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3 P Deformation @ Www 07 Met Tk

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  • 1. Elastic Deformation. Plastic Deformation. Deformation By Slip. Deformation By Twinning. Diff B/w Slip And Twinning. All About Metallurgical & Materials Engineering Download This & More Stuff @ WWW.07MET.TK
  • 2. Elastic And Plastic Deformation
    • Metals deform both elastically and plastically.
  • 3. Elastic deformation
    • Elastic deformation takes place at low stresses and has three main characteristics, namely
    • I) It is reversible .
    • ii) Stress and strain are linearly proportional to each other according to Hook’s Law
    • iii) It is usually small i.e < 1 % elastic strain.
  • 4. Plastic or permanent deformation
    • Once the yield or flow stress has been exceeded, plastic or permanent deformation occurs.
    • It take place by one of two simple processes,
    • I) Slip or glide
    • ii) Twinning.
  • 5. Deformation by Slip
    • The usual method of plastic deformation in metals is by
    • Sliding a block of the crystal over one another along definite crystallographic planes( slip planes) in the specific slip direction and
    • A step is produced in the polished surface.
    • When view a polished surface from the top through a microscope, the step shows up as a line , which is called as a slip line.
  • 6.
    • Slip plane: is the plane of greatest atomic density
    • Slip direction: is the closet-packed direction
    • Slip system: Slip plane together with the slip direction
    • To prove that these slip directions lie in the slip plane hu + kv + lw = 0 .
  • 7. Face centered cubic Structure
    • The four {111} octahedral planes are with high atomic density.
    • Three <110 > directions are the close-packed direction.
    • Therefore, the fcc lattice has 12 slip system.
    • Copper , Brass , aluminum are f.cc metals.
  • 8. Body centered cubic structure
    • The {110} planes have the highest atomic density in bcc structure, but they are not greatly superior in this respect to {112} , {123} planes.
    • However, in bcc structure four <111> directions is closed packed direction in bcc structure.
    • Slip in bcc metals is found to occur on the {110}, {112}, and {123} planes. Total twelve planes.
    • Therefore, 48 possible slip system.
  • 9. Hexagonal Closed-Packed metals
    • Basal plane (0001) , only plane with high atomic density in hcp structure.
    • The axes<1120> , three closed packed direction.
    • Three Slip system in HCP structure.
    • The limited number of slip systems is the reason for the extreme orientation dependence and low ductility in hcp metals.
  • 10.
    • The active slip plane depends on the value of the axial ratio.
    • zinc and cadmium c/a is 1.856 and 1.886 respectively, the planes of greatest atomic density are {0001} basal planes and slip takes place on these planes.
    • When the axial ratio is appreciable smaller than the ideal value of c/a = 1.633 then other slip planes operate.
    • In zirconium c/a = 1.589 and titanium c / a = 1.587, the slip takes place on the {101- 0} prism planes at room temperature.
    • However at high temperature e.g 225 0C in megnasium the {101- 1} pyramidal plane plays an important role in slip.
  • 11.  
  • 12. Deformation by Twinning
    • Deformation twinning is a process in which a region of crystal undergoes a homogeneous shear that produces the original crystal structure in a new orientation.
    • In the simplest case, this results in the atoms of the original crystal (parent) and those of the product crystal (twin) being mirror images of each other by reflection in a composition plane.
  • 13.
    • -Twinning occurs in a definite direction on a specific crystallographic plane for each crystal structure.
    • -Twinning stress is lower than the stress for slip and occur at low temperatures or high stain rates.
    • -The lattice strains needed to produce a twin configuration in a crystal are small, so that the amount of gross deformation that can be produced by twinning is small.
    • - The important role of twining is to place a new slip systems in a favorable orientation with respect to the stress axis so that additional slip can take place.
  • 14. Difference in Twinning compare to Slip
    • -In slip orientation of the crystal above and below the slip plane is the same after deformation.
    • -An orientation difference exist across the twin plane.
    • -Slip is usually considered to occur in discrete multiples of the atomic spacing.
    • -Movements are less than an atomic distance in twinning.
    • -Slip occur on a relatively widely spread planes .
    • -Twining involve every atomic plane in the deformation.
    • -There is a delay time of several milliseconds before a slip band is formed.
    • -Twin can form in a time as short as few milli second .
  • 15.
    • Mechanical twins are produced in BCC or HCP metals under conditions of rapid rate of loading (shock loading) and decreased temperature .
    • FCC metals are not ordinarily considered to deform by mechanical twinning (they deform by annealing twin).
    • Mechanical twins have been produced in copper by tensile deformation at 4 K and by shock loading.
  • 16.
    • -The lattice strains needed to produced a twin configuration in a crystal are small, so that the amount of gross deformation that can be produced by twinning is small.
    • For example, the maximum extension which it is possible to produced in a zinc crystal when entire crystal is converted in to a twin on the plane {1012} is only 7.39 percent.
    • The important role of twining comes in plastic deformation not from the strain produced by the twinning process but from the fact that orientation changes resulting from twinning may place a new slip systems in a favourable orientation with respect to the stress axis so that additional slip can take place. Thus, twinning is important in overall deformation of metals with a low number of slip systems, such as HCP metals. However it should be understood that only a relatively small fraction of the total volume of a crystal is oriented by twinning, and therefore hcp metals will, in general, posses less ductility than metals with a greater number of slip systems.
  • 17. All About Metallurgical & Materials Engineering Download This & More Stuff @ WWW.07MET.TK

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