First-Principles Study on the Grain Boundary Embrittlement of Metals by Solute Segregation: Part I. Iron (Fe)-Solute (B,
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INTRODUCTION
IT is well known that the strength of metals is changed by the segregation of some solutes at grain boundaries. One of the famous examples is temper embrittlement of low alloy steels due to phosphorous (P) segregation at grain boundaries. It is also known that sulfur (S) is a very strong embrittling element for iron and nickel (Ni) grain boundaries, and that boron (B) and carbon (C) are inversely strengthening elements. Analysis of the fracture surfaces by Auger electron spectroscopy (AES) with argon ion sputtering shows that the grain boundary (GB) segregation of solutes occurs within a few atomic layers at the GB plane.[1] For this reason, the GB embrittlement is considered to be caused by the change in the cohesive properties of atoms within a few atomic layers at the GB plane. However, for many years, it has not been known why and how some solute elements decrease/increase the cohesive properties of atoms at the grain boundaries. Many theoretical studies based on the electronic structure calculations of grain boundaries have been done since the 1980s. One of the most famous examples is the electronic structure calculations of metal-solute clusters (Ni4S, Ni8S, etc.) by Messmer and Briant.[2] They insist that the origin of the GB embrittlement of Ni by S segregation is the electron transfer from Ni to S that weakens the adjacent Ni-Ni bonding in the GB. On the other hand, Crampin et al.[3] insist that the increase of directional bonding by solute segregation in the GB prevents the production or migration of dislocations; it results in the hardening that causes GB embrittlement. MASATAKE YAMAGUCHI, Senior Researcher, is with the Center for Computational Science and e-systems, Japan Atomic Energy Agency, Tokai-mura, Ibaraki-ken 319-1195, Japan. Contact e-mail: [email protected] Manuscript submitted November 24, 2009. Article published online August 10, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
However, these studies cannot explain quantitatively the relation between the macroscopic change in the mechanical properties of metals and the microscopic change in the electronic properties of the grain boundaries. In 1989, Rice and Wang showed their theoretical model of GB embrittlement by solute segregation.[4] They insisted that the most important physical quantity in the GB embrittlement is the ideal work of interfacial separation (2cint) of the GB, which is defined as the energy difference between the fracture-surface energy and GB energy. (In this article, we call the 2cint ‘‘the cohesive energy of grain boundary.’’) They roughly estimated the 2cint from experimentally observed surface and GB segregation energies of solutes (C, P, S, antimony (Sb), and tin (Sn)) in iron or steels, and then showed that the change in the 2cint with increasing segregation was roughly proportional to the experimentally observed shift in the ductile-to-brittle transition temperature (DBTT). This indicates that the embrittling elements reduce the fracture-surface energy more than the GB energy, and that inve
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