Cold Working is Actually Strain Hardening

1. s large hardening s y1 s small hardening y0 e Cold Working is Actually Strain Hardening Basic equation relating flow stress (strain hardening) to structure is: so = si +aGbr1/2 • so is the yield stress • si is the “friction stress” – overall resistance of lattice to dislocation motion • a is numerical constant 0.3 – 0.6 • G shear modulus • b is the burger’s vector • r is the dislocation density • Yield stress increases as r increases:

2. Effects of Cold Work As cold work is increased • Yield strength (sy) increases • Tensile strength (TS) increases • Ductility (%EL or %AR) decreases

3. Other Cold Work Effects • Usually a small decrease in density (few 10ths of a percent) • An appreciable decrease in electrical conductivity (increased number of scattering centers) • Small increase in the thermal coefficient of expansion • Because of increased internal energy – chemical reactivity is increased (decreased resistance to corrosion)

4. s-e Behavior vs. Temperature 800 -200C 600 -100C Stress (MPa) 400 25C 200 0 0 0.1 0.2 0.3 0.4 0.5 Strain 3 . disl. glides past obstacle 2. vacancies replace atoms on the obstacle disl. half 1. disl. trapped plane by obstacle • Results for polycrystalline iron: • sy and TS decrease with increasing test temperature. • %ELincreases with increasing test temperature. • Why? Vacancies help dislocations move past obstacles. Climb of Edge Dislocations Never Screw Positive Climb

5. Strain Energy Related to Cold Work • Mentioned that ~10% of the energy imparted during cold working is stored as strain energy • Amount of strain energy is increased by increasing the severity of deformation, lowering the deformation temperature, and by impurity additions • The strain energy increase is stored in the highly deformed microstructure – dislocation tangles • Metastable microstructure! Figure: Stored energy of cold work and fraction of the total work of deformation remaining as stored energy for high purity copper Source: Reed-Hill & Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

6. Annealing • Can we release the stored strain energy? YES! • The material is in an unstable state – but there is an activation energy barrier to releasing that energy • By heating the material and adding energy to the system we can increase the probability of moving past the activation barrier • Heat treating cold worked material is called Annealing

7. Release of Stored Energy • What happens as we heat up cold worked material? • Curve to the left is an anisothermal anneal curve • Two samples – one cold worked and the other not • Samples are heated continuously from low temperature to a higher temperature • Energy release is determined as a function of temperature • Difference in power to heat the specimens at same rate Figure: Anisothermal anneal curve for electrolytic copper Source: Reed-Hill & Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

8. Annealing Temperature (ºC) 100 200 300 400 500 600 700 600 60 tensile strength 50 500 ductility (%EL) tensile strength (MPa) 40 400 30 ductility 20 300 Recovery Grain Growth Recrystallization Annealing Stages • The cold worked state is thermodynamically unstable. • With increasing temperature it becomes more and more unstable • Eventually the metal softens and returns to a strain-free condition • Complete process is known as Annealing • Annealingis easily divided into 3 distinct processes: • Recovery • Recrystallization • Grain Growth

9. Recovery • Defined as:Restoration of physical properties of a cold worked metal without any observable change in microstructure • Electrical conductivity increases and lattice strain is reduced • Strength properties are not affected • Involves: • Dislocation Annihilation • Polygonization: • Removal of grain curvature created during deformation • Regrouping of edge dislocations into low angle boundaries within grains • Reduces the energy of system by creating reduced energy subgrains Source 1: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986. Source 2: Reed-Hill & Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

10. Recrystallization Recrystallization is: • The replacement of the cold worked structure by the nucleation and growth of a new set of strain free grains • Density of dislocations is reduced • Strain hardening is eliminated • The hardness and strength is reduced and the ductility is increased • Driving force for recrystallization is the release of stored strain energy • Note this is also the driving force for recovery and therefore they are sometimes competing processes Source 1: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986.

11. How does it work? • Nucleation of strain free grains occurs at points of high lattice curvature • Slip line intersections • Deformation twin intersections • Areas close to grain boundaries • Several models (unproven) that propose mechanisms for nucleation: • Grain boundary bulging due to a local variance in strain energy • Sub-boundary rotation and coalescence Source 2: Reed-Hill & Abbaschian, Physical Metallurgy Principles, 3rd Edition, PWS Publishing Company, 1994.

12. Recrystallization 0.6 mm 0.6 mm 33% cold worked brass New crystals nucleate after 3 sec. at 580C. • New grains are formed that: -- have a small dislocation density -- are small -- consume cold-worked grains.

13. Further Recrystallization 0.6 mm 0.6 mm After 8 seconds After 4 seconds • All cold-worked grains are consumed.

14. TR º TR = recrystallization temperature º

15. Variables for Recrystallization Six main variables influence recrystallization behavior: • The amount of prior deformation • Temperature • Time • Initial grain size • Composition • Amount of recovery or polygonization prior to the start of recrystallization Because the temperature at which recrystallization occurs depends  Recrystallization temperature is not a fixed temperature like melting point The practical definition for recrystallization temperature is: The temperature at which a given alloy in a highly cold worked state completely recrystallizes in 1 hour. Source: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986.

16. Affect of Variables on Recrystallization • Minimum amount of deformation is required • The smaller the deformation, the higher the temperature required for recrystallization • Increasing annealing time decreases required recrystallization temperature. Temperature is more important than time. Doubling annealing time is approximately equivalent to increasing annealing temperature 10oC • Final grain size depends most on the degree of deformation and to lesser extent on the annealing temperature. The greater the deformation & the lower the annealing temp., the smaller the recrystallized grain size. • The larger the original grain size, the greater the amount of cold work required to produce same recrystallization temp. Source: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986.

17. Affect of Variables on Recrystallization • The recrystallization temperature decreases with increasing purity of the metal. Solid solution alloying additions ALWAYS raise the recrystallization temperature. • The amount of deformation required to produce equivalent recrystallization behavior increases with increased working temperature • For a given reduction in cross-section – different metal working processes produce different effective deformations. Therefore, identical recrystallization behavior may not be obtained. Source: G. Dieter, Mechanical Metallurgy, 3rd Edition, McGraw-Hill, 1986.