ELECTRON MICROSCOPY STUDIES OF LATH MARTENSITE

Abstract
Crystallographic and substructural features of lath martensite formed in an Fe-20Ni-5Mn (wt.%) alloy containing small amounts of lath martensite in a matrix of austenite have been investigated using transmission electron microscopy. The average austenite/martensite orientation relationship determined by an accurate method is (111)_f || (011)_b and [[MATH]01]_f 3.9⁰ from [[MATH][MATH]1]_b, while the habit plane of the laths is close to (575)_f or ([MATH]54)_b. Adjacent laths within a packet of laths adopt the same variant of the orientation relationship, but may be misoriented relative to each other by about 2°. The martensite laths contain screw dislocations in all four <111>_b directions, but one set of the four is clearly dominant. The martensite/austenite interface on one side of a lath is planar, while that on the opposite side is irregular. Dislocation tangles in the austenite associated with the irregular interface suggest that the laths thicken mainly in the direction away from the initial planar interface. The martensite/austenite interface contains a single set of dislocations with Burgers vector a_b/2 [1[MATH]1] = a_f/2 [0[MATH]1], which form loops around the laths. The observed interface dislocation structure satisfies the requirements for a glissile semi-coherent interface.