Deformation Behavior of Duplex Zircaloy-4-Oxygen Alloys
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I.
INTRODUCTION
I N the event of a postulated loss of coolant accident (LOCA) Zircaloy fuel sheathing is expected to reach temperatures in excess of 1273 K and to oxidize rapidly. In view of this, many recent studies of the high temperature mechanical properties of zirconium alloys containing different concentrations of oxygen have been reported. 1.2.3 Most of the investigations reported so far deal with the single phase a or/3 material. The duplex alloys of the (a +/3) region, so called because these are composed of two different phases of comparable grain size, have received rather limited attention. This work deals with the flow behavior of the duplex a +/3 alloys in which the two constituent phases are in thermal equilibrium. It is the purpose of this investigation to: (i) generate reliable strength data (or vs e at different temperatures and strain rates) for fuel modeling inputs, (ii) assess the role of volume fraction and the relative strength of the constituent phases and the nature of the interphase boundary on the strength of the two phase alloys, and (iii) consider the proper limits of usefulness of the pseudobinary Zircaloy-4-oxygen phase diagram.
II.
EXPERIMENTAL PROCEDURES
The alloys were prepared by arc melting commercial Zircaloy-4 and zirconium oxide to produce nominal oxygen concentrations up to 2 wt pct. The as-cast alloys had Widmanst~itten structures resulting from the transformation of/3 phase into oL phase during cooling. Fine precipitate particles, which were believed to be intermetallic compound, preferentially deposited along the needle or plate boundaries. Upon annealing at temperatures in the (a +/3) field, these boundaries and particles were favored nucleation sites for /3 phase. Therefore, the two-phase microstructures at the test temperatures were inherited at least partly from the cast structure. Three alloys having oxygen concentrations of 0.12, 0.4, and 0.88 wt pct were similarly made by Wah Chang, and were supplied in hotD. TSENG is Research Associate and K. TANGRI is Professor and Research Director, Metallurgical Sciences Laboratory, Department of Mechanical Engineering, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2. Manuscript submitted April 28, 1981. METALLURGICAL TRANSACTIONS A
extruded condition. The Wah Chang alloys had equiaxed grains after annealing. Cylindrical compression specimens 8 mm in diameter and 16 mm long were machined from these ingots. Testing was carried out in a vacuum compression jig specially designed for quenching the specimens following high temperature tests. Before testing, each specimen was annealed in the compression jig in vacuum ( - 2 • 10 -6 torr) for a duration listed in Table I. Compression tests were conducted in an Instrom at constant displacement rates. Strain-rate increment tests were employed on a single specimen to obtain the strain-rate vs steady-state stress relationship. As soon as the test was completed, the specimen was immediately quenched into water. This caused the/3-phase to transform into the martensitic a ' st
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