Recrystallization Theoretical & Practical Aspects

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1 Theoretical & Practical Aspects , Microstructure & Properties I Fall 2006 Supplemental Lecture A.D. Rollett, M. De Graef Materials Science & Engineering Carnegie Mellon University 1

2 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

3 Basics 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. 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

4 Microstructures The microstructure gradually changes from one with elongated, deformed grains to one with undeformed, equiaxed grains. Aluminum Handbook, Hatch (1984). 4

5 Annealing Processes 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. 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

6 : 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? boundaries are slowed down by the presence of solute and most practical materials have significant amounts of solute. 6

7 : 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

8 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

size as a function of prior deformation level The grain size after recrystallization decreases with increasing prior strain, i.e. the nucleation density increases.

9 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 600 C (1.5h) after 2 (top), 6, 8 & 10% (bottom) reduction in tensile strain. Note that these are very small strains compared to commercial practice. Next slide shows industrial data on grain size, also for commercial purity aluminum. 9

10 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.

11 Strain effect on kinetics takes place more rapidly as the deformation strain increases. This work was performed at Carnegie Tech. [Humphreys] 11

12 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

13 Temperature Effect on Rex kinetics 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

14 Impurity effects on recrystallization V (cm.s -1 ) decreasing Fe content 1/T F. R. Boutin, J. Physique, C4, (1975) C increasing Cu content R. Vandermeer and P. Gordon, Proc. Symposium on the Recovery and of Metals, New York, TMS AIME, (1962) p

15 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

16 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

17 Source of stored energy Martin, Doherty & Cantor distinguish between microstructural changes driven by chemical energy and change driven by strain energy. 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

18 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 = 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 µm, which is a very large (too large!) critical radius. Therefore nucleation in recrystallization must be heterogeneous. 18

19 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

20 Growth of New s The interface between a new grain and the deformed (unrecrystallized) material ( matrix ) is a grain boundary. 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

21 Laboratory 1 - duction 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

22 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

23 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

24 - 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

25 What is a Boundary? 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

26 Crystal orientations at a g.b. g B TJ ACB g D g B g A -1 g C TJ ABC g A 26

27 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]. 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

28 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 c1=c0/15.*(1.-ln(c0/15.)) Energy Mobility c2= *exp(-.5*(c0/15)^9) Angle ( ) 28

29 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

Chapter 8 Strain Hardening and Annealing

Chapter 8 Strain Hardening and Annealing This is a further application of our knowledge of plastic deformation and is an introduction to heat treatment. Part of this lecture is covered by Chapter 4 of