Fracture Modes in Ll 2 Compounds
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FRACTURE MODES IN L12 COMPOUNDS C.L. Briant and A.I. Taub General Electric Company Research and Development Center P.O. Box 8 Schenectady, New York 12301
ABSTRACT This paper reports a study of grain boundary segregation and fracture modes in L12 intermetallic compounds. Data obtained on Ni3 A1, Ni 3 Si, Ni3Ga, Ni3 Ge, and Pt3Ga will be presented. It will be shown that the amount of boron segregation and its ability to improve cohesion depends on the total composition of the compound. The beneficial effects of boron can be counteracted by the presence of borides on the grain boundaries. Carbon additions also produce some improvement in ductility in Ni 3Si.
INTRODUCTION Most high purity L1 2 intermetallic compounds are very brittle and fail by intergranular fracture. This brittleness has been one reason why these compounds have had limited applications as structural materials. In 1979 Aoki and Izumi [1] reported that additions of boron to the L12
compound Ni3 A1 increased its ductility and changed its fracture mode from intergranular to
transgranular. This work triggered a large amount of research directed at studying the fracture behavior of many intermetallic compounds that were doped with boron or other elements that might provide a similar improvement in cohesion. To date, the most extensive research has been on the L12 compounds. We present in this paper a summary of our own work, the overall goal of which was to determine the factors that control the segregation and fracture mode in these compounds. The investigation of embrittlement and segregation and their interrelationship has only recently begun in intermetallics, but studies of this type have been carried out for many years in disordered iron and nickel base alloys[2].The results of these studies provide a framework that we can use to think about the results that we obtain on intermetallics. For example, it has been shown that in disordered alloys the amount of segregation that occurs depends primarily on the activity of the segregant in the matrix. This activity can be varied by changing the bulk composition of the segregant, the temperature at which segregation occurs, and the total composition of the alloy. The total composition is also important because it can determine the degree to which a given segregant can affect grain boundary cohesion. These studies have also shown that grain boundary precipitates can exacerbate grain boundary embrittlement. Yet one important difference between the disordered alloys and intermetallics is that many intermetallics have intrinsically brittle grain boundaries, and we are looking for elements that would improve cohesion. In high purity disordered alloys the grain boundaries are usually highly cohesive, and we worry about segregants that might weaken them. In this paper we will report results from experiments that were based on many of the concepts described in the preceeding paragraph. We will show that the ability of boron to segregate and also its ability to improve cohesion depends on the total composition of
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