CHE 333 Class 16

1. CHE 333 Class 16 Plastic Deformation of Metals and Recrystallization

2. Shear Stress and DislocationsDislocations are moved by Shear Stresses • = applied stress = F/A sn = stress normal to plane tr = shear stress acting in the plane shaded The applied stress can be resolved using the angle the plane makes with the applied stress l, and the angle between the plane normal and the applied stress j. tr =s(coslcosj)

3. Critical Resolved Shear Stress It is this resolved shear stress that moves dislocations, when the stress magnitude reaches a critical level, the Critical Resolved Shear Stress. Each material has its own value, so this is a material parameter. When l and j are both 45, • = 2 tr The maximum value of t ocurrs at 450 to the applied stress. At stress in imposed on a material, it will firstly experience “ Elastic Deformation” . At the Yield Stress, dislocations start moving in metals and when the “Plastic Deformation” starts in the material as the threshold Critical Resolved Shear Stress is exceeded sy = 2 tcrss Critical Resolved Shear Stress is a function of material and the slip system.

4. Failed Sample Metal A failed sample is compared to a new untested sample. Note the failure is at 45o to the applied stress. The local deformation in this case is very near the failure point. ROA Data would be very difficult in this case. Elongation at failure would be more useful

5. Dislocation Motion. At the yield stress, dislocations start moving on slip planes in slip directions. The slip planes are the densest packed and the slip directions are the ones of greatest density. When a polycrystaline material is above the yield stress, then slip occurs which is the movement of dislocations along slip planes by the critical resolved shear stress being exceeded and so activating slip systems on slip planes. In the figure several slip systems are active. Note that slip lines stop at grain boundaries. This is due to the planes changing their orientation with respect to the stress, so the critical resolved shear stress is no longer at the magnitude for continuation of slip. However, with increasing stress applied densest packed planes in the next grain will exceed the critical resolved shear stress and so slip will continue.

6. Displacements from Slip As dislocations move along slip planes, they eventually emerge at a surface and leave a step with the magnitude of the Burgers vector for each one. So with large numbers of dislocations moving, then the material will change shape as shown in the figure. Plastic deformation therefore leads to shape change such as used in manufacturing by bending, rolling forging, drawing and many other . techniques. These are called cold working techniques. Cold working is therefore carried out at stress levels above the Yield Stress but below the UTS. Cold working is usually involves compressive stresses to avoid opening cracks – rolling, forging, extrusion.

7. Cold Work. After cold work, the structure has many slip lines and a large increase in dislocation density from 106 to 109 /cm2Thegrains also change shape as the plastic deformation allows the material to move. If a material is rolled between two rollers it will elongate, become thinner and the grains will change from equiaxed to ellipsoidal or cigar shaped. The yield and tensile strength will have increased while the elongation to failure will decrease. Sometimes this will be the end point. In other cases further cold work will be required and this will require other actions to stop the material from failure.

8. Recrystallization. Recrystallization is a process where materials regain the mechanical properties associated with the weakest and most ductile condition to enable further cold work. It is a thermal process after cold work. The material is placed in a furnace for a period of time. The mechanical properties change with both temperature and time and also as a function of previous cold work. The temperature is often about 0.3 to 0.5 the melting temperature in oKelvin There are three stages to the process, Recovery, Recrystallization and Grain Growth.

9. Recovery. In this first stage, dislocations rearrange themselves by thermal processes. Diffusion of atoms is possible, so the dislocations move and form what are called “cells” which are the nucleii of new grains. The mechanical properties do not change much during this stage of the process.

10. Recrystallization

11. Grain Growth

12. Mechanical Property Changes Recovery – little change, just dislocation rearrangement Recrystallization – significant changes, new small grains formed, ultimate tensile and yield both decrease to softest condition along with hardness. Elongation to failure or ductility increases. Process sometimes called “Full Anneal” Annealing is thermal processing to change a property. Stress Relief Anneal – after cold working to reduce residual stresses, just a recovery treatment. Recrystallization temperature depends on material and cold work, usually 0.3 to 0.5Tm in Kelvin

13. Dynamic Recrystallization. If a material is worked, that is, deformed at the same time as it is hot, above the recrystallizarion temperature, the material will not work harden, but will recrystallize at the same time it is being worked. This is dynamic recrystallization. It is called “hot working”. In this case “hot” is relative to the recrystallization temperature, not absolute temperature. A metal can be red hot but still be cold worked because it s below its recrystallization temperature. Another case of dynamic materials is pure, FCC metals such as gold. These have high elongations to failure and so can absorb many dislocations which form cells and eventually new grains just by extreme amounts of work. The best example of this is gold leaf, which is gold continually deformed from thick to very thin sheets. Silver can be worked the same way as well as platinum. This dynamic recrystallization was very important in the jewellery industry.