Papers by Author: Jürgen Eckert

Abstract: Mechanical treatments such as deep rolling are known to affect the strength and toughness of metallic glass due to the residual stress. It is well known that compressive residual stress states usually enhance the mechanical properties in conventional metallic materials. We present investigations on the change of fracture behavior related with mechanical properties of “brittle” bulk metallic glass by cold rolling at room temperature. Improvement of the intrinsic plasticity is observed not only after constrained cyclic compression but also after cold rolling. Moreover, neither nanocrystallization nor phase separation occurs during deformation. By these findings we provide a unique fundamental basis by considering the introduction of structural inhomogeneity and ductility improvement in metallic glasses. The experimental evidence clearly supports that such an inhomogeneous glassy can be produced by residual stress in well known “brittle” bulk metallic glasses, and does not depend on a specific pinpointed chemical composition.

Abstract: The work hardening ability under room temperature compression of ductile Cu47.5Zr47.5Al5 and Cu47Ti33Zr11Ni8Si1 bulk metallic glass-forming alloys has been studied and compared. Both alloys exhibit high fracture strength, distinct work hardening and large plastic strain. Systematic investigations on the microstructural evolution reveal the occurrence of nano-scale heterogeneities, of both structural and chemical nature, which enables multiplication, branching, and restriction of the shear bands, thus controlling the plastic instability of metallic glasses. Phase separation in the liquid state leading to chemical inhomogeneities has been revealed for as-cast Cu47.5Zr47.5Al5 samples. In the case of Cu47Ti33Zr11Ni8Si1, a composite-type microstructure with in-situ formed nano-scale precipitates embedded in a glassy matrix is responsible for the distinct work hardening recorded on the stress-strain curves. The present results support the important role of nano-scale heterogeneities for promoting efficient work hardening in Cu-based metallic glass composites.

Abstract: The formability of several Zr-based bulk metallic glasses in the supercooled liquid region has been estimated. Using the data obtained from compression tests, normalized processing maps based on a dynamic materials model (DMM) have been constructed to evaluate feasible forming conditions. Laboratory-scale hot extrusion of the Zr44Ti11Cu9.8Ni10.2Be25 BMG has also been carried out to clarify the effectiveness of the normalized processing maps established in this study. The influence of thermal properties and microstructural differences on the formability of BMGs is interpreted in terms of a normalized temperature within the supercooled liquid region.

Abstract: Nanostructured or partially amorphous Al- and Zr-based alloys are attractive candidates for advanced high-strength lightweight materials. The strength of such materials is often 2 – 3 times higher than the strength of commercial crystalline alloys. Further property improvements are achievable by designing multi-phase composite materials with optimized length scale and intrinsic properties of the constituent phases. Such alloys can be prepared by quenching from the melt or by powder metallurgy using mechanical attrition techniques. This paper focuses on mechanically attrited powders containing amorphous or nano-(quasi)crystalline phases and on their consolidation into bulk specimens. Selected examples of mechanical deformation behavior are presented, revealing that the properties can be tuned within a wide range of strength and ductility as a function of size and volume fraction of the different phases.

Abstract: This article deals with the materials science and engineering of glass-forming alloys in Fe-(Nb)-(Al, Ga)-(P, C, B, Si), Fe-(Cr, Mo, Ga)-(P, C, B) and Fe-(Co, Ni)-(Cu)-(Zr, Nb)-B bulk metallic glasses (BMG) systems with high thermal stability of the undercooled melt against crystallization. Different liquid quenching techniques (melt-spinning or copper-mold casting) as well as hot pressing of the powder obtained by milling of the melt-spun ribbons were used to prepare samples in various shapes. Synthesis of the investigated BMG alloys is discussed according to Inoue’s empirical components rules for the achievement of the large glass forming ability (GFA). Thermal and microstructure characterization (performed by DSC, TMA, XRD and Mössbauer spectroscopy) was used to correlate GFA, microstructure and thermo/thermo-magnetic treatments with optimum soft magnetic properties.