| Transmission electron microscopy has been used to study the morphology and the crystallography of "lath martensite" in low and medium carbon steels in the "as-quenched" and 200(DEGREES)C tempered conditions. The steels have microduplex structures of dislocated lath martensite (a (,M)), to an abrupt change in the orientation by a large rotation, again, about (,M), which is a common direction in a packet of laths. The habit plane in austenite is {lcub}111{rcub}(,(gamma)) (parallel to {lcub}110{rcub}(,(alpha)')) and the long direction of the laths is in (,(alpha)') (parallel to (,(gamma))).; Considerable C partitioning into the (gamma)-films ((TURN)1 wt%) and to the (alpha)'/(gamma)-interface ((TURN)2 wt%) has been revealed by convergent beam electron diffraction and atom-probe studies. Partitioning largely occurs during the transformation, although some C diffusion into (gamma) is unavoidable during room temperature holding. Austenite, trapped between the laths is stabilized chemically (by lowering the M(,s) temperature) and thermally (by pinning the high-C interface probably by clusters). Austenite is highly deformed to accommodate the stresses created as a result of the shear transformation of the surrounding matrix (mechanical stabilization). Consequently, the overall stabilization is determined by the cooperative operation of all of the above mechanisms. |
