GB2246367A - Age hardenable beta titanium alloy - Google Patents

Abstract

Alloys comprising titanium, vanadium, chromium and silicon are age hardenable and have good high temperature properties. A typical composition is 35%V, 15% Cr, 1% Si, balance Ti. The silicon is present as a separate dispersed silicide phase which is produced by a controlled recrystallisation process. The separate phase improves the strength of the alloy. <IMAGE>

Description

1 1 - AV Ace Hardenable Beta Titanium Allov

Technical Field

This invention relates.to age hardenable beta titanium alloys which contain substantial amounts OEL vanadium and chromium and which also contain a small amount of siJAcon.

temperatures in excess titanium allovs contain B,c.kground ArtPure titanium transt-orms from the alDha cryst-;-1 -ure to the bea crvs- s t ru c X-al structure at of about 16210F. Commerc,-t-=" alloying elements which increase or decrease the stability of the beta phase so that the transformation temperature is increased decreased and the beta phase is stable at lower or higher temperatures. Certain commercial titanium alloys are referred to as beta alloys because they contain large amounts of beta phase stabilizers and 4 1 1.

p are largely beta phase at room temperature.

However this type of alloy is not 100% beta phase but includes someamount of the alpha phase which acts as a strengthening phase under use conditions. Since this alpha phase disappears with increasing temperature, leading to a decrease in prooerties, these alloys are not generally useful at el-evated temperatures. The subject of the prior beta titanium alloys is discussed in "The Beta Titanium Alloys" b F. H. Froes; et al, Journal of Metals 1985 pp. 28-37. There are no.known commercial titanium alloys which are true 100% beta phase alloys under all temperatures.

Titanium alloys possess desirable properties for many aerospace applications and are extensively used in gas turbine engines, esoecially for compressor blades and vanes and related hardware. one drawhadk to the use of titanium is its high reactivity which means it can undergo sustained combustion under conditions which are encountered in gas turbine engine compressors. In partic.ular, air is compressed to higli temperatures and pressures and flows at a high velocitv in such comoressors. Under these conditions commerical titanium alloys burn uncontrollably if ignited. Ignition generally occurs by friction or galling between relatively moving.parts. Such "in reference" friction results from component failure or foreign body ingestion (especially bird strikes). In gas turbine applications, relative speeds between -s can range from 1,000 to 3,000 feet per moving part minute (183 - 54.9 km/hr) and high levels of kinetic energy are present. Such combustion is of-grq.,at concern to gas turbine 4 i i i 1 1 1 i i 1 1 i 1 i 1 i 1 designers but to date it has been an inherent physicalcharacteristic of all commercial titanium alloys.

-The applicants are exz)erienced in the des'.gn of gas turbine engines and havedeveloped a test for titanium alloy combust-,bilit.i in which air is flowed at a velocity of 450 feet per second (494 km/hr) at a pressure of 100 psi (6.9 x 105 Nm-2) and a temperature of 8500C over a thin sheet sample which is ignited using tained combustion occurs the a laser beam. If sust alloy is deemed to be burnable. This test will be used hereinafter to define whether or not an alloy i burnable.

British Patent No. 1,175,683 to Imperial Metal. Industries describes a titanium alloy which can contain 25-40% vanadium, 5-1555% chromium, up to lo% al"M4nu-n, balance titanium. U. S. Patent No. 3,6414,1531 desc.ribes abrasion resistant materials produced by nitriding titanium alloy substrates. Certain of the subst-rates mav contain substantial a=unts of vanadium and chromium. U. 5. Patenc 3,673,0318 deals with a braze material for joining granhite and refractory meta-s which can consisc of 10-45% vanadium, 5-20% chromium, balance titanium.

Cur copending U.K. Application No. 871-3686 discloses hic.n strength nonburning beta titanium alloys containing substantial amount.3 of vanadium and chromium.

Disclosure of Invention

An age hardenable beta titanium alloy is disclosed based on a ternary composition containing major amounts of vanadium, chromium and titanium and Jng further containing 0.3-3% silicon. Other alloy.. elements may be added in minor amounts for various purposes. A typical alloy composition is 35% 1 1.

p i 4 1 i 1 va-,iadiurr.i, 15% chromium, 1% silicon, balance titanium. The alloys of the inventionmay. be solution treated and aged to produce exceptionally strong articles containing a fine dispersion of a silicon-containing strengthening phase. Further, most alloys of the invention are nonburning under conditions typical of those encountered in gas turbine engine compressor applications.

The foregoing, and other fe-atures and advantages of the present invention will become more apparent from the following description and accompanying drawings. All percent figures are weight percent values unless otherwise indicated. Listsof alloy ingr&.------ts given herein do not include incidental ingredients and impurities.

Brief Description Of The Drawings

Figure 1 shows the base alloy composition.

Figure 2 shows yield strength as a function of temperature for two alloys.

Figure 3 shows ultimate tensile strength as a function of temDerature.for two alloys.

Figure 4 shows elongation as a function of temperature for two alloys.

Figure 5 is an electron photomicrograph showing the strengthening phase.

Best Mode For Carrying Out The Invention,

The invention relates to a composition of matter and heat treated articles made from this composition of matter. The composition of matter, the invention alloy, has a composition which is illustrated in Figure 1, 'which is a portion of the ternary 1 1 i 1 1 i i i 1 1 1 1 i 4 _r 1 W vanadium-chromium-titanium diagram. The invention compositions lie within the polygon defined by points A, B,' C, D, E. Table I defines these points. A typical composition lying within these points is 35% vanadium, 15% chromium, 50% titanium. A preferred base composition is defined by points F-G-H-I-J. Alloys in this region have a particularly useful combination of properties. Preferably the alloys contain more than 13% Cr to ensdre that they are nonburning and most preferably the alloys contain more than about 15.1% Cr.

To this basic composition must be added a small, but critical amount of silicon in order to provide enhanced properties through age hardening. From 0.3-3% weight percent silicon and preferably from 0.5-2% silicon are added to an alloy selected within 1 with the silicon additions tne range shown in F being in partial substitution for titanium. Thus an exemplary invention alloy would comprise 35% vanadium, 15% chromium, 1% silicon, 49% titanium.

Of course the alloys of the invention may have small amounts of other elements added without detracting from their inventive nature. In particular, small amounts of carbon will be found to improve the stability of the alloy and to maintain a fine grain size. more particularly, carbon enhances the post creep ductility of these alloys, ameliorating any embrittlement which might otherwise occur. For this purpose from 0.05-2.0% carbon may be.added.

_r 1 1. 1 Ot-her alloying elements in the amounts listed in Table. II may also be added to the material. However, for purposes of the invention it is preferred that the alloying elements added do not produce extraneous phases (except for carbides and the silicon strengthening phase) in amounts in excess of about 1 volume percent; preferably alloying elements in Table II are added in amounts which do not cause use optically discernable' extraneous phases. It is clear'that if all the elements listed in Table II were added in theirmaximum amount the character of the alloy would be changed drastically and doubtless many unknown, unDredictable detrimental phases would form. Accordingly, a skilled artisan can easily determine through optical metallography whether added alloying elements have produced extraneous phases. Particularly promising elements are Re, Zr, Hf, and Nb.

The processing of tLtanium alloys is well known in the art and widely p. ract-ced on a commercial scale." - the invention may be processed according The allovs of to standard commercial practice. In fact, because Cie preferred alloys are notably less reactive than most commercial titanium alloys, some relaxation of commercial processing restrictions may be tolerated. In particular, the low reactivity of the alloy may mean that processing equipment e.g. crucibles and molds can be fabricated from ceramic refractory materials as opposed to the metal skull melting technique which is generally required with commercial titanium alloys. Promising results have been obtained 1 1 i 1 i - 1 i 1 k# using ceramic shell molds of the type used with nickel base superalloys. Graphite and other forms of carbon are also candidate materials for parts which contact molten alloys of the invention. The preferred alloys however, do possess the normal titaniurl affinity for and sensitivity to interstitial elements such as oxygen and nitrogen and are therefore preferably melted and cast under inert or vacuum conditions. The preferred alloys are uniquely cold workable and may for example be cold rolled 80% without cracking.

A key feature of the preferred alloys is that a strengthening phase based on Ti 5 si 3 is formed which dramatically increases the strength of the alloy. silicon-containing alloy behaves like a typical age hardenable alloy in that the alloy can be heated to an elevated temperature to dissolve the strengthening phase, rapidly cooled to ambient temperature to suppress the reprecipitation of the strengthening phase and then heated t.o an intermediate temperature to cause the strengthening phase to reprecipitate in a controlled fashion. The strengthening effects in the present alloying system have been most widely evaluated using\an alloy having a nominal composition of 35% vanadium, 15% chromium, 1% silicon, balance titanium. The heat treatment temoeratures desc---Ibed 1 below were developed with respect to this particular alloy although experiments with other different base compositions have not revealed significant differences.

7 - -c p The alloy may be heat treated by heating to a tempe ratu re i h e xces s c f abou t 19 5 0 0 F (10650C) to dissolve the strengthening phase. A time of about one hour will normally be sufficient although in specific cases where very coarse particles must be dissolved, longer times and/or higher temperatures may be required. Cooling from the solution treatment temperature at a rate in excess of about 50OEF (28OC) per minute and preferably in excess of 100 OF (561IC) per minute is sufficient to effectively suppress reprecipitation of the strengthening phase. Such cooling can be obtained in still air for moderate size articles, but for thicker articles forced air-cooling or other more effective quenching techniques may- be required. Once cooled to below about 500'F (260IC) the alloy may be reheated to a temperature between about 1100-1500 OF (5900C 82001U', for a neriod of time ranging from about 15 minutes to about 16 hours -he strengthening phase.

to cause reorecipitation of 1 k_ Prior to this agEng heat treatment, the alloy may be cold worked to a desired shape. Preferred agEing cond i t ions are 1200-1400 'F ',6500(-- - 76WC) for 0.5-2 hours.

resultant precipitate has a particle size of from 50-2500A0 and is present in from about 0.2 to about vol.%.

The processing of the invention alloys and the resultant properties will be illustrated in the following example. Samples of two alloying compositionswere cast in the form of one pound buttons using skull melting techniques. One alloy contained 35% vanadium, 15% chromium, 50% titanium, the other contained 35% vanadium, 15% chromium, 1% silicon, 49% _r i p i i 1 1 1 1 1 i 1 1' W titan ium. The alloys were forged at a,)out 2050OF (11200c) to produce a reduction in height of about 50%. The forgings were air cooled to room temperature and mechanical property specimens were prepared. A portion of the forged material was cold rolled about 80% and anneal ed at 1400OF (760OC) for one hour and further mechanical test samples were prepared. The mechanical test samcles were tested at a variety of temperatures and the results are presented in Figures 2, 3 and 4. The samples taken from the forged specimens provide data representative of material in the solution treated condition. Samples taken from the sheet tative of the material pr3vide data represent 4 material. Figure 2 of solution treated and aged illust-rates the yield strength as a function of temperature of these different alloys conditions. Two things are immediately apparent from Figure 2, first, that the silicon-containing mater4al is substantially terial. For example stronger than the silicon-E------ mat at 4 0 0 OF (20.00C11 the silicon- free material had a yield strength of about 105 ksi (0.7 MPa) while the siliconcontaininc material had a yield strength cE either 120 ksi o 1-11 ksi (0.8 MPa or 1.1 MPa) depending upon heat treatment. The increased strength of the solution treated material (relative t2 the Si free material) results from solid solution hardening. Second, the silicon- containing mater4a! 2C3 a the showed an age hardening response. Thus, at 400OF solution treated silicon-containing materia.11. had a strength of 120 ks i (0.8 MPs) but the heat treated material had 5.

a yield strength of 153 approximately a U% increase. Above about 10OVF (540OC) the two curves for the 1 0 9 silicon containing material cross because aE dynamic aiTeing during testing. The solution treated samples display the age hardening response at temperatures in excess of about 800F (430IC) and this increases their mechanical properties up to about 1200' (650IC), the maxinnum.

test temperature. The alloys will exhibit.

room temperature y.ield strengths in excess of about lSO ksi in the heat treated condition.

Figure 3 gives t.L same type of information about - e the ultimate tensile strength of these alloys. Again the silicon containing material is stronger than. the silicon free material and t'ie silicon containing displays a marked ace harte-ning resnonse.

Figure 4 shows tl-,-- elongation as a function of temperature of the varlous samples previously discussed in Figures 2 and 3. The solution treated a nd SClUt4 on treated plus aced samples were of d-41---nt geometrics which affeecteed the elongation values. Comoarisons 1--ween the two dif----ent :111 "he s4;con specimen geometrics are. noz meaningl. I containing aced mater'--='- snows a eloncation whicn is - i aced material'.

mucn less than t.'aat o he silicon free This low but acceotable level of ductilitv imoroves With increasinc temoerazure. Coriverselv, for the solution treated siliccri cc.-ir-a-ining material, tne elongation value drops off over about 1000'F (540'C) as a consequence of ac,,ej-ncr during testing. The silicon free solution treated material displays relativeiv unifo.= elongation indicating a lack of age hardening response.

- 10 1 i 1 1 i il 1 !Alp Table III presents selected data for various alloys falling within the bounds of the invention and for two, (35V-15Cr-5OTi, and Ti-6-2-4-2) which do not. The general effectiveness of age hardening is apparent and the preferred alloys are clearly superior to a similar silicon free alloy and to a commercial high strength titanium alloy. I n g e n e r a 1, pref err ad, alloys will have room teMDerature yield strengths in excess of 150 (11.0 MPa) in the heat treated conditions; 1200 OF (6501C) yield strengths in excess of 100 ksi (0.7 MPa); and 0.5% creep times at 100011F/40 ksi (540OC/0.3 MPa) in excess of 40 hours.

Figure 5 is a photomicrograph, of Ti-35%V-15%Cr-l%Si material in the aged condition, taken at a-magnification of 100,000 illustrating the strengthening phase in the present alloy which is believed based on Ti:Si 3 11 - -01 t 1.

TABLE 1

Weight Percent p v Cr A 25 12 B 22 17 c 30 25 D 37 19 A E 2 12 F 24 14 G 24 17 H 30 22 36 19 14 1) - -e- i 9 1 i 1 1 1 1 1 1 w TABLE 11 (wt - %) Broad Praferred B 0-0.6 0-1-0.5 c 0-2.0 0.1-1.5 C o 0-7.0. 0 - 5-6. or Hil- 0-1.5 0-1-1.0 MO 0-4 0.55-2.0 Nb 0-12 0.-10.0 0 0-0.22 0.08-0.2 Re 0-5.0 0.25 2.0 W 0-5.0 0.25 2.0 Zr 0-2.0 0.2 1.0 H j: 0-1.5 0.1 - 1.0 111 0-7.0 0.5 - 6.0 1 1 TABLE Ill

IF Time to Room.5% C.-eeo Temp. 12001F at 10001F/ Y.S. Y.S. 40 ks-i 35V-15Cr 130 ksi 95 ksi 50 hou 35V-15Cr-ISi S.T. 140 141 -42 35V-15Cr-lSi S.T.+Age 182 113 42 30V-15Cr-SCb-0.5Si S.T. 151 103 133 30V-15Cr-5Cb-0.5Si S.T.+Age 160 108 133 35V-15Cr-2Mn-Hi S.T.+Age 160 -- -- Ti 6-2-4-2 60 25 1 rs S.T. means solution treated Y.S. means yield strength 14 - j i 1 1 1 1 j 1 i i 3 1 1 1 i 1 i i 1 i 1 1 i - 1. -1r i 1 k# 1 j 1 Cl a ims 1. -A high strength beta titanium alloy compr-JSLng: a titanium-chromiumvanadium composition falling within the area defined by points AB-C-D-E in Figure 1 and further containing 0.3-3% silicon and up to 2% carbon in partial substitution for titanium, said alidy being In the heat treated conditions and containing a strengthening silicide dispersion.

Claims (1)

1. 2. A beta titanium alloy as claimed in Claim 1 which contains more than

   about 13% Cr.

   3. A beta titanium alloy as claimed in Claim 1 which contains more than about 15.1% Cr.

   4. A beta titanium alloy as claimed in any one of claims I tc which further contains one or more elements set out in Table 11, -s not being present in amounts said Tacle II element sufficient to create more than about 1 volume percent of extraneous phases, said Table 11 elements being added in partial substitution for titanium.

   5. A beta titanium alloy as claimed in any one of claims 1 to 4 whose chromium and vanadium content falls within region F-G-H-I-J in Figure 1.

   A 1 6. A heat: treated, high strength beta titanium allcy article comprising: chromium an. vanadium withinregion 11 j-i-B-C-D-- in Ficure 1; b - 0.3-3R) silicon present largely in ipitate- particles havinc 0.2-4 vol.% of: prec an average s-ze of 2-5-2000A' C. 0-2% carbon; and said ar in excess cE ac-ut 1-50 ks-;.

   d. balance essencially titan-Jum.

   havinc a room t--z.-iice-r-=zure vield 7. An article as claimed in Claim 6 whose chromium and vanadium levels are within recion F-G-H-PJ in Floure 1.

   8. An article as claimed in Clairn 6 or 7 which flurther contains one or more elements from Table IIr said Table 1:

   elements not be-;ng present in amounts suTicient to create more than about 11. volume percent clE new phases.

   E heat treating a beta titanium alloy A method OE 'e'4ne,-4 bv point is, Ls A-B-C-D-E: in Figure 1 and which fu.-zlier contains 0.3-3% silicon anc 0-1% carbon, in partial' subscitution for titanium, including the steps OE:

   a. solution treating the articles by heating tc temperatures at which all silicon containingprecipitates are dissolved; cooling at a rate sufficient to essentially 4 suppress precipitation of s,.licon containing particles; and heatinc to a temperatures between about 110. and 1500'F to cause controlled recreciDitation of said screnal-hening Phase.

   1 i i i I 4 i i i 1 1 10. A method as c!aimed in C11airn 9' in which the chromium and vanadium levels fall wit-hin recion F-G-H-I-J in Fri-ure to 11. A high strength beta titanium alloy, substantially as hereinbefore described with reference to the accompanying drawings.

   12. A method of heat treating a beta titanium alloy substantially as hereinbefore described with reference to the accomioanying drawings.

   rport Gwent NP9 - be obtained from.

   Published 1992 at The Patent Office. Concept House. Cardiff Road "e%% IRH Further copies rna Sales Branch. Unit 6. Nine Mile Point. Cu7nfelinfach. Cross Keys. Newpor- E. NP I -,HZ- Printed by Mull.iDlex techniques]Ed. St Mary Cray. Keni