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An experimental methodology was developed which is simple and yet accurately elucidates the stress-state dependency of the deformation behavior of metallic glasses or similar amorphous systems. The methodology includes in situ observations of the evolution of the deformation behavior of metallic glasses under a cylindrical indenter. The set-up not only collects information on the load–displacement response of the system to the deformation, but also resolves the much-needed information on the temporal and spatial evolution of the shear bands. By employing various indenter radii, the critical shear stress for the nucleation of the shear bands is obtained. Experimental results for a zirconium based metallic glass where found to be consistently independent of the indenter radius and in close agreement with the uniaxial yield values.

Correlation between Barkhausen emission (BE) signal, microstructure and mechanical properties of a series of low carbon steel bars after receiving different heat treatments are studied for the purpose of further developing the nondestructive BE technique to monitor the stress–strain characteristics of steel components. It is found that the BE signal, which is proportional to the number of unpinning events of domain walls (DW), is more intense in samples with small grains which have been annealed at lower temperatures, than in those with large grains which have been annealed at higher temperatures. The small-grain samples have larger BE signals, because they have large fractional volume of grain boundaries that act as sites for the DW unpinning. A qualitative model is proposed to describe the BE signal obtained from these ferritic steel samples. BE measurements are also made in-situ on each sample while it is under increasing tension. The BE signal increases in sample under tension. In the case of small-grain sample, the increase is mainly due to the re-orientation of domain configurations that facilitates the magnetization process. In the case of large-grain sample, tensile stress re-orients the domain configurations as well as creating new DWs that increase the number of unpinning events. In this work, it is also found that the BE–strain plots of the steel bars annealed at temperatures ranging from room temperature to 600 °C exhibit behaviors which are similar to their corresponding stress–strain curves.

Barkhausen emission (BE) measurements are made on a series of plain carbon steel samples with different carbon contents. Different heat treatments are used to produce a ferrite–pearlite structure for the samples with hypoeutectoid compositions (C 0.76 wt.%). It is found that as the carbon content increases, the BE signal increases for the hypoeutectoid steel samples but decreases for the hypereutectoid samples. The maximum BE signal is detected in samples with carbon content closest to the eutectoid composition (0.76 wt.%C) at which the volume fraction of pearlite is largest. Therefore, it is evident that the BE signal increases monotonically with pearlite content for both hypoeutectoid and hypereutectoid samples. However, no trivial or obvious relationship is found between the BE signal and the carbon content. It is also observed that the BE peaks of the hypereutectoid samples tend to shift to the lower field region as the content of carbon increases, such shift is due to the pinning effect of the increasing proeutectoid cementite on the domain walls.

Magnetoacoustic emission (MAE) measurements have been made on mild steel bars of various thicknesses h, subjected to a slowly varying magnetic field. The MAE signal was found to decrease with increasing h. For smaller h, B as well as dB/dt are large, and the MAE signals are therefore strong. For larger h, the MAE signals are weaker because dB/dt is smaller. It is found that the peak height is proportional to h/sup -0.73/spl plusmn/0.02/. This result is in good agreement with predictions by two theoretical approaches, namely (i) calculations based on considering the magnetic circuit formed by the c-core magnet, the nonmagnetic spacer and the specimen in series, and (ii) calculation of the amount of magnetic field penetrating into the specimen. In this study, the lateral dispersion of magnetic induction penetrating inside the bar was also estimated. The result shows that B decreased in the lateral direction in the same manner as its dependence on h. The sensitivity of MAE to specimen thickness and width enables this technique to be used for size determination in cases where more direct methods are not applicable. >

When domain wall pinning is due to random arrays of inhomogeneities, the dynamical process of unpinning could be described by the weak or the strong domain wall pinning models. Based on these two models, the values of activation volume V/sub w/ (for weak pinning) and V/sub s/ (for strong pinning) of bowed domain wall of thermally annealed iron-rich FeAu specimens were estimated. The results were then used to calculate their corresponding fluctuation held H/sub fw/ (using V/sub w/) and H/sub fs/ (using V/sub s/), for each of the specimens. Comparing the values of H/sub fw/ and H/sub fs/ with the Barbier's plot which relates magnetic viscosity to the coercive field H/sub 0/ of a wide variety of magnetic materials, it was found that H/sub fw/ of the low age specimens plotted against their H/sub 0/ exhibited similar behaviour as in the Barbier's plot. However, when values of H/sub fs/ were used in the plotting, similarity with the Barbier's result was only found in specimens after prolonged aging. It is also found that when the localized demagnetizing effect on the bowed portion of the domain wall due to the presence of the surface free-poles is to be considered in the derivation, the fluctuation field is smaller than that without such consideration by a factor of F/2F/sub 0/ in the case when the material has a weak pinning regime, where F and F/sub 0/ are the pinning force per unit area with and without the demagnetizing consideration respectively, and by a factor of 1/4 in the case of strong pinning. >