The effects of b and zr on the creep and fatigue crack growth behavior of a Ni-base superalloy
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I.
INTRODUCTION
D U R I N G the late 1950's metallurgists discovered that small additions of boron and zirconium were extremely beneficial to the creep rupture properties of nickel-base superalloys. The common observation was that these elements helped to prevent premature cavity and microcrack formation on the grain boundaries, and thus improved the rupture life and ductility. ~Much of the earlier work on minor element effects in superalloys is summarized in a review by Holt and Wallace. 2 The optimum concentrations of B and Zr vary somewhat with the base composition of the alloy, but most alloys with these additions typically contain on the order of 0.006 wt pet B and 0.05 wt pct Zr. While the helpful effects of these elements are recognized, the reason why B and Zr minimize grain boundary cracking has remained a subject of debate. Some of these explanations will be discussed later. It does seem clear from the evidence in hand that no single mechanism can explain all the observed effects, and that B and Zr may provide several different positive effects depending upon the particular alloy chemistry and test conditions. To date all the investigations of the effects of B and Zr have used smooth or machined notch creep rupture specimens. In recent years considerable attention has been paid to the creep and fatigue crack growth characteristics of superalloys. These studies have demonstrated that compositional and microstructural variations can produce order of magnitude differences in the crack growth rates. In view of the potent effects of B and Zr on rupture life, one would expect that the crack growth behavior would be significantly affected by these elements, and that the crack growth rate measurements could provide additional insight into the mechanisms by which these elements improve the properties. In the present study four heats of a nickel-base superalloy based on the NIMONIC* alloy PE16 composition were *Trademark of the Inco family of companies.
made containing neither B nor Zr, a B addition, a Zr addiS. FLOREEN, Research Fellow, and J. M. DAVIDSON, Research Metallurgist, are both with Inco Alloy Products Company Research Center, Sterling Forest, Suffern, NY 10901. Manuscript submitted July 15, 1982. METALLURGICALTRANSACTIONSA
tion, and a B plus Zr addition. The creep behavior of smooth specimens and the creep and fatigue crack growth rates at 650 ~ were determined both in air and in high purity helium, together with supplemental microstructural and deformation studies.
II.
EXPERIMENTAL P R O C E D U R E S
The materials used in this study were prepared as laboratory melted vacuum induction heats using high purity charge materials. To minimize compositional variations between the heats they were prepared as two 27 kg melts that were each split to give a total of four ingots with different B and Zr concentrations. The final analyzed compositions of the alloys are given in Table I. The B and Zr additions were 0.006 and 0.05 wt pct, respectively. The base composition without any B and Zr added contained
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