Nickel-based superalloy development

8.4.2 Nickel-based superalloy development

A major application of superalloys is in turbine materials, jet engines, both disk and blades. Initial disk alloys were Inco 718 and Inco 901 produced by conventional casting ingot, forged billet and forged disk route. These alloys were developed from austenitic steels, which are still used in industrial turbines, but were later replaced by Waspaloy and Astroloy as stress and temperature requirements increased. These alloys (Table 8.4) were turbine blade alloys with a suitably modified heat treatment for disks. However, blade material is designed for creep, whereas disk material requires tensile strength

Advanced alloys 461 Table 8.4 Composition of some Ni-based Superalloys

Composition (wt%)

B Others Waspaloy Balance

Ni-Alloy Cr

– Fe 20 Nb 5.2 Inco 901

– Bal. Fe

coupled with low cycle fatigue life to cope with the stress changes in the flight cycle. To meet these requirements Waspaloy (Table 8.4) was thermomechanically processed (TMP) to give a fine grain size and a 40% increase in tensile strength over the corresponding blade material, but at the expense of creep life. Similar improvements for disks have been produced in Inco 901 by TMP. More highly alloyed nickel-based disks suffer from excessive ingot segregation, which makes grain size control difficult. Further development led to alloys produced by powder processing by gas atomization of

a molten stream of metal in an inert argon atmosphere and consolidating the resultant powder by HIPing to near-net shape. Such products are limited in stress application because of inclusions in the powder and, hence, to realize the maximum advantage of this process it is necessary to produce ‘superclean’ material by electron beam or plasma melting.

Improvements in turbine materials were initially developed by increasing the volume fraction of γ ′

in changing Nimonic 80A up to Nimonic 115. Unfortunately, increasing the (Ti +Al) content lowers the melting point, thereby narrowing the forging range, which makes processing more difficult. Improved high-temperature oxidation and hot corrosion performance has led to the introduction of aluminide and overlay coatings, and subsequently the development of IN 738 and IN 939 with much improved hot-corrosion resistance.

Further improvements in superalloys have depended on alternative manufacturing routes, particu- larly using modern casting technology. Vacuum casting was first used to retain high Ti and Al contents without oxidation loss. With 9–11% (Ti +Al), a 70% volume fraction of γ ′ has been produced in IN 100 (Nimocast PK 24) which does not require supplementary solid solution strengtheners and therefore gives a saving in density. 1

Additions of high melting point elements such as W extend the high-temperature capabilities at the expense of density. M 200 contains 12% W and 1% Nb but has limited ductility around 760 ◦ C, which can be improved by additions of hafnium. The significant improvement in ductility and reduced porosity produced by Hf has led to its addition to other alloys (e.g. Mar 001 (IN 100 + Hf) and Mar 004 (IN 713 + Hf) and M 002, which contains 10% W, 2.5% Ta and 1.5% Hf).

Creep failures are known to initiate at transverse grain boundaries and, hence, it is reasonable to aim to eliminate them in the turbine blade to gain further improvement in performance. Technologically this was achieved by directional solidified (DS) castings with columnar grains aligned along the growth direction with no grain boundaries normal to that direction. By incorporating a geometric constriction into the mold or by the use of a seed crystal it has been possible to eliminate grain boundaries entirely and grow the blade as one single crystal (see Chapter 2).

The elimination of grain boundaries immediately removes the necessity for adding grain-boundary strengthening elements, such as C, B, Zr and Hf, to the superalloy. The removal of such elements raises the melting point and allows a higher solution heat-treatment temperature with consequent improvement in chemical homogeneity and more uniform distribution of γ ′ precipitates. Particularly

1 A Pratt and Whitney version of IN 100 (B1900) replaced Ti with Ta to improve the castability.

462 Physical Metallurgy and Advanced Materials important, however, is the control of the growth direction along the [1 0 0] direction. The [1 0 0]

alignment along the axis of the blade gives rise to an intrinsic high creep which enables thermal stresses caused by temperature gradients across the blade to be minimized.

Single-crystal blades have now been used successfully for both civil and military engines, SSR99 replacing Mar M002 but with improved tensile, creep and fatigue properties and a lower-density alloy RR2000 replacing IN 100.