Tensile properties of mechanically alloyed/milled Ods-Ni-Based alloys
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I.
INTRODUCTION
OXIDE dispersion strengthened (ODS) superalloys are designed for long-term, high-temperature applications at high stress levels. High creep strengths will be achieved if the ODS alloy contains a fine dispersion of high-temperature stable particles (usually yttria) and if the structure consists of coarse elongated grains. According to theoretical and experimental work, a mean particle distance between 100 and 250 nm of 10 to 20 nm yttria particles and a grain aspect ratio of minimum 10 guarantees good high-temperature properties51-4] Today, ODS superalloys are produced by ball milling of metal and yttria powders, canning, degassing, and powder consolidation either by hot extrusion or by hot isostatic pressingJ 5] The chosen consolidation technique primarily depends on the shape of the desired product; sheet materials are made by hot isostatic pressing and rolling, and rods and wires may be produced by hot extrusion, forging, wire rolling, or drawing. After consolidation, the alloys are worked to semifinishes, which are heat treated after the last working step. The aim of this final heat treatment is a secondary recrystallization process, by which a structure of coarse, highly elongated grains is formed. The core of the ODS processing technology is the milling process, in which metal powders and yttria are ball milled in dry, inert atmospheres. If powder blends of yttria and a single metal component, which might be a pure metal or a prealloyed powder, are milled, a fine and homogeneous dispersion of the yttria particles in the metal matrix is desired. If powder blends of yttria and two or more metal components, which might also be elemental or prealloyed powders, are milled, not only a fine and homogeneous yttria dispersion but also the formation of a solid solution by
J. ZBIRAL, Researcher, is with the Department of Powder Metallurgy, Institute of Chemical Technology and Inorganic Materials, University of Technology Vienna, A-1060 Wien, Austria. Manuscript submitted November 15, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
mechanical alloying is desired.tr] Mechanical alloying is promoted by repeated fracturing, rewelding, and deformation of the powder particles, which creates a fine layered structure, which is continuously refined during further milling. If the interfacial area between the milled metal components is large enough, a solid will be formed by diffusion, which is enhanced by temperature rises due to the dissipation of the kinetic energy of the ball-particle-ball collisions.t7,s,9]
The dispersion of yttria particles not only influences strength but also the recrystallization response of ODS superalloys. Basically, the secondary recrystallization process is driven by the energy of the grain boundaries, while grain growth is hindered by the yttria dispersion. If the driving forces are larger than the backstressing forces, which implies that the primary grain size is below a critical one, secondary recrystallization is possible. But the desired structure of coarse, elongated grains w
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