Patent Publication Number: US-4728359-A

Title: Method of producing a dispersion-hardened metal alloy

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method of producing dispersion-hardened metal alloys, particularly for high-temperature structural parts of complicated shape, in which particles of a second phase are integrated into a metallic matrix by means of a colloidal suspension. 
     BACKGROUND 
     It is known that, by a uniform, fine distribution of hard particles, and particularly oxidic particles, in metallic alloys, the resistance of the alloys to deformation can be considerably increased, particularly for use in structural parts operating at high temperatures. A survey of the prior art has been published in &#34;Materials Engineering&#34; February 1982, pages 34-39. 
     The main problem in the production of such alloys resides in providing a suitable distribution of the hard particles within the metallic matrix. The distance between the particles must be sufficiently small and uniform and the percentage by volume of the hard particles must be limited. 
     The following known procedure is employed for introducing the hard particles into the metal matrix. 
     A mechanical alloying of the oxide particles and the alloy is effected by grinding the oxide particles with the alloy so that a continuous fusing takes place and granules containing the oxide particles and the alloy are produced. The granules are reduced to powder form in a high-energy mill, ball mill, attritor or the like. The powder contains the oxide particles in the desired degree of fineness and distribution. This method, however, is very difficult to carry out and does not provide sufficient homogeneity and reproducibility. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a method of producing dispersion-hardened metal alloys which, in simple manner, yields homogeneous powders and the desired particle distribution in the matrix, and thus to structural parts capable of resisting high temperatures. 
     This object is achieved by the method comprising: forming a solution of a metal alloy; adding oxide particles which are not reactive with the alloy into said solution, adding a deglomerating agent to said solution so that the particles are suspended therein and form a resultant colloidal solution, atomizing said colloidal solution to form atomized particles, removing liquid solvent from the atomized particles and; reducing said atomized particles to produce metal powder in which the oxide is dispersed in the alloy matrix. 
     The essential advantage of the invention resides in the fact that one obtains a metallic powder with inclusion of oxide particles which has reproducible properties, particularly with respect to uniformity of the distribution of the oxide particles and thus of dispersion hardness. The metal powder obtained in this manner can be formed into compact shaped bodies by known methods of powder metallurgy. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention will be described with reference to the example which follows hereafter in which preferred embodiments of the method are set forth. 
     EXAMPLE 
     A solution of a metal alloy is formed by dissolving the metal alloy in hydrochloric acid. The metal alloy is intended to form the matrix of a metallic powder suitable for the production of a dispersion-hardened metal alloy. The metal alloy is a nickel chromium alloy in which the nickel and chromium are in a ratio of 80:20. Other suitable elements which can be included in the nickel chromium alloy are cobalt, aluminum and titanium. 
     Oxide particles of Y 2  O 3  or ThO 2  of a size of between about 0.01 μm to about 0.1 μm are admixed in the solution of the metal alloy. The particles are added to the solution in an amount such that the particles represent up to 75% by volume of the metal alloy. 
     In order to prevent agglomeration of the oxide particles in the solution, the particles are separated by adsorption of similarly charged molecules, i.e. electrically charged particles of the same polarity, and held in suspension in the solution. For this purpose, deglomerating agents such as trisodium orthophosphate or aluminum nitrate are added to the solution. 
     The colloidal solution or slip thus produced is then atomized in a trickle tower and is either directly transferred into a reaction chamber or is first dried and converted in another reaction vessel into metal powder with oxide inclusions dispersed uniformly therein. 
     The metal powder obtained in this manner can then be worked by known methods of powder metallurgy to form compact bodies of desired shape, for instance, by injection molding, extrusion, extrusion molding, sintering, cold isostatic pressing or hot isostatic pressing. The selection of the method of compacting depends, in particular, on the purpose of use of the final product, namely as a high-temperature structural part. The method determines the shape and size of the final product and provides the desired mechanical and other properties as well as the desired density and surface finish. 
     Modifications of the disclosed method can be effected without departing from the scope of the invention, provided that the same properties of the powder or the structural parts are obtained and the method of particle separation in the colloidal solution is employed. 
     The invention is also not limited to the metal alloy disclosed in the Example and other materials, particularly other nickel base alloys or super alloys, can form the metal matrix. Other elements, as mentioned above, can be employed in the alloy. The solvents for the metallic alloys, the construction of the atomizing equipment and its operating parameters as well as the powder reduction of the atomized particles into metal are known per se. 
     The alloy produced in accordance with the invention is particularly suitable for use in motor and turbine construction, for instance for airplane turbine blades.