US3275434A - Molybdenum-base alloy - Google Patents

Description

Sept. 966 w. H. CHANG 3,275,434

MOLYBDENUM-BASE ALLOY Filed April 13, 1964 EELHT/ VE fiBUNDQNCE l l l l I I l l 2000 2250 2500 2750 3000 3250 3500 3750 4000 4200 TEMPE/QATUEE, "F

His A t rzvnek United States Patent 3,275,434 MOLYBDENUM-BASE ALLOY Winston H. Chang, Cincinnati, Ohio, assignor to General Electric Company, a corporation of New York Filed Apr. 13, 1964, Ser. No. 359,080 5 Claims. (Cl. 75--176) .This is a continuation-in-part of application Ser. No. 227,046, Chang, filed Sept. 28, 1962, now abandoned, and assigned to the assignee of the present invention.

This invention relates to molybdenum alloys and, more particularly, to molybdenum alloys having a combination of high temperature strength and low temperature duetility as a result of the alloying of molybdenum with judiciously selected amounts and proportions of the elements, titanium, zirconium and carbon.

Some of the advantages of alloying molybdenum with titanium and molybdenum with zirconium have been set forth in United States Patents 2,678,269 and 2,678,271, respectively. Furthermore, these two patents mention that carbon in very small quantities can be present. In addition with regard to carbon, United States Patent 2,947,624 indicates that certain advantages can be obtained from the use of carbon in molybdenum alloys in the range of 0.30 weight percent or higher.

In the modern design of molybdenum alloys the mechanisms of solid solution alloying and precipitation alloying have been extensively investigated. One of the molybdenum alloying systems on which much work has been done contains a minor but significant amount of titanium, a somewhat smaller amount of zirconium and some carbon. Several modifications in this alloy system have been investigated and some are presently being commercially exploited. However, in my persent investigations concerning the phase relationships and metallurgy of molybdenum alloys which can be precipitation hardened by combination of titanium-, zirconiumand molybdenum-carbides, I have found that such levels of titanium as have been advocated in the prior art are deleterious to both strength and ductility. Relatively small amounts of zirconium stabilize the precipitated titanium carbides and also form more stable complex carbides of zirconium and titanium. By appropriate small but critical adjustments of the levels of titanium, zirconium and carbon and the ratios of zirconium plus titanium to carbon, improvements over similar prior art molybdenum alloys have been obtained. However, many of the improvements have been in the direction of higher strength at elevated temperatures accompanied by lowered workability and ductility at room temperature and below.

Furthermore, many of the previously known alloys in the molybdenum-titanium-zirconium-carbon system contain significant amounts of MOZC. Interactions between Mo C and other precipitating phases are generally undesirable. Massive deposits of Mo C often are formed as a molten ingot freezes, and the carbon is not readily redistributed through the martix on decomposition of the *Mo C to other precipitates during aging. Also, unstable lower temperature precipitates that convert to Mo C at moderate temperatures do not provide the desired strength levels. Preferential distribution of Mo C in the grain boundaries is quite deleterious to properties. Thus, molybdenum alloys that can be hardened by precipitation from solution in the absence of ditficult-to-control interactions between different precipitated phases are greatly to be desired, and are not generally found among previously known alloys.

It is the desire of development metallurgists to produce alloys of unusual strength at high temperatures. However, it is important to the designers of highly stressed articles, such as turbine buckets for gas turbine engines, that an alloy b'e ductile at lower temperatures to avoid brittle fractures during the starting of such apparatus. Furthermore, the designers of articles for use in flight apparatus prefer materials which have a high strength-to-density ratio so that lighter weight parts can be fabricated.

It is a principal object of the present invention to provide an improved molybdenum-base alloy having unusual high temperature strength up to about 3500 F. along with good room temperature ductility.

It is another object of this invention to provide a high strength molybdenum-base alloy having a high strengthto-density ratio.

Another object of the invention is to provide molybdenum-base alloys with optimum contents and proportions of titanium, zirconium and carbon such that they can be treated to develop desired properties at elevated temperatures and at room temperature and below.

Another object of the invention is to provide a precipitation-hardenable molybdenum-base alloy in which the formation of undesirable Mo C is avoided.

Still another object of the invention is to provide a precipitation-hardenable molybdenum-base alloy which is not deleteriously susceptible to aging during use at elevated temperatures.

These and other objects and advantages will be more readily recognized from the following detailed descrip tion, tables and examples which are representative of but are not intended to be limitations on the scope of the present invention.

The drawing is a graphic representation of the interrelations of carbide precipitates in various molybdenumbase alloys including the alloys of the present invention.

It has been found that molybdenum-base alloys containing judiciously selected concentrations and proportions of titanium, zirconium and carbon have properties unexpected from that which was previously known. The alloys of the present invention consist essentially of about, by weight, 1.25-2% Ti, 0.40.7% Zr, ODS-0.2% C, the atomic ratio of (Ti+Zr)/C being from about 2:1 to about 6:1, with the balance Mo. Percentages givenin this specification are by weight except where otherwise indicated.

In titanium and carbon-bearing dispersion-hardened molybdenum-base alloys, the major dispersion phase is titanium carbide. However, an excess of carbon with regard to the amount of titanium will allow the formation of massive Mo C which is not efiect-ive for strengthening in the manner of the finely dispersed titanium carbide. It has been recognized that a careful and judicious selection of .amounts and proportions of the elements titanium, zirconium and carbon results in the formation of a (Ti, Zr) complex carbide.

The inclusion of too small an amount of titanium will not provide enough titanium carbide for strengthening and also will permit the formation of Mo C. Too high a titanium content will lower the alloys melting point, form massive carbides, and increase the diffusion rate to the detriment of high temperature strength.

Zirconium in relatively small quantities stabilizes the titanium carbide phase. But zirconium within the range of the present invention forms a complex zirconiumtitanium carbide which is more stable than titanium carbide.

The function of carbon in the alloy of the present invention is that it is necessary for carbide dispersion. A critical amount of carbon must be present with the range of titanium and zirconium in order to form a line dispersion of the complex titanium-zirconium carbide. If there is too large an amount of carbon or two low an amount of titanium and zirconium, too much Mo C forms. This molybdenum carbide does not provide the strengthening effect of the complex titanium-zirconium carbide.

'Even though the .titanium-zirconium-carbide dispersoid 1 is much harder than the molybdenum matrix, it also is desirable that the precipitated particles be of a size small enough that there be few if any dislocations in each par-;

ticle, and so that dislocation movement in the particle is very limited if existent at all, thereby greatly increasing the etfective hardness ofthe dispersed particles. A

finer particle size also leads to smaller interparticle spacing, thereby improving the mechanical properties of the V alloys. "'Relative'to titanium-zirconium complex carbide precipitates in alloys of the invention, molybdenum carbide.

generally precipitates in the form of massive, sometimes feathery particles which serve little useful purpose in strengthening the alloy or in making it ductile. Although molybdenum carbide, serves a purpose in some related alloys as an intermediate phase for the purpose of promoting con-trolled aging reactions, it is not generally desirable as a constituent of a final alloy in condition for applica-- tion. Moreover, residual molybdenum carbide left after 1 controlled heat treatment in processing can cause. deleterious on-the-j-ob aging and embrittling of such alloys by conversion to other carbides at elevated tempera-.

ture during application, particularly with the aid of straininduced aging. Although for some applications it may be desirable to have an alloy that will age during use, it is also quite desirable to have alloys that will not age during use for other applications.

4 utilized in the formation of useful and controllable precipitates, and in which unwanted aging reactions can be avoided.

The following Table I is representative of the number of the alloys which were considered in the study ofthe alloy of the present invention.

Alloys 1, 2,1 3A and 4 were made in 251 pound heats by melting in a vacuum arc furnace; The composition given for alloyTZC is one of several with which the name TZC has beenused in the art. Alloys 1, 2, 3Aand '4 were then extruded and swaged to a total of, 93% reduction in area to A" bars. These bars weretested for strength and ductility after being stress relieved $2100";

F.fOI' about one hour. The results of these tests,,which were conducted in a vacuum at the temperatures shown,

are given in Table 11.

TABLE IL-TENSILE TEST RESULTS ON SWAGED ALLOYS Temp. U.T.S., 0.2% Y.S., El. (percent RA Strength to Alloy F.) ksi. ksi. in 1) (percent) Density Ratio (in.)

avoided. The precipitated carbon is present in the form of (Ti, Zr)C. By the choice ofappropriate combinations of composition and thermal and mechanical treatments,

alloy bodies can be produced within the limits of the pres cnt invention in which the carbide present is efliciently .75 tures from about room temperature (78.F.) to 3500 F5 In Table 11 and subsequent tables, U.T.S. means:

Reduction in Area and 'ksi. means ,thousands of pounds per square inc The densities .of alloys 1, 2,,

3A,.3B and 4 were respectively, in pounds per square inch: 0.359; 0.3 60; 0.360; and 3.342; 7

By comparison, the strength of alloy 3A at temperais unusually high and unexpectedly better than other alloys tested.

Alloy 3B was produced by consumable arc melting in a vacuum furnace to a 31-pound ingot. The ingot was extruded at 3500 F. to sheet bar at an extrusion ratio of 4 to 1. The resulting bar, 1.93 inches wide by 0.94 inch thick, was bisected along the plane of thickness and rolled to 0.05 inch thick sheet. The rolling procedure consisted of 50% reduction in thickness at 2750 F. at 10% per pass, 50% at 2500 F. at 10% .per pass, and final reduction to 0.05 inch thick at 2200 F. at 10% per pass. The heating was done in a hydrogen furnace, with the material being soaked at temperature for 5 to 10 minutes between passes. Highquality sheet was produced with a material recovery of greater than 90%. The results indicated that the alloy is quite fabricable. With appropriate adjustments in techniques, the melting difliculties should be readily overcome.

Bend tests were preformed on the sheet at room temperature in both the stress-relieved condition (2100 F./ 1 hour) and the recrystallized condition (3000 F./1 hour). In the stressed-relieved condition the alloy sustained a full bend around less than a IT radius (T equals thickness of the sheet), while the recrystallization treatment only raised the radius for full bend to 2T, indicating no significant embrittlement on recrystallization.

Tensile tests were performed on 0.05 inch thick sheet of alloy 3B as stress-relieved at 2200 F. for onehour and as-recrystallized at 3200 F. for one hour. The results are presented in Table HI below. SR indicates the stress-relieved condition and RX indicates the recrystallized condition. The specimens had 0.5 inch long gauge lengths, inch wide. The major surfaces were as-pickled in the rolled condition, and the tests were performed on an Instron machine at a nominal strain rate of 0.05 per minute. Elevated temperature testing was done in a vacuum of about 10* torr.

TABLE III.'IENSILE PROPERTIES OF SHEET ALLOY 313 Test Condi- Tensile TensilefDen- 0.2% Yield Elonga- Texnp., tion Strength, sity Ratio, Strength, tion,

F. ksi. iu. 1[)- ksi. Percent The tensile data on the alloy 3B sheet compare very favorably with those on the alloy 3A swaged material shown in Table II for the following reasons:

First, as in the swaged condition of alloy 3A, the 0.05-in. sheet material possesses an excellent combination of high-temperature strength and low-temperature ductility which appears far superior to that of any other molybdenum sheet alloy reported in the literature. The high-temperature strength is illustrated by the tensile strength of 28,600 and 14,300 p.s.i. at 3000 and 3500 F., respectively, corresponding to a tensile strength to density ratio of 80,000 and 40,000 inches at the respective temperatures. This remarkable high-temperature strength is accompanied by room-temperature tensile elongation of almost 20% (probably corresponds to a 10-15% elongation in a l-in. gage length) which agrees well with the bend test results. Second, the literature is full of examples which show much lower strength in sheet form than in bar or plate form. The sheet alloy 3B material is shown to have strength and ductility properties very similar to those of the strongest swaged alloy 3A condition up to 3000 F. Above the latter temperature, the swaged material becomes superior mainly because of its higher recrystallization temperature of 3500 F., as compared with 3000" F. or 3200 F. for the sheet material. In this connection, it may be noted that the titanium and carbon contents in alloy 3B are 1.28% and 0.08%, respectively, which are lower than the 1.6% and 0.13% present in alloy 3A. The excellent properties of the sheet material, in spite of the possibly adverse chemical difierences, further accentuate the importance of processing and heat treatment in controlling microstructure and properties.

Table IV compares alloys 3A and 3B with some other titanium-zirconium-carbon-molybdenum alloys. The data on alloy 3B are from sheet specimens, those on the other alloys are from swaged specimens.

TABLE IV Strength to Alloy Temp. U.T.S., ksi. E1. percent Density F.) in 1 inch Ratio The data of Tables II and HI show that alloys of the invention have an unexpected and unusual combination of high strength at low as well as at high temperatures along with good low temperature ductility not found in closely similar alloys. It has been found that a preferred range in which alloys of the invention are included consists essentially of, by weight, 1.252% titanium, 0.4-0.7% zirconium, 0.080.-2% carbon, with the atomic ratio of (Ti+Zr)/C being from about 3:1 to about 5:1, with the balance molybdenum. The higher titanium bearing alloys as represented by alloys 4, 45, 6, 44'and 1720 were very much weaker and had a much lower strength-to-density ratio than do the alloys of the present invention. This is true even for alloy 44 which has the same general zirconium and carbon content as the alloy of the present invention but with a higher titanium content. Referring to Table IV, for example, it is seen that alloy 44 has less than half the strength of alloy 3A at 3000 F. with a change of less than about 1.5% titanium.

Alloy TZC has too low a content of alloying additions and too low a ratio of titanium plus zirconium to carbon to possess the desired properties. Specifically, the ratio leads to the formation of Mo C which is difficult to control and the undesirable characteristics of which have been discussed above. The higher titanium content alloys without zirconium, alloys 4 and 1720, had such high ratios of titanium to carbon that it was quite difiicult to produce a fine dispersion of precipitate. These alloys have relatively low recrystallization temperatures and unattractive strength properties.

Similarly, alloys 4 and 45 having lower carbon and alloy 6 having higher carbon content are substantially weaker than the alloy of the present invention. Alloys 1 and 4 show dramatically the effect of the absence of zirconium in the presence of titanium and carbon, and in particular with regard to alloy 4, the effect of high titanium and low carbon in the absence of zirconium.

Although the presently claimed compositions may appear superficially to be similar to those of prior art, they are in fact critically different as shown in part by a par-. ticular kind of susceptibility of alloys of the invention to alterations in their mechanical properties through ther- 7 mal and mechanical treatments. This susceptibility or capability is,in itself, a valuable property. It is present in alloys within the composition limits stated inthe specification and the claims, and having the specified ratios between the Group IV-A metals (zirconium and titanium) and carbon. These ratios as claimed establish a critical relationship between the Group IV-A metals and carbon which determines the type and morphology of carbides which will be precipitated with various types of heat treatments. The claimed ratios are all above a threshold .ratio which is the maximum which would generally allow precipitation of Mo C. Therefore, as stated above, all the carbides precipitated in alloys of the invention are carbides of the Group IV-A metals. Previously known alloys in the molybdenum-titanium: zirconium-carbon system which do not meet the ratio or proportion limits in addition to the composition limits produce alloys that are different in kind from those of the present invention. zirconium plus titanium to carbon ratio substantially less various heat treated and worked conditions.

For example, an alloy having a.

more, the present invention is directed toward alloys having a carbon content low enough in relation to the zir conium and titanium contents to substantially avoid .the

presence of Mo C, as distinguished from alloys that sists essentially of, by weight, 1.5-2% Ti, 0.4-0.7% 'Zr,

0.1-0.2 C with the balance .Mo. These composition limits, of course, establish a range of ratios of (Ti+Zr) /C. The two extreme limits of this range on an atomic basis are about 2.13:1 and about 5.94:1. 'In like manner, the

ratio of (Ti+Zr)/ C inherent in the composition of alloy 3A'is about 3.70:1, and inalloy 3B about 5:1.

In order, to enable a better understanding of the criticality of the amounts and proportions of alloying additions in the alloys of the invention, consideration will now be given to various types of carbide precipitation reactions in molybdenum-base alloys containing various amounts and proportions of titanium, zirconium and carbon. The drawing integrates the information discussed below and should be referred to for perspective and for comparison of the characteristics of the various alloys. The addition of titanium in Mo-C alloys first results in the appearance of TiC at low temperatures, with Mo C remaining as the high-temperature equilib rium carbide. .In the TZC alloy which had a (T i +Zr)/ C ratio of 1.8, the stability ranges were characterized by complete TiC dissolution at 3500 F. and persistance of Mo C down to about 3000 F. The effect of further increase in Ti is-seen to raise the equilibrium solution temperature of TiC, while restricting the stability range of Mo C. Thus, in alloys 1 and 2 which contain 1.6- 1.8% titanium with (Ti+Zr)/C ratios of 3.2-3.5, the TiC phase did not completely dissolve until about 3750 F., while the Mo C was no longer stable below 3300 F.

This effect of titanium continues until a concentration is reached which eliminates the M0 0. phase completely, as in the case of alloys 4 and 1720. It is interesting to note that while a substantial amount of M0 0 remained stable in alloy 1 which had a Ti/C ratio of 3.5,none existed in alloy 1720 which had a Ti/C ratio of 5. The threshold Ti/C ratio for eliminating the Mo C phase thus appears to lie at about 4.5 in the Mo-Ti-C system.

Compared with titanium, zirconium proved to be much more potent in stabilizing the monocarbide at the expense of M0 0. The diiference, indeed, appears to be zirconium in alloy 3A was suflicient to eliminate Mo C which was a predominant high-ternperature carbide in Furthermore, although the atomic concentra-' alloy 1. tion of titanium in alloy 3 (about 3.2% was some five times that of zirconium, the monocarbide was found to. be zirconium-rich, as evidenced by the] large tlattice parameter and the strong zirconium emission intensities: These results indicate that the threshold Zr/C ratio inthe Mo-Zr-C system is considerably below the Ti/C ratio in the Mo-Ti-C system. More recent data indicate the threshold Zr/C ratio to be below 2.5,"as compared E with the Ti/C threshold ratio. of 4.5 in the Mo-Ti-C.

system.

In alloys of the invention, the zirconium limits claimed,

particularly the lower limit of 0.4%, are critical to the properties, structure, andcharacteristics of the alloys.

:Below 0.4%, and generally outside of the composition range of 0.40.7%, zirconium does not give the desired aging kinetics for the formation and growth of the com-( plex carbides, and does not give the desired stability and morphology to the precipitates. Moreover, insuflicient amounts of zirconium will not insure against the presence of deleterious amounts'of M0 0, the undesirable charac-' teristics of which have been discussed above.

As compared to. alloy 2, alloy 3A represents an interesting case by virtue of its moderately higher (Ti+Zr) /C ratio and zirconium content. These two characteristics influence the carbide nucleationinsuch a favorable manner that the alloy develops a fine and extensive dispersion both by partial precipitation on cooling and uponwork ing or aging: Thus, the carbide is utilized more eflectively in alloy 3A than in alloys 1 or 2,1 as it is not tied 1 up in large clusters of decomposedMo c sometimes seen in the worked or aged conditions of the latter two alloys.

From lattice parameter studies, it is apparentthat essentially all the zirconium addition in alloy 3A is consumed in the .(Zr, Ti.)C phase, thereby indicating thatthe,

amount of zirconium dissolved in the molybdenum matrix is so small as to make insignificant any possible soluof alloy 2 can be improved, though witha slight sacrifice of strength at room temperature, through the use of -Heat Treatment B. However, through the use of such heat treatment, there is an improvement in high temperature strength as well, although alloy 3A within thepreferred range of this invention is unexpectedly im-. proved with all heat treatments shown. V .Heat Treatments A, B and C, following extrusion at 3500-3650 F., are as follows:

Heat Treatment A: As-extruded material swaged 93% at 2500 F.-2100 F.; stress-relieved at .2100 F. for, one

hour in hydrogen- Heat Treatment B: As-extruded material aged at'2500" V F. for 50 hours before swaging 93% and stress-relieved as in Heat Treatment A."

Heat Treatment C: Assextruded material swaged 88%; at 2500 F.-2100 F.; stress-relieved at12200 F. for one hour in hydrogen.

As used in Table TABLE V Temp. Heat U.T.S. 0.2% Y.S. El. (percent RA. Strength to Alloy F.) Treatment (ksi.) (ksi.) in 1") (percent) Density Ratio (inches) One-hour vacuum annealing at various temperatures was found to lead to recrystallization in alloy 3B at temperatures between 3000 and 3250 F. depending on variations in heat treatment before the rolling process. Material which was given a vacuum anneal of 3200 F. for one hour prior to rolling recrystallized at about 3000 F after rolling, while the material that was rolled directly from the extruded condition recrystallized at about 3250 F. Although the recrystallized grain size was slightly larger in the directly rolled material, neither material underwent substantial grain growth in one hour at temperatures below 3750 F.

The heat treatment and processing of the alloys of this invention involve control of the interplay between dispersion hardening and strain hardening and can be applied to usefully vary the properties of alloys of the invention.

Thus, the present invention involves a dispersion-hardened molybdenum-base alloy of unusual characteristics in an unexpectedly critical range of compositions and proportions in order to control the amount, type and distribution of complex carbides formed.

Although this invention has been described in connection with specific examples, those skilled in the arts of metallurgy and heat treatment will recognize the variations and modifications of which the present invention is capable without varying from its scope.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An improved molybdenum-base alloy consisting essentially of about, by weight, 1.252% titanium, 0.4-

0.7% zirconium, ODS-0.2% carbon, the atomic ratio of titanium plus zirconium to carbon being from about 2:1 to about 6:1, with the balance molybdenum.

2. An improved molybdenum-base alloy consisting essentially of about, by weight, 1.25-2% titanium, 0.4- 0.7% zirconium, ODS-0.2% carbon, the atomic ratio of titanium plus zirconium to carbon being from about 3:1 to about 5:1, with the balance molybdenum.

3. An improved molybdenum-base alloy consisting essentially of about, by weight, 1.6% titanium, 0.6% zirconium, 0.13% carbon, the atomic ratio of titanium plus zirconium to carbon being about 3.7: 1, with the balance molybdenum.

4. An improved molybdenum-base alloy consisting essentially of about, by weight, 1.28% titanium, 0.58% zirconium, 0.08% carbon, the atomic ratio of titanium plus zirconium to carbon being about 5:1, with the balance molybdenum.

5. The alloy of claim 1, wherein the precipitating phase is essentially all a complex carbide of zirconium and titanium.

References Cited by the Examiner V UNITED STATES PATENTS 2,678,269 5/1954 Ham l76 2,678,271 5/1954 Ham 75176 2,947,624 8/1960 Semchyshen 75-176 2,960,403 11/ 1960 Timmons 75176 DAVID L. RECK, Primary Examiner. W. o. TOWNSEND, Examiner.

Claims (1)

1. AN IMPROVED MOLYBDENUM-BASE ALLOY CONSISTING ESSENTIALLY OF ABOUT, BY WEIGHT, 1.25-2% TITANIUM, 0.40.7% ZIRCONIUM, 0.05-0.2% CARBON, THE ATOMIC RATIO OF TITANIUM PLUS ZIRCONIUM TO CARBON BEING FROM ABOUT 2:1 TO ABOUT 6:1, WITH THE BALANCE MOLYBDENUM.

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