Low dislocation mobility

Two outstanding properties of FeB metallic glasses are their low magnetic permeabilities and their low acoustic attenuations. The former results from their lack of magnetic anisotropy and has led to their use in power transformers, theft detectors, and various electronic devices. The latter results from the very low dislocation mobility in them. 

Another way to achieve the strongly time-dependent hardness range, preceding the achievement of the constant time independent minimum value of hardness predicted by equation (4.18), is to make hardness tests at elevated temperatures. As the test temperature increases above where T is the melting point of the sample, the hardness rapidly decreases and becomes time dependent until a temperature is reached where the dislocation mobility becomes high, the yield strength becomes very low, and hardness approaches the constant, time-independent value. It may not be easy with some ceramics to achieve the constant hardness zone because elevated temperature may permit diffusional creep, when once again the hardness will become obviously time dependent, and hardness values lower than that predicted by equation (4.18) will be achieved. 

Returning now to the question of dislocation mobility, the following argument shows, in a general way, why the slip motion of an isolated dislocation can occur under a low force per unit length. Figures 3.4 and 3.5 suggest that molecules that are sufficiently behind or far ahead of the dislocation will be in their normal equiUbrium positions, while those close to the dislocation line are sheared out of their normal positions. 

Meir and Clifton [12] study shocked <100) LiF (high purity) with peak longitudinal stress amplitudes 0.5 GPa. A series of experiments is reported in which surface damage is gradually eliminated. They find that, while at low-impact velocities the dislocations in subgrain boundaries are immobile and do not affect the dislocation concentration in their vicinity, at high-impact velocities ( 0.1 km/s) dislocations emitted from subgrain boundaries appear to account for most of the mobile dislocations. 

We first consider strain localization as discussed in Section 6.1. The material deformation action is assumed to be confined to planes that are thin in comparison to their spacing d. Let the thickness of the deformation region be given by h then the amount of local plastic shear strain in the deformation is approximately Ji djh)y, where y is the macroscale plastic shear strain in the shock process. In a planar shock wave in materials of low strength y e, where e = 1 — Po/P is the volumetric strain. On the micromechanical scale y, is accommodated by the motion of dislocations, or y, bN v(z). The average separation of mobile dislocations is simply L = Every time a disloca-... 

Where is the ratio of the irradiated to unirradiated elastic modulus. The dislocation pinning contribution to the modulus change is due to relatively mobile small defects and is thermally annealable at 2000°C. Figure 13 shows the irradiation-induced elastic modulus changes for GraphNOL N3M. The low dose change due to dislocation piiming (dashed line) saturates at a dose <1 dpa. 

A dislocation can move no faster than its core (the region within one to two atoms of position c in Figure 5.7) so the mobility is determined by whatever barrier is presented to the core. Since the core is very localized, so must be the barrier if it is to have a substantial effect. This is why local covalent bonding leads to low mobility while the non-local bonding in metals gives high mobility. 

At low temperatures, the surface mobility of the atoms is limited and the structure grows as tapered crystallites from a limited number of nuclei. It is not a full density structure but contains longitudinal porosity on the order of a few tens of nm width between the tapered crystallites. It also contains numerous dislocations with a high level of residual stress. Such a structure has also been called botryoidal and corresponds to Zone 1 in Figures 6 and7. 

The carrier concentration n = Nt, — and the compensation ratio 6 which were obtained by measurements of the Hall coefficient and carrier mobility, respectively, were found to be functions of Fas or Pas- The compensation ratio 0 exhibits a minimum and the carrier concentration n a maximum, at the optimum temperature 617 °C, shown in Fig. 3.47. From the compensation ratio and the carrier concentration, the concentrations of the ionized donors, A, are calculated as a function of Tas, as shown in Fig. 3.48.At the optimum temperature 617 °C, Aq shows a maximum and Aa a minimum. The total concentration of ionized impurities, N, = N/ + N, remains essentially constant in the measured Tas range. This result suggests that vacancy-related mechanisms are associated with the formation of dislocations during growth and the compensation process. Thus, high quality crystal GaAs with a low density of dislocations has been grown by precise... 

Particle irradiation effects in halides and especially in alkali halides have been intensively studied. One reason is that salt mines can be used to store radioactive waste. Alkali halides in thermal equilibrium are Schottky-type disordered materials. Defects in NaCl which form under electron bombardment at low temperature are neutral anion vacancies (Vx) and a corresponding number of anion interstitials (Xf). Even at liquid nitrogen temperature, these primary radiation defects are still somewhat mobile. Thus, they can either recombine (Xf+Vx = Xx) or form clusters. First, clusters will form according to /i-Xf = X j. Also, Xf and Xf j may be trapped at impurities. Later, vacancies will cluster as well. If X is trapped by a vacancy pair [VA Vx] (which is, in other words, an empty site of a lattice molecule, i.e., the smallest possible pore ) we have the smallest possible halogen molecule bubble . Further clustering of these defects may lead to dislocation loops. In contrast, aggregates of only anion vacancies are equivalent to small metal colloid particles. 

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