Vi cấu trúc nói nhóm 2

BẮC
Hi every one. Today our group wanna give you a presentation about observation
microstructure. Our group includes Nguyễn Hữu Bắc, Đặng Tiến Đạt, Phùng Thị
Hương, Đào Duy Khánh, Đặng Bùi Nhật Lê, Vũ Thị Thùy, Phạm Bá Tuấn.
The content of the presentation consists of 4 parts. I will present to you the first
part: microstructure. The next part will be presented by Lê, Tuấn, Hương. Then,
Thùy anh Khánh will take responsibility for sample preparation. And the last one is
Đạt. He will talk about the procedure for the metallographic preparation.
First of all, we would like to introduce the microstructure. Microstructure is the
very small-scale structure of a material, defined as the structure of a prepared
surface of material as revealed by an optical microscope above 25× magnification.
Microstructures form through a variety of different processes. Microstructures are
almost always generated when a material undergoes a phase transformation
brought about by changing temperature or pressure, by deformation or processing
of the material and by combining different materials to form a composite material.
The term ' microstructure' is used to describe the appearance of the material on the
nm-cm length scale. A reasonable working definition of microstructure is:"The
arreangement of phase and defects within a material"
A ‘phase’ is taken to be any part of a material with a distinct crystal structure
and/or chemical composition. Different phases in a material are separated from one
another by distinct boundaries.
A ‘defect’ is taken to mean any disruption to the perfect periodicity of the crystal
structure. This includes point defects such as vacancies and interstitials, planar
defects such as surfaces, twin boundaries, grain boundaries, and dislocations.
And what is microstructural characterisation? Microstructural characterization
involves qualitative and quantitative analysis of surface topography, porosity,
crystal defects, and interfaces. ... Crystal defects mostly form either as a result of
imperfections during the crystal growth process or as a consequence of structural
phase transitions. Microstructural characterization is usually achieved by allowing
some form of probe to interact with a carefully prepared specimen. The most
commonly used probes are visible light, X-ray radiation, a high-energy electron

beam, or a sharp, flexible needle. These four types of probe form the basis for
optical microscopy, X-ray diffraction, electron microscopy, and scanning probe
microscopy.


LÊ
Now, we move on to the structure observation technique. In this part, we will
mention 3 techniques. They are light microscopy, hardness testing and quantitative
microscopy.
First, I wanna talk about the light microscopy. The light microscopy is an
instrument for visualizing fine detail of an object. It does this by creating a
magnified image through the use of a series of glass lenses, which first focus a
beam of light onto or through an object, and convex objective lenses to enlarge the
image formed. Light microscopy has numerous applications. The most important
application is the determination of the structural phases present and the constitution
of the bulk of the metal. Although numerous sophisticated electron metallographic
tools are now available to an investigator, the light microscope remains the single
most important device.
Now, let's go into detail about this instrument.
First, about the illumination, various light sources are available to the
metallographer. The most common light sources are Low-voltage tungsten
filament lamp, Carbon arc, Xenon arc, Quartz-iodine lamp, Zirconium arc lamp. In
condenser system, the condenser is a lens designed to focus light from the
illumination source onto the sample. The condenser may also include other
features, such as a diaphragm and filters, to manage the quality and intensity of the
illumination.
Filters are often required to modify the light for optimum visual examination or
photomicrograph. Selective filters, either absorption or interference, alter the light
to provide wavelengths for which the objective lenses are corrected. Selective
filters are often used to match the color temperature of the light source to that

required by the film. These filters can also be used to increase contrast between
phases of different colors.
The next component is objective lens. The objective lens forms the primary image
of the specimen and, thus, is the most critical item of the microscope. The
objective collects as much light as possible coming from any point on the specimen
and combines this light to form the image. The working distance means the
distance between the front surface of the objective lens and the sample, can be an
important lens parameter. As a rule, as the magnifying power of the objective
increases, the working distance decreases. The objectives are usually mounted on a
rotating nosepiece turret that can hold from four to six objectives.


Next, the next one is eyepieces. The major function of the eyepiece is to magnify
the primary image produced by the objective so that the eye can use the full
resolution of the objective. A virtual image of the specimen is formed at the point
of most distinctvision, approximately “two hundred fifty” mm from the eye.
About the stage, it must be sturdy so that vibrations are not encountered. Stage
movement should be smooth and precise. The stage surface is usually fitted with
an X and Y graduated scale for making measurements or locating features. Special
stages are available with a micrometer screw control for precise measurement.
In microscopy, the term 'resolution' is used to describe the ability of a microscope
to distinguish detail. Resolving power is the ability to produce separated images of
multiple structures under the best conditions and is usually expressed as the
number of uniformly spaced similar parallel black lines per unit length on a white
background that can be separated in the image. Besides, magnification also plays
an important role in image visibility, since the degree of visual perception of the
human eye varies from person to person. The depth of field is the distance along
the optical axis over which details of the object can be observed with adequate
sharpness. Factors that affect resolving power influence depth of field as well but
in the opposite direction, i.e., increasing the resolving power decreases the depth of

field.


TUẤN
Now, we continue with the working principle. The compound microscopes shown
here magnifies an object in two stages. Light from a mirror is reflected up through
the specimen, or object to be viewed, into the powerful objective lens, which
produces the first magnification. The image produced by the objective lens is then
magnified again by the eyepiece lens, which acts as a simple magnifying glass. The
magnified image can be seen by looking into the eyepiece lens. The secound is
formed by the ocular.
To obtaine good photomicrographs, three separate effects are used to create the
visual impression of good focus over the field of view:
 The depth of field of the objective
 The adjustment of the fine-focus control while viewing
 The slight change in focus due to eye accomodation
In obtaining sharp photomicrographs, the microscopist must control the following
variables:
• Eliminate vibrations
• Align illumination
• Match illumination color to objective corrections
• Maintain cleanliness of optics
• Correct adjustment of field and aperture diaphragms
• Focus precisely, generally with the aid of a focusing telescope
There are some examination modes light microscopy, such as bright-field
illumination, dark-field illumination, Polarized light, Phase-contrast illumination,
Interference techniques, Light-section microscopy, Fluorescence microscopy. In
this part, we will focus on the bright-field illumination. Bright Field (B.F.)
illumination is the most common illumination technique for metallographic
analysis. The light path for B.F. illumination is from the source, through the

objective, reflected off the surface and returning through the objective and back to
the eyepiece or camera. This type of illumination produces a bright background for
flat surfaces with the non-flat features (pores, edges, etched grain boundaries)
being darker as light is reflected back atan angle.


HƯƠNG
Now, we turn to the other technique: hardness. In its most general sense, hardness
implies resistance to deformation. As applied to metals, hardness is a measure of
resistance to permanent deformation (i. e. plastic). Hard materials exhibit high
strengths. Hardness also has other connotations—resistance to scratching,
resistance to cutting, ability to cut softer materials, brittleness, lack of elastic
damping, wear resistance, lack of malleability, magnetic retention, and so forth.
These pictures are the system of Vicker and Rockwell’s hardness.
Next, we continue with quantitative microscopy. To determine the quantitative
morphometric (i.e., number, size, orientation) of biological structure (i.e., cells
nuclei, collagen fibers) in an automated unbiased fashion. The use of a light
microscope for the quantitative analysis of specimens requires an understanding of:
light sources, the interaction of light with the desired specimen, the characteristics
of modern microscope optics, the characteristics of modern electro-optical sensors
(in particular, CCD cameras), and the proper use of algorithms for the restoration,
segmentation, and analysis of digital images.


THÙY
Let’s move on the sample preparation
First of all, we need to select the sample. Specimens should be chosen from
locations that are most likely to show the maximumvarieties within the material
being studied. Specimens should be taken as closely as possible to the fracture or
to the initiation of the failure. Before taking the specimens, study of the fracture

surface should be complete, or, at the very least, the fracture surface should be
documented. In many cases, specimens should be taken from a sound area for a
comparison of structures and properties.
Next, in the sample preparation, there are 5 steps: sectioning, mounting, grinding,
polishing, etching.
Samples need to be cut accordingly to the area of interest and for handling
convenience. Depending upon the material, the sectioning operation can be
obtained by Fracturing, Shearing, Sawing, Cutting or Wire Saws. Proper sectioning
is required to minimize damage, which may alter the microstructure and produce
false metallographic characterization. Proper cutting requires the correct selection
of abrasive type, bonding, and size; as well as proper cutting speed, load and
coolant. The step of this process, we need to define the intended sectional plane
and mark it on the bar. Then, the bar to the abrasive saw to cut out the small
sample; fix the bar securely, prepare the machine, select the cutting parameter and
start the first abrasive cut. Intensive water cooling and appropriate parameter are
important to keep the material as cool as possible.
Next step is mounting. When working with bulk samples, mounting may not be
required; however, if the sample is small or oddly shaped, mounting may be
necessary. The purpose of mounting is protecting the material’s surface and edges,
filling spots that have pores on the material and handling irregular-shaped samples,
conveniently. Mounting process consists of cleaning, adjusting specimen,
adhensive mounting and vacuum impregnation. The majority of metallographic
specimen mounting is done by encapsulating the specimen into a compression
mounting compound (thermosets or thermoplastics), casting into ambient castable
mounting resins (acrylic resins and polyester resins).


KHÁNH
The next step is grinding. Grinding is a very important phase of the sample
preparation sequence because damage introduced by sectioning must be removed

at this phase. The grinding step is accomplished by decreasing the particle size
sequentially to obtain surface finishes that are ready for polishing. Care must be
taken to avoid being too abrasive in this step, and actually creating greater
specimen damage than produced during cutting. We need to grind the sample to
reduce the damage created by sectioning, creat a flat, reflective, smooth and
scratch-free surface and handle irregular-shaped samples, conveniently. After
grinding, the specimen is thoroughly washed with water, followed by alcohol and
then allowed to dry. The drying can be made quicker using a hot air dryer.
Then, the sample is polished to produce a flat, reasonably scratch-free surface with
high reflectivity. When an unpolished surface is magnified thousands of times, it
usually looks like a succession of mountains and valleys. By repeated abrasion,
those "mountains" are worn down until they are flat or just small "hills." The
process of polishing with abrasives starts with a coarse grain size and gradually
proceeds to the finer ones to efficiently flatten the surface imperfections and to
obtain optimal results. Cleaning between polishing stages is more critical than
between grinding stages because carryover is a bigger problem. Carryover can also
be caused by abrasive on the operator's hands; thus, both the sample and the
operator's hands should be washed between steps. If automatic devices are used,
the fixture must also be washed. The mount or fixture can be held under running
water and swabbed with cotton, followed by similar treatment with alcohol. With
porous samples or when gaps are present between the sample and the mount,
ultrasonic cleaning should be used. Careful cleaning is important if good results are
to be obtained.
The last step is etching. The metallographic etching is a chemical technique used to
highlight features of metals at microscopic levels. The specimen is covered or
dipped into a protective layer of etching liquid (this varies from the
electrochemical, chemical, physical or cathodic vacuum). The sample must be
thoroughly cleaned before etching, a satisfactory etchant must be selected and
prepared, and the etching technique must be carefully controlled. Following
etching, the sample must be washed free of any residue and dried.

ĐẠT
Let’s us give a detailed sample preparation.
As a typical example, we are going to investigate a bar made from steel. The plane
carbon steel C45E with 0,45% of carbon. The material tester define the intended
sectional plane and mark it on the bar. In this case, it's the longitudinal section.
then, takes the bar to the abrasive saw to cut out the small sample. we need to fix
the bar securely, prepare the machine, select the cutting parameter and start the
first abrasive cut. Intensive water cooling and appropriate parameter are important
to keep the material as cool as possible for further processing, the specimen has to
be mounted in resin. To ensure that the resin will adhere well to the specimen
surface, the specimen is placed in an ultrasonic cleaner bar for a few seconds.
alcoho and ultrasonic wave have to remove fat and lose particle from the surface.
To mount the specimen, small plastic mold are suitable. the material tester picks up
the clean specimen and carefully place it in to one of the mold. the intended plane
of examination is at the bottom. then he posly put the resin in to the mold. a thin
layer of grese at the end surface of the mold act as the release agent. this ensure
that the cure resin can lay to be release easily from the mold. now the mold
containing the specimen goes in to the lightcuring unit. under the action of the
intense blue light, the resin polimerize within half an hour. the resin has cured and
embedded the specimen well.
Because the top surface is still an even and slightly sticky, the material tested
grinds it there until it is even. The material tester uses a rotating water lubricated
disk equipped with coarse grain silicon carbide paper. Now comes the most
important part of the preparation of the intended plane of examination, all materials
has that begins with comparatively abrasive paper to make sure to press the
specimen commonly and evenly until the abrasive paper compensating the
tendency for the top of the Tilt. Now having achieved an even relatively rough
surface, the specimen is ready for the next step with final paper, insert a new

abrasive paper onto the disc rotating the specimen by 90 degrees leads to new
grinding groups perpendicular to the old ones. In this way the material test can
easily check whether the old grinding grooves have been removed completely. it is
important to use sufficient water flushing during grinding. the specimen is then
ready for polishing. An absolute prerequisite for polishing is a thoraly clean
specimen the ultrasonic bath and running water help to remove any residue from a
ground surface, otherwise hard particle might be press in to the polishing cluff and


scratch the surface. Hand should be washed as well so that no unwanted particle
find their way to the sample and the polishing cloth.
Now we can start with the first polishing operation. he chosed the polishing disk
equip with hard cloth of low resilient, moisten the cloth with suitable lubricant and
add 2 splashes of diamond suspension of particle size 6 micrometer. on the counter
rotating motion, he press it with high pressure on to the polishing cloth. the
diamond particle have settled on the cloth and now abrading the surface. after
about a minute, all grinding gruel have been removed, and the specimen surface
already has a shiny appearance. but still there are many find scratched to be found
resulting from the 6micrometer diamond abrasive. to remove them, material tester
washed specimen and his hand again very carefully and polishes the specimen for a
second time. now he uses a softer polishing pad, final diamond suspension of 3
micrometer particle size and less pressure. after the the second polishing operation,
the specimen surface has an almost mirror like appearance. No more scratches can
be seen with naked eyes. Never the less, the specimen has to be clean again and go
to a third and final polishing operation. this time, fifteen nanometer aluminum
oxide abrasive is used. only then is the surface prepared to a sufficient quality. at
the end of the mechanical preparations, special care has to be taken to clean the
specimen properly. on the one hand, all abrasive particle has to be removed. On the
other hand, the freshly surface must not be scratch or damage. under running
water, the material tester gently wipes the specimen with cotton wool, carefully

rinses it with alcohol. from now on, the sample is called a metallic graphic
specimen or micro section.
THÙY
It is now alow to go under the microscope for the first time. now on the monitor, in
the polish stage, there isn't much to be seen in this material. only some elongated
nonmetalic inclusion can be observe. but if these inclusions are a special interest,
then the polish stage is the perfect one. Inclusion may best be seeing here.
However, if one wants to see the crystal, the grain and grainboudary, then the
micro section has to be etched. To do this, material tester protect himself with a lab
coat, put on safety glasses and pour the agent into a glass bowl. The agent consists
of solution of 10% of concentrated nitric acid in alcohol. Now he turns on the
water tap, pick up the freshly polish microsection with the gripping tongs and
emerges it for a few seconds into the agent. Then he rinses microsection
thoroughly the water and afterward with alcohol. Now, this sample is well-


prepared for the next step: observation. After observation, we have this image. As
you can see, the darker region is perlite. Pearlite is a two-phased, layered structure
composed of alternating layers of ferrite and cementite. The white region is ferrite
crystal. They consist of almost the pure iron.