Recrystallization Theoretical & Practical Aspects - Materials Science ...

Intro 

Recrystallization 

Lab 1 

Grain 

Boundaries 


Theoretical & Practical Aspects 

27-301, Microstructure & Properties I 

Fall 2006 

Supplemental Lecture 

A.D. Rollett, M. De Graef 

Materials Science & Engineering 

Carnegie Mellon University 

1

Intro 


Lab 1 

Grain 

Boundaries 

Objectives 

• The main objective of this lecture is to introduce you 

to the process of recrystallization and to prepare 

you for a laboratory exercise on this topic. 

• You will have mastered the material in this lecture if 

you can describe the process in qualitative terms, 

can relate it to thermomechanical processing in 

general and know how to apply Johnson-Mehl- 

Avrami-Kolmogorov analysis to the kinetics. 

2

Intro 


Lab 1 

Grain 

Boundaries 

Recrystallization Basics 

• Recrystallization is essential to thermomechanical 

processing of metallic materials. Plastic 

deformation stores energy in the form of 

dislocations and also distorts the shape of the 

grains. Recrystallization restores the material to 

an undeformed state. 

• Static recrystallization occurs on heating the 

deformed material to an elevated temperature. 

• Dynamic recrystallization occurs during the 

plastic deformation. This only occurs for hot 

deformation at temperatures greater than 0.5 of the 

melting point. 

3

Intro 


Lab 1 

Grain 

Boundaries 

Microstructures 

The microstructure 

gradually changes 

from one with 

elongated, 

deformed grains to 

one with 

undeformed, 

equiaxed grains. 

Aluminum Handbook, Hatch 

(1984). 

4

Intro 


Lab 1 

Grain 

Boundaries 

Annealing Processes 

• Recrystallization is one example of a process that occurs 

during annealing of materials. Annealing is simply the 

exposure of a material to elevated temperature for a specified 

period of time. Various thermally activated processes 

occur during annealing that the materials engineer seeks 

to control in order to optimize properties. 

• Other processes include recovery, grain growth, carburization, 

and sintering. 

• Recovery is the decrease of dislocation density that occurs by 

motion and annihilation of individual dislocations. 

• Grain growth is the coarsening of the grain structure by 

motion of grain boundaries. 

• Carburization is an example of a change of chemical 

composition near the surface brought about by the presence of 

a high chemical potential for carbon (e.g. by having CO in the 

furnace atmosphere) during annealing. This is important in 

steels for producing high hardnesses at the surface of a 

material. 

5

Intro 


Lab 1 

Grain 

Boundaries 

Recrystallization: mechanisms 

• The basic mechanism of recrystallization is the 

(long-range) motion of grain boundaries that 

removes dislocation density from the material. 

• A consequence of the requirement for longrange 

boundary migration is that 

recrystallization is a thermally activated 

process. 

• In most materials, temperatures > T m/3 are required 

for recrystallization to proceed at a measurable rate. 

• Why? Grain boundaries are slowed down by the 

presence of solute and most practical materials 

have significant amounts of solute. 

6

Intro 


Lab 1 

Grain 

Boundaries 

Recrystallization: measurement 

• How can we measure recrystallization? 

• The traditional method is to perform optical 

metallography on sectioned samples. 

Recrystallized grains appear as approximately 

equiaxed grains with uniform color. Unrecrystallized 

grains appear as deformed grains with irregular 

contrast. 

• Measurement is primarily the area fraction of 

recrystallized versus unrecrystallized material. 

Stereology tells us that this area fraction is 

equivalent to the volume fraction of recrystallized 

material. 

• An easier measurement is hardness which 

decreases during the recrystallization process. 

7

Intro 


Lab 1 

Grain 

Boundaries 

Recrystallization Characteristics 

• In order for the boundary between a new grain (nucleus) and 

the deformed material to be able to move, it must be a high 

angle boundary. This is a consequence of the properties of 

boundaries, to be described later. 

• The requirement that new grains have high angle boundaries 

means that the final grain size and the rate at which 

recrystallization takes place is highly dependent on the strain 

level. 

• Higher strains mean greater lattice rotations (from dislocation 

slip) inside grains and higher stored energies. Therefore the 

probability of generating new grains increases with strain and 

the driving force increases. 

• Increasing probability for nucleation translates directly into 

increased density of nuclei and therefore smaller recrystallized 

grain size. 

8

Grain size as a function of prior 

deformation level 

• The grain size after 

recrystallization decreases 

with increasing prior strain, 

i.e. the nucleation density 

increases. 

• Example of commercial 

purity Al, recrystallized at 

Intro 600°C (1.5h) after 2 (top), 6, 

Recryst- 8 & 10% (bottom) reduction 

allization in tensile strain. 

Lab 1 

• Note that these are very 

small strains compared to 

Grain commercial practice. 

Boundaries 

• Next slide shows industrial 

data on grain size, also for 

commercial purity aluminum. 

9

Intro 


Lab 1 

Grain 

Boundaries 

Strain dependence 

Aluminum Handbook, Hatch (1984). 

• In most materials, the grain size after recrystallization 

decreases as the strain increases. For most applications, 

small grain size is desirable. Certain applications, however, 

require large grain size, and so small strains are sometimes 

used. 

• Note that the heating rate has essentially no effect on the 

10 

outcome of recrystallization.

Intro 


Lab 1 

Grain 

Boundaries 

Strain effect on kinetics 

• Recrystallization takes place more rapidly as the deformation 

strain increases. This work was performed at Carnegie Tech. 

[Humphreys] 

11

Intro 


Lab 1 

Grain 

Boundaries 

Temperature dependence 

• The growth of new grains requires motion of grain 

boundaries. Boundary migration occurs by the 

transfer of atoms across the boundary which is a 

diffusion-like process. 

• Solutes have a strong effect on boundaries because 

the interaction leads to segregation (generally an 

excess of solute on the boundary). In effect, moving 

the boundary forces the solute to move with it. 

• A suitable measure of the “reaction rate” is the time 

for 50% recrystallization. 

12

Intro 


Lab 1 

Temperature Effect on Rex kinetics 

Grain 

Boundaries 

• Recrystallization is a 

thermally activated 

process and therefore 

proceeds more rapidly 

as the temperature 

increases. 

• Note that the rate of 

recrystallization is 

measured by the time 

required for 50% 

recrystallization. 

[Humphreys] 

13

Intro 


Lab 1 

Impurity effects on recrystallization 

Grain 

Boundaries 

V (cm.s -1 ) 

1/T 

decreasing Fe content 

F. R. Boutin, J. Physique, C4, 

(1975) C4.355. 

increasing Cu content 

R. Vandermeer and P. Gordon, Proc. 

Symposium on the Recovery and 

Recrystallization of Metals, New York, 

TMS AIME, (1962) p. 211. 

14

Intro 


Lab 1 

Grain 

Boundaries 

Nucleation & Growth 

• Based on the microstructural characteristics (a 

different type of material appears as dispersed 

‘particles’ and grows to the point of replacing the 

deformed material), recrystallization is classified as 

a ‘nucleation & growth’ phenomenon. 

• Although treating recrystallization as a nucleation & 

growth process is perfactly adequate, more detailed 

examination shows that it is actually a continuous 

coarsening process. The coarsening is, however, 

so highly heterogeneous that classification depends 

on the length scale at which it is characterized. 

15

Intro 


Lab 1 

Grain 

Boundaries 

Nucleation & Growth 

• Two steps are required for recrystallization to 

proceed: 

– (a) nucleation of new grains that are dislocation-free 

– (b) growth of the new grains into the dislocated matrix 

Deformed matrix 

New grains 

16

Intro 


Lab 1 

Grain 

Boundaries 

Source of stored energy 

• Martin, Doherty & Cantor distinguish between 

microstructural changes driven by chemical energy 

and change driven by strain energy. 

• Recrystallization is a process of microstructural 

change driven by strain energy. 

• We will examine the details of plastic deformation 

later in the course. For now, it is sufficient to know 

that plastic deformation requires a high level of 

dislocation activity on at least 5 slip systems in each 

grain. Dislocations intersect and multiply leaving 

behind a highly irregular structure. This storage of 

dislocation line length is the direct cause of work 

hardening and is the strain energy that drives 

recrystallization. 

17

Intro 


Lab 1 

Grain 

Boundaries 

Nucleation Issues 

• Nucleation in recrystallization must be a 

heterogeneous nucleation process because the 

driving force is too small to sustain homogeneous 

nucleation. 

• Estimate of driving force, E: 

– Energy per unit length of dislocation ≈ Gb 2 

– Thus energy/volume, E ≈ Gb 2 ρ 

– Typical cold worked dislocation density, ρ = 10 15 .m -2 

– For Al, G = 27GPa, b =0.28nm,E ≈ 2 J.m -3 ( = 2MPa) 

– For a boundary energy σ = 0.5 J.m -2 , the critical nucleus 

size, r crit = 2σ/Ε 2 ≈ 0.25µm, 

which is a very large (too large!) critical radius. 

• Therefore nucleation in recrystallization must be 

heterogeneous. 

18

Intro 


Lab 1 

Grain 

Boundaries 

Heterogeneous Nucleation 

• Nucleation of recrystallization therefore occurs on 

defects in the material. 

• Sites for nucleation: 

– Prior grain boundaries (strain induced boundary migration, 

SIBM) 

– Deformation bands or shear bands, i.e. regions of nonuniform 

rotation of the lattice 

– Coarsening (recovery) of a subgrain structure 

– Coarsening (recovery) of dislocation structure near to 

coarse particles; this is called Particle Stimulated 

Nucleation (PSN). 

19

Intro 


Lab 1 

Grain 

Boundaries 

Growth of New Grains 

• The interface between a new grain and the 

deformed (unrecrystallized) material (“matrix”) is a 

grain boundary. 

• Grain boundaries vary in their mobility, M, i.e. the 

constant of proportionality between migration rate, v, 

and driving force, E. 

• Assume a linear relationship: v = ME. 

• The driving force is exactly the stored energy 

estimated previously. 

• (HAGB) High angle grain boundaries (θ>15°) are 

typically far more mobile than (LAGB) low angle 

boundaries (by orders of magnitude). 

20

Intro 


Lab 1 

Grain 

Boundaries 

Laboratory 1 - Introduction 

• The objectives of the first Lab are as follows: 

– Demonstrate recrystallization 

– Develop metallography skills 

– Ability to measure grain size 

– Ability to measure hardness 

– Demonstrate effect of strain on recrystallized grain size 

– Demonstrate effect of temperature on recrystallized grain 

size 

– Demonstrate effect of strain on hardness 

– Demonstrate effect of temperature on hardness 

– Demonstrate the Hall-Petch effect 

– Promote critical thinking about the reasons for the variations 

in grain size and hardness observed 

21

Intro 


Lab 1 

Grain 

Boundaries 

Lab. 1- Apparatus 

• The approach is to deform a rectangular piece of 

brass and then anneal it in a temperature gradient. 

The brass specimen is machined to have a wedge 

shape so that when it is deformed (rolled), the strain 

varies from the thin side to the thick side. 

• Download the Lab Manual from Blackboard in order 

to obtain further details. 

22

Intro 


Lab 1 

Grain 

Boundaries 

Lab. 1- Apparatus, contd. 

• The specimen is suspended by a wire within an 

induction coil. The lower end dips into a beaker of 

water in order to maintain one end at (a maximum 

temperature of) the boiling point of water. 

Induction 

Coil 

Water 

HOT 

COLD 

Procedure: apply 

heat and observe 

the temperature at 

the top of the 

specimen (T/C). 

Stop the heating 

once T=900°C; cut 

the wire so that the 

specimen falls into 

the water and is 

quenched. 

23

Intro 


Lab 1 

Grain 

Boundaries 

Lab 1 - Expected Results 

• Hardness: before recrystallization, the hardness should 

increase with increasing strain (hint - relate this to what you 

know about stress-strain behavior in metals). 

• After recrystallization, the grain size will increase with 

increasing temperature, but decrease with increasing prior 

strain. 

• Also after recrystallization, the hardness will increase with 

decreasing grain size (Hall-Petch effect) in the recrystallized 

areas. 

• Note: a critical part of the Lab is to obtain high quality images 

of the grain structure so that you can measure grain size. The 

metallography involved requires skill and effort. Also, 

computer-based submissions are required (Word, or LaTex). 

24

Intro 


Lab 1 

Grain 

Boundaries 

What is a Grain Boundary? 

• Grain boundaries control the process of 

recrystallization. This suggests that it is worth 

knowing something about the structure and 

properties of boundaries. 

• Regular atomic packing disrupted at the boundary 

by the change in lattice directions. 

• In most crystalline solids, a grain boundary is very 

thin (one/two atoms). 

• Disorder (broken bonds) unavoidable for 

geometrical reasons; therefore large excess free 

energy. This interfacial energy is analogous to the 

surface tension in a soap bubble (and many 

investigations on grain growth have been made with 

soap froths). 

25

Intro 


Lab 1 

Grain 

Boundaries 

Crystal orientations at a g.b. 

g D 

TJ ABC 

g B 

g Bg A -1 

g A 

TJ ACB 

g C 

26

Intro 


Lab 1 

Grain 

Boundaries 

Grain Boundary Structure 

• High angle boundaries: can be thought of as two 

crystallographic planes joined together (with or w/o a 

twist of the lattices). 

• Low angle boundaries are arrays (walls) of 

dislocations: this is particularly simple to understand 

for pure tilt boundaries [to be explained]. 

• Grain boundary energy increases monotonically with 

misorientation as a consequence of increasing 

dislocation density. 

• Transition: in the range 10-15°, the dislocation 

structure changes to a high angle boundary 

structure. 

• The grain boundary mobility increases abruptly at 

this transition in structure. 

27

Intro 


Lab 1 

Grain 

Boundaries 

LAGB to HAGB Transitions 

• The Read- 

Shockley equation 

describes the 

energy of low 

angle boundaries. 

• An exponential 

function is useful 

for describing the 

sharp transition in 

mobility from lowto 

high-angle 

boundaries 

Energy, Mobility 

1 

0.8 

0.6 

0.4 

0.2 

c1=c0/15.*(1.-ln(c0/15.)) 

Energy 

Mobility 

c2=1.-0.99*exp(-.5*(c0/15)^9) 

0 

0 5 10 15 20 25 

Angle (°) 

28

Intro 


Lab 1 

Grain 

Boundaries 

Example: tilt boundary = array 

of parallel edge dislocations 

b 

• Low angle boundaries are arrays of parallel edge 

dislocations if the rotation between the lattices is 

small and the rotation axis lies in the boundary 

plane. In this example, the rotation axis between 

the two crystals is perpendicular to the plane of the 

picture. 

29

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