Structure and properties of yttria-zirconium dispersions

Abstract

Uniform dispersions of fused and sized yttria particles in zirconium have been produced by a powder metallurgy route in which particle-free matrix volumes are efficiently broken up in an intermediate stage of rehydriding and recrushing. High-density specimens sintered at 1200 °C or at 1000 °C followed by rolling at 950 °C have been examined by replica and transmission electron microscopy. From the observed rounding of the larger particles and disappearance of many of the finer ones it appears that yttria is not chemically stable in zirconium under present fabricating conditions. The main effect of the particles on the microstructure was to pin moving grain boundaries during sintering and so reduce the average grain size in the dispersions. In the hot-rolled specimens numerous dislocation arrays, forming low-angle subgrain boundaries, were present. These too appeared to interact to some extent with the particles. Stress rupture tests at 500 °C on rehydrided and resintered specimens showed no effect of the yttria particles for concentrations up to 2 vol %. At higher concentrations the properties progressively improved culminating in a ten-fold increase in rupture time at 5 vol % yttria. The hot-rolled specimens showed more complex behaviour in which the rupture time first decreased to a minimum at about 2 vol % yttria and then increased again. By 5 vol % the rupture time was, respectively, 3 and 50 times greater than for hot-rolled and sintered zirconium without yttria. Corresponding tests on selected zirconium alloys showed that the best hot-rolled dispersions were comparable in rupture properties to hot-rolled Zircaloy-2 and annealed Zr/2.5 wt % Nb. They are still much inferior, however, to ultra high strength alloys such as annealed Zr/1.5 wt % Al/0.5 wt % Mo.