RAWDON: STANDARD ZINC-BRONZE 631 



ing gradually from the base to the apex; it is 11 to 12 mm. long, 

 3,5 mm. broad, and 1.5 mm. thick; the radicle is slender and 

 ascending; the cotyledons are thin and very brittle; the testa is 

 membranous and pale brown. 



The illustration showing habit and habitat is from a photo- 

 graph kindly furnished by Dr. Townsend, who has supplied also 

 the geographic and habital data given above. 



TECHNOLOGY. — Standard zinc-bronze: Relation of micro- 

 structure and 7nechanical properties. Henry S. Rawdon, 

 Bureau of Standards.^ 



To complete the study of the mechanical properties of zinc- 

 bronze (Cu 88, Sn 10, Zn 2) as influenced by the method of casting, 

 temperature of pouring, and other varying factors of foundry 

 practice, a detailed study of the micro-structure of a large num- 

 ber of test bars was made. The results show that the method of 

 preparation affects the structure only indirectly by the rate of 

 cooling, amount and distribution of foreign inclusions, etc. 



In the cast condition the alloy consists of an aggregate of 

 relatively large crystals, each of which comprises a dendritic 

 matrix consisting of a solid solution of tin (and zinc?) in copper, 

 embedded in which are numerous particles of a hard brittle 

 eutectoid. When stress is applied and continued beyond the 

 "elastic limit," the matrix is plastically deformed while the 

 eutectoid enclosures are shattered and broken transversely 

 across. When broken in the tension test, the bars become 

 wrinkled and roughened in a characteristic manner which is to 

 be attributed to the orientation of the crystal-structure and 

 properties. The examination of a good many broken bars 

 shows that, with few exceptions, when fracture occurs it takes 

 place within the crystals and is not a mere pulling apart and 

 separation of adjacent crystals. 



Since the fracture occurs through the crystals, i.e., along 

 cleavage planes, rather than between them, the size of crystals 

 is an important factor in determining the ultimate strength of 



1 To appear in full as a Technologic Paper of the Bureau of Standards. 



