254 ANNUAL UEPOKT SMITHSONIAN INSTITUTION, 1951 



POLISH-ETCH METHOD 



According to this method, a metal or alloy is simply cut with a saw 

 through some section intended for study. The saw-cut surface is then 

 ground flat and further polished with increasingly fine emery papers 

 and buffing cloths until the polishing scratches are so minute as to 

 be no longer visible at even relatively high magnification. Such treat- 

 ment produces, of course, a superficial layer of higlily distorted metal 

 which completely conceals the true internal formations. This layer 

 is then removed by carefully selected chemical reagents, determined 

 through much research to provide certain characteristic effects depend- 

 ing upon the microscopic structure of the metal. 



The reagent eats off the thin and disorganized superficial layer and 

 then attacks the underlying metal — but to a degree dependent upon 

 subtle differences in composition and structure. The result is a differ- 

 entiated mottling, whose pattern is characteristic for the condition and 

 hence is metallurgically informative. This is shown in plate 1, figure 

 1, for an alloy of equal parts of bismuth and antimony, at a magnifica- 

 tion of 225 diameters. The specimen was cut with a saw, ground, 

 polished, and etched with a solution of iron chloride in hydrochloric 

 acid. The superficial layer was entirely removed, and the underlying 

 metal was attacked in the elaborate manner shown. The peculiar 

 light-colored skeletons are known as dendrites because of their tree- 

 like form, the word coming from the Greek "dendron," meaning tree. 



Dendrites express a uniform peculiarity in the growth of crystals 

 which causes them to grow from the fluid state in the form of branch- 

 ing growths. Wlien an alloy — which is a mixture of two or more 

 metals — solidifies from its liquid, the first solid to construct the den- 

 drite is richer in the metal having the higher melting point. The 

 remaining liquid is relatively rich in the metal of low melting point, 

 and it is this that fills in between the branches of the dendrite. Wlien 

 an etching reagent is chosen which attacks one of the metals more 

 than the other, the difference between the trunk and the interbranch 

 material of the dendrite is made visible by the difference in chemical 

 attack. 



FRACTOGRAPHIC TECHNIQUE 



While the polish-etch method has yielded a tremendous amount of 

 information on the constitution of metals, it also has important limi- 

 tations. For example, the remarks that have just been given will make 

 it clear that the polish-etch technique would supply relatively little 

 information for a pure metal. No constitutional differences exist, 

 and attack by a chemical reagent would accordingly be uniform. 

 About the only exception is a preferential attack at the boundaries of 

 the individual grains and the fact that the separate grains are distin- 



