title: Microstructure and cleavage in lath martensitic steels.
abstract: In this paper we discuss the microstructure of lath martensitic steels and the mechanisms by which it controls cleavage fracture. The specific experimental example is a 9Ni (9 wt% Ni) steel annealed to have a large prior austenite grain size, then examined and tested in the as-quenched condition to produce a relatively coarse lath martensite. The microstructure is shown to approximate the recently identified 'classic' lath martensite structure: prior austenite grains are divided into packets, packets are subdivided into blocks, and blocks contain interleaved laths whose variants are the two Kurjumov-Sachs relations that share the same Bain axis of the transformation. When the steel is fractured in brittle cleavage, the laths in the block share {100} cleavage planes and cleave as a unit. However, cleavage cracks deflect or blunt at the boundaries between blocks with different Bain axes. It follows that, as predicted, the block size governs the effective grain size for cleavage.
authors.affiliation: Department of Materials Science and Engineering, University of California, Berkeley, CA 94620, USA.
authors.forename: John W
authors.initials: JW
authors.lastname: Morris
authors.affiliation: Department of Materials Science and Engineering, University of California, Berkeley, CA 94620, USA.
authors.forename: Chris
authors.initials: C
authors.lastname: Kinney
authors.affiliation: Department of Materials Science and Engineering, University of California, Berkeley, CA 94620, USA.
authors.forename: Ken
authors.initials: K
authors.lastname: Pytlewski
authors.affiliation: Department of Mechanical Engineering, Kagoshima University, Kagoshima, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Adachi
keywords: 10.07; cleavage; fracture; lath martensitic steels; microstructure