Morphological survey of U–2Nb alloy under isothermal transformation

The morphology of isothermally transformed products of U–2Nb alloy in the early stage of phase transformation was systematically investigated in the temperature range of 450–635 °C. Two kinds of products were identified corresponding to different transformation mechanisms. The first kind is acicular-like under Light Optical Microscopy (LOM). Careful examination demonstrated that each acicular structure contains many parallel units in lath or plate shape. Such structure has transformed through monotectoid reaction, which occurs at temperature between 550 and 635 °C in current work. Starting from 550 °C, isothermal holding at lower temperature results in another kind of structure, which is blocky or nodular-like under LOM. Higher magnification reveals that the structure is lamellar. Nucleation and growth characteristics of these two kinds of structure were discussed based on observations in morphological length scale. Similarities with acicular structures observed in other well understood alloys, e.g. Widmanstätten and bainitic ferrite in Fe–C system, were identified. On the other hand, the lamellar structure shows similarities to products transformed through discontinuous precipitation as has been well established in many binary systems with substitutional solute, e.g. Al–Zn system.