Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 53

Line defects or dislocations are imperfections in the linear arrangement of atoms in the crystal. There are two main types of dislocations that represent the extreme forms of line defects: edge dislocations and screw dislocations. Dislocations play an important role in plastic deformation and allow deformation to occur at lower stresses than would be anticipated in a perfect crystal as only a few atoms are moved from their equilibrium positions at a given time.

Edge dislocations can be visualized as an extra partial plane of atoms. As shown in Figure 2.6, the partial plane continues into the direction of the page such that the edge dislocation line runs into the plane of the page. Creation and motion of edge dislocations across the crystal results in relative shear or slippage of planes of atoms past each other. There are generally two types of dislocation motion: slip/glide and climb.

In dislocation glide, as shown in Figure 2.6a, the slip plane is the plane in which the dislocation moves and is generally the most closely packed plane in the crystal. A family of close‐packed planes can all act as potential slip planes. As such, the crystal system of the material determines how many glide planes are possible. However, the orientation of the differential stress determines which glide planes are active or not. The edge dislocation moves in the slip plane in the direction of the shear and generally corresponds to one of the shortest lattice translation vectors in the material. The Burgers vector (b) represents the magnitude and direction of the lattice distortion resulting from a dislocation in a crystal lattice. The Burgers vector and the edge dislocation are perpendicular to each other and define the slip plane. Real dislocations will not always be in a straight line and can move along more than one slip plane.

Figure 2.6 Schematic of the basic motion or propagation of an edge dislocation. (a) Dislocation glide or slide as a result of shear stress. (b) Dislocation climb involving material transfer from the dislocation core.

In edge dislocation climb, as shown in Figure 2.6b, the dislocation moves up or down out of its slip plane to a parallel slip plane. This involves the creation or annihilation of a row of vacancies and is energetically difficult.

Figure 2.7 Schematic of the basic motion or propagation of a screw dislocation during plastic deformation.

Screw dislocations are harder to visualize, but result when a part of the crystal shifts relative to its other parts such that the dislocation motion is perpendicular to the applied stress (Figure 2.7). The screw dislocation lies in its slip plane and the dislocation line is parallel to the Burgers vector. Screw dislocations may move from one glide plane to another or in more than one slip plane at a time in a process called cross slip, which is common at high temperatures and/or high stresses. In practice, real dislocations can occur as an intermediate or a combination of edge and screw dislocations. When the orientation of the dislocation line changes, an edge dislocation can continue as a screw dislocation and vice versa.

Dislocations generally require proper lattice ordering to move through a material at low energy. Barriers to the motion of dislocations can be created by the presence of other defects in the crystal, such as point defects, grain boundaries, other dislocations, and the precipitation of a secondary phase. In particular, precipitates can act as locations through which the dislocation might become pinned or must bend in order to continue moving. Bending around precipitates can often lead to the formation of dislocation loops formed around precipitates.