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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:

  1. F$CEMENTITE_P0, F$M23C6_P0 (fraction of cementite and M23C6 precipitated, respectively)
  2. X$CEMENTITE_P0$CR, X$BCC_A2$CR (Cr content of cementite precipitates and matrix, respectively)
  3. X$BCC_A2$C (C content of matrix)
  4. NUM_PART$CEMENTITE_P0, NUM_PART$M23C6_P0 (number of cementite and M23C6 precipitates, respectively)
  5. 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.

 

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