Crystallographic analysis of upper bainite in Fe-9%Ni-C alloys

Abstract

In order to determine the orientation relationships among ferrite, austenite and cementite in addition to the habit plane of bainite laths, bainite in Fe-9 mass%Ni-C alloys has been examined mainly by means of transmission electron microscopy. Since little austenite has been retained in the present alloys, the orientation of austenite was estimated by assuming the Kurdjumov-Sachs ferrite/austenite relationship with fixing the ferrite/cementite orientation relationship and the habit of upper bainite lath. The results obtained are as follows: An upper bainite lath is formed by the coalescence of needlelike ferrite subunits elongating in a 〈111〉α//〈110〉γ, on a {111}γ plane, the resultant habit of a bainite lath being a 〈110〉γ, {111}γ. The ferrite subunit needle exhibits parallelogram cross section. One set of the parallel needle surfaces inclines slightly from the {111}γ plane and the other set lies almost parallel to the plane expected as that for lattice invariant shear plane expected by the application of phenomenological theory of martensite. At lower temperatures, cementite platelets precipitate on the latter sets of the ferrite needle surfaces, and the coalescence of them leads to a lathlike bainite containing cementite platelets in a specific orientation. The ferrite/cementite orientation relationship is close to that of Isaichev and cementite can be related to austenite by the Pitsch relationship within the deviation less than one degree. Thus, the three phases, i.e., austenite, ferrite and cementite, are mutually almost coherent in upper bainite.