260 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



contrast to plate 3, figure 1, where it traversed the boundary. This 

 elaborate pattern is believed to represent a frozen record of the growth 

 pattern during the time that the micelles of the liquid state were 

 orienting and transfixing to form the solid. The roughly parallel 

 bands are believed to result from pulsations in the solidifying front. 

 This metal was cast — it will be recalled from earlier description — 

 under the conditions of an electric arc at extremely high temperature, 

 and it solidified in a water-cooled copper crucible. These are violent 

 freezing conditions for a metal melting near 2620° C. (4750° F.). 

 The story of solidification read from the fractograph in plate 3, 

 figure 2, would show this grain to have formed from the upper left 

 corner toward the lower right, the micelles rotating and orienting with 

 one another sufficiently to produce a single crystal, but remaining 

 slightly displaced from one another and misfitted sufficiently to set up 

 a special pattern of weakness, which then showed itself fracto- 

 graphically by deflection of the fracture traverse in accord with the 

 pattern misfit. 



Finally, in plate 4, a pair of fractographs adds further description 

 to the micellar concept, and in addition shows an unusual application 

 of the fractographic technique. A specimen of plain iron (Armco 

 ingot iron) was annealed at 1250° C. for 2 hours and slowly cooled 

 in the furnace to remove effects of previous mechanical strain and 

 to increase the grain size. The metal was then embrittled by forcing 

 atomic hydrogen into its structure. This was accomplished by making 

 the specimen the cathode (negative electrode) in an electrolytic cell. 

 Iron absorbs hydrogen, but only atomic hydrogen ; and, on the sur- 

 face of the cathode, protonic or atomic hydrogen is deposited by the 

 electric current passing through the solution — here 10 percent sodium 

 hydroxide. It is known from extensive research that this atomic 

 hydrogen enters among the atoms of the iron, probably diffusing 

 through interatomic interstices, and then later precipitates at certain 

 well-defined places within the body of the grain — the intermicellar 

 boundaries, according to the micellar theory. The result is a marked 

 loss of ductility; and it is said that the metal is suffering from 

 hydrogen embrittlement. 



The iron in plate 4 was fractured while embrittled with hydrogen. 

 The two fractured halves of the metal were separately mounted on the 

 microscope, and a fractograph was taken of the same facet on the 

 two matching halves of the fracture. These tAvo fractographs are 

 mounted facing each other in plate 4, constituting obverse and reverse 

 views of the fracture traverse through the single grain. Thus, one 

 can follow the various markings as they appear to either side of the 

 fracture. Most markings appear on both sides, but some do not ; and 

 there are provided some informative differences. 



