CHE 333 Class 14

Plastic Deformation of Metals and

Recrystallization

Shear Stress and DislocationsDislocations are moved by Shear Stresses

s = applied stress = F/Asn = stress normal to planetr = 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)

Critical Resolved Shear StressIt 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,s = 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 asthe threshold Critical Resolved Shear Stress is exceeded

sy = 2 tcrss

Critical Resolved Shear Stress is a function of materialand the slip system.

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. ROAData would be very difficult in this case. Elongation at failure would be more useful

Dislocation Motion.At the yield stress, dislocations start movingon slip planes in slip directions. The slip planesare the densest packed and the slip directionsare the ones of greatest density.When a polycrystaline material is above the yield stress, then slip occurs which is themovement of dislocations along slip planesby the critical resolved shear stress beingexceeded 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 respectto the stress, so the critical resolved shear stressis no longer at the magnitude for continuationof slip. However, with increasing stress applieddensest packed planes in the next grain will exceed the critical resolved shear stress and so slip will continue.

Displacements from SlipAs 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.

Cold Work. After cold work, the structure

has many slip lines and a large increase

in dislocation density from

106 to 109 /cm2 The grains 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.

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.

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.

Recrystallization

Grain Growth

Mechanical Property ChangesRecovery – little change, justdislocation rearrangement

Recrystallization – significantchanges, new small grainsformed, ultimate tensile and yieldboth decrease to softest condition along with hardness.Elongation to failure or ductilityincreases.

Process sometimes called “Full Anneal”Annealing is thermal processing to change a property.Stress Relief Anneal – after cold workingto reduce residual stresses, just a recoverytreatment.Recrystallization temperature depends onmaterial and cold work, usually 0.3 to 0.5Tmin Kelvin

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.