Metal treatment – Process of modifying or maintaining internal physical...
Reexamination Certificate
2000-03-14
2001-11-13
Mai, Ngoclan (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
C148S514000, C419S029000
Reexamination Certificate
active
06315838
ABSTRACT:
BACKGROUND OF INVENTION
This invention relates generally to thermal treatment of various compositions, materials and/or material systems and, more particularly, to such treatment without external stresses and without phase transformation.
Thermal treatments, both with or without imposed external stresses, are commonly used to densify materials which contain internal pores or cavities. For instance, despite their outstanding creep resistance, oxide-dispersion-strengthened aluminum materials have limited creep ductility due to the formation and subsequent growth and linkage of creep cavities in service.
One approach to extending the creep-life of various engineering components is ex-situ treatments to close creep cavities after some service time. The technical literature contains many examples of the shrinkage and closure of cavities formed during service by isothermal heat treatment with or without superimposed hydrostatic pressure. Although isothermal heat treatment at ambient pressure is simple and inexpensive, the time required to fully close creep cavities is often prohibitive. Hot isostatic pressing (HIP) can more rapidly close porosity, but at an increased cost. Further, studies show that cavity shrinkage is strongly affected by the internal residual stress state, potentially increasing the rate of densification or causing cavity growth rather than shrinkage.
The technical literature provides another approach: densification of polymorphic metal powders by a process of thermal cycling while simultaneously applying an external stress. The related deformation mechanism for densification is referred to as transformation superplasticity, and results in a solid/solid phase transformation upon application of the external stress.
SUMMARY OF THE INVENTION
As apparent from the above, there are a considerable number of limitations in the prior art associated with metal densification. There is a demonstrated need for a general methodology to densify a variety of material systems efficiently, at low relative cost, and without application of external stresses.
Accordingly, it is an object of the present invention to provide a general methodology which can be used with a variety of material systems to reduce internal porosity, thereby overcoming various deficiencies, problems and shortcomings of the prior art, including those outlined above. It will be understood by those skilled in the art that one or more aspects of this invention can meet certain objectives, while one or more other aspects can meet certain other objectives. Each objective may not apply equally, in all instances, to every aspect of the present invention. As such, the following objects can be viewed in the alternative with respect to any one aspect of the present invention.
It can also be an object of present invention to provide a method of treating a material system to increase the relative density of the material at a more rapid rate than can be achieved by isothermal treatment at comparable temperatures.
It can also be an object of the present invention to densify materials cavitated under creep conditions and/or other service conditions.
It can also be an object of the present invention to enhance densification and/or the rate thereof as compared to other treatment methods of the prior art.
It can also be an object of the present invention to provide a general methodology for densification of a wide range of diverse material systems, with the parameters thereof as could be predicted by those skilled in the art with an understanding of the operative densification mechanisms.
Other objects, features, benefits and advantages of the present invention will be apparent in this summary, and the following descriptions and examples, and will be readily apparent to those skilled in the art having knowledge of various densification methods and/or the use of such work material systems of the type described herein. Such objects, features, benefits and advantages will be apparent from the above as taken in conjunction with the accompanying examples, tables, data, and mathematical and physical relationships, and all reasonable inferences to be drawn therefrom.
In part, the present invention includes a method of reducing internal material porosity, with reduction substantially without phase transformation. The inventive method includes: (1) providing an internally porous material; and (2) annealing the material over a temperature range providing activation energy sufficient to induce mass transport within the material, with the temperature changing during annealing. In this manner, the material porosity can be reduced substantially without phase transformation.
In various preferred embodiments of the present invention, the porous material is a polycrystalline single-phase material with anisotropic thermal expansion properties. In various other embodiments, the material is multi-phased, such that the material can include metallic phases, ceramic phases and/or combinations thereof which have different thermal expansion coefficients. In highly preferred embodiments of the present invention, the multi-phase material is a metal and metal oxide composite. A mismatch in thermal expansion can induce internal stress, to enhance densification.
Regardless, preferred embodiments of the present invention can include non-cyclic temperature change, over a range of temperatures, while the porous material is annealed. Alternatively, with equal effect depending on a particular material system, the temperature can change in a cyclic manner, over an indicated range. Whether with cyclic non-cyclic temperature change, the annealing process can be repeated for a time sufficient to density the particular material system. Internal pores and/or cavities can be reduced to the extent desired. In contrast to the prior art, desired densities can be achieved in a more cost efficient and more timely manner.
In part, the present invention is also a method of shrinking cavities introduced to a material system over time under service conditions such as creep. Whether the material is a single- or multi-phased composition, annealing is conducted within a given temperature range dependent upon the material system. The range includes temperatures which induce internal stress between phases of the compositions. Modifying such temperatures during annealing regenerates internal stresses of the type described herein necessary to induce mass transport. Such a method can be used to treat or rejuvenate materials previously stressed under creep conditions or otherwise damaged by adverse environmental factors.
In part, the present invention is also a method of using thermal treatment of the type described herein to enhance the rate of densification of a porous material. The method includes: (1) providing an internally porous material; (2) annealing the material over a temperature range, with the range of temperature inducing stresses internal to the material, and the temperature changing during annealing; and (3) densifying the material, substantially without application of external stress. As described more fully above, such a method can be used to treat a wide variety of material systems. Likewise, annealing can include either cyclic or non-cyclic temperature changes to effect the desired result.
In part, the present invention is also a method of assessing densification and related parameters for a variety of materials and/or material systems. The guidance and direction provided herein would allow one skilled in the art—through straightforward application of the principles, models, concepts and mathematical and physical relationships set forth herein—to determine such parameters and assess the resulting densification of a material/material system. One skilled in the art would expect variations in temperature range, cycle time, rate of temperature change, etc. depending upon a given material or system. Determination of such parameters, however, would be no more involved than efforts otherwise expended by those engaged in such endeavors. Any further experimentation would be n
Dunand David C.
Schuh Christopher
Mai Ngoclan
Northwestern University
Reinhart, Boerner, Van Deuren, Norris & Rieselbach, s.c.
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