T16: Effect
of microstructure and conditions (Part 2)
This tutorial was created on
MatCalc version 5.23 rel 1.026
license: free
database: mc_sample_fe.tdb; mc_sample_fe.ddb
Contents:
- Simultaneous precipitation of two phases
- Effect of dislocation density
- Effect of grain diameter
- Subgrains and elongation factors
Before starting...
Re-open the workspace saved from Tutorial 15 and
save it under a new name.
1. Simultaneous precipitation of cementite and M23C6 (~54 min.)
Setup
Open 'Global > Phase status' and
create a precipitate phase from M23C6. In
the 'precipitate' tab,
enter '25' for the number of size
classes, and attach the phase to the domain 'ferrite'.
In the 'Nucleation' tab, set the nucleus composition model to 'ortho-equilibrium',
and the nucleation sites to 'Grain boundaries'.
In 'Global > Precipitation domains > Structure',
reset the dislocation density to its default value of '1e12' m-2.
The default grain diameter is '100e-6' m, and it is this which governs
the density of M23C6_P0 nucleation sites. Create a new XY-plot
window with five plots in it, and insert series into them as follows:
- F$CEMENTITE_P0, F$M23C6_P0 (fraction of cementite and M23C6
precipitated, respectively)
- X$CEMENTITE_P0$CR, X$BCC_A2$CR (Cr content of cementite precipitates
and matrix, respectively)
- X$BCC_A2$C (C content of matrix)
- NUM_PART$CEMENTITE_P0, NUM_PART$M23C6_P0 (number of cementite
and M23C6 precipitates, respectively)
- R_MEAN$CEMENTITE_P0, R_MEAN$M23C6_P0 (mean radius of cementite
and M23C6 precipitates, respectively)
Run the kinetic simulation, with an isothermal heat treatment
at 600°C as before, and an end time of 3.6e+10 s.
Interpretation of results
Plot 1: Cementite appears rapidly and reaches a steady-state precipitate
fraction. M23C6 starts to form after around
1 hour at 600°C;
this is at the expense of cementite, which redissolves. By the
end of the simulation, all the cementite has dissolved and M23C6 has
reached its equilibrium phase fraction.

Plot 2: This shows that cementite enrichment has just started
by the time the cementite phase has dissolved completely.

Plot 3: The depletion of the matrix in carbon occurs in two stages.
The first of these corresponds to the formation of cementite, and
the second to the formation of M23C6.

Plot 4: The maximum number of M23C6 precipitates
is significantly greater than that of cementite precipitates (note
log scale). The reduction in the number of cementite precipitates
occurs in two stages:

Plot 5: The cementite precipitates have begun to coarsen by the
time they dissolve. Coarsening of M23C6 begins
around 104 hours into the heat-treatment.

2. Effect of dislocation density (~44 min.)
Duplicate and lock the series and re-run the simulation with a
dislocation density of 1e14 m-2, leaving the grain size the same.
Results:
The onset of cementite precipitation occurs earlier with a higher
dislocation density, but the kinetics of cementite dissolution
and of M23C6 precipitation are unchanged.
The plot below shows the dramatic increase in the number of cementite
precipitates formed.

With a higher dislocation density, the cementite particles do
not become so large during the growth stage. However, coarsening
begins earlier, and the curve of radius versus time for the coarsening
precipitates eventually becomes parallel with that for the lower
dislocation density.

3. Effect of grain diameter (~42 min.)
Reopen 'Global > Precipitation domains > Structure'.
Reset the dislocation density to 1e12 m-2, set the grain diameter
to 10e-6 m and re-run the simulation.
Results:
As might be expected, reducing the grain diameter
accelerates the precipitation kinetics of M23C6, by providing more
nucleation sites. It also accelerates the dissolution of cementite,
which begins to dissolve when M23C6 starts to precipitate.

The plot below shows the effect on the number of M23C6 of
reducing the grain size from 10-4 to 10-5 m.

In the plot of radius against time, the same effect can be seen
in M23C6 as occurred for cementite in the
previous simulation: a higher nucleation site density results in
smaller precipitates during the growth stage, but coarsening starts
earlier and the radius eventually follows the same curve irrespective
of the number of nucleation sites. The maximum cementite radius
attained is reduced, because there is less time available for coarsening
of this phase before dissolution sets in.

4. Subgrains and elongation factors
In the 'Global > Precipitation domains > Structure', the
other adjustable parameters in the 'Microstructure
parameters' section are:
- Grain size elongation factor, i.e. aspect ratio of grains.
This is useful in, for example, extruded structures, in which
grains tend to be elongated in the direction of extrusion.
- Subgrain diameter. Subgrains are
structures such as the packets and blocks of martensite in prior
austenite grains, or polygonised structures formed during recovery
of cold-worked materials. In the 'Global > Phase status > Nucleation' tab, subgrain boundaries as well as grain boundaries, can be
selected as nucleation sites.
- Subgrain size elongation factor: analogous to the grain size
elongation factor, for subgrains.
Re-open 'Global > Phase status > Nucleation'.
In the 'Nucleation
sites' section for M23C6_P0, select 'subgrain
boundary' in addition
to 'grain boundary', to allow this
phase to nucleate on subgrain as well as grain boundaries. (Click
twice in the box to give a black, not grey, tick-mark.) The default
subgrain size in 'Global > Precipitation
domains > Structure' is 1e-6 m; accept this value and re-run
the simulation.
The plots below compare calculations in which the dislocation
density is 1e-12 m-2 and the grain diameter is 10e-6
m, with and without nucleation on subgrain boundaries (s. g. b.).
It can be seen that providing an additional set of nucleation sites
accelerates the M23C6 precipitation kinetics,
increases the number of M23C6 precipitates,
and has a marked effect on the maximum value of mean radius attained
by cementite precipitates before they redissolve.



Elongated subgrains, obtained by increasing the subgrain elongation
factor to 10, have a very small effect on precipitation and coarsening
rates but a significant influence on the number of precipitates.

‹‹ to the Top ›› ‹‹ to
the Tutorial files ›› |