The slide contains basic sample preparation procedures for examining under optical microscope, SEM etc

1. Metallographic sample preparation- UZr
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A.V.VINOD

2. Objective of the presentation
To explain steps involved during the specimens preparation
 Examination and evaluation criteria of the sample
Some literature data on data U-Zr Metallographic examination
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3. Precision Metallurgical Sample Preparation, also called Metallographic Specimen Preparation, is
a key step in performing reliable metallurgical testing. This type of testing often involves evaluating
the microstructure of materials through the use of optical magnification or scanning electron
microscopy (SEM)
Metallographic sample?
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4. Surface finish, also known as surface texture or surface topography, is the nature of a surface
as defined characteristics of surface roughness.
To reveal the microstructure for the examination, sample should have high surface finish
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5.  Surface roughness is usually measured by profilometry
Usually Roughness is expressed in two parameters
1) Ra- Arithmetical mean deviation
2) Rq- Root mean squared
Both are expressed in the units: micro meter (µm)
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6. Introduction
Many steps in the preparation of metallographic specimens described here are also applicable in
other types of metallographic studies, such as electron microscopy, micro hardness testing,
quantitative measurement of constituents of structures, and electron microprobe analysis.
Preparation of metallographic specimens generally requires five major operations
 Sectioning
 Mounting
 Grinding
 Polishing
 Etching
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7. Sectioning
Separate test pieces are taken from the casting for sectioning as required for producing metallographic
specimens.
The cutting should be performed such that there is no heat generation of the sample
Which will change the microstructure
Mounting of Specimen
The primary purpose of mounting is to make it convincement to handle specimens of arbitrary shape and/or small
sizes during various steps of metallographic sample preparation and examination.
Bakelite and diallyl phthalate are the generally used as molding material
Methyl methacrylate , polystyrene, poly vinyl chloride are some transparent moulds used
The standard sizes of mould used for holding the samples are 25 mm, 32 mm, 38 mm
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9. Grinding and polishing Machine
Accessories
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10. Grinding
 These are the important steps for the sample preparation which will be deciding factor for qualification of the
sample
 The aim is to cut he surface suitably for examination under optical microscope or scanning electron microscope
The grinding employees the sequence of grinding stages of increasing fineness to get uniform and scratch free
surfaces
The abrasive grinding is done over the abrasive sheet usually SiC supported over a thin sheet by a mylar film
The sheets used for grinding is described based on their girt sizes, which usually refers to the particle size of the a
abrasive material ( SiC in this case)
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11. ANSI: American national standard institute
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13. Sequential steps involved in grinding
1) Fix Abrasive paper of grit size 120 on the wheel and open the water valve
2) Set the RPM of the between 100-200
3) To grind the sample apply minimum force and hold the sample opposite direction of the rotation
4) After grinding for 2 min, switch off the grinding machine; clean the sample under running water
5) Then change the grit size of 240,320, 400,600, 800, and 1200 and grinding time limited for each grit
size is 4, 6, 8, 10, 12, 15 min respectively.
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14. 6) Each of the grinding stage the sample should be rotated about an angle of 90o
7) The sample is then examined under microscope
8) If scratch free surface is obtained, then the sample is take for polishing
9) If scratch still persist, then the grinding is continued with 1200 grit to ensure scratch free
surface and polishing to be continued.
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15. Polishing of the sample
1) The grinding wheel is changed to polishing wheel
2) 6 micron velvet cloth is fixed on the polishing wheel
3) 6 micron Diamond paste and lubricant oil is applied on the velvet cloth to reduce the friction
4) The rotary disc of the machine is set at speed of 300-400 RPM
5) Clean the sample with acetone to remove the oil
6) The above procedure is followed for 1 micron, 0.5 micron and 0.25 micron velvet cloths.
7) After final polishing the samples were examined under microscope
8) The perfectly polished sample is looked for surface smoothness
9) Further etching procedure based on the sample is used for reveling the grain boundaries and grain
structure and examined under optical microscope or SEM
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16. Electrochemical polishing/ Etching process
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17. Etching:
Metallographic etching is a chemical technique used to highlight features of metals at microscopic levels. By
studying the character, quantity, and distribution of these different features, metallurgists can predict and
explain the physical properties and performance failures of a given sample of metal.
Metallographic Etching Processes
There main etching processes used in metallographic sample preparation
 Chemical etching
 Electrolytic etching
 Heat tinting
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18. Chemical Etching
This typically involves immersing the sample in an etchant such or swabbing the surface with an etchant. The
etchant selectively corrodes microstructural features. Immersion time or etching time is highly dependent on the
system and in most cases requires experience.
Electrolytic Etching
Electrolytic etching and electropolishing are in effect the same process, except that electrolytic etching uses
lower voltages and current densities.
Most electrolytic etching processes use direct current electrolysis. The process uses the specimen as the anode,
with the cathode being a highly insoluble, but conductive material.
Heat Tinting
Heat tinting, sometimes called thermal etching is the process of oxidizing a sample in a furnace. This induces
oxidation of surface features at different rates, to reveal various features.
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19. U-Zr Etching procedures
1) Electrochemical etching:
Electrolyte: 10 g citric acid, 10 g nitric acid and 90 cc distilled water.
Procedure: The voltage was 3 V, the current density 0.2 A/cm2 and the etching time 5 to 15 seconds.
Inference : Claimed to be best when electrolytic polishing procedure is adopted as described
Polishing: Electrolyte of 1 0 g pyrophosphoric acid, 10 g chromic oxide, 40 cc ortho posphoric acid, 100 cc
sulphuric acid and 100 cc distilled water
The voltage was 1 0 V, the current density 0. 25 A/cm2 and the time of polishing about 1 minute. During
polishing the specimen was gently rubbed with a piece of cotton to remove the reaction products formed
Ref: PHASE TRANSFORMATIONS IN A URANIUM - ZIRCONIUM ALLOY CONTAINING 2 WEIGHT
PER CENT ZIRCONIUM. G. Lagerberg, AKTIEBOLAGET ATOMENERGI STOCKHOLM • SWEDEN •
1961
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20. U-Zr etching:
Etching solution: Water + phosphoric acid 1:1 v/v
The solution used for both electro polishing and electro etching with constant DC Voltage of 25 V and 2V
respectively
Reference: Phase Transformations in U-Zr Alloy System
Chandra Bhanu Basak
BARC NEWS LETTER
ISSUE NO. 316 • SEP. - OCT. 2010
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21. Etching of U-7.5% Nb -2.5% Zr alloy
Etchant : 10% Oxalic acid
Procedure: electrochemical etching
By passing 3-4 volts for 3-4 seconds
Reference:
Hardness Response Surface for U-7.5Nb-2.5Zr Alloy: A Study of
Recovery/Recrystallization and Phase Transformation Interactions
Metallurgical Transactions E
September 2015, Volume 2, Issue 3, pp 147–156 |
Etching of U-50wt%Zr, U- 60wt%Zr and U-70wt%Zr
Etchant: 5% H3PO4 aqueous solution as electrolyte
Procedure : Electro-etching was carried out using and SS304 as cathode with a constant
potential of 2V.
Reference:
Physical Metallurgical Studies of Zr-Rich U-Zr Alloys
Bagchi AC*, Prasad GJ, Khan KB and Singh RP
Nuclear Fuels Group, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India
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24. EXPECTED QUESTIONS AND EXPLANTIONS
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 D-values can be thought of as the diameter of the sphere which
divides the sample's mass into a specified percentage when the
particles are arranged on an ascending mass basis. For example,
the D10 is the diameter at which 10% of the sample's mass is
comprised of particles with a diameter less than this value. The D50
is the diameter of the particle that 50% of a sample's mass is
smaller than and 50% of a sample's mass is larger than.

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26. Surface Finish Units From RA To RZ
Ra – Average Roughness
Ra is also known as Arithmetic Average (AA) or Center Line Average (CLA). It is the average roughness in the area
between the roughness profile and its mean line. Graphically, Ra is the area between the roughness profile and its
centerline divided by the evaluation length. The evaluation length is normally five sample lengths where each sample length is equal
to one cutoff length.
Ra is by far the most commonly used Surface Finish parameter. One reason it is so common is that it is fairly easy to take the
absolute value of a signal and integrate the signal using analog electronics, so Ra could be measured by instruments that contain no
digital circuits.
Ra, while common, is not sufficient to completely characterize the roughness of a surface. Depending on the application, surfaces
with the same Ra can perform quite differently. Here are 4 surfaces with the same Ra and quite different shapes:
All four surfaces have the same Ra but quite different shapes…
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27. To distinguish these differences, more parameters are needed.
Rmax – Vertical distance from highest peak to lowest valley
Rmax is particularly sensitive to anomalies such as scratches and burrs that may not be obvious from measures such
as Ra that rely on averages.
Rz – Preferred by many Europeans
Rz is often preferred to Ra in Europe and particularly Germany. Instead of measuring from centerline like Ra, Rz
measures the average of the 5 largest peak to valley differences within five sampling lengths. While Ra is relatively
insensitive to a few extremes, Rz is quite sensitive since it is the extremes it is designed to measure.
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