SUMMARY Nearly 70 years ago, the first dispersion-strengthened ...

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INTRODUCTIONFor the purpose of this report, dispersion strengthening is defined as the process of strengthening a metalor an alloy by incorporating a fine insoluble phase dispersed uniformly throughout the matrix of theparent metal or dispersant. The dispersoid is usually present in small amounts, e.g., less than 10 volumepercent, and is stable (i.e., inert) at tempel'1atures approaching the melting point of the matrix. Thedispersoids are not coherent with the matrix and are so small that electron-microscopy techniques may berequired to identify them. This report does not cover precipitation hardening, which is a form ofdispersion strengthening in which the precipitated particles usually redissolve in the matrix at hightemperatures. The precipitated particles mayor may not be coherent with the matrix.The dispersion strengthening of metals has been an active field of research and development for the lasttwo or three decades. Much work has been done with many materials. For example, a cursory check ofthe literature reveals that dispersion strengthening has been explored on over 20 of the most widely usedindustrial metals. Moreover, for some metals like aluminum, copper, and nickel, several hundredreports and publications are available. Obviously, an in-depth analysis covering all of the work done onall of these metals would be a formidable task.As a compromise, this report presents an overview of the major developments which include the dispersionhardening of 21 metals on both commercial and experimental levels. Properties obtained recently(primarily within the past 10 yearst or from difficult-to-obtain sources, have been given preferencealong with work not covered in previous reviews. This report also identifies the most recentreviews which have been prepared for each of these metals. Each section has its own reference list toassist the reader interested only in particular metals. Finally, for copper and nickel, a bibliographyof added references, not cited in the overview, has been provided as an additional information source.The units of measurement in the report are those used by the original authors. However, in order toderive some consistency, where English units were used in the original, 51 units have been added inparentheses in the text of this report. This has also been done in the captions of figures and tables. Intables, where columns of stress data appear in English units or in kg/mm 2 , a column showing megapascals(MPa or MN/m 2 ) has been added. Factors for these conversions are: 1 kg/mm 2 (or kp/mm 2 ) x9.8067 = 1 MPa and 1 ksi x 6.89 = 1 MPa. Units on figures have been left as they appeared in theoriginal.
BACKGROUND INFORMATIONWhat we now designate as "dispersion-strengthened metals" probably originated with the developmentof "ductile tungsten". (1) Ductile tungsten lamp filaments were developed more than 65 years ago at theGeneral Electric Laboratories by C. G. Fink and W. D. Coolidge.(2) The 1913 patent(3) on the processmentioned a product "containing additional material which will prevent coarse crystallization of thetungsten at high temperatures". Shortly thereafter Fink (4) indicated that "large quantities of drawnwire, flexible and strong, are being daily produced and used as filaments in incandescent lamps". Healso suggested further applications.The effect of Th02 additions to tungsten was studied by Zay Jeffries. (5) He observed that theseadditions retarded grain growth at high temperatures and consequently resulted in longer lives for thefilaments. At that time, thoriated tungsten wires containing about 0.7 percent Th02 were mainlyused for incandescent lamps. Wires for cathodes usually contained 1.5 percent Th02. Additions ofTh02 were limited to a maximum of about 2 percent since it was difficult to work tungsten with higherTh02 contents. These alloys were produced from a mixture of W03 and an aqueous solution ofTh(I\J03)4 which was dried, pulverized and sifted. The W03 was reduced to tungsten by sintering themixture in hydrogen. The Th02 remained stable through the sintering process. However, Fink found(6)that if rods of ductile tungsten were heated high enough in an oxygen-free atmosphere, the tungstenand thorium alloyed.In 1930, Smith(7) proposed internal oxidation of powders as a means of dispersion strengthening. Theprocess was later applied by Rhines(8) and Meijering(9) using copper and silver alloys and by De Jong(10)on beryllium and copper alloys.The use of an inert oxide as a dispersoid was not realized until later, as reported in a historical reviewby Goetze!. (11) He discusses what may have been forerunners of dispersion-strengthened aluminumgoing back to 1940 and possibly even to a report by Masing in 1909. In 1942, a patent was issued ondispersion-strengthened (OS) Pt-Th02(12), as a logical extrapolation of techniques used on DS-W-Th02for the lamp industry.Around 1946, Irman(13) developed an Al203-strengthened aluminum named SAP, an acronym foreither "sintered aluminum powder" or "sintered aluminum product". SAP is a registered trade name ofAluminum Industrie A.G. of Neuhausen a. Rheinfall, Switzerland. The manufacturing process for SAPconsists primarily of preparing the aluminum powder by a grinding process in a special atmosphere toproduce an oxide film about 100 Angstroms (0.01 micron) thick on the surface of the powder particles.Further dry grinding breaks up the oxide-coated particles to produce a powder consisting of isometricparticles that are not only oxidized on the surface, but also contain fine oxide particles in the interior.After compacting, sintering, and hot working, the resultant material contains a fine dispersion of verysmall particles of A1203' This SAP process was the first of the mechanical, or nonchemical, processesused successfully to provide a uniform dispersion of very small particles of oxide in a metal. Manyinvestigators then decided to try adding other refractory oxides, to obtain a similar effect. The process,of course, is confined to those metals or alloys that form stable oxides that will serve as dispersoids.The dispersion of AI203 conferred strength to the SAP system up to the melting point of the aluminummatrix. SAP was the first dispersion-strengthened material designed as a structural load-bearing system.Considerable research has been done by companies in the U.S. and elsewhere to further improve thismaterial since it first appeared on the market. SAP alloys, however, have not found widespread usage.* References are listed at the end of this section and each section in the report.2
Page 1: SUMMARYNearly 70 years ago, the fir
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SUMMARY Nearly 70 years ago, the first dispersion-strengthened ...

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