摘要:

Crystals of the general composition {Y3−2xCa2x} [Fe2−yIny] (Fe3−xVx)O12were grown by the PbO flux method. Electron microprobe studies revealed zoning patterns of varyingxwithin these crystals, while the indium content was found uniform among all crystals from the same crucible. Averagexandywas determined by measuring the saturation magnetizationMsand the Curie temperature. Three series of crystals were produced withy=0, 0.11 and 0.35, andxranging from 0.19 to 0.82. Ferrimagnetic resonance between 4 and 300 K yielded the anisotropy constantsK1andK2. These single‐crystal data were supplemented by the results of line‐shape analysis of polycrystalline samples with lowMs. AtT=0, |K1(x) | is nonlinear with a maximum atx≈0.2. The reduced temperature dependence of the reduced anisotropy field was found independent of composition.

ISSN:0021-8979    

DOI:10.1063/1.1659419

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The anisotropy properties of fine magnetic particles can be determined by measurements of the initial reversible susceptibility. The effects of magnetostatic interactions on the reversible magnetization in low magnetic fields near zero and saturation remanence are treated in detail with regard to packing fractionpand particle axes orientations. It is shown among other results that for any symmetrical orientation distribution, the reciprocal initial reversible susceptibility decreases linearly withp. Thus, by extrapolation top=0 (the susceptibility without interaction effects) the effective anisotropy constant of the particles can be obtained. From the temperature dependence of the susceptibility it is conjectured that &ggr;‐Fe2O3has ⟨110⟩ easy directions of crystalline anisotropy. The corresponding anisotropy field as a function of temperature between 77° and 400°K was thus determined from susceptibility measurements. For CrO2particles the anisotropy data are already known and are shown to be in good agreement with the present results.

ISSN:0021-8979    

DOI:10.1063/1.1659420

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The domain structure of thin metal platelets having positive cubic anisotropy constantsK1changes discontinuously on application of external stress. The paper deals with the calculation of the eigenvalues of the stress and of the wall energy corresponding to these discrete domain configurations determined by the finite size and shape of the platelets.

ISSN:0021-8979    

DOI:10.1063/1.1659421

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Experimental evidence is adduced to suggest that electrodeposited iron‐nickel alloys may contain &agr;‐phase regions in a &ggr;‐phase matrix. This observation is taken as evidence of large variations in composition on a scale of 1000 Å or less, and calculations are presented which show that it may be feasible to explain the observed uniaxial anisotropy in electrodeposited iron‐nickel alloys in terms of a magnetostrictive mechanism even though the average plating composition be that of a zerostrictive material.

ISSN:0021-8979    

DOI:10.1063/1.1659422

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Observations have been made of the changes in shape that develop in initially cylindrical specimens of In&sngbnd;0.5% Pb and In&sngbnd;5.0% Pb alloys extended axially at overall strain rates in the range 1.2×10−7−1.7×10−1sec−1at a temperature of 0.69Tm. To deal with the dynamic nature of neck formation and with the possibility that necks once formed may become dormant, the concept of neck activity is introduced. Two types of instability are observed. The first instability is related to the initiation of several active necks in a uniformly deforming specimen. It may occur at small strains (<0.05) and apparently is strain‐rate sensitive. The second instability occurs at large strains and involves the unequal rate of growth of concurrently active necks, resulting in the final ascendancy of a single neck. The second instability is related to elevated temperature ductility. At high strain rates, where work hardening is dominant, it corresponds approximately to Conside`re's criterion, but at low strain rates, where viscous effects are significant, a postponement with respect to the Conside`re condition is observed, the ductility increasing with decreasing strain rate.

ISSN:0021-8979    

DOI:10.1063/1.1659423

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Variational principles and variational bounds are obtained for the effective elastic moduli of quasiisotropic composite materials.

ISSN:0021-8979    

DOI:10.1063/1.1659424

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Hertizan fracture theory, previously developed for ideal brittle solids fractured under essentially environment‐free conditions, is here modified to allow for an interaction between the growing cone crack and a reactive environment. Two alternative models, both of which account for the observed detrimental effect of an environment on the Hertzian strength but which predict slight differences in the growth of the cone crack, are offered. The first is based on a surface‐energy‐lowering concept and the second on a subcritical‐crack‐growth concept. The theoretical implications of these two models are examined in the light of Hertzian fracture experiments on abraded glass slabs in different test environments. Observations of the growth of the cone crack in fatigue experiments favor the second model for glass. The advantages of the Hertzian test as a tool for the study of environmental effects on the fracture strength of brittle solids in general are discussed in relation to more conventional fracture tests.

ISSN:0021-8979    

DOI:10.1063/1.1659425

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

A mechanism for relating the redistribution of ions in a periodic stress gradient to internal friction peaks is proposed. The calculated internal friction peak is broad and asymmetric, and the frequency of maximum loss is proportional to the conductivity of the material, as is found experimentally, for silicate glasses. There is good agreement between the calculated and measured peak temperatures for some glasses, although further work is needed to test the theory completely. It is possible that this mechanism makes an important contribution to delayed elasticity in other ionic conductors.

ISSN:0021-8979    

DOI:10.1063/1.1659426

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

Deformation and fracture behaviors of ZnO whiskers were investigated by the performance of tensile, compressive, and bending tests. The whiskers could be elastically strained to as much as 15% in bending without undergoing fracture or plastic deformation. They are generally brittle, except in a few cases. Results of compression tests show that slip may take place on the basal plane as well as on the prism plane. In a tensile test a specimen is found to undergo shear along pyramidal planes. Besides the usual planar fracture, a specimen, when failed in tension, showed cup‐and‐cone fracture, which is a feature of ductile materials. On the basis of the available evidence it is suggested that cup‐and‐cone fracture may not be a feature of ductile materials only; it may occur, as in the present case, in brittle materials, with perhaps a different mechanism.

ISSN:0021-8979    

DOI:10.1063/1.1659427

出版商:AIP    

年代:1970

数据来源: AIP

摘要:

The elastic constants of pyrolytic graphite which has been highly ordered by annealing under compressive stress have been determined by ultrasonic, sonic resonance, and static test methods. Ultrasonic tests yielded the elastic constantsc11,c12,c33,c44= 1/s44, and the stress derivative ofc33. The moduli 1/s11andc44were determined from the free flexural resonant vibrations of bars, and the shear modulusc44also was determined from the fundamental torsional resonance of the bars and from the resonance of compound torsion oscillators composed of thin graphite disks with steel end pieces. Static tension, compression, and torsion tests on the pyrolytic graphite yielded a complete set of compliances (s11,s12,s13,s33, ands44). The following self‐consistent set of elastic constants (cijin units of 1011dyn/cm2;sijin 10−11cm2/dyn) has been deduced from the results:c11= 106±2,c12= 18±2,c33= 3.65±0.10,c13= 1.5±0.5,c44= 0.018 to 0.035, 1/s11= 102±3, 1/s33= 3.65±0.10,s12= −0.0016±0.0006,s13= −0.0033±0.0008, ands66= 0.023±0.002; with the possible exception ofc44, these constants are expected to be typical of the graphite single crystal. The shear strength for shear between basal planes was found to be in the range of 90–250 g/mm2.

ISSN:0021-8979    

DOI:10.1063/1.1659428

出版商:AIP    

年代:1970

数据来源: AIP