Mechanical Alloying

Mechanical alloying is a high-energy ball milling process, which can be used to produce materials with unique compositions and microstructure. The process was originally developed to produce nickel and iron-base oxide-strengthened alloys (Ref 26). The application of the process to titanium-base alloys is relatively recent and generally involves loading a mix of powders and a grinding medium (usually hardened steel balls) into a container under a protective argon atmosphere. A small amount of a process control agent is normally added to prevent vent excessive cold welding of the powder particles during the milling operation. The material is then processed on a high-energy ball mill. Alloying occurs as the result of repeated welding, fracturing, and rewelding of the powder particles (Ref 27). Much of the work to date has involved the development of unique structural and microstructural characteristics such as supersaturated solid solutions, metastable crystalline phases, dispersion strengthened alloys, amorphous phases, and nanostructured materials. Extensive reviews of the application of mechanical alloying to titanium are contained in Ref 27, 28, 29, 30. Mechanical alloying is also being studied as a technique for producing nanostructured intermetallic alloys (Ref 30, 31) and composites such as Ti-6Al-4V/SiC (Ref 32) and titanium aluminide/titanium silicide (Ref 33). Figure 20 shows an example of mechanically alloyed powder. Figure 21 shows an example of the extremely fine microstructure that can be attained using mechanically alloyed powder.

Fig. 20 Example of mechanically alloyed powder. Starting powders of titanium hydride and a master alloy were blended in the proportion to produce a mixture of 66 vol% TiAl and 34 vol% Ti5Si3. Source Ref 32

Fig. 21 Nanostructured grains in Ti-55Al (at.%) after mechanical alloying and hot isostatic pressing. Source: Ref 26

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