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Development Perspectives of Titanium Alloys

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Development Perspectives of Titanium Alloys TITANIUM'99: SCIENCE AND TECHNOLOGY DEVELOPMENT PERSPECTIVES OF TITANIUM ALLOYS V.N. Moiseev*, N.V. Syisoyeva* ' . *All-Russian Institute of Aviation Materials (VIAM), Russia, 107005 Moscow, Radio str., 17 . ANNOTATION Titanium metallurgy is being intensely developed at the present time, especially in foreign countr:ies. A great number of titanium alloys often repeat each other in their physical and mechanical properties. Each country (Russia, the U.S., Great Britain, France, and.in recent years, China and Japan) creates its owri ranges of industrial titanium alloys that often differ only in the combination of alloying elements rather than in the set of their properties. The present paper is an attempt to determinate the main . directions of advancements in titanium alloys which, in the opinion of the authors, are of great. interest for modem engineering - · Key words: titanium alloys, classification, solid solutions, chemical compounds For many years, industrial titanium alloys have been considered exclusively as consisting of~- and P'­ solid solutions. This is understandable because "solid solution" titanium alloys have been assumed to possess the best combination of physical, mechanical, ·and chemical properties that are very interesting for modem industry. - Today, interest is drawn toward titanium alloys based on their chemical compounds, iri particular, aluminides and nickelides. Alloys of this type possess a combination of physical, mechanical, and chemical properties that are very interesting for modem industry. · Recent studies show that a set o( high physical and mechanical characteristics can be obtained for titanium alloys represented by a mixture of solid-solutions and chemical compounds. Titanium alloys. .with the structure of a- and P-solid solutions and a low amount of chemical compound have been:s.tudied the best. The overwhelming majority of high-temperature titanium alloys belong to its group.·· · Thus, in order to cover all the basic types of industrial titanium alloys, we should classify them in accordance with their structural properties into -alloys represented by a-, {a+P)-, and P-solid solutions (group J); . -alloys based on solid solutions with this or that content of a titanium chemical compound (group 2); -alloys based on a titanium chemical compound (group 3). This classification is helpful in formulating the path of development of each group and determining the methods for meeting the requirements imposed on the alloys. · SOLID SOLUTION TITANIUM ALLOYS As a nile, these are structur~l- alloys characterized by high · strength and ductility, satisfactory weldability, susceptibility to a strengthening heat treatment (a+P-alloys), good thermal stability in operat_ion, and some other properties requisite for modem structural materials. As a rule, solid solution titanium alloys preserve high strength up to a temperature of 300-400°C. Alloys of this type highly alloyed with elements that increase the recrystallization temperature (Al, Sn; Zr, etc.) preserve these properties even to 450°C. At a higher temperature the strength of solid solution alloys decreases substantially. Solid solution titanium alloys are classified into a-, (a+P)-, and P-alloys in accordance with the type of structure in:the equilibrium state. With allowance for the metastable phase diagram the ~lassification of this group can be made more detailed. Such a classification reflects not only the kind of structure, but also makes it possible to predict the physical and mechanical properties of titanium alloys of this type depending on the heat- _treatment regime. _ . .. · Table 1 shows· the ·basic iussian industrial solid solution alloys in accordance with this classification. 1t is based on the proportion ofthe amounts-~f (l- and p-phases iri the structure of the alloy ·and the special-features of structural transformations that occur in the alloy in heat treatment. ' · · · 48 TITANIUM'99: SCIENCE AND TECHNOLOGY Table 1 ., Group I Grade of alloy Averaged chemical, wt. % a-. alloys VTl-00 Unalloyed titanium The same · . VTI-0 - .. VT5 Ti-5Al VT5-1 Ti:- 5Al - 2.5Sn Pseudo-a-alloys OT4-0 Ti - 0.8Al - 0.8Mn (Kp<0.25) OT4-l Ti- l.5Al- l.0Mn OT4 Ti - 3.5Al - l .5Mn ,, VT20 Ti-6.0Al-2.0Mo- IV- lZr (a+P)-alloys of VT6C Ti - 5Al - 4.0V martensitic class VT6 Ti - 6Al - 4.5V (Kp=0.3-0.9) VT14 Ti - 4.5Al_- 3Mo - 1V VT16 - Ti - 2.5Al - 5Mo - 5V ··-· VT23 Ti:- 5.5Al- 2Mo- 4.5V- I Cr- 0.7Fe (a+P)-alloys of VT22·· Ti- 5Al ..:.·sMo-5V - lFe-·tcr intermediate class VT221 Ti -2.5Al- 5Mo- 5V ...c,Ife- I Cr · (Kp=l.O~l.4). VT30 Ti - l lMo - 6Sn '- 4Zr- Pseudo-P-alloys VT35 Ti~ 3Al·- l .5Mo - 15V - 3Sn - 3Cr '. ~ . (Kp~l .5-2.4) VT32 Ti - 2.5Al - 8.5Mo - 8.5V - ·l .2Fe - l .2Cr ' VT15 Ti - 3Al - 7Mo - l l Cr P-al_loys (K8=2.5-3.0) 4201 Ti-33Mo The so-~alled r~ference coefficient of stabilization ~f the P- phase (Kp) shows the proportion of the amount of P-stabilizing element in the given alloy to its concentration in the alloy· of a critical composition. The effect of dispersion strengthening attained in (a+P)-titanium alloys as a result of hiµ-dening 'arid aging is determined by the volume ofthe metastable phase decomposed in aging, the disp·ersity of the segregated 0 • particles, and-some other factors, and is a predictable quantity. The facts presented above allow us to suppose that the potentialities of conventional solid solution , titanium alloys have been exhausted to a considerable degree. Attempts to create industrial solid solution structural titanium alloys using various combinations of alloying elements do not seem promising. Research in the direction of heterogenizing the structure of solid solution titanium alloys by metastable 1 11 structural components (a -,a -, ro-, P-metastable phases) that are also solid solutions is of any interest. In alloys with this structure, the strength and ductility are higher, arid it becomes possible: to attain a new set of properties that are quite·promising for modem engineering, such'·as high damping'properties, an elevated crack resistance, etc. SOLID SOLUTION TITANIUM ALLOYS WITH ACHEMICAL COMPOUND . This class of titanium alloys is based on a-, (a+P)-, and p~solid solutions with a certain amount of disperse formations of a chemical compound which provides a substantial increase in the strength and the high­ temperature properties. Modem deformable high-temperature titanium. aHoys generally contain a very low ~mount of a chemical compm1nd in an a- or (a+P)-matrix._ Sometimes the high-temperature strength is increased even after • . the fjrst stage of formation of the chemical compound. · · · · - · · · · · The alloying elements that form chemical compounds in. titanium are usually aluminium (Th Al), silicon (TisSi:i), carbon (TiC), and boron (TiB). In multicomponent alloys, other chemical compounds can· be formed. Unfortunately, it should be noted that the kinetics of the formation of chemical compounds has been studied insufficiently well even in industrial high-temperature alloys. · Chemical compounds are formed in melting of ingots, are quite stable, and cannot be controlled (dissolved) in the subsequent heat or-thermomechanical treatment. An exception is a Th Al compound (az­ phase), which dissolves easily when heated. · ··· High-tempera~e titanium alloy~· based on a~ and (a+P)-solid sohition~ with' intermetallic strengthening.possess a quite high long-term strength and creep_at a temperature up to 500-600°C. 49 TITANIUM'99: SCIENCE AND TECHNOLOGY Today, industrial high-temperature titanium alloys are classified only in accordance with their solid­ solution7base, like_ stru_ctural titanium alloys (Table 2). Table 2 Group 2 Grade of alloy. .'Averaged chemical composition, wt.% Pseudo-ex-alloys VT18U Ti - 6.7AI- 4.0Zr- 2.5Sn - 0.7Mo - l.ONi- 0.15Si .. (K13=0.25) VT36. ' Ti-6.2Al-3.6Zr-2.0Sn -0.7Mo~ 5.0Ni-0.15Si Alloys of martensitic class VT8. ~ -· . Ti - 6.3AI - l.2Zr - 1.2Sn - 3.2Mo- 0.15Si (K13=0.3-0.9) VT9.· Ti - 6.4Al - l.5Zr - 3.0Mo - 0.25Si VT8M Ti.:._ 5.4Al - l.2Zr - l.2Sn - 4.0Mo - O. l 5Si - • r VT3-l Ti.,.. 6.5Al - 2.5Mo - l .5Cr""" 0.5Fe - 0.3Si .. .. VT25Y ; Ti - 6.5Al- 3.7Zr..:. l .7Sn -4.0Mo - l .OFe- 0.2Si At the present time, the base of high-strength titanium alloys is an aluminum-saturated ex-solid solution . with a low amount of P-phase. For this reason, all industrial alloys are classified as pseudo-ex-alloys or martensite alloys with low additives of.P-stabilizing elements. As a rule, they are all used in an annealed state. Further· development oftitanium·alloys of this kind is connected with the problem of purposeful control of the interrnetallic .strengthening. · · . In our opinion, granule metallurgy is a very promising method of controlling the kinetics of the formatioi1 of chemical COll!p<>unds. A supertapid cooling of granules from a liquid state provides supersaturated­ solutions that decompo_se in the-subsequent artificial aging with the formation of disperse particles of a' chemical compound. Changing the aging regime, we can control the size and shape of the disperse particles of the chemical compound in wide range. - For example, VT22PT (titanium alloy VT22 alloyed additionally by 0.25%C and 0.20%B) produced by the gr!lllule _tec.~nology has the following mechanical properties: cr.=1300-1350 MPa, cr0 _i=l250-1300 MPa, 2 8=8.0-9.5%, lj/=27-31 %, KCU:"17-20 J/cm , E=l28-130 GPa, a low-cycle fatigue equal to 11,630-14,200 cycles at K1=3.2, and ~max=450 MPa. .: . ' . · · · It is also supe~ior to alloy VT22 in high-temperature strength (Table 3). · Table 3 -;1 't~V cr ... ~v cr 'TJV . Alloy cr/vv I .1 cr/uu I .
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