Microstructural characterization of 5 to 9 pct Cr-2 pct W-V-Ta martensitic steels
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I.
INTRODUCTION
First wall and blanket structure materials of fusion reactors are expected to become highly radioactive during service. The ferritic steels first considered for fusion applications in the United States were the commercial Cr-Mo steels: 2.25Cr-1Mo (2.25 wt pct Cr-1.0 pct Mo-0.1 pct C), 9Cr-1MoVNb (9 wt pct Cr-1.0 pct Mo-0.2 pct V-0.06 pct Nb-0.1 pct C) and 12Cr-1MoVW (12 wt pct Cr-1.0 pct Mo0.25 pct V-0.5 pct W-0.5 pct Ni-0.2 pct C). Because the disposal of radioactive waste materials poses serious problems, an important consideration in the design of the first wall and blanket structure is to minimize the induced radioactive decay times of the radioisotopes that would be formed during service.[1] The alloying elements that result in radioisotopes with long decay times are nickel, molybdenum, nitrogen, copper, and niobium. One approach to developing reduced-activation steels is to replace the elements that result in slowly decaying radioisotopes with elements that cause fast decaying ones without compromising the mechanical properties.[1] Substitution of tungsten for molybdenum has been suggested[1] because it behaves similarly to molybdenum in many steels.[2] Other alternatives proposed are the omission of nickel as an alloying element and the substitution of vanadium and tantalum for niobium.[1] Reduced-activation steels were developed that were variations of the conventional ferritic/martensitic steels, with
the molybdenum replaced by tungsten and niobium replaced by tantalum.[3] Previous studies of such steels have indicated that a 9Cr-2WV steel with a nominal composition of 9 wt pct Cr-2 pct W-0.25 pct V-0.1 pct C had tensile and impact toughness properties similar to the 9Cr1MoVNb steel that it would replace.[3] The addition of tantalum to the same nominal composition resulted in a steel (9Cr-2WVTa) with superior impact properties.[4] Neutron irradiation of ferritic steels generally causes an increase in the ductile-brittle transition temperature (DBTT) and a decrease in upper shelf energy (USE) as determined in a Charpy impact test. The low DBTT of the unirradiated 9Cr2WVTa translated into a small shift in DBTT after irradiation. The 9Cr-2WVTa steel had the smallest change in DBTT and USE ever observed for this type of steel (conventional or reduced activation) after irradiation in a fast reactor.[3] The objective of this study is to characterize the microstructure and chemistry of the matrix and precipitates in unirradiated 5 to 9 pct Cr reduced-activation steels using a combination of transmission electron microscopy (TEM) and atom probe field ion microscopy (APFIM). It is also of interest to gain a better understanding of these steels by comparing the experimental analytical results obtained from microanalytical techniques with phase equilibria calculations based on the ThermoCalc software.[5] II.
R. JAYARAM, Assistant Research Professor, is with the Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261. R.L. KLUEH, Senior Member of Research Sta
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