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of Figure 15, B. The first two are of more frequent occuirence than the last in the authors' experience, the first being associated with slower quenching rates. The parallel plates of Figure 15, B , permit a fairly accurate measurement of the angles between the Widmanstatten plates to be made. These prove to be six in number with the follom ing angular distances from an arbitrary reference line: O D , 9", 46.5", 54.5", and 60". These findings are consistent with the formation of alpha plates along dodecahedral beta planes. The interesting grain boundary transformation reaction illustrated in Figure 16 for a n isotheimally transformed l2Y0 copper alloy is offered as the final example of the versatility of hydrofluoric acid-base etches for titanium. Kot only is i t interesting for the physical metallurgy involved but also for the lines t h a t might be mistaken for polishing scratches. Actually these are swab marks through the product of the etching reaction and
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in this particular alloy could be avoided only with the greatest difficulty. I n such cases swabbing can sometimes be eliminated.
Summary Metallographic polishing and etching procedures have been outlined and illustrated for high- and commercial-purity titanium and titanium-base alloys in a variety of metallurgical conditions I t is concluded that many useful analogies t o the metallography of the older metals exist and that the future holds the promise of more than a few metallographic findings peculiar to titanium alone.
Acknswledgmemt The permission of the Remington Arms Company, Inc., to publish this paper is acknowledged. RECEIVED September 26, 1919
Bruce W. Gonser Battekle Memorial Institute, Columbus, Ohio
T h e favorable atomic size of titanium and its other fundamental properties favor alloying with nearly all the useful metals. Practically, the relatively high melting point of 1728" C. discourages alloying with those metals having high volatilization rates below this temperature. Reaction of titanium with nitrogen, oxygen, and carbon is of great importance, not only in raising difficulties in handling but i n profoundly affecting the mechanical properties by forming interstitial solid solution alloys. A transformation poimt a t 885' C. for the pure metal may
be drastically reduced or raised by alloying additions, This transformation in some alloys makes possible considerable strengthening, by heat treatment, and small grain size, by retaining some of the beta constituent on cooling. A large field of useful alloys is indicated and is being explored rapidly. Among the most useful are combina tions of metal additions, particularly those having considerable solid solubility in either of the allotropic forms of titanium, plus small amounts of carbon, oxygen, or nitrogen.
HE growing importance of ductile titanium as a metal of the immediate future is now well known. I t s good metal properties, the abundance of its ores, and its comparative lightness have rightfully attracted the interest of chemical, mechanical, and aeronautical engineers, as well as of metallurgists. After securing the reasonably pure or ductile metal by methods which are commercially feasible, the next logical step has been to explore alloying possibilities. These are almost limitless. As with most of the useful metals-iron, copper, aluminum, and zinc-the pure metal has a useful field, but the alloys probably will far exceed the parent metal in breadth of application and usefulness. An amazingly large amount of research is in progress on titanium alloys for so young a metal. Only a few results have been released for publication, however, and progress is being made too rapidly t o cover the status of the various titanium alloy systems at this time. It is the purpose of this paper to point out some of the general factors involved in alloying, rather than to make a comprehensive list of properties that can be obtained.
atomic diameter range have a size factor that is favorable to forming considerable solid solution with titanium by atomic substitution (9). In addition, there is reason t o expect some of the elements of small atomic size, as carbon and hydrogen, t o form solid solutions interstitially. In general, then, this leaves only a few metals of comparatively large atom size-the rare earths and most of the alkali and alkaline earth metals-being unpromising alloying constituents from the standpoint of atom oiza arid solid solubility. Some of the elements which may be unfavorable for forming solid solutions with titanium may form intermetallic compounds. These may be useful as minor additions in alloy building. No metal has been noted so far t h a t will not alloy with titanium to some extent. A polymorphic metal, titanium has a hexagonal, close-packed crystal structure at normal temperatures, but changes to a bodycentered cubic structure above about 885" C. This offers an opportunity for development of properties by heat treatment of some alloys. However, interesting and useful as the transformation may be, it adds complexity t o alloying considerations. Titanium is a Group IV-A transitional element, considering it from the standpoint of atomic structure. As such, it forms solidsolution type alloys with other transitional elements like chromium, molybdenum, tungsten, columbium, tantalum, zirconium, iron, and manganese. The bonding with these on alloying includes exchange force coupling between unpaired d-electrons, 88 well as by the usual metallic electron bonding.
Some Fundamental Consideratilmm T h e position of titanium among other metals of the Periodic System is very favorable for alloying. As shown in Table I, which gives the interatomic distances of 40 metals and a few nonmetals (6,8), practically all of the common and most useful metals have interatomic distances within 15% of that of titanium. This is important in showing that the metals within this 15%
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An interesting observation along these lines was made from some powder metallurgy work at Battelle Institute (4) on the alloys of titanium with the Group V and Group VI transitional metals-columbium, tantalum; and chromium, molybdenum, and tungsten, respectively. Here it was noted that the strengthening effect was greater for Group VI metals than for. the Group V metals. Presumably the Group IV metals, zirconium and hafnium, would have a still lesser strengthening effect, and this 8eems to be the case. Also, the lighter elements in any one group gave the greater strength. In Group VI, chromium was the most effective in adding strength, molybdenum next, and then tungsten. In Group V, columbium was a more effective strengthening addition than tantalum.
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only does it pick up a n excesaive amount of oxygen and nitrogen with disastrous effect on ductility long before the melting point is reached (unless heated in a vacuum or completely inert atmosphere), but the molten metal reacts with every known refractory crucible material. This has required the development of unusual handling methods. Alloying Procedure. The Erst successful method for alloying titanium with other metals was by powder metallurgy. This has been used by Kroll (IS), the Bureau of Mines (16), P. R Mallory and Company ( I 4 ) , and others. The usual procedure hm been to mix the constituent metal powders, compact a t a pressure of 30 to 50 tons per square inch, and sinter in vacuum for at least an hour or two at a maximum temperature of about 1100" t o 1200" C. Recently, Long (16) described a method of sheath rolling, whereby the mixture of inetal powders t o form the desired alloy are sealed into a welded metal container and coasolidated by hot working. A temperature of only 800 t o 1000' C. is used and advantages are gained in time and by forming the alloy by hot rolling while it is being compacted. This generd method of alloy formation is useful but inherently expensive. Also, difficulties in diffusion may give inhomogeneous alloys. This is particularly true when alloying with high melting metals like tungsten, molybdenum, tantalum, and chromium. Attempts t o make titanium alloys by melting the constituent metals in refractory crucibles in all cases have met with e m tamination. I n some c s e s , this contamination may not be too serious. Brace ( I , $ ) has described the induction melting of some fifty titanium alloys in thorium oxide whereby useful results were secured, even though there was obvious contamination of the metal from oxygen and thorium. Sutton (19) hrts described the induction melting of titanium sponge in graphite without getting more than 0.4 to 0.7% carbon in the ingots. A similar method obviously can be used in making alloys. O
Figure 1. Types of Titanium Diagrams A . Complete alpha and beta solubility B . Complete beta solubility
A general classification of alloying constituents can be made on the basis of compatibility of structure with that of titanium. Thus, only zirconium and hafnium with the same type of structure, transition, and favorable atom size have complete solubility in titanium under all conditions-that is, in both the alpha and beta phases. This gives a n interesting type of minimum point in both freezing and transformation curves of the equilibrium d i a g r a m shown schematically in Figure 1A. Alloying elements that extend the range of solubility of the hi& temperature, or beta, allotrope form a very important group. Some having the same crystal structure, body-centered cubic-molybdenum, tungsten, tantalum, and columbiumhave complete solubility with p-titanium. This type of phase diagram is shown in Figure 1B. Another group with a less favorable combination of atom size and crystal structure-as chromium, iron, manganese, and possibly nickel-extend the beta Eeld but form intermetallic compounds a t higher alloy concentrations, rather than having solid solubility in all proportions with the beta phase. Among the most important elements that extend the alpha Eeld are oxygen and nitrogen, which alloy interstitially. The extent of their solid solubility is very high, amounting to about 20 atomic yofor nitrogen and 40 atomic % for oxygen. Carbon also alloys interstitially, but has surprisingly little solubility in both alpha and beta forms of titanium. This, unfortunately, prevents realization of the same important type of hardenability by heat treatment from titanium-carbon alloys that is obtained in steels.
Praetioal Considerations Titanium has a rather high melting point, 1725OC., hence, alloying with metals of low boiling point presents obvious difficulties. This has discouraged attempts t o alloy such metals as zinc, cadmium, magnesium, arsenic, and mercury with titanium, even by powder metallurgy methods. The greatest practical difficulty in making alloys of titanium, however, is caused by its reactivity a t high temperatures. Not
I 20
:
Ioc
8C
6C
40
2c
C
I
0.25
I
0.5
I
0.75
I
I
I
Akrnic Per Cent
Figure 2. Tensile Strength and Elongation of Alloys ef Iodide Titanium with Oxygen, Nitrogen, and Hydrogen
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Possibly the most successful method of avoiding contamination in making titanium alloys by melting has been by arc melting in argon in a water-cooled copper crucible. Equipment for doing this in a small dkhed container was described by Kroll (19) in 1940. Considerable work has been done a t Battelle since then in developing equipment for melting various sized ingots of titanium and its alloys in water-cooled copper crucibles (18). The excellent heat conductivity of copper permits it to stay sufficiently cold as a crucible to inhibit alloying, or even wetting, by molten titanium. Segregation is still a problem for alloys that melt over a wide
/
2 5C
I
/
200
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Effects of Some Alloying Additions on Mechanical Properties I t is difficult to evaluate the few results that have been reported on properties of titanium alloys. Usually there has been sufficient contamination, particularly with nitrogen and oxygen, to cloud the effect of the purposely added constituent. Since the gaseous constituents are so important their effects should be considered first. Oxygen, Nitrogen, and Hydrogen. The effects of these elements singly on some of the properties of relatively pure titanium prepared by the tetraiodide decomposition method have been described by Jaffee and Campbell (11) and by Cross (4). Figures 2 and 3 show the chief results on some mechanical properties. Wire specimens of titanium were heated in vacuum, and known amounts of the desired gas were added and diffused into the metal to secure the compositions shown. In later work it was found to be equally effective t o add oxygen or nitrogen as the oxide or nitride, respectively, to a melt of titanium. Hydrogen, a t least in additions of up to only 1 atomic %, has no pronounced effect on strength, hardness, and ductility, as indicated by percentage of elongation. Since hydrogen is absorbed reversibly by titanium, an excess can be removed by melting or heating in vacuum somewhat below the melting point. Although both nitrogen and oxygen have a strong effect on the mechanical properties, nitrogen seems to be considerably more severe in adding strength and losing ductility. This does not necessarily mean that oxygen and nitrogen contamination of titanium is always bad. On the contrary, their presence within limits may add desired strength. Oxygen tends to refine the crystal structure and nitrogen promotes the formation of needles of a-titanium. As pointed out by Jaffee and Campbell ( I O ) , the unaorkably brittle range is reached with concentrations of above 0.5 weight % ’ of either gas. The trouble is, oxygen and nitrogen contamination
I50
Table I.
100
0.25
0.5 0.75 Atomic Per Cent
Figure 3. Average Cross-Sectional Hardness Values for Alloys of Iodide Titanium with Oxygen, Nitrogen, and Hydrogen
For experimental work in alloy exploration with iodide titanium, small arc furnaces making an alloy melt of only a few grams have been used advantageously. Melting is extremely rapid, and a surprisingly large number of properties can be determined from the small castings secured. There are some limited possibilities in alloying by codeposition when producing titanium in the massive form by the iodide decomposition method. Purity of Metals Used. The serious embrittlement of titanium by contamination with oxygen and nitrogen, as well as by a number of other alloying elements, is now well recognized. Ductile metal, now commercially available and made by msgnesium reduction of titanium tetrachloride, is reasonably free from contarninetion and is useful in determining general alloying effects, However, there are still sufficient impurities present in such metal to give misleading results a t times when attempting to obtain the true effect of an alloying addition. The purest titanium available, that made by thermal decomposition of the tetraiodide (3, 6), is essential for fundamental work on alloying and for determining precise properties in simple alloying systems.
Atomic Size of Some Alloying Elements ( 8 )
Element Hydrogen (6) Oxygen (6)Nitrogen ( 0 ) Carbon Boron Beryllium Silicon Gallium Germanium Iron Chromium Sickel Manganese Cobalt Arsenic Copper Vanadium Molybdenum Tungsten Rhenium Zinc Platinum Aluminum Tantalum Columbium Gold Silver Tit aniu in Antimony Mercury Tin Cadmium Lithium Bismuth Zirconium Indium Hafnium Magnesium Tliallium Lead Thorium Cerium Sodium Calcium Potassium
Interatomic Distance ?’ & Difference from Titanium
d , A. 0.46 1.32 1.42 1.54 1.8 2.25 2.35 2.44 2.45 2.48 2.49 2.49 2 . 5 (approx.) 2.5 2.51, 3.16 2.55 2.63 2.72
2.74 2.75 2.78 2.77 2.86
2.86 2.86 2.88 2.88
2.88,2.95 2.90, 3.36 3.0 3.02, 3.17 3.04 3 03 3.11, 3.47 3 . 1 3 , 3.19 3.14 3.17 3.20 3.42 3.49 3.60 3.64 3.71 3.93 4.62
84 54 51 47 38 22 18 15
*
15
14 13.5 13.5 13 13
;; (i-9) 9 6 5 5 5 4 1 1
1 0 0
1, 17 4 5, 10 5 5
8, 20 9 , 11 9 10 11 19 21 25 26 29 36 60
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+70~
Figure 4. General Arrangement of Most of the Possible Alloying Elements in ReIation to Atomic Size of Titanium
.
can occur so easily in making the titanium, preparing the alloy, o r processing it, that attainment of desired properties by alloying is constantly jeopardized by their presence in unpredictable amounts. Metal Additions. I n early work on, making ductile titanium by the iodide decomposition process as reported by Van Arkel (.% littleI) attention , was given to the effect on mechanical properties by metal additions. Such work was discouraged by the small amount of the pure metal that was available and by serious contamination on melting. Kroll ( l a ) reported in 1937 the general effect on rollability and hardness of a number of metal additions-aluminum, beryllium, chromium, cobalt, copper, iron, manganese, molybdenum, nickel, silicon, tantalum, tungsten, vanadium, and zirconium. However, the alloys were formed by powder metallurgy from titanium of 310 Brinell hardness, produced by sodium reduction of the tetrachloride, and results are indicative only. In later work some alloys were made by Kroll ( l a ) by arc melting with titanium of 180 to 200 Brinell hardness produced by magnesium reduction of the tetrachloride. Qualitative results were reported on the general hardening effect of thorium and columbium, as well as of several additions mentioned in the previous group. At the Symposium on Titanium, sponsored by the Office of Naval Research and held December 16, 1948, several papers dealt with titanium alloys. With the exception of investigations on the
effect of nitrogen, oxygen, and hydrogen, the work was based OM alloying with titanium powder or sponge (made by magnesium reduction of the tetrachloride) under conditions which should b e regarded as giving preliminary information. Although some results were given on mechanical properties of the alloys made, the conditions of alloying and testing were so divergent t h a t n o summary tabulation of comparative properties could be made. From the various scattered results that have been reported, a n d some unpublished information, the following general comments may be made on the effect of alloying some of the metals and metalloids given in Table I:
CARBON.Because of limited solubility, additions of more than a few tenths of 1%result in carbide formation. With the concen-
trations of carbon now bein obtained when melting carefully in graphite containers, a mocferate strengthening is imparted t o titanium without serious effect on hot working properties, BORON (14). Additions of 0.1 and 0.5% show some hardening but indicate no particularly advantageous effects. A 5% addition has made the casting unworkable cold, BERYLLIUM (18, 14). The solidus point is drastically reduced by beryllium, down to 975" C. with a 3% addition (by powder metallurgy). Alloys of up to 1% ber llium are readily cold worked, give ft somewhat higher strength &an does titanium alone after working, and show some indication of giving increased strength by heat treatment. SILICON. Titanium is embrittled by silicon but alloys of up to 1% can be cold rolled. A strength of over 160,000 pounds per
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square inch tensile strength was obtained by adding 1% silicon, without entire loss of elongation ( 1 4 ) . Silicon also reduces the melting range, down to 1225” C. with a 3% addition. IRON.In small amounts iron shows some promise as a strengthening addition. However, Larsen and co-workers ( 1 4 ) found a 10% addition to injure cold working properties. CHROMIUX ( 2 , 4, IS, 14). I n amounts up to 10% chromium rapidly increases the tensile strength and yield point of titanium, with corresponding decrease in ductility. It alloys well, giving increased strength with cold workability over a moderately wide range and appears to be one of the most promising primary alloying elements. NICKEL. Iiroll ( I S ) considered nickel to be one of the best alloying elements for titanium. A 5y0nickel alloy gave excellent rollability. Larsen and eo-workers ( 1 4 ) report on the basis of a 10% addition that nickel affects cold workability adversely. Long (15) suggests 6 to 8qo nickel as the limit for high temperature use. , MAYGANESE.Except for heavy losses of manganese through volatilization on melting, this was found to be one of the most interesting alloying metals by Larsen and eo-workers (14). Alloys of up to 15% manganese were made and hot rolled; a tensile strength of 164,000 pounds per square inch with 4.5y0 elongation was reported for a 5.6% alloy. COBALT.A sintered bar containing 10% cobalt showed poor cold working properties ( 1 4 ) ,but ICroll ( 1 3 ) found a 57c cobalt alloy t o hot roll very well. COPPER.So far, no alloys with copper have indicated any particularly useful characteristics. VANADIUX ( 2 4 ) . Additions of 95% grade vanadium up t o 5yc showed some increased strength but serious and rapid loss of ductility when cold rolled. ALUMIXUM ( 1 4 ) . Alloys with up to 6 or 7% aluminum have been readily hot worked and cold worked, giving indications of usefulness as a primary addition. MOLYBDENUX (4).This metal is useful as a primary addition as it alloys readily w-ith titanium and strengthens it increasinglv with increasing concentrations, a t least to 20%. TUNGSTEN ( 4 ) . Because of its wide beta solubility range, tungsten is also useful as a primary addition. A 20% addition by powder metallurgy has given 150,000 pounds per square inch tensile strength with 4.5y0 elongation as hot rolled and furnace cooled. One 4.7y0tungsten alloy with some nitrogen-oxygen contamination, made by arc melting, gave 198,000 pounds per square inch tensile strength with 2 7 , elongation as hot forged. TANTALUM ( 4 , I d ) . Alloys of up to 20y0 tantalum have been prepared which behave similarly to titanium alone in both hot and cold rolling properties. Co1,cniBIvu (4, 12). Additions of up t o the 20% concentration of columbium reported so far strengthen titanium moderately, and the alloys can readily be hot rolled or cold rolled. Ductility seems to be lowered only slightly by rather large additions of columbium. TIN, Although melting at a low temperature, the high volatilization point of tin makes alloying with titanium possib!e. Such alloys show only moderate strengthening properties. ZIRCONIUM.Although zirconium alloys readily with titanium throughout the composition range, there has been no evidence of the development of unusual properties. Moderate hardening and strengthening with loss of ductility has been reported for a 10% zirconium addition ( 1 4 ) . LEAD. Addition of a few per cent of lead to titanium is poisible It increases strength moderately. THORIUM.Kroll ( l b ) states that alloys of titanium with thorium are very soft, but no further data are available 011 mechanical properties. I n general, alloying additions within the limits teuted so far have not markedly improved the resistance of titanium to oxidation. McPherson and Fontana (16) found some indication of improvement by a 17% chromium addition, but their data were incomplete, Brace (1)has noted a very substantial improvement in oxidation resistance on melting titanium in a thoria crucible This has been tentatively ascribed to thorium pickup, but oxygen (and nitrogen) contamination may be a factor also. Alloying additions that raise the transformation temperature, likewise appear to raise the temperature for rapid oxidation, since the alpha allotrope seems to be the most resistant to oxidation ( 7 ) . Alloys made by powder metallurgy are much more sensitive t o oxidation than
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those made by melting. Alloys riracic. by melting can be worked practically to the transformation t,emperature without protection, or above this temperature if the contaminated outer layer is ground off. High strength a t high t,emperature has not been a property of commercially pure titanium. By comparatively large additions of chromium, molybdenum, and tungsten, high temperature strength and resistance to creep is greatly increased ( I ) . N o information is yet available on the fatigue properties of titanium alloys, or of creep properties, other than of the highly alloyed titanium-chromium-molybdenum-tungsten group. Since alloying may drastically raise the tensile and yield strengths of titanium without much change in density, the strength-weight ratio is, in general, very favorably affected. Alloying usually decreases the modulus of elasticity slightly; hence, is of benefit only in exceptional cases when equipment design is based on the modulus-density ratio. The stat,us of titanium alloys, then, is one of current rapid progress in general exploration of mechanical propert’ies from which no definite conclusions can be drawn as yet regarding the most important alloys of the future nor their properties. Initial work, already published, is fragmentary, and many of the results are open t o doubt because of the complexity of elements present from contamination and limited scope of testing conditions. As better control is obtained over both the alloying constituents apd the technique of alloying, more consistent and logical results are being obtained. Fundamental work on alloy systems is progressing rapidly and many publications of this nature can be expected soon. Indications are that the most interesting and practical alloys will be solid-solution primary additions, or mixtures of them, plus smaller additions of some of the leas soluble elements to gain specific properties. Fortunately, there are enough of both types of constituent,s to keep metallurgists busy almost indefinitely and to evolve alloys that, should meet a wide divervity of application.
Biblisgraphy ( 1 ) Brace, P. H., Ofice iVuvuZ Research, Rept. Titanium dyriiyosium. pp. 132-43 (March 1949). (2) Brace, P. H., T r a n s . ElectrochevrL. Soc., 94, 170-6 (1948). (3) Campbell, I. E., Jaffee, R . I., Blocher, J. M., Jr., Ourland, J.. and Gonser, B. W., I b i d . , 93, 271G3.5 (1948). (4) Cross, 1%.C., Ofice Y a z ~ nResearch, l Rept. Titanium Symposium, pp. 125-31 (hlarch 1949). (5) Engel, N.S . ,presented at, Battelle hiernoris1 Iristitate, Columbus, Ohio (1949). (6) Gonser, B. W., Ofice .YavaZ Besearch, IZept. Titanium Symposium, p p , 60-9 (March 1949). (7) Greiner, E. S.,and Ellis, IT. C., Metals Technol., 15, 6 (1948). (8) Hume-Rothery, W., “Structure of Metals and Alloys,” pp. 40-8. Institute of M e t a l s , 1945. (9) Hume-ltothery, W,, X n b b o t t , G . W., and Channel-Evans. K . hI., P h i l . T r a n s . Roy.SOC.,233(A), 44 (1934). (10) Jaffee, R. I . ,and Campbell, I. E., I r o n A g e , 164, 51 (1949). (11) ,Jaffee, R . I . , and Campbell, I. E., T r u n s . Am. I n s t . iWining M e t . Engrs. I n s t . Metals Dia., 185, 64G-54 (1949); J . Metuls. 1, 646--54 (1949). (12) Kroll, TV., T r a n s . Electrochem. Soc., 78, 35-46 (1940). (13) Kroll, W., 2 . Metallkunde, 29, 189-92 (1937). (14) Larsen, E. I., Swaay, E. F., Busch, L. S.,and Freyer, It. H., Ofice N a v a l Research, Rept. Titanium Symposiurn, pp. 10524 (March 1949). (15) Long, J. R., Ibid., pp. 27-48. (16) McPherson, D. G., and Fontana, AM. G., Ibid., pp. 12-17. (17) Metal Progress, 56, 345-68 (1949). (18) Simmons, 0. W., Greenidge, C. T., and Eastwood, L. W., Ofice Naval Research, Rept. Titanium Symposium, pp. 77-91 (March 1949). (19) Sutton, J. B., Ibid., pp. 73-6. ( 2 0 ) Van Arkel, A. E., “Reine Metalle,” p. 187, Berlin. Jiilius Springer, 1939. RECEIVED Yeptemher 26, 1 9 W
