Thermomechanical- Static and Dynamic Structural Changes

Question 1

Where do dynamic and static events occur for rolling?

Answer 1

Dynamic events take place during deformation so in the region between the rolls.
Static events take place after the deformation has occurred so are after the rolls.
Examples are dynamic and static recrystallisation

Question 2

How does the microstructure changes before during and after rolling?

Answer 2

The original grains before rolling are deformed and become elongated by the rolling process. Later new grains can form and grow by static RX to form new equiaxed grains. Could also get dynamic RX in which case this happens between the rolls

Question 3

Graphs for true stress vs true strain for work hardening

Answer 3

Both have concave curve up initially as the grains deform (elongate) a bit. For high Z conditions, this curve keeps going up as grains continue to be deformed and elongated. This is pure work hardening. For low Z conditions, the curve reaches a peak at εp then starts decreasing a bit as dynamic RX starts then levels off once dynamic RX complete. The peak in the graph is usually an indicator of dynamic RX

Question 4

Stress strain graph for dynamic recovery

Answer 4

Small concave curve up at start then levels off to basically straight horizontal line. Entire graph is higher for larger strain rate but same shape. Behaviour typical for α-iron, ferritic steel and Al as these all have high stacking fault energies

Question 5

What does dynamic recovery mean for RX?

Answer 5

When dynamic recovery takes place, all the deformational energy is expended through recovery so there is none left to drive (dynamic?) RX

Question 6

Why might there be a peak in the stress strain curve even when dynamic RX is not occurring?

Answer 6

It could just be dynamic recovery but at a high strain rate as this generates heat increasing the temperature of the metal and reducing its strength. The peak will be at εs

Question 7

What happens during recovery?

Answer 7

Polygonisation occurs which is where dislocations rearrange themselves into low energy configurations. This doesn’t change the overall dislocation density and so there is very little change in mechanical properties. At strains greater than εs (where peak in graph may be) polygonisation looks to have made a network of almost subgrains but the boundaries are clusters of dislocations

Question 8

How do hardness, resistivity, cell size, density and energy release vary with temperature through recovery and RX?

Answer 8

Property vs temperature with recovery region (wider) left of RX (narrower). Hardness stays fairly constant into and through recovery then drops quickly through RX then settles after. Resistivity decreases through recovery a bit then more through RX then settles. Cell size constant until just before RX then does steep linear increase. Density increases a bit through recovery then more through RX then settles. Energy releases increases just before recovery then fairly constant then increases a lot more into RX then comes back down still within RX.

Question 9

Why ides resistivity decrease through recovery and RX?

Answer 9

The as-deformed material has more point defects like vacancies which increase resistivity. Recovery reduces the number of these defects so resistivity decreases (conductivity increases)

Question 10

What microstructure changes occur due to work hardening and recovery and where do they take place relative to the stress strain curve?

Answer 10

Concave curve up until εm then fairly level. Throughout the process grains elongate. Left of εm dislocation density increases and subgrains develop. Right of εm dislocation density remains constant and subgrains remain. These are about equiaxed with constant means size and mean misorientation. This is for constant strain rate and T

Question 11

When dynamic RX occurs, describe how the true stress vs true strain graph varies with strain rate and what the grains are doing at each stage of the graph

Answer 11

Steep curve up to peak then short exponential curve down to horizontal. This graph moves up for increasing strain rate and the peak is further right (higher strain to peak stress). RX starts before the peak and the peak is where there is a great enough fraction of RX grains to compensate for work hardening from elongated original grains. Once horizontal section reached all grains are equiaxed RXd grains with constant mean size. Original grains elongate until this point

Question 12

Where do new grains from RX first form and how do they progress?

Answer 12

They form on the grain boundaries to form a necklace of new grains. As strain increases a second stage of cascade forms on both sides of this necklace progressing into grains. Third and fourth stages of cascade form until the centre of grains is reached by new grains.

Question 13

Why are the observable microstructural changes produced by static recovery small?

Answer 13

Because dynamic recovery has already taken place during deformation

Question 14

What can static recovery result in and lead to?

Answer 14

Results in the develop,ent of nuclei for static RX when the prior strain exceeds some low critical value. Static recovery also leads to some softening which is easily detected if a second deformation is applied following some delay period after the end of the first deformation (e.g. roll 1 then delay before roll 2)

Question 15

Formula for fraction recrystallised by static RX

Answer 15

JMAK equation
X=1-exp(-C(t/t(f))^k)
Where t is time
t sub f is time required for a fraction f to have RXd
C=-ln(1-f) and is 0.693 when f=0.5
k is Avrami coefficient which is typically about 2 although values as low as 0.7 have been observed for some alloy steels

Question 16

Three characteristics of JMAK kinetics

Answer 16

There is an incubation period τ before any static RX occurs (this time is less when more deformation (% cold work) has occurred).
RX begins and ends slowly.
RX reaches a maximum rate in the middle (50%) of transformation

Question 17

Fraction recrystallised vs log time graph for static RX

Answer 17

Basically cumulative frequency shape curve which moves side to side depending on conditions

Question 18

Effect of temperature and strain rate on fraction recrystallised vs log time graph for static RX

Answer 18

Higher temperature brings graph to left (sooner) so incubation period less.
Higher strain rate has the same effect

Question 19

Recrystallised grain size vs elongation graph and explanation

Answer 19

Starts very high at some critical amount of cold work. Then steep exponential decay curve down as elongation increases. This is because as the amount of strain increases, more nuclei are created leading to a finer recrystallised grain size. The temperature at which RX starts decreases with increasing amount of strain. See slide 15

Question 20

Effect of temperature on time to reach 50% RXd on fraction recrystallised vs log time graph for static RX

Answer 20

Follows equation
1/t50%=Aexp(-Q(R)/RT)
So whole graph shifts left for increasing temperature and time to reach 50% RXd is reduced

Question 21

How does purity affect RX temperature?

Answer 21

As purity increases RX will commence at lower temperature. Caused by solutes segregating to dislocations and slowing their motion.
RX temp vs per cent purity graph is concave curve down

Question 22

How does grain size affect fraction recrystallised vs log time graph for static RX?

Answer 22

As grain size decreases there is more GB area per unit volume which leads to faster RX kinetics and therefore graph is shifted to the left

Question 23

What are the single and double-hit tests for?

Answer 23

Simulating a metalworking process at some temperature and strain rate indicative of that process. Plot a stress vs strain curve for both

Question 24

Single-hit test

Answer 24

Total strain for the operation is induced at once with no delay time in between. Get a single concave curve up

Question 25

Double-hit test

Answer 25

Do some amount of strain then have some delay time between the second amount of strain. Get concave curve up then vertical drop then another concave curve up on the stress vs strain graph

Question 26

How does delay time affect the double-hit test?

Answer 26

It dictates the effect if precipitation. A long delay time means that restoration processes can get rid of any precipitation that resulted from the first deformation. This results in a lower second curve up. A short delay time means that precipitation is dominant and leads to a higher second curve up which ends higher than the single-hit test would have. This represents hardening

Question 27

How to work out fractional softening from single and double-hit tests

Answer 27

Take the double-hit stress strain graph. The area under the first curve is A1 and under the second curve is A2.
For the single-hit test, split the area under the single curve into an equivalent A1 at the start and the remaining area A3.
Fractional softening=((A3-A2)/(A3-A1))x100.
This will be negative for hardening and positive for softening

Question 28

How does temperature affect hardening or softening in double-hit test?

Answer 28

Higher temperature means less supersaturation so less precipitation will occur in the test so there will be less hardening due to precipitation. Means the second curve will not be as high compared to the first one than for lower temperatures. Both curves are also lower down the graph for stress due to the higher temperature

Question 29

How to find the recrystallisation stop temperature from overall fractional softening vs deformation temperature graph

Answer 29

There will be different curves for different delay times. 0.2 or 20% softening is due to recovery so T(RXN) will be where the curves cross y=0.2. The y-axis goes from negative (hardening) down to positive (softening).