Heat treatment of alloys having elements for improving grain boundary strength

The present invention relates to a heat treatment of alloys, especially nickel base superalloy and, more particularly, to castings having a columnar grain microstructure.

U.S. Pat. No. 4,597,809 describes single crystal castings made from a nickel base superalloy having a matrix with a composition consisting essentially of, in weight %, of 9.5% to 14% Cr, 7% to 11% Co, 1% to 2.5% Mo, 3% to 6% W, 1% to 4% Ta, 3% to 4% Al, 3% to 5% Ti, 6.5% to 8% Al+Ti, 0% to 1% Nb, and balance essentially nickel with the matrix containing about 0.4 to about 1.5 volume of a phase based an tantalum carbide as a result of the inclusion in the alloy of about 0.05% to about 0.15% C and extra Ta in an amount equal to 1 to 17 times the C content.

Single crystal castings produced from the aforementioned nickel base superalloy exhibit inadequate transverse grain boundary strength. The present inventors attempted to produce directionally solidified (DS) columnar grain castings of the nickel base superalloy. However, the directionally solidified (DS) columnar grain castings produced were unacceptable as DS castings as a result of the castings exhibiting essentially no transverse grain boundary strength and no ductility when tested at a temperature of 750 degrees C. (1382 degrees F.) and stress of 660 MPa (95.7 Ksi). The transverse grain boundary strength and ductility were so deficient as to render DS columnar grain castings produced from the aforementioned nickel base superalloy unsuitable for use as turbine blades of gas turbine engines.

WO 99/67435 discloses nickel base superalloy castings having boron added to improve transverse stress rupture strength and ductility of DS castings. The castings are heat treated at 1250° C. for 4 h so that a fully solution of the secondary phase (γ′-phase) is performed. Due to the occurrence of grain boundary cracks after the fully solution heat treatment the producibility is so deficient as to render DS columnar grain castings produced from the aforementioned nickel base superalloy unsuitable for use as turbine blades of gas turbine engines.

An object of the present invention is to provide a heat treatment of alloys, especially of as-cast alloys, e.g. DS columnar grain castings based on the aforementioned single crystal nickel base superalloy, having substantially improved transverse stress rupture strength and ductility as well as producibility to an extent that the DS castings are acceptable for use as high temperature applications such as turbine blades of a gas turbine engine.

The present invention involves a heat treatment of cast alloys, such as superalloys, having at least one addition, which improves grain boundary strength such as boron in the nickel base superalloy described here above in a manner discovered to significantly improve transverse stress rupture strength and ductility of directionally solidified (DS) columnar grain castings produced with a heat treatment, which solves a secondary phase only partly, e.g. no fully solution heat treatment is performed.

Boron is often added to superalloy compositions in an effective amount to substantially improve transverse stress rupture strength and ductility of directionally solidified columnar grain castings produced from the boron-modified superalloy. The boron concentration preferably is controlled in the range of about 0.003% to about 0.0175% by weight of the superalloy composition to this end.

In conjunction with addition of boron to the superalloy composition, the carbon concentration preferably is controlled in the range of about 0.05% to about 0.11% by weight of the superalloy composition.

A preferred nickel base superalloy in accordance with an embodiment of the present invention consists essentially of, in weight %, of about 11.6% to 12.70% Cr, about 8.50% to 9.5% Co, about 1.65% to 2.15% Mo, about 3.5% to 4.10% W, about 4.80% to 5.20% Ta, about 3.40% to 3.80% Al, about 3.9% to 4.25% Ti, about 0.05% to 0.11% C, about 0.003% to 0.0175% B, and balance essentially Ni. The boron modified nickel base superalloy can be cast as DS columnar grain castings pursuant to conventional DS casting techniques such as the well known Bridgman mould withdrawal technique.

DS castings produced in this manner typically have a plurality of columnar grains extending in the direction of the principal stress axis of the casting with the <001> crystal axis generally parallel to the principal stress axis. DS columnar grain castings pursuant to the present invention preferably exhibit a stress rupture life of at least about 100 hours and elongation of at least about 2.5% when tested at a temperature of 750 degrees C. (1382 degrees F.) and stress of 660 MPa (95.7 Ksi) and will find use as turbine blades, vanes, outer air seals and other components of a industrial and aero gas turbine engines.

The above objects and advantages of the present invention will become more readily apparent form the following detailed description taken with the following drawings.

Exemplarily as alloy a nickel base superalloy is chosen which consists essentially of, in weight %, of about 9.5% to 14% Cr, about 7% to 11% Co, about 1% to 2.5% Mo, about 3% to 6% W, about 1% to 6% Ta, about 3$ to 4% Al, about 3% to 5% Ti, about 0% to 1% Nb, and balance essentially Ni and B present in an amount effective to substantially improve transverse stress rupture strength of a DS casting as compared to a similar casting without boron present.

The inclusion of boron, as an addition, which improves the grain boundary strength in the alloy, is chosen in an amount discovered effective to provide substantial transverse stress rupture strength and ductility of a DS columnar grain casting produced from the alloy as compared to a similar casting without boron present.

Preferably, the nickel base superalloy is modified by the inclusion of boron B in the range of about 0.003% to about 0.0175%, preferably 0.010% to 0.015%, by weight of the superalloy composition to this end.

In conjunction with addition of boron to the superalloy composition, the carbon C concentration is controlled in a preferred range of about 0.05% to about 0.11% by weight of the superalloy composition. Also Silizium Si, Zirkonium Zr and Hafnium Hf can be used as addition.

Furthermore all combinations of B, C, Si, Zr, Hf are possible.

The transverse stress rupture strength and ductility as well as the producibility of DS castings produced from nickel base superalloy with the modified heat treatment are provided to an extent that the castings are rendered acceptable for use as turbine blades and other components of gas turbine engines.