The morphology and crystallography of lath martensite in alloy steels

The morphology and crystallography of lath martensite in two Mn-containing interstitial free steels and a maraging steel were examined in detail by a combination of transmission electron microscopy, electron backscatter diffraction in a scanning electron microscope and optical microscopy. Indirect determination of the orientation relationship between the lath martensite and the austenite was made by analysis of misorientation distributions, and a similar orientation relationship was found for the three alloys in accordance with previous observations in low-carbon steels. Furthermore, the formation of six variants in a given packet and the preferential arrangement into blocks of low-misorientation variant pairs demonstrate a universality of morphology and crystallography of lath martensite. The presence of six variants in a packet can be accounted for by the minimization of the total shape strain introduced during the transformation. The lath boundaries developed within the volume of a certain variant show alternating misorientations and a mixed tilt and twist character characterizing these structures as low-energy dislocation structures.