Use of spherical indentation technique for measurement of property variations of gamma-TiAl

Abstract

Novel lightweight intermetallic titanium aluminides, so-called gamma-TiAl, provide good strength and creep resistance up to 700 °C. Their stress–strain behavior at room temperature is however strongly confined in elongation due to their low ductility. For studying the stress–strain behavior of gamma-TiAl, the viscoplastic mechanical properties are determined using spherical indentation testing. The identification problem is solved on the continuum level by neural network analysis, which is based on a unified viscoplasticity model. The identified material parameters are validated by comparing the predicted stress–strain behavior with conventional compression tests at different deformation velocities. It was found that the average response of the indentation tests is in good agreement with the compression tests of round bars. Using a spherical indenter tip of R=0.2 mm, a small volume is tested, offering possibilities for investigation of local property variations due to processing. The experimental indentation curves exhibited wide hysteresis loops, revealing the existence of pure kinematic hardening. Since tensile fracture strength for gamma-TiAl is very low, microcracking occurred during loading as well as during unloading, significantly contributing to the unloading compliance.