| 其他摘要 | Gum Metal is a new type of multi-functional metastable β titanium alloy. One of its typical compositions is Ti-23Nb-0.7Ta-2Zr-1.2O (mol.%). The Invar property induced by cold working makes this alloy a candidate as a new kind of low thermal expansion alloy. In order to control the composition accurately, only powder metallurgy is adopted to synthesize the alloy at present. Additionally, there are a few essential arguments about the room-temperature plastic deformation mechanism and the nature of low thermal expansion is also not clear. In this thesis, the alloy was prepared by a conventional melting process, the room-temperature compression plastic deformation mechanisms of Ti-(22.4~24.5)Nb-0.7Ta-2Zr-1.4O (mol.%) alloys and the unique thermal expansion property of Ti-22.4Nb-0.73Ta-2Zr-1.34O alloy were investigated.
Ti-22.4Nb-0.73Ta-2Zr-1.34O alloy after cold compression to ~77% strain showed the following properties, weak work hardening, high plasticity and strength, nonlinear and super elasticity, low elastic modulus in the direction vertical to the compression direction, and anisotropic thermal expansion property. In the directions vertical and parallel to the compression direction, the alloy exhibited low and high thermal expansion below 300˚C, respectively. This phenomenon was a result that the anisotropy of discomposition in the textured stress-induced α" martensite (SIMα") and β phase during SIMα"→β and β→α" transformations superposed on the normal thermal expansion of the material. Aging treatment with ω phase precipitation would damage the low thermal expansion property. This was caused by the decrease in volume fraction of textured SIMα" and the increase in the stability of residual SIMα" and β phase that inhibited SIMα"→β transformation during heating and β→α" transformation during cooling.
The room-temperature compression true stress- true strain (σt-εt) curve of Ti-22.4Nb-0.73Ta-2.0Zr-1.34O alloy was composed of elastic stage, stress plateau stage, first strain hardening stage, strain softening stage and second strain hardening stage orderly, i.e. multi-peak stress oscillations. As increase of the Nb content, the stress plateau stage disappeared, and the strain softening stage and stress softening value were decreased. Multi-peak stress oscillations phenomenon was caused by the change of the dominant deformation mechanisms during compression. At the stress plateau stage, the alloy was deformed by multiple plastic deformation mechanisms, including 1/2<111> dislocations slipping, SIMα" and stress-induced ω phase (SIω) transformations as well as {332}<113>β and {112}<111>β twinning. The first strain hardening stage mainly resulted from a mass of dislocations tangles. At the strain softening stage, the alloy was mainly deformed by kinking. The second strain hardening stage was corresponding to the formation of shear bands. As increase of the Nb content, {332}<113>β twinning, SIMα" and SIω transformations, and {112}<111>β twinning were suppressed orderly and 1/2<111> dislocations slipping was a common deformation mode.
Both SIω transformation and {112}<111>β deformation twinning needed the 1/2<111> perfect screw dislocations dissociated into the extended dislocations. As increase of the Nb content, both the stability of β phase and fault energy were increased, the splitting of perfect dislocations became difficult and the constriction of extended dislocation became easy, which resulted in changes of the characters of SIω and {112}<111>β deformation twins. In Ti-22.4Nb-0.73Ta-2.0Zr-1.34O alloy both SIω and {112}<111>β deformation twin were flat and independent, in Ti-22.85Nb-0.68Ta-1.94Zr-1.49O alloy some flat SIω was along the boundary of {112}<111>β deformation twin, and in Ti-24.5Nb-0.7Ta-2Zr-1.4O alloy SIω was not found and some {112}<111>β deformation twin showed zigzag shape. Zigzag-shaped {112}<111>β deformation twin could be formed through screw dislocations cross slipping mechanism. |
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