Creep-fatigue behavior of directionally solidified and single crystal intermetallic Ni 3 AI(B, Hf) at an intermediate te
- PDF / 2,108,545 Bytes
- 8 Pages / 613 x 788.28 pts Page_size
- 42 Downloads / 290 Views
I.
INTRODUCTION
Ni3AI, an L12 ordered intermetallic, has gained renewed attention recently. It is known that this material has a positive temperature dependence of yield strength that reaches a peak at approximately 760 ~ has a high temperature oxidation resistance, requires little alloying additions of strategic elements, and possesses a relatively low density compared to conventional gamma prime strengthened superalloys. The recent interest has been spurred in part by trace element alloyed polycrystalline nickel aluminides, which reduce the propensity for intergranular failure. Microalloying with boron 1'2'3has been shown to improve grain boundary cohesion. Boron has been observed to segregate to grain boundaries4'5'6 and appears to increase the mobility of grain boundary dislocations. 7 The extent of ductility and fracture behavior of polycrystalline Ni3A1 was found to depend on stoichiometry also. 1'2 A substoichiometric aluminum content of 24 at. pct provided the most improved ductility in a B-doped Ni3A1./ Hafnium, on the other hand, has been shown to be the most effective solute in raising the yield strength and creep resistance of Ni3A1 at high temperatures. 8'9 Although single crystals are ductile at room temperature, they experience a loss in ductility near peak temperatures. 1~ This ductility minimum is aggravated in polycrystalline Ni3A1, even with boron and hafnium additions, by dynamic embrittlement of grain boundaries when tested in air at temperatures above 600 ~ 8'11 Although some high temperature fatigue and crack growth data exist for ordered intermetallic alloys, very little is known about the creep-fatigue resistance of Ni3AI(B, Hf), especially near peak temperatures. The present investigation begins to determine the intrinsic creep-fatigue behavior of directionally solidified (columnar grained) and single crystal forms of the Ni3AI(B, Hf) material. *UDIMET is a trademark of Special Metals Corporation. RICHARD S. BELLOWS and ERIK A. SCHWARZKOPF, Graduate Research Assistants, Center for Strategic Materials, and JOHN K. TIEN, Henry Marion Howe Professor of Metallurgy and Materials Science, and Director of The Center for Strategic Materials, are with Henry Krumb School of Mines, Columbia University, 500 West 120 Street, New York, NY 10027. Manuscript submitted January 26, 1987. METALLURGICALTRANSACTIONS A
The test procedure used in this study was first described and employed by Paulson e t a l . 12 for determining the creepfatigue behavior of UDIMET 115, a conventional, wrought, approximately 45 pct gamma prime strengthened nickelbase superalloy. UDIMET 115 is considered to be a high strength nickel-base superalloy at 760 ~ The difference between this procedure and conventional fatigue testing is that, as the cyclic stress range decreases, the mean stress increases. Thus, the damage at high temperatures attributed to the cyclic stress (fatigue) and the damage attributed to the mean stress (creep) can be separated. This testing protocol results in maximum creep-fatigue information per test matri
Data Loading...
