AD730™ - A new nickel-based superalloy for high temperature engine rotative parts

Abstract

The enhancement of efficiency in gas turbine engines requires the development of new superalloys capable of withstanding higher temperatures. The development of new industrial cast and wrought (C&W) disk alloys with required combination of strength, creep and fatigue properties at 700°C is highly desired due to the expensive cost of powder metallurgy. AD730, which is the newly nickel base superalloys developed by Aubert & Duval, was therefore designed to offer a better combination between high temperature properties at 700°C and cost compared to other C&W superalloys. This paper describes the alloy design based upon the chemistries of the previous experimental alloys Ni30 and Ni33 [1-2]. The control of expensive elements contents and the presence of iron in AD730™ alloy confer to this alloy an attractive cost compared to other C&W superalloys for disk applications. Gamma prime solvus was decreased compared to Ni33 in order to improve hot workability and (Ti+Nb)/Al ratio was decreased compared to Ni30 and Ni33 in order to avoid any risk of Eta-phase precipitation. It was actually observed that the precipitation of the needle-shape Eta-phase predicted by the thermodynamic databases was not in agreement with experimental results obtained on various alloys of the AD730™ chemical system. Industrial ingots with a diameter equal to 500mm were produced (Vacuum melting and remelting) and converted to evaluate the mechanical properties and the ability for the conventional C&W route. A special attention is made in this paper to the AD730™ workability which was highly evaluated with various industrial forging process routes (close-die forging, ring-rolling⋯etc). Heat treatment optimization was then performed on this alloy in regard to tensile and creep properties. The effect of solution heat-treatment temperature and cooling rate after solution heat treatment were investigated on AD730™. Solution heat treatment temperature has a slight effect on the tensile strength if the temperature is lower than the gamma prime solvus. Yield strength remains stable and close to 1100MPa at 700°C. Solution heat-treatment was therefore optimized in regard of grain size in order to increase creep properties. As most of superalloys strengthened by gamma prime phase, cooling rate after solution heat-treatment has to be as fast as possible to get the highest tensile and creep properties. Oil quenching can be easily performed on AD730 without any issues due to the moderate gamma prime content in the alloy (35-40%) and the fine grain size. Tensile, creep, long-term aging performed on a forged disk heat-treated in optimized conditions, are presented and discussed in this paper. A comparison with Udimet720™ properties and 718Plus™ ones show that AD730™ alloy presents a higher combination between cost and mechanical properties at 700°C than current C&W superalloys.