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UNIQUE ATTRIBUTES OF TITANIUM

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Update time : 2019-04-30 11:52:23
With regard to stress-corrosion cracking (SCC), commercially pure and most titanium alloys are virtually immune unless there is a fresh, sharp crack in the presence of stress. If the titanium is cracked in air, the protective oxide will immediately re-form, and SCC may not occur. If the crack is initiated in sea water, for instance, then SCC could occur on certain high-strength alloys or high oxygen grades of commercially pure titanium. Even here, the SCC may be mitigated if the part is not loaded immediately. Dawson and Pelloux4 showed that fatigue crack growth of Ti-6Al-6V-2Sn can be reduced when tested at a low frequency as long as the stress intensity is below that of the stress corrosion threshold. This is attributed to re-passivation (re-formation of the oxide) in the sea water at the lower frequency whereas there is insufficient time for this to occur at higher frequencies.

The modulus of ß-alloys can be altered significantly. Ti-15V-3Cr-3Al-3Sn with 60% cold work had a tensile strength of ~1,070 MPa with a modulus of ~76–83 GPa. When aged at 480°C the strength and modulus were ~1,515 MPa and 103 GPa, respectively. Titanium alloys containing Nb, Zr, and Ta, referred to as gum metal, developed for the medical industry, have elastic moduli as low as 40–50 GPa depending on orientation and processing. These moduli are close to that of bone, making it ideal for prosthetic applications. Cold work decreases the modulus while increasing the strength.5 

The crystallographic texture of the hexagonal close-packed (HCP) a-phase can have a very significant effect on properties in different directions. Larson6 modeled the modulus of a single crystal of commercially pure titanium and determined that when stressed along the basal pole the modulus is ~144 GPa, but when stressed orthogonal to the basal pole it is ~ 96 GPa. Differences in ultimate tensile strength, which are also an indicator of crystallographic texture, between the longitudinal and transverse direction of about 205 MPa have recently been observed for rolled strip, with continuous rolling in one direction which can result in a strong texture. 

The Bauschinger effect, while not necessarily unique, seems to have a stronger effect in titanium alloys than other alloy systems. It is attributed to the limited number of slip systems in hexagonal close-packed (HCP) low temperature α-phase. If a tensile specimen is pulled in tension and the test is stopped prior to failure, and a compression specimen is taken from the gage length of the tensile specimen, a significant drop in the yield strength is observed. A tensile strain of 0.5% at room temperature can reduce the compression yield by 30%. This is attributed to the dislocations in the material going in the reverse direction following the same slip path, meaning dislocation barriers do not have to be overcome in the early stages of deformation. The same phenomenon is observed if one strains a compression specimen and then pulls a tensile from its gage length. This effect can be eliminated or mitigated by forming at elevated temperature, or subsequent annealing. Consequently, at least in the aerospace industry, when a titanium part is formed, it is subsequently annealed to avoid this large yield reduction. It does not affect ultimate tensile strength. 

Solid metal embrittlement has been a problem with titanium and its alloys, with the most prominent example being cadmium. Intimate contact (forcing the titanium into the cadmium or vice-versa) and high tensile stresses are required for this to occur.

 
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