Engineering Materials -Comparison between Crystal Slip and Twinning

Slip and twinning can be done in parts like type of defect, crystal axis change, stress requirement, visibility and for threshold value of stress occur. In this article, let’s look at comparison between crystal slip and twinning on surface of metal after deformation.

What is slipping?

Slip is defined as shear deformation that moves atoms by many interatomic distances in one plane to another crystal plane. This slip system refers to slip planes or lines at which dislocation motion occurs. It leads to plastic deformation. In crystals, slip system is an important mode of deformation. It is most crucial deformation mechanism in metals and metallic alloys.

Plastic deformation begin with critical resolved shear stress is needed. Slip is permanent displacement of one part of crystal relative to other part. This slip involves sliding of one plane of atoms over other and plane on which slip occurs are called slip planes. Slip is most common manifestation of glide.

What does Twinning Means?

Twinning is mechanism of plastic deformation occurring in single crystals is twinning. In this process part of atomic lattice is deformed so that it forms mirror image of under formed lattice next to it. It is shear force that can produce atomic displacements. Check Engineering Materials – Atomic Structure Introduction 

As a result, twinning may be produced by mechanical deformation. Twinning is also known as mechanical twinning. Like slip, twinning also occurs along certain crystallographic planes and directions. These planes and directions are called as twin planes and twin directions. In plastic deformation, twinning is important role that causes changes in plane orientation so that further slip can occur.

Differences between Crystal Slip and Twinning

Different metals have different slip system due their different crystallographic structures. These different types of lattice cause different slip system in material. Difference between slip and twinning is given below. Also read Engineering materials: Brittle Fracture Mechanics and Considerations 

  • Slip is important among the two and crystal slip is line defect.
  • Slip is observed in boy centered cubic (BCC) and face centered cubic metals (FCC)
  • All atoms in one block move over same distance.
  • When viewed under microscope, slip appears as thin line.
  • Crystal slipped lattice has same orientation
  • During slip each atom moves same integral number of atomic distances along slip plane.
  • It need lower shear stress required for slipping to occur
  • Occurs in metals having more number of slip systems
  • On other hand, crystal deform by twinning
  • Slip occurs in several milliseconds

Twinning

  • Twinning is defect grain boundary defect
  • Different planes of atoms moves fractional distances depending on their distance from twinning plane
  • During twinning, atoms in each plane in block move through different distances proportional to their distance from twinning plane
  • Twinning is observed in Hexagonal close packing metals (HCP)
  • Crystal axis is deformed after twinning
  • Twinning can be seen as broad lines when viewed under microscope
  • In twinned region, lattice orientation changes and it minor image of original lattice
  • It requires higher sheer stress and stress is necessary to spread twinning is lesser than that need starting it.
  • Valve of stress is need for twinning to occur
  • Occurs in metals having less number of slip system
  • Twinning takes place in few microseconds

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