High Tempeature Ordered Compounds for Advanced AERO-Propulsion Applications
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HIGH TEMPE§ATURE ORDERED Q9MPOUNDS FOR ADVANCED AEPO-PROPULSION APPLICATIONS and D.M. SHAH D.L. ANTON * United Technologies Research Center, E. Hartford, CT, ** Pratt & Whitney, E. Hartford, CT ABSTRACT Advanced aero-propulsion engine designs now being considered for implementation require structural materials with high temperature strength and creep properties above 1300"C, in excess of the range where nickel base superalloys are now being used. A number of ordered single phase compounds having melting points above 1500"C have been identified which both hold promise for engine applications and are representative of a number of different crystal structures such as B2, C15 (Laves), A15, C1 and DO19. Elevated temperature characterization has been conducted on these compounds which includes ductile/brittle transition temperature determination, minimum creep rate analysis, elastic modulus, tensile strength and cyclic oxidation testing. The results of these tests have led to insight in the processing methods required for these compounds and selection of compounds based on elemental constituents and crystal structure.
INTRODUCTION The need for ever increasing gas turbine combustion temperatures for enhanced efficiency and performance will necessitate both static and rotating materials that retain dimensional stability and resistance to surface attack at temperatures exceeding current Ni base superalloys; =1O0"C. A number of intermetallic compounds have been identified as potential gas turbine engine materials [1,2] and their elevated temperature properties are currently being evaluated. This list of candidate compounds is quite long and being composed of the transition, refractory, precious, alkali, rare earth and semi-metals. Additionally their crystal structures range from the complex cubic structures such as A12, A15 and Cl; the Laves C15, C14 and C36; the Sigma like phases of D~b, D85 and D8m to the Ll like phases such as DO and DO29 . Many of these compounds wih be found to be unsuital1e for eievated temperature applications, but with this variety of composition, structure and bonding, our understanding of these alloys will be enhanced immeasurably from our current limited understanding of crystal structure and often erroneous phase diagrams. The properties of most interest to aero-engine and alloy designers, are the ductility or fracture toughness, tensile strength, creep strength as functions of temperature, elastic modulus, and oxidation resistance. Tensile loading, as can be obtained simply in bend testing yields modulus, strength, the ductile brittle transition temperature (DBTT) and fracture characterization. Steady state creep rate analysis has been shown to yield great insight to the high temperature deformation mechanisms. Through compressive creep testing, many of these alloys can be tested efficiently using both temperature and stress as independent variables. Finally cyclic oxidation tests in conjunction with X-ray analysis of oxides rapidly identify those compounds which maintain adherent oxides throug