Variant selection in surface martensite.

Annick P Baur,Roland E Logé,Cyril Cayron

doi:10.1107/s160057671701398x

Abstract

A significant variant selection is reported in isothermal martensite formed on the surface of an Fe-30% Ni sample. The selection phenomenon is modelled using different descriptions of the martensitic phase transformation. In particular, matrices based on the phenomenological theory of martensite crystallography, the Jaswon and Wheeler distortion, and the continuous face centred cubic-body centred cubic distortion are compared. All descriptions allow good predictions of the variant selection. However, the Jaswon and Wheeler distortion and the continuous distortion better account for other features of the surface martensite, such as the {225}γ habit plane and the accommodation mechanism by twin-related variant pairing.

Highlights

Variant selection in martensitic steels is a well documented phenomenon because of its implications for the industrial processing of steel
We show a significant variant selection in isothermal surface martensite formed on the free surface of an as-cast Fe–30% Ni sample without any applied stress
We consider the invariant plane strain and the invariant line strain based on the phenomenological theory of martensite crystallography (PTMC) (Bowles & Mackenzie, 1954), the Jaswon and Wheeler distortion (Jaswon & Wheeler, 1948), and the continuous distortion associated with the Kurdjumov–Sachs orientation relationship (Cayron, 2015)

Summary

Introduction

Variant selection in martensitic steels is a well documented phenomenon because of its implications for the industrial processing of steel It is reported when stress is applied to the material either before (Wittridge & Jonas, 2000; Miyamoto et al, 2012) or during transformation (Gey et al, 2005; Mishiro et al, 2013). The present study shows this phenomenon and proposes a predictive model, based on the maximization of the extension of a material fibre oriented along the normal to the sample’s free surface. This model is used to compare different crystallographic descriptions of the transformation. The study shows that all four descriptions of the transformation are able to predict correctly the trend of variant selection in surface martensite; the Jaswon and Wheeler distortion and the continuous distortion are more appropriate for accounting for other features of the transformation, such as the habit plane and accommodation by twin-related variant pairing

Methods

Results

Conclusion