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# 3 P Deformation @ Www 07 Met Tk

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### Transcript

• 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.