ON THE CRYSTALLINE STRUCTURE OF METALS. ::7 I 



Tin- e\|>erimente on iron were extended to certain M.-.-I-. In very mild steel slip- 

 I i.i i ids <>an 1*5 readily observed in what are generally called the "ferrite" areas, which 

 remain white after light etching. This is shown in h'g. 45. The Hrst effect of strain 

 is to develop the inter-granular junctions in these white areas, then more severe 

 st ruining makes the slip-binds apjiear. In steels containing larger projiortions of 

 r.nlH.ii, tlie scale of the granular structure of the " ferrite " diminishes and the 

 slip-binds become correspondingly minute, requiring the highest jxjwers of the 

 microscope for their observation. We have not been able to observe anything 

 of tin- nature of slip-lmiids in the dark or " pearlite " areas of steel, but the 

 i-orrespondence which has been recognised by OSMOND to exist tatween the struc- 

 ture of "|>earlite" and that of typical eutectic alloys, taken with facts to be 

 desmlied lx?low, points to the possibility that "pearlite" may also yield plastically 

 l>\ slipping. 



Slip-bands have also l>een observed in various s|>ecimens of brass and bronze. 

 The l>ehaviour of eutectic alloys under plastic strain is of special interest, because 

 these bodies apparently differ so widely in structure from pure metals. Our observa- 

 tions have been made on the eutectics of lead-tin, copper-silver, and lead-bismuth. 

 T'ir micro-structure of such eutectics has l>een described by OSMOND. 



Fig. 4G is from a sjiecinien of lead-tin eutectic kindly prepared for us by Messrs. 

 Hr.M-.icK and NEVILLE; the surface was obtained by casting against glass, and was 

 lightly etched with a I per cent, solution of nitric acid. Figs. 47 and 48 illustrate the 

 most obvious effect of strain on such structures ; the surfaces have not been etched, 

 tin- differentiation of the two constituents by differences of level l>eing here entirely 

 due to strain. It will be observed tlyit the scale of this structure is similar to that 

 of the slip-binds seen in pure metals, and examination of strained specimens shows 

 that plastic yielding is associated with slips occurring between layers of the two con- 

 stituents. A close examination of strained specimens has enabled us to detect 

 slip-binds in the light-coloured constituent. By adopting the device of slow cooling, 

 which has led to such excellent results in. the hands of Messrs. HEYCOCK and NEVILLE, 

 \\e have succeeded in producing specimens of eutectics in which the characteristic 

 -tincture is developed ujxm a much larger scale. Fig. 49 exemplifies this in the 

 eutectic of bismuth and lead, and shows slips which occur in the white constituent as 

 ,i consequence of straining. This photograph illustrates a feature very characteristic 

 of eutectic alloys ; a parallel system of slip-luinds extends over many patches of the 

 white constituent, thus pointing to the fact that the crystalline elements are similarly 

 oriented throughout considerable areas of at least one of the two constituents of the 

 alloy. This suggests that the alloy as a whole has comparatively coarse granular 

 structure, and the same conclusion is borne out by observing the general character of 

 a surface under lower magnification* (such as 100 diameters), when its structure is 

 revealed either by straining or etching. The surface is then seen to be divided into 

 lather Imp- more or less polygonal areas, each covered with a system of ribs radiating 



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