| Amorphous metals are the ultimate nanostructure for mechanical strength. Their macroscopic ductility can be limited, however, by the formation of shear bands. If these materials are to be used in structural applications, the formation of shear bands must be understood and ultimately controlled. The mechanical properties of sputtered thin films of amorphous Pd0.77Si0.23 (a-PdSi) were studied using tensile testing. By depositing thin films of metallic glass on compliant substrates, fracture of the composite specimens was avoided. This testing technique allowed, for the first time, measurement of homogeneous flow and homogeneous deformation-induced softening at room temperature in this material. The stress-strain behavior and formation of shear bands does not depend on sample size, strain rate (over a range of 10 -5 to 8 x 10-5 sec-1 ), or film adhesion. By geometrically confining thin films of metallic glass with layers of a crystalline metal, the formation of shear steps at the interface is counteracted elastically, and the formation of shear bands is suppressed. A Cu/a-PdSi/Cu trilayer exhibited a yield strength of 1180 MPa, a flow stress of 1680 MPa, and a plastic strain of 13% without fracture. Multilayers of crystalline Cu and a-PdSi were deformed to large strains by cold rolling and bending. Transmission electron microscopy showed the layered structure remained intact and that no shear bands formed. In rolled Cu/Nb multilayers extensive grain rotation about the transverse direction occurs on micron-scale Cu grains. This grain rotation is reduced in nanoscale Cu/a-PdSi multilayers presumably due to a change in the distribution of dislocations in the Cu grains. The strength of the crystalline/amorphous multilayers was modeled with confined layer slip, in which deformation occurs by the motion of threading dislocations in the thin crystalline layers. |
