Grain growth, pore coarsening

Question 1

What is a sintered microstructure?

Answer 1

~ a section through grains contacting along surfaces

~ GB surface is a curve in the microstructure section
~ 3D intersection curve is a point in the microstructure section

Question 2

What happens to large vs small grains during sintering?

Answer 2

~ large grains have more sides

~ small grains (<6 sides) grain shrinks, GBs tend to bow outward
~ large grains (>6 side) grain grows, GBs tend to bow inward

Question 3

Why do grain boundaries sustain unsatisfied bonds?

Answer 3

~ lattice mismatch

~ reduction in grain boundary area reduces energy and is thus thermodynamically favored

Question 4

What is the power law for “normal” grain growth?

Answer 4

~ D^n = Kt + c
~ grain growth slows with time

~ D: grain diameter
~ t: time

Question 5

What happens as the smallest grasins are annihilated?

Answer 5

~ the remaining grains tend to be more equal in size, attenuating the driving force for grain growth

Question 6

What is GB surface motion?

Answer 6

~ atom detachment from one lattice, then attechment across the GB to the other lattice

Question 7

What happens to grain boundaries meeting at sharp angles?

Answer 7

~ they undergo grain boundary rotation to dull the intersection angle

~ the result is curved grain boundaries that tend to meet at 120degree angles

Question 8

When does abnormal (exaggerated, discontinuous) grain growth occur?

Answer 8

~ occurs when the starting particle size distribution is too broad

~ produces a coarse-grained microstructure with a diminished network of GB sustaining diffusion paths for sintering

~ abnormal grain growth has been developed for single crystal growthy

Question 9

What is the effect of pores?

Answer 9

~ porosity impedes grain growth

~ pores are dragged by the moving GB
~ pore migration with the boundary is generally by evaporation/condensation or surface diffusion

~ if pore moves, GB area decreases

~ with grain growth, pores can coalesce

~ for same % porosity, fewer large pores have less surface area than larger quantity of small pores

Question 10

What is Ostwald ripening?

Answer 10

~ pore coarsening: large pores grow at the expense of small ones via vacancy diffusion

~ once porosity is closed, if trapped gas is insoluble in the sintering solid, the pore pressurizes, and resists further shrinkage