The orientation relationship between lath martensite and austenite in low carbon, low alloy steels

Abstract

The orientation relationship between lath martensite and the narrow films of retained austenite in low carbon, low alloy steels has been determined to an accuracy of approximately ± 1 2° using Kikuchi line patterns obtained with a 7 nm probe in a 200 kV STEM set up for microdiffraction. The mean of the orientation relationship determinations was close to the Greninger-Troiano relationship with the extreme results showing a spread of ±2° and lying between the neighbouring Kurdjumov-Sachs and aishiyama-Wassermann relationships. Laths with the same habit plane formed packets and, in general, adjacent laths in such a packet were of the same orientation. However a few cases were observed where adjacent laths had markedly different orientations. In all these cases the two orientations represented a pair of orientation relationship variants in which the same close packed plane in the austenite was parallel to a close packed plane in the martensite and the same close packed direction in the austenite was approximately parallel to a close packed direction in the martensite, i.e. (111) γ nearly parallel to (101) α [11 10] γ 1.5 to 3° from [ 111] α and (111) γ nearly parallel to (101) α [1 10] γ 1.5 to 3° from [ 111]α . These two orientation relationship variants approximated to a twin relationship between the adjacent laths and in the less accurate conventional electron diffraction spot patterns could easily have been interpreted as a twin relationship. Single surface trace analysis showed that the long direction of the laths was always the [1 10] γ direction which is approximately 2.5° from [11 1] α , while the habit plane of the laths was consistent with (557) γ—i.e. an ( hhl) γ plane approximately 10° from the (111) γ which is parallel to (101) α.