Long-Term Creep and Oxidation Behavior of a Laves Phase-Strengthened NiAl-Ta-Cr Alloy for Gas Turbine Applications
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Long-Term Creep and Oxidation Behavior of a Laves Phase-Strengthened NiAl-Ta-Cr Alloy for Gas Turbine Applications Martin Palm and Gerhard Sauthoff Max-Planck-Institut für Eisenforschung GmbH Max-Planck-Str. 1 D-40237 Düsseldorf Germany ABSTRACT A Laves phase strengthened NiAl-Ta-Cr alloy (IP 75) has been developed for structural applications in gas turbines as well as in heat exchangers. It has a lower density and a higher thermal conductivity as well as a higher melting point than conventional superalloys. Different methods for alloy production have been established including investment casting (IC) and a powder metallurgical method (PM). Creep properties have been determined in compression and tension. Tension tests up to 10000 hours were performed at 900 °C and 1000 °C for both PM and IC materials. The oxidation behavior of the PM and IC material was studied for up to 1000 hours by thermogravimetry in air at constant temperature between 600 °C and 1300 °C. The results are compared with results obtained from long-term isothermal and cyclic oxidation experiments in air at 1200 °C for 17900 hours. Microstructures and scales were examined by light optical and scanning microscopy, X-ray powder diffraction and electron probe microanalysis. INTRODUCTION One of the most demanding applications for materials is in gas turbines, where materials have to withstand temperatures above 1000 °C in an oxidizing environment under heavy stresses for long times. For such applications, Ni-base superalloys are used which are restricted to temperatures below 1200°C. For higher temperatures NiAl-base alloys are considered which show not only higher melting temperatures and higher thermal conductivities, but also lower densities than Ni-base superalloys. Current developments include eutectic alloys, oxide-dispersion strengthened alloys and alloys which are strengthened by precipitates of second phases. The two latter concepts should yield a high creep resistance at and above 1000°C, which is a prerequisite for hightemperature applications. Following the concept of strengthening by precipitates a new NiAl-base alloy of nominal composition 45 Ni, 45 Al, 7.5 Cr and 2.5 Ta (always at.%) – IP 75 for short – has been developed [1-4]. Compared to IN 738 LC, a superalloy currently used for gas turbine applications, IP 75 has a much higher melting point of 1638°C (IN 738 LC: < 1315°C), a lower density (6.3 vs. 8.1 g/cm3) and a higher thermal conductivity (57 vs. 27 W/mK at 1000°C), while coefficients of thermal expansion, specific heat capacities and Young´s moduli are of the same order. Though properties for IP 75 have been well established, long-term creep and oxidation data have not yet been available. The present work is directed at clarifying the long-term creep and oxidation behavior at high temperatures in oxidizing environments, which is part of a materials development program in progress. N6.8.1
EXPERIMENTAL Rectangular creep samples of 5 mm x 5 mm x 10 mm for compression tests and “dog-bone” tensile specimens of 4 mm diameter and 35 m
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