Effects of Alloying Elements on Iron Grain Boundary Cohesion
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*Dept. of Evanston, "**Dept. of Evanston,
Physics & Astronomy, Northwestern University, IL 60208 Materials Science & Engineering, Northwestern University, IL 60208
1. INTRODUCTION: a long time, understanding the mechanisms of impurity-promoted embrittlement in iron and the consequent cohesion(decohesion) effects has been a challenge for materials scientists. The role alloying elements play in impurity-promoted embrittlement is important due to either their direct intergranular cohesion(decohesion) effects or effects upon embrittling potency of other impurities. Some alloying elements like Pd and Mo are known to be helpful for intergranular cohesion in iron and some other alloying elements like Mn are known to segregate to and weaken iron grain boundaries dramatically[1]. There have been intensive investigations on these mechanisms for a long time and especially, with the progress in computing techniques in recent years, calculations on more realistic models have become possible[2-4]. In this paper we briefly present our studies on some selected alloying-element/iron grain boundaries(GB) and free surface(FS) systems. The effects of Pd, Mo, Mn and Cr on the Fe E5 (031) grain boundary and its corresponding
(031) free surface are examined, using a
combination of molecular dynamics(MD) and first-principles electronic structure calculations. Section 2 gives a brief introduction to the methods used and Section 3 gives the main results. 2. METHODOLOGY: To understand the behavior of impurities and alloying elements on iron grain boundaries at the first-principles level, a realistic structural representation of the system is important. For grain boundaries, unlike other kinds of interfaces, an unrelaxed model or a layer-only relaxed model is far from realistic because atom rearrangement occurs in directions both perpendicular and parallel to the interface. Ideally one would use first-principles theory to compute forces on all atoms, rearrange atomic positions accordingly, and search out both local and global energy minima. While feasible for small molecules, more approximate methods are needed for low-symmetry extended systems. Here we have chosen a two-step process: In the first step, we used MD to obtain an energy-minimized configuration(or relaxed configuration) 243
Mat. Res. Soc. Symp. Proc. Vol. 492 © 1998 Materials Research Society
with plausible interatomic potentials. Then in the second step, we carried out first-principles electronic structure calculations on the model obtained. We go further by modifying the empirical interatomic potentials, based on results from the first-principles calculations, repeating steps one and two to improve the results. The essential idea of ordinary MD is to numerically solve the Nbody problem of classical mechanics. The relaxation process we have used to find equilibrium geometry is a mixture of the ordinary molecular dynamics and static relaxation methods. When doing MD the most important and also the most difficult task is to choose a proper set of interatomic potentials whic
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