Domain Wall Pinning in Inhomogeneously Deformed Amorphous Alloys

Inhomogeneous deformation in amorphous alloys is characterized by local regions of intense shear. Experiments on VITROVAC 0040 (Fe40Ni40B20) supplied by Vacuumschmelze (Hanau, Germany) show a direct correlation between the number density of the shear bands and the coercive field after inhomogeneous deformation by cold rolling. The deformation process is also shown to induce an off axis magnetic anisotropy whose mean value is large compared to other residual and induced anisotropies in these materials. On this basis a domain structure is proposed for deformed material, and a model for coercivity based on domain wall pinning at residual stresses remaining after deformation is shown to lead to reasonable estimates of the coercive force. INTRODUCTION Metallic amorphous alloys display a characteristic inhomogeneous deformation mode when deformed under high stresses at temperatures below the glass transition temperature. Theoretical modelslP2 for the deformation process are based on ideas of stress driven creation of free volume leading to a dilation of the matrix. The dilation regions extend and interconnect until they span the ribbon and allow macroscopic shear to occur on a plane. External evidence for macroscopic shear is seen in the shear steps that are found on the surface of inhomogeneously deformed material. Luborsky et a1 3 have shown that material deformed by cold rolling has a large coercive field, Hc, that increases with the level of deformation. In an extension of this work Gibbs et a14 demonstrated that the domain wall pinning interaction leading to changes in H lies in the bulk of the ribbon, and is associated predominantly with local fluctuations in strain. We present here measurements that give information on the nature of these pinning centres and on the general way in which they interact with the domain structure to determine the magnetization process. EXPERIMENTAL & RESULTS The experiments were carried out on VITROVAC 0040 (Fe40Ni40B20) supplied by Vacuumschmelze, Hanau, Germany. The shear band distribution after varying amounts of deformation was studied using the following technique. A narrow rectangular depression was formed on the ribbon surface by masking with resist and electropolishing in a 10% perchloric in glacial acetic acid solution at -30°c. About 6p.m of material wcs removed sufficient to ensure that the region was not marked during rolling. The depression did not affect the rolling process as its width was much smaller than the ribbon width. The level of deformation was controlled by varying the number of passes through the rolling mill5 at a fixed closure pressure. The thickness of rolled ribbon was uniform to within lpm. The variation in the shear band configuration was determined by optical microscopy viewing under green light. The shear bands were at 90° to the rolling direction, their mean spacing was found from an average of 200 bands for each specimen. A few specimens were also viewed in SEM to check that bands with small shear steps were not being missed, a small degree of bifurcation of shear bands was observed that was not obvious in the optical micrographs. After rolling, the DC magnetic hysteresis loop 4 -1 was measured in fields up to 2.10 Am using a 4 standard technique . Values of Hc and K, the mean off-axis magnetic anisotropy, were obtained from these curves; the latter from the area under an interpolated reversible curve. The rolled ribbons were also measured with a tensile load applied at the ends to investigate the effect of additional applied tensile strain on H and K. The variation of ribbon thickness and coercive field with the number of passes through the rolling mill are shown in Fig.1. The coercive field is very sensitive to the level of deformation rising by two orders of magnitude for five passes, (as Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19808178 received value 4 ~m-').