INTERPHASE BOUNDARY STRUCTURE AND HABIT PLANE OF LENTICULAR MARTENSITE IN IRON-BASED ALLOYS

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In iron-based alloys, the morphology of martensite changes from lath to lenticular and thin plate with a decrease in Ms temperature. In the present study, in order to make clear the reason of martensite morphology change, substructure and crystallographic features such as habit plane, orientation relationship, interphase boundary structure, etc) were observed in detail by means of transmission electron microscopy using mainly Fe-Ni alloys. Main results obtained are as follows.1.Lenticular martensite plate consists of three parts, i.e., (1)the mid-rib which is completely twinned, (2)the twinned region in which some twins are extended from the midrib, (3)the untwined region which contains a high density of dislocations. Midrib is the region at which the transformation is initiated, and has the same morphology (thin plate) and crystallography ({3,10,15}habit plane, G-T orientation relationship) as those of thin plate martensite.2.The orientation relationship of lenticular martensite is G- … More T at the mid-rib but gradually deviates towards K-S near the M/A interface. This change is related to the change in the substructure of martensite.3.Although the interface of lenticular martensite is usually smooth in the scale of optical microscopy, it becomes irregular when the Ms temperature is increased near room temperature, and exhibits many {225}A facets. The {225}A plane corresponds to the habit plane of lath martensite.4.The interphase boundary of lenticular martensite consists of two sets of screw dislocations which accommodate transformation strain. This boundary structure is same as that of lath martensite.5.It was concluded that the change of marteniste morphology results from the change in the mode of lattice invariant shear during transformation. Martensite morphology becomes thin-plate when the twinning occurs, and the lath shape when the slip occurs. When the lattice invariant shear changes from twinning to slip during the growth, morphplogy of martensite becomes lenticular. Less