In Focus

TITANIUM IS THE NINTH ELEMENT, AND THE FOURTH MOST ABUNDANT METAL ON THE EARTH’S CRUST (AFTER IRON, ALUMINUM AND MAGNESIUM): LET’S FIND OUT MORE ABOUT ITS HISTORY AND CHARACTERISTICS.

It was back in 1791 in Cornwall that the parish priest William Gregor, a Cambridge graduate and amateur chemist, examining the sand of the local Herford river, identified an element, up until that time unknown, and which we now know to be ilmenite, a mixed oxide of iron and titanium (FeTiO3) which contains about 20% titanium. Four years later, the German chemist Martin H. Klaproth also isolated a substance previously unknown, now known as rutile (TiO2). He demonstrated that the ilmenite of the parish priest Gregor and the rutile he discovered were minerals (oxides) of the same metal which he called titanium. When choosing the name he was inspired by the Titans, the six sons of Uranus (the Sky) and Gea (the Earth), who in Greek mythology were defeated by Zeus and thrown into the Tartarus.

The discovery to the fine-tuning of a process for the production of pure titanium, however, took over a century: the production process in fact turned out to be highly complex due to the tendency of this metal to react with atmospheric gases and in particular with oxygen.

It was only in 1910 that Matthew A. Hunter produced a sufficiently substantial and pure amount to initiate studies on the properties of this new metal. The process consisted in reducing titanium tetrachloride with sodium.

Later on William J. Kroll fine-tuned a process in which he replaced sodium with magnesium and in 1937, in St. Petersburg, the first two tons of titanium were produced. The Kroll process, despite being very expensive, is still the most commonly used method for the industrial production of titanium. It should be noted that this is a production process that is far less efficient than the production of steel in the blast furnace, which can easily explain the reasons behind the high costs of titanium components, especially when compared with those made of steel.

The actual industrial development of titanium, however, is thanks to the US army industry at the beginning of the 1940s and, later on in the middle of the cold war, to the Russians (starting from the early 1950s). For several decades in fact, titanium was classified as a strategic metal for the production of armaments and excluded from any significant distribution at a civil level.

The most significant example of this evidence is represented by the Lockheed SR-71 reconnaissance plane, better known historically as the “Blackbird”, made of 93% of its weight in titanium alloy, which broke a number of records such as the fastest speed ever reached by a piloted plane, 3,530 km / h, and the maximum height of almost 26,000 m. To be more accurate, three different titanium alloys were used for this aircraft: Ti-6Al-4V, Ti-5Al-2,5Sn and Ti-13V-11Cr-3Al (also known as B120VCA).

Aereo Lockheed o "Blackbird"

Diag. 1: Lockheed SR-71 or “Blackbird”

Moreover, for at least three decades (from the mid-1950s to the late 1980s), titanium was rated as a flagship material for the American aerospace industry.

At the end of the 1980s, with the end of the Cold War, the military constraints imposed up to that point on titanium were progressively lifted, hence allowing the development of many other fields of application including biomedical, architectural, chemical, jewellery, infrastructure and leisure equipment (tennis rackets, golf clubs, bicycle frames). Also the automotive sector, and in particular that of racing cars, has and continues to play the leading role in the development of titanium alloys for various components.

PHYSICAL AND MECHANICAL PROPERTIES OF TITANIUM

Titanium has a density of around 4.5 g/cm3, higher than other light metals of structural interest such as aluminium or magnesium, but almost half compared to that of steels. Titanium also has excellent mechanical resistance to yield and fracture, an elastic modulus of around 115 GPa and excellent resistance to various forms of corrosion.

Titanium also boasts very low thermal conductivity and thermal expansion coefficients, around 22 W/mK and 8,410-6 C-1 respectively, that is equal to almost one order of magnitude and 1/3, respectively, of those of aluminium .

As for the crystalline structure aspect, titanium concedes an allotropic transformation in which the crystal lattice passes from a compact hexagonal (CH), at room temperature, to a body-centred cubic (BCC) structure, at temperatures that strongly depend on the specific chemical composition of the alloy. Typically, in the case of pure titanium, the compact hexagonal crystallographic structure (or  phase α) is maintained up to 882°C, a temperature to which the name β-transus is conventionally assigned (Fig. 2).  At a higher temperature than this, the stable structure is a body-centred cubic (phase β) structure, which is maintained up to 1670°C, a value which corresponds to the melting point of pure titanium.

Diag. 2: Allotropic transformation in commercially pure titanium

ADDITION OF ELEMENTS AND FORMATION OF ALLOYS

Alloy elements are classified as α or β-stabilizers based on the relative effects on the α-β transition temperature or on the different solubility during the two different phases. It should be highlighted that, in binary alloys, unlike pure titanium, the transition from phase α to β does not occur exactly at a precise temperature (e.g. 882°C), but within a range of temperatures: in this case the β-transus temperature of the alloy is conventionally defined as that beyond which the microstructure of the material consists entirely of phase β.

The substitutive element Al and the interstitials O, C and N are powerful α-stabilisers and raise the temperature of β-transus, as shown in the outline state diagram seen in Fig. 3. The β-stabilising elements, on the other hand, lower the transition temperature. The latter are subdivided into β-isomorphs (V, Mo, Nb, Ta) and β-eutettoids (Mn, Fe, Cr, Co, Ni, Si, H), depending on the shape of the binary state diagram related to titanium. Finally, other elements, such as Sn and Zr, behave in a basically neutral manner, determining only a slight lowering of the β-transus temperature.

Diag. 3: Examples of the different types of binary state diagrams, highlighting the effect of the main alloy elements on the β-transus temperature and on the stability of phases α and β.

From what has just been illustrated, it is easy to note that by duly varying the type and content of alloying elements, it is possible to obtain alloys at ambient temperature with a structure consisting entirely of phase α, or two-phase alloys with the simultaneous presence of phases α and β, or finally completely phase β alloys.

Titanium alloys can reach mechanical tensile strengths that even exceed 1200 MPa, with an elastic modulus that, depending on the structure and the relative phases, can vary between 80 and 145 GPa. In this regard, Fig. 4 shows the performance of the specific resistance (intended as yield strength in relation to density) with the temperature for different types of metal alloys.

Diag. 4: Specific resistance based on the operating temperature of titanium alloys compared with other types of metal materials commonly used in industrial activities.

The main properties of commercial titanium alloys can be summarised as follows:

  • The elastic modulus that increases with the content of interstitial elements (C, N, O and H) and aluminium, but which can also decrease following the addition of β-stabilisers.
  • High hardness that generally increases with the addition of β-stabilising elements (some alloys, as will be explained in more detail below, can be hardened by means of a thermal solubilisation treatment with subsequent aging).
  • High creep resistance of up to 0.6Tf (Tf is the melting temperature expressed in Kelvin degrees).
  • High resistance to specific mechanical fatigue (fatigue limit compared to density), which constitutes the strong point of titanium alloys, as it is superior to any other metal alloy with the same mechanical strength.

Wait for the next episode to find out more details on the characteristics and properties of the various Titanium alloys!

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