Second phase particles in zirconium alloys.

Transmission electron microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, and x-ray diffraction have been used to study the zirconium alloys, Zr-1wt%Nb. Zr-2.5wt%Nb, Zircaloy-2, Zircaloy-4, and bulk stoichiometric $\rm Zr(Fe\sb{1-x}Cr\sb{x})\sb2$ alloys, where x = 0, 0.25, 0.45, and 0.75. Both Zr-2.5Nb alloy pressure tubing and Zr-1Nb fuel sheathing contain two main phases, $\alpha$-Zr and $\beta$-Zr. Zr(FeCr)$\sb2$ Laves phase precipitates and Zr$\sb3$Fe intermetallic phase were found in cold-worked Zr-1Nb alloy. Only Zr$\sb2$Fe precipitates were found, however, in Zr-2.5Nb pressure tubing, which contains more Fe and Cr than Zr-1Nb. The volume fraction of Zr$\sb2$Fe was much smaller than the volume fraction of Zr(FeCr)$\sb2$ precipitates in Zr-1Nb. On annealing of the Zr-2.5Nb at 560$\sp\circ$C both Zr(FeCr)$\sb2$ and Zr$\sb2$Fe precipitates were formed. It is proposed that on annealing the $\beta$-Zr phase (containing Fe and Cr in solution) transforms to $\alpha$-Zr + $\beta$-Nb + Zr(FeCr)$\sb2$ phases. Three kinds of second phase particles were found in the Zircaloy-2. These precipitates were: (i) a Zr$\sb2($FeNi)-type precipitate with a composition, $Zr\sb2(Ni\sb{0.6-0.5}Fe\sb{0.4-0.5}).$ The $Zr\sb2(Ni\sb{0.6-0.5}Fe\sb{0.4-0.5})$ precipitates have a body-centered tetragonal structure with a space group I4/mmm $(D\sbsp{sh}{17},$ No. 139); (ii) Zr(CrFe)$\sb2$ Laves phase particles with a composition of $Zr(Cr\sb{0.55-0.57}Fe\sb{0.45-0.43})\sb2;$ and (iii) Zr$\sb3$P precipitates with a simple tetragonal structure with a = 1.080 nm and c = 0.5410 nm. These zirconium phosphide particles were found in a Zircaloy-2 sample containing only 5 ppm phosphorus. They were also found in a high phosphorus (160 ppm) Zircaloy-4 sample. The hexagonal structure is the stable structure for bulk ZrCr$\sb2$ Laves phase at low temperature, but upon annealing at temperature higher then 850$\sp\circ$C, this hexagonal structure transforms first to a cubic structure and then to a fcc twinned structure. Cast and heat-treated bulk $\rm Zr(Fe\sb{1-x}Cr\sb{x})\sb2$ alloys, where x = 0.25, 0.45 and 0.75, have the 2H structure: ZrFe$\sb2$ has the 3R structure. But $\rm Zr(Fe\sb{1-x}Cr\sb{x})\sb2,$ where x $\ne$ 1, precipitates in dilute zirconium alloys can have polytypic modifications and stacking-faults. An electron diffraction method was developed to identify the structures of unknown phases and was applied to the identification of a number of the second phase particles in zirconium alloys. By this method, a series of reciprocal vectors, which should be in a principle reciprocal plane of a crystal, and their inter angles can be obtained. Using this data the principle planes of the crystal can be constructed. Thus, the elements of both the reciprocal and the direct lattice cells of the unknown phases can be obtained.Dept. of Mechanical, Automotive, and Materials Engineering. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis1990 .M455. Source: Dissertation Abstracts International, Volume: 52-11, Section: B, page: 5962. Thesis (Ph.D.)--University of Windsor (Canada), 1990