Transcript CHE 333 Class 17 - Chemical Engineering

CHE 333 Class 16
Plastic Deformation of Metals and
Recrystallization
Shear Stress and Dislocations
Dislocations are moved by Shear Stresses
 = applied stress = F/A
n = 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 = (coslcosj)
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
y = 2 tcrss
Critical Resolved Shear Stress is a function of material
and 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. ROA
Data would be very difficult in this case. Elongation at failure would be more useful
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.
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.
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 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
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.