Sept., I9o2.] Structure of IFIetals and Binary Alloys. 20x

Mining and Metallurgical Section. Read at the Slated Meeting, held Wednesday, December H, #9oz,

Upon the Structure of/~etals and Binary Alloys.

BY WILLIAM CAMPBELL, B.Sc. (Durham), F.G.S.

( Concluded f r o m p. tee. )

In Group I I I no eutect ie in the t rue sense of the word exists. The examples of this group are:

Bismuth and antimony. Silver and gold.

W h e n an alloy of gold and silver commences to solidify, dendri tes compara t ive ly rich in gold begin to form. The mother- l iquor is enr iched in silver. As the t empera tu re falls, the dendri tes cont inue to grow larger and larger, b u t as each coat ing of metal is less rich in gold than the one previous, and as diffusion (in the case of slow cooling) pro- duces a un i fo rmi ty of compc~sition or an equi l ibr ium in the dendrites, these la t ter become richer and richer in silver, till at the point where the whole alloy becomes solid they have the composit ion of the original alloy. Of course, i f equi l ibr ium is not establ ished we get mixed crystals, or in other words the dendri tes are richer in gold at the center than at the outside. "

A four th group migh t be added- -where a compound is formed and we get two divisions :

(Q An excess of the first metal or the compound crystal- l izing out in the euteet ic of this metal and the compound.

(2) An isomorphous mix ture of the compound and the second metal .

To this group would belong an t imony and silver and perhaps t in and silver. T h u s this group is really a. combi- nat ion of Groups I and III.

202 Campbell : [J. F. I.,

Charpy groups together alloys with abnormal curves of fusibility. Amongs t them are:

Copper and tin. Antimony and tin. Copper and zinc. Zinc and silver.

T I N AND ZINC.

The cool ing curve of the series (Fig. 33) is composed of two inclined branches meet ing at 8 per cent. zinc. The

Freezing-point Curve : Lead-Tin (alloys research). L e a d

326° ~ 8 ~ 3o0 ° T i n

2500 " C 231° 2o0 °

180 °

Zinc Freczln~- point'Curve : Zinc-'l~n. 4z9 ° A

4oo ° i

35 ~°

3oo0 T i n

25 °° z32 o 200 o

20~ o

IOO 90 80 70 60 5 ° 4 ° 30 20 Io O

Percentage composition by weight.

FIG. 33.

horizontal branch corresponding to the freezing of the eutectic occurs at 2o5 ° C., and extends from almost pure tin to almost pure zinc ; in other words, as soon as a very small quantity of one metal is added to the other a eutectic is formed,.for the concentration of the solid solutions formed is very dilute.

Sept., 19o2.] £tructure o f Metals and Binary Alloys. 203

T h e n be tween tin and the eutec t ic alloy conta in ing 8 per cent . zinc we find grains, and then dendr i tes of tin set in an increas ing g roundmass of eutect ie . The solidification of t h e t in is marked by the break B C in Fig. 33, whils t tha t of t h e eutect ic is denote d b y the break b C. Below b C the alloys are solid; be tween B C b we have a mixture of solid grains ~)r dendr i tes of tin in a l iquid whose composi t ion will depend upon the tempera ture .

Be tween zinc and the eutect ic alloy we find grains, den- dr i tes , and then long, a t tenua ted , lath-like bodies of zinc su r rounded b y increasing amounts of eutect ic . The solidi- fication of the zinc is marked by the break A C, whi ls t tha t <)f the eutec t ic is denoted by a C. Then, as before, below a C all the alloys are solid. W h e n an alloy cools down from the l iquid state, it remains ent i re ly l iquid till its tempera- tu re reaches tha t of the curve A C. For example, take the al loy conta in ing 80 per cent. zinc. At 39 °0 C. the first grains ~)f zinc begin to form and the mother- l iquor is enr iched wi th regard to tin. As the t empera tu re falls, the zinc grains grow a n d the mother - l iquor becomes r icher and r icher in tin. T h u s at 35 °0 C. it will contain abou t 47 per cent. of tin ; at 3oo ° C. abou t 72 per cent. ; whi ls t at j u s t above 2o5 ° C. i t con ta ins abou t 92 per cent. A t 205 ° C., tha t is, the tempera- t u r e a C b, the mother- l iquor solidifies and the t empera tu re r ema ins cons tan t till the whole mass is solid ; then the tem- pe ra tu re f a l l s normal ly in the solid mass. Fig. z 9 shows an alloy conta in ing 20 per cent. zinc, 80 per cent. tin. T he long, lath-like grains of zinc are seen wi th their dis t inct ten- d e n c y towards parallel growth. Fig. I7 would well i l lustrate the appearance of an alloy conta in ing be tween 6o and 7o p e r cent. Zn.

L E A D A N D TIN.

The cooling curve for these two metals , taken from the R e p o r t of the Al loys Resea rch Commi t t ee (London), is s h o w n in Fig-. 33. I t consists of two inclined branches m ee t i ng at the eutec t ie point, 68 per cent. tin, 32 per cent. lead. The horizontal branch cor responding to the solidifi- ca t ion of the eutect ic occurs at I8O ° C. and p robab ly extends

• 2o4 Campbe l l ." [J. F. I.,

fur ther than shown, for it would appear in this ease also tha t the concentra t ion of the solid solutions formed was very minute . The mean ing of this curve, is the same as tha t of the tin-zinc series. Alloys conta in ing more than 68 per cent. Sn remain l iquid unt i l their t empera tu re reaches C B. A t tha t point pure tin or almost pure tin crystall izes out of the mother-l iquor and cont inues to crystallize out till the mother- l iquor reaches the composit ion of 68 per cent. Sn, which it does at 18o ° C. A t tha t point it solidifies as a eutect ic of a l te rna te laminae more or less curved of lead and tin, wi thout any fall in tempera ture . W h e n solidification is complete the t empera tu re falls normally. Alloys contain- ing less than 68 per cent. Sn remain l iquid till they reach the t empera ture of branch A B, when lead commences to crystallize out, enr iching the mother-l iquor in tin. Crystal- l iza t ion cont inues unt i l the mother- l iquor has a composit ion of 68 per cent. Sn at I8O ° C., when it solidifies as before.

The surface s t ructures of these alloys when cast, and also when slowly cooled, give a good indicat ion of their in ternal s tructure. Fig. 2o shows the surface s t ructure of the alloy 15 per cent. Pb ; 85 per cent. Sn when cast. The large den- drites of tin, different ly orientated, are seen s t and ing ou t above the surface. The euteetic, in cooling and sol idifying, has shrunk and sunk benea th the level of the dendri tes , which thus s tand out in relief. F i g . 2 I shows the surface of an alloy from the other end of the series. I t conta ins Sn, 26 per cent. ; Pb, 74 per cent. I t is taken from a small ingot. In this case the dendri tes are those of lead, which s tand out above the level of the eutectic. The difference in size of the dendri tes of t in and lead is great.

F i g . 22 shows the effect of copper upon common solder. Five per cent. of copper was added to the solder (2 Pb : I Sn) g iv ing an alloy, Pb 63 per cent., Sn 3~ per cent., Cu 5 per cent., in which three cons t i tuents can be seen. The long, rough crystals of a compound of copper and tin have crys- tallized out first of all ; then the excess of lead has solidified as grains and dendri tes, and last of all, the eutect ic has solidified.

Sept., x9o2.] Structure o f Metals and Binary Alloys. 2o5

LEAD AND ANTIMONY.

T h e c u r v e of f u s i b i l i t y is c o m p o s e d of two b r a n c h e s w h i c h c u t a t a s h a r p ang le a t t h e e u t e c t i c point , 13 p e r cent . Sb a t 228 ° C. a c c o r d i n g to Ro land -Gosse l i n ,* an d 247 ° C. a c c o r d i n g to S t e a d . t Fig. 34 shows t he curve.

mtimony '

629. 5 :

600 °

500 °

4o0 °

300 °

Bismuth

266 °

Cooling Curves : Antimony-Lead. Bismuth-Tin.

600 ° C.

500 ° C.

400 °

Lesd •

327 °

300 °

i 247 ° Stead} 228°{Gautier.) [ 232°

Tiu 2 O O ° 2O0 °

i 4 0 o ' . . I4 o°

~n i . o 20 4 ° 60 80 ioo Per cent. Composition by weight.

FIG. 34.

Al loys c o n t a i n i n g 0 to 13 pe r cent . Sb are e x t r e m e l y diffi- cu l t to po l i sh a nd e t ch p rope r ly . T h e y are seen to cons i s t of g r a i n s a nd t h e n d e n d r i t e s of l ead in a m a t r i x c o m p o s e d of a l t e r n a t e b r i g h t and d a r k laminae, Sb and Pb. Fig. 23 shows

*Gautier: Bull. de la .)oc. d'gncourage., 1896 , Tome I, 5 e S6rie. t Journal Society Chem. Industry, March and June, 1897.

206 Campbell : [J. F. t.~

the 1o per cent. Sb alloy, and the large skeleton crystals o f lead are seen to have solidified in the eutectic. This cure t - tic corresponds to the formula Pb4Sb, bu t this does no t mean tha t a compound exists, bu t tha t the eutee t ic happens to have a percen tage composi t ion which cor responds to a formula. An exactly similar case is tha t of Levol's. alloy.

A b o v e 13 per cent. Sb 'the s lowly cooled alloys show a dis t inct layer of harder metal at the upper surface. Th i s hard whi te layer increases in thickness as the total anti- mony in the alloy is increased, unti l at abou t . 56 per cent. Sb the whole mass appears br ight . U n d e r ' t h e microscope these layers are s e e n to consist of more-or less .well-formed cubes imbedded in the eutect ic . S t e a d has proved tha t these crystals crystall ize out 9 5 a homogeneous mother- liquor, b u t by grav i ty float tg/ the top. It is not a case of two liquids, as, for examplo / l ead and zinc.

The crystals, on analysis, were found to contain at mos t o'2 per cent. Pb, showi6g tha t the an t imony crystal l izes ou t in a lmost the pure State, or, in o ther words, tha t the con- centra t ion of the solid solut ion of lead in an t imony is on ly 2 in I,OOO, and tha t above o'2 per cent. lead we ge t a eutec- tic. The specific g rav i ty of these cubes of an t imony is 6"5, whi ls t tha t of the eutec t ic is 1o"5, which explains why the an t imony is found on {he top of the slowly cooled alloys. On analyzing the lower port ion of any of the alloys from i3 to near ly 5o per cent. Sb, S tead found the composi t ion to b e 12"7 to I2"8 per cent. Sb, which gives us the exact eu tec t i e point.

W h e n these alloys are cast the cubes of a n t i m o n y are compara t ive ly small and are evenly d i s t r ibu ted through- out the mass, thus a l lowing the alloys wi th the lower per- centages of an t imony to be used as bear ing metals. Fig-. a4 shows the surface of an ingot conta ining 75 per cent. Sb , 25 per cent. Pb. The cubic g rowth of crystals of an t imony resembles closely tha t of pure b i smuth . As in the o t h e r i l lus t ra t ions of surface s t ructures , the eutee t ic on cool ing has shrunk and the an t imony crystals s tand out in relief. Fig. 30 is very similar to the appearance of the b r igh t l ayer

Sept., x9o2.] Structure o f Metals and Binary Alloys. 207

of antimony cubes in slowly cooled alloys containing from x5 to 30 per cent. of antimony.

TIN AND BISMUTH.

The curve consists of two branches meet ing at 143 ° C.* (Rudberg). The eutectic contains about 46 per cent. bis- muth, 54 per cent. tin, and under high powers has a peculiar granular appearance. From tin to the eutectic we find

96o'7 ° C.

s i l v e r

90o o

8oo o

700 °

6oo o

500 °

Z i n c ,

¢oo c

300 °

Silver-Lead. Aluminium-Zinc.

800 °

700 °

A l u m l n l u t

600 °

5oo o

400 ° Lead 326 ° C.

300 °

o 2o 40 60 80 Ioo

Percentage composition by weight.

F I G . 3 5 "

grains and then dendrites of tin crystallizing out in the eutectic. The alloys are similar in appearance to Fig. 23.

Above 46 per cent. Bi, irregular white crystals of bismuth make their appearance. In this case, however, their density is greater than that of the eutectie, and consequently they form and sink to the bottom when the alloy is slowly cooled.

* Poggendorf's ~lnnalen de Physik und Chemic, xviii, 240. Annales de Chimie el Physique[2], xlviii, 353.

208 Campbell: [J. F. I.,

W h e n cast they appear regular ly t h r o u g h o u t the mass. The cooling curve, Fig. 34, is based upon Dr. Gaut ier ' s f igures in the " F u s i b i l i t y of Metallic Alloys."

ZINC AND ALUMINIUM.

The two branches which compose the curve meet at abou t 5 per cent. A1. The eutect ie melts at 38o ° C., some 39 ° below zinc, according to Heycoek and Neville, ~ or 389 ° C., according to Roland-Gosselin. These alloys are ex- t remely difficult to polish. T h e y are best e tched with ni t r ic acid, towards the zinc end of the series, and with caust ic soda at the a lumin ium end. The alloys consist of an excess of A1 or of zinc in a eutect ic consis t ing of laminm of the two metals. Fiff. 25 shows the alloy 4 per cent, A1, 96 per cent. Zn. Grains of zinc appear in the typical eutectic.

SILVER AND LEAD.

The curve consists of two branches, mee t ing at 2"8 per cent. Ag, the eutect ic point. This melts at 3o3 ° (Hey- cock and Neville), and the eutect ic line extends from almost pure lead to about 96 per cent. Ag. Between o and 2"8 per cent. A g we find lead crystal l izing out first as rounded grains, then as dendri tes in an increasing ground- mass consis t ing of a l ternate br igh t and dark iamina~.

Above 3 per cent. of Ag, b r igh t whi te cubes make thei r appearance, toge ther wi th well-formed octahedrons. These are silver, bu t contain some lead in solid solution. As the silver contents are increased, these oetahedrons increase, t end ing to crystallize out along definite directions, and forming in this way large skeleton crystals of silver. Fig. 26 shows this mode of g rowth in an alloy conta in ing IO per cent. Ag, 9 ° per cent. Pb. The skeleton crystals become more compact, the octahedrons forming them become less perfect, and there is a grea t amoun t of interference as the silver is increased. A t 5 ° per cent. the silver forms a dense

*Journal Chem. Soc., 1897, p. 383 .

'Sept., i9o2. ] Structure o f Metals and Binary Alloys. 209

mesh t h r o u g h o u t the alloy. This increases in th ickness and the eutec t ic d iminishes till at 96 per cent. Ag the whole mass appears to be made up of si lver gra ins and dendri tes. These alloys are bes t e tched by pro longed act ion . f acetic acid.

ANTIMONY AND COPPER.

The curve of fusibil i ty, as de te rmined by Le Chatelier , consists of three branches, which cross at the two eutec t ie points abou t 25 and 7I per cent. Cu. The s u m m i t of the middle branch occurs at abou t 60 per cent. Cu. Accord ing to Stansfield, the two eutect ic points occur at 25 and abou t 69 per cent. Cu, and the s u m m i t at abou t 57 p e r cent. Cu. Fig. 36 shows Dr. Stansfield's curve. The lower branches have as ye t not been accounted for; bu t quench ing the alloys involved, af ter comple te solidification, b u t above the lower breaks, . revealed new s t ruc tures and this seems to point to a r ea r r angemen t in the solid s imilar to tha t in the copper-tin series.

W i t h regard to the mieros t ruc ture of the series, Charpy and Stead differ wi th respect to the al loys near the inter, media te summit . Stead 's work is summar ized as fol lows:

zoo to 75"8 2er cent. 5 b . - - A n t i m o n y crystall izes in a eu- teetic of 24"z per cent. Cu.

75"8 to 48"5 per cent. Sb.--SbCu2 crystall izes in the eutectic, and increases wi th the copper, till at 5 I'5 per cent. Cu the whole mass is SbCuz, the purple compound.

48"5 to 38"5 per cent. Sb. - -The purple compound SbCu2 crystall izes imperfec t ly in wha t appears to be a second defi- nite compound, SbCu3, which is whi te ; the purple compound decreases and finally d isappears when the Sb reaches 38"5 per cent., the whole mass be ing composed of a compound , whi te in f rac ture and when polished, bu t purple on e tching wi th HCI.

Sb 38"5 to 3 r per cent. Sb. - -The s t ruc tures of the alloys be tween 38"5 and 36 per cent. Sb are similar in appearance ; t h i n veins envelop the grains, and from these veins needle- l i k e processes pene t ra te into the grains for a very shor t dis- t a n c e . As the an t imony is decreased below 36 to 3x per VoL CLIV. No. 9z~. I4

g

<

o~ L)

2 I O Campbell.: [J. F. I. ,

cent., microl i ths of some c o m p o u n d appear in the center of the grains, and the ve ins enve lop ing the latter increase in thickness and in their copper contents. W h e n 3I per cent. Sb is present, the microl i ths are present in increased quantity, and at this point it would appear that we have the second eutectic with a very complicated structure.

3x to o per cent. S 3 . - - A s the copper is increased above 6 9 per cent. it at first falls out of the euteet ic in detached iso-

I IO0 ° C.

C o p p e r

iooo o

900 °

800 °

7O0 °

600 o

5oo °

4o0 °

300 °

Tin

232 ° C.

251"C"

64.0°c

Copper-Antimony. Nickel-Tin.

116C

500°C

I45 °0 (

N i c k e

12000

9500

(63 °0 (

A n t i c

7oo °

350 °

200 °

o 20 40 60 80 Io0

Percentage composition by weight. FxG. 36.

lated globulites . T h e y are not pure copper, but contain Sb and arrange themse lves in definite l ines and angles, A s the copper is increased, they form dendritic crystals. The mierol i ths are absent and have probably been absorbed by the yellow-colored dendrites at the m o m e n t of solidification. Th e color of the dendrites passes from yel low to red as the

Sept., x9o~.] Structure of 34retals and Binary Alloys. 2I !

Sb is fur ther reduced, and finally the last o'I or o'2 per cent. are left beh ind at the borders of, bu t not separa ted from, the grains of copper, where it p robab ly exists as an t imonide of copper in solid solution. (Stead : Journal 5oc. Ckem. Industry, Dec. 3 I, i898.)

Charpy's conclusions are as fol lows: In the case of alloys conta in ing less than 25 per cent.

copper , pure crystals of an t imony are separa ted when solidi- fication begin's, increasing gradual ly in size as the tempera- ture decreases ; the port ion remain ing liquid, therefore, gradual ly becomes r icher in copper unti l the composi t ion of

t h e eutec t ic alloy is reached; it then solidifies at a cons tant tempera ture , th rough a s imul taneous crystal l izat ion of Sb and SbCu.. In the case of alloys conta in ing from 25 per cent. to 6o per cent, Cu, a s imilar phenomenon occurs, only it is the definite compound SbCu~ which separa tes from the mol ten mass as soon as the freezing point is reached. W h e n from 6o per cent. to 7o per cent. of copper is reached, the same compound is separated, bu t is in this case su r rounded by a second eutect ic alloy made up of copper and the com- pound SbCu~. Finally, when more than 7 ° per cent. of cop- per is present, a port ion of the la t ter is first depos i ted when solidification sets in, unti l the port ion remain ing l iquid has reached the composi t ion of the second eutect ie alloy (Metal lograpMst, Vol. I, p. zoo).

TIN AND N I C K E L .

Accord ing to Charpy, the micros t ruc ture and the curve of fus ibi l i ty seem to show that they have a const i tu t ion very similar to tha t of the copper-ant imony alloys. The eutee t ic points occur at 2 per cent. and 7o per cent. Ni, whi ls t the s u m m i t of the in te rmedia te curve occurs at abou t 43 per cent. Ni.

In F~K. 36 the curve according to Gaut ie r is shown.

A L U M I N I U M A N D A N T I M O N Y .

Gaut ie r points out tha t the curve of fus ibi l i ty of these alloys is remarkable , since near ly all its points correspond

12 Campbell: [J. F. I.,

to t empera tu re s h igher than those of the fus ion of' the two metals . He says that the curve indicates the format ion of a compound SbA1, whose fusion-point is s l ight ly lower than tha t of copper. Dr. Mathews ' curve is shown in Fig. xa of his paper.

On a microscopic examina t ion these alloys fall into two g roups :

(Q o to 8I"5 per cent. an t imony, in which increas ing amoun t s of the compound SbA1 crystal l ize out, first as short rod-like crystals, then as more or less i r regular bars and pla tes wi th much parallel growth. The g r o u n d m a s s is a lumin ium or a lumin ium eontainir ig some SbA1 in solid solut ion. The g roundmass d isappears be t ween 75 and 8o per cent. Sb and the alloy appears homogeneous .

(2) 8i" 5 to Ioo per cent. an t imony, in which we pass from the compound th rough a series of alloys composed of crys- ta ls of SbA1 in an inereas ing g roundmass till we reach pure an t imony. The g roundmass could not be resolved into two components , and p robab ly consists of an t imony conta in ing some of the compound in solid solution.

The series is r emarkab le in many ways; for when the a n t i m o n y reaches abou t 6o per cent. the alloys soon become ro t ten and rapid ly d is in tegra te into a fine black powder . T h i s is due to oxidation, according to Gau t i e r ; for if the alloy be we ighed before and af ter d is in tegra t ion it will be found to have ga ined in weight . If the fresh alloy be sealed up in vacuo, no change takes place. Again, when the anti- m o n y in the a l loy is be tween 50 and 8o per cent. a grea t expans ion takes place dur ing solidification and par t of the l iquid res idue is squeezed out and solidifies as a b u t t o n on top of the alloy. T h a t it takes place dur ing the solidifica- t ion of the crystals of SbA1 is shown b y the fact tha t the b u t t o n contains a large percen tage of the compound as well f o r m e d crystals. Last ly, it is seen tha t the curve consists of a rise f rom the a lumin ium end to the alloy conta in ing 33 per cent. Sb. Th is is a summit , and the curve falls to 37 per cent. Sb, af ter which it r ises again to 8I'5 per cent. Sb. The reason for this has not ye t been explained, for under the microscope the all0ys b e t w e e n 33 and 4o per cent. Sb pre-

Sept., I9o2.3 Structure o f Metals and Binary Alloys. 2i 3

sent the same characteris t ics . A similar th ing occurs in the alloys of tin and a luminium.

S I L V E R A N D T I N .

The curve, like tha t of the silver-lead series, consists of two branches, the one curved, mee t i ng at the eutect ic po in t 5"5 per cent. Ag. T h e eutect ic melts at 222 ° C. and ex tends from almost pure t in to 65 per cent. Ag, where it d isappears .

S i l v e r - T i n .

S i l v e r - A n t i m o n y .

960'7 ° C.

Silver

900 °

800 °

7oo °

600 °

500 °

4oo °

3oo o

~o0o

9oo o

8oo °

7oo o

A n t i m o t

600 °

500 °

400 °

3O00

T i n

200 °

Ioo 8o 60 4o 2o o

Percentage of silver by weight. F I G . 3 7 .

Accord ing to Charpy, we have a compound Ag~Sn at 65 per cent. Ag, and thus the series can be d iv ided i n t o - -

(x) Alloys which are i somorphous mix tures of A g and Ag2Sn- -be tween Ioo and 65 per cent. Ag.

(2) Al loys of the compound Ag2Sn and Sn, which are qui te normal.

Be tween o and 3"5 per cent. A g we find the excess of tin crystal l izing out as grains in the eutectie. A b o v e 3"5 per cent. A g we find long, s lender needles, p robab ly of the corn-

214 Campbell: [J. ~'. h,

pound Ag2Sn crystal l izing out. The n u m b e r and size of these hard, b r igh t crystals increase with the total si lver in the alloy. T h e y assume curious tree-like shapes, finally be- coming uni ted and forming dendrites. A t abou t 50 per cent. they occur as rounded grains in close contact. The eutec t ic diminishes, and at 65 per cent. A g we find the alloy homogeneous . This alloy may ei ther be a definite compound or it may be a solid solut ion of tin in silver. F rom this poin t onwards to pure silver the alloys are i somorphous wi th sil- ver. Fig. 29 shows an alloy of the first group conta in ing 15 per cent. Ag, 85 per cent. Sn, and contains the cur ious ly g rouped crystals of the compound set in the eutectic. T he curve, Fig. 37, is based on the freezing-point curve of Hey- cock and Neville.

A N T I M O N Y A N D S I L V E R .

In Fig. 37 we having the cooling curve based on the figures of Heycock and Neville. I t is composed of two b ranches : the one from pure an t imony to 55 per cent. Ag, the eutec t ic point, be ing normal ; the other from silver, hav ing a de- c ided angle at 72 per cent. Ag, corresponding to the formula Ag3Sb. The eutect ic mel ts at 485 ° C., whils t the 72 p e r ' cent. A g alloy mel ts at 56I'5 ° C.

Accord ing to Charpy, be tween 72 per cent. AK and o per cent. A g we have excess of ei ther AK~Sb or Sb crystal l izing o u t in the eutectic, in the same manner as all the alloys of Group I ; be tween Ag:~Sb and pure silver we have a series

o f i somorphous mixtures of the definite compound wi th silver. It may be, however , tha t here we are deal ing with a case similar to the an t imony end of SbSn series, and that we have dendr i tes of silver crystal l izing out in the compound Ag.~Sb. This point will be set t led when a complete cooling curve has been taken of this end of the series.

TIN A N D A N T I M O N Y .

Between o and 7"5 per cent. of an t imony the alloys crys- tallize out in the same forms as pure tin ; in o ther words, the tin will re tain some 7"5 per cent. of Sb in solid solution, p robab ly in the form of the compound SbSn. W h e n the

Sept., I9o2.] Structure o f Metals and Binary Alloys. 2x 5

an t imony is increased above 7"5 per cent., some very hard, b r igh t cubes are found at the surface of the alloy when slowly cooled, bu t When cast t he cubes are found sca t te red t h r o u g h o u t the alloy. The b r igh t layer of cubes increases in th ickness wi th the an t imony in the alloy, till at abou t 3o per cent. Sb it reaches the base of the alloy, and the ground- mass is seen occupying the inters t i t ia l spaces. A t abou t 40 per cent. the form of the cubes begins to change, and at 45 per cent. Sb the alloy consists of squat, thick bars or plates crossing at all angles, the spaces be tween being still occu- p ied by a g roundmass similar to that found in the lower percen tage alloys, A t abou t 52 per cent. Sb, a new con- s t i tuent , p robab ly ant imony, is found making up the core of the bars or plates, b u t there still remain traces of the groundmass , which finally d isappears at about 55 per cent. Sb. The cores of the bars cont inue to increase and develop into the usual crys ta ls of an t imony and, at abou t 95 per cent. Sb, become cont inuous . In the joints be tween them traces of a matr ix are seen, which finally disappear as we approach pure ant imony.

Fiff. 3 o shows a vert ical sect ion through the center of a s lowly cooled alloy conta in ing 2o per cent. Sb, 8o per cent. Sn. The b r igh t cubes are seen set in the softer matrix. S tead found that the common freezing point of this matr ix be tween 7"5 per cent. and 5o per cent. Sb to be 256 ° C., which is remarkable since it is h igher than that of pure tin by 25 ° C. On analyzing the cubes isolated from a 25 per cent. Sb alloy, he found their composi t ion to be approximate ly SbSn. H e says that a homogeneous mass corresponding to SbSn can- not be obtained, for on mel t ing the metals in that proport ion the resul t ing alloy consisted of the peculiar plates and the dark mat r ix ; tha t the crystals up to 30 per cent. Sb are SbSn, bu t in the ne ighborhood of 40 per cent. Sb the forms begin to change and the contents of an t imony increase. This is cont rary to Behrens, ¢ who isolated from the IO per cent. alloy cubes wi th the formula SbSn~, and from the 64 per cent. Sb alloy a res idue of the formula SbSn.

~#/etallographist, Vol . I I I , p a g e I i .

216 Campbell." [J. F. 1.,

According to Stead, the specifie gravity of the cubes is 6"96 , which is l ighter than that of the groundmass, and thus they float to the top of the slowly cooled alloys, just as the antimony in the lead-antimony alloys. Fig. 3z shows the surface structure of the cast 25 per cent. Sb alloy, in which the cubes are quite distinct. In Pig. 32 we have the 7o per cent. Sb alloy, showing the crystals of ant imony set in a matrix, which is probably SbSn, containing much ant imony in solid solution. Stead puts forward the alternate view that Sb and SbSn may form isomorphous compounds with each other, and when the antimony reaches a certain point it crystallizes out in a separate state.

The alloys of tin and phosphorus and of tin and arsenic probably belong to the same group as tin and antimony.

TIN A N D P H O S P H O R U S . TIN AND A R S E N I C . *

When phosphorus is added to tin, a hard constituent, consisting of brilliant white plates, similar to graphite, is formed. Stead has studied the series from 0'04 per cent. to 5 per cent. phosphorus. The structure of these alloys can be developed by polishing alone, for the phosphide formed stands out in relief from the soft matrix of tin between. On etching with dilute acid this groundmass of tin (probably containing some phosphorus in solid solution, for its melt- ing point is 4 ° C. above that of pure tin), turns black, and the hard white crystals of phosphide stand out in strong contrast. When analyzed the bright white plates proved to have the composition Sn~P2. When the alloy is slowly cooled the phosphide invariably commences to grow at the outer portion of the alloy and travels in straight lines toward the center. The plates have a decidedly hexagonal form. Fig. 28 shows an alloy containing 2 per cent. phos- phorus; it has been cast, and so the phosphide appears as comparatively small thin plates throughout the ground- mass.

When arsenic is added to tin, thick rough plates are formed, having the composition Sn3As2. The groundmass consists of tin, probably containing some arsenic in solid

Stead : Journal Society Chemical Industry, March, 1897.

Sept., i9o2. ] Structure o f 3~etals and B i nar y Al loys . 217

solution, because, as in the ease of phosphor-t in, the melt- ing point has been ra ised some 4 ° C. S tead was able to in t roduce as much as 43 per cent. As into tin. Fig'. 27 shows a slowly cooled alloy, in which the thick rough plates of arsenide run in all directions, with a tendency towards par- allel growth. The alloy contains 20 per cent. As.

C O P P E R AND TIN.

The mici 'ostructure of the copper-tin alloys has been s tudied by Behrens, Charpy, Stead, Heycock and Neville. The earlier explanat ions were based on Le Chatelier's curve of fusibil i ty, which consists of three branches forming b y thei r in tersect ions two points corresponding to alloys with 3 and 72 per cent. of copper. These two points correspond to the two eutect ic alloys of the series. The existence of the compound SnCu3 (6I"7 per cent. Cu) has been proved, for at this point we find discont inui t ies in the variations of many propert ies, such as the e lec t romot ive force of disso- lut ion (Laurie); the electric conduc t iv i ty (Mathiessen); t he specific gravi ty (Riche). Hence it was thought that t he series consisted of two simple sets of alloys, viz.: the alloys of copper and SnCu~ and the alloys of SnCu~ and tin, and was similar in s t ruc tura l variat ion to the alloys of copper and ant imony.

This view of the cons t i tu t ion of this series of alloys does not explain the complete freezing-point curve pub- l ished in the four th repor t of the Al loys Research Commit- tee, Ins t i tu t ion of Mechanical Engineers , shown in Fig. 3 8 . I t was not unti l recent ly that an explanation of the branches b, d and e was offered.

o to I per cent. Co p p e r . - - W h e n x per cent. of copper is present the first eutect ic alloy is ob ta ined ; that is, the one with the lowest freezing point. Be tween pure tin on the one hand and this alloy conta in ing i per cent. Cu on the other, tin is found crystallizing, first in grains, then in den. drites in the eutectic. The cooling curves of all the inter- media te alloys show two breaks.

z to 8 per cent. Copper.-- W h e n the percentage of copper is increased above that of the eutect ic alloy, thin, bright, hol-

218 Campbell .. [J. F. I.,

low crystals are seen. In section they are horseshoe-shaped, and at first occur isolated; then they tend to form groups which appear in section as three- and six-rayed stars. The i r composit ion varies also, increas ing in copper from 34 to 44 per cent.

8 to 4 ° per cent. Copper.--A third cons t i tuen t is seen when the copper exceeds 8 per cent. W e have the eutect ic or g roundmass enclosing the br ight porphyri t ic crystals char- acteristic of the i to 8 per cent. al!oys, bu t these br igh t crystals are seen in places to have grown on and around a different kind of crystal. I t is not a ease of one crystal vary ing in composit ion from the center to the faces, for a sharp line of junc t ion can be seen between the two con- s t i tuents . On oxidat ion this new cons t i tuen t becomes very dark, and is easily d is t inguished from the o ther two con- s t i tuents of the alloy. As the percentage of copper is increased, the more easily oxidized crystals increase in number and size; whereas the br igh t crystals begin to decrease toge ther wi th the euteetic. I t would, therefore,. appear tha t in alloys conta in ing more than 8 per cent. of copper, the first cons t i tuen t to crystallize out is the central, easily oxidized crystals. This causes the first rest in the cooling curve de. Then the br igh t crystals solidify, causing the second hal t on the horizontal branch d e . Las t ly the eutectic solidifies and the thi rd ha l t is reached on oranch f J. As branch d e is horizontal , it would seem tha t the br ight crystals have a definite composit ion when above 8 per cent. Cu is present in the alloy, bu t when branch ere joins the outer curve at e and falls to f , these crystals no longer have a definite composition, but their percentage of copper falls wi th their t empera ture of solidification from Sn~Cu 3 to SnCu.

The upper curve in Fig. 38 shows the relat ion between the percentage of copper in the alloy to the percentage of copper in the crystals which have been isolated. A break occurs at 8 per cent. copper, at the in t roduct ion of the th i rd cons t i tuent to the alloys. These results agree very closely with those of Stead. ~

Journal Society of Chemical Industry, June, i897.

S¢pt., I9o2.] Structure of Metals and Binary A/lays. 2t 9

A s the copper approaches 4o per cent., the central plate- l ike crystals are grouped together in parallel bunches , unti l at 4 0 per cent. Cu they are very thick and cover more than hal f the field. In the eutect ie be tween them the small, br ight , hol low crystals are seen.

4x to 6r" 7 per cent. Cbpper.~The difference between the a l loy containing 4o per cent. and that containing 4t per

Percentage of copper in alloy.

I0o0

9o0

8o0

700

6oo

5oo

400

30o

20o

40~. 30g ZO'/. 10%

i s6X

I :-.:a~, Liouid -".. ;~__ : I . , i Q m ( t + \ - . . ~ - 45%

,sa;. \ . 2"- "X 4{ - . \

.., ~ - . . \~,. so',.a . ~ ~ . - . X

,, rid, ~ L,*+u,d ,..3o~ ,, a+ a ' ~ a: ~ "

d~ e' "Liouid + ,Solid N ~ ¢ ,

\ R • i

IOCO c C.

9 0 o

8o0

700

600

500

4 0 o

30o

SII.

20o

]. • xoo 80 60 40 20 0

Percentage of copper. F I G . 3 8 .

cent. Cu is very marked. The crystals in the Tatter are , smal l and lath-shaped, arranged more or less in groups and

L

are separated from each other by eutectic. T h e y are com- posite as before, but the white const i tuent surrounds the dark as an envelope of uniform thickness, not as a rough i~erUstation. N o s ingle prismatic crystals of the white c o n s t i t u e n t have been seen in the eutectic. W i t h each

2 2 o Campbell : [J. F. I.,

addi t ion of copper the groups of crystals become more and more compact and the amoun t of eutect ic d iminishes unt i l at 56 per cent. Cu it disappears a l t o g e t h e r : Therefore branch f f of cooling curve ends at 56 per cent.

The br ight cons t i tuent of the crystals grows smaller and smaller ; at 56 per cent. Cu it takes the place of the eutect ic and forms the groundmass , whils t at about 6i per cent. Cu it disappears and we have a homogeneous mass of SnCu~. Hence branch e d ends at 60 per cent. at d.

Seeing tha t these alloys up to 56 per cent. Cu show four breaks in their cooling curves, one would na tura l ly expect to find four different cons t i tuents in each. Only three, however, can be dis t inguished. Quenching below the first and second breaks gives a difference in s t ruc ture only. As in the alloys conta in ing 6I" 7 per cent. Cu and o n w a r d s , branch d of the freezing-point curve corresponds to a re- a r r angemen t in the solid, and as the difference between the 4o per cent. alloy and those of a h igher copper contents is one of s t ruc ture only, we may assume tha t the second re ta rda t ion in the cooling curve dd ~ is one of rearrange- men t also.

6z" 7 to 6S'2 per cent. Copper. SnCu 3 to 5nCu~ . - -The changes which take place between these two points can only be observed when the alloys are very slowly cooled. The alloys set as a whole at the first break on c d. and tend to rear range themselves subsequent ly in the solid. Near SnCu~ we see the dark grains of this compound sur rounded by an envelope of a br ight material , probably SnCu4, whose format ion is indicated by dl in the cooling curves. Each addi t ion of copper brings in more and more of the b r igh t const i tuent . Above 65 per cent. Cu we find, in places, a s t ruc ture like tha t of a eutect ic which accounts for the horizontal branch d 2of the curve. A t 68"2 per cent. Cu we have a homogeneous alloy, very brittle, t ak ing a beaut i fu l polish, and this very probably is a definite compound, SnCu4. W h e n the alloys of this group are quenched a t definite temperatures , very m a n y curious and beaut i fu l

• ~Stead: J . S. C. I . ; June, 1897.

Sept., I9O2.] Structure o f Metals and Binary Alloys. 221

s t ruc tu res are me t with. For example, if the 66 per cent. Cu alloy be quenched below the first break, say at 72o ° C., then a clear cell-like s t ruc ture is seen. If it is quenched be tween the first and second break, say at 65 o° C:, then the s t ruc tu re consists of a ne twork of parallel s t ra igh t lines, a l t e r n a t e l y l ight and dark. Each large area has two or more sets of lines, which are different ly or ien ta ted for different areas. It may be called the " S c h i l l e r " s t r u c t u r e of these allo);s. If the alloy be quenched below the second break, bu t above d2, say at 6o0 ° C., the s t ruc ture is a lmost the same as tha t met with in the s lowly cooled alloy, except tha t no sign of any eutect ic can be seen. If the 68"2 per cent. al loy be quenched j u s t af ter the first break, i ts struc- ture is tha t of dark rounded grains pass ing impercep t ib ly in to a l ight-colored groundmass , clearly showing tha t the al loy is in a me tamorph ic state. T h u s it seems qui te cer- tain tha t in this g roup the alloys rear range themse lves in the solid.

68"2 to 75 per cent. Copper.--In this g roup we pass f rom the h omogeneous SnCu,, consis t ing of i r regular and e longa ted grains, t o the so-called second eutect ic at 74 to 75 per cent. Cu. Be tween these two alloys we find the compound SnCu, crystal l iz ing in the eutect ie . Nea r 68"2 per cent. Cu the al loys are composed of polygonal grains, wi th boundar ies of b r igh t SnCu4. The i r central par ts are composed of dendr i tes and rose t tes of SnCu4 set in the eutectic. Near 74 per cent. Cu the alloys are composed of a t t enua ted rose t tes and gra ins of SnCu4 set in a compara t ive ly large amoun t of eutect ic . T h u s the upper par t of branch d, be tween d2 and d3, marks the poin t at which these alloys rearranged themse lves into grains i somorphous wi th SnCu4, whi ls t the horizontal branch d 3 marks the format ion of the eutectic, bo th changes hav ing t a k e n place in the solid. A t c there occurs a small horizon- tal branch, the mean ing of which is no t qui te certain. A microsect ion of an alloy quenched be tw een this and the ou t e r curve shows dark rounded grains set in a l ight ground- mass. Heycock and Nevi l le say tha t the ou te r curve b c indica tes the format ion of skeletons rich in copper; bu t when the a l loys are quenched below the horizontal branch c, then

2~2 Campbell. [J. F. I,,

we have uniform solid solutions. I t is to be noted tha t on the surfaces of alloys conta in ing 7o per cent. or more cop- per, a network of dendri tes or skeleton crystals resembl ing those on the surface of a pure metal is to be seen. I t was soon noticed tha t the in ternal s t ructure of the alloys from 7o to 75 per cent. Cu showed no trace of these dendri tes , a n d s o the surfaces of severa l were rubbed down, polished and etched so as to lay bare their in ternal s t ructure. In each case it was the same as tha t of the center of the alloy, which shows tha t these dendri tes have split up and rear- ranged themselves after solidification, and all t ha t remains of them is this surface s tructure.

75 to zoo per cent. Copper.--Above 75 per cent . Cu two new const i tuents make their appearance and ,the alloy assumes a yellow t in t and begins to lose its brit t leness. In section we find yellow grains sur rounded by a br igh t whi te border, set in the second eutectic, in which small whi te grains also occur. Now this eutect ic at 76 per cent. Cu is much larger in character than the 74 per cent. alloy, and this may account for the fact tha t the eutect ic break rises some 3 °° C. as it passes from 74 to 75 per cent. Cu.

As the total copper is increased the yellow grains ncrease, f o rming dendri tes and skeleton crystals, the whi t e borders and grains m e r g e together , and the eutect ic decreases till at about 9o per cent. Cu it disappears. The yellow grains of copper become darker and darker (contain less and less tin in solid so lu t ion) t i l l they reach copper color. The l ight borders diminish and disappear about 95 per cent., leaving copper dendri tes alone. These dendr i tes vary in composit ion from center to outside, and so the cen ter etches a darker color. These dendr i tes darken with increase of copper till about ioo per cent. is reached, when we have the characteris t ic s t ructure and color of pure copper.

Quenching these alloys at different t empera tures proves tha t copper grains and dendri tes begin to crystallize out as soon as an alloy has reached the tempera ture of the outer curve a b ," tha t these cont inue to grow till the tempera ture falls to b'b, when the whole mass becomes solid; a t th is

Sept., i9o2. ] Notes and Comments. 2z 3

point we have crystals of copper in a homogeneous ground- mass. A b o v e the curve a b the alloy is liquid, be low b' b the alloy is solid, whi ls t be tween a b and b' b we have a mix- ture of solid and liquid. The eomposi t ion of the eopper dendr i tes crystal l iz ing ou t of any par t ieular alloy would be given b y the composi t ion line a b'.

Below b' b . the g roundmass consists of a solid solut ion much r icher in tin than the grains of copper in it. I t is p robable tha t these grains of eopper cont inue to g row in the solid unti l at abou t 5oo ° C. the g r o u n d m a s s has arr ived at the composi t ion 75 per cent. Cu, when it splits up into a eutect ic composed of laminee of SnCu4 and Cu, conta ining a eonsiderable a m o u n t of Sn in solid solution. Th is change in the solid is marked by branch d4.

The mean ing of branch d~ is obscure. It may indicate the format ion of the b r igh t whi te grains seen in the 76 and 77 per cent. alloys.

In their recent papers before the Roya l Soeie ty of Lon- don and elsewhere, Heycock and Nevi l le ~ have clearly

• proved tha t the g rea t changes which take ptaee when a bronze reaches the t empera tu re of the curve d' . . . . d4 are dntirely in the solid. T h e y go fur ther than this b y deter- mining the var ious subs tances or phases which exist in the var ious alloys, e i ther chilled or s lowly cooled. The i r modi- fication of the cooling-curve d iagram by the addi t ion of a curve indicat ing the posi t ion of the end of solidification of each alloy and their explanat ion of this new diagram, go far in clear ing up many of the n u m e r o u s difficulties me t wi th in the alloys of copper and tin.

ALUMINUM AS A SUBSTITUTE FO R COPPER.

The possible subst i tut ion of a luminum for copper in electrical work is a question which has been discussed from t ime to t ime in the press; frequently, we regret to say, by writers who apparent ly know very li t t le about the facts. I t has also been referred to by electricians, some of whom have been able to speak from actual experience. From the expert tes t imony it appears that

~ Proceedings Royal Society of London, Vol. 68. British Association, Glasgow Meeting, 19Ol, Chem. Section.