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As indicated in the General Electric and Siemens Westinghouse discussions, firing temperatures used in the ATS gas turbines ne-cessitate materials changes in the hot gas path, particularly in the first two turbine stages. Moreover, new manufacturing techniques are needed to affect the materials changes. While single crystal and directionally solidified turbine blades are being used on aircraft engines, these parts are far smaller and one-tenth the weight of ATS utility-scale machines, and require less dimensional control.
To support the major systems development efforts, Oak Ridge National Laboratory has coordi-nated a materials and manufactur-ing technology program to hasten the incorporation of single crystal cast components into the ATS hot gas path.
Single Crystal Casting

General Electric and PCC Airfoils (GE-PCC) teamed up to address the challenges of bringing cost-effective single crystal (SX) technology to land-based gas tur-bine engine applications. As noted by General Electric, the require-ments for grain perfection and those for accurate part geometry compete with one another and create formi-dable challenges to successful, wide-spread use of large, directionally solidified (DS) and single crystal (SX) parts.
The GE-PCC work has pro-duced a number of findings and ad-vances in casting technology that will enable General Electric to in-corporate higher-yield SX and DS components into their ATS unit. Early work determined that signifi-cant improvement in oxidation re-sistance resulted from reducing sulfur levels to 1.0–0.5 ppm in the super nickel alloy used. GE-PCC developed a low-cost melt desulfu-rization process to replace expen-sive heat treatment methods for sulfur removal.
In parallel, GE-PCC advanced the casting and silica core processes to enable SX manufacture of com-plex-cored and solid airfoils for land-based turbine applications. Also explored was the use of alu-mina ceramic formulated core ma-terials to provide enhanced stability and dimensional control. Prototype testing showed promise for com-mercial application. Liquid metal cooling (LMC) was evaluated for application to DS processing. LMC provides increased thermal gradi-ents without increasing the casting metal temperature by improving heat input and removal from cast-ings. Casting of large stage-2 buck-ets for a 9G prototype machine was successfully demonstrated.
Siemens Westinghouse is de-veloping a process to fabricate com-plex SX blades and vanes from small, readily producible castings using transient liquid phase bond-ing. Transient liquid phase bond-ing was developed in the 1970s by Pratt & Whitney for aircraft engine components. The bonding media contains a melting point depressant and a carefully selected subset of the parent metal chemistry to attain 90 percent of the base metal properties. In the fabricated component ap-proach, bond planes are placed in insensitive locations.

Howmet Research Corpora-tion is pursuing ways to enhance SX and DS casting technology toward improving yields for the large ATS hot gas path components. Activities are focused on: (1) improving cur-rent Vacuum Induction Melt (VIM) furnace capability and control; (2)
　
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