第一单元MetalsandMetalAlloys金属和金属合金Metalsareelementsthatgenerallyhavegoodelectricalandthermalconductivity.Manymetalshavehighstrength,highstiffness,andhavegoodductility.金属就是通常具有良好导电性和导热性的元素。许多金属具有高强度、高硬度以及良好的延展性。Somemetals,suchasiron,cobaltandnickel,aremagnetic.Atlowtemperatures,somemetalsandintermetalliccompoundsbecomesuperconductors.某些金属能被磁化，例如铁、钴和镍。在极低的温度下，某些金属和金属化合物能转变成超导体。Whatisthedifferencebetweenanalloyandapuremetal?Puremetalsareelementswhichcomefromaparticularareaoftheperiodictable.Examplesofpuremetalsincludecopperinelectricalwiresandaluminumincookingfoilandbeveragecans.合金与纯金属的区别是什么？纯金属是在元素周期表中占据特定位置的元素。例如电线中的铜和制造烹饪箔及饮料罐的铝。Alloyscontainmorethanonemetallicelement.Theirpropertiescanbechangedbychangingtheelementspresentinthealloy.Examplesofmetalalloysincludestainlesssteelwhichisanalloyofiron,nickel,andchromium;andgoldjewelrywhichusuallycontainsanalloyofgoldandnickel.合金包含不止一种金属元素。合金的性质能通过改变其中存在的元素而改变。金属合金的例子有：不锈钢是一种铁、镍、铬的合金，以及金饰品通常含有金镍合金。Whyaremetalsandalloysused?Manymetalsandalloyshavehighdensitiesandareusedinapplicationswhichrequireahighmass-to-volumeratio.为什么要使用金属和合金？许多金属和合金具有高密度，因此被用在需要较高质量体积比的场合。Somemetalalloys,suchasthosebasedonaluminum,havelowdensitiesandareusedinaerospaceapplicationsforfueleconomy.Manyalloysalsohavehighfracturetoughness,whichmeanstheycanwithstandimpactandaredurable.某些金属合金，例如铝基合金，其密度低，可用于航空航天以节约燃料。许多合金还具有高断裂韧性，这意味着它们能经得起冲击并且是耐用的。Whataresomeimportantpropertiesofmetals?Densityisdefinedasamaterial’smassdividedbyitsvolume.Mostmetalshaverelativelyhighdensities,especiallycomparedtopolymers.金属有哪些重要特性？密度定义为材料的质量与其体积之比。大多数金属密度相对较高，尤其是和聚合物相比较而言。Materialswithhighdensitiesoftencontainatomswithhighatomicnumbers,suchasgoldorlead.However,somemetalssuchasaluminumormagnesiumhavelowdensities,andareusedinapplicationsthatrequireothermetallicpropertiesbutalsorequirelowweight.高密度材料通常由较大原子序数原子构成，例如金和铅。然而，诸如铝和镁之类的一些金属则具有低密度，并被用于既需要金属特性又要求重量轻的场合。Fracturetoughnesscanbedescribedasamaterial’sabilitytoavoidfracture,especiallywhenaflawisintroduced