SECOND EPISODE IN OUR FOCUCS ON TITAN: we start to concentrate ourselves on the classification of its alloys.
Commercially pure titanium (indicated according to ASTM ratings as CP grade “x”, where “x” is an indicative number of any impurities present) is mainly appreciated for its extraordinary characteristics of resistance to corrosion.
There is no universally recognised classification system for titanium alloys, as different national standards are used.
One of the most convenient methods is to divide the alloys into groups according to the phases that comprise the microstructure of the material at room temperature. Let’s remember that the stability of the phases is strictly influenced by the α-stabilising and β-stabilising elements that have been intentionally added to the alloy.
In this sense, it is possible to group the effect of the various α-stabilising and β-stabilising elements by defining the parameters “Aluminium Equivalent” and “Molybdenum Equivalent”:
Aleq = 1,0(%Al) + 0,17(%Zr) + 0,33(%Sn) + 10(%O + %N) (Eq. 1)
Moeq = 1,0(%Mo) + 0,67(%V) + 0,44(%W) + 0,28 (%Nb) + 0,22(%Ta) +2,9(%Fe) + 1,6(%Cr) + 1,25(%Ni) + 1,7(%Mn) + 1,7(%Co ) – 1,0(%Al) (Eq. 2)
where the percentages of the single elements are expressed in weight.
To calculate the β-transus temperature starting from the chemical composition of the alloy, the following can be used as an initial approximation:
βtr = 882 + 2,1(%Al) – 9,5(%Mo) + 4,2(%Sn) – 6,9(%Zr) – 11,8(%V) – 12,1(%Cr) – 15,4(%Fe) + 23,3(%Si) + 123,0(%O) (Eq. 3)
In particular, the Molybdenum Equivalent allows us, as shown in Fig. 1, to introduce a generic pseudo-binary state β-isomorphic type diagram to subdivide the titanium alloys into three large groups.
The three main groups, respectively called α alloys, α + β alloys and β alloys, are identified on the basis of the phases that are present at room temperature. The type of such aforementioned phases depends on the content of the various alloy elements and, ultimately, on the Molybdenum Equivalent value.
The β-series alloys can, in turn, be divided into β-metastables, or high-strength, and β-stables, although the former are far more important and more commonly used than the latter.
Strictly speaking, the β-metastables would present, if slowly cooled from the β field down to ambient temperature, a two-phase α + β structure, but following a rapid cooling process, achievable – for example – by means of a solution quenching from the β field, they appear to consist exclusively of a β-metastable phase (also called β retention).
Experimentally it has been shown that this condition can be obtained when the Molybdenum Equivalent is between 10 and 30.
Some authors prefer instead a more articulated classification introducing two further groups: near-α alloys, which have a two-phase structure but with modest quantities of β phase, since the content of β-stabilising elements is not higher than 1-2% , and near-β alloys, characterised by Molybdenum Equivalent values of between 5 and 10 (Fig. 2).
It should be highlighted that, as a rule, the near-α family could be included in that of the α + β alloys, while the near-β group could be included in that of the β-metastable alloys, hence returning to the simplified classification shown in Fig. 1.
Table 1 shows some of the main commercial alloys currently in use for each of the introduced groups. It should be noted that in this schematic classification the commercially pure titanium has been included in the α alloys group.
COMMERCIALLY PURE TITANIUM AND α ALLOYS
The main classes of commercially pure titanium are grades 1 to 4, which differ in terms of iron content and interstitial elements: in particular, the oxygen content increases from 0.18% (grade 1) to 0.40% ( grade 4), which significantly increases the unitary yield strength, which rises from 170 to 480 MPa, while the elongation at break drops from 24 to 15%.
The significant resistance to corrosion (passivation) of titanium, especially in the commercially pure form, is guaranteed by the formation of a thin layer measuring a few dozen nanometres of surface oxide (TiO2) resistant up to temperatures of around 530°C. At higher temperatures it loses its structural continuity, drastically reducing the corrosion resistance of the material.
Grade 1 is the most ductile of the different types of commercially pure titanium and has the highest formability among all titanium alloys, combined with high values of resistance to corrosion and toughness. It is generally produced in the form of flat products for chemical processes, desalination plants, architectural panels and in the medical and marine sectors.
Grade 2 can be considered to be the most produced and used commercially pure titanium. It shares all the excellent grade 1 properties with a significant increase in the unit yield load. It is therefore a material that features good weldability, mechanical tensile strength, ductility and formability. It is often produced in bars for a wide range of sectors, from architecture, to the biomedical, marine, automotive, aeronautical and chemical industries.
Grade 3 is an evolution of grade 2 with a further increase in mechanical strength. It is commonly used in the aerospace, marine, medical and chemical sectors.
Grade 4 maintains good formability and weldability properties, although it is a variant with higher mechanical strength than the other three. It is widely used as a commercially pure alloy suitable for cryogenic tanks, aircraft components, heat exchangers, condensation tubes and surgical instruments.
As shown in Fig. 4, the addition of 0.2% of palladium (grades 7 and 11) makes it possible to achieve an exceptional resistance to crack corrosion even in highly reducing environments; also the addition of 0.1% ruthenium (grades 26 and 27) gives the same effect as palladium.
(a) CP Titanium grade 2
(b) Ti-0.2Pd (grade 7)
Fig. 4. Effect of the addition of palladium in increasing the resistance to crack corrosion, intended as pH and temperature limits, in a de-aerated sodium chloride solution.
In α type alloys, the alloying elements guarantee an effective increase in the mechanical characteristics of the compact hexagonal crystal structure with a strengthening mechanism for a solid solution. Among these the main ones are the aluminium and oxygen α-stabiliser elements, but also the neutral elements such as tin and zirconium. As an initial approximation, a criterion can be adopted whereby for each additional percentage point of each of these elements, the increase in mechanical resistance is between 35 and 70 MPa.
However, there is an intrinsic limit to the possibility of adding alloy elements, given that if the value of the Aluminium Equivalent parameter (Eq. 1) exceeds 9% we will witness the formation of fragile intermetallic compounds with an ordered lattice.
Although some of these alloys show the possibility of forming martensitic structures following rapid cooling from high temperatures, these are usually used in the annealed status, with the microstructure consisting of equiaxial α-phase grains. The presence of impurities such as iron or the intentional addition of small quantities of β-stabilising elements, such as for instance the Ti-3Al-2.5V alloy, results in the presence at ambient temperature of a small quantity of β phase, never exceeding 2%, which mainly forms along the edges of the α phase grains. These β phase particles play a beneficial role in the annealing heat treatment, making it possible to contain an excessive enlargement of the α phase grains, which would negatively impact the final mechanical characteristics of the material.
As regards the sensitivity of alpha alloys to hydrogen, it should be noted that this element shows a tendency to form metal hydrides (TiH2 and TiH) with thin plate morphology, which can lead to the onset of fragile failure mechanisms when put into operation.
Generally speaking, α alloys boast high toughness levels, even at very low temperatures, and are easily weldable. Cold deformability is obviously limited, since these are alloys with a compact hexagonal crystalline structure, intrinsically characterised by a limited number of sliding systems.
Conversely, over the last two decades, the most commonly used alloy, namely Ti-5Al-2,5Sn, has witnessed a drastic reduction in use in favour of other alloys of the α + β group, such as Ti-6Al -4V, which are more easily deformable and creep resistant. In fact, among the few fields of application where it can still be used, is for the storage of liquids in cryogenic conditions. The Ti-5Al-2.5Sn ELI (i.e. in the low interstitial element content version), in fact, maintains high levels of toughness up to the typical temperatures of liquid hydrogen (-253°C).
A further detailed description of the other titanium alloys will be available soon!