PHASE EQUILIBRIA IN ZIRCONIUM-RICH ZIRCONIUM-CHROMIUM-OXYGEN ALLOYS

Phase equilibria in the system ZrCrO have been studied by metallography, X-ray and electron diffraction, electron-probe microanalysis, hardness and thermal analysis in the composition ranges 0-2.25 Wt.% chromium and 0-2400 p.p.m. oxygen. A binary (α-zirconium + ZrCr2) eutectoid forms over a temperature range that expands with oxygen concentration up to about 135 °C for a 2400-p.p.m.-oxygen alloy. Oxygen also increases the chromium concentration of the eutectoid from between 0.7 and 0.95 wt.% for a o-p.p.m.-oxygen alloy and 1.25 wt.% for a 2400-p.p.m.-oxygen alloy. The oxygen concentration of the α solid solution has a derived maximum of about 9000 p.p.m. at approximately 900 °C. The crystal structure determined for ZrCr2 (MgCu2 type, a = 7.21 A ̊) confirms earlier work. The impurity iron dissolves completely in ZrCr2 but, up to 3 wt.% iron, has no observable effect on the lattice parameter. Outstanding microstructural features are the formation of a twinned martensitic product in quenched 0.2 to 1.5 wt.% chromium alloys, the existence of a hexagonal omega phase (a = 5.02 A ̊, c = 3.12 A ̊, c a = 0.622) in a quenched 2.25-wt.%-chromium alloy and the appearance, at certain cooling rates, from the β and α + β regions of a grain-boundary alpha-like" phase. © 1969."