Cang Fan

Ductility of bulk nanocrystalline composites and metallic glasses at room temperature

Mechanical properties of bulk Zr60Cu20Pd10Al10 nanocrystalline composite and Zr55Ni5Cu30Al10 metallic glass were measured by compression tests at room temperature. The Zr60Cu20Pd10Al10 as-quenched alloy obviously exhibits plastic strain while no distinct plastic deformation is recognized in the Zr55Ni5Cu30Al10 metallic glass. Moreover, the plastic strain increased by increasing the volume fraction of nanocrystals and achieved maximum value in the early stage of the nanocrystallization. High-resolution electron microscopy showed that, different from the microstructure of Zr55Ni5Cu30Al10 metallic glass, nanocrystals with main grain sizes of about 2 nm were embedded in the amorphous matrix of the bulk Zr60Cu20Pd10Al10 alloy which showed the maximum plastic strain.

Nanocrystalline composites with high strength obtained in Zr–Ti–Ni–Cu–Al bulk amorphous alloys

Nanocrystalline composites with the grain size less than 10 nm were produced by annealing of Cu-mold cast Zr70−x−yTixNi10Cu20Aly (X=5–7.5 and Y=10–15 at %) bulk amorphous alloys. The nanostructured alloys show increased tensile strength at the volume fraction of nanoparticles less than 30%. The microstructure of the amorphous alloys was found to contain medium range order (MRO) domains, which uniformly distributed in the amorphous matrix. We suggest that MRO domains provide nucleation sites for precipitation of the primary crystals and lead to the formation of nanocrystalline composites.

Nanocrystal composites in Zr–Nb–Cu–Al metallic glasses

Adding Nb to Zr–Cu–Al amorphous alloys induces the primary transformation occurring on heating. Nanocrystalline-amorphous composites were produced by annealing Zr70−yNbxCu30−xAly (X=5–7.5 and Y=8–12 at.%) metallic glasses. Their structures, after completing the primary transformation by annealing, were found to consist of fine crystals with a scale less than 15 nm dispersed homogeneously in an amorphous matrix. The nanostructured alloys show increased tensile strength and hardness with good bending ductility at a volume fraction of nanoparticles less than 50%. We suggest that the stronger attractive interaction between Zr–Al is the reason of the presence of the quenched-in embryos in as-solidified samples, and the quenched-in embryos therefore provide nucleation sites for nanocrystal formation.

High-strength bulk nanocrystalline alloys containing compound and amorphous phases

Abstract Partial crystallization of cast bulk amorphous alloys in ZrAlCuPd and ZrAlCuPdFesystems was found to cause bulk nanocrystalline alloys with high tensile strength (σf). The nanostructure alloys consist of nanoscale bet- Zr2(Cu, Pd) surrounded by the remaining amorphous phase. The crystallization of a ternary Zr60Al10Cu30 amorphous alloy occurs by the simultaneous precipitation of Zr2Cu and Zr2Al phases with large particle sizes of 400 to 500 nm and hence the addition of Pd is essential for formation of the nanostructure. The Pd has much larger negative heats of mixing against Zr and the resulting ZrPd atomic pair seems to act as preferential nucleation sites leading to the precipitation of Zr2(Cu, Pd). The nanostructure alloys in the cast cylinder of 4 to 5 mm in diameter keep good ductility in the volume fraction (Vf) range of the compound below 30 to 40 %.

Influence of the liquid states on the crystallization process of nanocrystal-forming Zr–Cu–Pd–Al metallic glasses

Rapidly solidified ribbons of nanocrystal-forming Zr–Cu–Pd–Al metallic glasses were prepared at various liquid temperatures (TL). Differential scanning calorimetry (DSC) traces show clearly the influence of the liquid states on the thermal properties and crystallization process. Namely, with increasing TL, the exothermal peaks of the DSC traces shift to higher temperatures, the super-cooled-liquid region ΔTx increases, and the decomposition of the metastable compound Zr2(Cu, Pd) becomes more difficult. These results suggest that the liquid state strongly controls the crystallization process of the nanocrystal-forming metallic glasses. This behavior may originate from the variation of the quenched-in nuclei, which highly depends on the short-range-order domains in liquid with different TL. We suggest that the stronger attractive interaction in Zr–Pd, which exhibits large negative mixing enthalpy, leads to the short-range order domains.

Effects of Nb addition on icosahedral quasicrystalline phase formation and glass-forming ability of Zr--Ni--Cu--Al metallic glasses

This work shows that the crystallization process of Zr–Ni–Cu–Al metallic glass is greatly influenced by adding Nb as an alloying element. Based on the results of the differential scanning calorimetry experiments for metallic glasses Zr69−xNbxNi10Cu12Al9 (x=0–15 at. %), the crystallization process takes place through two individual stages. For Zr69Ni10Cu12Al9 (x=0), metastable hexagonal ω-Zr and a small fraction of tetragonal Zr2Cu are precipitated upon completion of the first exothermic reaction. Contrary to this alloy, the precipitation of a nanoquasicrystalline phase is detected when 5–10 at. % Nb is added. Furthermore, the crystallization temperature Tx, supercooled liquid region ΔTx and reduced temperature Tg/TL (Tg is the glass transition temperature, TL the liquidus temperature) increase with increasing Nb content. These results indicate that adding Nb content to Zr–Ni–Cu–Al metallic glasses not only induces quasicrystalline phase formation, but also enhances glass-forming ability.

Investigation of short-range order in nanocrystal-forming Zr60Cu20Pd10Al10 metallic glass and the mechanism of nanocrystal formation

Rapidly solidified ribbons from the nanocrystal-forming Zr60Cu20Pd10Al10 alloy prepared at various melting liquid temperatures were used to study the influence of the liquid state upon quenched-in nuclei. With lowering quenching liquid temperatures, the small-angle x-ray scattering shows increased related periodic composition fluctuations and the radial distribution function analysis from x-ray diffraction method reveals that the coordination number of Pd around Zr increases. These results provide evidence for the stronger attractive interaction in Zr–Pd, which exhibits large negative mixing enthalpy, leads to the formation of (Zr, Pd)-rich domains of short-range order in the liquid. They remain in the amorphous phase as quenched-in nuclei and therefore contribute to nanocrystalline formation.

Fracture behavior of a nanocrystallized Zr65Cu15Al10Pd10 metallic glass

Fracture surfaces of a nanocrystallized Zr65Cu15Al10Pd10 metallic glass were observed on an atomic scale with field ion microscopy. Based on geometrical and phase characteristics of the fracture morphologies, it can be determined that a crack propagates along the interfaces between nanocrystals and the amorphous matrix in the alloy with optimized microstructures. Combining with macromechanical properties, it is proposed that the strengthening effect of nanocrystals in the Zr-based metallic glass arises from a strong interaction between nanocrystals and local shear bands during deformation processes of the glassy matrix.

High-strength bulk nanocrystalline alloys in a Zr-based system containing compound and glassy phases

Abstract Polycrystalline alloys with tensile strength ( σ f ) in Zr–Al–Cu–Pd and Zr–Al–Cu–Pd–Fe systems were formed by partial crystallization of cast glassy alloys. The alloys consist of nanometer scale Zr 2 (Cu,Pd) surrounded by a glassy phase. The particle size and interparticle spacing of the compound are less than 10 and 2 nm, respectively. The crystallization of a ternary Zr 60 Al 10 Cu 30 amorphous alloy occurs by the simultaneous precipitation of Zr 2 Al and Zr 2 Cu with particle size of 200 nm and hence the addition of Pd is essential for formation of the nanostructure (NS). The NS cylindrical alloys of 2–3 mm in diameter keep good ductility in the volume fraction ( V f ) range of the compound below 40%. The σ f and Youngs modulus ( E ) increase from 1760 MPa and 81.5 GPa, respectively, at V f =0% to 1880 MPa and 89.5 GPa respectively, at V f =40% for the Zr 60 Al 10 Cu 20 Pd 10 alloy. The formation of the NS alloys with high σ f in coexistence with the compound is presumably because the remaining glassy phase contains free volumes by quenching from the supercooled liquid. The synthesis of the high-strength bulk NS alloys is important for future development of new high-strength materials.