Page:Proceedings of the Royal Society of London Vol 69.djvu/342

326 composition I, and accounts for the fantastic angular shapes seen in the a combs of slowly cooled alloys. Our photograph 1, in Plate 2, of the ' Proceedings,' vol. 68, shows this angularity well. In this photo- graph the a combs are dark. When the temperature falls below 500 C. (the eutectic line of Roberts-Austen and Stansfield) the residual ft decomposes into a very minute eutectic of a and the tin- rich body D', which we believe is Cu 4 Sn. Thus at all temperatures below b'C/' this group of alloys form a complex of a + D'.

3. The LC Alloys, containing from about 13 to 15 '5 atomic per cents, of Tin.' These alloys commence their solidification with the formation of a crystals, but at the C temperature these are completely changed into fi of the percentage I, and then the (3 reacts along the lines Ic and CD with the residual liquid. When the temperature has fallen to the solidus Ic the alloy is a uniform solid solution. It is a mass of ft crystals, chemically identical, but forming crystalline grains differently oriented and therefore showing, after etching, differences of brightness on tilting or rotating. (The Stead effect.) This uniform solid solution continues to exist until the temperature falls below the line 1C', which

s comparable to a freezing-point curve, inasmuch as on cooling to a point on this line, the uniform (3 becomes saturated with a, and below the line the a crystallises out in large copper-rich crystals. Finally, as before, at 500 C. the residual /3 breaks up into the C' eutectic. Below the b'C' line these alloys, like the previous group, consist of the complex a + D'. We have verified all this repeatedly, chilled samples of the alloy Snu showing everything very clearly. The C' eutectic has often, of course, been previously observed in unchilled alloys, but, so far as we know, without its real nature being dis- covered.

4. The CD Alloys, containing from 15 -5 to 20 atomic per cents, of Tin. These alloys begin to solidify by forming large, comparatively copper-rich combs of /3, and when just solid are a uniform mass of /3 or 7 crystals. This uniform solid solution persists so long as the tempera- ture is above the line C'XD'. These solid solutions are very homo- geneous, and we have not been able to find any difference between ft and 7 crystals. When the temperature falls below C'XD', the phase D' crystallises out in ribands at the borders of the crystal grains of solid solution, as well as in the form of a fern leaf or rosette scattered through these grains. The effect is well shown by photograph 6, Plate 3, of the ' Proceedings,' vol. 68, and the cause of the pattern is discussed in that paper. The substance between the crystals of D' is, however, uniform until the temperature X is reached, when the residual solid solution breaks up into the C' eutectic complex of a and D'. The fact that the eutectic point X of the CD alloys is a little lower than C' is evident in the pyrometric curves of Roberts- Austen and Stansfield, and we have abundantly verified it. But Professor Roozeboom suggests,