AN X-RAY DIFFRACTION STUDY OF A MARTENSITIC TRANSFORMATION IN URANIUM ALLOYS

The martensitic bcc (γ) to orthorhombic (α') crystal structure change in uranium alloys is similar to the bcc to hcp transformation in Ti and Zr. One characteristic is that the change can be understood in terms of a Burger's shear mechanism. It has been proposed theoretically and with some experimental support that by progressively restricting the magnitude of the shear (by alloying, for example) a continuous series of displacively-produced transition states could be developed as follows : γ → γ° → α" → α' where γ° and α'' are tetragonal and monoclinic distortions of the cubic and orthorhombic phases, respectively. Previous studies have not been clear as to whether the intermediate transformation is continuous or discontinuous. In this work an alloy containing 6.4 wt% niobium in uranium was prepared and heat treated so that the γ° to α'' transformation could be induced at room temperature by an applied stress. The structure change was detected by insitu x-ray diffraction analysis. Lattice parameter measurements indicated that the transformation was martensitic involving a significant deviatoric component of the lattice distortion, a volume change, and a discontinuous rather than continuous change in structure