摘要:

The mobility during glide of uniformly moving dislocations or dislocation segments supposed not to be obstructed by any Peierls' barrier is estimated. For a straight freely moving dislocation, the strong anharmonicities in the core region, the thermoelastic (edge dislocation) and the phonon viscosity effect give rise to a drag stress at ordinary temperaturesT∼&thgr;, &thgr; being the Debye temperature, of the order&sgr;∼110&egr;×V/cin insulators. In metals the thermoelastic effect is negligible, while the core anharmonicity effect and the phonon viscosity effect will be of the same order of magnitude as in insulators. In the above formula, &egr;=thermal energy density,V=dislocation velocity, andc=velocity of shear waves. The scattering of phonons by the dislocation also causes a drag stress at ordinary temperatures of the order of magnitude of the above formula.All of the above mentioned contributions to the drag stress go rapidly to zero with decreasing temperature. However, if the dislocation is constrained by the Peierls' barrier except at freely moving kinks, the kink mobility determines the dislocation mobility. It is shown that the scattering of phonons of a half‐wavelength longer than the kink width causes a drag stress which may outweigh all other contributions up to ordinary temperatures, and which persists with decreasing temperature asTdown to a temperature ∼&thgr;b/D, whereb=the lattice spacing constant andDis the kink width.

ISSN:0021-8979    

DOI:10.1063/1.1728907

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

A study has been made of the grain‐boundary behavior in bi‐crystals of sodium chloride and magnesia held under load at elevated temperatures. Grain‐boundary sliding was observed in both materials, of a form similar to that found in metals. The sliding occurred not smoothly, but in discreet jumps followed by periods of zero movement. In sodium chloride the over‐all sliding rate was found to depend upon four variables examined: stress, temperature, atmosphere, and boundary misorientation. Two variables examined, stress and boundary misorientation, were found to influence the over‐all boundary sliding rate in magnesia. Void formation as a result of sliding was observed in a sodium chloride sample which contained a well‐defined jog in its boundary. Implications that the present results bring to bear upon general considerations of the high temperature deformation and fracture behavior of polycrystalline nonmetallic materials are discussed.

ISSN:0021-8979    

DOI:10.1063/1.1728908

出版商:AIP    

年代:1962

数据来源: AIP

摘要:

The mechanism of crystal nucleation in a titania‐magnesia‐alumina‐silica glass has been studied by light scattering. Predominantly isotropic scattering centers become anisotropic during the initial crystallization process. This is interpreted as crystal nucleation within glassy concentrations fluctuation of titania. Titania‐containing glasses nucleate readily because of the tendency of titania to segregate in amorphous regions which are more unstable toward crystallization than the original homogeneous glass. The data can be described by a model of scattering by a center surrounded by a region of polarizability of opposite sign. This model is utilized to obtain measurements of the crystallization rate and average crystal size.

ISSN:0021-8979    

DOI:10.1063/1.1728909

出版商:AIP    

年代:1962

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1728911

出版商:AIP    

年代:1962

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1728913

出版商:AIP    

年代:1962

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1728915

出版商:AIP    

年代:1962

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1728917

出版商:AIP    

年代:1962

数据来源: AIP

ISSN:0021-8979    

DOI:10.1063/1.1728918

出版商:AIP    

年代:1962

数据来源: AIP