Question

4) A) Describe and illustrate the edge -type dislocation (only this type). What types of strain fields surround this type of dislocation? (5 points) B) Cold working can place residual compressive stresses within a material. Is this a good thing or a bad thing (and explain your answer)? (5 points)

Answer 1

A) One of the most common crystal structure defects is known as an edge dislocation. This occurs when there are extra atoms inserted into a plane in the crystal lattice.When there are lot of edge dislocations in a metal, it will deform much more easily than a similar metal containing fewer edge dislocations. Edge dislocations make it easier for a material to deform because they stretch out bonds between adjacent atoms and make it easier for atoms to slide past one another.

The edge defect can be easily visualized as an extra half-plane of atoms in a lattice. The dislocation is called a line defect because the locus of defective points produced in the lattice by the dislocation lie along a line. This line runs along the top of the extra half-plane. The inter-atomic bonds are significantly distorted only in the immediate vicinity of the dislocation line.

For idealized edge component, one entire plane has been pushed into the other planes above the glide plane but not below (tensile + compressive stresses).

The elastic displacements around edge dislocations in isotropic m aterials include all three normal strains εxx, εyy and εzz, and the shear strains in the x‐y plane γx y.

B) Cold working is good. Cold working is deformation in an alloy accompanied by work hardening—that is, increased strength. Cold working is used as the final working step when tight dimensional tolerances and manipulation of alloy properties are required. Adjusting the amount of deformation and the rate at which the deformation is performed can control both strength and ductility. Cold working also produces a good surface finish.Common cold-working processes used for nickel alloys are rolling, drawing, tube reducing, and forging. Factors affecting the choice of process include alloy characteristics, equipment availability, and shape of the finished form .