Effects of copper on proeutectoid cementite precipitation
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I.
INTRODUCTION
W H E N hypereutectoid steels are cooled from the austenite range, proeutectoid cementite is often precipitated at austenite grain boundaries. The first morphology observed is usually that of grain boundary allotriomorphs. LThese precipitates nucleate at austenite grain boundaries and grow, more or less smoothly, along them. 2 Lengthening along the grain boundary proceeds more quickly than thickening normal to it, and this causes the allotriomorphs to take the shape of lenses. The allotriomorphs quickly lengthen and impinge upon each other, leaving the grain boundaries covered with thin (on the order of a micron) films of proeutectoid cementite. After longer transformation times, proeutectoid cementite may also precipitate intragranularly, usually in the form of Widmanstiitten plates.~ This evolution of microstructure is analogous to that which occurs during the precipitation of proeutectoid ferrite in hypoeutectoid steels) '3 Proeutectoid cementite grain boundary allotriomorphs have been identified as a source of embrittlement in high carbon steels. ~ Proeutectoid cementite grain boundary allotriomorphs thicken rapidly at first, but much more slowly at the later stages of the transformation. 6 The initial thickening rates have been found to be considerably lower than those calculated under the assumption that growth is controlled exclusively by the diffusion of carbon through austenite, s'6 Thickening rates lower than those allowed by the diffusion of carbon through austenite have also been found for proeutectoid ferrite grain boundary allotriomorphs in hypoeutectoid steels. 2'3'7 These sluggish thickening kinetics have been attributed to the effects of interphase boundary structure. 2'3'5 7 It is thought that the interphase boundary surrounding grain boundary allotriomorphs consists of a J.A. WASYNCZUK is with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720 and Materials and Molecular Research Division, Lawrence Berkeley Laboratory, I Cyclotron Road, Berkeley, CA 94720. R.M. FISHER is Staff Senior Scientist, Center for Advanced Materials, Lawrence Berkeley Laboratory, 1 Cyclotron Road, Berkeley, CA 94720. G. THOMAS is Professor, Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720 and Scientific Director, National Center for Electron Microscopy, Materials and Molecular Research Division, 1 Cyclotron Road, Lawrence Berkeley Laboratory, Berkeley, CA 94720. Manuscript submitted February 4, 1986.
METALLURGICALTRANSACTIONS A
mixture of incoherent and partially coherent portions, z The incoherent portions have high mobility and can migrate freely under the action of carbon diffusion. 2 The partially coherent portions are manifested as planar facets which can migrate normal to themselves only by a ledge mechanism. 2,3 The necessity to migrate by a ledge mechanism causes these facets to act as a structural impediment to growth. 2'3 However, the exact amount and distribution of partially coherent
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