Structure and mechanical properties of unidirectionally solidified Fe-Cr-C and Fe-Cr-X-C alloys
- PDF / 1,695,704 Bytes
- 7 Pages / 613 x 788.28 pts Page_size
- 63 Downloads / 215 Views
I.
INTRODUCTION
UNIDIRECTIONALLY solidified eutectics (also known as in situ composites) are playing an increasingly important role in the development of new materials. These composites are characterized by possessing unusual and highly anisotropic mechanical properties derived from their fiberlike structures. In particular they are materials with potential applications in high temperature environments (e.g., gas turbine components) or under severe loads. Since the original work of W. Tiller m on the growth of eutectics by unidirectional solidification there has been a great deal of interest focused mainly on the fundamental understanding of solidification. Numerous reports have been written on the nucleation, [2-6] g r o w t h , t2,7-~2] segregation, [13' 24' 15] classification, [16' 17' lS] and interface stability [13' 19-21] of eutectics or on their crystallography.~22'23"241These investigations are summarized by comprehensive reviews on this s u b j e c t . [22,23,24]
Although there is extensive work on the undirectional solidification of eutectics, their mechanical properties have not been fully investigated. In these composites a volume fraction of high strength continuous fibers embedded in a relatively soft matrix give rise to a two-phase material inherently stronger than current engineering alloys. The strength of the composites mentioned is strongly dependent on their fiber diameters 128]as well as fiber contents.[29] In most conventional composites the mechanical properties can be properly described by the rule of mixtures (i.e., composite strength depends only on the mechanical properties and volume fractions of each of the two phases). However, in very fine microstructures (-~ 1 /zm fiber diameters) the measured strengths are greater than predicted from the rule of mixturesJ 3~ The deviations between experimentally measured and predicted composite strengths are attributed to deformation constraints which arise from differences in elastic and plastic properties across the fiber/ matrix interface, t3~ In addition there is evidence [61 of an
EDWARD FRAS, Professor, and EDWARD GUZIK, Associate Professor, are with the Academy of Mining and Metallurgy, Reymonte 23, Cracow, Poland. HUGO E LOPEZ is Associate Professor, Department of Metallurgical Engineering, Saltillo Institute of Technology, Apartado 84-C, Saltillo, Coah., Mexico. Manuscript submitted March 16, 1987. METALLURGICAL TRANSACTIONS A
increase in composite strength as fiber spacings (h) decrease as a result of an increase in the extent of constrained regions. Thus, an optimum eutectic composite should contain the highest volume fraction possible of well-oriented and thin fibers strongly bound to the matrix. In spite of the great possibilities opened by these new materials and the vast scope of the investigations carried out in this respect (i.e., the microstructures of nearly 300 in situ composites are known t25'26'271) our knowledge of their properties is still scarce. Table I shows the mechanical properties reported for in situ composites co
Data Loading...
