Alloys or metallic compositions – Nickel base – Chromium containing
Patent
1992-09-29
1994-07-19
Dean, Richard
Alloys or metallic compositions
Nickel base
Chromium containing
148410, C22C 1905
Patent
active
053307119
DESCRIPTION:
BRIEF SUMMARY
This invention relates in a first aspect to a nickel base alloy suitable for making castings and in a second aspect to a casting made from such an alloy. The invention relates in particular to a high strength, weldable casting alloy, having superior stress rupture, tensile and fatigue properties.
BACKGROUND OF THE INVENTION
Cast nickel-base alloys and in particular the so-called nickel-base superalloys have been widely used in applications where resistance to high temperatures is required. Such applications are largely found in the hotter parts of gas turbine engines, in particular vanes and blades in aircraft engines. Superalloy castings have also been favoured for lower temperature (c. 600.degree. C.) applications for static structural parts such as casings, compressor and turbine exit guide vanes and bearing housings. For such applications, in addition to good creep resistance, weldability, fatigue resistance and low thermal expansion properties are required.
The compositions of such superalloys are chosen to meet specific engine requirements, and it is generally recognized that improvement in one property of a superalloy is usually at the expense of one or more other properties. For instance, it is difficult to make a nickel-base superalloy possessing good casting and welding properties whilst at the same time exhibiting high tensile strength and creep resistance.
Alloying elements in nickel-base superalloys have various roles, which may be summarised as follows.
Typically, nickel-base superalloys consist of the following phases:
1) Gamma matrix phase. This is typically high in nickel, chromium, cobalt, tungsten, and molybdenum. Rhenium and ruthenium may also be present in some applications. Nickel, cobalt, chromium, tungsten, molybdenum, and rhenium all affect the properties of the superalloy matrix.
2) Gamma prime precipitate strengthening phase. This is typically high in nickel, aluminum, titanium, niobium, tantalum, and vanadium. Some chromium and cobalt will be present. Hafnium will be present in the gamma prime phase in alloys that contain hafnium. The properties of the gamma prime phase are affected by the presence of these elements.
The gamma matrix is hardened by large, heavy, refractory elements (e.g. tungsten, molybdenum, rhenium) which distort the crystal structure--i.e. solid solution strengthening. The limits of addition of these elements is indicated by the onset of phase instability, where embrittling phases occur. This limit is predicted by a phase computation procedure which is known in the prior art whereby freedom from formation of embrittling phases is predicted if the composition has a low calculated value of the average electron vacancy number (Nv) of the matrix. Such refractory elements also slow down chemical diffusion which is beneficial for weldability and in controlling creep.
The gamma prime precipitate is hardened by the elemental content. The important feature of the precipitate is that it imparts strength to the matrix. The strength of the structure is a function of the amount of precipitate present, its size and shape distribution, and the stability of the structure in service. All of these factors are affected by the chemical balance.
Grain boundaries are strengthened by the presence of carbon, boron, hafnium and zirconium, and carbides such as those of chromium, tungsten, molybdenum, titanium, tantalum, niobium, vanadium, and hafnium.
It is desirable for good castability of a superalloy that it has a moderate freezing range of about 80.degree. C. to give low porosity. Low boron, zirconium, and carbon content gives hot tear and weld fissure resistance. A low carbide content during solidification gives low porosity.
Good weldability of a superalloy is indicated by a low aluminum/titanium ratio and low aluminum plus titanium total contents since this gives a low gamma prime volume fraction producing a weaker, more ductile alloy which is better able to accomodate the stresses produced during the weld thermal cycle. However, alloys of this nature are often weak and not
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Dean Richard
Phipps Margery S.
Rolls-Royce plc
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