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1、单击此处编辑母版标题样式单击此处编辑母版副标题样式2021/3/10讲解：XX1Chapter 7 Deformation and strengthening mechanismsFundamentals of Materials Science and Engineering Why study deformation and strengthening mechanism?lMost of metallic products are produced through deforming the metal somehow to obtain desired shape, size and
2、strength. But what factors would influence the process of deformation?lYou may have an experience that an iron wire will get tougher and it will break finally when it is bended for times. Do you know why?lStudy on deformation and strengthening mechanism will help us use and develop materials better.
3、2021/3/10l22Fundamentals of Materials Science and Engineering What should you be able to do after studying this chapter?lDescribe dislocation motion from an atoms perspective.lUnderstand the relationships between dislocation motion and plastic deformation of metallic materials.lDefine slip system an
4、d explain the influence of slip system on the deformability of materials.lExplain mechanism of grain boundary strengthening, solid-solution strengthening, strain hardening.lDescribe recovery and recrystllization in terms of both the alternation of microstructure and mechanical characteristics of the
5、 material.2021/3/10l33Fundamentals of Materials Science and Engineering Important terms and concepts:lCold working 冷加工lCritical resolved shear stress 临界分剪切应力lDislocation density 位错密度lGrain growth 晶粒生长lLattice strain 晶格应变lRecovery 回复lRecrystallization temperature 再结晶温度lResolved shear stress 分剪切应力2021
6、/3/10l44Fundamentals of Materials Science and Engineering lSlip 滑移lSlip system 滑移系lRecrystallization 再结晶lsolid-solution strengthening 固溶强化lstrain hardening 应变强化lstrengthening by grain size reduction 细晶强化lannealing 退火ltemper 回火lquench 淬火lageing 时效 2021/3/10l55Fundamentals of Materials Science and Eng
7、ineering lnatural ageing自然时效lartifical ageing人工时效ldislocation multiplication 位错繁殖lmisorientation 位相差异lequiaxed grains 等轴晶粒lelongated grains 拉长晶粒lstrain field 应变场lStress field 应力场2021/3/10l66Fundamentals of Materials Science and Engineering Key knowledge points:Deformations mechanisms for metals1. Hi
8、storical: l1930s, theory and actually measured discrepancy in mechanical strengths l1950s, dislocation be examined by electron microscope2021/3/10l77Fundamentals of Materials Science and Engineering 2. Basic concepts of dislocationslPlastic deformation corresponds to the motion of large numbers of d
9、islocations. lAn edge dislocation moves in response to a shear stress applied in a direction perpendicular to its line. lIn an edge dislocation, localized lattice distortion exists along the end of an extra half-plane of atoms.lThe mechanics of dislocation motion are represented in Fig.7.12021/3/10l
10、88Fundamentals of Materials Science and Engineering The mechanics of dislocation motionFig.7.1 Atomic rearrangements that accompany the motion of an edge dislocation as it moves in response to an applied shear stress. (a) The extra half-plane of atoms is labeled A. (b) The dislocation moves one atom
11、ic distance to the right as A links up to the lower portion of plane B; in the process, the upper portion of B becomes the extra half-plane.(c) A step forms on the surface of the crystal as the extra-plane exists.2021/3/10l99Fundamentals of Materials Science and Engineering lSlip- the process by whi
12、ch plastic deformation is produced by dislocation motion is termed slip. lslip plane-the crystallographic plane along which the dislocation line traverses is the slip plane. lMacroscopic plastic deformation simply corresponds to permanent deformation that results from the movement of dislocations, o
13、r slip, in response to an applied shear stress. 2021/3/10l1010Fundamentals of Materials Science and Engineering lDrawing a carpet in the way of (a) and (b) may help you understand the motion of dislocation better.2021/3/10l1111Fundamentals of Materials Science and Engineering Dislocation motion is a
14、nalogous to the mode of locomotion employed by a caterpillar ( Fig.7.3)2021/3/10l1212Fundamentals of Materials Science and Engineering lThe motion of a screw dislocation in response to the applied shear stresses shown in Fig.7.2b. the direction of movement is perpendicular to the stress direction. l
15、For an edge, motion is parallel to the shear stress. However, the net plastic deformation for the motion of both dislocation types is the same. lThe direction of motion of the mixed dislocation line is neither perpendicular nor parallel to the applied stress, but lies somewhere in between. 2021/3/10
16、l1313Fundamentals of Materials Science and Engineering Fig.7.2 The formation of a step on the surface of a crystal by the motion of (a) an edge dislocation and (b) a screw dislocation. Note that for an edge, the dislocation line move in the direction of the applied shear stress ; for a screw, the dislocation line motion is perpendicular to the stress direction.2021/3/10l1414Fundamentals of Materials Science and Engineering lDislocation density -the number of dislocation in a material is expresse
