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The transition
from austenite to phases stable at lower temperatures also takes place
when the material is cooled at a constant rate. The diagram illustrates
the transition zone (yellow) and also shows two constant cooling rate trajectories
(they are curved because the time scale is logarithmic). If the cooling
rate is less than 35 C/s the final material will be pearlite - the thermodynamically
stable low temperature material. At cooling rates faster than 140
C/s the transition is to a metastable material known as martensite. Between
35 and 140 C/s the system at room temperature is a mixture of these two materials.
The
microstructure of martensite in austenite is shown in the micrograph.
The black grains are the martensitic phase and the white the non-transformed
austenite. The transformation does not involve diffusion processes
but lattice displacement of atoms by shear produced by the thermal contraction
of the crystal structure during cooling. Martensite does not start to form
in the eutectoid alloy until about 210 C. The "lenses" of martensite
grow at the velocity of sound and they are associated with high elastic
strain, making the material hard and brittle. |
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