INDUSTRIAL A-VD ELVGIATEERINGCHE-WISTRY
ADril. 1926
393
Properties of Some Sand-cast Alloys of Aluminium Containing Silicon and Magnesium' By Samuel Daniels WAR DEPARTMENT, AIR SERVICE, MCCOOK FIELD, DAYTON, OHIO
The effects of the addition of silicon in amounts up atures. During a g i n g t h e to 5 per cent to aluminium bearing a constant of 0.5 dissolved MgzSi is reprecipimagnesium appear per cent of magnesium are described. The alloys as tated as submicroscopic partisimultaneously in cast have no outstanding mechanical properties, but in cles and the strength and aluminium, the characterishardness are increased, while tics which they impart inthe heat-treated condition, because of the solubility dividually2 are disturbed, to of silicon and of magnesium silicide, they present inthe ductility is decreased, teresting possibilities. The metallographic characterreinforcing the effectsalready a n ex t e n t depending upon engendered by the quenching the amount in which they are istics of the series are outlined. present and upon the conoperation. The attributes of this alloy have recently been tent of such additional ele-. ments as copper, iron, and zinc. The cause of this condition described a t length by Archer and Jeffries.6 is the formation of the compound MgZSi, whose solubility in Hanson and GaylerG are responsible for the equilibrium aluminium varies with temperature from a maximum of 1.6 diagram of the aluminium-silicon-magnesium series shown per cent (0.94 per cent of magnesium, 0.66 per cent of silicon) in Figure 1. These investigators, in their work on the a t 580" C. (1076" F.) down to about 0.5 per cent at 30" C. constitution and age-hardening of these alloys, determined (86' F.), and with the composition of the alloy.3 Inasmuch the conditions under which hardening occurs. The diagram as silicon is almost always a constituent in aluminium ingot, represents the projection on a horizontal plane of the conthe alloying of magnesium produces this compound and stitution of the alloys immediately after the solidus has generally an increase in both strength and hardness and a been passed. The heavy black lines demarcate the several loss in ductility. phase fields, while the broken lines delimit the solubility Though there is a fairly large number of quaternary of the compound MgzSi a t 150" C. (302' F.). The effect and even more complex commercial aluminium alloys which of excess magnesium is to cause the solubility of MgzSi a t contain added magnesium and also resident or added silicon, high temperatures to decrease rapidly, the solubility being there is only one ternary alloy of aluminium, silicon, and diminished from the maximum of 1.6 per cent of Mg&i magnesium (to disregard the impurity, iron) that is of (0.94 per cent Mg, 0.66 per cent Si) to 1.1 per cent with an industrial note-the proprietary 98 aluminium-1 silicon-1 excess of only 0.7 per cent of magnesium and to practically magnesium alloy.4 This material may be readily hot- or nil when 5 per cent of magnesium is present. Magnesium cold-worked. I n the rolled (and in the cast) form it responds silicide appears as a separate constituent under all conditions remarkably to heat treatment, depending upon whose nature in the alloys still richer in magnesium, even when the silicon and other factors it affords a range in ultimate strength content is only 0.2 per cent. Silicon in excess of the amount of from 14,000 to 50,000 pounds per square inch, with ac- necessary to form Mg8i apparently increases the solubility companying percentage of elongation of from 30 to 10 per of that compound at both high and a t low temperatures, but
H E N silicon and
Figure 1-Constitution
M A G N E S I U M P E R CENT. of AI-SI-Mg Alloys Just below the Solidus.
cent and Brinell hardness of from 15 to 100, or higher. Its specific gravity is about 2.69. After this alloy has been quenched from about 525' C. (977' F.) t o retain the soluble MgZSi in solution, it is subject to age-hardening a t room temperature and more strongly a t slightly elevated temper-
'
Received December 8, 1925. Published by permission of the Chief of Air Service, War Department. 3 Daniels, THIS JOURNAL, 16, 1243 (1924); 17, 485 (1925). a Hanson and Gayler, J . Inst. Metals, 26, 321 (1921); 28, 213 (1922); 30, 139 (1923), and Discussions. U. S. Patent 1,472,739 (1923).
'
(Hanson and Gayler)
not to more than slight degree. The authors further state that the hardness conferred by aging a t atmospheric temperature increases with the quantity of MgzSi in solution up to the limit of solubility at the quenching temperature. British investigators' have shown that complete age-hardening (after quenching) requires, depending upon the composition of the alloy, 4 days or more a t room temperature and only 6
'
A m . Inst. Mining Met. Eng., Paper 1416-E (February, 1925). J. Inst. M e l d s , 26, 321 (1921).
Institute Mechanical Engineers, 11th Report to Alloys Research Committee, 1921, p. 241.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
394
an hour at a temperature of 200' C. (392' F.);that aging at slightly elevated temperatures gives greater strength and hardness than aging at atmospheric temperature; and that aging at 200' C . (392' F.) nearly triples the strength and hardness. The present paper is the third of a series of investigations into the properties of the sand-cast alloys of the aluminiumsilicon-magnesium system, undertaken by the Material Section, Engineering Division, Air qervice, U. S. -4. I n this article are discussed five alloys of aluminium containing nominally 0.5 per cent of magnesium and from 0.25 to 5.0 per cent of silicon.
a previous article.* Temperature measurements were made with a bare chromel-alumel thermocouple and a potentiometer. Methods of Testing
CHEMICALAmLYsIs-The various materials and melts were analyzed for copper, silicon, magnesium, and iron by standard methods, with the results noted in Tables I and 111.
Melt
2821 2822 2823 2824 2825
Method of Alloying
For this investigation there were sand-cast in the form of test specimens five melts of aluminium-silicon-magnesium alloys, in which the magnesium content was calculated a t a constant of 0.5 per cent, while the silicon content !vas varied a t 0.25, 0.5, 1.0, 3.0, and 5.0 per cent. These melts were numbered 2821, 2822, 2823, 2824, and 2825, respectively. Table I gives the composition of the aluminium ingot, the aluminium-silicon hardener, and of the magnesium metal stick used'in the manufacture of the several alloys. The aluminium ingot was of "Special" grade under Air Service Specification 11,010-B, containing 99.54 per cent of aluminium, by difference, and small quantities of copper, iron, and silicon. The hardener contained 18.45 per cent of silicon and was prepared by dissolving 25 pounds of metallic silicon in an equal amount of aluminium at 982' C. (1800' F.), Seventy-five pounds of aluminium were melted in another furnace and added, at a temperature of about 732' C. (1350' F.), to the first molten mixture. After having been thoroughly stirred and well skimmed, the alloy was pigged into thin ingots to prevent segregation. No analysis was made of the magnesium, but the manufacturer reported that over 99.0 per cent of this element was present. Subsequent use in analyzed melts proved that the purity was probably 99.5 per cent, or higher. Melt
MATERIAL Alingot AI-Sihardener Mg stick
,.
..
0.14 18.45
..
0.28 0.48
..
Melt
2821 2822 2823 2824 2825
15 15
15 10 15
738 746 741 746 752
1360 1375 1365 1375 1385
704 704 704 702 704
1300 1300 1300 1295 1300
Method of Casting Test Specimens
Three molds of Air Service standard tension test specimens were cast to size (1.28 em., or 0.505 inch in diameter) from each melt. There are three test bars to the mold, with individual risers and common pouring sprue, as shown in
0.55 0.54 0.61 0.56
0.60 0.60
O:k7
Analysis a n d Mechanical Properties of AI-Si-Mg
Allovn -------
Elongation Ultimate in 5.08 cm. (2in.) Brinell Specific Si hig strength Per cent Per cent Lb./sq. in. Per cent hardness gravity A s sand-cas1
.. .. .. .. .. .... .. ..
.. .. .. .. .. ..
..
14.3 30 7.3 34 3.0 41 2.8 48 3.0 48 CW 300-8) 15.7 34 10.7 51 5.5 63 5.0 65 4.8 59
2.64 2.64 2.62 2.62 2.60 2.64 2.64 2.60 2.63 2.60
F.)
27.5 20.5 16.2 2.0 2.8
22 22 21 15 15
2.64 2.64 2.61 2.38 2.35
The specimens as sand-cast were tested within 2 days after having been cast; and the heat-treated specimens 7 days after the completion of treatment. Such test specimens as were warped during heat treatment-Melt 2824, Mold 3 (annealed); Melt 2825, Specimen 2A (quenched and aged); and Melt 2825, Mold 3 (annealed)-were straightened, cold, before test.
The various alloys were made in 10-pound lots by charging the proper amounts of aluminium ingot and of hardener together in a plumbago crucible and melting rapidly in an oil-fired furnace. The molten bath was in no case carried above 752' C. (1385' F.). It was thoroughly stirred and skimmed just before the addition of the magnesium, again stirred and skimmed, and then finally cast from a temperature of about 704' C. (1300' F.), A detailed schedule of the melting procedure is indicated in Table 11.
~
0.5 0.5 0.5 0.5
16,450 0.27 0.48 0.55 17,460 0.49 15,440 0.91 0.54 0.61 20,150 2.90 21,410 4.67 0.56 A s quenched and aged (552'-96 2821 18,900 2822 27,250 2823 29,360 .. .. 30,550 2824 2825 .. 30,600 A s annealed (55Za-96 11 680 2821 11:790 2822 11,050 2823 5,120 .. 2824 5,810 2825 a 5520 C. = 1025°F.
99.54
~
0.49 0 91 2.90 4.67
1 0
3.0 50
2821 2822 2823 2824 2825
99:0+
Table 11-Foundry Practice (Furnace, Monarch: fuel, oil; weight of charge, 1'0 pounds) Time in C - - - - T ~ ~ ~ ~ ~ ~ furnace Max. furnace -PouringO C. ' 1. C. ' F. Melt Minutes
0.50
Table IV-Chemical
Composition of Raw Materials Aluminium CoDDer Silicon Iron Magnesium ( d l f f )
O,-&
Table 111-Analyses of Melts Investigated (Per cent) SILICON CONTENTMAGNESIUM CONTENT Calcd. Actual Calcd. Actual Iron 0 25 0 27 0.5 0.48 0.44
XECHANICAL TEsTINc-Tensile, Brinell hardness (10 mm. ball, 500 kg. load, applied for 30 seconds), and specific gravity tests gave the data in Table IV. Each value therein is the average for three bars, except in the case of hardness, where the figure is from one impression made on a sanded flat on the grip end of the middle bar in the mold, and in the case of specific gravity, for which only one determination was made for any given condition of treatment.
Table I-Percentage
2063 2415 919
T'ol. 18, S o . 4
Method of Heat Treatment
~
The purpose of the heat treatment was to ascertain whether quenching and aging would benefit the tensile properties of the alloys and also to learn the effect of annealing. The heat treatments gave an approximation of equilibrium a t the quenching temperature and for slow cooling. ~ Six test specimens from each melt were wrapped three to the bundle (and to each treatment) in iron wire and then heated for 96 hours a t 552' C. (1025' F.), close to the melting point (550' C., or 1022' F.) of the ternary eutectic of aluminium, silicon, and magnesium silicide, in an electric furnace automatically controlled to within 5.56' C. (10" F.). One set of these bars (Mold 2) was then quenched in cold water and directly aged in an electric oven a t 149' C. (300' F.) for 8 hours; the other set (Mold 3) was very slowly cooled in the furnace over a period of 7 days, down to room temperature. 8
Daniels, THISJOURNAL, 16, 1243 (1924).
April, 1926
IND USTRId4L AND ENGINEERING CHE-VISTRY
Preparation and Examination of Metallographic Specimens Transverse sections about 1.27 cm. (0.5 inch) long were taken from the riser end of the middle bar in the mold from each melt and in each condition of heat treatment. These polished metallographic specimens were examined under low and high (oil immersion, 1.9 mm. objective) magnification, and both as unetched and as etched for 10 seconds in a 2 per cent aqueous solution of hydrofluoric acid. Foundry and Mechanical Properties CASTALLOYS-KO experimental castings were made from any of the alloys, but the piping effect for those containing less than about 3 per cent of silicon was fairly considerable. At 3 per cent of silicon the tendency to pipe was much lessened, and the alloy containing 4.7 per cent of silicon was even more satisfactory. No quantitative measurements of soundness were made, but visual examination of polished specimens gave indication that no sensible variation in porosity accompanied change in composition when the additions of silicon were appreciable. As for chemical analysis, it was found that the actual composition of the alloys was quite close t o that calculated (Table 111). The magnesium content, calculated to a constant of 0.5 per cent, ranged between 0.48 and 0.61 per cent; whereas, the actual silicon content in the materials
raw materials, there was considerabIe pick-up in iron from the stirring rod. The mechanical properties of the alloys as sand-cast are given in Table IV and are plotted graphically in Figure 2. Their ultimate strength was about 16,000 pounds per square inch for silicon contents up to about 1 per cent. With about 3 per cent of silicon the strength rose to some 20,000 pounds per square inch, which figure was also approximated by the alloy containing 4.7 per cent of silicon. The Brinell hardness curve roughly paralleled that of ultimate strength, but the major portion of accretion of hardness had occurred by the time about 1 per cent of silicon was present. The most pronounced effect of silicon additions resided in the matter of ductility. When the silicon content was increased to 0.49 per cent from 0.27 per cent, the percentage of elongation dropped from 14.3 to 7.3 per cent. With 0.91 per cent of silicon, the elongation decreased again, to 3.0 per cent, there to remain for further increases in silicon content up to 4.7 per cent. As to the effect of the addition of 0.5 per cent of magnesium to the aluminium-silicon alloys as sand-cast, the situation is clearly marked in Table V. The presence of this small amount of magnesium materially increases the strength and the hardness even more substantially, on the order of 50 per cent, for silicon contents up to about 5 per cent. On the other hand, the percentage of elongation is a t least halved. The magnesium gives to the fractures a blue tinge, which is made more pronounced when the alloys become richer in silicon. Table V-Comparison
containing not more than about 3 per cent of silicon did not differ from that calculated by more than 0.1 per cent. Melt 2825, however, contained 4.67 rather than 5.0 per cent of silicon, which discrepancy was probably caused by segregation in the hardener. The iron content of the several melts fell between 0.44and 0.60 per cent, most of the results near the higher limit. To judge from the analysis of the
395
of Mechanical Pro erties of AI-Si and Al-SiMg Alloys as Sand-East
Melt
Si Per cent
2137 2796 2822 2797 2823 2794 2824 2795 2825
0.14 0.50 0.49 1.20 0.91 2.80 2.90 4.80 4.67
M g
Per cent Xone Sone 0.55
None
0.54
None 0.61
None 0.56
Elongation in 5.08 cm. Ultimate strength (2 in.) Brinell Lb./sq. in. Per cent hardness 11,OEO 12,240 17,460 13.830 15;440 16,370 20,150 18,310 21,410
29.2 18.0 7.3 12.5 3.0 10.2 2.8 7.8 3.0
20 23 34 28 41 31 4s 34 48
HEAT-TREATED ALLOYS-The changes in mechanical properties resulting from heat treatment are to be seen in Table IV and are compared graphically in Figure 2 with those of the various alloys as sand-cast. During this heat treatment none of the specimens were warped except 2925-28, a quenched and aged bar; 2824-3A13B, and 3C; and 2825-3A, 3B, and 3C-the last six bars being annealed (furnacecooled) samples. The fractures of all quenched and aged bars were normal except that of specimen 2825-28, which was bluish in color, coarsely crystalline, and porous. The fractures of specimens from Melts 2822 and 2823 in any condition of treatment were satisfactory, but in Melts 2824 and 2825 they were much more blue for the annealed state than for the cast or for the quenched and aged and also were coarsely crystalline and porous. The surfaces, too, of these last specimens were crinkly and pitted in some places. The blue color of the annealed bars is attributable to the large amount of particles of silicon which were in part precipitated from solid solution and also coagulated during the slow cooling. The effect of quenching and aging was hardly to affect the mechanical properties of Melt 2821 (0.27 Si, 0.48 Mg), practically a magnesium silicide alloy, and markedly those of the remaining melts which contained silicon in excess of that necessary to form Mg2Si. I n such melts the ultimate strength was increased over that of the cast alloys uniformly to about 30,000 pounds per square inch. At the same time the elongation of Melt 2822 (0.49 Si, 0.55 Mg) was improved
Ffsure 3-0.91 Si. 0.50 MP. X I00 Smd-cast. 16,44o-8.0--41. Iron-bearing eoirstil~lmts binary AI-% eutectic. ternary e ~ t e c t i eof Ai, MglSi. and Si, and blue-way compound
FiRure b--4.67 SI, 0 3 6 M a . x IOU Sand-cast. 21.41Wd.048. Iron-bearing constituents, finely divided Lernsiy eutectic, marser Ai-Si eutectic, and biu$-grsy ,-apound. Mg&i as skeietonr. porsiblya binary
Annealcd.
11,05~b16.2-21. Same con-
stitllents es in Figure 4, but With reprecipi.
tvfed and coagulated Mg&
Finure 7-4.61 Si, 0.56 Mg. X I00 Quenched and aged. 30,600-4.8-50. Needles oE iron-bearing conrtituents. rounded excess Si, atid ronsidrrabie blue-gray m m peuod. No MsSi present
and Si
Figure 84.67 Si,0.56 Mg.
x 100
eutectic
from 7.3 to 10.7 per cent. The treatruent sirnilarly enhanced to 5 per cent the orisinal (as cast) elongation of the other alloys in the series, but additions of silicon to produce a total of more than possibly 1 per cent apparently, for the treatment given, are not justified from the standpoint of resulting tensile properties. The Brinell hardness of the magnesium silicide alloy (Melt 2821) was slightly higher after quenching rand aging. In the excess siiicon alloys the hardness was increased from 34 to 51 in Melt 2822, from 41 to 63 in Melt 2823, from 48 to 65 in Melt 2824, and from 48 t.o 59 in Melt 2825. Most of the inoreme in hardness caused hy additions of silicon, whether the alloys were in the cast or in the quenched and aged condition, eventuated by the time 1 per cent of silicon was present. The specific gravity of the various alloys as cast, which decreased with increase in percentage of silicon, was not appreciably altered by thermal treatment. Annealing caused the ultimate strength and hardness of Melts 2821, 2822, and 2823 to revert to about 11,W pounds per square inch-the strength of sand-cast aluminium itself-with attendant iniprovement in the percentage of elongation, compared with that of the alloys as cast. The ductility, however, declined with increase in percentage of silicon.
Melts 2824 and 2825 were adversely affected by this annealing, as is clearly demonstrated hy their tensile properties, the fractures of test specimens, and by their specific gravity. The behavior of these latter melts was probably induced by the melting at 550" C . (1022" e.)of the ternary eutectic of aluminium, silicon, and magnesium silicide. The temperature of soaking, 552' C. (1025' Y,), may also have affected unfavorably the properties of Melts 2824 and 2825 as quenched and aged. Metallography Figures 3 to 14, inclusive, illustrate the inetallography of the series. All the photomicrographs are unetched structures. Melt 2821 (0.27 Si, 0.48 M g ) as sand-cast contained skeletons and a few needles of the iron-hearing compounds, very little blue-gray constituent* (Si02?'@)and LfgsSi except in a segregated area near a pipe, and some eutectic complex of aluminium and silicon. The particles of silicon in this complex were very small and generally arranged elliptically.
* I X x . Trunr. Am.Irs1. Mining Met. Ens.. 69, 965 (1928). Daniels, Tsrs J o o a l m ~11, , 485 (1925).
FiBure 9 4 . 9 1 Si, 0.54 Me. X 500 Sand-cast. Dupler iron-bearing needles a i d the teinliiyeuterticof AI, Mgzd (black). and Si
F i m r e 1 2 4 . 6 7 SI, 0.56 Mg. Anneded.
FiQure 1 1 4 . 6 7 Si, 0.56 Ma.
x
500
Duplex needles and skeleton$ X, the &-si eutectic, and ruushly heregonsl p r t i c l e s a< the blucgray constituent Sand-enst.
of PeAh and
X 1000
Blue-gray eonsiiiueiit closely
sociated with Si particle (dark)
as-
Annealed. Duplex iron-bearing iieedlrs inside Iparticle of Si nnd d w l e x skeleton with Fe.41. core and lighter sheath of X
Silicon was sometimes associated with Mg&, possibly as a ternary eut,ect.ic with aluminium Many iron-bearing skeletons were found about a pipe, but these were in the niaiii broken up by heat t,reatment, which, when it involved queimhing, caused the disappearance of the Si and Mg2Si; and when it involved furnace cooling caused copious precipitation and coagulation of WSgaSi in rounded or acicular particles, and of a veiy small amount of silicon iinmediately adjoiniug some of the more simhle particles of MgSi. Sone of the blue-gray constituent could be definitely detected in the heat-trratcd specimens. Under oil immersion the iron-hearing skeletons appeared t.o be duplex, having watery-blue r i m of the compound X and purple cores of FeAb. Mclt 2822 (0.4'3 Si, 0.55 Mg) as cast possessed a structure similar to that of Melt 2821, from which it differed in that, of the forms of the iron-bearing constituents, needles predominated over skeletons and in that the quantity of each of tho hard compounds was greater. The Mg2Si occurred either as fairly large skeletons or as fine particles interspersed with silicon (ternary eutectic), generally clustered about needles either entirely of X or duplex in character. The effect of quenching was to cause the Si and Mg8i to he completely ret.ained in solution and the effect of t.he high
Annealed. Duplexskeletou with FeAlrcare end X sheath. Si particle ,solid black) end binary M g s i skeleton (stippled blnck)
temperature was to round and dissociate the iron-bearing needles and skeletons. The blue-gray constituent occurred sparsely in the quenched and aged specimen. It should be noted that Melt 2822 as quenched and aged had an ultimate strength of 27,250 pounds per square inch and an elongation of 10.7 per cent, while Melt 2821, in similar condition, had an ultimate strength of 18,900 pounds per square inch and an elongation of 15.7 per cent. The explanation of the failure of the latter melt to respond equally as well as Melt 2822 to heat treatment lies in their differing cont,ents of silicon and the consequent effect upon the formation of Mg,Si. Theoretically, in the absence of iron, Melt 2821, containing 0.48 per cent of magnesium, would require about 0.18 per cent of silicon to form the compound Mg2Si. While 0.27 per cent of silicon is present in the alloy, much of it is rendered inoperative chiefly when it enters into the constitution of the compound X and when it appears as elemental silicon. It is not until enough silicon is present, as in Melt 2822, that the requirements for silicon by the magnesium are met, and then the full possible quota of Mg8i exerts its powerful influence upon the results from heat treatment. It is difficult., however, to visualize how silicon, except in excess, can be precipitated in the original material when magnesium is known to have such an intense
INDUSTRIAL AND ENGINEERING CHEMISTRY
398
affinity for silicon, unless it be on the basis that both the silicon and the magnesium are prone to segregation. As cast, Melt 2823 (0.91 Si, 0.54 Mg) showed (Figure 3) rectangular or nodular blue-gray constituent and some ekeletons and needles of FeAls, a more abundant quantity of needles and irregularly shaped particles purely of X, duplex needles of F e d 3 and X, the AI-Si eutectic in moderate amount, and MgzSi in fairly large skeletons (binary eutectic?) and in small particles closely associated with silicon, as if in a ternary eutectic (Figure 9). Some of these latter complexly constituted colonies of hard particles also embraced minute fragments of X . More of the blue-gray constituent and of the &-Si eutectic was present in this than in Melt 2822, and the particles were coarser. No MgzSi or Si was observed in the quenched and aged specimen (Figure 4), which, like the annealed specimen, contained both skeletons and needles, principally the latter, of tbe iron-bearing constituents, and all of such particles were generally composed partly of FeAb and partly of X. Annealing caused the precipitation and coagulation both of Mg&i and of Si (Figure 5 ) , there being an abundance of the latter as compared with the amount in Melt 2822, annealed. This type of structural change is responsible for the loss in strength to 11,OOO pounds per square inch. The duplex character of many iron-bearing particles has already been noted. This condition was distinguishable in the unetched specimens, but was much more emphasized by etching in 2 per cent aqueous hydrofluoric acid for 10 seconds. The FeA& cores turned brown, while the X sheaths remained watery blue. The action of this reagent in aluminium-silicon-iron (as impurity) alloys containing magnesium is just the reverse of its action in alloys of t,he same base, yet without magnesium.
Vol. 18, No. 4
Melt 2825 (4.67 Si, 0.56 Mg) presented many interesting metallographic peculiarities. As cast, there were duplex iron-bearing needles, skeletons and needles of X, skeletons and small rounded particles of MgzSi-those in the latter form grouped with silicon in the ternary eutectic-he and coarse binary ALSi eutectic, the predominating constituent, and a fairly considerable quantity of the blue-gray compound (Figures 6 and 10). After quenching and aging the MgzSi was not visible, but the silicon in excess of the solubility limit (about 1.5 per cent) was simply coagulated (Figures 7 and 11). The annealing treatment caused grain growth (Figure 8). Figure 12 gives evidence of the formation of the blue-gray constituent prior to that of the aluminiumsilicon eutectic. In Figure 13 the duplex iron-bearing needles are shown also to be antecedent to this eutectic. Figure 14 portrays a duplex skeleton with FeA13 core and enveloping X,MgzSi (right), and a silicon particle enmeshed by X . In conclusion, it should be mentioned that the slight solubility of MgzSi and the ready solubility of the blue-gray constituent lead to confusion when the specimen is etched in dilute hydrofluoric acid for 10 seconds. When there is doubt whether the black spots are polishing pits, cavities, or MgZSi partially or completely blackened by the finishpolishing with magnesia and distilled water, the specimen should be repolished down to 65 F alundum and then examined for the characteristically blue MgBi. Acknowledgment Grateful acknowledgment is herewith made to Clifford McMahon and to John L. Hester for their assistance in this experimentation.
Laboratory Preparation of Viscose’ By Earle H. Morse NUTLBY, N. J.
I
N a recent article Snellz has described the preparation of viscose from cotton cellulose. The process of making viscose from sulfite cellulose differs somewhat from that for making viscose from cotton. The writer has prepared in the laboratory viscose for experimental work which should be essentially the same as that made in the plant. Since commercial viscose is ordinarily made from sulfite cellulose, sulfite cellulose must be used in the experimental batches. The preparation of laboratory batches of viscose of a grade favorably comparable with the commercial batches differs in certain important details from the commercial production, owing to mass reactions and the limitations of the mechanical equipment for handling the larger batches. The methods here described have been in actual use by the writer for several years and have also been tested in other laboratories. From 200 to 800 grams of sulfite cellulose have been used per batch and both American and foreign pulps have been employed. With different grades of sulfite cellulose it is sometimes necessary to change details slightly to obtain identical results, but with little changes the process outlined here will produce commercially comparable products from any suitable grade of sulfite cellulose. It is not a t all difficult to make viscose, but some experience is necessary to make uniform batches. 1
Received October 31, 1925.
f
THISJOURNAL, 17, 197 (1925).
If viscose is to be prepared as a routine operation it is advisable to secure the best possible apparatus. However, if only a few batches are desired it is possible to prepare viscose with the simplest of makeshift apparatus. The process and equipment described here will give good results where a considerable number of routine tests and batches are to be made. Notes as to simpler equipment which has been found satisfactory for a smaller number of batches or an occasional test are added. Treatment with Caustic Soda About 800 grams of sulfite cellulose pulp are cut into sheets about 15 cm. (6 inches) square. The cellulose as used should contain about 10 per cent of moisture, so that about 890 grams will be required to give 800 grams of dry cellulose. The sheets are piled into a wire mesh basket about 20 cm. (8 inches) deep and immersed in 8 liters of a caustic soda solution. A basket made of 4-mesh black iron wire is satisfactory. The tank into which the basket is dipped and which holds the alkali solution is preferably an open-top tank welded from light (No. 8 or 12) black sheet iron. Glass or wooden containers should be avoided. Although about an 18 per cent sodium hydroxide solution is suitable for commercial work, a concentration of 20 to 21 per cent is preferable for these small batches. Ordinary
