热加工专业英语第2版教学课件作者王晓江第1章节热加工基础lesson1-3课件

iticstructuresjointogetheratb)andc).Developmentisstoppedbyinterferencewithadjacentstructuresandthecontainerorthegrainboundaries晶界SpaceLattices.Eachgrainconsistsofmillionsoftinyunitcellsmadeupofatomsthatarearrangedinadefinitegeometricpatternofimaginarypoints.Theatomicarrangementsfoundinmostmetalsare:a)body-centeredcubic(BCC),b)face-centeredcubic(FCC),andc)hexagonalclose-packed(HCP)(seeFig.1-3-2).Fig.1-3-2TheatomicstructureofsomemetalsUponcooling,thespacelatticestructuresexpandinalldirectionsoftheaxesofthelatticeuntildevelopmentisstoppedbycontactwiththecontainerorbycontactwithadjacentgrowinggrains.Wherethecrystalsmeetorintersect,itisimpossibleforanotherspacelatticetodevelop.Thus,atthegrainboundariestheatomicdistancesarenotcorrectandtheatomsarenotinastablepositionbutareinahigherstateofenergy.