Metals- Material Properties

Question 1

What are the important mechanical properties?

Answer 1

Stiffness, strength, ductility, hardness, toughness, fatigue, creep

Question 2

Which property is not sensitive to composition and microstructure?

Answer 2

Stiffness

Question 3

Bond energy curve

Answer 3

Potential energy vs interatomic separation. Steep curve down to minimum below x-axis. Then upside down decay curve back towards x-axis. Is summation of repulsive energy (high at low r and 0 just right of minimum) and attractive energy (-1/r^2 curve)

Question 4

What does bond energy curve determine?

Answer 4

Melting temperature, elastic modulus, thermal expansion coefficient

Question 5

What causes thermal expansion?

Answer 5

Consequence of change in average separation between its constituent atoms or molecules. Molecules vibrate with increasing amplitude with increasing temperature so overall object expands

Question 6

Linear expansion coefficient equations

Answer 6

α=(1/l)dl/dT
For small changes in t:
Δl=αl0ΔT
l is length
l0 is initial length
T is temperature 
Can also have area and volume expansion of material

Question 7

How can α change depending on direction along material?

Answer 7

Need to consider expansion parallel or perpendicular to fibres in some materials. Kevlar has -ve parallel and large +ve perpendicular to fibre

Question 8

How does α vary with bond energy?

Answer 8

It generally decreases with increasing bond energy

Question 9

What explains different stiffnesses of different materials?

Answer 9

Effects of bond energy and lacking of atoms

Question 10

What else can change the value of Young’s modulus for a material?

Answer 10

It may be different in different directions of loading relative to the crystal structure

Question 11

Face centre cubic metals, lattice parameter, coordination number, atoms per unit cell

Answer 11

Al, Cu, Pb, Ni, Pt, Ag, Pd
a=2rt(2)R
CN=12
4 atoms per unit cell

Question 12

Hexagonal close packed metals, lattice parameter, coordination number, atoms per unit cell

Answer 12

Be, Ti, Zr, Mg, Zn, Co
a and c ideally c/a=1.633
CN=12
6 atoms per unit cell

Question 13

Body centred cubic metals, lattice parameter, coordination number, atoms per unit cell

Answer 13

Cr, W, Fe(α), Ta, Mo
a=4R/rt(3)
CN=8
2 atoms per unit cell

Question 14

Why is atomic arrangement important?

Answer 14

Play important role in determining microstructure and properties of material. Controls tensile behaviour, fracture behaviour and density. Materials with same atomic arrangements usually behave in similar ways when loaded and have similar methods for improvement of properties. Crystal structure can cause anisotropy in properties

Question 15

What is polymorphism?

Answer 15

A physical phenomenon where a material can have more than one crystal structure. A material that shows polymorphism exists in more than one type of space lattice in the solid state

Question 16

When is a polymorphic change allotropy?

Answer 16

If the change in structure is reversible with temperature or pressure

Question 17

3 crystal structures of iron

Answer 17

BCC as α when below 912C
FCC as γ when above 912C
BCC as δ when above 1538C

Question 18

Change in length vs temperature graph of iron

Answer 18

Diagonal line up (shallow) while α. Small drop at start of γ then diagonal line up. Small increase at start of δ to continue as the original line would have. Contraction due to FCC being close-packed and BCC not.

Question 19

Polymorphism in Tin

Answer 19

Normally BCT with density 7.30g/cm^3. When cooled to 13.2C goes to diamond cubic and expands 27% volume and density is 5.77g/cm^3