CH11 - Transformations

Question 1

Homogenous nucleation?

Answer 1

nucleation takes place at any defect-free point in the matrix (and directly from solution)

Question 2

Exsolution?

Answer 2

when former homogenous phase separates into 2 or more solid solution phases under sub solidus conditions.
(Lots of little crystals in host crystal. Always organised with respect to each other and host)

Question 3

Nucleation and growth?

Answer 3

the mechanism in which most first order and reconstructive transformations take place. A new phase is initiated by a local fluctuation in composition and/or structure which develops into parent.

Question 4

What does formation of critical radius nucleus depend on?

Answer 4

Depends on if embryo can overcome activation energy barrier. Ea decreases as undercooling increases

Question 5

Limitations to thermodynamic predictions about exsolution?

Answer 5

Nucleation - undercooling is required for new phase to nucleate and overcome strain energy Kinetics - Formation of 2 phases requires diffusion to nucleation site. This can take a long time at low temperatures

Question 6

what does coherency of the interface depend on?

Answer 6

nature of the interface (structural similarity) and shape of the precipitate

Question 7

Coherent solvus?

Answer 7

situation with no energy barrier; much greater for homogenous nucleation due to greater undercooling

Question 8

Heterogeneous nucleation?

Answer 8

involves nucleation and growth of the product phases within the parent phase. Takes place on defects

Question 9

What are defects?

Answer 9

Disruptions in the lattice. Include point, line and planar defects. Different to bulk structure

Question 10

what happens if nucleation destroys part of a defect?

Answer 10

Activation energy barrier is reduced

Question 11

where is nucleation likely to start?

Answer 11

Always expected to begin at grain junctions/boundaries as the amount of undercooling needed is less for heterogenous nucleation. low energy sites

Question 12

Does thermodynamics or kinetics have more control on nucleation at equilibrium temperatures?

Answer 12

Thermodynamics have more control because diffusion may be high at equilibrium temp, however rate of nucleation is low because of high dG.
But at LOW TEMPERATURES it is more kinetically controlled because the dG is low. Low diffusion and low nucleation

Question 13

Solid solution?

Answer 13

homogenous mix of 2 or more elements in a solid state with a single crystal structure

Question 14

What are the types of interfaces by migration?

Answer 14

Glissile (athermal) and Non-Glissile (thermal)

Question 15

Interface-controlled growth?

Answer 15

controlled by rate at which atoms of the parent phase can cross the interface and become part of the new phase. Characteristic of displacive and reconstructive polymorphic transformations

Question 16

Diffusion-controlled growth?

Answer 16

In an exsolution transformation, growth may require long-range diffusion due to differences in chemical and physical structure. This may control the rate

Question 17

Can interface-controlled growth dominate when two phases have different compositions?

Answer 17

Yes. When the interface is incoherent, it does not present a structural obstacle. Migration Is controlled by rate of supply of atomic ingredients. Incorporation on a coherent plane results in instability, making migration difficult. Thus growth is interface controlled

Question 18

Stepped interfaces?

Answer 18

lamellar plates with a dislocation at each step. The interface can then growth perpendicular to itself by the lateral migration of these steps. Found in pyroxenes, amphiboles and feldspars
- Form due to low mobility of coherent and semi-coherent interfaces
- Control degree of coherency - increase coherency

Question 19

After initial nucleation, growth of exsolution depends on?

Answer 19

1. Diffusion of atoms to growth site
2. Incorporation of these atoms into the surface of new phase

Question 20

What if rate of diffusion is less than incorporation?

Answer 20

Diffusion controlled reaction

Question 21

What is rate of diffusion is greater than incorporation?

Answer 21

Interface controlled reaction

Question 22

Why does diffusion occur?

Answer 22

The bulk diffusion of atoms results from thermal oscillations which may have sufficient amplitude to cause an atom to jump randomly from one site to the next

Question 23

Fick’s First Law?

Answer 23

the rate of flow is proportional to concentration gradient

Question 24

Fick’s Second Law?

Answer 24

When the concentration profile is concave down in a given region, the concentration decreases with time.

Question 25

Interdiffusion?

Answer 25

Two materials are placed into close contact and the concentration profiles of a specific ion across the interface are analysed after heating for a specific amount of time.wo materials are placed into close contact and the concentration profiles of a specific ion across the interface are analysed after heating for a specific amount of time.

Question 26

Interdiffusion coefficient?

Answer 26

when two species contribute to the rate of homogenization and thus the value of D

Question 27

Anisotropy in crystal diffusion?

Answer 27

Different energetic barriers to exchange in diff directions

Question 28

What does interface drift demonstrate?

Answer 28

Shows that vacancy assisted diffusion is the main mechanism in the bulk

Question 29

Which diffusion pathway is most important?

Answer 29

Grain boundary. At low temperatures rate of grain boundary diffusion is faster than bulk (most occurs via grain boundary diffusion). But at high temperatures bulk is faster than grain boundary

Question 30

How do defects move along a surface/ crystal structure?

Answer 30

Movement described as the "random walk". This arises from the finite probability that an atom will exchange with its neighbour. This probability is controlled by bond strength and anisotropy

Question 31

How can olivine transform to wad/ringwoodite, AND which is most likely?

Answer 31

Nucleation and growth or reconstructive transformation by partial dislocations (CCP to HCP). Most likely to proceed by transformation as it has the smallest kinetic barrier. Low energy as only a small part of crystal needs to 'unzip'