Presented before the D i v b i o n ofInd.trirl and Engineering Chethe Amerieai Chamicrl Society, Atlantic City,
at the 114th Meeting e€
N-J.
POTENTIAL USES OF TITANIUM META -
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0. C. Ralston and F. .I. Cservenvak Bure~uof Mines, Washington, D. C.
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ITANIUM, a metal inadvertently but correctly named after the Titans, may naturally be expected to have many usea. As a pure metal it is unusually strong,and because it haa a specific gravity only a little over half that of steel it has the advantage of exceptionally high strength-weight r a t i v t h e real baais of comParjson. Another great advantage is ita cormxion mistance, which places titanium in the mame c k aa the s t a i n l m steels. It is obvious, therefore, that the biggest immediate field of usefulnem is in aviation, where high strength, light weight, and re. sistance to corrosion are all important. The anme applies to marine conditions, but the necemity for getting nothing but top quality is not so pressing. This paper gathers some details on the usea of titanium IYI dis-
Potential ueea of titanium a n h a d on the favorably combined properties of high strength, light weight, and
cl& by the literature and reported by those who are actually tasting ita suitability for numerow purposes. Many of the p o t e h l u868 have minor i m p o h c e 88 far hs amounta of metal are concerned, but everything taken together indicates a pmmking future for enterprisers. The Bureau of Mines pioneered and developed the fvst commercially adaptable procese for the production of high-purity titanium metal. It haa been the bureau's policy to distribute new products to qualified technical and scientific organisationa to expedite industrial interest and exploitation. Thin practice bas accelerated commercial development in the c m of electrolytic manganese and is doing the same for titanium. The potential u ~ e sof titanium are based msinly on the favor-
given. Although this d a t i v e l y new metal with umkue and highly deshble propertiem h s a promisins fuNle, its pduction today is too m t l y to warrant the c o n c l d ~ ~ ~ that it will have almost universal applications and mmpek with steel, alumhum, and copper where these chcppsr
wbtance to corrosion. U w of titanium as d h l o d by the Literature and reported by those who are aotuaUy tenting its suitability for numerous purposes are cited. The field ofusefulness by the Army, Navy, and Air Fora is
metals can function satisfactorily. -
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February 1950
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INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
ably combined outstanding properties of high strength, light weight, and excellent resistance t o corrosion. Its development should have a n important bearing on the future welfare of the nation not only as a substitute for presently used strategic materials in short domestic supply, but also for construction materials having a combination of properties not possessed by other available metals and alloys. It is estimated (4)that titanium is the fourth most plentiful metallic element in the earth's crust suited for structural uses. Commercial ores of titanium are widely distributed in the United States and the cost of developing them has been charged in part to the titanium oxide industry, which, up t o the present, has been the main use for titanium. The titanium industry has been featured year after year by the record-breaking consumption of titanium dioxide in pigments for paints, the only large quantitative use (17). Production of the oxide, therefore, has provided excellent sources of ore for the development of the metal.
Indnstrial Uses
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The availability of high-purity titanium will advance its use far beyond prior applications where alloys or impure titanium metal have been previously utilized. These prior uses are wellknown and include ferrotitanium as a scavenger or purifier for steel, as a primary precipitation hardening element, and as a ferrite former (22). I n the form of ferro alloys, titanium is used as an alloying element in austenitic stainless steels (23)to stabilize carbon and prevent intergranular corrosion. It is also employed in heat-resisting alloys, weld-rod coatings, and in permanent magnets (11). Titanium belongs t o that class of elements that causes unmixing when added in small amounts t o melts of iron and ferrous sulfide, converting the ferrous sulfide into titanium disulfide (35) similar to the action of manganese. The metal is principally used in steel because of its property of forming stable carbides and nitrides; it acts also as a deoxidizer. Steel with a titanium content of only 0.02% reduces the equilibrium oxygen content in a melt just as much as 0.2%silicon ($99). Titanium is claimed t o be suitable for use in chromium-free, heat-resisting steels (26). It was also found most effective in a group of alloying elements used t o reduce grain size in highchromium, heat-resistant steel (24). It was found possible to get high creep-strength values for certain steels containing titanium (9). I n low-carbon steels tested for high-temperature use, it is believed that titanium will prevent hot shortness due t o sulfur and that, therefore, manganese above the usual residual content of about 0.15% is unnecessary (5). The addition of titanium stabilizes enameling iron ( 2 7 ) and steel (6) by converting the carbon t o a more stable form. Most defects in the vitreous enameling iron result from reaction of an iron-carbon aggregate with the molten glass. Addition of titanium in enameling steel stabilizes the carbon and stops its reaction with the oxides in the enamel coating that forms gas and blisters. Porosity is eliminated in Monel castings by use of titanium t o combine with the hydrogen and nitrogen. I n aluminum alloys, titanium acts as a grain refiner and strengthener. Small quantities of titanium added t o copper or nickel give strong age-hardening alloys which may be utilized as a substitute for tin (16). At 900" C., titanium spreads in a thin layer over copper surfaces, which suggests the titanizing of ferrous plates t h a t have first been given a copper coating. Titanium forms extremely hard nitrides and carbides, the latter being useful for special cutting tools. A high-temperature, 70% nickel alloy, Inconel X, uses titanium now introduced as ferrotitanium. The availability of unalloyed pure titanium low in nitrogen and oxygen might mean t h a t 7 to 10% titanium could go into the production of this alloy (14). .Incone1 X possesses outstanding spring characteristics a t elevated temperatures and may have application in the gas turbine and jet engine fields.
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Tests indicate that addition of titanium decreased grain size and increased flexural strength of cobalt silumin and manganese silumin (la),. The gettering or gas-absorbing property of titanium is well known. Gases such as oxygen, nitrogen, carbon dioxide, etc., are absorbed by titanium in the production of high-vacuum tubes. Chips of titanium can be used t o remove oxygen and nitrogen from helium and argon. During World War I1 there was considerable interest in Germany on certain potential uses of high-purity titanium ($1). In the vacuum tube industry, tests were conducted with pure titanium in sheet form for use as functional elements and as getters. Addition of the metal to certain steels seemed t o enhance their high-temperature resistance properties and t o lower their creep rates. Some of the German work included cladding steel with high-titanium alloys of copper, nickel, and cobalt t o provide fairly good heat-resistant surfaces. It was also indicated that light armor plate might be prepared by incorporating titanium into a steel surface, followed by carburizing and heat treatment. Preliminary experiments indicated t h a t laminated plate formed from sheets of titanium showed promise for this purpose. Consideration was also given to the application of titanium in jet propulsion aircraft. It was thought that the metal might have useful applications in the production of mirrors because of its excellent resistance t o corrosion and condensation. A very thin layer of the ductile metal was evaporated onto a glass surface t o make such mirrors. A low-carbon German steel with 0.5% titanium was used ( 1 ) as sheet material in jet planes for high-temperature service and for turbine wheels. It was reported (3) that when small amounts of titanium were added t o Thomas steel, the product could be used t o make machine gun barrels. The German war metallurgy (16)brought out a type of alloy that was protectively chromized by gaseous chromous chloride or chromic chloride. Various basis metals were chromized most effectively and an important type was a steel-containing titanium. The influence of titanium as grain refiner is evident in all of these processes. I n one instance, the titanium proportion is definitely specified t o be less than enough t o combine with the carbon content of the steel. High-temperature creep strength was attained by hot-working the metal above 1100" C. A heat treatment for titanium-bearing chromized steel was also described, heating at 500' t o 900 O C. and then quenching in air, water, or oil. Products manufactured from such steel were heat- and scale-resistant and good for exhaust parts of internal combustion engines. Pearlitic nickel-vanadium steel with less than 0.2y0 carbon, u p t o 1.5% vanadium and 3% nickel (in which the vanadium could be replaced by titanium) was corrosion-resistant and had high impact strength a t lower temperatures. It was designed for vessels and tubing for refrigerator coolers. It is claimed t h a t a basis metal of austenitic manganese steel with 12 t o 22% manganese and under 0.2% carbon should contain a grain refiner like titanium, zirconium, vanadium, or molybdenum t o impart resistance to corrosion at high and low temperatures when chromized, high hot strength, toughness, and high fatigue strength. The phenomenon of marking glass with pure titanium is well known. The Elass should be clean and free of grease or oil, and if it is wet with a mild alkali it is possible t o draw consistent lines without pressure. The mark is essentially a streak of smeared titanium, but the surface is also scratched, as can be noted by dissolving the metal with dilute hydrofluorir acid. Many minute scratches are formed transverse t o the direction of marking. Rubbing titanium metal against other hard surfaces often produces smears which are difficult t o remove. Such smears have been investigated (28)on diamond, corundum, hematite, magnetite, rutile, chrysoberyl, spinel, quartz, topaz, diopside, beryl, albite, axinite, epidote, garnet, and tourmaline. Titanium
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INDUSTRIAL AND ENGINEERING CHEMISTRY
smears can be used to make high electrical resistances by marking glass with a metal point or pencil. The smearing of titanium parallels the seizure of aluminum in the early days of experimenting on aluminum engine blocks and pistons. That difficulty was cured by alloying and heat treatment. Therefore, any trouble that might be encountered with pure titanium under similar conditions or in operating titanium shafting against bearing metals should not indicate that all forms of the metal are unsuitable for such purposes. The combination of stainlessness, high proportional limit, and low modulus makes titanium an interesting material for springs where a considerable expansion in relatively low loads is required. Main and hair springs for watch and clock products may be improved by replacing the steel wire now used with titanium. Titanium or alloyed titanium hair springs may be used in control equipment such as time devices for water softeners that are usually installed in basements, which in many cases are damp, and give trouble due to rusting of the hair spring. Titanium has been suggested for use in boiler and boiler-feedwater systems as replacement for 18-8 stainless steel, which is about the only metal resistant to high purity and aerated water. Some suggested uses for titanium in the boiler plant include valve seats, corrugated gaskets, and pump parts. I n applications where titanium proves insufficient, consideration should be given to its sister metal, zirconium. Since titanium resists attack by dilute sulfuric acid and also by concentrated acid, especially in the range of 40 t o 60%, it might be used in recuperator elements for preheating air in power plants. Some of these elements are alternately exposed to temperatures ranging from below the dew point of the flue gases to 1000" F., and there is a tendency for condensation and absorption of sulfur dioxide from the flue gases, forming acid in varying concentrations, which attacks the metal, The thin oxide film which will form a t the maximum temperature might be of assistance in resisting the acid attack. It is hoped that titanium can be used for measuring chambers and for powdered-metal gears in water meter gear trains, both of which suffer greatly from corrosive waters. A small percentage of titanium is used in steel tubing a t a steam power plant ( 2 ) t o superheat steam from 850' to 1050' F. in the last two stages of a three-stage system. It is believed that titanium can be used as a rotary shaft seal operating in fuming nitric acid because of its chemical resistance t o this acid and its possibility of resisting frictional wear. The resistance to frictional wear, mainly dependent on the service conditions, may be increased by surface hardening in a cyanide bath (7) or introducing oxygen by heating under controlled conditions. Temperature control is necessary t o obtain the desired surface hardness without scaling. The ability of titanium to be surface-hardened suggests its use for parts subject to frictional wear, such as cutting tools, dies, bearings, pistons, cylinders, and other engine components (8). Titanium appears to have a future in the design of portable machine tools where strength and light weight combined with a definite proportional limit are desirable. It is also being considered for use in sensitive geophysical instruments. Titanium powder, with its sister metal, zirconium powder, has been used experimentally in a low-melting alloy matrix in producing sparking flints for cigaret lighters. Experiments showed that a satisfactory flint can be produced as a substitute if cerium or misch metal became scarce. The pyrophoric properties of these flints are satisfactory, although their softness leads t o rapid consumption. An optical firm is experimenting a i t h titanium evaporation onto glass surfaces to produce high-index films. Successful tests have been made on sealing hard glass t o titanium wire by a n electronic tube manufacturer, Because of its wetting properties, titanium has found use in an alloy for sealing window glass to metal frames ( 2 3 ) . Approximately 201, titanium is preferred in a copper alloy ( I S ) used in metalizing glass. The allov is easily
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atomized into droplets small enough to eliminate largely local shattering of glass by droplets of hot metal. I t has been estimated that there are 12,000,000 orthopedic patients in this country, of whom 2,000,000 wear full braces. Because of its light weight titanium could replace stainless steel in the manufacture of such braces. It is also believed that titanium might have a place in the prosthetic dentistry field in making stronger appliances for the mouth. Titanium may be used for medical and dental equipment Fhere strength and freedom from corrosion have prime importance. Preliminary tests indicate that titanium would be satisfactory for metal coil springs in oviduct diaphragms. It has been suggested for spectacle case3 where strength and light weight are required. It is claimed that aluminum cases are too soft and steel cases too heavy. The extra cost of titanium is compensated by the added protection given in rimless glasses. Titanium should also be useful in making fancy frames for women's glasses. Although aluminum frames are light, they reportedly become dull and lose their polish. I n cutting lens shapes the glass blanks may be marked with a titanium pencil. The special inks now being used for making glass blanks come off during handling and edging. Corrosion studies with various foods ( I O , I Q ) , such as pineapple juice, cider vinegar, lard, tea, coffee, grapefruit juicc, and lactic acid showed no attack on titanium. This suggests that titanium could be used for food handling and processing equipment in restaurants and lunch counters. Its low thermal conductivity indicates many possible household uses, such as handles for pots and pans. The development of titanium may be an important contribution to the electronic industry for making miniature capacitors of smaller size than now obtainable. Tests have been made on oxidizing the surface of titanium to a thickness of 0.5 mil of oxide and by firing a silver film on the exposed side in nitrogen atmosphere. It was reported that higher capacities per unit volume of the composite metal and oxide were produced than is possible using ceramic material. X-ray targets of titanium have been prepared and have proved to be satisfactory in practice, showing no signs of deterioration. Only a thin layer of titanium is necessary on a water-cooled copper base because of the low thermal conductivity of titanium. A considerable reduction in the amount of scattered radiation is indicated by the use of titanium radiations in x-ray diffraction photographs. It is believed that the intensity of radiation from a synchrotron may be increased by enclosing barium aluminate in a titanium foil jacket with a heater buried in the center. There is reason t o expect that the barium, which diffuses through small holes punched in the titanium, will condense on the surface of the titanium and provide a greater source of electrons than can be obtained from a pure tungsten or a coated filament that can he provided in the available space. The stability of the discharge in lamps is greatly improved by the use of titanium electrodes (20). Titanium may also be used as a capacitor electrode material and as an electrode for disintegrator drills. Interest is also being shown in titanium for the fabrication of thin-window Geiger tubes for counting beta particles and in a radioactive isotope of vanadium which may be conveniently obtained from a titanium target by deuteron bombardment. Titanium should find use in textile machinery ( 8 ) for highspeed, light-weight spindles, spools, warp beams, and other working parts. It may be used for sporting equipment such as light-weight noncorroding golf clubs, tennis rackets, and fishing rods. Its corrosion resistance, together with its ability to surface-harden, is being utilized in making pen points and styluses.
February 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY Military Uses
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Potential applications of titanium by the Army, Navy, and Air Force were reported a t a recent symposium on titanium sponsored by the Office of Naval Research. The property most attractive to the Navy and Air Force is the outstanding resistance of titanium t o corrosion by sea water and saline atmospheres. Corrosion tests on titanium in sea water have shown scarcely any effect on the metal and indicate that titanium is more resistant to salines than any common engineering material. The life span of naval equipment subject t o corrosion could be greatly prolonged by the use of titanium (SO), and this metal would thus compete economically with materials now produced a t a lower cost. Numerous possibilities exist for the use of titanium on ships. Possible applications are for lightweight piping systems handling salt water, condenser tubes operating with high water velocities, replacing Monel or stainless steel in plumbing fixtures, substituting for stainless steel in pump rods or rotor shafts, and for water-lubricated and antifriction bearings of high load capacity operating in salt water. Titanium wire may be used for shipboard radio aerials if tests prove that it is resistant to stack gases. Small, high-speed propellers may be made from titanium if satisfactory resistance t o hydraulic cavitation is shown. T o date, few, if any, alloys tested have better corrosion resistance than pure titanium for the above applications. Numerous applications of titanium for aircraft are now being considered ( I @ , the majority of which involve substitution of titanium for other materials in structures where intermediate temperatures, weight, or corrosion problems are encountered. Where a high strength-weight ratio or resistance to salt water corrosion is required, titanium could be substituted for lightweight alloys and steel. At high speeds the organic protective coatings on leading edges made of light alloys are removed by dust and rain impingement, and the efficiency of the airplane is decreased because of increased turbulence and drag resulting from roughening of the edges. The present high-strength light alloys begin t o weaken a t temperatures between 300 O and 400’ F. Elevated temperatures are becoming more common in aircraft as a result of aerodynamic heating a t high speeds, use of heat for deicing, proximity of structural parts t o jets and afterburners, and higher temperatures in the compressors of jets and turbines. Titanium is preferable t o steel for moderate-temperature service because of its lightness and corrosion resistance. Titanium has also been suggested for special aircraft applications, such as high-speed hot-air heater wheels, armor plate, electrical components, pontoons, cables, structural braces, wing coverings, and fuselage construction. Army engineers are interested in titanium for such equipment as truck bodies, girders, and other members of portable bridges. Air-borne equipment used by future armies undoubtedly will utilize light-weight, strong titanium advantageously. The following quotation from a letter from the Office of the Chief of Ordnance, Department of the Army, will show the vital interest of the armed forces in uses of titanium: The Ordnance Department, Department of the Army, is intensely interested in the potential uses of titanium metal for military applications. I n the past it has directed its efforts in research and development to the more basic problems such as preparation of equilibrium diagrams for the more promising alloys, effects of impurities, and other metallurgical investigations. It will continue t o emphasize the more fundamental aspects of this research t o the utmost of its capabilities t o rovide data urgently needed in the development of titanium a& s. The Ordnance D e artment is particularly interested in d e use of this metal as a repyacement for other metals where light weight, high strength, hardness, corrosion resistance, shock resistance, etc., are required. Ordnance equipment of the future is pointing more and more towards air-transportability. The over-all weight
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of equipment which the foot soldier must carry into battle must be reduced to render him more effective. The combat man must have more effective protection with lighter weight against whatever missile the enem might employ. I n order to accompEsh these objectives, the Ordnance Department must depend largely upon industry and others t o devote as much of their efforts as possible to: perfecting economical methods for reducing titanium ore and producing the metal; the proper alloying of titanium metal to secure maximum toughness, hardness, strength, resistance t o impact, corrosion resistance, etc. ; and appropriate welding techniques and fabrication procedures for the various alloys which may be developed. The apparent advantages to be gained by the use of titanium in military equipment are so marked that the significant future improvement of e uipment may well be predicated on an early solution to the probqems outlined above.
Conclusion Commercially pure titanium is a new base metal whose alloys have not yet been developed. Extensive development and testing are now being conducted on high-temperature alloys. The study of alloys based on titanium is still in its infancy, and some very useful alloys may be developed that will be as far superior t o the pure metal as stainless steel is to iron. When the essential information on titanium alloys is available, the usefulness of the new metal can be fully evaluated. Although considerable progress in alloying is being reported, the results are not conclusive enough to permit making definite recommendations. Commercially pure titanium is a very good metal for a wide variety of uses, but alloying might greatly improve it and increase its advantages. The principal improvements noticed in alloying have been in its strength rather than its corrosion resistance. From certain articles that have appeared in newspapers and the popular press, one might be led to believe that titanium is destined to replace the better-known structural metals and have almost universal applications. Although this relatively new metal has unique and highly desirable properties, its preparation today, even with the advances made in its recovery from natural sources, is too costly to warrant the conclusion that it will compete with steel, aluminum, and copper for purposes where these cheaper metals can function satisfactorily.
L i t e r a t u r e Cited (1) Alexander, F., U. 8. Dept. Commerce, O.T.S., PB 10567 (1945); Main Rept. PB 1635; Bib., 1, p. 858. (2) Anon., Iron Age, 163, 64 (1949). (3) Bacon, N. H., et al., BIOS Final Rept. 685, 53-4 (1946) : U. 8. Dept. Commerce, O.T.S., PB 49219; Bib., 3, 954 (1946). (4) Clarke, F. W., U.S. BeoE. Survey, Bull. 770, 36 (1924). (5) Comstock, G. F., Metal Progress, 56, 67-71 (1949). (6) Comstock, G. F.,and Wainer, E., Iron A g e , 155, No. 7, 60-3, 152-3 (1945). (7) Dean, R. S., U. S. Patent 2,453,896 (1948). (8) Dean. R. S.,and Silkes. B.. U. 8. Bur. Mines. Inform. Circ. 7381 (1946). (9) Fischer, W. A., U. S.Dept. Commerce, O.T.S., PB 7044 (OTIB Rept. 2478) ; Bib., 1,567. (10) Gee, E. A., Golden, L. B., and Lusby, W. E., IND.ENG.CHEM., 41, 1668-73 (1949). (11) . . German Patent Amlieation 566.029: U. 9. Dent. Commerce. O.T.S., PB 13706‘(1943). (12) Gurtler, G., and Juna-Konig, - W., U.S. Dept. Commerce, O.T.S., PB 39044 (1937).(13) Haven, C. D., U.9. Patent 2,369,350 (1945). (14) International Nickel Co. of Canada, Ltd., address to shareholders, April 1949. (15) Lawrence, P. H., and Samuel, R. L., BIOS Final Rept., 1534 (1946): U. 5. Dept. Commerce, O.T.S., PB 97015 (August 1946). (16) Metal Hydrides Inc., Beverly, Mass., “The Hydride Process and Its Products.” (17) Meyer, H. M., Eng. Mining J . , 150, No. 2, 89 (1949). (18) Promisel, N. E., “Report of Symposium on Titanium,” pp. 5-11, sponsored by Office of Naval Research, Washington, D. C., 1949. (19) Remington Arms Co., Tech. Bull., p. 22 (1949).
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, (1948).
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(26) (27) (28) (29)
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
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.
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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
