TiAl(Mn) 金属間化合物の引張特性のひずみ速度依存性

Abstract

Tensile tests of a γ-base Ti-45 at%Al-1.6 at%Mn intermetallic compound with duplex microstructure prepared by a reactive sintering method were carried out at strain rates ranging from 3.5×10−4 s−1 to 3.2×102 s−1 using a servohydraulic high rate testing machine. Both the ultimate tensile strength (UTS) and the 0.2% proof stress (YS) were found to increase with increasing testing strain rate. The strain rate sensitivities of UTS and YS were found to have an identical value of 1.2×10−2. The tensile elongation, around 1.5% in value, also showed a slight positive dependence on the strain rate. The experimental study on the microstructure before and after the tensile test was performed by both transmission electron microscopy and scanning electron microscopy. It was demonstrated that the predominant deformation mechanism both in the equiaxed γ grains and in the lamellar laths is caused by the deformation twinning, although the observed dislocations in the deformation twin variants may also contribute to the mechanical behavior. It is believed that the low strain rate sensitivity of the mechanical properties in the tested material stemmed from the twinning mechanism, which operates at high strain rates. It was demonstrated that though the deformation twins may easily propagate across the twin boundary in the γ grains, the process becomes difficult when the grain boundaries are encountered. The stress concentration is likely to result in leading to the crack initiation at the grain boundaries and finally introducing intergranular cracking. On the other hand, in the lamellar region, the deformation twins, normally perpendicular to the lamellar laths, were found to propagate frequently across the γ inter-variant interfaces, whilst the stress concentration at the γ⁄α2 interfaces was introduced. The microstructure observation was found to be consistent with the fractography.