Effect of Alloying on Physical Properties of NiAl
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EFFECT OF ALLOYING ON PHYSICAL PROPERTIES OF NiAI
W.S. WALSTON AND R. DAROLIA GE Aircraft Engines, Cincinnati, OH 45215
ABSTRACT Several alloying additions were made to NiAl and the effect on physical properties was evaluated. The values obtained were compared to available data on NiAI and NiAl alloys. Negligible effects were observed on thermal expansion and dynamic modulus with the additions studied. The high thermal conductivity of NiAl can be advantageously utilized in many applications, and it was found that additions of Hf lowered thermal conductivity only slightly, while larger decreases were found with other additions. However for the NiAl alloys currently under consideration for applications, significant advantages in physical properties are still maintained over nickel-base superalloys. INTRODUCTION The unique physical properties of NiAI make it an attractive choice for high temperature structural applications. Significant improvements in strength have recently been attained by alloying additions, such that these alloys are being considered to replace nickel-base superalloys in the turbine section of aircraft jet engines. The impact of these alloying additions on the physical properties is important in determining if NiAl alloys still maintain significant advantages over superalloys. Of primary interest are density and thermal conductivity, although thermal expansion and modulus are important as well. There have been relatively few studies on the physical properties of NiAl and even fewer on the effect of alloying additions. This paper presents data on different levels of several additions, primarily Hf, Ti and Re. EXPERIMENTAL PROCEDURES The alloys in this study were single crystal castings made using a modified Bridgman technique. In general, these alloys contain 50 at.% Ni with alloying additions substituted for Al. Unless otherwise noted, specimens were EDM wirecut in the orientation. All specimens received a 1315"C/50 hr homogenization heat treatment, which also served to solution the primary precipitates. Density was determined by the water displacement method. Resonance measurements for calculating the dynamic Young's modulus were performed and no corrections made for thermal expansion. Thermal expansion measurements were performed on a dilatometer at heating rates of 30 C/min. Specific heat was measured using a differential scanning calorimeter, and thermal diffusivity was measured using the laser flash diffusivity method. Neither of these sets of data were corrected for thermal expansion, and thus the thermal conductivity was not corrected for thermal expansion, as is common practice. Thermal conductivity was calculated as a function of temperature from the specific heat, thermal diffusivity and room temperature density data.
Mat. Res. Soc. Symp. Proc. Vol. 288. @1993 Materials Research Society
238
RESULTS AND DISCUSSION Density Several investigators have measured the density of NiAl as a function of stoichiometry.1 ,2, 3 The room temperature density of stoichiometric NiAl is 5.90 g/cm 3 .
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