Stress-induced martensitic transformation and impact toughness of cast irons and high-carbon Fe-Ni-C steel
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I. INTRODUCTION
BOTH the stress-induced (or strain-induced) martensitic transformation and the associated transformation toughening have been long observed and studied over the last several decades, both in ferrous alloys and in ceramics. The first published work on transformation toughening was in transformation-induced plasticity (TRIP) steels.[1–7] This was later extended to white cast irons.[8] But, the more extensive work on transformation toughening was done on ceramics, and a number of models have been developed since the 1980s.[9–18] Transformation toughening is the most efficient way to improve the toughness of this brittle ceramic material. There are some common and some different features relating stress-induced transformation and the consequent transformation toughening in ferrous alloys and in ceramics. The common feature is that the transformation in both types of materials can generate transformation strains. Kelly and Rose[19] have explicitly indicated that the transformation strain—or, more correctly, the energy absorbed on transformation and the degree of crack-tip shielding produced—give rise to the observed toughening. The different features are as follows.[19] (1) In ceramics, self-accommodating variants are normally generated from the initial transformation, so that the overall shear component of the shape strain can be reduced. By comparison, in ferrous alloys, the formation of self-accommodation variants is less likely. Instead, the initial transformation may “trigger” additional martensite plates that do not entirely reduce the overall shear component of the shape strain. (2) The lattice-invariant shear in ferrous alloys is much larger than in ceramics. Thus, substructures are generated. (3) In steels, the martensite is very brittle, while the parent austenite is tough. However, in ceramics, both the new and the parent phases are brittle. These features M.-X. ZHANG, Research Fellow, and P.M. KELLY, Associate Professor, are with the Department of Mining, Minerals and Materials Engineering, University of Queensland, ST. Lucia, QLD, 4072, Australia. Manuscript submitted November 28, 2000.
METALLURGICAL AND MATERIALS TRANSACTIONS A
define and differentiate the mechanism of transformation toughening in ferrous alloys and ceramics. Regardless of what materials are used, the understanding of transformation toughening is based on crack-tip shielding or energy absorption associated with the propagation of a crack.[19] The original transformation-toughening models for TRIP steels, developed by Zackay and co-workers[5,6,7] and Antolovich et al.,[1–4] are very similar. In both models, only the shear component of the shape strain of the martensitic transformation was involved, and the dilatational component was recognized to be small compared to the transformation shear. Gerberich et al.[6] calculated the contribution of the martensite, the transformation shear, and the austenite as energy-absorbing media and found that the transformation shear is 5 times as effective as those plastic dissipation pro
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