2l8 



NATURE 



[April 14, 192 1 



The Internal Physics of Metals. 



THE general discussion on the failure of metals 

 under internal and prolonged stress, held on 

 Wednesday, April 6, was of special interest for several 

 reasons. In the first place, being arranged jointly by 

 the Faraday Society, the Institution of Mechanical 

 Engineers, the Iron and Steel Institute, the Institute 

 of Metals, the Institute of Shipbuilders in Scotland, 

 and the East Coast Institution of Engineers, it con- 

 stituted a symposium which united the physicist, the 

 metallurgist, and the engineer in the discussion of a 

 problem which can be solved only by the co-operation 

 of all three. The problem itself, also, is of no small 

 interest, whether viewed from the practical point of 

 view of the engineer who is concerned with the 

 adequate safety and permanence of his works, or 

 from the scientific point of view as a question of the 

 internal physics of metals and of solids in general. 



Briefly, we have first the long-known phenomenon 

 miscalled "season-cracking" in brass. A cold-drawn 

 rod or tube, or a spinning such as a cup, may appear 

 to be perfectly sound and good when first made, but 

 after a time, which may be a matter of hours or of 

 years, it breaks, seemingly spontaneously. Such 

 fracture we now know is the result of the prolonged 

 operation of an internal stress which existed in the 

 finished article as the result of undue deformations 

 app^ed to the metal during manufacture, and this 

 stress has in time proved suiificient to pull the con- 

 stituent crystals apart. This is a tvpe of fracture 

 quite different from that which the same metal under- 

 goes if broken in the ordinary way in a tensile test, 

 when fracture occurs through the crystals themselves, 

 and not through their junctions. 



Until 1919 this phenomenon stood as an isolated, 

 but important, fact in connection with brass, but 

 then it was discovered that other metals, such as 

 certain aluminium alloys, lead, and even steel, could 

 undergo similar inter-crystalline fracture after the 

 lapse of time if left, under suitable conditions, exposed 

 to a sufficiently severe and continuouslv acting stress. 

 In view of these discoveries Rosenhain and Archbutt 

 put forward the suggestion that inter-crystalline frac- 

 tures of this tvpe arise as a consequence of the exist- 

 ence of an amorphous layer between adjacent metallic 

 crystals ; such a layer is regarded as consisting of a 

 highly viscous, under-cooled liquid, and should, there- 

 fore, be subiect to a minute amount of movement — 

 either true viscous flow or visco-elastic displacement — 

 under the action of lon«-continued stress. If, then, 

 the form of the crystal boundaries is such as to 

 favour easv relative displacement, inter-crystalline 

 fracture will ultimatelv result, while if the boundaries 

 between crvstals are irregular or rough, displacement 

 will soon be checked and no fracture occur. Rosen- 

 hain and Archbutt found that in their aluminium alloy 

 they could produce at will a micro-structure with smooth 

 boundaries in which failure under stress might occur 

 within an hour, while in another condition the same 

 material would resist failure for many years, and 

 probably indefinitely. Similar results were obtained 

 with lead, and in the rase of steel also indications 

 of a powerful effect arising from the nature of the 

 crystal boundaries were found. 



More recently Moore and his collaborators at Wool- 

 wich have shown that the selective action of certain 

 chemical reagents, such as mercury salts and am- 

 monia on inter-crystalline material, in the case of 

 brass, plays a most important part in the process of 

 "season-cracking"; indeed, they go so far as to say 

 that, in brass at all events, such chemical action is 

 essential to the occurrence of the phenomenon. In 

 reply to this contention Rosenhain and Archbutt have 

 recently shown that while even in their special alloy, 

 in which the phenomena are most strictly analogous 

 to those in brass, but more rapid, and therefore more 

 readily studied, chemical action — in that case by air 

 or water vapour, or both — also affects the process, yet 

 it serves, not as the prime cause, but as an accelerator. 

 Specimens of their alloy which fail, when left in the 

 air, in a few hours, withstand the same stress for 

 several days when kept in a high vacuum or in 

 hydrogen ; yet they ultimately fail even in the total 

 absence of chemical action, and it is suggested that 

 severely stressed brass will do so also, given time 

 enough. 



The main discussion, however, did not turn upon 

 the relatively minor differences between the views of 

 Moore and of Rosenhain, but rather upon the general 

 question of the existence of the supposed inter- 

 crystalline amorphous layer and its properties. Here 

 it seems that some of the metallurgists who wished 

 to dispose of this theory on a priori grounds — that 

 the existence of such a layer in "highly crystalline" 

 materials like metals was not possible — adopted a 

 somewhat unintelligent and unscientific attitude. 

 They cannot surely claim to have so intimate a know- 

 ledge of the behaviour of atoms during crystallisation 

 as to entitle them to say that when two growing 

 crystals approach each other the process of crvstal- 

 lisation must continue until the last layer of atoms 

 is in some way forced to assume some orientation 

 common to both the adjacent space-lattices. Nor 

 can they dispute that a highlv viscous liquid may 

 behave as a hard and brittle quasi-solid under forces 

 as ordinarily applied, i.e. at relatively rapid rates, and 

 may vet undergo flow or visco-elastic displacements 

 if sufficient time is allowed. 



It is not. perhaps, possible to say that the actual 

 existence of amorphous inter-crystalline layers in 

 metals is proved, but it must be admitted that there 

 is more than a strong prima facie case for the theory, 

 and, further, that it serves to explain and unifv a very 

 large range of phenomena which otherwise lack ex- 

 planation or correlation. The theory of an amorphous 

 inter-crystalline layer must at least be regarded as 

 an extremely helpful hypothesis which has been gain- 

 ing steadily in strength from the accumulation of 

 experimental evidence during the past ten years. 

 Whether it will ever be possible to place it on a surer 

 foundation it is difficult to predict, but our methods 

 of studying the internal structure of matter have 

 made such great progress in recent years that more 

 is to be anticipated. Meanwhile, so far as inter- 

 crystalline fracture under prolonged stress is con- 

 cerned, it remains the only tangible explanation which 

 was put forward during the discussion. 



D' 



Mongolian 



>R. F. C. CRUIKSHANK read a paper on 

 March 22 at a meeting of the Royal Anthropo- 

 logical Institute entitled "The Ethnological 'Signi- 

 ficance of Mongolian Imbecility." He pointed out 

 that Robert Chambers eighty years ago directed atten- 

 tion to the occurrence in England of persons who in adult 



NO. 2685, VOL. 107] 



Imbecility. 



life are yet a "kind of children" and "of the Mon- 

 golian type." In 1866 Dr. Langdon Down definitely 

 described a type of idiocy that he called Mongolian, 

 and that has been recognised ever since by physicians. 

 The homologies of these imbeciles have been discussed 

 by medical men from various points of view, but it 



