coherent) precipitates. Chromium also form, Cr₂₃C₆, which strengthens grain boundaries. Thus a major portion of the gross engine weight consists of alloys containing chromium.

The required oxidation resistance of the cobalt- and nickel-base alloys is also obtained through either the development of alumina or chromia scales on the alloy or the application of a coating on the alloy (NRC, 1995). Chromium metal plays a critical role in the formation of these protective scales. Even if alumina is the final scale, chromium metal minimizes transient oxidation (NiO or spinels) and promotes the early formation of alumina. In addition, chromium metal provides hot-corrosion resistance to attack by such impurities as salts in the air and sulfates in the fuel. There are possible substitutes for many elements (e.g., tantalum for molybdenum, tungsten for rhenium) but not for chromium metal.

There are three reasons why high purity levels are needed for the chromium metal used in aircraft gas-turbine engines. First, these alloys are normally vacuum-melted, and the final form is either a cast or wrought product. The success of a forging (or other aggressive forming) operation requires the avoidance of hot cracking or incipient melting, which is dependent on the quality of the starting materials, especially the iron, oxygen, and silicon contents. The components with the most demanding combinations of temperature and stress are the single-crystal superalloys, which are used for high-pressure turbine blades and vanes. These materials are dependent on very low impurity levels in order to avoid incipient melting, fatigue crack initiation at oxides, and oxide spallation in the cyclic temperature environment of engine operation.

Second, inclusions are one of the limiting microstructural features for ductility, fatigue life, or creep-rupture. There are high- and low-density inclusions (based on radiographic appearance), but a particularly insidious type is dross (i.e., large stringers of oxide; see Figure 2-2). These stringers can be

FRONT MATTER (R1-R8)

CONTENTS (R9-R14)

EXECUTIVE SUMMARY (1-6)

1 THE MARKETPLACE FOR CHROMIUM METAL (7-18)

2 SPECIFICATIONS FOR HIGH-PURITY CHROMIUM METAL (19-28)

3 ELECTROLYTIC CHROMIUM-METAL PRODUCTION (29-38)

4 ALTERNATIVE TECHNOLOGIES FOR CHROMIUM-METAL PRODUCTION (39-44)

5 RELIABILITY OF SUPPLIERS OF HIGH-PURITY CHROMIUM METAL (45-50)

6 POTENTIAL ECONOMIC SCENARIOS FOR THE U.S. SUPPLY OF CHROMIUM METAL AND THE ROLE OF THE NATIONAL DEFENSE STOCKPILE (51-57)

REFERENCES (58-58)

APPENDIX: BIOGRAPHICAL SKETCHES OF COMMITTEE MEMBERS (59-60)