568 TRANSACTIONS OF SECTION B. 



which has been observed at ordinary temperatures, and whether the allotropic 

 transformations of iron affect its behaviour under strain. The method of Ewing 

 and Rosenhain for observing the effects of strain on metals consists in preparing 

 a properly polished surface on a specimen of suitable shape, and subsequently 

 exposing the specimen to plastic deformation, the changes in the microscopic 

 features of the surface being studied during, and more especially after, the 

 application of the strain. In order to apply this method to hot metal it was 

 necessary to devise means for heating the specimens in such a manner that even 

 at temperatures well above 1000° C. the original polished surface would remain 

 - free from oxidation. The method adopted was that of heating the specimen elec- 

 trically in a very high vacuum, and in this way specimens could be heated up to 

 1100° C. without injury to the surfaces. For the purpose of applying the desired 

 strain to the specimen while heated in vacuo, an apparatus was devised whereby 

 a compressed spring could be released at any desired moment by electrically fusing 

 a wire which had kept the spring compressed; by means of a suitably arranged 

 lever the release of this spring was caused to strain the specimen to any desired 

 extent, while the operation could take place when the specimen had attained any 

 desired temperature. The actual temperature of the specimen was determined 

 by attaching to the rear or unpolished side minute particles of various salts of 

 known fusion points and observing which of them had melted. The specimens 

 used in this apparatus were thin, short strips of sheet metal, and as these were 

 heated by a direct electric current passing through them, the ends of the speci- 

 men remained a good deal colder than the middle, owing to the conduction of 

 heat from theso ends into the comparatively massive frame of the apparatus. In 

 this way strain was applied to a strip of metal varying continuously in tempera- 

 ture from the middle towards each end. 



The material used for the earlier experiments was a pure variety of very mild 

 steel such as that employed for transformer-sheets, but this had been annealed in 

 hydrogen in order to remove all traces of oxide. Subsequently other materials — 

 all of them approximating in composition to pure iron — were employed, notably 

 some specially prepared electrolytic iron of a very high degree of purity. The 

 photo-micrographs accompanying the paper showed that the behaviour of these 

 different materials was closely similar. 



Before describing the effects of strain, the effects of simple heating in vacuo 

 must be considered, since this alone produces changes in the surface appear- 

 ance. Those portions heated up to the Ac, point (770° C. in one of the materials 

 used) show no changes; those portions which had attained a temperature between 

 Ac 2 and Ac, (941° C. in one of the materials) show a double system of boundaries 

 due to the volume-change which iron undergoes on passing through the change 

 from the o to the /3 state. Those parts of the specimen which had been heated 

 above Ac, showed, particularly in cases where the vacuum had not been as high 

 as possible, a slight amount of tinting due to oxidation, and this revealed a third 

 structure superposed on the structure just mentioned and (in the case of etched 

 specimens) on the original o structure. This last, which is the structure of y iron, 

 shows the characteristic twinned structure of this material. 



The effects of strain indicated the three regions of temperature corresponding 

 to the three allotropic forms of iron in a very definite manner. In the region of 

 o iron observation shows that on passing along the specimen in the direction of 

 increasing temperature the amount of deformation, as evidenced by the number 

 and depth of the slip-bands formed, increases steadily, but the slip-bands retain 

 the curved and forked appearance characteristic of o iron. The amount of 

 deformation increases rapidly with increasing temperature until at a point of the 

 specimen coinciding with the a/0 transformation, the signs of deformation cease 

 quite abruptly. For a short length of the specimen there are no slip-bands, and 

 the material appears to have undergone no plastic deformation. On passing to 

 still hotter portions of thy specimen, slip-bands again appear, and when the 

 region of y iron is reached the bands again become numerous, but now show the 

 regular, rectilinear character and the characteristic features of twinned crystals, 

 thus differing very strikingly from the slip-bands seen in a iron. 



The conclusions drawn from the observations here indicated are : — 

 1. That iron at temperatures up to 1100° C. behaves as a crystalline aggregate 

 and undergoes plastic deformation by a process of slip on the cleavage or gliding 



