Titanium has two dominant allotropic phases α+β. These alpha and beta phases, and their orientations are responsible for the difference in properties between titanium’s many alloys. The alpha phase of titanium is close-packed hexagonal and its beta phase body centred cubic. In pure titanium, the alpha phase is stable below 882°C degrees and the beta above 882°C degrees.
Alloying elements cause titanium’s atoms to organise themselves in either of these two phases. Beta stabilisers lower the beta transition temperature allowing the beta phase to exist at ambient temperatures. These include vanadium, molybdenum, iron, chromium and manganese.
Elements that promote the alpha phase are called alpha stabilisers these include aluminium, oxygen, nitrogen and carbon. Some alpha stabilisers like oxygen, carbon and nitrogen are considered contaminants. This is because they create microscopic hardening within the alloy which reduces the metal’s ductility and makes it more susceptible to cracking. In Alpha, alloys Aluminium promotes the stability of titanium’s alpha phase beyond 882°C, which improves strength and resistance to corrosion.
The most commonly used titanium alloys are alpha beta, among them Ti-6AL-4V and Ti–6Al–2Sn–4Zr–2Mo. Ti-6Al-4V accounts for half of all titanium alloy manufactured. With Alpha Beta titanium alloys the phases exist alongside each other and can be further processed to give the broadest range of properties making it suitable for a wide broad range of final applications. Alpha beta titanium’s microstructure can be altered through heat treatment to give the optimum qualities of strength, corrosion resistance, weldability and ease of fabrication.
Beta titanium consists almost entirely of beta phase Ti. The structure of the Base Centred Cubic crystals means that there are more planes for the alloys lattice to deform. This means they offer superior mechanical properties. Because of this beta phase alloys are widely used in orthodontics and medicine, where their low modulus of elasticity is closer to that of bone than many steels.
Beta alloys for medical applications forego vanadium and aluminium as they can have adverse effects on the human body. Vanadium ions in human tissues can alter the kinetics of the enzyme activity associated with the inflammatory response cells; whilst the presence of aluminium increases the likelihood of developing Alzheimer’s disease. Alloys, therefore, make use of non-toxic Numidium, Zirconium, Tantalum, Molybdenum and Tin.
A Titanium Alloys ductility and hardness are further affected by Lamellar, Equiaxed and Bimodal structures within the metal. Bimodal consists of plate-like structures of Titanium crystals. Lamellar consists of parallel elongated crystal structures. Equiaxed microstructures have small crystals which are achieved through lower cooling rates and low recrystallisation temperatures these can be as small as 20 microns in size. The process of heat treatment reduces the size and consistency of alpha boundaries which give poor fatigue properties through slip system and isotropic plasticity.
Meta-stable beta alloys consist entirely of Titanium’s beta phase. These alloys have excellent strength properties and excellent formability due to the BCC microstructure.