Kazutaka Fujita

Malleable hypoeutectic Zr–Ni–Cu–Al bulk glassy alloys with tensile plastic elongation at room temperature

A new bulk glassy alloy (BGA) showing macroscopic tensile plastic elongation at room temperature has been developed in the hypoeutectic Zr–Ni–Cu–Al alloy system. The hypoeutectic Zr–Ni–Cu–Al BGA shows a high Poissons ratio, a low Youngs modulus, and is highly malleable in compression. It exhibits a Poissons ratio of 0.39, a Youngs modulus of 73 GPa, and a distinct tensile plastic elongation of about 1.7% at room temperature with necking due to the operation of many shear bands. The tensile plastic deformability seems to originate from modifications of the glass structure with increasing the number of Zr–Zr atomic pairs in the hypoeutectic composition.

Tensile Plastic Deformation of Zr70Ni16Cu6Al8 Bulk Metallic Glass at Cryogenic Temperature

Zr70Ni16Cu6Al8 bulk metallic glass (BMG) exhibited clear tensile plastic deformation at room temperature. The tensile plastic deformation was induced by the specimen sliding along one main shear band, which completely penetrated the specimen. Also, it was surmised that there is a viscous medium in a main shear band. Therefore, it can be considered that the viscous medium in the shear band plays an important role in the mechanical properties of the BMG under tensile loading. In this study, tensile tests of the Zr70Ni16Cu6Al8 BMG were carried out at the temperatures of 293 and 133 K to investigate the effect of the viscous medium in the shear band on the mechanical properties of the BMG. As the results, it was found that the strength of the BMG at 133 K was 21% higher than that at 293 K. Furthermore, at 133 K, highly denser, longer, and many branching and curving shear bands were observed over a wide area of the specimen surface in comparison with those at 293 K. In addition, at 133 K, a few main shear bands were observed on the fracture surface.

Propagation of microscopic fatigue crack under periodic overstressing in quenched and tempered 0.45%C steel.

Propagation of microscopic fatigue cracks by periodic overstressing was studied with quenched and tempered 0.45% C steel. Acceleration of crack propagation occured in 0.45% C steel as in the case of 0.15% C steel previously reported, when the crack length was more than 50μm. The lower limit of understress that caused such an acceleration was reduced steeply as the crack length became longer. The acceleration in 0.45% C steel was less than in 0.15% C steel in accordance with the previous result on macroscopic cracks in which the acceleration was less in materials of higher strength. In the cases of short cracks (below 200μm) and long cracks (above 200μm) loaded with low understress, the microscopic fracture surface consisted of small facets, which were supposed to be related to the crystal structure of steel, and cracks observed on the specimen surface propagated intermittently, suggesting that microstructure had a significant effect upon crack propagation under intermittent overstressing. In the case of long cracks loaded with high understress, the microscopic fracture surface consisted of large facets and cracks propagated steadily, indicating that microstructure had less effect. No appreciable difference was observed in crack opening behavior before and after overstressing, and the acceleration of crack propagation was not likely to be related to the crack closure phenomenon.

Propagation of microscopic fatigue crack under periodic overstressing. Overstress and understress values and acceleration of crack propagation.

Propagation of microscopic fatigue cracks by periodic overstressing was studied under different overstress and understress conditions using a low carbon steel. When the overstress value was large, a significant acceleration of crack propagation (more than one hundred times) occurred even if the understress value was lower than the threshold stress, similarly as in the case of macroscopic cracks. The lower limit of the understress that caused such a significant acceleration was reduced as the overstress was increased, but it was independent of the crack length as far as the crack was microscopic (less than 50μm). For longer cracks, the lower limit was considerably reduced by increasing the crack length. When the overstress value was low, a significant acceleration in short cracks occurred only when the understress value was nearly equal to the threshold stress, but in longer cracks it took place even the understress value was lower than the threshold stress. The microscopic fracture surface consisted of small facets, which was supposed to be related to crystal structures. Observation of the specimen surface showed that cracks propagated intermittently, hesitating at the boundary of microstructure. These observations indicate that microstructure has a significant effect upon microscopic crack propagation under intermittent overstressing.