Unique properties of titanium
The general corrosion resistance has already been discussed. Regarding stress corrosion cracking (SCC), commercial pure titanium alloys and most titanium alloys are actually immune, unless new, sharp cracks appear in the presence of stress. If titanium ruptures in the air, the protective oxide will immediately reform and SCC may not occur. For example, if cracks are initiated in seawater, SCC may occur in certain high-strength alloys or high-oxygen grades of commercial pure titanium. Even in this case, SCC can be alleviated if the parts are not loaded immediately. The results of Dawson and Pelou4 show that as long as the stress intensity is lower than the stress corrosion threshold, the fatigue crack growth of Ti-6Al-6V-2Sn can be reduced during low-frequency testing. This is due to re-passivation (reformation of oxides) in the lower frequency seawater, and there is not enough time for this to happen at higher frequencies.
The modulus of ß alloy can be changed significantly. The tensile strength of Ti-15V-3Cr-3Al-3Sn with 60% cold working is about 1,070 MPa and the modulus is about 76–83 GPa. When aged at 480°C, the strength and modulus are ~1,515 MPa and 103 GPa, respectively. The titanium alloy containing Nb, Zr, and Ta developed for the medical industry is called colloidal metal. Depending on the orientation and processing technology, its elastic modulus is as low as 40-50 GPa. These moduli are close to bones, making them ideal for prosthetic applications. Cold working reduces modulus while increasing strength. 5
The crystal structure of the hexagonal close-packed (HCP) a phase has a very significant effect on the performance in different directions. Larson6 simulated the modulus of a commercial pure titanium single crystal and determined that when a force is applied along the base, the modulus is about 144 GPa, but when it is orthogonal to the base, the modulus is about 96 GPa. Recently, it has been observed that the difference in ultimate tensile strength between the longitudinal and transverse directions of rolled strip (which is also an indicator of crystalline texture) is about 205 MPa, and continuous rolling in one direction will produce strong texture.
Although the Bauschinger effect is not necessarily unique, it seems to have a stronger influence in titanium alloys than in other alloy systems. This is due to the limited number of slip systems in the hexagonal close packed (HCP) low temperature α phase. If the tensile specimen is stretched in the tensile state and the test is stopped before failure, and the compressed specimen is removed from the gauge length of the tensile specimen, a significant decrease in yield strength is observed. A tensile strain of 0.5% at room temperature can reduce the compression yield by 30%. This is due to the fact that the dislocations in the material follow the same slip path and move in opposite directions, which means that dislocation barriers do not have to be overcome in the early stages of deformation. If you apply strain to a compressed specimen and then pull the tensile specimen from its gauge length, the same phenomenon will be observed. This effect can be eliminated or alleviated by molding at high temperature or subsequent annealing. Therefore, at least in the aerospace industry, when titanium parts are formed, they are subsequently annealed to avoid this drastic reduction in yield. It does not affect the ultimate tensile strength.
The embrittlement of solid metals has always been a problem of titanium and its alloys. The most prominent example is cadmium. This happens requires close contact (forcing titanium into cadmium and vice versa) and high tensile stress.
