Investigation on the nucleation mechanism of deformation-induced martensite in an austenitic stainless steel under sever
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G. Yang and Y.L. Gao Central Iron and Steel Research Institute, Beijing 100081, China
S.D. Wub) and S.X. Li Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China (Received 22 June 2006; accepted 17 November 2006)
The nucleation mechanism of deformation-induced martensite was investigated by x-ray diffraction and transmission electron microscope in an ultra-low carbon austenitic stainless steel deformed by equal channel angular pressing at room temperature. It was found that two types of martensite transformation mechanism, stress-assisted and strain-induced, occurred via the sequences of ␥ (fcc) → ⑀ (hcp) → ␣⬘ (bcc) and/or ␥ → ␣⬘. In both cases, the crystallographic relationships among ␥, ⑀, and ␣⬘ followed the Kurdjumov-Sachs orientation relationships: {111}␥ //{0001}⑀ //{011}␣⬘ and 〈110〉␥//〈112¯0〉⑀//〈1¯11〉␣⬘.
I. INTRODUCTION
Austenitic stainless steel (SS) is susceptible to martensite transformation (MT) under plastic deformation at low temperature, which is used as an effective method to obtain high ductility, known as the transformationinduced plasticity effect.1,2 Two mechanisms of MT, stress-assisted and strain-induced transformation, have been confirmed in various iron alloys.3,4 It is defined that the stress-assisted nucleation is the nucleation at the same sites that trigger the spontaneous transformation on cooling but assisted by the thermodynamic effects of applied stress, whereas the strain-induced nucleation involves the creation of new nucleation sites by plastic deformation.3 Usually, the mode of MT induced by deformation depends on the stress state, strain, strain rate, and deformation methods. For instance, in 304 SS, the modes of ␥ (fcc) → ⑀ (hcp) → ␣⬘ (bcc) and ␥ → deformation twin → ␣⬘ were proposed to be the transformation mechanisms for uniaxial tension 5 and wiredrawing,6 respectively. Through severe plastic deformation (SPD), metallic materials can be deformed up to a very high strain that is hardly achieved by the conventional deformation methAddress all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2007.0094 724
J. Mater. Res., Vol. 22, No. 3, Mar 2007 http://journals.cambridge.org Downloaded: 25 Mar 2015
ods.7,8 Recently, it was found that shear deformation achieved by equal channel angular pressing (ECAP) was the most effective route to trigger MT in 304 SS compared with uniaxial tension and compression.9 Inverse phase transformations of ␣⬘ → ⑀ and ␣ → ␥ under high pressure torsion were also found in 304L SSs and nanocrystalline (NC) Fe-C composite,10,11 respectively. Under SPD, the formation of NC martensite from the parent austenite via MT was observed in several SS as well.12,13 However, all these references about MT in nanocrystallites or about the achievement of NC martensite through SPD are lack of investigations on the MT mechanism, especially at the early stage of nucleation. This article will investigate the nucleation mechanism of ma
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