M. Heilmaier

Effect of crystalline precipitations on the mechanical behavior of bulk glass forming Zr-based alloys

Abstract Production of bulk amorphous Zr-based alloys with a significant supercooled liquid region was carried out by die casting into a copper mold. Fully amorphous and partially crystalline samples were prepared. The microstructure was analyzed by X-ray diffraction (XRD), scanning (SEM) and transmission electron microscopy (TEM), and chemical analysis with special emphasis on the size and composition of the crystallites. The mechanical behavior was investigated by constant compression rate tests at strain rates from 1 x 10−4 s−1 to 3 x 10−4 s−1 at room temperature using cylindrical specimens of 3 mm diameter and 6 mm height. Independent of the chosen composition the samples exhibit relatively low Youngs moduli of about 70 GPa, flow stresses around 2 GPa, and elastic strains of up to 3%. Specimens with a high volume fraction of crystalline phases are extremely brittle. In contrast, almost fully amorphous samples show microplasticity up to 1% strain without significant work hardening. A first explanation is given for the influence of crystalline phases upon crack initiation and propagation.

Supersaturated solid solution of niobium in copper by mechanical alloying

Abstract Alloys with both high strength and high conductivity have been produced by mechanical alloying. In the present study, copper was mechanically alloyed with 5, 10 and 20 at.% Nb using a planetary ball mill. The Cu–Nb phase diagram shows a negligibly low mutual solubility in the solid state, but high energy ball milling can largely extend the region of solid state solution. Previously, it was observed that niobium partly dissolves in the copper lattice during milling. The present investigation demonstrates that this limit can be extended to a strongly supersaturated Cu solid solution of up to 10 at.% Nb provided the appropriate mechanical alloying method is applied. The change in the powder microstructure was followed by scanning and transmission electron microscopy (TEM) as well as by X-ray diffraction (XRD) analysis. In the case of Cu–5%Nb and Cu–10%Nb a homogeneous single-phase microstructure was obtained after 30 h of milling. Elemental Nb could no more be detected, indicating the formation of a metastable supersaturated Cu–Nb solid solution.

Deformation behavior of Zr-based metallic glasses

Abstract Multi-component bulk Zr-based glassy alloys exhibit a wide supercooled liquid region, which provides excellent stability against crystallization. Microstructures consisting of a metallic glass matrix with crystallite precipitates of different shapes and sizes are synthesized. The compressive deformation behavior of metallic glassy matrices with and without second-phase precipitates is reviewed. At room temperature, fully amorphous alloys exhibit microplasticity without significant work hardening. In contrast, alloys with a large volume fraction of crystallites introduced either in dendritic form during casting or as nano-scaled precipitates during post-annealing of fully amorphous structures are brittle. At high temperatures, amorphous and partially crystalline alloys exhibit Newtonian viscous flow at low strain rates. At both temperatures the deformation behavior is described within the framework of the free volume model.

Bulk nanostructured Zr-based multiphase alloys with high strength and good ductility

IFW Dresden, P.O. Box 27 00 16, D-01171 Dresden, Germany(Received August 21, 2000)(Accepted in revised form December 25, 2000)Keywords: Casting; Metallic glasses; Crystallization; Bulk nanostructured alloys; MechanicalpropertiesIntroductionAn increase in strength along with good ductility has recently been observed for a number of alloys withmultiphase nanoscale microstructures (1–4). In particular, partial crystallization of bulk metallicglass-forming alloys is a promising way for producing new bulk nanophase materials with outstandingmechanical properties at room temperature (5,6). Already homogeneous glassy alloys exhibit abeneficial combination of high yield strength, microplasticity on the order of about 1–2%, rather lowYoung’s moduli and high fracture toughness values as well as high wear resistance (7–9). Partialnanocrystallization can further improve these properties (1–4) and allows to obtain bulk nanostructuredalloys with promising potential as new engineering materials. As an example, this work reports on thesynthesis of bulk nanostructured (Zr/Ti)-Cu-Al-Ni alloys through devitrification of cast bulk glassyspecimens, and presents results for their mechanical properties at room and elevated temperatures.ExperimentalRods with 3 mm diameter and 50 mm length were prepared by arc-melting pure elements under argonatmosphere and casting into a copper mold. In order to restrict the oxygen content of the alloys to lessthan 0.2 at.% oxygen high purity starting materials (99.99% purity) were used and the casting wasperfomed under an argon atmosphere of 99.9999% purity. This gave fully amorphous specimens aftercasting. The samples were characterized by x-ray diffraction (XRD, Co Ka radiation), transmissionelectron microscopy (TEM) and differential scanning calorimetry (DSC) using a heating rate of 20Kmin

Microstructure and mechanical properties of Cu-Ag microcomposites for conductor wires in pulsed high-field magnets

The influence of heat treatment and cold deformation on microstructure and mechanical properties of Cu-Ag alloys with more than 20 wt.% Ag has been investigated. Due to the embedding of the Cu-rich dendrites by the Ag-Cu eutectic high-angle grain boundaries are absent in the as-cast state of these compositions. Hence, the continuous precipitation of Ag particles is favored in comparison with discontinuous precipitation reactions observed in Cu alloys containing less than 8 wt.% Ag. The tensile strength after heavy cold deformation is enhanced by a beneficial superposition of several effects: (i) The solid solution hardening of the Cu-rich dendrites, (ii) the age hardening of silver precipitates inside the dendrites and the high work hardening rate of the peak-aged state, (iii) the phase boundary hardening of the Ag-rich eutectic, and (iv) the increase of cooling velocity after solidification of continuously cast billets which determines the extent of the Ag supersaturation in the copper matrix. As a consequence, with a continuously cast starting billet of 12 mm diameter an optimum value for the ultimate tensile strength of 1.3 GPa is obtained after a total drawing strain of only η = 4.3.

Nanostructured materials in multicomponent alloy systems

Abstract Production of bulk nanostructured composites with amorphous matrix revealing a significant supercooled liquid region was carried out by die casting and mechanical alloying (MA) and subsequent consolidation at elevated temperatures. Multiphase materials with different volume fractions of nanocrystalline precipitates were prepared from as-cast fully amorphous samples by isothermal annealing. In contrast, MA powders were blended with insoluble metallic and oxide particles upon milling. The different samples were analyzed by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscope (SEM) and transmission electron microscopy (TEM), and viscosity measurements. The mechanical behavior of cast samples was investigated using constant (true) compression rate tests at strain rates ranging from 1×10 −5 to 3×10 −3 s −1 . The Newtonian viscous flow behavior of the composites at temperatures around T g is mainly triggered by the homogeneous flow of the supercooled liquid. The easy flow behavior above T g was used to consolidate bulk dense specimens from mechanically alloyed composite powders. Microhardness measurements at room temperature reveal a substantial increase of the hardness of the composites due to the uniform distribution of nanoscale particles.

Thermal Relaxation and High Temperature Creep of Zr55Cu30Al10Ni5 Bulk Metallic Glass

The free volume model is applied to isothermal relaxation and hightemperature creep. For this analysis, the time dependent flow behaviourof Zr55Cu30Al10Ni5 bulk metallic glass (BMG) near the glass transition temperature (Tg) is expressed as a trade off between stress inducedgeneration and diffusion controlled annihilation of free volume. Thestrain rate-stress relation over a wide strain rate-range (10−7to 10−2 s−1) was established for three different temperatures near Tg. It was found that the thermal relaxation behaviour and creep kinetics arecontrolled by the mobility of atoms with an activation energy of 161 kJ/mol.

Order strengthening in the cast nickel-based superalloy IN 100 at room temperature

Abstract The isothermal aging behavior of the cuboidal ordered γ′ precipitates of the cast nickel-based superalloy IN 100 was investigated at temperatures between 800 and 1000°C. Despite the complication connected with morphological changes of the γ′ particle microstructure due to coarsening and raft formation a classical Ostwald ripening law was confirmed. Comparing the calculated critical resolved shear stress (CRSS) increase with the experimental data exhibits the functional dependencies on the mean γ′ size as predicted by theory [B. Reppich, in: R.W. Cahn et al. (Eds.), Materials Science and Technology, vol. 6, in: H. Mughrabi (Ed.), Plastic Deformation and Fracture of Materials, Wiley-VCH, Weinheim, Germany, 1993, pp. 311–357]: (a) a parabolic increase of the CRSS for cutting of fine under-aged γ′ particles (weak pair-coupling); (b) a transition to cutting by strongly pair-coupled dislocations for increasing particle sizes; and (c) a deviation from the hyperbolic CRSS decrease due to strong pair-coupled cutting towards Orowan by-passing for non-coupled single dislocations at over-aged particles.

Deformation behaviour of particle-strengthened alloys: a Voronoi mesh approach

Abstract A modelling approach based on the discretisation of a particle-strengthened material using a Voronoi mesh is considered. The model is applied to studying the effects of particle arrangement on the deformation behaviour of a particle-strengthened alloy, particularly on its creep resistance. On a local scale, i.e. within an individual Voronoi cell containing a single particle, a dislocation density-based constitutive model developed previously is applied. Interaction with adjacent Voronoi cells due to plastic incompatibilities is also included. The model predicts a dependence of the creep resistance on the character of particle distribution, notably, on the degree of particle clustering. Application to oxide dispersion strengthened (ODS) nickel-base alloys MA 754 and PM 1000, which are very similar in their metallurgical characteristics, including the volume fraction of oxide dispersoids, but differ in the dispersoid arrangement, shows an excellent predictive capability of the Voronoi mesh modelling technique.

Characterization of microstructures, mechanical properties, and oxidation behavior of coherent A2 + L21 Fe-Al-Ti

Two Fe-Al-Ti alloys with coherent αFe,Al (A2) + Fe 2 AlTi (L2 1 ) microstructures have been produced and the evolution of the microstructure with aging time has been studied by light optical and scanning electron microscopy and hardness measurements. The compressive flow strength, creep properties, brittle-to-ductile-transition temperatures (BDTT), and oxidation behavior of the alloys have been evaluated. The results show that the investigated alloys show good flow strength, high creep resistance, and good oxidation resistance. However, their BDDT is high compared to binary Fe-Al-based alloys and compared to other Fe-Al-Ti alloys no increase in creep resistance was achieved by generating coherent microstructures. The latter effect is due to the breakup of the coherent microstructures when the temperature varies because the compositions and consequently the volume fractions of the phases vary markedly depending on temperature.