Metals- Recovery and Recrystallisation Flashcards
Methods of thermomechanical processing
Rolling, cogging, upsetting (open or close die forging)
Problem with deforming into requires shape
Following deformation the properties of the material are not optimised and annealing is required to obtain an optimum microstructure and properties. Also during the deformation, increasing dislocation density can make it harder to deform using same force
What is annealing and what does it normally result in?
The process where a deformed component is heated and held at that temperature for a specified length of time. Often observed that hardness and yield strength of metal decrease and ductility (expressed by strain to failure) increases.
The two processes that occur during annealing
Recovery and recrystallisation
What does recovery refer to?
Changes in the material properties which occur after deformation and before recrystallisation
When does recovery typically occur?
At about 0.3xTmelt
What properties change during recovery?
Very few visible microstructural changes but yield strength and electrical resistivity can change dramatically
Electrical resistivity change from recovery
Use this change in resistivity to observe recovery. During deformation resistivity increases due to disruption of the regularity of the structure as a result of the presence of dislocations and strain fields that reduce the mean free path of propagation of electrons. Reduction in resistivity of material hence indicates reduction in dislocation density and hence stored deformation energy
The processes of dislocation density reduction during recovery
Dislocations interact at increasing rate at higher T so migrate and annihilate causing a reduction in dislocation density.
To further reduce their energy dislocation arrange themselves in cell structures in process known as polygonisation.
Lastly the cells now forming subgrains in the material can grow
Polygonisation
Widespread rearrangement of dislocations inti low energy configurations leads to polygonisation of the crystal. The rearranged dislocations form low angle or tilt boundaries. Energy of these boundaries doesn’t vary linearly. Often more energetically favourable for dislocations to arrange in fewer but more highly oriented boundaries
Sub-grain growth
Newly formed subgrains still have energy associated with them. This is the driving force for their growth. In theory all grain boundary energy should be eliminated as it forms the driving force for grain growth. However once the grains reach a certain size the mobility of the interface might be stopped by even small energy barriers. Think subgrains form and grow on GBs from rearrangement of dislocations
Methods of measuring recovery
X-ray diffraction measures misorientation of subgrains in-situ as function of time.
Differential scanning calorimetry (DSC) measures activation energy for recovery (careful calibrations required as energy input required for recovery small (0.3Tmelt)).
Resistivity and conductivity measurements track recovery
What is recrystallisation?
Process by which the existing grain structure in a material is replaced by new grains. Occurs a T between 0.4-0.7Tmelt
Driving force for recrystallisation
Like for recovery it’s the reduction in stored deformation energy in the material (mostly due to reducing dislocation density)
Dynamic or static recrystallisation
If occurs during deformation at high temperature it’s dynamic recrystallisation. Otherwise static recrystallisation
Difference between recrystallisation and recovery
Recrystallisation results in the formation of new grains rather than rearrangement of dislocations into subgrains. Also if sufficient recovery has occurred then recrystallisation can’t initiate
Growth of preexisting nuclei
Recrystallised microstructures usually result of growth of preexisting nuclei formed during deformation or recovery and/or strain induced grain boundary migration (SIBM). Orientation sir nuclei often agree with existing orientations of deformed grains but nuclei that grow must form in regions of high orientation gradients. Growth occurs by boundary migration
SIBM
Stain induced boundary migration. Believed to occur when strain energy stored in adjacent grains is different enough so that the reduction in stored energy when a bulge forms is sufficient to make up for the increase in boundary energy. Bulge into grain with higher dislocation density
What are extent of recrystallisation and final grain size dependent on?
The prior grain structure and amount of deformation in the material
Effect of prior deformation and prior grain size on recrystallisation
Critical amount of deformation is required before recrystallisation can be initiated. Amount before determines number of nuclei in material. If insufficient nuclei present they grow into large grains.
If prior grain size (pre-RX) has large grains it will result in a coarse RX-grain structure
Grain size vs elongation or deformation graphs for recrystallisation
Grain size vs %elongation: like exponential decrease curve, can recrystallise at lower elongation if use higher T but result sin larger grains.
Grain size vs deformation: vertical line at critical deformation then exponential decrease curve from that.
Twinning from recrystallisation
Markedly different to twins obtained during deformation. Think they form to reduce grain boundary energy and stop once energy lowered sufficiently. Or they form to allow boundaries with increased mobility. No tapering at GBs and form wider bands
Features of recrystallisation process
Nucleation of new grains. Followed by unhindered near-linear growth until grains start impinging on each other and reduce further growth
JMAK model
Used to simplistically describe the kinetics of recrystallisation assuming nucleation of new grains occurs at random (often not the case).
Xv=1-exp(-Bt^n)
Where X sub v is fraction of recrystallisation
B is a constant
n is Avrami exponent usually 4 (lower for coarse materials)
Graph of Xv vs log(t)
Starts at origin, curves up (nucleation) to near-linear, inflection, reducing gradient (impingement of grains)
