• PDF / 8,407,028 Bytes
• 11 Pages / 576 x 792 pts Page_size
• 36 Downloads / 231 Views

DOWNLOAD

REPORT

---

Ivan E. Locci, S. V. Raj, and Michael V. Nathal NASA-Lewis Research Center, 21000 Brookpark Road, Cleveland, Ohio 44135 (Received 16 December 1991; accepted 25 March 1992)

A multiphase oxidation resistant composition (Nb-67Al-7Cr-0.5Y-0.25W) based on NbAl3 was prepared by both induction melting and rapid solidification processing (RSP), followed by grinding to 75 /xm powder and consolidating by powder metallurgy techniques (hot pressing, hot isostatic pressing, and Ceracon pressing). Constant strain rate compression tests conducted on consolidated materials in the temperature range 300-1300 K indicated that the RSP material exhibited significantly higher strength and ductility than the induction melted alloy up to 1200 K. Bend strengths measured on induction melted material were significantly lower than the corresponding compressive strengths, suggesting the brittle, flaw-sensitive nature of this alloy. The NbAlCrYW alloy exhibits a brittle-to-ductile transition around 1000 K. The constant load creep tests conducted on the induction melted alloy in the 1200-1300 K temperature range indicated that this alloy shows a power law creep dependency with a stress exponent, n, of 3.2. It was found that the specific strength of this alloy is competitive with other aluminide intermetallics. I. INTRODUCTION The intermetallic compound NbAl3 is an attractive material for advanced aerospace structural applications because of its favorable combination of high melting temperature (>1873 K), low density (4.5 kg/m 3 ), and low coefficient of thermal expansion (10.5 x 10~6 K"1). However, NbAl3 is extremely brittle and does not possess good oxidation resistance. The overall objective of this work is to understand and develop low density NbAl3-base alloys with greatly improved oxidation resistance to serve eventually as matrices for structural composite applications in advanced air breathing engines at temperatures up to 1500 K. Although NbAl3 has the lowest oxidation rate among the binary Nb-Al alloys, it does not form exclusive layers of protective A12O3 scales.1 Recently, Perkins et al.2 have shown the feasibility of forming alumina scales on Nb-Al alloys at greatly reduced Al contents. Hebsur et al.3 were successful in maintaining the high aluminum content and therefore low density, while achieving the capability of growing protective A12O3 scales by a ternary addition of 7 at. % Cr to NbAl 3 . A quaternary addition of 0.5 at. % Y to Nb-Al-Cr promoted oxide adherence, reduced scale growth rates, and eliminated convolutions within the oxides.3"5 An addition of 0.25 at. % W to the NbAlCrY alloy appeared to be beneficial to strength and processibility while not deleteriously affecting the oxidation behavior. The isothermal oxide growth kinetics on a NbAlCrYW alloy4 are the same as those of conventional A12O3 form1696 http://journals.cambridge.org

J. Mater. Res., Vol. 7, No. 7, Jul 1992 Downloaded: 13 Mar 2015

ing alloys.5 This alloy also exhibited excellent cyclic oxidation resistance at 1473 K, being nearly equivalent to NiAl + 0.1