Trans.) Leadtin eutectic. Magnified 75 o diameters.
It is evident that every mixture except the eutectic mixture C will have two halts in its cooling, and that its solidification will take place in two stages.
The two sloping lines cutting at the eutectic point are the freezing-point curves of alloys that, when they begin to solidify, deposit crystals of lead and tin respectively.
Eutectic point gives the second halt in cooling, due to the simultaneous formation of lead crystals and tin crystals.
The eutectic alloy itself, fig.
2 (Plate), shows the minute complex of the tin-lead eutectic, photographed by J.
Osmond, shows the structure of a silver-copper alloy containing considerably Eutectic more silver than the eutectic.
The analogy between the breaking up of a solid solution on cooling and the formation of a eutectic is obvious.
6, consisting of two downward sloping branches meeting in the eutectic point, and that of thallium and tin, the upper curve of fig.
There are thus two eutectic alloys B and D, and the alloys with compositions between B and D have higher melting-points.
It is probable that all the alloys of compositions between B and D, when they begin to solidify, deposit crystals of the compound; the lower eutectic B probably corresponds to a solid complex of mercury and the compound.
The higher eutectic D may correspond to a complex of solid thallium and the compound; but the possible existence of solid solutions makes further investigation necessary here.
It also possesses a splendid purple Here, the large dark masses are the silver or silver-rich substance that crystallized above the eutectic temperature, and the more minute black and white complex represents the eutectic. It is not safe to assume that the two ingredients we see are pure silver and pure copper; on the contrary, there is reason to think that the crystals of silver contain some copper uniformly diffused through them, and vice versa.
The pearlite when highly magnified somewhat resembles the lead-tin eutectic of fig.
Alloys represented by points on Ee, when they begin to solidify, deposit crystals of lead and bismuth simultaneously; Ee is a eutectic line, as also are E'e and E"e.
The alloy of the point e is the ternary eutectic; it deposits the three metals simultaneously during the whole period of its solidfication and solidifies at a constant temperature.
The presence in an alloy of a eutectic which solidifies at a much lower temperature than the main mass, implies a great reduction in tenacity, especially if it is to be used above the ordinary temperature as in the case of pipes conveying super-heated steam.
The solid is then known 800 as a eutectic alloy.
A liquid in which the com position is nearly that of the eutectic shows the changes in the rate of fall of tempera ture as it is allowed to cool.
This process continues till the composition of the liquid phase reaches that of the eutectic, when the whole mass solidifies on the further loss of heat without change of temperature, giving a very definite freezing point.
Proceeding along the curve in either direction, we come to a non-variant or eutectic point.
Thus in interpreting complicated freezing point curves, we must look for chemical compounds where the curve shows a maximum, and for a eutectic or cryohydrate where two curves meet at a minimum point.
The residual liquid would thus become richer in B, and the tem perature and composition would pass along the curve till E, the eutectic point, was reached.
The eutectic point may never be reached.
Eutectic here freezes 7 Austenite+Cementite Pro-eutectoid Cementite forms progressiuely 882930 1200 0 U a 5 6 ?
Eutectic,a earIitei-Cementit .(/a primarg, Oxide, Cementite(t 400 300 20 even if rich in austenite, is strongly magnetic because of the very magnetic a-iron which inevitably forms even in the most rapid cooling from region 4.
1 A " eutectic " is the last-freezing part of an alloy, and corresponds to what the mother-liquor of a saline solution would become if such a solution, after the excess of saline matter had been crystallized out, were finally completely frozen.
So much of either metal as is present in excess over the eutectic ratio, freezes out before the eutectic; (2) that though thus constant, its composition is not in simple atomic proportions; (3) that its freezing-point is constant; and (4) that, when first formed, it habitually consists of interstratified plates of the metals which compose it.
If the alloy has a composition very near that of its own eutectic, then when solidified it of course contains a large proportion of the eutectic, and only a small proportion of the excess metal.
If it differs widely from the eutectic in composition, then when solidified it consists of only a small quantity of eutectic and a very large quantity of the excess metal.
A " eutectoid " is to such a transformation in solid metal what a eutectic is to freezing proper.
Measures the percentage of the excess of ferrite or cementite for hypoand hyper-eutectic steel and white cast iron respectively.
When the gradually falling temperature reaches 1430° (q), the mass begins to freeze as -y-iron or austenite, called " primary " to distinguish it from that which forms part of the eutectic. But the freezing, instead of completing itself at a fixed temperature as that of pure water does, continues until the temperature sinks to r on the line Aa.
The freezing of molten cast iron of 2.50% of carbon goes on selectively like that of these steels which we have been studying, till the enrichment of the molten mother-metal in carbon brings its carbon-contents to B, 4.30%, the eutectic 1 carbon-content, i.e.
At this point selection ceases; the remaining molten metal freezes as a whole, and in freezing splits up into a conglomerate eutectic of (1) austenite of about 2.2% of carbon, and therefore saturated with that element, and (2) cementite; and with this eutectic is mixed the " primary " austenite which froze out as the temperature sank from v to v'.
The white-hot, solid, but soft mass is now a conglomerate of k1) " primary " austenite, (2) " eutectic " austenite and (3) " eutectic " cementite.
Here the mass consists of (1) primary austenite, (2) eutectic austenite and cementite interstratified and (3) pro-eutectoid cementite.
This formation of cementite through the rejection of carbon by both the primary and the eutectic austenite continues quite as in the case of 1.00% carbon steel, with impoverishment of the austenite to the hardenite or eutectoid ratio, and the splitting up of that hardenite into pearlite at Ari, so that the mass when cold finally consists of (1) 1 Note the distinction between the " eutectic " or alloy of lowest freezing-point, 1130°, B, with 4.30% of carbon, and the " eutectoid," hardenite and pearlite, or alloy of lowest transformation-point, 690° S, with 0.90% of carbon.
In the black-and-white ground mass the white is the eutectic cementite, and the black the eutectic austenite, now split up into pearlite and pro-eutectoid cementite, which cannot here be distinguished from each other.
4.-The constitution of hypo-eutectic white or cementitiferous cast iron (washed metal), W.
The black bat-like areas are the primary austenite, the zebra-marked ground mass the eutectic, composed of white stripes of cementite and black stripes of austenite.
Both the primary and eutectic austenite have changed in cooling into a mixture of pearlite and pro-eutectoid cementite, too fine to be distinguished here.
Smaller and smaller proportion of the whole, and the austenitecementite eutectic which forms at the eutectic freezing-point, 1130° (aB), increases in amount until, when the carbon-content reaches the eutectic ratio, 4 30%, there is but a single freezing-point, and the whole mass when solid is made up of this eutectic. If there is more than 4-30% of carbon, then in cooling through region 3 the excess of carbon over this ratio freezes out as " primary " cementite.
In it the normal constituents are, for region II., molten metal+primary austenite; for region III., molten metal+primary graphite; for region IV., primary austenite; for region VII., eutectic austenite, eutectic graphite, and a quantity of pro-eutectoid graphite which increases as we pass from the upper to the lower part of the region, together with primary austenite at the left of the eutectic point B' and primary graphite at the right of that point.
Hastening its cooling by casting it in a cool mould, favours the formation of cementite rather than of graphite in the freezing of the eutectic at aBc, and also, in case of hyper-eutectic iron, in the passage through region 3.
Like the hardening of steel, it hinders the transformation of the austenite, whether primary or eutectic, into pearlite+cementite, and thus catches part of the iron in transit in the hard a state.
The reason is that the particles of temper graphite which are thus formed within the solid casting in its long annealing are so finely divided that they do not break up the continuity of the mass in a very harmful way; whereas in grey cast iron both the eutectic graphite formed in solidifying, and also the primary graphite which, in case the metal is hypereutectic, forms in cooling through region 3 of fig.
27; the eutectic graphite is much FIG.
Thus it is extremely probable that the primary graphite, which forms large sheets, is much more weakening and embrittling than the eutectic and other forms, and therefore that, if either strength or ductility is sought, the metal should be free from primary graphite, i.e..