Characteristics of lath martensite: Part I. crystallographic and substructural features

This investigation, using an Fe-20 pct Ni-5 pct Mn (wt pct) alloy, was carried out to provide more detailed and accurate information on the crystallographic features of ferrous lath martensite than is presently available. The martensite observed was typical of that found in low carbon steels, but with the present alloy substantial amounts of retained austenite are found, which can be used as a crystallographic reference basis. Analysis of some twenty laths showe_d_the average austenite-martensite orientation relationship to be (lll)_f ‖ (011)_b: [•101]_f 3.9 deg from [•1•1l]_b, using an electron diffraction method involving an error of only a fraction of a degree. Adjacent laths within a packet of lath martensite were found to exhibit the same variant of the orientation relationship although such laths may be misoriented relative to each other by up to 2 deg. Thick layers of austenite found between adjacent laths indicate that the laths do not form by self-accommodation. The lath martensite habit plane is irrational, close to (575)_f [equivalently (•154)_b], but since the habit plane is of the type hkh, 12 apparent habit planes are observed although 24 variants of the orientation relationship may be found. The martensite-austenite interface on one side of a given lath is relatively planar, while that on the opposite side is irregular, suggesting that the laths thicken mainly in one direction. The martensite laths contain screw dislocations in all four ( 111 )_b directions, but one set of the four with Burgers vector α/2 [•111]_b is clearly dominant as a result of accommodation deformation imposed by the large lath martensite shape strain. Austenite dislocation arrays associated with the straight and irregular lath interfaces are very different, again suggesting that the thickening of a lath takes place mainly in one direction away from the initial straight interface.