Crystallization during bending of a Pd-based metallic glass detected by x-ray microscopy

Without the availability of slip systems and dislocation glide as in crystalline materials, metallic glasses resist irreversible deformation to elastic strains of 2% or more before undergoing heterogeneous plastic flow via the formation of shear bands. Observation of crystallite formation under compressive load was previously obtained by transmission electron microscopy. In this Letter, we present results of nondestructive x-ray diffraction microprofiling of the section of a bent glassy Pd40Cu30Ni10P20 ribbon in transmission using a synchrotron microbeam. Crystallization was clearly detected but only on the compression side of the neutral fiber. The experimental results and crystal nucleation frequency analysis are consistent with massive nucleation in shear bands forming under compressive stress but mainly for metallic glasses that show a large supercooled liquid temperature range ΔT=T(x)-T(g) between glass transition at T(g) and crystallization at T(x). The phenomenon is sensitively dependent on the volume change that accompanies crystallization in the supercooled liquid temperature range where the much larger liquid-state thermal expansion coefficient significantly increases the specific volume difference between the liquid and crystalline states. The results are also consistent with the many reports of extensive strain to fracture of metallic glasses under compressive load but not under tension.