Lab Script- New Deformation and Recrystallisation vRQ v Jan 2014

Laboratory 1st year- Spring Term Name of Student

Experiment Structure and Properties of Metals-Deformation and Recrytallisation (Brass rolling)

Lab group

Academic in charge

Dr Rongshan Qin Date of lab

Demonstrator Mon- Izzati Nadzri

Tues- Giovanni Luongo

Report due date

After one week from the lab, before midnight of the day.

Experimental data checked

............................................................. ...................................................

Experiment Demonstrator’s signature date

Important information Please fill in your information in the appropriate boxes above and cross out the inappropriate

demonstrator’s name. Your lab report is due within one week of the lab date. The instructions on how to write a lab report state that they should be no more than 1,500 words long.

1,500 should be a target, but you are allowed to go a small amount over this. But please do not go over 2,000 words.

In order to gain a credit for this practical you must complete the data collection and preliminary assessment of data before the end of the allotted time period.

Your results should be checked by a demonstrator or staff before you leave the lab and the box on the above initialled.

The completed report with discussion and conclusions must be submitted through blackboard BY THE DATE & TIME GIVEN ABOVE. Failure to do so may result in a loss of marks.

PlagiarismYou are reminded that all work submitted as part of the requirement for any examination (including coursework) of Imperial College and the University of London must be expressed in your own words and incorporate your own ideas and judgements.

Plagiarism, that is the presentation of another person’s thoughts or words as though they were your own, must be avoided with particular care in coursework, essays, and reports written in your own time. Note that you are encouraged to read and criticise the work of others as much as possible. You are expected to incorporate this in your thinking and in your coursework and assessments. But you must acknowledge and label your sources.

Direct quotations from the published or unpublished work of others, from the internet, or from any other source must always be clearly identified as such. A full reference to their source must be provided in the proper form and quotation marks used. Remember that a series of short quotations from several different sources, if not clearly identified as such, constitutes plagiarism just as much as a single unacknowledged long quotation from a single source. Equally, if you summarise another person’s ideas, judgements, figures, diagrams or software, you must refer to that person in your text, and include the work referred to in your bibliography. Departments are able to give advice about the appropriate use and correct acknowledgement of other sources in your own work.

The direct and unacknowledged repetition of your own work which has already been submitted for assessment can constitute self-plagiarism. Where group work is submitted, this should be presented in a way approved by your department. You should therefore consult your tutor or course director if you are in any doubt about what is permissible. You should be aware that you have a collective responsibility for the integrity of group work submitted for assessment.

The use of the work of another student, past or present, constitutes plagiarism. Where work is used without the consent of that student, this will normally be regarded as a major offence of plagiarism.

Failure to observe any of these rules may result in an allegation of cheating. Cases of suspected plagiarism will be dealt with under the College’s Procedure for Dealing with Examination Offences and may result in a penalty being taken against any student found guilty of plagiarism.

Student’s declaration I declare that this assignment is entirely my own work in accordance with the above regulations. I have read the safety notice accompanying this experiment and the hazards have been explained to me

by the demonstrator.

Student’s signature....................................................... Date..........................................

1 1st year- Deformation and recrytallisation lab script- 2013/2014

Department of Materials

Rules for safe conduct to be observed in Deformation and Recrystallisation

If you do not take care during this experiment you could hurt yourself. Therefore, do not start the experiment before a demonstrator has introduced the class.

1. You MUST READ the experiment risk assessment form, especially noting the section that identifies hazard.

2. The eye protection (laboratory goggles), gloves and lab coats is a requirement to perform this experiment.

3. Hold the sample using the tongs provided when etching

4. Abrasion of fingertips can occur if contact is made with SiC paper or diamond impregnated cloth so take extra care when working on grinders and polishers

5. Students must keep their fingers away from the rollers to avoid crushing of fingers in rollers. No loose clothing, hair is allowed when performing this experiment,

6. Note: non-compliance with the safety instructions in this document or given verbally by the demonstrators or academic staff in the lab, will lead to expulsion from the laboratory.

7. The instructions given by the demonstrator must be followed. No experiments can be started before the introduction by the demonstrator is completed.

8. If any electrical equipment fails to operate, contact the demonstrator or a technician or member of academic staff. Do not attempt any repairs yourself.

9. If you have read and understood all safety instructions, then please sign COSHH and risk assessment identification sheet before you start the experiment.

Check list

Before you leave the lab you should have completed all the sections below. When you have done so, get your data checked by your demonstrator before you leave.


Deformation and Recrytallisation

(Brass rolling)

AimTo learn the effects of deformation on the microstructure and properties of α Brass

ObjectivesAt the end of this experiment, you should be able to demonstrate

The effect of cold deformation on the hardness and microstructure of an α Brass The effect of annealing time and temperature on the recrystallisation of α Brass

1. Introduction

Part 1. Effect of Deformation on Microstructure and Properties

Figure 1. Stress-strain curve

Fig. 1 illustrates a typical example of stress strain curve for a tensile test. Initially we have a region of elastic

deformation, which continues until a critical stress is reached (the yield stress) beyond which plastic deformation occurs

and permanent deformation of the sample occurs. In this section we will look at the mechanisms by which this plastic

deformation occurs. During plastic deformation planes of atoms slide over each other by the action of shear stresses on

certain types of plane known as slip planes. Consider a tensile test sample with cross sectional area A to which a tensile

force F is being applied (see figure 2 below). Looking a plane within the sample (see shaded area in figure 2 below) this

force can be resolved into forces both normal and in plane. If the plane is at an angle with the cross section, then the

area of this plane is A/cos . Similarly the in plane force is Fsinand hence the shear stress is equal to

Fsin/(A/cos) (or Fsincos/A or Fsin2/2A). As a result it can be seen that the maximum resolved shear stress

will be F/2A, acting on a plane at an angle of 45°C to the applied force (maximum value of sin2 1 when 2= 90). As

a result slip will generally occur along planes of atoms at an angle of ~45° to the applied force.

Theoretically the stress required to cause plastic deformation by simultaneous slipping of all the atoms in one plane over

those in the next plane can be shown to be many thousand times greater than the stresses observed in reality. The

reason for this discrepancy is due to the presence of defects in the crystal structure known as dislocations, which allows 3 1st year- Deformation and recrytallisation lab script- 2013/2014

slip to occur by a successive movement of atoms. In simple terms a dislocation can be considered as an extra plane half

plane of atoms present on one side of the slip plane (see Fig. 2 below). When a shear stress is applied along the slip

plane this half plane of atoms moves sequentially through the crystal giving rise to deformation. Dislocations are always

present in a crystal due to irregularities during the growth of the crystal. Plastic deformation occurs by large numbers of

dislocations moving along slip planes. During plastic deformation further dislocations are generated and the number of

dislocations present within the crystal increases with increasing plastic strain.

The strength of a material is determined by the stress required to move dislocations through a structure. There is an

intrinsic stress required to move a dislocation through a single crystal of a pure metal. Any barriers to dislocation

movement such as other dislocations, solute atoms (atoms in solid solution), second phase particles (precipitates) or

grain boundaries will increase the stress required to move a dislocation through the structure. In a real material some or

all of these features will be present and act cumulatively to increase the overall strength of the material.

Figure 2. Tensile test piece with cross sectional area A, applied force F and shear stresses normal and in plane


Figure 3 Deformation of a metal sheet by cold rolling

Figure 4 Grain structure before deformation

Part 2. Effect of Cold Rolling on the Hardness and Microstructure of α Brass

In this part of the experiment we look at the effect of deformation on the hardness and microstructure of an α Brass. We start with an annealed (i.e. soft) section of α brass. α brass is an alloy of Cu with 30 wt.% Zn. This level of Zn addition is less than the solubility of Zn in Cu, and therefore is a solid solution alloy. It is a single phase alloy comprising grains of α with the Cu crystal structure, hence the name. This class of brass is used extensively in numerous applications. These include radiator cores and tanks, fasteners, locks, hinges, ammunition shell cases, and plumbing accessories to name but a few. This has then been cold rolled to different thicknesses to introduce different levels of cold work into the microstructure.


Final Height

Original Height

The amount of deformation imparted to a material during rolling is given by:-

%Reduction (or % Cold Work) = (Original Height - Final Height) X 100Original Height

Figure 5. Deformation by cold rolling and the % deformation calculation

Figure 6. Composition of wt.% Zn

Part 3. Effect of Time and Temperature on the Recrystallisation of α Brass

After a material been cold rolled the strength has been increased and the ductility reduced. For a 30% Zn α brass the yield strength increases from 70 MN/m2 in the annealed (un-deformed condition) to ~310 MN/m2 in the cold worked condition. This is accompanied by a corresponding decrease in ductility from 45% to 6%. Before the sheet is further shaped into a component it may be desirable to try and return the microstructure and hence properties back to the condition it was in prior to cold deformation. This can be achieved via a process called recrystallisation. During cold rolling a large amount of work is done on the material and some of this energy is stored in the material in the form of dislocations. Basically the deformed microstructure is in a higher energy configuration than the original un-deformed microstructure. Therefore there is a thermodynamic driving force for the microstructure to return to a lower energy state. For this to occur the atoms need to rearrange to remove the dislocations and grow new dislocation free grains. For most metals this can only occur at an appreciable rate at temperatures above room temperature. If the material is held at an elevated temperature (but well below the melting point) the growth of new dislocation free grains can occur. This type of heat treatment is called annealing and the process by which new grains form recrystallisation. When recrystallisation occurs the strength (and hardness) of the material decreases and the ductility increases.

2. Experimental Procedure

Part 1

There is no experimental work for this part. After reading the introduction of the Part 1, please answer to the questions given in the results section.


Part 2

In this part of the experiment we look at the effect of deformation and subsequent annealing on the hardness and microstructure of α brass.

1. We start with a section of annealed α brass. You will cold roll the material to 10%, 30%, 50%, and 70% reduction, and then anneal the 70% reduced material for 2, 5, 10, 20, 50, and 70 minutes at 300°C. You will do hardness test for the material in all these conditions

Code Deformation Original Height (mm) Final Height (mm) % Reduction

A As annealed

B Cold Rolled 10%

C Cold Rolled 30%

D Cold Rolled 50%

E Cold Rolled 70%

2.The hardness of each one of the conditions is to be measured. Hardness test the samples using a 5 kg load and take the average of three readings (Hardness is a very good indicator of the mechanical properties (i.e. yield strength) of a material, basically the harder the material is the stronger it is). Collect the results and complete the table below.

Sample Deformation VHN1 VHN2 VHN3 Average

A Annealed Cold Rolled 0%

B Cold Rolled 10%

C Cold Rolled 30%

D Cold Rolled 50%

E Cold Rolled 70%

3. One section should be cut from the as annealed 0% (sample A), 10% (sample B), 30% (sample C), and 50% (sample D) reduced α brass, and 7 sections cut from the 70% (sample E) reduced sample. All seven sections from sample E are to be annealed at 300°C for the duration indicated in the table below:

Sample reference

Annealing Time at 300°C (minutes)

1 2

2 5

3 10

4 20

5 50

6 70

4. While these samples are being heat treated mount the as deformed samples A through to E in bakelite. After mounting the samples should be ground down to a 1200 grit paper and then polished using 6 and 1 micron polishing cloths. The samples should then be etched in a solution of alcoholic ferric chloride for 3 to 5 seconds, washed with water and then methanol and left to dry.

5. You are now going to investigate the effect of rolling reduction on the grain structure in the α brass. All the samples that have been ground, polished and etched in alcoholic ferric chloride should then be examined under a light microscope. In this part examine and sketch the microstructure (See results section) of samples A (annealed cold rolled 0%), C (cold rolled 30%) and E (cold rolled 70%). NB: Use the same magnification for all your examinations.

Part 3

1. In this part of the experiment you will look at the microstructure of an α brass that has been fully annealed (i.e. fully recrystallised). By now you should have removed all the samples from the furnace and mounted them ready for polishing. The polishing procedure is the same as that described above. The samples you will look at


have been cold rolled 70% and annealed at 300°C for 20 and 70 minutes (sample E4 and E6 respectively). Examine these samples under the light microscope (remember to use the same magnification that you used for the previous microstructural examinations).

2. In this part of the lab you are going to investigate the effect of time and temperature on the rate at which the α brass (deformed by 70%) softens during annealing. A series of samples taken from the α brass deformed by 70% have been annealed at different temperatures for different lengths of time. You are responsible for testing one of the temperature conditions. Collect the results and complete the table below.

Code Annealing Temperature (°C) Annealing Time (min) VHN1 VHN2

VHN3 Average

E 300 0

E1 2

E2 5

E3 10

E4 20

E5 50

E6 70

3. Results

Please answer the following questions in your lab book and transfer them to your lab report.

Part 1

1.Referring to the stress strain curve for a tensile test on a metal given in Figure 1 explain why after the onset of plastic deformation the stress increases as the % elongation increases, where % elongation is defined as

% Elongation=(Final length−Original length)

Original length×100

(Hint: answer requires discussion about dislocations)

2. If we assume we start with an equiaxed grain structure prior to testing sketch the grain structure you would expect before and after testing to failure:-

Grain StructureBefore Testing

Grain StructureAfter Testing

3. If the material was deformed to 15% elongation and then the stress released, what would the new Stress %Elongation curve be. (draw this new curve on the graph above- Figure 1). What will happen to the stress required to cause plastic deformation (Yield Stress), and the elongation to failure (ductility)?


4. In commercial metal production larger pieces of metal are cold worked by processes such as rolling to reduce the thickness of the metal to the required dimension. From what you have just considered what effect will this cold rolling have on the strength and ductility of the material?

Part 2

1. When you have collected all the hardness data above plot this data using the template given below

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

250

0 10 20 30 40 50 60 70 80

% Cold Rolling

Ha

rdn

es

s (

Hv

)

2. Comment on the effect of cold rolling on the hardness and microstructure of α Brass:3. Using the hardness graph above estimate the hardness of a 20cm thick piece of α Brass that has been cold

rolled to a final thickness of 8 cm. (check formula for % reduction during cold rolling on page 6)

4. Sketch the microstructure of samples A (annealed cold rolled 0%), C (cold rolled 30%) and E (cold rolled 70%).

NB: Use the same magnification for all your examinations.

SAMPLE A(Cold Rolled 0%)

Magnification

Micron Marker:-


SAMPLE C(Cold Rolled 30%)

Magnification

Micron Marker:-

SAMPLE D(Cold Rolled 70%)

Magnification

Micron Marker:-

5. Draw a schematic of the three samples in the circle below


Part 3

1. Sketch the micro structure of the Sample E4 and E6. (remember to use the same magnification that you used for the previous microstructural examinations).

SAMPLE E4 (300°C/20MIN)(Recrystallised)

Magnification

Micron Marker:-

SAMPLE E6 (300°C/70MIN)(Recrystallised)

Magnification

Micron Marker:-

Draw a schematic of the sample in the circle below.

2. Compare this microstructure with the original undeformed microstructure (sample A), and the microstructure of the sample deformed by 70% (sample E).

3. For the annealing temperature of 300°C plot the hardness against log time on the graph below. (for 0 time use 0.1 min).


4. From these curves determine the time taken for the specimen to become 50% recrystallised. This is the time taken for the hardness to drop to ½ (HD+HR), where HD is the hardness of the deformed materials and HR is the hardness of the fully recrystallised material. Enter this data into the table below:-

Temperature (°C) Time for 50%Recrystallisation (Mins)

275

300

325

350

Why do we use the time for 50% recrystallisation as opposed to 100%?

5. The growth of new undistorted grains during recrystallisation involves the movement of atoms in the solid state. The ease with which such movement can take place depends critically on the thermal energy which the atoms have, due to the temperature of the material. This dependence upon temperature is called thermal activation. For thermally activated processes, such as recrystallisation, the relationship between the rate of the process and the temperature is given by:

Rate of Reaction = A . Exp(-Q/RT)

A is a constant, Q is the activation energy for the process, and R is the universal gas constant (8.314 J/mol/K).

NB: T is Temperature in K

This equation is called the Arrhenius Equation.

Given this relationship how would you calculate the activation energy for recrystallisation in α brass? 12 1st year- Deformation and recrytallisation lab script- 2013/2014

6. Having ascertained the correct method complete the table below and plot the appropriate graph in your lab book. Remember to label axes.

Temperature (°C) Time for 50%Recrystallisation (Mins)

y-axis x-axis

275

300

325

350

7. From this graph calculate the activation energy for recrystallisation in α brass.

4. Discussion and Reporting requirements

Write a full lab report to Departmental guidelines (see Notes for Tutees), integrating the microstructural sketches and hardness measurements into a stand-alone report. Also include all your results in your report.

Also answer to the questions given under the results section.

5. Reference/ Further Reading

Bert Verlinden, Julian Driver, Indradev Samajdar, Thermo-Mechanical Processing of Metallic Materials, Elsevier, 2007

R. E. Smallman, A. H. W. Ngan, Modern Physical Metallurgy, Butterworth-Heinemann Ltd, 2013.


Lab Script- New Deformation and Recrystallisation vRQ v Jan 2014

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