Patent Number: 051223341
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides a novel zirconium-base alloy which has improved creep strength due to the addition of gallium. The improved creep strength is achieved without severely degrading the corrosion resistance of the alloy. In the field of fuel tubing for nuclear reactor applications, such tubing is required to have a combination of good corrosion and mechanical properties for optimum performance. Zirconium alloys based on 0-0.5 wt. % Sn and various combinations of Fe, Cr and V show improved corrosion resistance compared to Zircaloy-4, but have extremely poor strength and creep properties. U.S. Pat. No. 4,584,030 (the disclosure of which is hereby incorporated by reference) discloses that the composition of Zircaloy-4 contains about 1.2-1.7 wt. % Sn, about 0.12-0.18 wt. % Fe and about 0.05-0.15 wt % Cr. U.S. Pat. No. 4,584,030 refers to U.S. Pat. No. 3,148,055 (the disclosure of which is hereby incorporated by reference) with respect to the composition of Zircaloy-4. U.S. Pat. No. 3,148,055 discloses a zirconium-base alloy containing 1.3-1.6 wt. % Sn, 0.07-0.12 wt. % Cr, 0.12-0.40 wt. % Fe, a maximum of 0.007 wt. % Ni, 0.16-0.25 wt. % oxygen, a maximum of 0.012 wt. % Si, a maximum of 0.05 wt. % C+N, balance Zr. U.S. Pat. No. 4,938,920 (the disclosure of which is hereby incorporated by reference) discloses a zirconium-base alloy containing 0.10-0.16 wt. % oxygen, 0-1.0 wt. % Nb, 0-0.8 wt. % Sn, at least two metals from the group consisting of Fe, Cr and V having 0.2-0.8 wt. % Fe, 0-0.4 wt. % Cr and 0.3 wt. % V with a total Fe, Cr and V being 0.25-1.0 wt. % and a total of Nb and Sn being 0-1.0 wt. %. U.S. Pat. No. 4,938,920 discloses a specific zirconium-base alloy containing 0.25 wt. % Sn, 0.2 wt. % Fe, 0.15 wt. % V, 0.1 wt. % oxygen and less than 50 ppm Ni. According to the invention, it has unexpectedly been discovered that small additions of Ga to zirconium-based alloys improves the creep strength of such alloys. In particular, additions of Ga of 0.25-0.5 wt. % to dilute zirconium-based alloys such as Sn containing zirconium-based alloys unexpectedly improves the creep strength of such alloys. For instance, it has been found that creep strains after approximately 300 hours exposure at 400.degree. C. and 120 MPa stress are four to six times higher for alloys without gallium additions, as shown in Table 1. The corrosion resistance of the alloys according to the invention are equal to or better than standard Zircaloy-4 material as determined in a standard 400.degree. C. autoclave test. Table 1 establishes that alloys in accordance with the invention exhibit much lower creep stain compared to similar alloys without gallium additions and compared to Zircaloy-4. In addition, Table 1 establishes that the corrosion resistance of the alloys according to the invention are comparable to the corrosion resistance of Zircaloy-4 and to the corrosion resistance of similar alloys which do not include gallium. Accordingly, the gallium addition unexpectedly does not degrade corrosion resistance to an appreciable extent. The test results shown in Table 1 were measured after conducting a 400.degree. C. autoclave test for 3 days, 28 days and 31 days. TABLE 1 __________________________________________________________________________ CREEP CREEP WEIGHT GAIN TIME STRAIN mg/dm.sup.2 ALLOY COMPOSITION HRS. % 3 days 28 days 31 days __________________________________________________________________________ S1-V4 .23% Fe, .20% V, 311 3.95 22.0 .14% O S4-6 .24% Fe, .24% V, 306 2.88 11.2 23.9 .23% O S4-10 .26% Sn, .24% Fe, 306 0.89 12.8 26.0 .25% V, .27% Ga, .17% O S2-3 .53% Sn, .41% Fe, 311 8.36 24.7 .20% Cr, .12% O S4-7 .50% Sn, .35% Fe, 306 1.44 15.7 28.9 .18% Cr, .28% Ga, .11% O S4-8 .47% Sn, .38% Fe, 306 1.27 15.5 28.4 .18% Cr, .56% Ga, .11% O S4-9 .53% Sn, .37% Fe, 312 1.17 28.1 .19% Cr, .26% Ga .17% O Zircaloy-4 317 0.45 13.6-17 29.1 __________________________________________________________________________ The alloys in Table 1 were tungsten electrode, arc melted into buttons and manufactured into strips by hot working and multiple steps of cold working with intermediate recrystallization anneals. The strips were finally annealed at 500.degree. C. for 3.5 hours prior to testing. Conventional processing, such as disclosed in U.S. Pat. Nos. 3,148,055; 4,584,030; or 4,938,920 or the other patents mentioned in the Background of the Invention, can be used to manufacture fuel tubing made of the alloys of the invention. Due to the low melting temperature of Ga, additional steps may be necessary to achieve the desired compositions. For instance, the Ga can be combined with other ingredients in the form of a master alloy of Zr-Ga. Compared to Zircaloy-2 and Zircaloy-4, alloys of the invention exhibit a similar microstructure with respect to the distribution of intermetallic particles. It is well known in the art, however, that the distribution and size of the intermetallic particles depends on the thermo-mechanical processing of the alloy as well as the alloy composition. The Ga in the alloys of the invention appears to contribute to solid solution strengthening of the alloy. This effect occurs even when the Ga is present in small amounts. Some of the Ga may be in particle form such as in the form of precipitates. While the invention has been described with reference to the foregoing embodiments, various changes and modifications may be made thereto which fall within the scope of the appended claims.