1IntroRecryst-allizationLab 1GrainBoundariesRecrystallizationTheoretical & Practical Aspects27-301, Microstructure & Properties IFall 2006Supplemental LectureA.D. Rollett, M. De GraefMaterials Science & EngineeringCarnegie Mellon University2IntroRecryst-allizationLab 1GrainBoundariesObjectives• The main objective of this lecture is to introduce youto the process of recrystallization and to prepareyou for a laboratory exercise on this topic.• You will have mastered the material in this lecture ifyou can describe the process in qualitative terms,can relate it to thermomechanical processing ingeneral and know how to apply Johnson-Mehl-Avrami-Kolmogorov analysis to the kinetics.3IntroRecryst-allizationLab 1GrainBoundariesRecrystallization Basics• Recrystallization is essential to thermomechanicalprocessing of metallic materials. Plasticdeformation stores energy in the form ofdislocations and also distorts the shape of thegrains. Recrystallization restores the material toan undeformed state.• Static recrystallization occurs on heating thedeformed material to an elevated temperature.• Dynamic recrystallization occurs during theplastic deformation. This only occurs for hotdeformation at temperatures greater than 0.5 of themelting point.4IntroRecryst-allizationLab 1GrainBoundariesMicrostructuresThe microstructuregradually changesfrom one withelongated,deformed grains toone withundeformed,equiaxed grains.Aluminum Handbook, Hatch(1984).5IntroRecryst-allizationLab 1GrainBoundariesAnnealing Processes• Recrystallization is one example of a process that occursduring annealing of materials. Annealing is simply theexposure of a material to elevated temperature for a specifiedperiod of time. Various thermally activated processesoccur during annealing that the materials engineer seeksto control in order to optimize properties.• Other processes include recovery, grain growth, carburization,and sintering.• Recovery is the decrease of dislocation density that occurs bymotion and annihilation of individual dislocations.• Grain growth is the coarsening of the grain structure bymotion of grain boundaries.• Carburization is an example of a change of chemicalcomposition near the surface brought about by the presence ofa high chemical potential for carbon (e.g. by having CO in thefurnace atmosphere) during annealing. This is important insteels for producing high hardnesses at the surface of amaterial.6IntroRecryst-allizationLab 1GrainBoundariesRecrystallization: mechanisms• The basic mechanism of recrystallization is the(long-range) motion of grain boundaries thatremoves dislocation density from the material.• A consequence of the requirement for long-range boundary migration is thatrecrystallization is a thermally activatedprocess.• In most materials, temperatures > Tm/3 are requiredfor recrystallization to proceed at a measurable rate.• Why? Grain boundaries are slowed down by thepresence of solute and most practical materialshave significant amounts of solute.7IntroRecryst-allizationLab 1GrainBoundariesRecrystallization: measurement• How can we measure recrystallization?• The traditional method is to perform opticalmetallography on sectioned samples.Recrystallized grains appear as approximatelyequiaxed grains with uniform color. Unrecrystallizedgrains appear as deformed grains with irregularcontrast.• Measurement is primarily the area fraction ofrecrystallized versus unrecrystallized material.Stereology tells us that this area fraction isequivalent to the volume fraction of recrystallizedmaterial.• An easier measurement is hardness whichdecreases during the recrystallization process.8IntroRecryst-allizationLab 1GrainBoundariesRecrystallization Characteristics• In order for the boundary between a new grain (nucleus) andthe deformed material to be able to move, it must be a highangle boundary. This is a consequence of the properties ofboundaries, to be described later.• The requirement that new grains have high angle boundariesmeans that the final grain size and the rate at whichrecrystallization takes place is highly dependent on the strainlevel.• Higher strains mean greater lattice rotations (from dislocationslip) inside grains and higher stored energies. Therefore theprobability of generating new grains increases with strain andthe driving force increases.• Increasing probability for nucleation translates directly intoincreased density of nuclei and therefore smaller recrystallizedgrain size.9IntroRecryst-allizationLab 1GrainBoundariesGrain size as a function of priordeformation level• The grain size afterrecrystallization decreaseswith increasing prior strain,i.e. the nucleation densityincreases.• Example of commercialpurity Al, recrystallized at600°C (1.5h) after 2 (top), 6,8 & 10% (bottom) reductionin tensile strain.• Note that these are verysmall strains compared tocommercial practice.• Next slide shows industrialdata on grain size, also forcommercial purity aluminum.10IntroRecryst-allizationLab 1GrainBoundariesStrain dependence• In most materials, the grain size after recrystallizationdecreases as the strain increases. For most applications,small grain size is desirable. Certain applications, however,require large grain size, and so small strains are sometimesused.• Note that the heating rate has essentially no effect on theoutcome of recrystallization.Aluminum Handbook, Hatch (1984).11IntroRecryst-allizationLab 1GrainBoundariesStrain effect on kinetics• Recrystallization takes place more rapidly as the deformationstrain increases. This work was performed at Carnegie Tech.[Humphreys]12IntroRecryst-allizationLab 1GrainBoundariesTemperature dependence• The growth of new grains requires motion of grainboundaries. Boundary migration occurs by thetransfer of atoms across the boundary which is adiffusion-like process.• Solutes have a strong effect on boundaries becausethe interaction leads to segregation (generally anexcess of solute on the boundary). In effect, movingthe boundary forces the solute to move with it.• A suitable measure of the “reaction rate” is the timefor 50% recrystallization.13IntroRecryst-allizationLab 1GrainBoundariesTemperature Effect on Rex kinetics• Recrystallization is athermally activatedprocess and thereforeproceeds more rapidlyas the temperatureincreases.• Note that the rate ofrecrystallization ismeasured by the timerequired for 50%recrystallization.[Humphreys]14IntroRecryst-allizationLab