Fracture transitions in iron: Strain rate and environmental effects

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A number of recent mechanical property studies have sought to validate atomistic and multiscale models with matching experimental volumes. One such property is the ductile–brittle transition temperature (DBTT). Currently no model exists that incorporates both external and internal variables in an analytical model to address both length scales and environment. Using thermally activated parameters for dislocation plasticity, the present study attempts a small piece of this. With activation energy and activation volumes previously determined for single and polycrystalline Fe–3% Si, predictions of DBTT both with and without atmospheric hydrogen are made. These are compared with standard fracture toughness measurements similarly for samples both with and without atmospheric hydrogen. In the hydrogen-free samples, average strain rate varied by four orders of magnitude. DBTT shifts are experimentally found and predicted to increase 100 K or more with either increasing strain rate or exposure to hydrogen. I. INTRODUCTION

The ductile-to-brittle transition temperature (DBTT) is a materials property that has important consequences for safety and reliability in applications, but also requires an understanding of the energetics of fundamental deformation processes. It is strongly affected by both strain rate and environment, but a clear understanding of what the mechanisms are is still elusive. What is needed is an analytical model that accounts for all the important parameters governing the DBTT. Steps toward developing such a model are undertaken in this study; the authors contend that the critical concept for understanding the DBTT is crack tip shielding by dislocations, which is strongly affected by temperature, strain-rate, and hydrogen. The goal of this study is to develop such a model with a particular focus on the energetics of crack tip plasticity. This model will then be tested by applying it to previously published data, first on fracture toughness measured at different strain rates and second on fracture toughness with and without the presence of hydrogen, both as a function of temperature. First, we introduce the concept of crack tip shielding by dislocations, which will be the basic physical principle of the proposed model, and how this is linked to the DBTT. DBTT concepts have origins in the thermally activated dislocation plasticity models of Cottrell1 and crack-tip plasticity models of Dugdale–Barenblatt.2,3 From a materials science approach, St. John4 was possibly the first to discuss a definitive activation energy for the a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.142 J. Mater. Res., Vol. 29, No. 14, Jul 28, 2014

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DBTT process in single crystals. This was done for silicon, which undergoes a DBTT within a few degrees, known as a hard transition. This was later compared with soft transitions (more gradual) in metallic systems.4,5 Over the following three decades various dislocation nucleation6–8 and dislocat

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