MECH0007 Why does X1A contain equiaxed grains and X1, which is essentiallythe same metal, contain columnar grains?

MECH0007 Fundamentals of Materials - Materials Characterisation Lab

Metallography session

a) What is the purpose of "metallography"?

b) Why do the dendrites in X6/X7 not look like little trees?

c) Why does X1A contain equiaxed grains and X1, which is essentiallythe same metal, contain columnar grains?

d) What are the three forms of porosity seen in your specimens?

e) Which specimens contained interdendritic porosity?

f) What is coring?

g) Why is coring usually regarded as bad?

h) Explain why X2 cored and X3 is not?

i) Why are the dendrites in X7 not cored, when the ones in X6 are

j) What advantages are conferred on an alloy if it is of eutectic composition?

k) Explain how oil immersion improves the resolution of a lens.

l) Add in your sketch of specimen X6 here - remember that this specimen shows a number of important features in addition to a structure that is predictable by the use of the relevant equilibrium phase diagram - namely the non-equilibrium feature known as coring, as well as porosity and even shrinkage cracking visible at lower magnifications. Please annotate your sketch to highlight these features.

Mechanical Testingsession - (also see the separate lab-script for this session)

m). Carefully define the following quantities with respect to a typical tensile test: Young`s modulus, yield stress, tensile strength, percentage elongation and percentage reduction of area after fracture. You may wish to use diagrams to assist. Ensure you quote units.

n). Present your recorded data for the tensile test in the table below.

If you obtained a digital file with the test data, select representative data points for the table below (there is no need to record ALL data points below, given that some files could contain thousands of points depending on the strain rate).

Note that the testing machine recorded displacement vs load - you will need to plot stress versus strain - so populate the tables below correctly. Ensure you label your axes correctly and choose an appropriate scale for the plot.

Note also that the lab-script may have requested a value for the diameter - this was incorrect, as the specimen is of rectangular cross-section - so please quote the width and thickness values instead.

Plot your two graphs in the plot boxes below and provide the final values for the following quantities(in the answer boxes below). Show your working.

1. The quantities for mild steel and polyethylene (PE) of tensile strength, percentage elongation, and percentage reduction of area, after fracture.
2. The Young`s modulus (from your stress-strain plots) of steel and PE.
3. The value of the yield strength (if applicable) for each material.

See the Data Analysis section in your lab-script for more information.

O).Comment on the different portions of your two graphs. Discuss results obtained for both materials in relation to how necking and failure has occurred and how the material behaved under loading (such as which material has withstood more load and why). You should try and make comparisons between the two materials. You may employ sketches and diagrams to help your answer, or add photos if you took them.

p). Provide a brief uncertainty analysis for your tensile test results. Do this both from a qualitative viewpoint (by considering sources of error) and in a more formal quantitative manner, by considering and calculating the absolute and the percentage uncertainties in yourtensile strength results (show your working please).

q). State concisely and in general terms what useful information you have obtained and to what extent the tensile test was successful in terms of meeting the initial aims. Also suggest any improvements that you might make to the methods employed.

r). For the hardness tests, write in the data that you recorded during the lab session, and compute the average.

s). Using the resources available to you, research and hence explain, whether there is a relationship between the hardness value measured in a typical Vickers Hardness test and the actual yield strength and the tensile strength of a metal.

t). Briefly explain why the hardness test that you conducted is not suitable for ceramics and other brittle materials, or for the majority of polymer types.

u). Convert the Vickers Hardness values into Rockwell and Brinell hardness and comment on whether these conversions are reliable.