Weld microstructure of (Ni, Fe) 3 (V,Ti) long-range-ordered alloy
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I.
INTRODUCTION
LONG-range-ordered (LRO) alloys are a unique class of materials with an atomic arrangement distinctly different from conventional or "disordered" alloys. Different alloying atoms in LRO alloys arrange themselves periodically and form an ordered crystal structure. A series of LRO alloys with the general composition (Ni, Fe)3 (V, Ti) are being developed at Oak Ridge National Laboratory for elevated-temperature application. These alloys have ordered L12 structures and exhibit high strength, excellent ductility, low creep rates, and good fatigue properties at temperatures below their critical ordering temperature Tc ( - 6 7 0 ~ These alloys are also candidates for use in future fusion reactors I and exhibit relatively low swelling under neutron or heavy ion irradiation.2 An important area in the development of any commercial alloy is the ease with which it may be joined or welded. This paper describes attempts to weld (Ni, Fe)3 (V, Ti) and details of the weld microstructure. The alloy used in the investigation was designated LR037-5, an alloy designed to be the first commercial-grade LRO alloy in this class. Specifically, the alloy was manufactured using ferrovanadium feedstock which is considerably less expensive than pure metal raw materials.
II.
EXPERIMENTAL
A 0.4 kg ingot of LRO37-5 was arc cast under argon. A chemical analysis of the ingot is given in Table I. The ingot was clad in molybdenum sheet, hot rolled at 1100 ~ to a thickness of 2.5 mm, the cladding was removed, and the sheet cold rolled to a final thickness of 0.76 mm. The sheet was annealed at 1100 ~ and quenched into water to produce a disordered structure. The sheet was subsequently aged in helium at 630 ~ for one day, 600 ~ for one day, and 500 ~ for two days to produce the long-rangeordered structure. Autogenous gas tungsten arc (GTA) welds were made through the thickness of the sheet, inside a dry box having a 75 pct He-25 pct Ar atmosphere. The welds were made using a stationary arc over a traveling carriage with variable speed control. The welding parameters used were: arc voltage, 12 to 14 V, current, 40 A, and speed, 25.4 mm per minute.
Sections of the welds were prepared for metallographic examination by standard techniques using an etchant consisting of 40 pct conc HNO3, 40 pct H20, and 20 pct HF (by volume). Microhardness traverses were made across the welds using a Kentron microhardness tester with a 100 g load. Disks, 3 mm in diameter, were machined from the weld fusion zone, heat affected zone (HAZ), and base metal by electrical discharge machining (EDM). The disks were ground on 400-grit emery paper to a thickness of 0.3 mm and electropolished in a solution of 87.5 pct methanol and 12.5 pct H2SO4 (by volume) at - 1 0 ~ The voltage was - 1 5 V dc and the current - 1 0 0 ma. The polished disks were examined in an analytical transmission electron microscope (TEM) at 120 kV. Precipitate particles were extracted from the alloy by electrochemical dissolution of the metal matrix in a solution of 10 vol pct HC1 in methanol.
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