Quantitative analysis of variant selection in ausformed lath martensite

Abstract

Variant selection in lath martensite transformed from deformed austenite in a low-carbon low-alloy steel is examined quantitatively on the basis of electron backscatter diffraction analysis of martensite, in concert with a novel method for the reconstruction of austenite orientation. At a strain of 10%, variants whose (011)α plane was nearly parallel to the primary slip plane in austenite ((111)γ) were formed dominantly. At strains of 30% and 50%, variants whose (011)α plane was nearly parallel to the secondary slip plane ((−111)γ) as well as the primary slip plane were formed. Transmission electron microscopy observation of an austenite-stabilized alloy deformed under the same condition as the low-carbon low-alloy steel clarified that microband structures develop along the primary and secondary slip planes of austenite when its orientation is close to the main component of the deformation texture in austenite. A simple variant selection model is proposed in which martensite variants with habit planes nearly parallel to the primary and secondary slip planes nucleate preferentially on microband boundaries as a result of a smaller amount of activation energy and grow dominantly as a result of less inhibition from the microband boundaries.