864Powder Metallurgy

A number of powder metallurgy (PM) approaches have been evaluated for the titanium system including the blended elemental (BE), prealloyed (PA), rapid solidification (RS), mechanical alloying (MA), and vapor deposition (VD) techniques [12, 18].

Using a press-and-sinter technique, the BE approach allows fabrication of low-cost components from elemental and/or master alloy additions. However, because of the porosity resulting from this method, a result of the inherent salt from the Kroll or Hunter processes [1], generally initiation-related properties such as S-N fatigue are inferior to cast-and-wrought product.

The PA approach yields mechanical properties at least equivalent to those of ingot product. However, less than desirable cost advantages, in combination with a fear of the PM approach by design engines has resulted in few applications.

The powder metallurgy/rapid solidification (PM/RS) technique is a "far from equilibrium" approach that allows extension of alloying levels and much more refined microstructures than are possible using the ingot metallurgy (IM) technique. The greatly increased chemistry/microstructure "window" can lead to enhanced mechanical and physical properties in a variety of metallic systems.

Mechanical alloying (MA) with heavy working of powder particles results in intimate alloying by repeated welding and fracturing. This technique allows dispersoids to be produced, solubility extension, novel phase production, and microstructural refinement.

Production of alloys directly from the vapor allows even greater flexibility in microstructural development than RS or MA [20, 21]. A semicommercial scale electron beam vapor deposition process has been constructed to produce alloys that are not possible by ingot methods or even rapid solidification. One example is the production of low-density Ti-Mg alloys. Mg boils below the melting point of titanium, making production of a liquid alloy impossible by conventional methods.

None of the three processes discussed above has yet progressed from the laboratory.

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