Martensite

Question 1

How is martensite created?

Answer 1

If cooling is fast enough, austenite becomes more unstable as temp. Lowers and transforms to Martensite (alpha ‘)

Happens not by diffusion of carbon because there’s not time for it to diffuse out (carbon is trapped so it becomes a supersaturated solid solution)

Happens by a displacive transformation (by homogenous shear of the lattice)

Question 2

What is a supersaturated solid solution?

Answer 2

When austenite is cooled very quickly, carbon becomes trapped as interstitial impurities.

Question 3

What is a Gibbs Energy graph?

Answer 3

Gibbs Energy vs. Temperature (two lines of say alpha and gamma phase - where they cross is where Gibbs Energy is equal - should be at A3 temperature) - as lines get further apart, the difference in energy becomes substantial enough to change austenite to martensite

Question 4

What is degree of tetragonality?

Answer 4

Ratio of c length to a length of a bcc.

C/A = 1.005 + 0.045(wt%C)

Question 5

What is a reconstructive transformation?

Answer 5

Atomic bonds are broken and rearranged to a new lattice. Requires diffusion of atoms.

Question 6

What is a displacive or shear transformation?

Answer 6

Deformation of original lattice into new crystal structure.

Change affects macroscopic shape.

Question 7

What type of transformation forms Martensite?

Answer 7

Displacive - diffusionless and involves individual atom movement of less than one atomic spacing

Question 8

What is the interface called between austenite and martensite once the martensite is formed by shear?

Answer 8

Habit plane

Question 9

Realistically, where there are millions of grains, what does the constraint on a grain do to the habit plane?

Answer 9

The habit plane is curved and thin to minimise the strain energy as opposed to unconstrained transformations which have flat habit planes.

Question 10

What is a typical value of the shear component of the strain during a martensitic transformation?

Answer 10

0.25 - This is a very large elastic strain energy which is why the grain adopts the shape which will minimise the strain.

Question 11

What shape are Martensite planes?

Answer 11

Lath or lenticular plate shaped

Question 12

How do martensite grains grow?

Answer 12

Grow from one grain boundary until it reaches another grain boundary (looks like a grain of rice between both boundaries)

Question 13

What could be seen on the surface of a steel where a Martensitic grain had grown?

Answer 13

Visible displacement deformation upwards on the surface

Question 14

Describe a TTT diagram?

Answer 14

Time, Temperature, Transformation produced for each alloy to show what transformation would be happening at a certain temperature or time

Question 15

What is the only transformation that can change into martensite, ferrite, Bainite etc.

Answer 15

Austenite - to change, you’d have to reheat back to austenite city temp till it’s fully austenitised

Question 16

Describe how reconstructive and displacive transformations are different in terms of the TTT graph?

Answer 16

Reconstructive happen above the martensitic temperature and are dependent on the time you hold it at the temp. - more time = more transformation

Displacive transformations are relative only to temperature - if you hold them at a set temp. For a time, they will not transform anymore, they have to be cooled downwards to transform more

Question 17

What is Ms?

Answer 17

Martensitic start temperature - where Martensite transformation begins

Question 18

What is Ms dependent on?

Answer 18

Heavily reliant on the alloy content

Question 19

What is Mf?

Answer 19

Temp at which Martensite has finished transforming

Question 20

How do we know a transformation is diffusionless?

Answer 20

• Can form at very low temperatures where diffusion is not possible
• Can form at close to supersonic speeds >1000m/s
• Chemical composition of Martensite can be identical to parent Austenite
• Transformation is athermal, only a function of temp and not time

Question 21

What does the amount of Martensite transformed only depend on?

Answer 21

The temp to which the austenite is undercooked below Ms.