Plastic- Case Study 2: More on RX Flashcards
How does original microstructure control nucleation rate?
Nucleation commonly occurs at grain boundaries (an energetically favourable site). Means if original grain size decreases, number of nucleation sites increases so nucleation rate increases.
Second phase particles can also stimulate nucleation (particle stimulated nucleation). Higher localised strain on material in close proximity to second phase (undeformable) particles. Amount of localised strain related to size and volume fraction of second phase
Graph of nucleation rate vs Z or ε
Convex curve up from low to high going left to right. Graph higher (greater nucleation rate) for decreased original grain size, increased second phase particle size, increased second phase content.
Formula for growth rat being function of temperature
RG proportional to exp(-QR/R.T)
Where R sub G is growth rate
Q sub R is activation energy for hot working
What can slow down or prevent growth and why?
A dispersion of fine second phase particles can retard grain boundary migration and therefore slow down or prevent growth. This is grain boundary pinning
Graph of growth rate vs heat treatment temperature
Convex curve up shallow at start then gets very steep. Graph is lower for greater fine second phase particle content
What does overall time for RX depend on and give the formula for it
Is a combination of nucleation and growth and related to heat treatment temperature and Z.
tx=A.Z^-a.exp(QR/R.T)
Where A is a compositional constant that increases with increasing original grain size, decreasing coarse second phase particles/content and increasing fine second phase particle content
Graph to show time for RX
Pot of Z vs time. Has two exponential decrease lines. The lower and left one is start of RX and the upper and right one is the end. Left of start line is unrecrystallised and right of end line is recrystallised. Time interval between start and end lines at a certain Z is the overall time for RX. The lines move right and up for decreasing heat treat T, greater original grain size, less coarse second phase particle content and greater fine second phase particle content.
What is size of RX’d grains dependent on?
Nucleation rate with respect to growth rate.
Rapid nucleation and slow growth give fine grain size.
Slow nucleation and rapid growth gives coarse grain structure.
Recrystallised grain size vs Z graph
Exponential decrease curve. Moves up and right for increased heat treat T, increased original grain size, less coarse or fine second phase particle content
How does Z affect RX times? How to prevent RX during solution treatment
A high Z leads to very short RX times during post deformation heat treatments. It is very difficult to avoid RX under these conditions. To prevent RX during solution treatment need to extrude material under lower Z conditions (higher T and lower strain rate)
What values for Z avoid RX?
ln(Z) under 22.7
What temperature is used to calculate Z for this case study?
The exit temperature (T sub E) to account for temperature rises
How does avoiding RX during solution treatment limit the choice of parameters during extrusion?
The fastest speed of 35mm/s gives too high a ln(Z) (over 22.7) for the lower temperatures of 350 and 400C. Exit T for 450 and 500C already too high for 35mm/s. Using 17.5mm/s now only possible for 400 or 450C as ln(Z) too high for 350C and TE already too high for 500C
How is limite diagram amended to account for avoiding RX?
The original triangle has another line inside with positive gradient. Makes a new smaller triangle in bottom right in which extrusion is possible. Rest of the larger triangle is extrusion impossible due to RX during ST. Left of large triangle was insufficient pressure and right was extrude melting. This is for ram speed vs T
Equation for strengthening from subgrain boundaries
σ=σ0+k.d^-1/2
Subgrain boundaries have strengthening effect but not as effective as grain boundaries.
Value for k 0.08MN/m^-3/2 compared to 0.15 for GBs
However subgrains normally an order of magnitude smaller than grains
