I'KESIDENT's ADDKESS SECTION H. 623 



a highly plastic stage to a state of hardness and tenacity much 

 greater than it originally possessed. According to this thei3iy, the 

 Increased rigidity of the metal as hardened by overstrain results 

 from the partitioning up of the whole mass by thin rigid walls of 

 amorphous material. The theor}^ has caused some discussion, and 

 it appears possible that the partly amorphous form of metals that 

 have been subjected to severe hammering or other extreme straining, 

 owing to the grains being partly broken up into granules, may 

 account for the change in their physical and chemical behaviour, but 

 the evidence brought foi-ward seems to be quite against the existence 

 I if these hardened layers between surfaces of slip. 



The behaviour of iron and steel seems to show that immediately 

 after any plastic strain the slip surfaces are left in* a temporarily 

 weak condition, so that for a time such sliding seems to take place 

 more easily than before, but after a while these surfaces seem to 

 heal up, and the metal recovers its elasticity. It must be remem- 

 bered that the slip bands do not make their appearance until after 

 the elastic limit is passed, so that movements along the slip surfaces 

 imply that a plastic flow of the material is taking place similar to 

 that which occurs at the yield point. Now, if a bar of iron that 

 has been stressed beyond the yield point be again, very soon after- 

 wards, subjected to a fresh test, it is found that, so far from being 

 hardened, plastic flow takes place more easily than before, and the 

 elastic limit is temporarily lowered. The metal slowly recovers its 

 loss of elasticity at ordinary temperatures, but, if warmed, recovers 

 more quickly. Thus Muir showed that at ordinary temperatures 

 steel takes several days to recover its elasticity, but exposure to a 

 temperature of boiling water will bring about a recoveiy in a few 

 minutes, restoring it to a condition with higher elastic limit and 

 higher yield point than before. Aluminium has been shown to 

 behave in a similar way.* This certainly seems like a gi'adual 

 healing up of the slip sui'faces. If the specimen is subjected to vibra- 

 tion recovery is either retarded or altogether prevented. It has also 

 been shown in the case of mild steelf tliat recovery is absolutely 

 stopped if the overstrained specimen is kept at a temperature below 

 the freezing point. 



There seems a fair amount of evidence in support of Mr. Beilby's 

 mobile layer, but will the free moleciiles in this layer most probably 

 settle down into an amorphous film, or will they range themselves 

 in harmony with the crystalline formation by which they are sur- 

 rounded? Mr. KosenhaimJ draws the latter conclusion, and brings 

 foi-ward evidence from microscopic observation to show that after 

 elastic recovery the altered layer between surfaces of slip has disap- 

 peared. He educes two reasons to account for the raising of the 

 yield point by overstrain. Firstly, the fact that the slip bands are 

 ii'regularly spaced seems to show that slip takes place only on a 

 few out of the veiy large nuiiil^;-r of possible gliding surfaces, because 

 for some reason along these slip is easier than along others. The 

 temporary formation of a mobile layer and its subsequent reharden- 

 ing probably destroys these special conditions, and such a surface 



* Morlev and Tomlinson. Phil. Mao:., 190(>. 



tE. J. iVl'Claustland, Amer. lust. Min. Eug., 1906. 



J Journal Iron and Steel Institute. No. II., 1906. 



