Uwe Köster

Synthesis and characterization of particulate reinforced Zr57Nb5Al10Cu15.4Ni12.6 bulk metallic glass composites

The Zr[sub 57]Nb[sub 5]Al[sub 10]Cu[sub 15.4]Ni[sub 12.6] bulk metallic glass forming liquid is reinforced with WC, SiC, W, or Ta particles. Structure, microstructure and thermal stability of the composites are studied by X-ray diffraction, optical microscopy and differential scanning calorimetry. The metallic glass matrix remains amorphous after adding up to 20 vol.% of particles. The reactions at the interfaces between the matrix and the different reinforcing materials are investigated with scanning electron microscopy, transmission electron microscopy and electron microprobe. The mechanical properties of the composites are studied in compression and tension. The influence of the introduced particles on the thermal stability of the matrix as well as on the mechanical properties is discussed.

FORMATION OF QUASICRYSTALS IN BULK GLASS FORMING ZR-CU-NI-AL ALLOYS

Zr65Cu17.5Ni10Al7.5 as well as Zr69.5Cu12Ni11Al7.5 belong to the best glass forming alloys known. Glass transition temperatures of melt‐spun ribbons are 372 and 360 °C, respectively. TEM and x‐ray analysis of samples annealed above the glass transition temperature exhibit the formation of quasicrystalline microstructures with small amounts of crystalline phases. The metastable icosahedral phase is primitive with a quasilattice constant a=0.253 nm; its composition as determined by EDX is close to Zr69.5Cu12Ni11Al7.5. In both glasses, growth of the quasicrystals has been observed to be time‐dependent (r∝t1/2), thus indicating a diffusion controlled transformation.

Hydrogenation of amorphous and nanocrystalline Mg-based alloys

Abstract Amorphous and nanocrystalline Mg-based alloys were produced by rapid quenching (melt-spinning) and their hydrogenation properties were studied. The thermal stability and crystallization behaviour of the as-quenched and hydrogenated alloys were investigated as well. It was found that the as-cast nanocrystalline/amorphous Mg 75 Ni 20 Mm 5 (Mm=Ce, La-rich mischmetal) alloy possesses the best hydriding properties (hydrogenation kinetics and hydrogen absorption capacity) with maximum hydrogen capacity of 4.0 wt.% H. The difference in the hydriding properties of the as-quenched nanocrystalline and completely crystallized (with grain size in the range of 100–150 nm) Mg 75 Ni 20 Mm 5 alloys was found to be insignificant. The amorphous and crystallized (after heat treatment) Mg 87 Ni 12 Y 1 alloys show slower hydriding kinetics and lower hydrogen absorption capacity compared to the other Mg-based alloys studied. The amorphous Mg 87 Ni 12 Y 1 exhibits faster initial hydrogenation kinetics than the partially and fully crystallized alloys with the same composition, due to faster hydrogen diffusion in the amorphous phase, but the hydrogen absorption capacity of all Mg 87 Ni 12 Y 1 alloys having different microstructure is practically the same. The crystallization of melt-spun Mg 75 Ni 20 Mm 5 and Mg 87 Ni 12 Y 1 alloys is a two-step process. The primary crystallization of α-Mg (for Mg 87 Ni 12 Y 1 ) takes place at about 160°C, followed by transformation of the residual amorphous matrix into a metastable phase, assigned as fcc Mg 6 Ni (isomorphic with fcc Mg 6 Pd ( a o =2.0108 nm)). This intermediate metastable phase decomposes during further annealing (at about 300°C) into the equilibrium phases Mg 2 Ni and Mg. The product of the first crystallization reaction for the as-cast Mg 75 Ni 20 Mm 5 alloy is Mg 2 Ni, most probably realized by growth of the quenched-in Mg 2 Ni nanocrystals. The second reaction corresponds to transformation of the residual amorphous matrix into Mg 17 Mm 2 .

Mechanical properties of quasicrystalline and crystalline phases in AlCuFe alloys

Mechanical properties of icosahedral AlCuFe quasicrystals were investigated and compared with those of related crystalline intermetallic compounds by means of indentation and three-point bending techniques. Quasicrystalline phases possess high hardness (HV ≈ 1000) and high ability for elastic recovery (H/E > 0.08) but exhibit low toughness against unstable fracture (KIC ≈ 1 MPa m12). Icosahedral particles embedded in Al2Cu or in AlFe crystalline matrices lead to increasing resistance against subcritical crack propagation. In contrast to cleavage#1 fracture in crystalline Al13Fe4, the icosahedral quasicrystals exhibit a rough, quasicleavage#1 fracture surface. Evidences for plastic deformation in the quasicrystals were found and its mechanism is discussed.

Nanocrystalline materials by crystallization of metal–metalloid glasses

Abstract The formation of ultrafine microstructures by crystallization of metal-metalloid glasses was investigated by means of electron microscopy as well as in situ time-resolved X-ray diffraction. The results can be understood on the basis of nucleation and growth theories, taking into account the effect of recalescence during massive crystallization and the differences in the mode of crystallization and the diffusivity. In a polymorphic crystallizing Fe66Ni10B24 glass the finest microstructure can be achieved by annealing at temperatures significantly below the “nose” of the TTT diagram; the finest grain size can be calculated and observed to be in the range of about 0.1 μm. In glassy Fe73.4Cu1Nb3.1Si13.4B9.1 (FINEMENT) the combination of a reduced growth rate due to the niobium content as well as with increasing size of the primary crystals and an accelerated nucleation rate due to the copper additions allows the formation of extremely fine-grained microstructures in primary crystallizing metal-metalloid glasses at temperatures above the glass transition.

Surface crystallization of metallic glasses

Abstract The crystallization of metallic glasses proceeds by nucleation and growth processes in the bulk or at the surface. In most glasses, surface crystallization has been observed to occur even at temperatures far below any crystallization event in the bulk. Preferred nucleation and/or accelerated growth may occur at the surface because of a decrease in the total surface energy, the greater ease of diffusion and stress relaxation, or a local change in the surface composition. Local changes in surface composition by selective oxidation or segregation are assumed to possess the strongest influence on surface crystallization. Surface crystallization at the surface of melt-spun ribbons has been studied systematically in metal-metalloid as well as transition metal-transition metal glasses by means of quantitative optical and electron microscopy. Strong influences of quenching conditions, alloying, surface treatment or different atmospheres during annealing have been observed; such treatments may be used for controlling or avoiding surface crystallization.

Crystallization of highly undercooled metallic melts and metallic glasses around the glass transition temperature

Abstract Crystallization in highly undercooled melts can be studied either after severe undercooling of the melt or after heating up metallic glasses above their glass transition temperature. Whereas the crystallization of silicate glasses proceeds only above the glass transition temperature, the crystallization of metallic glasses can occur in both temperature ranges. Below the glass transition temperature, nucleation and crystal growth are controlled by diffusivity with an Arrhenius-type temperature dependence; above the glass transition, crystallization kinetics can be better described by the Vogel-Fulcher-Tammann equation which is usually used to describe the temperature dependence of shear viscosity. The different behaviour in comparison with silicate glasses is assumed to be due to the metallic bonding which allows atomic exchange of the glass-forming elements by diffusion even at temperatures below the glass transition temperature. Usually, metallic glasses are found to crystallize very rapidly at temperatures close to the glass transition, thus hiding the glass transition itself. For example, metal-metalloid glasses ( e.g. Fe 75 B 25 ) and zirconium based transition metal glasses ( e.g. Co 33 Zr 67 or Co 50 Zr 50 ) are known to crystallize within a few seconds in this temperature range. Zr 60 Ni 25 Al 15 glasses, however, can be held without crystallization for relatively long times in the highly undercooled state, i.e. in the temperature range above the glass transition temperature. During primary crystallization of metallic glasses, e.g. FINEMET (Fe 73.4 Cu 1 Nb 3.1 Si 13.4 B 9.1 , with size-dependent growth rates, the microstructure can be controlled by the addition of slow diffusing elements such as Nb and/or elements such as Cu or Au which enhance the nucleation rate.

Formation of quasicrystals in bulk glass forming Zr-Cu-Ni-Al alloys

Zr 65 Cu 17.5 Ni 10 Al 7.5 as well as Zr 69.5 Cu 12 Ni 11 Al 7.5 belong to the best glass forming alloys known. Transmission electron microscopy (TEM) and X-ray analysis of samples annealed above the glass transition temperature exhibit the formation of quasicrystalline microstructures. The metastable icosahedral phase is primitive with a quasilattice constant a = 0.253 nm; its composition is close to Zr 69.5 Cu 12 Ni 11 Al 7.5 . The growth of spherical shaped quasicrystals has been observed to be time-dependent (r ∞ t 0.5 ), thus indicating a diffusion controlled transformation. The nucleation of the quasicrystals in Zr 69.5 Cu 12 Ni 11 Al 7.5 was investigated by means of crystallization statistics and found to be heterogeneous and time-dependent. The nucleation rate reached a maximum at 410°C. The temperature dependence of nucleation and growth rates can be described best assuming a Vogel-Fulcher-Tammann equation with B = 5710 K and T 0 = 421 K. From differential scanning calorimetry (DSC) the enthalpy for the quasicrystal formation in Zr 69.5 Cu 12 Ni 11 Al 7.5 has been estimated to be 2.4 kJ mol -1 . Simulation of the observed nucleation and growth rates indicates an interfacial energy of about 13 mJ m -2 . The heterogeneous nucleation of quasicrystals might result from quenching-in clusters with icosahedral structure.

Recrystallisation mechanism and annealing texture in aluminium-copper alloys

The correlation between the mechanism of recrystallisation and the annealing texture of aluminium-copper alloys was investigated by transmission electron microscopy and selected area diffraction, and pole figure determination by X-rays. Continuous recrystallisation by sub-grain growth leads to preservation of the rolling texture, while recrystallisation by motion of a high-angle boundary produces a cube texture as in pure aluminium. The conditions under which the different modes of recrystallisation occur and the reasons for the formation of the two types of textures are discussed on the basis of microscopic mechanisms.

Crystallization and decomposition of amorphous silicon-aluminium films

Abstract The crystallization and decomposition of vacuum-deposited amorphous silicon-aluminium films have been examined by means of transmission electron microscopy. Depending on the aluminium concentration, the transformation of the metastable amorphous phase into the stable phases of aluminium and silicon proceeds by different reactions such as pre-crystallization of aluminium, polymorphous transformation into supersaturated crystalline solid solutions or eutectic decomposition. The temperature dependence of the eutectic crystallization was measured. The results are discussed in terms of the thermodynamics of amorphous-to-crystalline transformation.