PR. WALTER ROREXHAIN AXD MR. SYDXEY I, ARCHBUTT ON THE 



DESCRIPTION OF PHOTOMICROGRAPHS (PLATES 5-7). 



PLATE 5. 



(Fl(!S. 12 AND 15 OMITTED.) 



Fig. 8. The a phase with small quantity of eutectic in alloy 98. x 150. 



9. Decomposed /? phase in eutectic in alloy 95. x 1000. 

 10. Typical structure of eutectic. Alloy 95. x 200. 

 11. Typical structure of eutectic. Alloy 95. x 150. 

 ., 13. Hexagonal dendrite of /3 phase (Al 2 Zn s ). Alloy 95. x 200. 

 14. Dendrites of /3 crossing at angles near 60 degrees. Alloy 94. x 200. 

 16. Alloy 90 cooled from fusion in 45 minutes. /? embedded in eutectic and partially decomposed 



at the edges, x 150. 

 17. Alloy 86 cooled at the standard rate used for cooling-curves, ft almost completely decomposed. 



x!50. 



18. Alloy 86, under higher magnification, showing the laminated structure of decomposed /?. 

 x500. 



PLATE 6. 



Fig. 19. Alloy 80, slowly cooled after annealing at 430 C., showing the persistence of eutectic. x 150. 



20. Alloy 84, quickly cooled to a temperature just above 250 C. and then quenched, showing 

 sheaths of /? surrounding primary y. x 150. 



21. Alloy 84, slowly cooled from fusion, showing the more complete formation of ft and its 

 subsequent partial decomposition, x 150. 



22. Alloy 84, annealed for a considerable time at 430 C. and then slowly cooled, showing the 



complete formation and decomposition of /?. x 300. 

 23. Alloy 84, annealed at 430 C. and then quenched from a temperature just above 250 C., 



shoving the complete formation of /2 and the prevention of decomposition by quenching. 

 x300. 



24. Alloy 78, slowly cooled, showing dark cores of primary 7 surrounded by sheaths of decomposed 

 /3 and a residue of eutectic. x 150. 



Alloy 78, similar to fig. 24, under higher magnification, showing laminated structure of 

 decomposed /?. x 500. 



26. Alloy 77, annealed at 430' C. and quenched just above 250 C., showing the homogeneous 

 structure of ft or 8. x 600. 



27. Alloy 77, annealed first at 430 C. and then just below 250 C. Laminated or pearlitic 

 structure of decomposed /3 or 8. x 600. 



