Hip

72 lb

Finished Part 11 lb

Fig. 6.11. Material and Fabrication Savings with Powder Metallurgy1

Near Net Shaped Part 40 lb

40 lb

Fig. 6.11. Material and Fabrication Savings with Powder Metallurgy1

Nickel 64%

Chromium

Yttrium Oxidey 1%

Ball Mill

Rotating Impeller

Ball Mill

Steel Container Heated

Steel Container Heated

Mechanically

^ Finish g Machine

Mechanically

Alloyed P°wder Extrude Hot Roll at Heat Treat rx . n .. 1700-2000° F at 2400° F

Steel Ball

Bearings

Fig. 6.12. Mechanical Alloying Powder Metallurgy Process1

On subsequent impacts, the clean surfaces are welded together. This cold welding process increases the size of the particles, while at the same time additional impacts are fracturing particles and reducing their size. As the process continues, the microstructure of the particles is continually being refined. Very fine yttria (25 nm), which is added to the mixture, becomes entrapped between fragments of the composite metal powders. Milling is continued until every powder particle contains the same composition as the starting mix.

Powders for mechanical alloying include pure metals, master alloys and refractory compounds. Pure metals include nickel, iron, chromium, cobalt, tungsten, molybdenum and niobium, while master alloys are nickel based alloys with large concentrations of the more reactive metals aluminum, titanium, zirconium and hafnium. About 2% volume of fine yttria (Y2O3) is added to form the dispersoid. After milling, a uniform interparticle spacing of about 0.5 ^m is achieved.

After mechanical alloying, the powder is packed into steel tubes and extruded at 1950-2050° F with reduction ratios of 13 to 1. The consolidated extruded stock is then hot rolled into mill shapes at 1750-1950° F to produce product forms such as bar, plate, sheet and wire. Since most applications are for creep resistant structure, a grain coarsening anneal at 2400° F is applied after fabrication.

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