Michael Atzmon

The effect of compression and tension on shear-band structure and nanocrystallization in amorphous Al90Fe5Gd5: a high-resolution transmission electron microscopy study

Abstract Using both conventional and high-resolution transmission electron microscopy (HRTEM), the effect of bending at room temperature on the microstructure of amorphous Al 90 Fe 5 Gd 5 was investigated. In the compressive region, nanocrystallites formed at shear bands, along small cracks and at the fracture surface; in the tensile region, nanocrystallites were observed only at the fracture surface. Combining HRTEM with frequency filtering, low-density, nanoscale defects at shear bands were imaged. In the compressive region, both the shear bands and the undeformed matrix contain few defects. In the tensile region, there is a uniform distribution of defects within the shear bands. The preferential precipitation of nanocrystallites in the compressive region is attributed to a kinetic effect due to the uniformly distributed free volume in the shear bands. In contrast, the formation of the nanocrystallites at the fracture surfaces is likely due to adiabatic heating induced by fracture.

Thermodynamic and magnetic properties of metastable FexCu100−x solid solutions formed by mechanical alloying

Metastable solid solutions of Fe and Cu, which are immiscible in equilibrium, have been formed using high-energy ball milling of elemental powder mixtures. Single-phase face-centered-cubic (fcc) solid solution was obtained for 0<x≤60, and body-entered-cubic (bcc) solid solution for 75≤x<100. The transition from fcc to bcc occurred near x=70, where a mixture of fcc and bcc phases was obtained. The enthalpy of transformation to equilibrium was measured using differential scanning calorimetry. The average atomic volume of the phases exhibits a positive deviation from Vegard’s law, in qualitative agreement with the large positive enthalpy of mixing in this system. The magnetic moments and Curie temperatures for the metastable solid solutions have been determined and compared with those reported for Fe-Cu alloys formed by vapor deposition. Calculations of the formation enthalpy (ΔH) and free energy (ΔG) have been performed based on calphad data, with corrections based on our magnetization measurements. The cal...

Rate dependence of serrated flow in a metallic glass

Plastic deformation of amorphous Al 9 0 Fe 5 Gd 5 was investigated using nanoindentation and atomic force microscopy. While serrated flow was detected only at high loading rates, shear bands were observed for all loading rates, ranging from 1 to 100 nm/s. However, the details of shear-band formation depend on the loading rate.

Phase transformations induced by mechanical alloying in binary systems

Abstract Recent progress in the understanding of metastable and equilibrium phase formation induced by mechanical alloying (MA) of elemental powders is reviewed, with an emphasis on our recent results obtained using a combined experimental and modeling approach. Both the kinetic and thermodynamic aspects are examined. It is shown that alloy phase transformation can proceed under different constraints and via different mechanisms. In addition to the mechanism of a layer diffusion under metastable equilibrium, MA can be controlled by polymorphous constraints, or occur by self-sustained, exothermic reactions. It is demonstrated that, with properly determined thermodynamic functions of metastable phases, thermodynamics can be used to describe phase transformation by MA in limiting cases. The determination of thermodynamic functions of metastable phases is illustrated with several examples.

Formation and growth of amorphous phases by solid-state reaction in elemental composites prepared by cold working

Amorphous alloys of Ni-Zr and Cu-Zr have been synthesized by solid-state reaction of composite metal mixtures produced by mechanical deformation of metal powder mixtures and intercalated foil layers. The materials obtained were investigated by means of x-ray diffraction and differential scanning calorimetry. The results are compared with known data for thin films and rapidly quenched alloys.

Formation And Characterization of Amorphous Erbium-Based Alloys Prepared By Near-Isothermal Cold-Rolling of Elemental Composites

We report the formation of bulk single‐phase amorphous Cu‐Er and Ni‐Er alloys by extensive cold‐rolling of elemental foils. The reaction is driven by the negative enthalpies of mixing of the constitutent elements and occurs near ambient temperature. The crystallization behavior of the alloys obtained was studied by means of differential scanning calorimetry and found to agree closely with that of the corresponding sputtered and liquid‐quenched alloys. Radial distribution functions were measured for sputtered and rolled Cu72Er28 and were found to be in good agreement.

Thermodynamic properties of metastable Ag‐Cu alloys

The enthalpies of formation of metastable fcc Ag‐Cu solid solutions, produced by ball milling of elemental powders, were determined by differential scanning calorimetry. Experimental thermodynamic data for these metastable alloys and for the equilibrium phases are compared with both calculation of phase diagrams (CALPHAD) and atomistic simulation predictions. The atomistic simulations were performed using the free‐energy minimization method (FEMM). The FEMM determination of the equilibrium Ag‐Cu phase diagram and the enthalpy of formation and lattice parameters of the metastable solid solutions are in good agreement with the experimental measurements. CALPHAD calculations made in the same metastable regime, however, significantly overestimate the enthalpy of formation. Thus, the FEMM is a viable alternative approach for the calculation of thermodynamic properties of equilibrium and metastable phases, provided reliable interatomic potentials are available. The FEMM is also capable of determining such properties as the lattice parameter which are not available from CALPHAD calculations.

Deformation-induced nanocrystallization in an Al-based amorphous alloy at a subambient temperature

Abstract The compressive region of amorphous Al 90 Fe 5 Gd 5 , bent at −40 °C, was investigated by transmission electron microscopy. A high density of nanocrystals is observed within shear bands. Severe plastic deformation and precipitation of nanocrystallites are observed at the fracture surface. It is argued that deformation-assisted atomic transport leads to nanocrystallization.

Effect of strain rate on the formation of nanocrystallites in an Al-based amorphous alloy during nanoindentation

The effect of deformation by nanoindentation on nanocrystallization in amorphous Al90Fe5Gd5 was investigated by transmission electron microscopy. Massive precipitation of nanocrystallites is observed within the indents. Under the quasistatic condition used, a temperature rise due to adiabatic heating is likely negligible, confirming that plastic deformation can induce crystallization without a heating effect. The nucleation of nanocrystallites is significantly affected by the strain rate.

Evidence for self-sustained MoSi2 formation during room-temperature high-energy ball milling of elemental powders

We present evidence indicating that rapid, self-sustained, high-temperature reactions play an important role in the formation of tetragonal MoSi2 during room-temperature high-energy ball milling of elemental powders. Such reactions appear to be ignited by mechanical impact in an intimate, fine-grained, Mo–Si physical mixture formed after an initial milling period. Under certain conditions, limited propagation of self-sustained reactions in these uncompacted powder mixtures renders the compound formation seemingly gradual in bulk-averaged analysis. It is suggested that this type of reaction is an important mechanism in the mechanical alloying of highly exothermic systems. Results are discussed in comparison with similar reactions we observed in ball-milled Al–Ni powders, with self-sustained combustion synthesis previously reported for Mo–Si powders, and with interfacial diffusional reactions in Mo–Si powders or thin-film diffusion couples.