Biaxial creep testing of textured Ti-3AI-2.5V tubing

Abstract

Creep anisotropy of annealed Ti-3A1-2.5V tubing has been studied under biaxial stress conditions at 673 K using internal pressurization combined with axial loading. Biaxial strains were measuredin situ during creep using laser and linear variable differential transformer (LVDT) extensometers. Creep data were obtained for different stress ratios (α = σ_θ/gs_z @#@), and the steady-state creep rates were found to obey power law with a stress exponent of 4.5 ±0.2 essentially independent of the stress state. Trie experimentally determined creep locus constructed at a constant value of the dissipative work function(W) deviated significantly from isotropy, indicating anisotropy of the material caused by crystallographic texture. The anisotropy parameters(R andP) in the modified Hill’s equation were obtained from the locus fitted to the experimental data to be 5.9 and 1.0, respectively. The crystallographic texture of the material was characterized through inverse and direct pole figures using X-ray diffraction techniques. The crystallite orientation distribution function (CODF) was derived from the pole figure data and represented graphically in the form of Euler plots. This CODF, along with the lower-bound plasticity model, was employed for model predictions of the anisotropy parameters and the creep loci assuming the dominance of basal, prismatic, and pyramidal slip systems. The texture-based predictions differ from the experimental results, and probable reasons for the discrepancy are discussed.