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 . . 100 .. I cro.21100 MP a VT22PT. 1,145 ; . 1070 · 960 · 650 230 VT22 980 I 880 I 760 I 520 I. 120 We have developed-a new high-temperature alloy . .,The alloy. represents P-solid solution of tit~ium, niobium, molibdenum and aluminium, which is : disperse. hard~ned. by chemical compounds .~ carbides, borides, ~ili~ides and by the other. compounds which increase high-temperature strength. · ·
The alloy characteristicls are not worse the those of titanium alum in ides (Ti3Al), but at the same time it has sufficiently good ductility. . Ti-based alloy contains (weight %) the following: 6-7.5% of Al, 20-25% of niobium, 3-5% of molybdenum, and small additions of carbon, boron, and silicQn .. . ·Possible.level _ofproperties is given in the Tabl_e 4 .. . Table 4
Properties .. Temperature, -°C ... 20 600 . :700 .·· · cr., MPa · 1050~1150 ~ 1000 · ~950 cr0.2, MPa 1000-1050 ~950. ~ 870 8,% ~4 ~6 ~8 ljl, %- .. . ; ~6 ~8 ~ lO 1 cr 100 - 500 280 ..· >•
••• r
.· . - Jhe._use of ~ chemical compound in solid solution titanium alloys· as a component 'of a composite material is ofsome interest (Fig. 1). .· . For example, a titanium alloy with 4% Fe heat treated for a eutectoid ( cx+TiFe} has a good plasticity and a high high-temperature strength that exceeds considerable the high-temperature strength of the same alloy in a
50 T!TAN!UM'99: SCIENCE AND TECHNOLOGY
solid solution (a+J3) state. At the same time, it is possible to increase the volume of the eutectoid component in alloys of this type and raise the high-temperature strength to a still higher level. This can be attained by choosing a more heat-resistant eutectoid.
_._,i,,.,! •• ,
A B
Fig. 1. Microstructure of Ti-4%Fe alloy (x400): A - annealed state; B - after treatment for a eutectiod.
It should be noted that modem industrial titai111.irti ailays having a solid solution structure strengthened by a chemical compound have been insufficiently studied. therefore, research aimed at creating novel titanium alloys of type and a study of the kinetics of the formation of the chemical compound in the existing industrial alloys remain quite important.
TITANIUM ALLOYS BASED ON CHEMICAL COMPOUNDS
Comparatively recently, titanium chemical compounds were not considered as s structural material because of the unfavorable combination of mechanical properties. However, it turned out that in some cases these chemical compounds possess quite suitable and sotnetirfies even unique characteristics. The industry is interested in higb-tetnperatiife alloys based on titanium aluminide, alloys exhibiting the effect of shape memory based on titanium ilickelide, and fireproof alloys based on eutectiod (Table 5). Alloys of the latter type can only conventionaiiy be classified as belonging to the group based on a chemical compound, but their functional properties are determined by the eutectoid. ... Table 5 Base of group 3 Grade of alloy . A:".erag~~ t!tentic!'_I composition, wt.% (at least) Titanium aluminides: Ti3AI (arphase) VTi-1 'ti - I SAi '"'" 22Nb ~ l .SZr - l .SMo TiAl (y-phase) VTI-Kh fi - 48Al - 2Nb ~ 2Cr ( cohveritional grade) Titanium nickelides TNI Ti-55Ni TiNi TNIK Ti - 52Ni - :i.2Fe TNM3 Ti - 50Ni - 4.5Cu Titanium eutectoid a+TiiCu VTT-1 Ti - l 7Cu(Al,Mo,Zr) ( a+J3)+ TiCr2 VTT-Kh Ti-20V - 15Cr ( conventional grade)
It turned out that an additional alloying of Ti3 Al, TiAl, TiNi, TiiCu, and TiCr2 chemical compounds ar,d solid solutions can provide alloys with a set of properties close to industrial structural and high-strength titanium alloys. It should be noted that the industrial use of alloys based on titanium chemical compounds required the development of new technologies such as microgranulating from a liquid state with a high cooling rate, compacting of powders, granules and cast prefonns in gasostats at an elevated temperature, defonnation under isothennal conditions at a low rate, and other modem methods.
51 TITANIUM'99: SCIENCE AND TECHNOLOGY
Alloys of this type are in the initial stage of their development, and it is inexpedient to predict their potentialities at the present time, especially in light of the fact that titanium chemical compounds may have physical, mechanical, and chemical properties that differ in principle from the conventional industrial alloys. It should be stressed that the research directed at creating titanium alloys based on chemical compounds has good prospects.
52 TITANIUM'99: SCIENCE AND TECHNOLOGY
About the purposefulness of comparision of titanium· alloys in terms of aluminum and molybdenum equivalents.
Prof B~A: KOLACHEV•,Prof A.A. ll..YIN.Eng, V.A. VOLODIN••.Eng, D.V. RYNDENK.OV• .. ~-·.
' . ' .• ,, I • . • • •~Russian.State Technological University named after K.Tsiolkovsky, 121552~ M~scow, Orshanskaya st .. 3: ~• ,._Russia Normal Corporation, 603600, Nizhny Novgorod, Litvinova st., 74, Rus~i~: . .
ANNOTATION. In addition-to structural aluminu~ and molybdenum equivalents of titanium-alloys the-terms-of strengthening aluminum and '.molybdenum equivalents ofa-stab~lizers and :neutral strengtheners and 13- ,stab'ilizeis are introduced accordingly. The strengthening equivalents ·were evaluated by that' strengthening ·action which alloying elements cause on the ultimate strength of titanium alloys in comparison with aluminum and molybdenum action .. The commercial and some perspecnve experimental alloys in different countries are compared in the structural diagram in the coordinates structural equivalent of; ~luminum - struetural equivalent of molybdenum and in the strength · diagram in the . coordinates ~strengthefli_ng aluminum equivalent - strengthening.molybdenum equivalent».
Key words: structural aluminum and molybdenum equivalent. strengthening aluminum and molybdenum equivalent tensile, ultimate strength.
,_· .k...
. INTRODUCTION.
_ ~ 1:iWlium alloy~-- ar~ muiticomponent as a rule. The use o(qua~e!'D8fY !llld in~ie· complex phase di~grams to substantiate the' composition of alloys ·and their heat.freatineiit conditions· meets ~htiai 'difficulties. Therefore· the 'term ·ofmotybdmum equivaieiit" of 13·-stabiiizers. was introduced iri the .tirsf y~ ·Jr titanium and its aUoys application alrea = equivalent. of a-stabilizers and neutral· strengtheners - structural molybdenum .equivalent. of 13-stabilizers'' The comparison of alloys in the diagram one of the coordinates of which is presented by strengthening equivalent and the other by structural omids not fully correctly. In the present paper the term of strengthening molybdenum equivalent is introduced in addition.to the earlier proposed strengthening atominum equivalent. The commercial, semicommercial and some· perspective· experimental titanium alloys of· different countries · are compared on the diagram in the coordinates of strengthening alu~inum and molybdenum equivalents of alloying elements. 53 TITAN!UM'99: SCIENCE AND TECHNOLOGY 1. THE STRUCTURAL MOLYBDENUM AND ALUMINUM EQUIVALENTS . . . OF TITANIUM ALLOYS. . . At present, phase compositions of titanium alloys. their structure, transformation ability and properties are analyzed by Ti-Mo phase diagram mainly taking into account all the rest element action by mo~ybdenum equivalent. While using molybdenum equivalent, alloying elements are arranged into the order iccording to their influence on martensitic transformation. This influence is characterized by minimum required concentration of a given element in binary titanium alloy at which the monophase P-structure with co-clusters within it is retained by quenching from P field temperatures; this concentration was cailed the second critical one II, 2]. At tlie evaluation of molybdenum equivalent of multicomponent alloy, P-stabilizing action of different P~stabilizing elements is·believecfto be·additive and the influence of a-stabilizing elements and neutral strengtheners is ignored. In the papers published various values of critical concentration of alloying elements are given, which is due to different purity of the original materials and differences in experimental proeeclure(Table I). Table l. The second critical concentrations of b-stabilizers in binary titanium alloys Critical concentration C"cr. % wt Alloying Molchanova Ageev, Petrova Luke, Taggart, Polonis Froes, Bomberger [ 11 J; · ~olachev element [8] [9] .. (10] ·Bania [12] [S, 13, 14] .... -- .. V IS 19.3 16:0 IS .. lS.O Nb 36 36.8 -- 36-. 36.0 ., SO.O 45. 45:o.· Ta so 40.:50 ,· •'. Cr 8 9.0 8 6.3 · 6:5 Mo 10 11.0 13.6 10.0 11.0 w 25 26.6 .. 22.6 22.5 22.0 Mn 6 5.7 6.3-9.l 6.5 6.5 Fe 4 5.5 6.9-9.2 3.5 S.5 Co 6 6.0 · 6.5-8.5 7.0 9.5 Ni 8 7.3 8.5-9.7 9.0 8.5 In the )B:St Russian publications [14, IS] on t~e basis of the generalization of published data the following values of the~ critical concentration aie ~ed to be (wt.%): lSV; JoNb; 4STa; 6 ..SCr, l lMo; 22W; 6.SMn; 5.SFe; 9.SCo; 8.SNi. These concentrations s!_ve the following relation for the evaluation ·or"molybdenum equivalent of titanium alloys . . . ., . - . : . ' · [Mo]-.';.. o/oMo + %Nb/3.3 +°/oTa/4 + %W/2+%Vii.4 + %Cr/0.6 + %Mn/0.6 + %Fe/0.5 + %Co/0.9 + . . . . · %Ni/0.8 . . (1) . . - . . . . . From the.results of foreign publications the data recommended by Froes, Bomberger and·Barua [11, 12) are the closest to the above mentioned concentrations (see Table 1). ibis gives a some what different expression· for calculating· molibdenum ·equivalent of titanium alloys: · [Mo]eq =%Mo+ %Ta/4.5 + %Nb/3.6 + %W/2 + %V/1.5 + %Cr/0;63 + %Mn/0.6S + %Fe/0.35_+ %N"J/0.8 (2) The criticai concentration ofmilybdenum was assumed to be 10 % wt. in papers [1 l, 12}. It will be noted that one of the most known scientists in the titanium alloy metallurgy Jaffee [15) believed that the critical molybdenum .concentration was to be equal 11 % wt. . However, the discrepancy in the values of molybdenum equivalent evaiuated 1fy reiatiorui (i) and (2) is not great; at least this discrepancy does not cause the change from one class of alloys into the other one. The aluminum equivalent content of a-stabiTizers and neutral strengtheners introduced by Rosenberg Pl reflects the susceptibility of alloy to °'2·precipitation (ThAI), ·This equivalent is expressed in the_ form [Al]eq =%Al+ %Sn/'.l + %Zr/6 + 1or%O] (3) At the aluminum .equivalent content of the alloying elements more than 9%wt. near a high-temperature titanium alloys become thermally unstable because of a 2 precipitation during isothermal exposures, especially the 54 . TITANIUM'99: SCIENCE AND TECHNOLOGY simultaneous action of stresses. at'eilhanced temperatures corresponding to service conditions of compressors of gas turbine jets. The relation (3) was successfully used at .the design of high temperature near a titanium alloys, which indirectly proves its correctitude. In the coordinates "aluminum equivalent - molybdenum equivalent" titanium alloys not only near a but a+f3 and near 13-classes · may be compared. ·The structural diagrams reflecting the phase composition of multicomponent alloys in· a certain structure state are successfully used in phisical. metallurgy of. steels and cooper alloys. The diagrams of phase composition of chromium - nickel. steels in -the normalized state,. may- be an _example, the chemical composition of steels in it being expressed by chromium and nickel equivalents. The structural diagram of quenched titanium alloys in the coordinates "aluminum equivalent - molybdenum equivalent" is shoY.n in Figure I. [Al]cq 10 .,. . •• • • \... ••• •• • - '' ---··---·-- - _I -=\-\ \ • ·\. \ \ • • • • \ \ \ • • ,. r·-...... -. ··•.. ' ·, -.... ,1 • TT \ T\T ~ ·-·- ·-~ ': 10 IS 20 . 21 [Mo]eq Fig .. 1 The structural diagram of titanium alloys in coordinates "structural aluminum equivalent - structural molybdenum equivalent" I-- a- and near a-alloys; Il - a+j3-alloys; Ill - alloys of transition class; Ill - near 13-alloys. When designing this diagram it was taken into account first of all that the line dividing the phase fields with a 2-phase and without it is almost parallel to Ti-Me side of the concentration triangle for the most ternary systems of Ti-Al-Me. Further a great number of published data about the phase composition of quenched from 13-fields titanium alloys, both industrial and experimental ones were taken into account. The general data devoted to this problem were taken from the proceedings of the last Conferences on Titanium [12-15]. And at last the suppression of co-phase formation in titanium alloys at their alloying by aluminum and neutral strengtheners was taken into account. · The compositions of main titanium alloys of different countries of the world are indicated in the papers [ 14- 15). The aluminium and molybdenum equivalents of alloying elements are given too. The figurative points of these alloys are plotted in.Figure 1, the molybdenum equivalent contents being calculated by relation (I). · _ The most of proposed alioys belong to high temperature ones with the intervals of equivalents: (Al]eq = 6-9'A,; [M<>]eq = 0-4%. The density of the location of the figurative points of a+f3 and near 13-alloys is about equal. At the same time the tendency of lowering the aluminum equivalent content of alloys with increasing the molybdenum equivalent content is observed. The same diagram may be regarded as_ the classification one. The distinction between the statements following from the diagram shown and the accepted ones.consists in the following. At present near 13- alloys ~e assumed to be.alloys with molybdenum equivalent more then 11% (100/o in the foreign publications). In reality the concentration separating a+l3 and near 13-alloys slµfts ·to less values of [Mo ]c,q with [ Al]eq inttease. It is due to this cause that the alloys Transage 129, 134 and 175 with the molybdenum equivalent content 8,2; 8,6 and 9,3%wt. respectively belong toner~ ~-all<'r 55