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Abstract: The microstructure and the strength of the lath martensite in Fe–0.2C and Fe–0.2C–2Mn alloys were analyzed as a function of the prior austenite grain size. The size of martensite packets formed within individual austenite grains was controlled by the austenite grain size but not affected by the Mn addition. However, the further subdivision of packets into blocks differed significantly in the two alloys, and at a given austenite grain size a smaller block size was observed in the Mn containing alloy. The yield strength of the two alloys was related to the packet size and the block size, respectively, and the results suggested that the block size is the key structural parameter when analyzing the strength–structure relationship of lath martensite in low carbon steels. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

The morphology and crystallography of lath martensite in Fe-C alloys containing various carbon contents from 0.0026 to 0.61% were studied by analyzing electron back scattered diffraction patterns in scanning electron microscopy and Kikuchi diffraction patterns in transmission electron microscopy. As carbon content increases, the sizes of both packet and block decrease. In low carbon steels (0.0026–0.38%C), a block which is observed as having different contrasts under optical microscopy contains two groups (sub-blocks) of laths which are of two K-S variants with a misorientation of about 10 degrees. On the other hand, in the high carbon alloy (0.61%C), a block consists of laths of a single K-S variant. [Copyright &y& Elsevier]