which abound in TEM images.
Figure 1.17 Photograph of two rhinos taken so that, in projection, they appear as one two-headed beast. Such projection artifacts in reflected light images are easily discernible to the human eye but similar artifacts in TEM images are easily mistaken for "real" features.
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One aspect of this particular drawback is that, generally, all the TEM information that we
talk about in this book (images, diffraction patterns, spectra) is averaged through the thickness of
the specimen. In other words, a single TEM image has no depth sensitivity, as is apparent.
1.2.3.5 Specimen Preparation
All the above advantages of the TEM bring accompanying drawbacks. First of all, the price
to pay for any high-resolution imaging technique is that you only look at a small part of your
specimen at any one time. The higher the resolution, therefore, the worse the sampling abilities of
the instrument. We have an instrument that is a terrible sampling tool! This only serves to
emphasize that before you put your specimen in the TEM you must have examined it with
techniques that offer poorer resolution but better sampling, such as your eyes, the visible-light
microscope, and the scanning electron microscope. In other words, know the forest before you start
looking at the leaves on the trees.
Your specimens have to be thin if you're going to get any information using transmitted
electrons in the TEM. "Thin" is a relative term, but in this context it means "electron transparent."
For a specimen to be transparent to electrons it must be thin enough to transmit sufficient electrons
such that enough intensity falls on the screen or photographic film to give us an interpretable image
in a reasonable time. Generally this requirement is a function of the electron energy and the average
atomic number of the specimen. Typically for 100-keV electrons, specimens of aluminum alloys
almost up to 1 pm would be thin, while steel would be thin up to about several hundred nm.
However, it is an axiom in TEM that thinner is better, and specimens below 100 nm should be used
wherever possible, and in extreme cases, such as when doing HRTEM or electron spectrometry,
specimen thicknesses <50 nm are essential. These demands become less strict as the beam voltage
increases, but this is offset by the danger of beam damage.
So it should be obvious to you by now that while TEM and associated techniques are
tremendously powerful characterization tools when used properly, they should never be used in
isolation to solve a materials problem. You must understand your material at low magnification
with your eyes and with visible-light microscopy and scanning electron microscopy (SEM) before
venturing into TEM studies. Otherwise you may fall foul of some of the limitations we have just
listed [8].
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1.3 Análisis de XRD (X-Ray Diffraction)
1.3.1 Electromagnetic radiation
Discovered in 1885, these rays are invisible and travel in straight lines and more penetrating
than visible light.
What are x-rays?
X-rays are electromagnetic radiation which is exactly the same nature of visible light but of