Method of analysis of the regularity of lamellar eutectic structures by diffraction of a laser beam
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FAVIER, J. P. MORLEVAT, a n d J. D U V E R N O Y
It is shown that by exploiting the diffraction pattern given by a laser beam directly on a quasi periodical metallurgical structure, it is possible to quantify its regularity. This technique is first applied to the study of lamellar eutectic structures, where it defines the regularity and characterizes the misorientations at different scales. The extension of the method to the characterization of other types of directionally solidified structures is also discussed.
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INTRODUCTION
I N many eutectic alloys, directional solidification is likely to result in the formation of a fine anisotropic and regular structure of the lamellar or fibrous type. 1 If their regularity were sufficiently developed, these materials would have unique mechanical or physical properties. 2'3 However, the lamellar structures which have a period of some 1 to 10/zm are particularly sensitive to fluctuations in growth conditions--solidification rate, interface temperature, e t c . - and respond to such perturbations by locally adjusting the phase distribution. Consequently, from the point of view of fundamental knowledge, the relationship between fluctuations in growth conditions and the disturbance of the lamellar lattice is worth investigating. For this type of analysis, at both the lamellar and bulk scales, it is essential to find a quantitative, objective, and practical measurement procedure. Various authors have from time to time suggested more or less sophisticated methods depending on the particular character of the structure investigated. The most obvious feature of the eutectic structure is the periodicity of the lamellar lattice as defined by the interlamellar spacing. However, for some eutectics such as AuSna-Sn, a pseudo-periodic long range variation of the lamellar spacing in the same transverse section has been observed. 4 Racek and Serra undertook to study local variations of the period by means of a texture analyzer. This type of morphological study was developed further by Camard et al 6 to cover the definition of the structures of white cast irons, in particular to determine the size of the eutectic cells by using an angular variogram. At the scale of the lamellae, the other important structural characteristic is the lamellar fault created by local distortion, rupture, or splitting of the lamella. Berthou and Grnzleski 7 carried out a census of lamellar faults by counting them manually over intervals of 20 A by 20 A (where A is the interlamella spacing). Riquet 8 automated the method by using a Quantimet. As for Hopkins,9 he studied the degree of orientation of the lamellar structure by plotting the direction rose diagram. Nearly all these methods are either time-consuming measurements or suffer in the sense of the limited size of the J.J. FAVIER and J.P. MORLEVAT are Scientists, Commissariat & l'Energie Atomique, Centre d'Etudes Nuclraires de Grenoble, 85X, 38041 Grenoble Cedex, France. J. DUVERNOY is Maitre de Recherche, C. N. R. S. LA 214, Laboratoire d'Optique de la Facu
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