An Electron Microscopy Study of Graphite Growth in Nodular Cast Irons
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THE growth mechanism of graphite in nodular cast irons has led to controversial proposals since these materials were discovered in the mid of the last century. In fact, the nodules are constituted of sectors within which the (0002) graphite planes are roughly perpendicular to the radius of the nodule (see for example the review by Lux[1]). Accordingly, the apparent growth direction of the nodules is along the c basal direction of graphite and not along the a prismatic direction as is the case for lamellar graphite microstructure. It is generally accepted that crystallographic defects play an important role in graphite growth[2,3] and the most accepted model for spheroidal growth consists in spiral growth around screw dislocations emanating from the nodule center.[4] A very similar model proposed by Double and Hellawell[3] and Miao et al.[5] involves helicoidal growth. Adjacent sectors are separated between each other by high angle boundaries which have often been associated with twin boundaries[6–8] though some more complicated cases have also been observed.[9,10] As described below, alternative models have been recently proposed.
L. LAFFONT, R. JDAY, and J. LACAZE are with the CIRIMAT, Universite´ de Toulouse, CNRS / INPT / UPS, ENSIACET, 4 Alle´e Emile Monso, 31030 Toulouse, France. Contact e-mail: lydia. laff[email protected] Manuscript submitted September 29, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
There has been a renewed interest in spheroidal growth of graphite these last years that at first sight may appear as adding some confusion. While Qing[8] gives support to the screw dislocation mechanism, Amini and Abbaschian[11] claimed that spheroidal growth results from a roughening of the graphite/liquid interface when Stefanescu et al.[12] think demonstrating that the internal structure of graphite spheroids consists in foliated dendrites. This latter model is in fact not so far from the earlier proposal by Double and Hellawell[13] who suggested a mechanism based on repeated nucleation of graphite layers at the outer surface of the spheroids extending along the prismatic direction. This 2-D nucleation growth model would agree with transmission electron microscopy (TEM) observations that the sectors in the graphite spheroids are made of blocks elongated along the prismatic direction.[5,8,14–18] Some indirect support of this model has been gained by comparing the overall growth rate of spheroids with the rate of lateral extension of new graphite layers[19] and this has been recently more formally and quantitatively derived.[20] One of the most striking things concerning spheroidal growth of graphite is that nodular graphite may be obtained by solid-state graphitization of cast irons fully or partly solidified in the metastable systems where the carbon-rich phase is cementite. It is even more astonishing that in such a process, the melt must have been submitted to a spheroidizing treatment for giving nodular graphite through graphitization.[21,22] By solidstate graphitization of carbon steels, He et al.[2
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