Microstructure and Mechanical Properties of Laves Phase-strengthened Fe-Cr-Zr Alloys
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INTRODUCTION
ADVANCED structural materials with excellent high-temperature strength and resistance to thermal creep, corrosion, and neutron irradiation are essential for advanced reactors with enhanced safety margins, design flexibility, and economics. High-Cr (9 to 12 wt pct) ferritic-martensitic (FM) steels are important structural materials for nuclear reactors due to their advantages over other applicable materials like austenitic stainless steels in face-centered cubic (fcc) structure, notably for their resistance to void swelling, low thermal expansion coefficients, and higher thermal conductivity.[1–3] Their superior radiation resistance is primarily due to the body-centered cubic (bcc) structure of the matrix as reviewed by Raj et al.[3] However, current FM steels are not qualified for applications at temperatures above 893.15 K to 923.15 K (620.15 to 650.15 C) because they suffer significant strength reduction once beyond the temperatures and approaching the bcc–fcc transformation temperature.[4,5] Alloys with full bcc structure by eliminating the bcc–fcc transformation would be able to prevent such strength reduction and thus be allowable for use at higher temperatures. Laves phases are among potential intermetallic compounds being considered as strengthening elements for high-temperature structural alloys.[6–8] Room-temperature mechanical properties have been investigated in both Fe-Zr binary and Fe-Cr-Zr ternary alloys with
Laves phase as a strengthening element.[9–11] A significant strengthening effect of Laves phase was observed in an Fe90Zr10 alloy at room temperature, e.g., compression yield strength/plastic stain of 2.2 GPa/1.5 pct.[9] All compositions throughout the article are stated in at. pct. A slower cooling rate led to a lower strength but noticeably increased plastic strain of 1.9 GPa/9 pct at room temperature.[10] Room-temperature plastic strain could also be improved by changing alloy composition. Reducing Zr to Fe94Zr6 led to a remarkable increase in plastic strain compensated with some reduction in strength as 1.5 GPa/12 pct.[9] In contrast to the nearly 100 pct fine eutectic microstructure in Fe90Zr10, the Fe94Zr6 alloy contains fine eutectic microstructure and coarse primary bcc (Fe) phase. Partial substitution of Fe with Cr while keeping constant Zr content also enhanced the room-temperature plastic strain, e.g., Fe80Cr10Zr10 with 2.13 GPa/17 pct.[11] The enhanced ductility was attributed to the formation of Cr-stabilized C14 Laves. However, the knowledge on high-temperature mechanical properties of the Fe-Cr-Zr alloy is currently not available. This work investigates the temperature-dependent tensile properties and high-temperature creep strength of carefully designed Fe-Cr-Zr alloys in both as-processed and isothermally aged conditions, providing insights on their potential applications as high-temperature structural materials.
II. L. TAN and Y. YANG, Research and Development Staff, are with the Oak Ridge National Laboratory, One Bethel Valley Road, P.O. Box 2008, MS-6151 Oak Ridge, TN 378
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