Ferrite: Cementite crystallography in pearlite
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I.
INTRODUCTION
A. Orientation Relationships
THE decomposition of austenite in iron-carbon alloys can result in the formation of pearlite at the appropriate composition and transformation temperature. The two lamellar components are ferrite ( f ) , a simple bodycentered cubic structure with lattice parameter a = 0.28665 nm, space group Im3m, and cementite (c) which has a more complex structure whose orthorhombic unit cell contains 12 iron atoms and 4 carbon atoms, with space group Panm and lattice parameters a = 0.45235, b = 0.50890, and c = 0.67433 nm (here the convention a < b < c is adopted). [~ These two components form the well-defined lamellar structures. There are at least three different austenite grain-boundary conditions for pearlite to nucleate, namely, in association with proeutectoid ferrite, clean austenite boundaries, and proeutectoid cementite. The present article contributes to the understanding of the role interlamellar crystallography has in establishing pearlite orientation relationships and habit planes during the growth process. Although the importance and number of orientation relationships have fluctuated over the years, three have been consistently reported in the literature. They are BagaryatskyY 1 Isaichev, t3) and Pitsch-Petch t4'5~orientation relationships, which are traditionally expressed as [ 100]
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