Dislocation locks

These methods are now strongly suggesting a work-hardening function to account for anisotropy peaks as dislocations lock this feature becomes the dominant one in the most recent developments of hardness anisotropy theory that finally move away from the resolved shear stress models. 

KW Kear-Wilsdorf (dislocation locking MMC metal-matrix composite... 

Work hardening of our crystal system occurs as the dislocations move and eventually meet and lock. However these are still potential weaknesses and increasing applied stress or strain will again produce motion. Eventually catastrophic failure occurs as the material fractures. Here dislocations can come together to produce a crack. 

Since the (0 0 1) is not a slip plane, the product dislocation is immobile, or sessile. It provides an obstacle to the movement of other dislocations passing down the (1 1 1)and(1 1 1) planes. This particular case is known as the Lomer lock. 

Li J. C. M., Cross Slip and Cross Climb of Dislocations Induced by a Locked Dislocation, /. Appl. Phys. 32, 593 (1961). 

Whenever there is lateral or oblique bending, there is the possibility of twisting the neck. The associated torsional loads may be responsible for unilateral facet dislocations or unilateral locked facets [Moffat et al., 1978]. However, the authors postulated that pure torsional loads on the neck are rarely encountered in automotive accidents. 

When dislocations moving on inclined glide planes intersect one another they form nodes and locks. After the intersection is complete they will have created jogs or kinks in the other dislocation. The intersection process can instead result in the two dislocations knitting together to form part of a dislocation tangle the... 

There are some limitations to the use of a locking stylet. Once it is inserted into the lead, it may sometimes be difficult or impossible to reverse the locking mechanism and remove the device. Furthermore, if the inner conductor is broken or distorted, it is impossible to introduce or pass the stylet. Excessive force can dislocate the stylet, or the distal conductor coil can still unwind or even disconnect from the electrode. 

This can be unilateral or bilateral. Bilateral facet dislocation can often result in significant spinal cord injury. Bilateral facet dislocations with locked facets can be seen with flexion injuries and are due to significant ligamentous disruption in the posterior and middle columns (Fig. 20.13a-e). On radiographs bilateral facet dislocation is seen as anterior displacement of one vertebra with respect to the adjacent vertebra of more than 50%. Facet dislocations may be associated with facet fractures. Again these injuries are best demonstrated by CT. Spondylolisthesis is seen in association with flexion injuries and usually denotes significant soft tissue injury. Similarly retrolisthesis can be seen in extension injuries. 

There may be dislocation of the facet joints in lap belt injuries. This can be seen as widening, superior subluxation, perching or locking of the facets. Facet malalignment is best appreciated on CT reconstructions. Rotatory stability of the thoracolumbar spine is provided by the facet joints and these are injured in significant fracture dislocations caused by combined flexion and rotation. The major hallmark of fracture dislocation type of injuries is intervertebral subluxation or dislocation. 

Fig. 3.61 A Lomer-Cottrell lock a two partial dislocations before interaction, b after interaction of the leading partials forming a stair-rod dislocation...

Fig. 3.62 Dislocation pile-ups (paitials and their faults are shown) behind a Lomer-Cottrell lock, acting as obstacles to their movement...

As discussed in Sect. 3.3.14.5, dislocations become immobilized when they encounter Lomer-Cottrell locks. Figure 3.61 schematically illustrates the formation of a Lomer-Cottrell lock. 

Although Si is not necessarily a ceramic, it fulfills one requirement of the ceramic definition, namely it is covalently bonded. More illuminating illustrations of Lomer-Cottrell locks may be seen in Pt and Fe-Mn-Al-C steel. To observe Lomer-Cottrell locks in Pt, see Fig. 3.79 [54]. Using a newly-developed nanoscale deformation device, atomic scale and time-resolved dislocation dynamics were captured in situ by TEM during the deformation of a Pt ultrathin film with truly nanometer grains (diameter d< 10 nm). In larger grains (d 10 nm), full... 

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