Interfaces

Question 1

Vicinal surfaces:

Answer 1

> cut at a low angle offset from a low index plane/surface

> miscuts cause step edges

Question 2

Why is the surface energy of vicinal surfaces greater than the surface energy of low index surfaces?

Answer 2

broken bonds at step edges increase surface energy

Question 3

γ vicinal = γ low index + γ “steps”, in math: γ vicinal =

Answer 3

γ low index + (1/h * 1/a * n * ε/2)

Question 4

2 types of interfaces between solids and liquids during solidification:

Answer 4

1. atomically sharp (faceted)
   > liquid does NOT strongly interact with the solid phase’s surface
   > solid takes on shape similar to S/G case
2. diffuse interface (nonfaceted)
   > transition from L–>S occurs over a few atomic layers
   > all faces have same interfacial energy
   > solids take on spherical shape to minimize SA-V ratio

Question 5

The type of solid liquid interface depends on

Answer 5

entropy of fusion: ΔHf / Tm or ΔSf
> if ΔSf > 4R, sharp (very different structures, no blending)
> if ΔSf < 4R, diffuse (similar structures, interface blend)

Question 6

Richard’s Rule:

Answer 6

ΔSf ≈ 10 J/mol*k for atomic/elemental metals

Question 7

incoherent interface:

Answer 7

= γchemical + γstructural

Question 8

coherent interface:

Answer 8

= γchemical

Question 9

semicoherent interfaces: two types

Answer 9

1. PPT with similar or identical crystal structure but slightly different lattice parameters
2. PPT has a very different crystal structure, BUT one or a few of the planes in the PPT will match well to the symmetry/lattice paramter of the matrix-apply Wulff construction

Question 10

low angle grain boundaries

Answer 10

form periodic series of dislocations

Question 11

high angle grain boundaries

Answer 11

> no coherency at interface

> random grain boundaries

Question 12

coherent precipitates: ____ _____ and ____ ____ of the 2nd phase are the same as the matrix phase.
γtotal =

Answer 12

crystal structure; lattice parameter

γtotal = γchem

Question 13

incoherent precipitates: crystal structure and lattice parameter of 2nd phase are _______ than the matrix
γtotal =

Answer 13

considerably different

γtotal = γchem + γst

Question 14

semicoherent precipitate:

γtotal =

Answer 14

lattice parameter of precipitate can make periodic sets of bonds, but cannot fulfull every bond
γtotal = γchem + γst

Question 15

Low angle GBs

Answer 15

> low in energy

> bringing together vicinal surfaces

Question 16

Incoherent high angle GBs

Answer 16

> high in energy

> random GB

Question 17

Coherent/semi-coherent high angle GBs

Answer 17

> low in energy
2 planes that share a similar periodicity
low energy because of greater frequency of bonds
twin and coincident site boundaries

Question 18

Dislocation density 1/D =

Answer 18

sinΘ/b

Question 19

Dislocation energy is associated with the ____ _____ ____ introduced into the crystal lattice

Answer 19

internal strain energies

Question 20

At high dislocation densities, ____ _____ overlap and cancel each other out. Therefore, γGB _____ at high misorientation angles

Answer 20

strain fields; plateau

Question 21

Example of twin boundaries

Answer 21

the stacking of (111) planes in an FCC metal (close packed)

Question 22

Example of near-coincident boundaries

Answer 22

epitaxial thin films

Question 23

Interphase interfaces are formed during

Answer 23

solid state precipitation

Question 24

Types of interphase interface precipitates

Answer 24

1. coherent
2. semicoherent
3. incoherent