Q. What is slip? What is slip plane? What are principal if slip planes and slip direction for BCC, FCC and HCP crystal?

Ans. X-ray diffraction studies show that the crystalline order in the solid is not lost during plastic deformation, even though more imperfection is introduced. The atom movements are such that the crystal structure remains the same before and after plastic deformation.

      There are two basic modes of plastic deformation are called slip and twinning. Slip is a shear deformation that moves atom by many interatomic distances relative to their initial position. Steps are created at the surface of the crystals during slip, but the orientation of the all parts of the crystal remains the same before and after the slip. Twinning, on the other hand, change the orientation of the twinned parts. The movement of an atom relative to its neighbors is only a fraction of an interatomic distance. The slip mode of the deformation of the common mode in many crystals in ambient and elevated temperatures. At low temperature, the mode of deformation changes over to twinning in a number of cases. We shall consider deformation by slip only.

       Careful examination of the surface of a deformed crystal under the microscope shows group of parallel lines, which corresponds the steps on the surface. They are called slip lines. This indicates that the atomic planes within the crystal have sheared with respect to one another resulting in the surface steps. It is generally found that the slip planes are the closed packed direction. It turns out that the planes of the greatest atomic density, having the highest numbers of atoms per unit area, are the most widely spaced planes. The directions of the greatest atomic linear density have the smallest translation distance from one minimum energy position to the next. In ionic crystals, the slip planes and the slip directions are such that the ions of the same polarity do not become just opposed as nearest neighbors during shear, as this would mean a big increase in the potential energy of the crystal. The common slip planes and slip directions for the some simple crystals are given in table:

A s;ip plane and a slip direction that lies on it together constitute a slip system. For example, the combination {111 and {111} forms a slip system, but not {111} and {110}, as the {110} direction does not lie on the {111} plane, counting the slip system for the most densely packed slip planes only, there are 12 slip systems in FCC and BCC crystals, whole there are only3 in HCP crystals the NaCl crystal has 6 slip systems.

      In a polycrystalline material, slip in any crystal has to be accommodated by slip in neighboring crystals, if the grain boundaries are to remain continuous during slip. According to the von Misses criterion, a minimum of five independent slip is necessary to maintain the integrity of the grain boundaries during plastic deformation. In FCC a b, d BCC crystals, this condition is fulfilled. In HCP crystals, for slip on the basal closed packed plane, there are only two independent slip systems. These correspond to any two of the three closed packed directions. A slip displacement in the third direction can be expressed as the resultant of the slip along the other two directions and is therefore is not independent. In polycrystalline form, HCP materials can deform only by slip onb less common slip systems or alternatively by twinning.

    The stress at which slip starts in a crystal depends on the relative orientation of the stress axis with respect to the slip plane and the slip direction. When a tensile sress σ is applied to a crystal the shear stress τ resolve on a slip plane whose normal makes an angle of ?₁ with the stress axis, along a slip direction inclined at an angle ?₂ to the stress axis, is given by

                                      .τ =σ cos?₁ cos?₂

This resolved shear stress should reach a critical value called the critical resolved shear stress for plastic deformation to start. It is evident that all slip system in crystal will not have the same resolved shear stress for a given tensile stress along an axis. As the applied tensile stress is increased from zero, deformation will be initiated first on that slip system for which the resolved shear stress is a maximum and so reaches the critical value first.