Crystallographic Texture and Creep Anisotropy in Cold Worked and Recrystallized Zirlo

Abstract

Zirlo, a special zircaloy material alloyed with niobium, tin and iron is a successor of Zircaloy-4. Zirlo is materials used in fuel rod cladding, structural and flow mixing grids, instrumentation tubes, and guide thimbles. It increases margin to fuel rod corrosion limits and enhance fuel assembly structural stability in Pressurized Water Reactor. Zirconium and its alloys, being hexagonally close packed, have limited number of slip systems, and exhibit preferred orientations following thermo-mechanical treatments, which result in anisotropic mechanical properties. The objective of this project is to investigate the anisotropic mechanical properties, crystallographic texture, and microstructure of crept zirlo materials.

The anisotropic mechanical properties were investigated using uniaxial and biaxial creep tests. The specimen was loaded axially by a dead weight pan, and the hoop stresses was achieved by internally pressurizing the specimen with inert argon. Different axial and hoop stress, which produced different stress ratios (0, 0.67,0.75, 1, and 2) are selected for creep tests at 450°C. The axial displacement was measured by a linear variable differential transducer and the diameter change by a laser extensometer. Creep data are used to determine strain rate ratios vs stress ratios, the anisotropic parameters ( R and P), and creep loci for cold-worked and recrystallized zirlo.

The crystallographic textures were characterized in terms of inverse and direct pole figures using X-ray diffraction techniques. Inverse pole figures were constructed for specimens in the rolling direction, transverse direction, and normal direction for both cold worked and recrystallized tubes. Direct pole figures were constructed for specific reflection planes, such as basal (0002), prismatic (10 0) and pyramidal (10 2). Crystallite orientation distribution function (CODF) was derived from the pole figure data. Euler plots were obtained from crystallite orientation distribution coefficients (wlmn ) and subsequently therefore, ideal orientations were calculated. These CODFs were combined with the Lower-Bound model to predict creep anisotropy assuming the dominance of prismatic, basal and pyramidal slip systems. Creep strain rate ratios vs stress ratios, creep loci and anisotropy parameters (R and P) were predicted. The predictions based on the prismatic dominance matche with the experimental data very well.

Microstructure of the crept specimens was characterized by Transmission Electron Microscopy for different stress ratios ( 0, 0.75 and 1). The results show mainly dislocations in the matrix with no subgrain formation. The samples tested under equibiaxial loading revealed deformation twins. More detailed work is called for in characterizing the influence of stress-states and stress levels as well as cold work on deformation microstructures.