INDUSTRIAL AND ENGINEERING CHEMISTRY Rentachler, H. C., Henry, D. E , and Lilliendahl, W. C., Trans. Electroohem. Soc., 91, 7 pp: (1947) (preprint). Smatko, J. S., U. S. Dept. Commerce FIAT F i n d Rept. 798, (1946) ; PB 31246; Bib., 2, p. 588. Smith, R., Wyche, E. H., and Gorr, W. W., Trans. Am. Inst. Mining Met. Engrs., 167, 313-345 (1946).
Stewart, R. S.,Can. Mining J.,70, No. 8, 60 (1949). Svechnikov, V. N., and Alferova, N. S.,Stal, 7, 331-6 (1947). Vogel, R., and Kasten, G. W., Arch. Eisenhiittenw., 19, 65-71,
(26) (27) (28) (29)
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Volker, W., Ibid., 19, 69-72 (1948). Wainer, E., Bull. Am. Ceram. SOC.,25, 248-59 (1946). Webster, ’VV. A., and Macdonald, G. L., Nature, 160, 260 (1947). Wentrup, H., and Heiber, G., Arch. Eisenhiiftenw., 13, 69-72 (1939).
(30) Williams, W. L., “Report of Symposium on Titanium,” pp. 92104, sponsored by Office of Naval Research, Washington. D. C., 1949. RECEIVED Aueust 31, 1949
(1948).
Walter L. Finlay, John Resketo, and Milton B. Vordahl Remington A r m s Company, Znc., Bridgeport C o n n .
T h e optical metallography of titanium is, with a few modifications appropriate to its special characteristics, not markedly dissimilar to that of other metals in general. The special characteristics of titanium which are of controlling importance in its optical metallography are that titanium undergoes an allotropic transformation at 885” C., it possesses great reactivity at elevated temperatures, andit deforms at room temperature both by slip and
by twinning. Photomicrographs illustrate the effects of these characteristics. Metallographic techniques which have been developed are outlined and typical microstructures of commercially pure unalloyed titnnium in a variety of metallurgical conditions are presented and discussed. Interesting structures obtained with titanium-base alloys arepresented to show the applicabilityofthemetallographic technique to a vvide range of titanium-base structures.
T
HE optical metallography of titanium promises t o be a t least as complex as that of iron and many microstructural analogies exist between the two. Titanium, moreover, can be expected to exhibit several unique features and it is already evident that the study of its metallography will be both fascinating and rewarding. This paper outlines the metallographic techniques used in the Remington Arms’ research laboratory and presents a number of representative findings. It is necessary to emphasize a t this point, however, that the metallography of titanium has not yet been very well developed so t h a t all the information presented in this paper should be considered as tentative and subject to later correction as further information is gathered. Both high-purity and commercial-purity titanium as well as a number of alloys of both are discussed. The high-purity titanium was made by the decomposition of titanium tetraiodide on an incandescent titanium metal filament. It was prepared by the Battelle Memorial Institute. The commercial-purity titanium was purchased from the Pigments Department of the E. I. du Pont de Nemours & Company, Inc. Blloys were made by vacuum arc fusion from the highest purity alloying elements available.
deformation and no twins can be observed. In Figure 1, fj? however, a piece of the same material was slightly deformed as a result of being clamped in a vise while sawing off a specimen. The true condition of the material is shown by Figure 1, A , the clear areas of which are in sharp contrast to the heavily twinned regions shown in B. Not only can such twins obscure the true structure but they might also occasionally be mistaken for transformation structures or for Widmanstatten precipitation. Figure 2, A , for example, shows the herringbone pattern of mechanical twins which was formed when a particle of polishinggrain scratched the surface of a commercially pure specimen. This is more clearly shown a t higher magnification in Figure 2, B , where it can be observed that the crystal structure must be critically oriented to the direction of the scoring particle if the herringbone pattern of twins is to be formed. Thus in A the herringbone pattern is seen to be interrupted a t the center of the photomicrograph where a differently oriented grain intrudes. An additional factor in determining whether visible mechanical twinning occurs is the impurity content as discussed later in the paper.
Special Characteristics
Caution should be exercised in cutting the specimen so as n u t to introduce deformation twins which cannot be removed in subsequent polishing. Care should also be taken t o avoid the associated phenomenon of what might be termed polishing twinning. I n addition to these, the chief caie to be taken in metallographic preparation is in the elimination of scratches from the polishing papers-Le., the most difficult polishing stage is that between the papers and the final polish. The rough polishing procedure employed approximates that used for many other metals and includes the use of a belt sander (60 x), emery cloth (180 x and 320 x on glass), and emery papers l / O through 3/0. Intermediate polishing is usually carried out with 600-mesh Carborundum on a lead-foil lap on glass. Canvas has been tried
The characteristics of titanium which are particularly pertinent In a study of its optical metallography are the facts that it undergoes an allotropic transformation at 885” C. from the low temperature-hexagonal-close-packed phase to the high temperaturebody-centered-cubic phase; that it possesses great reactivity a t elevated temperatures; and that it deforms a t room temperature both by slip and by twinning. The first means that transformation products may be present in the microstructure; the second, that the commercially pure product invariably exhibits tnro or more phases; and the third, that mechanical twins resulting from specimen manipulation may obscure the true microstructure. Figure 1, A , shows the annealed structure of titanium of very high purity. This specimen was prepared with care to avoid
Titanium Metalhographie Teehniqae
I
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February 1950
acid attacks the metal; the nitric acid brightens the surface by removing stain and residue; and the glycerol acts as a vehicle and moderator, I n many cases i t is advantageous to eliminate the brightening effect of the nitric acid in which case Betch is employed: 1 part by volume 48% hydrofluoric acid 1 part by volume glycerol *
s
B
A
Figure 1.
High-Purity Iodide Titanium (75X)
(.
B
A
Commercially Pure Titanium
Annealed, unalloyed, and polishing twinned, B. (150X) A . (75X)
with some success also. The abrasive-water suspension should be made in a beaker and levigated by decanting after allowing the stirred suspension to stand for 3 t o 5 minutes. The abrasive should be introduced into the lead lap by covering it with the abrasive suspension, and rubbing i t into the lap with a clean flat piece of metal. Excess suspension may be washed away. The lap must be carefully handled and kept clean. Two steps are usually employed in final polishing:
.4
As-Cast. Polished, unetched, high-purity titanium presents completely blank surface. Commercial-purity titanium, on the other hand, may exhibit second phases polished in relief such as the titanium carbides shown for three as-cast ingots in Figure 3 with increasing carbon content. Etching such specimens reveals the structure shown in Figure 4. The matrix in Figure 4 does not have the obvious a - c a s t structure characterizing Figure 5. Unlike the former, however, the high alloy composition of Figure 5 does not transform from beta t o alpha in cooling from the freezing range to room temperature. Thus its as-cast structure, with its pronounced coring and fine eutectic lamellae, IS preserved. The beta dendrites formed on freezing the Figure 4 material, however, are completely erased by the transformation to alpha. The Widmanstatten plates resulting from this transformation have, in Figure 4, been partially obliterated by coalescence. Continuation of this trend by working and annealing below the transformation results in the equiaxed alpha shown subsequently. Worked. Hot and cold working tend tjo break up aggregations of inclusions such as the strings of carbides in Figure 3. In the volume of the metal, these strings constitute planes of low ductility and predispose towards fracture so t h a t their disintegration is desirable. Figure 6 shows an early stage of breakup at3 effected by hot forging, finishing just below the transformation. As a result of the relatively slow cooling from this temperature, the matrix is equiaxed alpha. Later stages are shown in Figure 7 and in the recrystallization series of Figure 8. Carbides are strikingly delineated by the sequence of A and B etches as can be seen by comparing Figure 4, which was prepared by the use of A-etch alone, with Figure 7 , B, for which both A and B etches were employed. Second phases can also be simulated by drying stains, a pitfall not peculiar to titanium. Figure 9 shows some A-etch drying stain carbides which subsequent polishing and etching removed completely t o reveal an equiaxed, single-phase field. Cold working equiaxed alpha by more tlian about 20% area reduction noticeably elongates the grains. Indeed, this is often the only metallographic evidence for the presence of cold work since, as Figure 8, A, shows, neither deformation bands nor twins are present in this specimen of 70% cold-worked, commercial-purity titanium. This is in sharp contrast to the proa
A . Unalloyed, as-deposited, A-etch 8. Unalloyed, as-deposited, and mechanically twinned, A-etch
Figure 2.
Some Representative Titanium Miorostrnotares
A waxed billiard cloth is employed with a silica abrasive suspension in water. Linde silica has been found t o be satisfactory. A few drops of denatured alcohol are placed on the waxed billiard cloth t o act as a wetting agent for the wax, dry Linde silica is added, and, finally, sufficient water is mixed in t o form a heavy slurry. Polishing should be carried out with circular motion, and, in general, the use of power on the wheel should be avoided. Gamal levigated alumina suspension or B-grade Linde yalumina suspension on Gamal cloth has been found to give a satisfactory final polish when the polishing is carried out with a circular motion on a slowly rotating, power-driven wheel.
Hydrofluoric acid appears to be the most effective etching agent for the metallographic evamination of titanium. The authors havedesignatedthe folloning solution as A-etch: 1 part by volume 48% hydro-
fluoric acid I part by volume concentrated nitric acid 2 parts by volume glycerol In this etchant, the hydrofluoric
B
A
Figure 3.
C
Commercially Pure Titanium with Titanium Carbide (75X) As-east, unetehed
A.
Ti-O.3C
8. Ti-0.5C
C. Ti-I.OC
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 4. Commercially Pure Titanium (75X)
Figure 5. Iodide Titanium A110)- (300 X )
Unallos ed, as-cast, A-etch
Ti-30XIo-2.8Si, B-etch
F i g u r e 6. Commercially Pure Titanium (75 X )
artifact-precipitate or polishing-scratch residueUnalloyed, as-forged, A-etch especially since the material is the highest purity iodide titanium with a low hardness of 78 T'lclrers and an estimated purity of 99.96yG. It is an odd and as yet unexplained circumstance that some but not all of the highest puiity iodide titanium samples contain the relatively large amount of precipitate shown. The dark etching of these precipitate platelets by A and their light etching by B establishes them as true second phases. This conclusion is confirmed by the coincidence of several platelet faces with the polishing plane in the lower grain of Figure 12, A and B. Quenched. Quenched titanium structures are many in number and are importantly affected by many variables. The base-line condition might be taken to be slow-cooling, cold-worked, highpuiity titanium from below (figure 10) and from above (Figure 13, A ) the transformation. The former results in small, clear, equiaxed grains: the latter in equally clear but much larger and more irregularly shaped grains. Rapidly quenching the former effects no metallographic change but rapidly quenching the latter giyes rise to the characteristic serrated grain struc-
B
A.
F i g u r e 7 . Iodide Titanium Alloy A Ti-O.5C
annealed unetched (150x1
6: Ti-0.5C: 50% cold'rolled, annealed for 1 hr. at 700' C., AB-etch (300X)
+
nounced tR-inning in Figure 1, B, and Figure 2, A and B. This absence appears to be associated with impurity (or alloy) content and is being investigated further. Annealed. The course of recrystallization of the i O % coldworked, commercial-purity material is shown in Figure 8, B-E. S o unusual features are exhibited. Figure 8, F , affords an interesting comparison with Figure 8, D, its much smaller amount of cold work being accompanied by a much larger grain size. Figure 8, A-F, was prepared with A-etch and the outlining of grain boundaries is not particularly good. Betch, by darkening the grains themselves to varying degrees, achieves a superior delineation. This is shown for high-purity titanium in Figure 10, the identical field being employed for both etches. Although B-etch dramatically darkens or-titanium as demonstrated in Figure 10, B , i t merely outlines @-titanium grains as shown in Figure 11
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A
B
C
E
F
1
D
F i g u r e 8.
C o m m e r c i a l l y Pure T i t a n i u m (75X)
IJnallnvedl. A-etch ._,__, . A . 70% cold rolled ~
B. C. D. E.
F.
~~~
70% cold rolled, 70% cold rolled, 70% cold rolled, 70% cold rolled, 10% cold rolled,
annealed for annealed for annealed for annealed for annealed for
1 hr. at 487' C. 1 hr. at 610" C. 1 hr. at 715' C. 1 hr. at 785' C. 1 hr. at 715' C.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 9. Iodide Titanium (500 ,- x . .,
Unalloyed cold rolled and annealed for'l hr. a t 700°'C., A-etch (with drying stains)
Figure 11. Iodide Titanium Alloy (150X) Ti-SOMo, held 15 min. at 950' C., quenched t o 550' C., held 15 min., air cooled, B-etch
1
221
ture shown in Figure 14, B. These serrations are alpha grain boundaries whose angularities possibly reflect the Widmanstatten character of the transformation giving rise to them. Quenching commercialpurity titanium below the transformation effects, as in the case of the high-purity material, no change associA B ated with the quench. But quenching commercialFigure 10. Iodide Titanium (75X) purity material from above Unalloyed, 30% cold rolled, annealed for 1 hr. a t 700' C. the transformation gives rise A. A-etch B. B-etch to different structures than that of high purity since the commercial grade differs in two pertinent respects from the high-purity iodide product. First, the transformation point of the highpurity material is widened out to a range (up to perhaps 885" * 25" (3.); and, secondly, the rate of transformation may be different in the commercial material. Both effects may be attributed to the oxygen a,pd A B nitrogen contamination of Figure 12. Iodide Titanium (300X) the latter. In any case the Cold rolled, heated 1 hr. a t 1000° C., quenched into water, heated 1 hr. a t 850' C., quenched into water Widmanstatten character of the transformation is much $: ~~e,ttcchh more evident in the commercial grade. A variety of structures is obtained: the basket-weave of Figure 14, A and B; the nonparallel acicular of Figure 15, A ; and the parallel acicular
.
B
R
Figure 13. A.
B.
Iodide Titanium (75X)
Unalloyed, furnace cooled from 1000° C.,A-etch Same as A except quenched into water from looOo C.
A
Figure 15.
B
Nonparallel, A, and Parallel, B, Acicular Structures of Titanium (75 X )
A-etch A. Iodide titanium, Ti-O.101, quenched into water from 1000° C. B. Commercially pure titanium, quenched into water from looOo C.
-A
Figure 14.
B
Commercially Pure Titanium
Unalloyed, quenched into water from 1OOO' C.. A-etch A. (75X) B. wax)
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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
Vol. 42, No. 2
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%
