FEATURES OF BAINITIC TRANSFORMATION IN Zr-Rh ALLOYS

N.I. Taluts and A.V. Dobromyslov

Institute of Metal Physics, Ural Division of Russian Academy of Sciences, 18 S.Kovalevskaya str., Ekaterinburg, 620219 Russia

In the zirconium alloys with beta-stabilizing elements the martensitic or bainitic structures can form during quenching from beta-phase region depending on cooling rate. As the bainitic transformation is the result of two processes - polymorphic transformation and eutectoid decomposition, the bainitic structures differ from those formed during equilibrium eutectoid decomposition and exhibit the features characteristic of martensitic transformation. We present here the results of structural studies of bainite transformation in quenched Zr-Rh alloys.

The alloys containing 0.5, 1.2, 2.2, 2.65, 3.2, 4.0, and 4.5 at.% rhodium were prepared from iodide zirconium and rhodium of 99.9% purity. The ingots produced were homogenized in a vacuum of 1x10-3 Pa at 1000 C for 3 h and quenched from this temperature in ice water. X-ray diffraction analysis, optical and transmission electron microscopy were used in studying the alloys structure.

The phase state and the structure of the quenched alloys were shown to depend on the rhodium content. In the alloys containing from 0.5 to 2.2 at.% rhodium the hexagonal alpha-phase and intermetallic compound Zr2Rh are formed. The metallographic and electron microscopy studies showed that these alloys have the bainitic structure. The metastable omega-phase is formed at the content of rhodium in the alloy ranged from 2.65 to 4.5 at.%, with the eutectoid decomposition being suppressed. The Zr-0.5 at.% Rh alloys reveals the structure of upper bainite resembling the structure of the packet-type martensite with the difference that the laths have more rounded ends, and the bands of precipitates are seen at the boundaries between the laths. In the Zr-1.2 at.% Rh alloy, the alpha-phase laths are smaller, their distribution inside the packets is more regular, and the packets themselves often form regular geometric patterns. The precipitates of intermetallic compounds at the boundaries of the alpha-phase laths are generally observed as a continuous network. The Zr-2.2 at.% Rh alloy has more complicate bainitic structure.

The crystallographic and morphological features of the structure of the quenched Zr-Rh alloys are compared with those of other zirconium and titanium alloys.