Phase Stability of Carbides and Nitrides in Steel
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0979-HH14-03
Phase Stability of Carbides and Nitrides in Steel Marcel Sluiter MSE-3ME, TU Delft, Mekelweg 2, Delft, 2628 CD, Netherlands
ABSTRACT Carbides, nitrides, and carbonitrides have great influence on the properties of steel but are relatively little studied at a fundamental level: experimentally because of small dimensions of the precipitates and other difficulties, theoretically because of complex structures and poorly defined compositions. Modern density functional calculations reveal significant trends that can be partially summarized in terms of atomic size, carbon affinity, and hybridization de-magnetization concepts. Phase stability and order-disorder phenomena in widely occurring Cr-based M23C6 carbides and Mo-based M6C carbides are used to illustrate predictions relevant to low alloyed steel. INTRODUCTION Transition metal carbides and carbonitrides appear in many technologically important materials, in steels, both the alloyed and the unalloyed varieties, in hard refractory materials and coatings. Many of the general features of carbides apply also to carbonitrides so that from here on I will refer carbonitrides implicitly when mentioning carbides. The structures of binary carbide phases exhibit certain trends: carbides of the group IV transition metals (TiC, ZrC, HfC) are cubic NaCl type, group V transition metals (V, Nb, Ta) also form carbides with the NaCl structure, but in addition hexagonal M2C carbides occur. Group VI transition metals (Cr, Mo, W) form several carbide phases, some of them forming under special conditions only, such as the various crystal structures of MoC. Carbides become progressively less stable with transition metals of the VII (Mn) and VIII (Fe,Co) groups. Theoretical studies based on electronic density functional approaches correctly calculate the stable structures and their lattice parameters. However, so far in the literature these studies have been limited mostly to the simplest carbide phases, typically those of the NaCl type [1] and of the MoC varieties [2,3] where the effect of carbon vacancies, and the stacking of hexagonal planes are the main issues. Only very recently have studies dealing with cementite (Fe3C) and other iron carbides appeared in the literature [4,5,6]. The carbides that occur in low-alloyed steels usually involve structurally much more complex phases involving more than a single metallic species. Some carbide phases in steel, such as M6C are not known in any binary system M-C, but occur only in ternary or higher order systems such as Fe-Mo-C as e.g. Fe3Mo3C. Until quite recently theoretical calculations on multicomponent carbides such as M6C with 112 atoms in the face centered cubic cell were unpractical, but now with advances in computational methodology and computer hardware these phases can be calculated with good precision. An intermediary approach, where ab initio calculated cohesive energies vs lattice parameter are used to obtain an approximate pair potential through an in principle exact Möbius-Chen inversion [7], has been used for sever
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