. Transactions. alloys with Sn from 0 to per cent. (CugSn) the curvesbelow the solidus represent changes in the solid alloy. Considering the Cu-Sn thermal diagram in small parts it atonce becomes simple, being merely compounded of a numberof simple curves. The Tiuo Solid Solutions, Aljiha and part of the diagram embraced by ABC—Aabe and just 836 NOTES ON THE METALLOGRAPHY OF ALLOYS. below it, shows that the conditions are similar to that repre-sented bj Fig. 2. In freezing, a solid solution rich in Cu (alpha or the solid I) sep-arates out when the temperature reaches the c
. Transactions. alloys with Sn from 0 to per cent. (CugSn) the curvesbelow the solidus represent changes in the solid alloy. Considering the Cu-Sn thermal diagram in small parts it atonce becomes simple, being merely compounded of a numberof simple curves. The Tiuo Solid Solutions, Aljiha and part of the diagram embraced by ABC—Aabe and just 836 NOTES ON THE METALLOGRAPHY OF ALLOYS. below it, shows that the conditions are similar to that repre-sented bj Fig. 2. In freezing, a solid solution rich in Cu (alpha or the solid I) sep-arates out when the temperature reaches the curve AB. Thisenriches the liquid in Sn,and at 795° C. there are dendrites ofalpha, saturated with Sn of composition «, and a liquid of com-position B. Then occurs a reaction : solid solution alpha (a) -\-liquid B = solid solution beta (/>). This reaction continues tillall of the alpha or all of the liquid is used up and the alloy J= Solid Solution, Alpha// = Solid Solution,Beta7/7= Cu4Sn IV= CusSn V= CuSn. G 232° Sn +liquid J -Sn + Eutectic 40 50 60 70 PERCENTAGE OF TIN Fig. 4.—The Copper-Tin Diagram. becomes solid. In the area marked by the solid I is the solidsolution rich in copper, or alpha. In the triangular area markedby the solid //is nothing but the beta solid solution, while be-tween these two areas is / 4- //, or the mixture of alpha andbeta (see Figs. 18 and 24). But this beta solid solution, which contains Sn, 22 per cent,at 795° C, becomes enriched in Sn as the temperature falls,and this it does by following down the curve hj and precipitat- NOTES ON THE METALLOGRAPHY OF ALLOYS. 837 ing some alpha. On arriving at J, the eutectoid point at 500° C,it suddenly breaks up into a mechanical mixture of alpha andanother solid which we will call Cu^Sn (although it may bemerely a solid solution and not a compound). This mechani-cal mixture is the eutectoid and corresponds to pearlite insteel. Beta Alone. In alloys between h and |; (Sn, from 22 to 25 per cent.) th
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