The transitional plasticity of ammonium chloride

2014 From a phenomenological theory of domain kinetics in a coherent two-phase field it is inferred that the creep rate at the start of macroplastic deformation goes to zero, as the temperature approaches the singular point of the thermal relaxation time. It is pointed out that the erosion of the domains decorating the edge dislocations can be followed through the intensity of the light scattered at 90 degrees. LE JOURNAL DE PHYSIQUE LETTRES TOME 40, 15 JUILLET 1979, Classification Physics Abstracts 62.20F 64.70K 78.35 The Cottrell theory of impurity atmosphere drag is remarkably successful in accounting for the microcreep of tin and the repeated yielding of iron near room temperature [1, 2]. It is taken here as a model for the construction of -a theory of coherent precipitate drag. I first assess the force of attraction between an edge dislocation and a nearby precipitate. Then I evaluate the strain rate at the onset of macroplastic deformation under the assumption that a slowly moving dislocation gains its freedom when the precipitate, trying to keep up with the dislocation, becomes noticeably stunded. Lastly I propose a study of the transitional opalescence of a crystal under a small external stress to determine how a typical precipitate fares during the passage from microplasticity to macroplasticity. The mechanical effects of a precipitate with a different composition are extensively discussed in the literature. On the other hand, deformation when the precipitate has the same composition as the matrix receives scant attention. To correct this imbalance I want to consider ammonium chloride during the discontinuous stage of its orientational ordering reaction. Because the transition line of the reaction has a positive slope, the crystallites on the constricted side of an edge dislocation pass from the disordered to the partially ordered form of the material at higher temperatures. As long as the ambient temperature exceeds the nominal ordering point at atmospheric pressure, a roughly cylindrical domain of the partially ordered modification appears alongside the dislocation. This domain can be expected to contribute to the elastic scattering of light, for its refractive index is different from that of the disordered matrix. It should also add to the elastic deformation of the material, as it tends to lock the dislocation at its side. According to Cottrell and Jaswon [1] a tin crystal exhibits microplasticity, when the average edge dislocation has to drag along an impurity atmosphere. By analogy, I expect ammonium chloride to manifest a slow creep in the coexistence region of the orientational transition, when a typical edge dislocation must drag along a domain of the minority phase. Probably the simplest way "to estimate the drag is through the change in the elastic strain energy, when the domain is displaced a minuscule distance s relative to the dislocation. As a first approximation, the stress around an isolated dislocation can be taken