| In-depth knowledge of lath martensite is an essential theoretical_basis to thecontrolling of the microstructure of many high strength steels and designing ofcomposition of steels and heat treatment processes. An important aspect of theoreticalstudy of martensite is on the crystallography of martensitic transformation, as the corefor understanding the properties and morphology of martensite.In this study, transmission electron microscopy (TEM) has_been applied for theinvestigation of orientation relationship (OR), interfacial structure and internaldislocations of lath martensite in Fe-20Ni-5.4Mn(wt%) alloy. It was found that the OR_between the martensite and austenite is close to G-T OR. The average ha_bit plane is(0.380.390.34)f|(-0.010.470.43)_b. Four sets of pure screw <111>_b/2dislocations, and two sets of [011]f/2and [10-1]f/2(nearly) screwdislocations have_been identified within martensite and austenite, respectively.The interface consists of a single set of parallel mixed-type dislocations havingBurgers vector [10-1]f/2|[11-1]_b/2. The spacing of the interface dislocationsvaries from6.0to15nm. Frame-_by-frame analysis of the austenite/lathmartensite interface during in situ heating using TEM is used to providedirection information on the mechanisms of the martensite interface motion. Itreveals that the interface migrated on only one side of the lath_by a ledgemechanism displaying start-stop growth_behavior; the highest velocity o_bservedwas0.79μm s-1.The shape strain accompanying surface relief, such as the magnitude and directionof the displacement vector, has also_been investigated in a quantitative way. Themorphology of the relief was studied_by the optical microscopy (OM) and the atomicmicroscopy (AFM). The crystallographic orientations of the matrix grain and the lathwere measured_by the electron_backscattered diffraction (EBSD), respectively, whichwas used to determine the orientation of the ha_bit plane, and the OR. Com_bining thedata from EBSD and AFM, it is concluded that the relief is produced_by a single_bcccrystal, which exhi_bits a tent-shaped relief. Based on an EBSD analysis, the OR and theha_bit plane are consistent with those got_by TEM. The largest shear angle for the relief is calculated to_be29.8, and the directions of com_bined displacement vector arescattered around [11-2]_b. The o_bserved maximum surface title angle is22.4, which issmaller than the calculated value. Considering the ha_bit plane is not perpendicular to thepre-polishing surface, the measured smaller value of tile angles is reasona_ble.The phenomenological crystallographic theory of martensitic transformation has_been applied to lath martensite. Experimental o_bservations on OR, ha_bit plane, theinterface dislocations and the shape strain of surface relief are in agreement with thetheory_by using the lattice invariant shear (-110)[110]_b. This mode is seen to_beequivalent to two equal amounts of shears in [111]_band [11-1]_bdirectionscontained in (-110)_b, which are the most common deformation mode in_bccstructure.This study also provides analytical expressions of crystallographic features_based on derivations in_both reciprocal and direct spaces in two dimensions (2D)for the phase transformations involving an invariant line strain with a pair ofparallel Burgers vectors in the ha_bit plane. Compared to existing2D models,this approach is more straightforward and simpler, and could_be applied tointerpret results in martensitic transformation from earlier studies.
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