2. Microstructure and Mechanical Properties

Question 1

what is meant by an equiaxed grain structure

Answer 1

when every crystal is approximately the same length in every direction

Question 2

what is meant by heterogeneous nucleation?

Answer 2

where atoms of a crystal nucleate on a solid surface

Question 3

state the main property of grain boundaries

Answer 3

high energy regions

Question 4

what is porosity

Answer 4

holes in metal formed in grain development due to insufficient molten metal in casting (as the volume shrinks during solidifcation)

Question 5

describe dendrite formation and explain where it happens

Answer 5

when a columnar grain splits off and grows in all directions, occurs in casting and only when a crystal goes from liquid to solid or from gas to solid.

Question 6

what is each crystal called in a polycrystalline structure?

Answer 6

a grain

Question 7

what is metallography and list the steps

Answer 7

process used to reveal the “grain structure”. steps: grind to flat surface, polish and then etching (with acid or alkali). Then use a metallographic microscope.

Question 8

grain boundaries stop what?

Answer 8

dislocation movement (in most cases)

Question 9

what is the effect of grain size on strength?

Answer 9

smaller grain size = higher strength/higher yield stress (for same metal) which levels off

Question 10

what is rolling texture?

Answer 10

elongated grains due to rolling of the equiaxed grain structure

Question 11

how does plastic deformation cause work hardening?

Answer 11

increases dislocations which cause a pile up, dislocations get in each others way which make it harder to move

Question 12

what is %cold work?

Answer 12

%change in area

Question 13

what happens to the yield stress and ductility after cold work?

Answer 13

work hardening, yield stress increases (until a point, leveling off), ductility decreases

Question 14

Define hardness

Answer 14

resistance to localised deformation

Question 15

Describe the difference(s) in grain structure typically produced when molten PURE metal is cast compared to when an ALLOY is cast

Answer 15

Pure metal - Columnar grains meet in middle Alloy - Columnar grains form but small equiaxed grains form in middle

Question 16

define recrystallisation temperature

Answer 16

temp at which 50% cold worked metal just fully crystallises in 1 hour

Question 17

what is hot work?

Answer 17

plastic deformation above recrystallisation temperature

Question 18

what atoms move by vacancy diffusion? describe this process

Answer 18

lattice atoms or substitutional atoms move into a vacancy, creating another vacancy for the next diffusion

Question 19

for vacancy diffusion, besides the vacancy, what is needed for this process?

Answer 19

activation energy, as the atom moves to a higher energy level in between moving back down into the vacancy

Question 20

what does the arrhenius equation state?

Answer 20

reactions occur faster at a higher temperature, increasing exponentially

Question 21

state the arrhenius equation and state what each symbol means

Answer 21

rate=Ae^(-Q/RT) A is constant; Q is activation energy, R is universal gas constant, T is temperature in Kelvin

Question 22

describe the arrhenius plot/graph

Answer 22

rearrange equation: ln(RATE) = lnA - Q/R • 1/T plot ln(RATE) against 1/T, negative gradient straight line

Question 23

define diffusivity

Answer 23

measure of how easily atoms move

Question 24

equation for diffusivity

Answer 24

D=Do • e^(-Q/RT), Do is the diffusivity constant

Question 25

what type of diffusion is carbon atoms diffusing through iron?

Answer 25

interstitial diffusion

Question 26

What is the driving force for the recrystallisation step in annealing?

Answer 26

stored strain energy