1. Field of the Invention
This invention relates to the field of metallurgy. More specifically, the invention comprises a method for achieving accelerated age hardening in superalloys made of nickel, chromium, and molybdenum by the addition of rhenium. The invention allows a greatly accelerated age-hardening process, while substantially reducing the risk of over-aging.
2. Description of the Related Art
Age hardening (also known as “precipitation hardening”) is used to produce various alloys with desirable properties. The process is used to mechanically strengthen malleable materials for structural applications. In addition to steels, precipitation hardening is commonly used for aluminum, titanium, and nickel alloys. The process produces fine particles of impurity phases, which act as barriers to the motion of crystallographic lattice dislocations.
Precipitation in solids can produce many different sizes of particles, which have radically different strengthening effects as demonstrated by the following equation:Δσ=2/(πE)−1/2(λapb/b)1/2r1/2f1/2 where E is the Young's modulus, λapb is the anti-phase interfacial energy, b is the Burger's vector, r is the size of the precipitates, and f is the volume fraction. Both r and f can be related to l, the distance between the precipitates. If the volume fraction is held constant, then one observes an optimized value for the size of the precipitates (r) at which the material reaches a maximum strength.
The optimal size of the precipitates formed depends upon the thermo-mechanical history of the alloy being hardened. In the prior art, alloys must be kept at elevated temperature for several hours to allow precipitation to take place. Thus, conventional precipitation hardening requires a substantial amount of energy (The large amount of time required is why the process is also referred to as “age hardening”).
On the other hand, if the process and alloys are altered so that the precipitates can form in a relatively short period of time, the temporal window for achieving an optimal result usually becomes very narrow. It is then easy to “over-age” the alloy. When a material is over-aged (held at the elevated temperature for too long), then both the size of the precipitates and the distance between the precipitates become too large and the Orowan process operates. At certain values for l and for r, the strength or hardness drops significantly to a value governed by a rule-of-mixture.
An example of a prior art nickel alloy that can be age hardened quickly is IN738LC. This is a nickel based alloy that can be age-hardened in less than 5 minutes at 850° C. Optimum hardness is obtained in about 80 seconds. On the other hand, the hardness will be substantially reduced if the process is carried forward for an additional 40 seconds. In fact, the window of effective age-hardening for this alloy is only about 60 seconds.
One may generally slate that the prior art discloses: (1) nickel alloys that can be age-hardened using a process that takes several hours and that are not very sensitive to over-aging (extending the process for an additional 10 hours or more does not significantly reduce the hardness), and (2) nickel alloys that have been altered to age harden very quickly, but which are very sensitive to over-aging (suffering reduced hardness if the aging window is inadvertently extended by as little as 40 seconds). A more useful nickel alloy would be one which (1) age hardens quickly, and (2) is not very sensitive to over aging.
The prior art also discloses accelerating the formation of precipitates in age-hardening by deforming the materials in order to increase the dislocation densities (which enhances the diffusion along the dislocation). In selected alloys, it is in fact essential to deform the alloy before the age-hardening process is applied. Unfortunately, deformation processes are also energy-intensive and therefore expensive. This approach does not represent the desired overall reduction in the amount of energy required for hardening.
The present invention uses a master alloy of nickel, molybdenum, and chromium (Ni—Mo—Cr). The inventors have discovered that the addition of rhenium to this master alloy in the right ratios and under the right conditions produces an unexpected and highly advantageous alteration in the alloy's age-hardening properties. As explained in detail in the descriptive sections to follow, the hardening properties found in the inventive composition and process result from the formation of long-range-ordered (“LRO”) precipitates of Ni2(Mo, Cr, Re). The prior art discloses various combinations of the elements, but fails to disclose or suggest the inventive process.
For example, U.S. Pat. No. 4,119,458 to Moore teaches alloys of nickel, chromium, and rhenium. Molybdenum is also disclosed in Moore, though the implied percentage of molybdenum is less than 8% by weight. The master alloy in Moore contains nickel, aluminum, vanadium, and cobalt. The Moore invention is directed to solving the problem of reaction between the molten metal and the crucible surrounding it during a re-melting process in order to form a regular secondary eutectic reaction. Moore does not teach age-hardening and in fact the compositions disclosed in Moore are not able to achieve the performance of the present invention since they do not contain enough Mo-like elements to form Ni2Mo-ordered precipitates.
Another example from the prior art is the article “Comparative Corrosion Behavior of Ni—Mo and Ni—Mo—Cr Alloy for Applications in Reducing Environments,” published in the Journal of Material Science, 2006, 41, 8359-8362 (written by Tawancy). The Tawancy article teaches the addition of chromium to enhance corrosion resistance by the delay of Ni4Mo precipitates. It does not suggest the inventive formulation or process related to age-hardening.
In summary, the prior art fails to disclose a Ni—Mo—Cr alloy that can be age-hardened rapidly while displaying resistance to over-aging. The present invention provides a precipitation hardening process which can be completed more rapidly than the known prior art, and which has a relatively broad time window for optimal results. The present invention achieves these results without requiring the use of mechanical deformation.