Tham khảo tài liệu 'modern physical metallurgy and materials engineering part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Atomic arrangements in materials 21 The transition can be abrupt but is often sluggish. Fortunately tetragonal tin can persist in a metastable state at temperatures below the nominal transition temperature. However the eventual transition to the friable low-density cubic form can be very Using the concept of a unit cell together with data on the atomic mass of constituent atoms it is possible to derive a theoretical value for the density of a pure single crystal. The parameter a for the bcc cell of pure iron at room temperature is 64 nm. Hence the volume of the unit cell is 55 nm3. Contrary to first impressions the bcc cell contains two atoms . 8 X 8 atom C 1 atom. Using the Avogadro constant NA 2 we can calculate the mass of these two atoms as 2 Na or X 10 24 kg where is the relative atomic mass of iron. The theoretical density mass volume is thus 7875 kg m 3. The reason for the slight discrepancy between this value and the experimentally-determined value of 7870 kg m 3 will become evident when we discuss crystal imperfections in Chapter 4. Diamond and graphite It is remarkable that a single element carbon can exist in two such different crystalline forms as diamond and graphite. Diamond is transparent and one of the Historical examples of tin plague abound . buttons coins organ pipes statues . 2The Avogadro constant NA is X 10 21 mol 1. The mole is a basic SI unit. It does not refer to mass and has been likened to terms such as dozen score gross etc. By definition it is the amount of substance which contains as many elementary units as there are atoms in kg of carbon-12. The elementary unit must be specified and may be an atom a molecule an ion an electron a photon etc. or a group of such entities. hardest materials known finding wide use notably as an abrasive and cutting medium. Graphite finds general use as a solid lubricant and writing medium pencil lead . It is now often classed as a highly .