Tham khảo tài liệu 'engineering materials 2e volume2 episode 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 16 Engineering Materials 2 back to . at 1391 C and titanium changes from . to . at 882 C. This multiplicity of crystal structures is called polymorphism. But it is obviously out of the question to try to control crystal structure simply by changing the temperature iron is useless as a structural material well below 914 C . Polymorphism can however be brought about at room temperature by alloying. Indeed many stainless steels are . rather than . and especially at low temperatures have much better ductility and toughness than ordinary carbon steels. This is why stainless steel is so good for cryogenic work the fast fracture of a steel vacuum flask containing liquid nitrogen would be embarrassing to say the least but stainless steel is essential for the vacuum jackets needed to cool the latest superconducting magnets down to liquid helium temperatures or for storing liquid hydrogen or oxygen. If molten metals or more usually alloys are cooled very fast - faster than about 106 K s-1 - there is no time for the randomly arranged atoms in the liquid to switch into the orderly arrangement of a solid crystal. Instead a glassy or amorphous solid is produced which has essentially a frozen-in liquid structure. This structure - which is termed dense random packing drp - can be modelled very well by pouring ball-bearings into a glass jar and shaking them down to maximise the packing density. It is interesting to see that although this structure is disordered it has well-defined characteristics. For example the packing density is always 64 which is why corn was always sold in bushels 1 bushel 8 UK gallons provided the corn was always shaken down well in the sack a bushel always gave X 8 gallons of corn material It has only recently become practicable to make glassy metals in quantity but because their structure is so different from that of normal metals they have some very unusual and exciting properties. Structures of solutions and compounds As
