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We have carried out a complete, detailed study of magnetomechanical damping on a set of annealed and deformed iron specimens. The results for macroeddy-current and microeddy-current damping are consistent with previous data on iron. The macroeddy peak gives a good indicator of achieving saturation. The microeddy damping offers a usable method for estimating domain size. The slope of damping versus amplitude in the hysteretic Rayleigh region is inversely proportional to the square of a measure of internal stress, as predicted by Smith and Birchak (SB). At higher amplitudes, in annealed and slightly deformed specimens, the damping increases more rapidly than in the Rayleigh region, a phenomenon not previously recognized, and contrary to existing models. The very long straight line in the Rayleigh region, implying a lack of any linear scale for the hysteresis, suggests that there is a fractal nature to the process. This suggestion is consistent with the SB theory. A possible model for the elementary hysteretic event is the reorientation of a quadrupole as the local stress changes sign; if this process involves a random sampling of the internal stress field, the lack of scale can be accounted for.

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The elastic stiffness coefficients of single-crystal iron have been measured at room (300°K) and liquid-nitrogen (77°K) temperatures using the ultrasonic pulse-echo technique. The three coefficients were determined from a total of seven independent modes. The values at 300°K are C11=2.28±0.02, C12=1.32 ±0.04, and C44=1.165±0.01, in units of 1012 dyn/cm2; the corresponding values at 77°K are 2.36±0.02, 1.34±0.04, and 1.19±0.01. The coefficients C11, (C11+C12+2C44)/2, and (C11+2C12+4C44)/3 of iron have been measured from 77° to about 673°K. C11 and (C11+2C12+4C44)/3, as well as the polycrystalline stiffness λ+2μ, depart from a linear dependence on temperature above about 300°C. It was not possible to say whether the same effect occurred in (C11+C12+2C44)/2 because of significant scatter in the data. The Debye temperature Θ0 was found from the stiffnesses extrapolated to 0°K, to be 471.5°K.

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The evidence for Snoek's model of interstitial position and diffusion in bcc metals is reviewed in the light of recent calculations. Other models are equally tenable in many instances, on the available evidence, if the internal friction relaxation strength associated with the tetrahedral sites is appreciable. To investigate the matter further, a systematic comparison of the models involving octahedral and tetrahedral sites is carried out using an elementary method for estimating strain energies about stable sites and about saddle points for diffusion. These estimates indicate that the tetrahedral sites are the more favored positions in many instances and that diffusion paths involving the tetrahedral sites may be important even when the octahedral sites are more favored. Next, a model of an interstitial in a tetrahedral site is developed for use in continuum mechanics, using the same forces as were hypothesized for the strain-energy calculation. It is deduced from this model that the relaxation strength associated with a tetrahedral site depends sensitively on an unknown angle, but that some values within a reasonable range for this angle do lead to significant relaxation strengths. It is also deduced that an impurity in a tetrahedral site interacts with strain gradients, as well as strains. A comparison of predicted patterns with measured relaxation strengths does not bear out Snoek's model but instead supports the hypothesis that in most systems based on the group VA metals the interstitial impurities occupy the tetrahedral sites.

Re-entrant spin glass (RSG) transitions in Ni-Mn and Au-Fe have been reassessed by acoustic measurements of the magneto-mechanical damping by domain walls. Stress-induced non-thermally activated domain wall dynamics is progressively replaced by an intense thermally activated relaxational response when the temperature approaches the RSG freezing point. A “frozen” state with negligible motion of domain walls on atomic and mesoscopic scales occurs in the RSG. We propose that RSG freezing has its origin in intrinsic properties of domain walls.