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 /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.

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. Recr
ystallization 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.