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Cr&stalline materials A crystalline material is one in which

the atoms are sit ated in a re$eating or$eriodic arra& o%er large atomic

distances3 that is, long4range orderexists, s ch that $on solidi5cation, theatoms #ill $osition themsel%es in are$etiti%e three4dimensional $attern, in#hich each atom is onded to its

nearest4neigh or atoms!

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All metals, man& ceramic materials,and certain $ol&mers form cr&stallinestr ct res nder normal solidi5cationconditions! For those that do notcr&stalli'e, this long4range atomicorder is a sent3 are (no#n as

noncrystalline or Amorphous materials

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Str ct res

The reg lar $atterns ado$ted & metal atoms #henmetals solidif&, is termed crystalline structures, and ho#the& solidif& into s ch $atterns!

7n a metallic gas, the $articles consist of single atoms8metallic gases are said to e monatomic with mostnon-metallic gases, such as oxygen and nitrogenconsisting of molec les each of #hich contains t#oatoms #hich are in a state of contin o s motion! As thetem$erat re falls, condensation occ rs at the oiling4$oint and in the res ltant liq id metal the atoms are

9 m led together #ill&4nill&! Since the& are held togetheronl& & #ea( forces of attraction at this stage, the liq idlac(s cohesion and #ill :o#!

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Since the atoms are no# arranged in a reg lar $attern, the& generall&occ $& less s$ace! Th s, most metals shrin( d ring solidi5 cation

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METAL STRUCTURES Most of the im$ortant

metals cr&stallise into oneof three di;erent $atternsas solidi5 cation ta(es$lace!

BODY CENTRED CUBICCRYSTA STRUCTUE!BCC"

a. A har# sphere cellunit cell

$. a re#uce# sphere unitcell

c. An a%%re%ate o& manyatoms

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'CC The face-centred cubic arrangement can be considered to have atoms

at each corner of a c e and also one in the centre of each face!

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