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Experimental
Techniques

 

     This chapter explains
formation of the alloys Cu-38%wtZn binary alloy and Cu-38%wtZn-2%wtPb
ternary alloy, heat treatment, experimental measures and their tools that
used to acquire the experimental results of the current scientific
investigations. The first step begins with the formation of the two brass
alloys with definite chemical composition, followed by organized measurements
to recognize the microstructural and mechanical properties of the selected brass
alloys. X-ray investigations were achieved to study the deviation of the structure of the test samples. Also, the practical procedures and tools are described
for creep and stress- strain tests.

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3.1 Preparation of Brass Alloys

     The brass
alloys with the composition (wt.%) of  Cu-38%wtZn, and                  Cu-38%wtZn-2%wtPb were established
from high purity Cu, Zn, and Pb (purity 99.99%) as raw materials, then combined
according to the indicated composition ratios shown in Table (3-1). The melting process was performed in a graphite
crucible at 1123?K for 20 min. The melting process was performed in a vacuum
arc furnace to produce rod-like sample under high purity argon atmosphere with
a diameter of nearly 10 mm. A cooling rate of 6-8?C/s was done, so
as to get samples having the totally precipitated phases to produce fine
microstructure.

 

 

 

 

Table (3-1): The brasses
studied chemical composition (wt. %)

 

Alloy

Zn

Pb

Cu

Cu-38%wtZn

38

Bal.

Cu-38%wtZn-2%wtPb

38

2

Bal.

 

This process allowed to a small quantity
of grain stabilization to happen and to let transformation to be approximately completed.
Then, the melted poured in a steel mold to form the chill cast ingot. The
homogenized cast ingots were then swaged to wires of 0.8 mm diameter and 50 mm
gauge length. All the samples annealed at 573?K for 4 hrs to eliminate the cold
work introduced during swaging, to stabilize the structure and to reduce the
residual stress induced in the sample preparation, then the samples should be left
to cool slowly to reach room temperature (RT). This step was done before
microstructure analysis and mechanical examinations.

 

3.2
Metallographic Samples Preparation

     With the purpose of getting perfect
microstructure photograph, the samples were established and polished carefully before
etching. The surface that we see in the microscope must be totally flat and
smooth firstly. Any irregularity will unclear the structure of the surface and
will complicate the viewer who attempts to examine the microstructure under the
microscope. The following steps were applied in order to get this smooth flat
surface.

 

3.2.1 Grinding

    
Grinding is used for eliminating damaged or deformed surface material,
while presenting only partial amounts of new deformation. The aim is to create
a flat surface in the samples with least damage that can be removed easily
during polishing in very short time, and this is succeeded with steps of better
abrasive grit. The steps in the process of grinding can be shortened as
follows:

Firstly, we grind the sample with wet
abrasive SiC 500 grit until the sample is smooth and the surfaces of the sample
are shown. Secondly, repeating the grinding with the better wet abrasive SiC 800
grit for 1 min. Lastly, grinding the sample with the better wet abrasive SiC 1500,
2000, 2400 and 4000 grit for 1 min.

 

3.2.2 Polishing

     After completing grinding process
with the silicon carbide papers, the polishing process achieved with soft cloth
pad (whose particle size is15 microns) Fig. (3-1).The imperfections and the
scratches are removed by the coarse polishing to produce a refined mirror-like
surface. Polishing includes contact between the sample and the rotational cloth
pad saturated with diamond dust and appropriate greasing polishing medium. The
surface should be washed carefully with water when it seems to be free from
scratches to confirm that the polished surface is free of any lubricant, and lastly
washed with alcohol tailed by drying using hot air blast. The polishing process
yields surface with deformed layer of metal which hides the basic crystal
structure and will later be melted by appropriately etching reagent. Lastly the
sample washed with methyl alcohol to keep the mirror-surface free from any impurity
and prepared for the etching processes.

Fig. (3-1):
Polishing machine.

 

3.2.3 Chemical Etching

     After grinding and
polishing, the flat refined surface is etched. The samples were etched with 20%
H2O2 (3%) – 40% NH4OH (conc.) and 40% H2O
solution. When acid is added to the flat sample surface, the etching process
will eat away some metallic structure more rapidly than others. The regions
that are dissolved most quickly will seem as obscure shadows under the
microscope. Those surfaces that react slowly to the acid seem light.

The etching process includes the following steps:

1.     Clean the sample
surface with alcohol then drying the sample in air.

2.     The sample is dropped
the etching reagent which is controlled in small glass vessel for 10-20 seconds
followed instantly by washing them with running water, then the samples held in
current of hot air from a hair drier dried.

 

3.3 The microscope        

     A microscope is a great
accuracy optical tool which uses a lens or a mixture of lenses to produce highly
magnified images of small samples or objects particularly when they are too
small to be seen by the naked eye. Digital microscopes are now obtainable which
use a CCD camera to check a sample, and the image is shown directly on a
computer screen without the necessity for optics such as eye-pieces. The
science of studying slight objects using such a tool is called microscopy. Let
us spot two types of it, the optical microscope and scanning electron
microscope.

 

3.3.1 Optical Microscopy

     The optical
microscope, often referred to as the “light microscope”, is a kind of
microscope which uses visible light and a system of lenses to magnify images of
small samples placed in the focal plane of the lens Fig. (3-2). Optical
microscopes are the oldest and simplest of the microscopes. Other microscopic
methods which do not use visible light include scanning electron microscopy and
transmission electron microscopy. An image of the object (sample) is formed by
the objective lens, which typically provides a magnification in the range 10x
to 100x. This magnified image is then viewed through the eyepiece (ocular),
whose magnification is usually 10x.

The total magnification of a microscope is obtained by
multiplying the objective and eyepiece magnifications according to the
relation:

 

M tot =   M obj
x M eye piece

   

 

Fig. (3-2):
Optical microscope (OM-Nikon).

 

 

     Microscope
magnification is how large the object will appear compared to its actual size. Typically
total magnification are in the range 100X to 1000X, because the image is  (usually)  not  presented  as  a
real image, but  rather  as  a  virtual  image  viewed
through the eyepiece. Briefly, operation of the optical microscope: the
objective lens forms a magnified image of the object (called the real
intermediate image) in or near the eyepiece; the intermediate image is examined
by the eyepiece and eye, which together form a real image on the retina.
Because of the perspective, the retina and brain interpret the scene as a
magnified virtual image about 25 cm in front of the eye Fig. (3-3).

     Magnification alone
is not enough: Magnifying an object without good resolution will simply produce
a large image of the object where details cannot be identified. Resolution refers to the ability of a
microscope to distinguish two separate points. Also phase contrast observation
is very important .It shows the differences in light paths (phase shift)
between refracted light rays that pass through the sample and direct rays from
the light source. Only fulfillment of these three conditions allows translation
of information as accurately as possible from object into an image which
represents that object.

     It is the only
microscope for real color imaging. It also fast, and adaptable to all kinds of
sample systems in any shapes or geometries. Easy to be integrated with digital
camera systems for data storage and analysis. On the other hand, it gives
low resolution, usually down to
only sub-micron or a few hundreds of nanometers, mainly due to the light
diffraction limit.

 

 

 

 

 

 

 

 

3.3.2 Scanning Electron Microscope (SEM)

    The scanning electron microscope is
considered today the powerful instrument in materials and life-science Fig. (3-4). SEM is an instrument for
observing and analyzing the surface microstructure of a bulk sample using a
finely focused beam of energetic electrons. The popularity of (SEM) comes from its capacity to obtain
tridimensional images of the surfaces of a wide range of materials. It is extremely
useful for the direct observations of surfaces because they offer better
resolution and depth of field than optical microscope. It creates the magnified
images by using electrons instead of light waves. The images created without
light waves are rendered black and white. SEM works under vacuum in a column
where an electron gun (at the top of the microscope) emits a beam of
high-energy electrons. This beam travels downward through a series of magnetic
lenses designed to focus the electrons to a very fine spot. Near the bottom, a
set of scanning coils moves the focused beam back and forth across the sample,
row by row. As the electron beam hits each spot on the sample, secondary
electrons are knocked loose from its surface. A detector counts these electrons
and sends the signals to an amplifier. The final image is built up from the
number of electrons emitted from each spot on the sample.  

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. (3-4):
Photographic image of Scanning Electron Microscope, model JSM-5410, in
El-Tebbin Institute for Metallurgical Studies (TMS).

A detailed explanation of
how a typical SEM functions show in Fig. (3-5).

 

1. The “Virtual
Source” at the top represents the electron gun producing a beam of high-energy
electrons.

2. The beam is condensed
by the first condenser lens (usually controlled by the “coarse probe
current knob”). This lens works in conjunction with the condenser aperture
to eliminate the high-angle electrons from the beam.

3. The condenser
aperture constricts the beam by eliminating some high-angle electrons.

4. The second condenser
lens forms the electrons into a thin, tight, coherent beam.

5. A user selectable
objective aperture further eliminates high-angle electrons from the beam.

6. The scan coils are
situated between the objective aperture and the objective lens in order to
induce wide deflection at the sample level by imposing only small movements to
the beam , that are then amplified by the lens.

7. The objective lens
focuses the scanning beam onto the part of the sample desired.

8. When the beam strikes
the sample and dwells for a few microseconds, interactions occur inside the
sample and are detected with various instruments.

9. Before the beam moves
to its next dwell point these instruments count the number of interactions and
display a pixel on a CRT (forming the image) whose intensity is determined by
this number (the more reactions the brighter the pixel).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

       

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. (3-5): Schematic shows the diagram of Scanning
Electron Microscope.

 

 

3.3.2.1
Energy Dispersive X-ray Spectrometry (EDS)

     Energy-dispersive spectrometry (EDS) is
the established technique for the determination of constituent elements and
elemental composition on the micro scale in the scanning electron microscope
(SEM). The EDS system is a safe instrument. You can do much more damage to it
than it can do to you. When the scanning electron microscope electron beam is
turned on, some X-rays are produced as a result of electron beam interaction
with the sample, but these X-rays are of relatively low energy and do not
escape the sample chamber. The EDS microanalysis is accomplished by measuring
the energy and intensity distribution of the characteristic X-ray signals
generated by a focused electron beam on the specimen feature to obtain its
elemental composition, where the signal is usually displayed on CRT (cathode
ray tube) or video output as a spectrum of X-ray intensity versus energy. The
X-ray characteristic was taken from elements with atomic number Z=6 (carbon)
and upwards.

 

3.4 X-Ray
Diffractometer Analysis  

     X-ray diffraction (XRD) is one of the earliest methods for
studying the structure of solids. These methods are often the only methods that
allow a further differentiation of materials under laboratory conditions. Phase
identification of the alloy samples was carried out by X-ray diffractometry
(XRD) at 40 KV and 20 mA using CuKa radiation with diffraction angle
(2?) from 25 to 85? and a constant scanning speed of 1?/min.
The model which is used is PHILIPS X’ Pert Diffractometer, this
technique is used for measuring size of crystalline unit cell, and its changes
with temperature, lattice parameter and hence thermal expansion coefficient as
showed in Fig. (3-6).

Bragg  assumed  according to Snell’ law that when X-rays hit
an atom, they make the  electronic cloud
move as does any electromagnetic wave. The movement of these charges reradiates
waves with the same (or elastic scattering). These re-emitted wave fields
interfere with each other either constructively or destructively, producing a
diffraction pattern on a detector or film. The resulting wave interference
pattern is the basis of diffraction analysis. X-ray wavelength is comparable
with inter-atomic distances(~1.5 Å) and thus are an excellent probe for this length
scale. The perpendicular distance, d, between atomic planes, parallel to
crystal face, generally increases when a crystal is heated .This distance can
be determined by the application of 
Bragg ,s law :

                                       

 

2d sin? = n ?

 

Where: ? is the wave
length of monochromatic  beam of 
X-rays, ? is Bragg’s angle and the value (n) indicates the order of
reflection.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. (3-6): X-ray
diffractometry (XRD), model (PHILIPS X’ Pert Diffractometer) in
El-Tebbin Institute for Metallurgical studies.

 

    3.5  Mechanical Tests

3.5.1 Creep Testing
Machine

   
 Fig. (3-7) shows the apparatus
used in the present tensile creep with an acceptable experimental error. The
sample was clamped horizontally at both ends between two frictional grips which
are carefully aligned inside a cylindrical furnace. One of two grips is fixed
by rod while the other was connected to another rod sliding smoothly and
axially inside a fixed cylinder. The other end of the sliding rod was connected
to a pan. The loads causing the different axial tensile stresses acting on the
samples which were put in the pan. An index (X) was fixed to the movable
rod. Sliding was smooth through a just fitting slat in the cylindrical guide.
In this way, any torsional motion of the test sample was prevented and any
tensile extension was allowed during the creep test. The position of the index (X)
served for the determination of the extension which was measured by using a
gage-length having sensitivity of 10-4 m. The creep machine was
fitted with a manual lever which used to avoid sudden tension of the test
sample. Creep testing was performed at 573,593,613,633and 653 ?K
after waiting time of 5 min for the test temperatures to be reached and stress
ranging from 2×10-5 to 3.6×10-3 s-1 .The
environment chamber temperature could be monitored by using a thermocouple
contacting with sample with temperature accuracy ±2?C. 

 

 

 

 

Fig.
(3-7): Creep machine.

3.5.2 Tensile Stress-Strain
Measurements

    
Tensile stress-strain tests were performed at constant temperatures 573,593,613,633,
and 653 °K and different strain rate ranging from 2×10-5 to 3.6×10-3
 s-1. Fig. (3-8) illustrates
the apparatus used in the present tensile stress-strain tests which are a
computerized locally made tensile testing machine with an acceptable
experimental error. Schematic diagram of the tensile testing machine is shown
in Fig. (3-9), in which samples were clamped horizontally between two
fractional grips G1 and G2. The first grip G1 is attached to a fixed
force sensor with accuracy ±0.03N which measures the force during elastic and
plastic deformation. The second grip G2 is attached to a rotary
motion sensor which measures the strain of the stretched sample. The two
sensors are connected with the interface which
collects data directly from the two sensors to a computer for drawing
stress-strain curve by using data studio software. Moreover, the sample and
grips were placed inside an electrical furnace composed of two half ‘s
each of power 1000 watt. One of the oven’s half ‘s was
fixed while the other half is removed to ensure that the samples are placed at
the same position and operates at the same conditions. The oven temperature was
controlled by temperature controller with temperature accuracy ±2 ?C.

Fig.
(3-8): Tensile stress-strain machine.

 

Fig. (3.9)
Schematic diagram of tensile stress-strain measurement.

 

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