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12/29/04

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Page 2087

Influence of Microstructure on Tensile and Creep Properties of a New Castable TiAl-Based Alloy M. GRANGE, J.L. RAVIART, and M. THOMAS Snecma Motors has been working on the development of -TiAl low-pressure turbine blades, including manufacturing optimization, castability evaluation of a selected alloy called G4, and heat-treatment optimization of mechanical and physical properties. The objective of this study was to evaluate microstructure variability regarding casting conditions and aluminum content. The response of cast microstructures to hot isostatic pressing (hipping) and subsequent heat treatments was determined and quantified using tensile and creep testing. Such investigations helped define an optimized heat treatment. Tensile and creep property assessment has shown a high-temperature potential for G4 alloy with respect to other  alloys. The G4 alloy also appears to be more creep resistant than conventional nickel-based superalloys on a specific basis. The enhanced creep properties under the optimized low-temperature treatment are mainly attributed to solid solution strengthening with Re, W, and Si elements and precipitation hardening with primary  phase decorating the primary dendrites.

I. INTRODUCTION

USING gamma titanium aluminides for high-temperature parts offers potential for weight saving in the aircraft industry.[1,2,3] TiAl-based alloys exhibit a low density (3.8 to 4.0 g/cc) combined with a high Young’s modulus, yielding specific mechanical properties equivalent to nickel-based superalloys or steels up to 800 °C.[4,5,6] Such materials are of interest also because of their good oxidation resistance and of their burn resistance when compared to commercial titanium alloys.[7] TiAl-based alloys being developed for commercial use contain a number of alloying additions with the primary aim of improving tensile ductility.[4,5,8] Considerable research activities have been conducted on alloy development (both alloying and heat treatment) to find a proper balance between mechanical properties such as yield stress, ductility, creep, and oxidation resistance.[9–12] In parallel, increased efforts were devoted to select the most suitable manufacturing route for each industrial application (aerospace structural parts, automotive engine parts, turbocharger rotors, etc.). Processes such as casting, forging, or ingot metallurgy were comparatively evaluated for -TiAl. Despite requiring hot isostatic pressing (hipping) (to alleviate structural heterogeneity and to heal shrinkage porosity) and chemical or mechanical methods to achieve final dimensions, casting is still the most cost effective route.[13] In the past 10 years, aeronautic and automotive -TiAl parts have already been manufactured and tested successfully (turbine blades, automobile valves, turbocharger wheel, etc.) mostly by using the casting route.[14,15,16] -TiAl is currently being used for specific automotive engine components (engine valves, turbocharger wheel, etc.).[17] However, factors such as temperature limit