{"id":6314,"date":"2019-07-19T15:32:13","date_gmt":"2019-07-19T13:32:13","guid":{"rendered":"https:\/\/www.tag.it\/?p=6314"},"modified":"2021-02-19T15:55:49","modified_gmt":"2021-02-19T14:55:49","slug":"titanium-alloys-classification","status":"publish","type":"post","link":"https:\/\/www.tag.it\/en\/titanium-alloys-classification\/","title":{"rendered":"TITANIUM ALLOYS: CLASSIFICATION"},"content":{"rendered":"

SECOND EPISODE IN OUR FOCUCS ON TITAN: we start to concentrate ourselves on the classification of its alloys.<\/h2>\n

Commercially pure titanium <\/b>(indicated according to ASTM ratings as CP grade “x”, where “x” is an indicative number of any impurities present) is mainly appreciated for its <\/span>extraordinary characteristics of resistance to corrosion.\u00a0<\/b><\/p>\n

There is no universally recognised classification system for titanium alloys,<\/b> as different national standards are used.<\/span><\/p>\n

One of the most convenient methods is to <\/span>divide the alloys into groups according to the phases that comprise the microstructure of the material at room temperature.<\/b> Let’s remember that <\/span>the stability of the phases is strictly influenced by the \u03b1-stabilising and \u03b2-stabilising elements that have been intentionally added to the alloy.<\/b><\/p>\n

In this sense, it is possible to<\/span> group the effect of the various \u03b1-stabilising and \u03b2-stabilising elements by defining the parameters “<\/b>Aluminium Equivalent” and “Molybdenum Equivalent”<\/i><\/b>:<\/b><\/p>\n

Aleq<\/sub>\u00a0= 1,0(%Al) + 0,17(%Zr) + 0,33(%Sn) + 10(%O + %N)\u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/em>(Eq. 1)<\/p>\n

and<\/p>\n

Moeq<\/sub>\u00a0= 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 ) \u2013 1,0(%Al)\u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/em>(Eq. 2)<\/p>\n

where the percentages of the single elements are expressed in weight.<\/p>\n

To calculate the \u03b2-transus<\/strong> temperature starting from the chemical composition of the alloy, the following can be used as an initial approximation:<\/p>\n

\u03b2tr<\/sub>\u00a0= 882 + 2,1(%Al) \u2013\u00a0 9,5(%Mo) + 4,2(%Sn) \u2013 6,9(%Zr) \u2013 11,8(%V) \u2013 12,1(%Cr) \u2013 15,4(%Fe) + 23,3(%Si) + 123,0(%O)<\/em>\u00a0 \u00a0 \u00a0 \u00a0 \u00a0<\/em>(Eq. 3)<\/p>\n

In particular, the Molybdenum Equivalent<\/strong> allows us, as shown in Fig. 1, to introduce a generic pseudo-binary state \u03b2-isomorphic type diagram to subdivide the titanium alloys into three large groups.<\/p>\n

The three main groups, respectively called \u03b1 alloys, \u03b1 + \u03b2 alloys and \u03b2 alloys, are identified on the basis of the phases that are present at room temperature.<\/strong> The type of such aforementioned phases depends on the content of the various alloy elements and, ultimately, on the Molybdenum Equivalent value.<\/p>\n

The \u03b2-series alloys can, in turn, be divided into \u03b2-metastables<\/em>,<\/em> or high-strength, and \u03b2-stables<\/em><\/strong>,<\/em> although the former are far more important and more commonly used than the latter.<\/p>\n

Strictly speaking, the \u03b2-metastables<\/strong> would present, if slowly cooled from the \u03b2 field down to ambient temperature, a two-phase \u03b1 + \u03b2 structure, but following a rapid cooling process,<\/strong> achievable – for example – by means of a solution quenching from the \u03b2 field, they appear to consist exclusively of a \u03b2-metastable phase (also called \u03b2 retention).<\/strong><\/p>\n

Experimentally it has been shown that this condition can be obtained when the Molybdenum Equivalent is between 10 and 30.<\/strong><\/p>\n

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Some authors prefer instead a more articulated classification introducing two further groups: near-\u03b1 alloys, which have a two-phase structure but with modest quantities of \u03b2 phase<\/strong>, since the content of \u03b2-stabilising elements is not higher than 1-2% , and near-\u03b2 alloys, characterised by Molybdenum Equivalent values of between 5 and 10<\/strong> (Fig. 2).<\/p>\n

It should be highlighted that, as a rule, the near-\u03b1 family could be included in that of the \u03b1 + \u03b2 alloys, while the near-\u03b2 group could be included in that of the \u03b2-metastable alloys, hence returning to the simplified classification shown in Fig. 1.<\/p>\n

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 \u03b1 alloys group.<\/strong><\/p>\n

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COMMERCIALLY PURE TITANIUM AND \u03b1 ALLOYS<\/strong><\/p>\n

The main classes of commercially pure titanium are grades 1 to 4, which differ in terms of iron content and interstitial elements:<\/strong> 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%.<\/p>\n

The significant resistance to corrosion (passivation<\/em>) of titanium<\/strong>, especially in the commercially pure form, is guaranteed by the formation of a thin layer measuring a few dozen nanometres of surface oxide<\/em> (TiO2<\/sub>) resistant up to temperatures of around 530\u00b0C.<\/strong> At higher temperatures it loses its structural continuity, drastically reducing the corrosion resistance of the material.<\/p>\n

Grade 1 is the most ductile of the different types of commercially pure titanium<\/strong> 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.<\/p>\n

Grade 2 can be considered to be the most produced and used commercially pure titanium.<\/strong> 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.<\/strong> It is often produced in bars for a wide range of sectors, from architecture, to the biomedical, marine, automotive, aeronautical and chemical industries.<\/p>\n

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Grade 3 is an evolution of grade 2 with a further increase in mechanical strength.<\/strong> It is commonly used in the aerospace, marine, medical and chemical sectors.<\/p>\n

Grade 4 maintains good formability and weldability properties, although it is a variant with higher mechanical strength<\/strong> 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.<\/p>\n

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<\/strong> (grades 26 and 27) gives the same effect as palladium.<\/p>\n\n\n\n
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