Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1994-06-20
2001-05-15
Kastler, Scott (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S421000
Reexamination Certificate
active
06231699
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the production of alloys, and, more particularly, to the preparation and heat treatment of alloys of the gamma titanium aluminide type.
Titanium aluminides are a class of alloys whose compositions include at least titanium and aluminum, and typically some additional alloying elements such as chromium, niobium, vanadium, tantalum, manganese, and boron. The gamma titanium aluminides are based on the gamma phase found at nearly the equiatomic composition, with roughly 50 atomic percent each of titanium and aluminum, or slightly reduced amounts to permit the use of other alloying elements. The titanium aluminides, and particularly the gamma titanium aluminides, have the advantages of low density, good low and intermediate temperature strength and cyclic deformation resistance, and good environmental resistance.
Gamma titanium aluminides have application in aircraft engines. They can potentially be used in applications such as low-pressure turbine blades and vanes, bearing supports, compressor casings, high pressure and low pressure hangars, frames, and low pressure turbine brush seal supports.
One area of continuing concern in the titanium aluminides, and particularly the gamma titanium aluminides, is their low-to-moderate levels of ductility. Ductility is the measure of how far a material can elongate before it fails, and is linked to other properties such as fracture resistance. The gamma titanium aluminides elongate only 1-4 percent at most prior to failure, and have a steeply rising stress-strain curve. Maintaining the strength and resistance of the material to premature failure are therefore highly dependent upon controlling the alloy ductility.
In this context of low-to-moderate ductilities, a key consideration is the ability to achieve and maintain some known minimum ductility. The development of specifications for the use of the material normally start with some minimum property that can be reached on a consistent basis, and then a safety factor is applied. Consistency and controllability become important considerations, and are not easily achieved due to the relatively low ductilities experienced in these materials.
There is therefore a need for an approach to achieving a minimum set of tensile properties, including as high a consistent elongation to failure as possible, in gamma titanium aluminide alloys. The approach must permit those properties to be achieved consistently and controllably in the alloys of interest. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides a processing and heat treatment procedure for gamma titanium aluminide alloys. This approach achieves ductilities of about 1-4 percent in these materials, which are comparable with the best elongations achieved by other techniques. Moreover, the processing of the invention permits the consistent development of specified minimum ductilities of at least about 1 percent.
In accordance with the invention, a method of producing a gamma titanium aluminide alloy article comprises the steps of providing a piece of gamma titanium aluminide alloy having a composition capable of forming alpha, alpha-2, and gamma phases and determining the alpha transus temperature of the gamma titanium aluminide alloy piece. The method further includes consolidating the gamma titanium aluminide alloy piece at elevated temperature to reduce porosity therein, and heat treating the piece at a temperature of from about 5 F. to about 300 F. below the alpha transus temperature for a time sufficient to generate a refined microstructure comprising from about 10 to about 90 volume percent gamma phase.
The consolidation preferably is accomplished by hot isothermal pressing at a temperature below the alpha transus temperature and the heat treatment temperature. More particularly, and further in accordance with the invention, the consolidating is accomplished by hot isostatic pressing the gamma titanium aluminide alloy piece at a temperature of from about 50 F. to about 250 F. below the alpha transus temperature and at a pressure of from about 20,000 to about 30,000 pounds per square inch, for a time of from about 1 to about 20 hours. The heat treating is preferably conducted at a temperature of from about 45 F. to about 200 F. above the temperature of the step of hot isostatic pressing.
A key to the present invention is the recognition of the high degree of variability among gamma titanium aluminide alloys, even those meeting narrow compositional standards. The alpha transus temperature of gamma titanium aluminide alloys varies strongly with changes in the aluminum content of the alloy. In turn, it is difficult to control the aluminum content of the alloy exactly in the melting operation. The result is that it is difficult to prescribe the aluminum content of the gamma titanium aluminide precisely, with certain knowledge that the prescribed aluminum content will be present in the final product. Prior processing techniques have not recognized this variability, resulting in a lack of consistency in the final product.
The present approach is designed to improve the properties of the final product, and improve the certainty of achieving the improved results. Two steps are taken with these characteristics in mind. First, the alpha transus temperature of the alloy is determined for each lot of material. The alpha transus temperature may be determined by any appropriate approach, with direct measurement of the transformation temperature preferred. Second, the subsequent processing temperatures are selected in relation to the determined alpha transus temperature, not in terms of a fixed temperature or temperature range to be applied in all cases. The result is both improved properties and improved consistency of these improved properties in the final product.
The present invention provides an important advance in the art of gamma titanium aluminide processing. The approach of the invention tailors the processing to the actual alloy processed and not to an intended or prescribed alloy, which may not be reached in the actual alloy processed. It is conventional metallurgical practice to select processing parameters in relation to the prescribed alloy, as in most instances the variability is not great. That approach is simply not sufficient in the present processing environment.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.
REFERENCES:
patent: 5205875 (1993-04-01), Huang
patent: 5226985 (1993-07-01), Kim et al.
patent: 5350466 (1994-09-01), Larsen, Jr. et al.
Austin Curtiss M.
Kelly Thomas J.
Larson, Jr. Donald E.
London Blair
Weimer Michael J.
General Electric Company
Hess Andrew C.
Kastler Scott
Narciso David L.
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