Chapter 8 魏茂國 - ndhu.edu.tw

1 1 Chapter 8 Annealing Stored energy of cold work The relationship of free energy to strain energy The release of stored energy Recovery Recovery in single crystals Polygonization Dislocation movements in polygonization Recovery process at high and low temperatures Recrystallization The effect of time and temperature on recrystallization Rerystallization temperature The effect of strain on recrystallization The rate of nucleation and the rate of nucleus growth Formation of nuclei Driving force for recrystallization2 Annealing The recrystallized grain size Other variables in recrystallization Purity of the metal Initial grain size Grain growth Geometrical coalescence Three-dimensional changes in grain geometry The grain growth rate Impurity atoms in solid solution Impurities in the form of inclusions The free-surface effects The limiting grain size Preferred orientation Secondary recrystallization Strain-induced boundary migrationChapter 8 3 Fig.

2 Stored energy of cold work and fraction of the total workof deformation remaining as stored energy for high purity copperplotted as functions of tensile Energy of Cold Work Stored energy of cold work- When a metal is plastically deformed at temperatures that are low relative to itsmelting point, it is said to be cold worked. A rough rule-of-thumb is to assume thatplastic deformation corresponds to cold working if it is carried out at temperatureslower than one-half of the melting point measured on an absolute Most of the energy expended in cold work appears in the form of heat, but a finitefraction is stored in the metal as strain energy associated with various lattice defectscreated by the deformation.

3 Fig. shows the relationship between the storedenergy and the amount of deformation in a specific metal (polycrystalline copper) for a specific type of deformation(tensile strain). The stored energy increases withincreasing deformation, but at a decreasing rate,so that the fraction of the total energy storeddecreases with increasing Energy of Cold Work - The amount of stored energy can be greatly increased by increasing the severity of thedeformation, lowering the deformation temperature, and by changing the pure metalto an Cold working is known to increase greatly the number of dislocations in a metal. Asoft annealed metal can have dislocation densities of the order of 1010to 1012m-2, andheavily cold-worked metals can have approximately 1016.

4 Accordingly, cold workingis able to increase the number of dislocations in a metal by a factor as large as 104to106. Since each dislocation represents a crystal defect with an associated lattice strain,increasing the dislocation density increases the strain energy of the A screw dislocation that cuts another screw dislocation may be capable of generatinga close-packed row of either vacancies or interstitials as it Since the strain energy associated with a vacancy is much smaller than that associatedwith an interstitial atom, it can be assumed that vacancies will be formed in greaternumbers than interstitial atoms during plastic of Free Energy to Strain Energy Relationship of free energy to strain energy- While plastic deformation certainly increases the entropy of a metal, the effect issmall compared to the increase in internal energy (the retained strain energy).

5 Theterm -T Sin the free energy equation may be neglected and the free-energyincrease equated directly to the stored Gis the free energy associated with the cold work, His the enthalpy, orstored strain energy, Sis the entropy increase due to the cold work, and Tis theabsolute Since the free energy of cold-worked metals is greater than that of annealed metals,they may soften spontaneously. Heating a deformed metal greatly speeds up itsreturn to the softened ( )HGHST 6 Fig. Anisothermal anneal curve. Electrolytic of Stored Energy Release of stored energy- Two of the more important studies of the release of the stored energy will be brieflyindicated. In the first , the anisothermal anneal ( ) method, the cold-worked metal is heated continuously from a lower to a higher temperature and theenergy release is determined as a function of temperature.

6 One form of theanisothermal anneal measures the difference in the power required to heat 2 similarspecimens at the same rate. One specimen of the 2 is cold worked before the heatingcycle, while the other serves as a standard and is not deformed. During the heatingcycle, the cold-worked specimen undergoes reactions that release heat and lower thepower required to heat it in comparisonwith that required to heat the standardspecimen. Measurements of the differencein power give direct evidence of the rate atwhich heat is released in the cold-workedspecimen (Fig. ).7 Fig. Isothermal anneal curve. High purity of Stored Energy It is noteworthy that some heat is released at temperatures only slightly above roomtemperature (Fig.)

7 - The other method of studying energy release involves isothermal annealing ( ). Here the energy is measured while the specimen is maintained at a constanttemperature. In Fig. , the data for this particular curve were obtained with the aidof a microcalorimeter with a sensitivity capable of measuring a heat flow as low as 13 Both the anisothermal anneal and the isothermal anneal curves of Figs. and maxima corresponding to large energy releases. These large energy releasesappear simultaneously with the growthof an entirely new set of essentiallystrain-free grains, which grow at theexpense of the original badly of Stored Energy The process by which this occurs is called recrystallization and may be understood asa realignment of the atoms into crystals with a lower free In Figs.

8 And , the area under each solid curve that lies to the left and above thedashed lines represents an energy release not associated with recrystllization. The partof the annealing cycle that occurs before recrystallization is called It is necessary to define the third stage of annealing-grain growth. In grain growth,certain of the recrystallized grains continue to grow in size, but only at the expense ofother crystals which must of Stored Energy - The release of stored energy in cold-rolled high-purity iron using a differentialscanning calorimeter (DSC) is shown in Fig. In this figure, where the heatgeneration by the metal shows as a negative deviation in the heat flow curve, after theinitial transient peak below 100 C, a broad exothermic peak extending from ~100 to280 C can be seen.

9 This broad exothermic peak is followed by a larger one extendingfrom ~300 to 480 C. The energy release in these 2 peaks has been calculated by theauthors as and J/mol, corresponding to 19 J/mol for the total stored The changes in the microstructure of thecold-rolled iron during heating, revealedby interrupted quenching, are shown inFigs. The deformed structure is retained up to300 C, but new grains form as the metal isheated Heat flow versus temperature for 80% cold-rolledultra-high-purity Optical view of microstructureof deformed iron at different annealingtemperatures; (a) as cold rolled, (B)annealed at 300 C, (C) annealed at370 C, (D) annealed at 410 C, (E)annealed at 460 C, and (F) annealedat 650 of Stored Energy - The new-formed grains can be clearly seen as isolated spots in Fig.

10 5C, while otherregions still show the deformed microstructure. At 460 C (Fig. ), most of thedeformed regions have been replaced by new grains which grow considerably largerat the higher temperature(Fig. ).- The 3 stages of annealingare recovery,recrystallization, and Introduction- Plastic deforming a polycrystalline metal specimen at temperatures that are lowrelative to its absolute melting temperature produces microstructural and propertychanges that include (1) a change in grain size, (2) strain hardening, and (3) anincrease in dislocation , other properties such as electrical conductivityand corrosionresistancemay be modified as a consequence of plastic These properties and structures may revert back to the precold-worked states by anannealing( ) treatment.