The purpose of this study was to investigate and discuss the relationship between the crystal structure of martensites and the material properties, such as the Young's modulus and internal friction, in quenched Ti Nb alloys with plasto elastic deformation. In the alloys, minimal Young's modulus was indicated by the a′ martensite of Ti15Nb, and maximal internal friction was indicated by the a″martensite of Ti18Nb. After a weak plastic deformation of 5 rolling reduction, the Young's modulus of Ti (15,18, 20)Nb decreased, and the internal friction of Ti20Nb increased though that of Ti (15, 18)Nb hardly changed. From the XRD profiles, the extreme values of the material properties were predicted to be associated with the unit volume and phases (hexagonal close packed or orthorhombic) of the materials. It was also predicted to be associated with the change of phase from a′ to a″and from a″to a′ ＋a″or a″ ＋a″with different lattice parameters. Cyclic tensile test results after weak plastic deformation revealed characteristic changes in elastic incline, that is, an increase from ～40 GPa in Ti15Nb, and a decrease until ～40 GPa from the initial value followed by an increase in Ti18Nb and Ti20Nb. The consequent changing mechanism of material properties was suggested from the viewpoint of the changing behavior of the phase transformation and transition on martensites with plastic deformation or loaded elastic deformation.

Keywords: be-ta type titanium alloy , omega phase, deformation mode, twinning boundary TEM specimen preparation: electropolishing (electrolyte; 6%HC1O4 , 35%Butanol, 59%CH3OH, 20 V, 223 K) TEM-utilized: Topcon EM-002B (200 kV)