The properties of titanium alloys are generally influenced by the individual properties of it’s phases i.e. α and β, the arrangement of these phases in the lattice and their volume fraction.
For example, the α phase has a lower density than the β phase due to the fact that the predominant metal Aluminium in the α phase has a lower density than the predominant metal Molybdenum or vanadium in the β phase.
Since the most important α-stabilizing element, aluminium, has only half the specific weight of titanium, α alloys have a lower density than β alloys, even more so since the latter are often extensively alloyed with heavy elements such as Mo or V.
Generally, α alloys – being the single-phase alloys, have moderate strength.
The dual phase α+β alloys are metastable and can be hardened to high strength levels.
The ductility is directly related to the microstructure of the alloy. Since the fracture toughness of Ti alloys is dependent on the microstructure and the aging condition, there is no firm correlation between the different alloy classes.
In general, coarse and lamellar microstructures show higher fracture toughness than fine microstructures. The high toughness of lamellar microstructures can be explained by the ability of this structure to deflect propagating cracks along differently oriented lamella packets. This causes a rough crack front profile that consumes extra energy for crack propagation.
High creep resistance is also observed for the dual phase microstructures with a discontinuous distribution of β alloy.
Titanium has a high affinity for oxygen. It means that even in air at RT a very thin, dense oxide layer (TiO2) is deposited on the metal surface. The layer of TiO2 acts as a barrier for corrosion and that’s why it is considered as an excellent corrosion resistant material.
Among the different phases of Ti alloys, α is more stable than β.
To generally improve the properties of materials, and titanium alloys in particular, there are essentially two ways to proceed: alloying and processing. Recently a third option has gained importance: the production of composite materials.
Alloying is the basic process to increase the strength e.g. solid-solution hardening and precipitation hardening etc.
Processing allows the creation of the desired property profile of materials. Different microstructures can be obtained for titanium alloys by means of thermomechanical treatment to render it for strength, ductility, toughness, super plasticity, stress corrosion, creep resistance, etc
In production of composite materials, different materials (usually a metal matrix and a solute phase) are combined to produce a composite with desired mechanical properties.
The yield strength of primitive Ti alloys ranges b/wc800 to 1200 MPa, with metastable β alloys showing the highest value of strength. For applications such as bolt or screw fasteners, the highest tensile and fatigue strengths are required
Alloying additions alone are not typically used to increase strength in Ti alloys. The alloy TIMETAL 125 (Ti-6V-6Mo-6Fe-3Al) was, however, specially developed for high strength fastener applications.
The Young’s modulus is a clear representation for the stiffness of a material. It is directly related to the bonding between the atoms in the lattice. A famous Ti-Al alloy Ti-48Al-2Cr2Nb, where the two main phases, α2 and ɣ have intermetallic bonds which are the strongest and thus so is the strength of the alloy.
The silicon content in the Ti alloy determines the mechanical behaviour towards the elevated temperature of near- α Ti alloys. Plus, the microstructure also has a strong influence. The Lamellar microstructure is obtained from cooling out of the phase field, and equiaxed microstructures is obtained as a result of a recrystallization process.
As Compared with equiaxed microstructures, the former shows superior creep behaviour due to their coarse structure. It means that there is a lower volume fraction of grain boundaries.
On the other hand, equiaxed show extra-ordinary fatigue properties as they have fine microstructures.
Generally, the resistance to fatigue crack initiation for titanium alloys decreases with coarsening of the microstructure, i.e. fine equiaxed microstructures have higher fatigue strength than coarse lamellar microstructures.
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