【文章內容簡介】 ion occurs. ? This plane is called the slip plane. It follows that the direction of movement is called the slip direction.Fundamentals of Materials Science and Engineering l slip system This bination of the slip plane and the slip direction. The slip system depends on the crystal structure of the metal and is such that the atomic distortion that acpanies the motion of a dislocation is a minimum. l For a particular crystal structure, the slip plane is that plane having the most dense atomic packing, that is has the greatest planar density. Fundamentals of Materials Science and Engineering l The slip direction corresponds to the direction, in this plane, that is most closely packed with atoms, that is, has the highest linear density. l For example: FCC crystal structure, {111}slip plane, 110slip direction, 12 slip system ( and table )l The more slip system, the more ductile of the metal. HCP metals have few active slip systems, are normally quite brittle. Fundamentals of Materials Science and Engineering Fundamentals of Materials Science and Engineering Fundamentals of Materials Science and Engineering 5. Plastic deformation of polycrystalline metalsl For polycrystalline metals, the direction of slip system varies from one grain to another. For each, dislocation motion occurs along the slip system that has the most favorable orientation (., the highest shear stress). ( Fig. )l Gross plastic deformation of a polycrystalline specimen corresponds to the parable distortion of the individual grains by means of slip. (Fig. )Fundamentals of Materials Science and Engineering l Figure slip lines on the surface of a polycrystalline specimen of copper that was polished and subsequently deformed.Fundamentals of Materials Science and Engineering Figure Alteration of the grain structure of a polycrystalline metal as a result of plastic deformation. (a) Before deformation the grains are equiaxed.(b) The deformation has produced elongated grains.Fundamentals of Materials Science and Engineering l During deformation, mechanical integrity and coherency are maintained along the grain boundaries. As a consequence, each individual grain is constrained, to some degree, in the shape it may assume by its neighboring grains. l Polycrystalline metals are stronger than their singlecrystal equivalents, which means that greater stresses are required to initiate slip and the attendant yielding. Fundamentals of Materials Science and Engineering 1. Strengthening by grain size reduction（細晶強化）Mechanisms of strengthening in metalsl Metallurgical and materials engineers are often called on to design alloys having high strengths yet some ductility and toughness. Ordinarily, ductility is sacrificed when an alloy is strengthened.l Important to the understanding of strengthening mechanisms is the relation between dislocation motion and mechanical behavior of metals. Fundamentals of Materials Science and Engineering l Macroscopic plastic deformation corresponds to the motion of large numbers of dislocations, the ability of a metal to plastically deform depends on the ability of dislocations to move. l Restriction or hindering dislocation motion renders a material harder and stronger. l The size of the grains, or average grain diameter, in a polycrystalline metal influences the mechanical properties. (Fig. )Fundamentals of Materials Science and Engineering The motion of a dislocation as it encounters a grain boundary, illustrating how the boundary acts as a barrier to continued slip. Slip planes are discontinuous and change directions across the boundary.Fundamentals of Materials Science and Engineering The grain boundary acts as