US 2943960 A
Description (OCR text may contain errors)
y 1960 M. J. SAARIVIRTA Q 2,943,960
PROCESS FOR MAKING WROUGHT COPPER-TITANIUM ALLOYS Filed Aug. 27, 195-"; 2 Sheets-Sheet '1 Wren/007% TIHZI.
ATTORN EY PROCESS FOR .MAKIN G WROUGHT COPPER- TITANIUM ALLOYS Matti J. Saarivirta, Plainfield, N.J., assignor to American Metal Climax, Inc., New York, N.Y., a corporation ,of New York r Filed Aug. 27, 1957, Ser. No. 680,439 .4 Claims. (Cl. 148-1211 ,This invention relates to copper base alloys of the age hardenable type and relates more particularly to binary alloys consisting of copper and titanium characterized by markedly improved workability both hot and cold. In one of its aspects, the invention concerns copper-titanium alloys in wrought form possessing excellent tensile strength, improved resistance to softening at elevated temperature, good ductility, moderateelectrical conductivity and other desirable properties. In another of its aspects, this invention concerns methods of treatment whereby very high strength Wrought alloys are produced in a very practical, eflicient and economical manner. The new and novel wrought alloys provide excellent materials for use in the fabrication of springs, electrical contactors, tools and for numerous other applications requiring particularly high strength combined with other desirable properties and characteristics.
It has long been known that age-hardenable coppertitanium alloys may be processed to provide materials of high strength, in some instances resulting in a tensile strength of about 200,000 p.s.i. or even higher depending upon the amount of titanium present in the alloy. In US. Patent No. 1,991,162 issued to Kroll, there are dis closed, for example, copper-titanium alloys containing varying amounts of titanium, and in the case of the alloy containing 4% titanium, a tensile strength of 150 kg./mrn. (approximately 213,000 p.s.i.) and an elongation of 1% is indicated. To achieve these mechanical properties, however, it was apparently necessary to age the alloy and thereafter subject the aged alloy to cold-stretching.
Although hot and cold working of copper-titanium alloys prior to the aging step has also been heretofore suggested, the resulting wrought alloys, as far as I have been able to determine, possess much lower tensile strength values compared to the wrought alloys produced by coldstretching the aged material.
As is well known to those skilled in the art, the step of cold-stretching is not readily or conveniently applicable to all forms of the material being processed, and in some instances, the effective use of cold-stretching is entirely impractical if not virtualy impossible. Moreover, coldstretching the aged alloy lowers the resistance of the alloy to softening at elevated temperatures and generally reduces its resistivity toward corrosion.
Although copper-titanium alloys of the type previously reported containing up to 5 or 6% by weight titanium may be subjected to hot and cold working or both, the extent to which .these operations may be carried out is limited by .the actual workability of these alloys. In the case of hot working, for example, preheated bars subjected to hot rolling fracture severely on the first or second pass involving relatively small reductions in area. At the same time a reduction of more than 50% by hot working is actually required to completely deform the cast crystal structure and this cannot be accomplished with the conventional copper-titanuim alloys due to their marked susceptibility to brittleness on hot working.
Extensive cold working of the conventional coppertitanium alloys is likewise precluded due to the tendency of the material to become embrittled rather rapidly especially when the titanium content is about 3% or higher. Thus, the application of cold working to the extent required in conjunction with appropriate heat treatment of the cold worked material to develop a suitable condition conducive to the production of high strength alloys is entirely impractical without resorting to cold-stretch-' ing the aged material which procedure, as previously indicated, is of limited applicability as well as being -detri-' mental to other properties of the resulting alloys.
It is a principal object of this invention to provide newand novel wrought copper-titanium alloys possessing ex-- cellent strength and other desirable properties without resorting to cold-stretching the previously aged alloy as heretofore required for producing comparable strength.
Another object of this invention is to provide an entirely practicable and commercially applicable process for the treatment of age-hardenable copper-titanium alloys prepared under oxygen-excluding conditions and con-' taining up to the maximum solid solubility of titanium whereby high strength wrought alloys having other desirable properties including improved resistance to soft ening at elevated temperatures are readily obtained.
Other objects and advantages will become apparent as this specification proceeds.
It has now been discovered that the workability of copper-titanium alloys containing up to the maximum solid solubility amount of titanium is drastically affected by the presence of oxygen in any form in the copper used in making the alloys. found that when high purity, initially oxygen-free copper is used in making copper-titanium castings under oxygenexcluding conditions and containing an amount of titanium Within the maximum solid solubility in such copper, such castings possess markedly improved workability.
When these castings are processed into wrought mate-'1 rials such as wires, rods, sheet or in any other desired form employing appropriate working and heating cycles as hereinafter described in detail, materials of excellent tensile strength together with other highly desirable properties are readily obtained. The enhanced workability accounts for many important advantages including (1) eliminating the need for cold stretching the previously age-hardened alloy for the purpose of providing high" tensile strength, (2) making possible the production of extremely high strength Wrought copper-titanium alloys by processing steps involving extensive working after solution annealing but before the age hardening treatment in the art as meaning a high purity copper which is substantially entirely free of oxygen by virtue of its having been processed by any of the known methods suitable for the purpose. Although electrolytic copper which has been processed in a reducing atmosphere such as OFHC brand copper provides excellent results, copper prepared 4 in an inert atmosphere or in a vacuum or by any other suitable means as processing with carbon may also be used. Copper treated with chemical deoxidizers such as phosphorus and the like wherein the scavenging substance remains in the copper as an oxygen-containing compound or complex though sometimes erroneously re- Patented Jui 5, 11960 v More specifically, I have ferred to as oxygen-free copper is not satisfactory for use in making the wrought' copper-titanium alloys herein disclosed.
The titanium content of the cast material suitable for use in making the wrought copper-titanium alloys comprising the present invention may be varied between 1 and 6% by weight, the lower limit representing the minimum titanium which must be present for any appreciable age-hardening to occur, whereas the upper limit represents approximately the maximum solid solubility of titanium in oxygen free high purity copper which is found to occur at about 900 .C. With the use of at least 3% titanium, minimum tensile strength values of 180,000 p.s.i. may be readily obtained in the wrought alloy upon employing appropriate working and heating cycles, while near the upper limit of titanium content the tensile strength approaches 220,000 p.s.i. For providing the most desirable combination of properties of the wrought material suitable for most applications, however, it is preferred to use from 3.8 to 4.3% titanium whereby cold drawn wire having a minimum tensile strength of 200,- 000 psi. and other desirable mechanical properties including a minimum elongation of 1% and a 180,000 p. s.i. yield strength may be readily obtained after appropriate age hardening heat treatment of the worked material. Slight overaging of the alloys during precipitation heat treatment may be utilized to improve elongation and electrical conductivity with only a relatively small decrease in tensile strength.
For preparing castings suitable for use in making the high strength wrought alloys mentioned above, the titanium may be added to the oxygen-free copper as good quality titanium metal sponge, a copper-titanium master alloy, or in any other suitable form of satisfactory purity. It is essential that oxygen be excluded throughout the melting, alloying and casting procedures. In general practice, the initially oxygen-free copper protected by an inert gas such as helium or argon is first melted at 1200-1400 C. in a graphite crucible and the desired amount of titanium is then added allowing for usual titanium losses. The melt is then stirred with a graphite rod and, after a short holding period, the alloy is cast while still protected by the inert gas. A holding period of less than /2 hour at the designated temperature is usually sufficient to effect complete dissolution of the titanium whereby homogeneous castings which are free of porosity and have good surface characteristics are consistently obtained. Large alloy castings, such as commercial size wirebars, containing up to 6 titanium are found to be free of inclusions as determined by microscopic examination of their crystal structures.
Preheating of the castings to about 800 to 900 C. prior to hot working the alloy and subsequent solution annealing operations should be carried out in a protective atmosphere to protect the alloys from oxidation at the elevated temperatures. Heat treating involving the use of temperatures appreciably below 600 C. may be performed, if desired, in the absence of such precautionary measures to exclude air since relatively little, if any, oxidation occurs at the lower temperatures during the time interval generally involved.
The improved hot workability of the oxygen-free copper-titanium alloys compared to similar alloys all processed under oxygen-excluding conditions is readily seen from the appearance of the hot rolled bars shown in Fig. 1 wherein bar A consists of an alloy prepared with initially oxygen-free high purity copper (OFHC brand) containing 4.3% titanium; bar B is an alloy consisting of electrolytic tough pitch copper containing 4.5% titanium; and bar C is an alloy of phosphorus deoxided copper containing 4.1% titanium. In each case, 1" diameter castings were preheated to 900 C. and hot rolled in a rod mill. As is evident, bars B and C show marked susceptibility to brittleness and fractured severely on hot working as evidenced by their appearance after either the first or second pass (20-30% reduction). At the same stage of working, however, bar A shows no sign whatever of cracking. In fact similar specimens were found to be hot workable with reductions up to 65-85% without any indications of failure. It will be appreciated that this extent of working permits the desired complete deformation of the cast structure in the initial hot working step. A fourth bar (not shown) consisting of fire refined copper alloyed with 4.2% titanium completely disintegrated upon hot working during the first pass involving only a 20% reduction. Microscopic examination of each of the fractured samples revealed a great number of inclusions accumulated in each of the alloys excepting the oxygen-free copper-titanium alloy which was substantially free of inclusions. It is readily apparent from the foregoing that the initial oxygen content of either fire refined or electrolytic tough pitch copper which usually ranges from .03 to .05 markedly impairs the hot workability of castings alloyed with the designated amounts of titanium. Interestingly enough, when the copper is chemically deoxidized as in the case of the phosphorus deoxidized specimen (bar C), the alloys made thereof with titanium actually possess rather poor hot workability.
For best results the alloys of this invention are preferably hot worked in the red heat i.e. between 700 and 900 C. particularly when the titanium content exceeds 3%, but care must be taken-to avoid heating much above 900 C. to preclude the formation of any liquid phase which would render the alloys hot-short.
In addition to the highly improved hot-workability of the present alloys, they further possess excellent cold working properties. It has been found that all these alloys, when uniformly recrystallized in the course of solution heat treatment to effect a complete dissolution of the titanium, may be cold worked very satisfactorily. Final reductions of more than may be applied without overworking the alloys. It is preferred, however, to start the initial cold working with smaller reductions of about 3050% utilizing solution anneals between working cycles to obtain uniform grain size. By way of illustration, hot rolled rods solution annealed at 900 C. may be initially cold rolled with about 40% reduction. After resolution annealing the material may then be further cold rolled with about 50% reduction, after which successive solution annealing and cold rolling may again be preformed with up to a final cold reduction of about 94% without failure or any indications of an overworked condition. By contrast, the other copper-titanium alloys of the type shown in Fig. 1 containing comparable amounts of titanium become embrittled rather rapidly thereby making the fabrication of the material as well as the development of any comparable combination of properties exceedingly difficult.
Although solution heat treatment temperatures between 850 to slightly over 900 C. may be employed, it is preferred to solution anneal the alloys of oxygen-free copper-titanium under non-oxidizing conditions at about 900 C. at which temperature the maximum solid solubility of titanium in oxygen-free copper occurs. The time required to effect complete solubilization of the titanium may be varied depending primarily upon the titanium content of the alloy and the solution annealing temperature employed. It has been found that two hours at 900 C. is generally conducive to best results though somewhat longer or shorter periods may be actually used.
In making the wrought alloys in accordance with the present invention, the preferred sequence of processing steps comprises preheating the cast material to about 900 C., hot working preferably in the red heat, solution annealing, quenching, cold working to the extent required with intermediate solution annealings if necessary and subsequently precipitation hardening the worked material. It should be understood, however, that other methods for producing the wrought alloys may also; be
employed. For example, rolled without initial hot rolling as by solution heat treating the cast material, say for 2 hours at 900 C., quenching in water and then directly cold rolling with about the castings may be coldof titanium following precipitation hardening of the semiples for 2 hours at 400 C. are shown in the diagrams comprising Figs. 3 and 4. Prior to the specified heat treatment the processing of the castings (1.3" diameter 50% reduction. The solution heat treatment and cold 5' bars) included preheating and hot working the material wonking cycle may be repeated to the extent required to 0.285 diameter rod, solution annealing at 900 C. for after which the alloy is subjected to appropriate age 2 hours, quenching in water, cold working to .203" hardening. Then too, following final cold reduction, an diameter rod, again solution annealing at 900 C. for 2 additional solution annealing step may be used immedihours and finally cold drawing to .081" diameter wire. ately prior to precipitation heat treatment of the ma- It will be seen from Fig. 3 that a minimum tensile strength terial whereby the aging characteristics of the resulting of 200,000 p.s.i. is obtainable with the alloys above 3.8% alloy are modified to the extent that a higher aging temtitanium and together with the other properties including perature is usually required to produce maximum hardhardness (Fig. 4) it will be readily apparent that the mess, alloys having 3.8 to 4.3% titanium comprise especially The elfect of age hardening at various temperatures 5 suitable materials where strength, hardness, elongation upon tensile strength of the alloys having from 0.2 to and electrical conductivity are important factors. While 5.8% titanium is indicated in the diagram shown as Fig. possessing strength not exceeded by any other copper 2. In each of the alloys there indicated by the various base alloys, the compositions of this invention unlike other curves, the Stings 13" di b were h t d copper-titanium alloys are characterized by excellent workcold rolled into 0.203" diameter rod with two interi y which greatly simplifies processing and fabrication mediate solution .anneals 1 ,9203" diameter was procedures and makes available an excellent wrought then again solution annealed at 900 C. tor 2 hours, alloy for varifms PP P quenched in water and cold drawn to 0.081" diameter followmg Speclfic examples are glven to more wire with an 84% final 001d reduction The tensile fully illustrate the properties of the copper base alloys strengths shown in the diagram were determined after of thls Invention: heating the wire specimens thus obtined :Eor two hours Example 1 at various temperatures indicated 011 1110 abscissa. It An oxygen-free copper 0f the following typical analywill be seen from Fig. 2, that at 0.72% or less titanium, sis
S, Pb, Sb, Bi, Sn, Fe, Ni, Mn, 0;,
D r- Derpercent percent percent perpercent percent percent cent cent cent little or no age hardening or increase in tensile strength was used in making the castings employed in making occurs; the efiect is greatest approximately between 3.0 the wrought materials herein described. By way of illusto 4.3%, and at 5.8% titanium the efiect of precipitatration, a 4 x 4" wirebar weighing 300 pounds was pretion hardening is appreciably reduced. 40 pared by melting the copper initially in a CO atmosphere.
Although temperatures between 300 and 600 C. may When the temperature reached between 1200 and 1400 be used in the precipitation heat treatment or age hard- C. the protective gas was changed to argon to preclude ening of the alloys containing from 1 to 6% titanium, reaction with titanium after which suflicient titanium in the preferred temperature range is 350 to 450 C. with the form of a 28% titanium 72% copper master alloy maximum age-hardening being obtained between 375 5 was added to provide a 4.5 titanium content (calculated). and 425 and especially at about 400 C. The time re- After stirring with a graphite rod, the melt was held at quired may obviously be varied depending upon the 1250 C. for'an additional 15 minutes after which the temperature employed, said temperature being largely alloy was cast in a water-cooled steel mold under argon determined by the previous treatment of the material and gas. The casting was free of inclusions and analysis of by .the specific property or combination of properties the bottom and top sections of the bar analyzed 4.30% deemed most pertinent for the intended application of Ti indicating that the retained alloying metal was evenly the wrought alloy. For developing maximum tensile dispersed throughout the bar and relatively little titanium strengths in the case of cold drawn wire, age hardening was lost in preparation of the casting. the worked material for about 2 hours at about 400 C. Smaller castings as 1 x 14" round castings weighing is conducive to best results. By extending the precipitaabout 3 pounds were similarly prepared containing var tion heat treatment at the same temperature, however, to ing amounts of titanium ranging from an actual content from 3 to 5 hours or even longer, ductility and electrical of from 0.2 to 7.3%. The melting, alloying and casting conductivity is appreciably improved with only relatively procedure was substantially the same as described above slight reduction in the maximum tensile strength. Where excepting that the alloys were cast in a copper mold. increased ductility and conductivity are of principal importance, the material may be processed more rapidly Example 2 by raising the precipitation heat treating temperature For determining the solvus and liquidus lines of the say to about 450 C. whereby the treatment time is rebinary system consisting of oxygen-free copper and titaduced to about 2 hours in the usual case. nium, samples prepared as described in the preceding ex- -For precipitation heat treatment of cold rolled sheet, ample were microscopically studied after heating to varithe heat treatment temperature may also be varied, prefous temperatures for from 2 to 20 hours. Up to 6.07% erably between 350 and 450 C. with maximum tensile Ti, hot and cold rolled rods of 0.285" diameter were used; strength being usually obtained in about 2 hours at 375 the alloys of higher Ti content being subject to fracturing to 400 C. Since the extent of previous cold working during hot rolling were studied in cast condition. The aifects the precipitation hardening characteristics in that specimens were first heated for 20 hours at 885 C. and the more extensively worked material requires a slightly at 900 C., quenched in water and then reheated at vari lower temperature compared to material subjected to relaous temperatures above and below 900 C. tively less working, the temperature and time conducive The maximum solubility of titanium was found to occur to best results must be selected accordingly. at 900 C., the amount being close to 6%; at higher Tensile strength, elongation, electrical conductivity and titanium contents, a hard undissolved phase consisting of hardness values of the alloys containing varying amounts Cu Ti persisted whereas below 6% only the alpha phase was obtainable upon appropriate heat treatment. The solid solubility limit of titanium below 575 C. was found to be between 0.3. and 0.7%, and at room temperatureitis less than 0.5%; very close to 2.9% is soluble at 750 C., about 2.9% at 800 C. and close to 5% at 850 C. The maximum solid solubility limit was further indicated by marked brittleness of the alloys upon hot or cold working containing in excess of 6% titanium whereas those containing within the maximum solid solubility amount could be worked upon appropriate solution annealing without difiiculty.
Example 3 A sample from a 4 x 4 x 60 inch wirebar containing 4.3% Ti which was preheated in a partially reducing atmosphere for 90 minutes to a temperature between 870 to 900 C. was forged into a 2 x 2" square with 50% reduction. Microscopic examination of the sample showed that the cast crystals in the center of the bar were not completely deformed indicating the need for working more than 50% to induce a homogeneous recrystallized structure. When worked to 65% reduction or higher, a uniformly recrystallized structure of the forging consisting of alpha plus Cu Ti precipitate in the alphagrain boundaries was obtained.
Example 4 Tensile Elongation Percent Cold Rolling Strength (percent (p.s.i.) in 2") It will be seen from the above that the tensile strength and elongation properties vary appreciably with the amount of final cold reduction applied prior to the age hardening step.
Example As seen from the preceding example, increasing the amount of cold working between solution annealing and precipitation heat treatment results in an increase in tensile strength and a lowering of the ductility of the alloy. Though the alloys and particularly the preferred compositions containing 3.8 to 4.3% Ti exhibit excellent hot and cold workability and extremely high tensile strength, the elongation after age hardening for 2 hours at the optimum temperatures for maximum strength is usually very low and often not even a measurable amount. As previously stated, however, elongation as well as other properties such as electrical conductivity may be improved by overaging the alloy.
The results achieved by extending the time of treatment at a given age hardening temperature is illustrated by the following data obtained on a 4.3% Ti alloy processed by hot and cold working with intermediate solution anneals and-with a final cold reduction of 84% to provide 0.081" diameter wire. Specimens of the wire were heat treated at 375 C. for the indicated periods and then tested for hours.
tensile strength, elongation and electrical conductivity. The values obtained are listed as follows:
Tensile Elongation Electrical Time (hrs.) at 375 0. Strength (percent Conduc- (p.S.l.) in 2") tivity (percent) Specimens heat treated at 400 C. for the specified periods of time possessed the following properties:
Tensile Elongation Electrical Time (hrs.) at 400 0. Strength (percent Conduc- (p.s.i.) in 2") tivity (percent) It will be apparent from the above that at 375 C. the elongation of the 4.3% Ti alloy may be improved to 1% by extending the age hardening time to a minimum of 5 During such treatment the electrical conductivity is also improved and the tensile strength is decreased only slightly.
By age hardening at 400 C., it will be seen that the time required to improve elongation of the same alloy is appreciably reduced. At this temperature a minimum elongation of 1% and improvement in conductivity to 10% with only a slight decrease in tensile strength is obtained within 3 to 5 hours.
Example 6 Specimens of the same alloy prepared as described in the preceding example subjected to heat treatment at 450 C. for 2 hours were lowered in tensile strength to 180,000 p.s.i. but elongation was increased to 3.6% and electrical conductivity to 20% by this treatment. The yield strength of the 84% cold drawn 0.081" diameter Wire determined after the 2-hour heat treatment at 450 C. was 170,000 p.s.i., after 5 hours at 375 C. it was 194,000; and 180,000 p.s.i. after 4 hours at 400 C.
Example 7 Alloys containing 3.0 and 4.3% Ti respectively cold rolled to 0.285" diameter rods were solution annealed for 2 hours at 900" C., quenched and then age hardened for 2 hours at various temperatures to determine the effect of precipitation heat treatment on the solution annealed material. Maximum precipitation hardening for both alloys resulting in hardness values ranging from 255 to 290 V.P.N. was found to occur at 450-500 C. By heating specimens of the solution annealed rod material at 475 C. for varying periods of time ranging from 0.5 to 50 hours to observe the effect of time, it was found that approximate maximum hardening of the material was obtained within an hour with only a small increase up to 4 hours. Heating over 16 hours was found to slowly decrease the hardness.
In the case of 0.182 diameter wire made with a 4.3% Ti alloy, precipitation heat treatment for 3 hours at 475 C. after solution annealing yielded a material of 133,600 p.s.i. tensile strength and 16.8% elongation.
Example 8 Sheet material of 0.10 and 0.30" thickness was made from 4.3% Ti castings by hot rolling on flat rolls with minimum reductions of 65% and as much as 85% and by cold rolling with intermediate solution annealing. After a final solution annealing for 3 hours at 900 C., the sheets were cold rolled with 85 and 94% reductions and then age hardened. In the case of 0.045" thickness sheet made from the 0.30" sheet cold worked to 85 final reduction, optimum strength of 195,000 p.s.i. with 1% elongation and a yield strength of 180,000 p.s.i. was obtained in 2 hours at 375 C. and upon increasing the age hardening temperature to 400 C., at the end of 2 hours the tensile strength was reduced to 170,000 p.s.i. but the elongation increased to 2%. The 0.10 sheet material similarly solution annealed at 900 C. for 3 hours after cold working to a final reduction of 94% to provide sheet of 0.019" thickness upon age hardening for 2 hours at 375 C. yielded a maximum tensile strength of 203,000 p.s.i., but the elongation was nil. By extending the age hardening time to 4 hours at the same temperature, however, material having a tensile strength of 195,000 p.s.i., elongation of 1% and yield strength of 189,000 p.s.i. was obtained. By increasing the age hardening temperature to 400 0., good properties including tensile strength of 190,000 p.s.i., minimum elongation of 1% and yield strength of 165,000 p.s.i. were obtained in only 2 hours.
It is apparent that many differing embodiments of this invention may be made without departing from the spirit and scope thereof and it is not intended to be limited except as indicated in the appended claims.
1. In the process of making copper-titanium alloys, the improvement comprising the steps of alloying initially oxygen-free copper under oxygen excluding conditions with suflicient titanium to provide a titanium content of from 1 to 6% by weight in the resulting cast alloy, preheating said cast alloy and hot working the same to ef- 10 fect a reduction in cross-sectional area of at least 2. In the process of making copper-titanium alloys, the improvement comprising the steps of alloying initially oxygen-free copper under oxygen-excluding conditions with sufficient titanium to provide a titanium content of from 3.8 to 4.3% by weight in the resulting cast alloy, preheating said cast alloy and hot working the same to efiect a reduction in cross-sectional area of at least 50%.
3. The process'of making wrought copper-titanium alloys which comprises the steps of alloying initially oxygen-free copper under oxygen-excluding conditions with sufficient titanium to provide a titanium content of from 1 to 6% in the resulting cast alloy, preheating said cast alloy and hot working the same to efiect a reduction in cross-sectional area of at least 50%, solution annealing the hot worked material at about 850 to 900 C., quenching, cold working and thereafter age hardening the alloy at a temperature between 300 and 600 C.
4. The process of claim 3 wherein the titanium content of the cast alloy is adjusted to form 3.8 to 4.3% by weight and the age hardening step is carried out at a temperature ranging from 350 to 450 C.
References Cited in the file of this patent UNITED STATES PATENTS 935,863 Rossi Oct. 5, 1909 2,201,555 Carson May 21, 1940 2,783,143 Johnson et al. Feb. 26, 1957 2,842,438 Saarivirta July 8, 1958 OTHER REFERENCES Journal of Metals, volume 4, July 1952, page 766, Titanium Copper Binary Phase Diagram, by Joukainen et al.
Metals Handbook, 1948, edition, by American Society for Metals, page 880.