578 C. H. Mathewson — Metallographic Description 



worked and unaltered regions, respectively. This may be 

 traced to a change in contrast values due to irregular move- 

 ments of the structural elements during cold working and the 

 appearance of dark-etching lines of deformation within the 

 grains. The photo-micrographs, figs. 44 and 45, afford a com- 

 parison between the altered and unaltered casting structure, 

 respectively, after etching with acidified ferric chloride. In 

 the latter figure there is abundant contrast between the small 

 number of grains which are collectively visible under this 

 moderate magnification (72 X ) • An unsound part of the object 

 was purposely chosen for representation in order that charac- 

 teristic oxidation fissures might be shown without recourse 

 to another micrograph. One of these fissures (black) extends 

 from the left hand edge towards the center near the bottom of 

 the figure. In fig. 44, one grain cannot be distinguished from 

 another and a general effect of elongation in the direction top- 

 bottom is seen in the arrangement of the light constituent 

 (a -+- 8 complex) lengthwise in this direction. This condition 

 was brought about by cold working. The lines of deformation 

 are not clearly resolved at this magnification. It was not 

 thought desirable to provide another photo-micrograph devoted 

 to this feature, since the same effect is shown in fig. 48, leaving 

 out the small recrystallized units which are not distinguishable 

 in the present object except near the spatula-tip. f 



The a -\- 8 complex is the transformation product of a 3 

 constituent which was the last material to solidify, and conse- 

 quently, filled in the interstices between dendritic grains of the 

 primary, a, material. Every change of state in an interstitial 

 material is likely to disturb its alignment with the surrounding 

 material and thus develop cavities. Aside from this, evolution 

 of gas (S0 2 , etc.), as a result of diminished solubility or of 

 reaction between dissolved impurities, is likely to occur most 

 freely during the latter stages of solidification when the corre- 

 sponding concentration values are high. As a matter of fact, 

 cavities could be seen in almost every patch of complex when 

 examined under high power. Very small ones would be indis- 

 tinguishable from the minute, dark-etching particles of a. The 

 rounded black spot in the uppermost branch of the large patch 

 shown in fig. 43 (300 X) is one of these cavities. A number 

 of similar black spots may be seen among the numerous patches 

 of complex shown under low power in fig. 41. 



Oxidation of the finely divided a of the a + 8 complex starts 

 at the surface of the casting and its inward creep is facilitated 

 * See p. 561. f See p. 562. 



