Metallurgy; an introduction to the study of physical metallurgy . out 25 per cent, of zinc (a kind of brass, therefore),which when very slowly cooled appears entirely homogeneous,but when rapidly soHdified exhibits the dendritic structureshown in the figure. The characteristic feature of suchstructures, however, is that the arms or branches of thedendrites are usually ill-defined, and that they He whoUy withinpolygonal boundaries which readily recall the crystal boun-daries of pure metals. The micro-structure of alloys of class (6) is rather more com-plex. Theoretically the addition to the pur
Metallurgy; an introduction to the study of physical metallurgy . out 25 per cent, of zinc (a kind of brass, therefore),which when very slowly cooled appears entirely homogeneous,but when rapidly soHdified exhibits the dendritic structureshown in the figure. The characteristic feature of suchstructures, however, is that the arms or branches of thedendrites are usually ill-defined, and that they He whoUy withinpolygonal boundaries which readily recall the crystal boun-daries of pure metals. The micro-structure of alloys of class (6) is rather more com-plex. Theoretically the addition to the pure metal, formingone end of a binary system of this type, of a minute trace of asecond element should result in the appearance in the micro-structure of traces of crystals of a second constituent. In-somecases this is true to a surprising degree of accuracy. Thus theaddition of five parts of carbon to 10,000 parts of pure iron atonce becomes visible under the microscope, while the presenceof one part in 1,000 is readily seen, as shown in the photograph PLATE 1 7 Tfltf^^^lb m 1 ^^^3 ^ ^/ji( />^ § ^n Fig. 20. Fig. 21. .V i > t< ? -/ r ..». ^.i. k- ; > ^ / I \ / i w ^. u f •r > / -A r-^N ^ t. y\ ?CN .^r • / V /1 X ^*. , r ?r /? / «? f • / Fig. 22. Fig. 23. {Tofacep. 74. PURE METALS AND ALLOYS 75 Fig. 22, Plate VI. The polygonal grains of the pure ironcrystals are readily recognised, but here and there betweenthem are seen small dark patches which are entirely absent inthe pure metal. These are patches of the carboniferous con-stituent, and in steels of increasing carbon-content the relativearea occupied by these dark patches rapidly increases. Thereason why the second constituent appears dark in this case,as in the majority of others, is simply due to the fact that, whenthe pohshed surface is attacked with an etching reagent, themore soluble, , more readily attacked, constituent is affectedalmost exclusively, while the other remains practically un-a
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectmetals, bookyear1922
