The Gas Carburization of Linear Cellular Alloys as a Novel Alloy Development Tool
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INTRODUCTION
ADVANCES in the formation of thin-walled metal honeycomb structures have allowed for the successful fabrication of linear cellular alloys (LCAs) in several material systems including, but not limited to, Fe-NiCr,[1–3] copper alloys,[4] maraging steels[5,6] and superalloys.[6] These alloys are made through the direct reduction of metal oxide powder mixtures or, in the case when the oxide cannot be reduced easily, the use of metal hydride or carbide powders. This results in a wide range of alloys that can be produced. Excellent compositional control of the parts has been demonstrated, and because of the high strength-to-weight ratios for some of the extrusion geometries used, the potential application of these materials is vast. Whereas many materials systems have been created using this technique, one large restriction of the process is the inability to add carbon to the LCAs prior to reduction. This ability would greatly enhance the range of alloys that can be produced using the technology. As a result, gas carburization is investigated as an alloying technique, rather than for its more conventional use as a surface hardening technique, for these structures. The thin-walled nature of these products allows for the adaptation of traditional techniques such that uniform carbon contents can be achieved through the cross LAURA C. DIAL, Postdoctoral Researcher, THOMAS H. SANDERS Jr., Regents Professor, and JOE K. COCHRAN, Professor, are with the Georgia Institute of Technology, Atlanta, GA 30332. Contact e-mail: [email protected] Manuscript submitted December 16, 2010. Article published online November 9, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
section of the walls of the structures in a reasonable amount of time. The success of this process allows for the fabrication of many different material systems than what was previously possible. One such phenomenon that occurs during gas carburization, which is known as metal dusting, is important to this research. Metal dusting causes serious material deterioration as a result of the disintegration of typically iron, nickel, and cobalt-based alloys into metal particles and carbon.[7–12] Metal dusting is a form of carburization, and in iron-based alloys, metal dusting occurs typically because of the separation of iron carbide into iron and carbon particles.[13,14] It is not a major concern in traditional gas carburization processes as parts are large in comparison to the extent of metal soot observed. However, the loss of material associated with metal dusting can ruin the integrity of a thin-walled structure, even if only small quantities are observed. In this study, the design of gas carburization experiments is such that metal dusting is avoided carefully. The heat treatment of gas-carburized LCAs of low-alloy steel composition to various microstructures characteristic of steels has been also carried out. A comparison of the mechanical and microstructural properties of the carburized and heat-treated LCAs with conventionally fabricated alloys
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