Microstructure controlled shear band pattern formation and enhanced plasticity of bulk metallic glasses containing in situ formed ductile phase dendrite dispersions

Results are presented for a ductile metal reinforced bulk metallic glass matrix composite based on glass forming compositions in the Zr-Ti-Cu-Ni-Be system. Primary dendrite growth and solute partitioning in the molten state yields a microstructure consisting of a ductile crystalline Ti-Zr-Nb b phase, with bcc structure, in a Zr-Ti-Nb-Cu-Ni-Be bulk metallic glass matrix. Under unconstrained mechanical loading organized shear band patterns develop throughout the sample. This results in a dramatic increase in the plastic strain to failure, impact resistance, and toughness of the metallic glass. PACS numbers: 81.40. – z, 81.05.Kf Zr41.2Ti13.8Cu12.5Ni10Be22.5 (V1) exhibits an exceptional bulk metallic glass (BMG) forming ability that has motivated investigations of its mechanical behavior [1– 3]. This alloy exhibits a 1.9 GPa tensile yield strength, and a 2% elastic strain prior to failure under tensile or compressive loading. However, as in all metallic glasses, V1 specimens loaded in a state of uniaxial or plane stress fail catastrophically on one dominant shear band and show little global plasticity. Specimens loaded under constrained geometries (plane strain) fail in an elastic, perfectly plastic manner by the generation of multiple shear bands. Multiple shear bands are observed when the catastrophic instability is avoided via mechanical constraint, e.g., in uniaxial compression, bending, rolling, and under localized indentation. This behavior under deformation has limited the application of bulk metallic glasses as an engineering material. This Letter presents results for a new class of ductile metal reinforced BMG matrix composites prepared via in situ processing. Under loading, the two-phase microstructure leads to spatial variations in elastic properties as well as the conditions for yielding, the ductile phase having a lower yield strain. The initiation and propagation of shear bands is controlled by the scale and geometry of the ductile phase dispersion with the result that deformation occurs through the development of highly organized patterns of regularly spaced shear bands distributed uniformly throughout the sample. The compositions in the Zr-Ti-Cu-Ni-Be system are compactly written in terms of a pseudoternary Zr-Ti-X phase diagram, where X represents the moiety Be9Cu5Ni4, characteristic of Zr41.2Ti13.8Cu12.5Ni10Be22.5. Results presented here are for alloys of the form Zr1002x2zTixMz1002yXy, where M is an element that stabilizes the crystalline b phase in Ti- or Zr-based alloys. The inset in Fig. 1 shows the x-ray diffraction pattern for the nominal composition Zr75Ti18.34Nb6.6675X25; i.e., an alloy with M Nb, z 6.66, x 18.34, and y 25. The diffraction pattern was obtained with an INEL diffractometer (Co-Ka radiation) on the cross sectioned surface of a 25 g arc melted rod of roughly cylindrical diameter, f 1 cm. The peaks shown [with (hkl) values labeled] are due to the bcc phase. A Nelson-Riley extrapolation yields a lattice parameter a 3.496 A [4]. Upon cooling from the high temperature melt, the alloy undergoes partial crystallization by nucleation and subsequent dendritic growth of the b phase in the remaining liquid. The remaining liquid subsequently freezes to the glassy state producing a twophase microstructure containing b-phase dendrites in a glass matrix. The final microstructure of a chemically etched specimen is shown in the scanning electron microscopy (SEM) image of Fig. 1. SEM electron microprobe analysis gives the average composition for the b-phase dendrites (light phase in Fig. 1) to be Zr 71Ti16.3Nb10Cu1.8Ni0.9. Under the assumption that all of the Be in the alloy is partitioned into the matrix we estimate that the average composition of the amorphous matrix (dark phase) is Zr47Ti12.9Nb2.8Cu11Ni9.6Be16.7. Both are quoted