The present invention relates generally to the fabrication of thin-walled articles of tungsten-nickel-iron alloy and more particularly to a method of fabricating such articles wherein the articles are of near theoretical density and possess essentially uniform properties.
Tungsten has proven to be a particularly useful material for various industrial applications and in the construction of nuclear reactors. The tungsten metal can be placed in a somewhat ductile form without seriously detracting from the desirable properties by alloying it with nickel and iron. An alloy composition found to be particularly useful is formed of essentially 95 weight percent tungsten, 3.5 weight percent nickel and 1.5 weight percent iron. However, alloy compositions with a nickel concentration in a range of about 2.1 to 7.0 weight percent and an iron concentration in a range of about 0.9 to 3.0 weight percent iron have proven to be useful in essentially the same areas as the 3.5 weight percent nickel and 1.5 weight percent iron compositions. Structures of the ductile tungsten-nickel-iron alloys are generally prepared by employing conventional powder metallurgical procedures. These procedures usually comprise the blending desired weight percents of elemental tungsten, nickel, and iron alloy powders, pressing the blended powders into a compact of the desired configuration and thereafter sintering the compact at a sufficiently high temperature to convert the minor phase of the alloy into a liquid to provide structural integrity to the sintered structure. Final densification, microstructure and properties of the structure are dependent upon the pressing and liquid phase sintering operation.
While tungsten-nickel-iron alloy structures of various configurations have been successfully fabricated by cold pressing and sintering the resulting compact, there have been some difficulties associated with the fabrication of structures having relatively thin walls, such as open cylinders and the like, because of the required liquid phase sintering operation. More specifically, the pressed compacts shrink approximately 40 volume percent during the liquid phase sintering step so that the thin-walled articles are usually subjected to extensive distortion and stresses which lead to deleterious cracking and other stress associated problems. Some techniques have been employed to overcome the distortion and cracking problems such as the use of multiple sintering steps and special sintering mandrels. However, even with such techniques, uniform, physical and mechanical properties are seldom achieved. For example, the density and tensile strength of the sintered structure usually vary by greater than 1 percent and 4 percent, respectively, over the structure.