Kinetics of martensite formation detailed analysis. Figure 8.26 shows a micrograph

8.3.3.3 Kinetics of martensite formation detailed analysis. Figure 8.26 shows a micrograph

One of the most distinctive features of the martensite taken after the formation of ˛-martensite and this,

transformation is that in most systems martensite is together with continuous observations, show that the

formed only when the specimen is cooling, and that martensite/matrix interface changes from f1 1 1g to the

the rate of martensite formation is negligible if cool- well-known f2 2 5g as it propagates. Clearly, one of

ing is stopped. For this reason, the reaction is often the important roles that the formation of ε-martensite

referred to as an athermal 1 transformation, and the plays in acting as a precursor for the formation

percentage of austenite transformed to martensite is of ˛-martensite is in the generation of close-packed

indicated on the TTT curve by a series of horizontal planes with ABAB stacking so that atomic shuffles

lines. The transformation begins at a temperature M s , can subsequently transform these planes to f1 1 0g bcc

which is not dependent on cooling rate, but is depen- which are, of course, stacked ABAB (see Figure 8.27).

dent on prior thermal and mechanical history, and on The ˛-martensite forms in dislocation pile-ups where

composition. For example, it is well established that the a/6h1 1 2i partials are forced closer together by the

the M s temperature decreases approximately linearly applied stress. The volume of effective bcc material

with increasing concentration of solutes such as car- increases as more dislocations join the pile-up until the

bon, nickel or manganese.

nucleus formed by this process reaches a critical size Speed of formation The observation that martensite and rapid growth takes place. The martensite initially

plates form rapidly and at a rate which is grows perpendicular to, and principally on, one side of

temperature-independent shows that thermal activation the f1 1 1g slip plane associated with the nucleus, very

is not required for the growth process. Electronic likely corresponding to the side of the dislocations with

methods show that the martensite needles form, in missing half-planes since ˛-martensite is less dense

iron–nickel –carbon alloys, for example, in about than austenite. 7 10 s and, moreover, that the linear growth velocity

is about 10 3 ms 1 even at very low temperatures. Such observations show that the activation energy for 200

0.5 µ m the growth of a martensite plate is virtually zero, and that the velocity of growth approaches the speed of sound in the matrix. Sometimes a ‘burst phenomenon’ is exhibited, as, for example, in iron–nickel alloys, when the stresses produced by one martensite plate assist in the nucleation of others. The whole process is autocatalytic and about 25% of the transformation can occur in the time interval of an audible click.

The effect of applied stress Since the formation of martensite involves a homogeneous distortion of the parent structure, it is expected that externally applied

1 In some alloys, such as iron–manganese–carbon and Figure 8.26 Electron micrograph showing an ˛-martensite

iron–manganese–nickel, the martensitic transformation plate, the austenite–martensite interface, and the faults in

occurs isothermally. For these systems, growth is still very the austenite matrix .

rapid but the nuclei are formed by thermal activation.

282 Modern Physical Metallurgy and Materials Engineering stresses will be of importance. Plastic deformation is

effective in forming martensite above the M s tempera- ture, provided the temperature does not exceed a criti-

cal value usually denoted by M d . However, cold work

above M d may either accelerate or retard the transfor- mation on subsequent cooling. Even elastic stresses, when applied above the M s temperature and main- tained during cooling, can affect the transformation; uniaxial compression or tensile stresses raise the M s temperature while hydrostatic stresses lower the M s temperature.

Stabilization When cooling is interrupted below M s , stabilization of the remaining austenite often occurs. Thus, when cooling is resumed martensite forms only after an appreciable drop in temperature. Such ther- mal stabilization has been attributed by some workers to an accumulation of carbon atoms on those dislo- cations important to martensite formation. This may

be regarded as a direct analogue of the yield phe- nomenon. The temperature interval before transfor- mation is resumed increases with holding time and is analogous to the increase in yield drop accompanying carbon build-up on strain-ageing. Furthermore, when transformation in a stabilized steel does resume, it often starts with a ‘burst’, which in this case is analo- gous to the lower yield elongation.