Solidification of Ni-Re Peritectic Alloys.

Differential thermal analysis (DTA) and microstructural and microprobe measurements of DTA and as-cast Ni-Re alloys with compositions between 0.20 and 0.44 mass fraction Re provide information to resolve differences in previously published Ni-Re phase diagrams. This investigation determines that the peritectic invariant between liquid, Re-rich hexagonal close packed and Ni-rich face center cubic phases, L + HCP → FCC, occurs at 1561.1 °C ± 3.4 °C (1σ) with compositions of liquid, FCC and HCP phases of 0.283 ± 0.036, 0.436 ± 0.026, and 0.828 ± 0.037 mass fraction Re, respectively. Analysis of the microsegregation in FCC alloys yields a partition coefficient for solidification, k = 1.54 ± 0.09 (mass frac./mass frac.). A small deviation from Scheil behavior due to dendrite tip kinetics is documented in as-cast samples. No evidence of an intermetallic phase is observed.

Polyamorphism and liquid-liquid transformations in D-mannitol.

The polyamorphism exhibited by D-mannitol between the normal melt quenched glass (GN) and the amorphous Phase X (GX) induced by annealing has been examined in a detailed series of differential scanning calorimetry (DSC) measurements covering a wide range of scanning rates. The glass transition of the (GN), TgN develops an increasing behavior upon annealing, but the glass transition of (GX), TgX changes little during annealing, implying that (GX) is a kinetically more stable glass. A series of interrupted thermal cycles has allowed for the identification of a liquid-liquid transition between the supercooled liquid of (GN), SCL-1 and that for (GX), SCL-2. The precise annealing conditions that could be reached by Flash DSC enabled the construction of the Temperature-Time-Transformation plot of D-mannitol for the transition between GN/(SCL1) and G X/(SCL2), as well as the transition between amorphous and crystalline phases revealing thermally activated behavior. Under the action of an applied stress, GX can be induced to transform irreversibly into the higher density GN.

Elastic and inelastic mean free paths of 200keV electrons in metallic glasses.

The elastic and inelastic mean free paths of three metallic glass alloys, Ni60Nb40, Pd82Si18 and Ni80P20, have been measured from focused ion beam prepared thin samples with measured thickness gradients. The elastic/inelastic mean free paths of the three alloys are 35±0.5/97±3, 26±0.5/148±3 and 40±0.5/129±2.5nm, respectively. Elastic mean free paths predicted from atomic scattering cross sections consistently underestimate the experimental data. A model based on the Wentzel atomic model was developed and the fit to available data is in much better agreement with experiments, with a maximum deviation of ~4nm. The parametrized model is thus capable of predicting the elastic mean free path for other amorphous materials. Existing models for the inelastic mean free path are not in good agreement with the experimental data.

Focus: Nucleation kinetics of shear bands in metallic glass.

The development of shear bands is recognized as the primary mechanism in controlling the plastic deformability of metallic glasses. However, the kinetics of the nucleation of shear bands has received limited attention. The nucleation of shear bands in metallic glasses (MG) can be investigated using a nanoindentation method to monitor the development of the first pop-in event that is a signature of shear band nucleation. The analysis of a statistically significant number of first pop-in events demonstrates the stochastic behavior that is characteristic of nucleation and reveals a multimodal behavior associated with local spatial heterogeneities. The shear band nucleation rate of the two nucleation modes and the associated activation energy, activation volume, and site density were determined by loading rate experiments. The nucleation activation energy is very close to the value that is characteristic of the ß relaxation in metallic glass. The identification of the rate controlling kinetics for shear band nucleation offers guidance for promoting plastic flow in metallic glass.

Structural investigation and mechanical properties of a representative of a new class of materials: nanograined metallic glasses.

A new class of materials: Au-based nanograined metallic glasses (NGMGs) were synthesized using magnetron sputtering with powder targets. A detailed study by x-ray diffraction and high-resolution transmission electron microscopy (TEM) documents the unique nanoscale granular structure of the Au-based NGMG. This material inherited the good mechanical properties of metallic glasses, showing a high hardness of ∼5.3 GPa and a low elastic modulus of ∼79 GPa. In addition, in contrast to most MGs the nanoglassy particles can deform along the loading direction, exhibiting unique tensile elongation up to 100%. During thermal crystallization of NGMG material, even smaller sized Au solid solution nanocrystals are formed within the glassy nanograins, offering a new way for production of the nanocomposites with tailored structural length scales.

Kinetics of heterogeneous nucleation on intrinsic nucleants in pure fcc transition metals.

Nucleation during solidification is heterogeneous in nature in an overwhelmingly large fraction of all solidification events. Yet, most often the identity of the heterogeneous nucleants that initiate nucleation remains a matter of speculation. In fact, a series of dedicated experiments needs to be designed in order to verify if nucleation of the material under study is based on one type of heterogeneous nucleant and if the potency of that nucleant is constant, e.g. for a population of individual droplets, or stays constant over time, e.g. throughout repeated melting/solidification cycles. In this work it is demonstrated that one way to circumvent ambiguities and analyze nucleation kinetics under well-defined conditions experimentally is given by performing statistically significant numbers of repeated single-droplet experiments. The application of proper statistics analyses based upon a non-homogeneous Poisson process is shown to yield nucleation rates that are independent of a specific nucleation model. Based upon this approach nucleation undercooling measurements on pure Au, Cu and Ni as model materials have confirmed that the experimental strategy and analysis method are valid. The results are comparable to those obtained by classical nucleation theory applied to experimental data that has been verified to comply with the assertions that are necessary for applying this model framework. However, the results reveal also other complex nucleant-sample interactions such as an initial transient undercooling behavior and impurity removal during repeated cycling treatments. The transient undercooling behavior has been analyzed by a nucleant refining model to provide new insight on the operation of melt fluxing treatments.

Nucleation-catalysis-kinetics analysis under dynamic conditions.

Even at large undercooling, nucleation during solidification is usually controlled by a heterogeneous process. A number of models have been devised to describe the catalytic potency of various potential nucleation sites. Most common analysis models are based upon continuum descriptions that neglect the atomistic structural features that are important at the critical nucleus size scale. Even when surface structures such as steps are considered, they are treated as static stationary features in catalysis models. At the same time, nucleation is recognized as a dynamic process. Experimental strategies to explore some aspects of nucleation dynamics have been developed based upon the droplet method. In droplet dispersions, heterogeneous catalysis can be examined in two-phase liquid-solid mixtures with well-characterized morphologies to reveal surface structure effects and catalyst dynamics.