AMERICAS ISSTITUTE OF M I S I S G A S D METALLURGICAL ESGINEERS Technical Publication No. 2309

Class E, Metils Technology, January 1948 DISCUSSION OF THIS PAPER IS INVITED. It should preferably be presented by the contributor in

person at the New York Meeting. February 1948. when an abstract of the paper will be read. If this is impos- slble. discussion in writing (2 copies) may be sent to the Secretary. American Institute of Mining and Metal- lurgical Engineers. 29 West 39th Street. Sew York 18. S. Y. Unless special arrangement is made. discussion of this paper will close April 25 1948. Any discussion offered thereafter should preferably be in the form of n new paper.

Solubility of Iron in Liquid Magnesium BY DAVID W. MITCHELL,* MEMBER AIME

(Sew York Meeting. February 1948)

WHILE pure magnesium does not corrode nucleating centers for crystallization. This rapidly the presence of even very small phase, if the foregoing hypothesis is correct, quantities of certain other metals acceler- would probably be a metal or an intermetal- ates corrosion remarkably. Because mag- lic compound. Of the metals likely to be nesium is such an electropositive metal found in magnesium alloys, iron is one of (EO = +2.34 volts)' the presence of sub- the most likely candidates for the role of stances that permit galvanic activity is ex- grain refiner for iron occurs in small quan- tremely undesirable. It is stated that as tities in all commercial magnesium and little as 0.017 pct of iron in magnesium magnesium alloys. causes a marked increase in the corrosion A. BeckS states that magnesium, in either rate.= Magnesium and its alloys are com- the liquid or the solid state, cannot dissolve monly melted in iron or steel pots; conse- iron and that iron exists as a finely divided quently the commercial metal has the suspension which does not settle out be- opportunity to pick up iron if i t has that cause of its extreme fineness. Beck also

v tendency. Quantitative data on the solu- presents a micrograph showing a large iron bility of iron in magnesium are of practical dendrite in magnesium but does not com- as well as of academic interest. ment on the crvstallization of iron from a

Some magnesium alloy castings tend to solution supposed to contain none. I t is have large grain size unless suitable treat- doubtful that dendrite formation from ment is given to the liquid metal just prior suspended iron would be possible. This - - to casting. One method df grain refine- suspension theory is the basis of patents4 ment consists of "superheating" the covering the removal of iron from magne- metal just prior to casting. "Superheat- sium by allowing the iron particles to act as ing" is heating to a temperature IOO to nuclei for the formation of primary mag- zoo°C above the casting temperature and nesium crystals which can be separated by holding for a short period of time. Super- segregation. Also covered by patents is the heated metal has a much finer grain than removal of iron by the addition of another

1 unsuperheated metal. A satisfactory expla- metal to form an intermetallic compound

r nation of this phenomenon has not been with the iron which, because of its increased forthcoming. One hypothesis attributes the particle size, is able to settle out of the grain rehement in the superheated metal molten metal. Here, too, quantitative in- to precipitation of some phase that is mis- formation on the extent of iron solubility in cible a t the temperature of superheat but liquid magnesium is of more than theo- not miscible just below the casting tem- retical interest. perature and which is thought to provide M. H a n ~ e n , ~ in his critical collection of

the data on binary alloys, states that iron is Manuscript received at the office of the

Institute September 22. 1947. not soluble in liquid magnesium. W. R. D. 'Lecturer in Metallurgy. University of Jones6 who studied the physical properties

I California. Berkeley. California. I References are at the end of the paper. of low iron magnesium alloys observed a

Copyright, 1948, by the American Institute qf Mining and Metallurgical Ehgineers, Inc. Pnnted In USA

2 SOLUBILITY OF IRON IN LIQUID MAGNESIUM

"globular type of constituent" in alloys containing 0.10 pct iron. Jones states that this constituent forms a eutectic structure which could be "divorced" upon heat treat- ment and that the globular constituent is probably a magnesium-rich compound of iron and magnesium. No micrographs show- ing the supposed magnesium-iron com- pound were published.

Quantitative data on the iron-magnesium system are very scarce. The only published experimental work is that of Fahrenhorst and Bulian7 (which was not available a t the time this investigation was started). They sealed magnesium metal in iron crucibles by welding, allowed the crucible and con- tents to remain in an electric furnace a t the temperature being investigated for a time sufficient for equilibrium to be established. The crucibles were quenched or otherwise cooled as desired and the magnesium was removed for analysis and examination. The solubility data obtained by Fahrenhorst and Bulian are given in Table I. Micro- graphs showing the occurrence of iron in the alloys were shown. One of the micro- graphs was almost the same as Fig s of this paper.

These same two investigators studied the crystal habit of iron in pure magnesium.8 They worked with very small quantities of iron obtained from the magnesium alloys by dissolving away the magnesium in am- monium chloride solution. Using X ray diffraction methods they found the iron obtained in this way had a body-centered cubic structure with a lattice constant of between 2.8600 and 2.8610 15. and so must have been pure or nearly pure a! iron; the accepted value for the lattice constant of a - iron is 2.8610 A. Chemical analysis of the crystals showed only 98 pct iron; the remaining 2 pct was considered to be iron and magnesium hydroxides.

Fahrenhorst and Bulian7 also report that Oettel, who was working on the recovery of magnesium by electrolysis, found the solu- bility of iron in magnesium to be 0.05 pct

a t 7s0°C, 0.1 pct a t 8s0°C, and 3.94 pct a t gso°C. Oettel also observed that a t g50°C the metal was only half melted. This obser-

TABLE I-Solubility of Iron in Liquid Magne- sium at Diferent Temperatures according to

Fahrenhorst and Bulian

vation, if correct, would indicate a magne- sium rich compound of iron and magnesium.

T~~

650-555 660-670 665-670 680-590 7 00 725 750

The metal used in these experiments was magnesium crystals produced by the carbo- thermic processQ a t the Permanente Metals Corp. plant a t Permanente, Calif. This magnesium is exceptionally pure, the sig- nificant impurities being nominally

Fe 0.006 pct Cu 0.0005 pct Ni < 0.001 pct

Per Cent Fe

o.oa.5-d.oa6 o . o a b o . 0 2 7 o ,028 o . 033-0.034 0.035-0.040 0 . 0 4 5 0 .05 1-0.060

The crystals were stored in a tight glass specimen jar in order to prevent oxidation of the metal surface. Only bright sur- faced crystals were taken for use in the experiments.

The crucibles which had a capacity of

.pC

Boo 850 900

rooo 1100 raoo

about 20 g of magnesium crystals, were made from Armco ingot iron. They were

Per Cent Fe

0.100-0.11a 0 .160 0 . 1 B b o . a o 1 o . 3ao 0 .560 0 . 8 4 0

made by boring a hole in solid bar stock and each crucible was fitted with a tight fitting lid. Fig I shows the shape and dimensions of the crucibles. Armco ingot iron was chosen for this use because it is relatively pure iron and is readily available in bar form. A typical analysis of ingot iron is 0.012 pct C, 0.017 pct Mn, 0.005 pct P, 0.025 pct S, balance Fe.'O

The crucibles were loaded with magne- sium crystals and the lids were arc-welded tight. No flux of any kind was added to the crucibles. Then the crucibles and contents

DAVID W. MITC

were put into an electric furnace a t the temperature being investigated and allowed to soak for a sufficient period of time for equilibrium to be established. At 702 to 708OC soaking for 26 hr gave 0.052 pct iron, 48 hr gave 0.054 pct iron, and 9759 hr gave 0.052 pct iron. At 8o0°C soaking for 18% hr gave 0.112 pct iron and for 9794 hr 0.115 pct iron. All other samples at 7o0°C or at lower temperatures were left in the furnace at least 48 hr and all samples at 8o0°C or at higher temperatures were allowed at least 24 hr residence time in the furnace.

The furnace in which the samples were heated contained a cylindrical stainless iron block around which was fitted a heating coil imbedded in alundum cement. The iron core contained cavities, each 4 in. in diam and 5 in. in depth, that were fdled with bath materials. For temperatures lower than 8o0°C a lead bath was used. At 8o0°C and a t higher temperatures the bath used was borax glass (Na2B r 0 1 ) . The baths were used to insure as nearly uniform temperature as possible throughout the samples. The fur- nace temperature was pyrometer con- trolled. Bath temperatures were measured by means of an iron cased pyod the end of which was kept against the iron crucibles containing the magnesium. Temperatures of the crucibles were maintained constant to + 2OC. At the end of the heating period the crucibles were removed from the fur- nace and immediately quenched in water so that freezing of the magnesium would occur as rapidly as possible thus allowing a mini- mum opportunity for segregation to occur. The crucibles were then opened by turning in the lathe. The magnesium was removed and cleaned by short immersion in I : I hy- drochloric acid. The metal was then cut up for chemical analysis and metallographic examination. The analysis for iron was made by titrating the hydrochloric acid solution of the metal with standard cerric sulfate solution using orthophenanthroline as indicator immediately upon complete solution of the magnesium. For some time

the analysis of the alloys gave difficulty. The end-points of the titrations were ob- served to disappear after standing for several minutes but the addition of more

standard solution brought back the end- points. Careful examination of the solutions revealed a considerable number of dark particles which were very magnetic. Some of these particles were removed from the solution, washed in distilled water and in alcohol and then were examined micro- scopically. They appeared to be metallic crystallites and were thought to be pure iron because they were so magnetic. Some of the crystallites were submitted to the United States Bureau of Mines for exam- ination by X ray diffraction methods. (The only X ray tube available here at the time was one with a copper target and not suited for use with an iron sample because the characteristic radiation of copper is very strongly absorbed by iron.) The X ray diffraction pattern obtained with chromium K a radiation revealed a body-centered cubic structure with a lattice constant of 2.8614 A. This agrees very closely with the accepted value of 2.8610 A for the lattice constant of pure a -iron.ll

The presence of the iron crystals in the solutions of the alloys was iirst considered

4 SOLUBILITY OF IRON IN LIQUID UGNESIUM

to be caused by the segregation of the pre- titrate the solution before the iron crystals cipitated iron during the cooling period, dissolved to an appreciable extent. Analy- even though the quench was a severe one. ses of duplicate portions of the sample dis- If this explanation were correct it would be cussed above without dissolving the iron

/Do0

9

6 '0°

$800

700

b d d o " ' " " " o./ az a3 Percenf Iron

FIG 2--MAGNESIUM-IUCH END OF MAGNESIUM-IRON SYSTEM.

necessary to dissolve all the iron in order to determine the solubility. The crystals of iron were dissolved by the addition of hy- drogen peroxide to the solutions of the alloys, the excess peroxide was destroyed by boiling and the iron was reduced with test lead and titrated as before. The per- centages .of iron found when using this procedure were very much higher than those obtained without dissolving the cry- stals but the results of several analyses of the same sample were very erratic. For example: iron analyses of triplicate por- tions from a sample that had been at goo°C for 24 hr gave 0.252, 0.620, and 0.478 pct iron. These erratic analytical results sug- gested that the iron crystals had not been in solution in the magnesium before quench- ing. The iron crystals dissolved very slowly in dilute hydrochloric and i t was possible to dissolve the magnesium rapidly and

crystals gave 0 .220 and 0.221 pct iron, and good agreement was obtained with other samples. The iron crystals must have grown in the melt during the soaking period because of fluctuations in temperature. When the temperature was high the melt dissolved iron from the crucible walls. When the temperature was low iron pre- cipitated throughout the melt, and when the temperature was high again iron was dissolved from the crucible walls and from the precipitated iron. As the temperature fell again the precipitated iron crystals grew at the expense of the crucible walls. Unquestionably the iron in solution just before quenching was precipitated as tiny iron particles but these were so small in comparison that they were dissolved before the larger ones were appreciably attacked. The iron solubilities obtained by analysis

DAVID W. MITCHELL-TP 2309 5

without dissolving the crystals were taken as the correct solubilities.

In Table 2 the experimentally deter- mined iron solubilities a t various tempera- tures are given. Fig 2 shows the position of the iron liquidus in the magnesium rich region of the iron-magnesium system plotted from the data of Table 2 and also

I from the data of previous investigations.

TABLE 2-Solubility of Iron in Liquid Magnesium at Different Temperatures

-Experimental Results

Fig 3 is a micrograph of iron-saturated magnesium quenched from 8o0°C. The large dark spots are primary iron crystals and the "salt and peppern'matrix is the

. magnesium-iron eutectic. Fig 4 shows the "salt and pepper" matrix a t high magnifi- cation. By extrapolating the iron liquidus in Fig 2 and taking the eutectic temperature to be practically the same as the melting point of pure magnesium, 650°C, the eutec-

I tic composition is seen to be 0.03 pct iron. Fig 5 shows the structure of an alloy furnace cooled from 800°C. This picture shows the

f growth of tiny eutectic iron crystals into larger ones during the cooling period leav- ing the matrix immediately surrounding them improverished in iron.

Fig 6 illustrates a structure observed in several samples which had been quenched from about 7m°C. With some samples the entire specimen showed this structure; in others only small patches appeared. Sam-

/ ples quenched from 800°C and from higher temperatures did not show this structure

nor did one sample quenched from 660°C. Fig 7 is the same structure a t high magnifi- cation. In the solution of one sample (No. 10) a considerable number of small black non-magnetic particles were found. The particles were recovered by centrifuging and decanting the solution and washing in the same way. An X ray diffraction pattern of this material obtained with iron K a radiation was not that of any of the sub- stances listed in the General Electric Co. index of di5raction patterns. The possibil- ity that the structure of Fig 6 is an eutectic between magnesium and a magnesium-iron compound or a peritectic structure had to be considered. This could be possible if the peritectic reaction was a slow one for in that event the primary iron crystals formed a t 8m°C and a t higher temperatures could persist until the metal mas too cold for further reaction to take place. Insoluble residues were collected from several other samples all showing the structure of Fig 5 and X ray di5raction patterns were ob- tained. All gave patterns identical with that of pure iron. However, nothing like the abundance of particles was obtained from any other sample as from the one in which the non-magnetic particles occurred. The likelihood is that some unknown impurity was accidentally put into the crucible with the magnesium.

If the precipitate in Fig 6 were a com- pound one would expect the hardness to be much greater than that of pure magnesium and very likely harder than the magnesium- iron eutectic shown in Fig 3, even though the latter is higher in iron. Ten hardness measurements were made on pure magne- sium, ten on a sample quenched from 800°C having the structure of Fig 3 and ten on a sampIe quenched from 700°C and having the structure of Fig 6. The hardness meas- urements were made with a Tukon micro-hardness tester. With this testing machine it is possible to test individual grains or parts of grains for hardness. In testing the metal quenched from 8o0°C

Sample Temp. No. OC

lo 11 a7 8 9 I0

7

Per Cent Pe

0.112 0.1'29 0.115 0.113 0.111 0.110 0.265

Per Cent I e

0.035 0.050 0.050 0.051 0.052 0.054 0.063

660 700 700 701 707 708 710

Sample No.

------ I 2 3 4 5 I4 15

Temp. OC

800 800 Boo 900 900 900 1000

6 SOLUBILITY OF IRON IN LIQUID MAGNESIUM

areas with large crystals of iron were does not indicate anything definite concern- avoided. The average Knoop hardnesses ing the existence of a compound. were 32.5 for pure magnesium, 47.4 for To settle definitely the question of the

FIG 3-IRON-SATURATED MAGWESIU>I QUENCHED FROY 800°C. loox. Etched with I : 10 HC1. Shows Mg-Fe Eutectic.

FIG ~-MG-FE EUTECTIC O F FIG 3 AT IOOOX.

metal quenched from Soo°C and 44.5 for existence or non-existence of a magnesium- metal quenched from */oo°C. Both alloys iron compound the following experiment are harder than pure magnesium but the was carried out :

.intermediate hardness of the 700°C metal An ingot iron capsule 6 in. long, in.

DAVID W. MITCHELL-TP 2309 7

od and W in. id was filled with magnesium contained in a pyrometer controlled elec- crystals and a tight lid was welded on it , tric tube furnace in such a position that the Two thermocouples were peened into small bottom was colder than the top. The silica

FIG j-IRON-SATURATED MAGNESIUM FURNACE COOLED FROM 800°C. XOOX. ETCHED WITH I : 10 HC1. SFIOlVs SECREGilTION OF I R O S .

FIG 6-IRON-SATURATED MAGNESIUhl QUENCHED FROM 70o0C. 250X. ETCHED \VITR I : I 0 HCI.

holes in the outside of the capsule, one near tube was evacuated t o prevent oxidation of the top and one near the bottom. The cap- the iron capsule. At the beginning of the sule was heated vertically in a silica tube experiment the temperature of the whole

8 SOLUBILITY OF IRON IN LIQUID MAGNESIUM ,

capsule was raised well above the melting of pure iron with no indication of another point of magnesium for a minute o r two so phase present. that the metal inside would melt into one The experiment was repeated with the body. Then the position of the capsule and lower end of the capsule a t 626 to 630°C

FIG ' ~ - ~ ~ E T A L QUENCHED EROX 700°C. 2000X. SAME SAMPLE AS I N FIG 6.

the furnace control were adjusted so tha t the temperature of the bottom of the capsule remained a t 648 to 652°C. The top of the crucible was a t 655-661Oc. The capsule was left in the furnace four days.

According t o the phase rule a condensed system of two components is invariant when three phases co-exist. Therefore, with the temperature gradient provided in this experiment, a peritectic reaction, if one exists, should proceed to the exhaustion of one of the reactants a t any temperature below the peritectic temperature.

When removed from the furnace the cap- sule was sawed in half lengthwise and the pencil of magnesium was pried out. One half of the bottom part of the pencil was polished and etched. The lower end con- tained an abundant second phase. Some of this second phase was separated by dis- solving away the magnesium and collecting it with a magnet. An X ray diffraction pat- tern of this material was identical with that

lest a peritectic reaction a t a temperature only slightly above the eutectic tempera- ture might have gone undetected in the first. The result was the same as before.

The solubility of iron in liquid magne- sium is small but appreciable. The iron liquidus in the temperature interval in- vestigated in this work is in excellent agree- ment with the only published experimental data on the magnesium iron system.

A eutectic structure, that of Fig 6, which has not appeared in the literature was observed. This structure is not funda- mentally different from the more usual one, as in Fig 3, but occurs only under certain conditions of cooling.

The fact that magnesium is insoluble or only very slightly soluble in solid iron a t temperature from 700 to goo°C has been verified.

DAVID W. MITCHELL-TP 2309 9

I t has been shown that a peritectic reac- :: Der Aufbau der Zweistoff- tion producing an iron-magnesium corn- legierungen. (1936). 675. Julius Springer.

Berlin. pound does not exist. The magnesium rich 6. W. R. D. Jones: The Effect of Iron on end of the magnesium-iron phase diagram Magnesium. The Metallurgisl, Supple-

is a simple eutectic with an eutectic com- ment to The Engineer (1938) 166, 157. 7. E. Fahrenhorst and W. Bulian: uber das . position of about 0.031 pct iron. Auftreten von Eisen in Reinmagnesium

und in Magnesium-Mangan-Legierungen. REFERENCES Ztsch. Metallkunt. (1941). January, 31.

. Uber das Auftreten von . I. W. Latimer: The Oxidation States of the 8. . -' I Elements and Their Potentials in Aque- Eisen in Reinmagnesium 11. Ztsch. t ous Solutions. (1938) 272. Prentice-Hall, Melallkunde, (1942) July 166.

Inc.. New York. 9. T . A. Dungan: Production of Magnesium 2. J. D. Hanaw'alt. C. E. Nelson, and J. A. by the Carbothermic Process at Perma-

Peloubet: Corrosion Studies of Mag- nente. Trans. AIME (1944) 159, 3 0 8 nesium and Its Trans' 10. N. Woldman and A. Dornblatt: Enginker- (1942) 147s 279.

3. A. Beck: The Technology of ~ a g n e s i u m ing Alloys. (1936) A.S.M. 97.

and I t s Alloys. (1940). 61 and 318. I I. G. L. Clark: (1940) Applied X-Rays. 327. F. A. Hughes and Co.. Ltd., London. McGraw-Hill Book Co., New York.