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We describe a sensor for measuring the electrical resistance of a conducting thin-film material as a function of temperature and composition. The sensor has excellent sensitivity and can be used at temperatures as high as the melting temperature of the material of interest. The sensor is fabricated by applying a simple lift-off process to a thin film. By combining combinatorial sputtering to fabricate composition spreads with arrays of sensors, the phase transformation behavior of complex alloys can be mapped. We demonstrate this capabilities by using the sensor to determine the glass transition and crystallization temperatures of several PdSiCu-based metallic glasses. We found that in two glass-forming systems, PdCuSi and NiZr, the ratio of the resistance of the crystallized to as-deposited material is correlated with the glass-forming ability. The ability to readily determine glass forming ability, suggests that the sensor is a powerful tool for measuring the glass-forming ability in a high-throughput manner over large compositional spaces.

In this work, we report a class of phase separated Cu94 − xZrxAl5.5Nb0.5 (x = 41, 44, 47, 50 at.%) bulk metallic glasses (BMGs) with extraordinary plastic strain up to 43% and work hardening effect featured in the compression strength compared with the yielding strength. Through the characterization of the microstructure, compositional distribution, free volume and activation energy of crystallization, the underlying mechanism for the plasticizing and work hardening has been investigated. It is confirmed that the structural heterogeneity due to the phase separation results in the formation of high density of free volume and low activation energy for crystallization. The deformation induced nanocrystallization (DINC) takes place in the whole region, especially in shear bands (SBs). The DINC may reduce the instabilization of SBs, resulting in the slow-down of the propagation of SBs and the initiation of more primary and secondary SBs from soft to hard region. This may be considered as the underlying mechanism for the plasticizing and work hardening of these BMGs. Keywords: Bulk metallic glasses, Phase separation, Plasticity, Work hardening, Deformation induced nanocrystallization

We have investigated the crystallization kinetics of Cu50Zr50 metallic glass thin films using nanocalorimetry. The crystallization process is growth-controlled during heating and nucleation-controlled during cooling, resulting in different critical heating and cooling rates to suppress crystallization. Measurements over a wide range of scanning rates (13 K/s to 21,000 K/s) reveal that crystallization does not follow Arrhenius kinetics upon heating. Instead, the behavior on heating is well described by a fragility-based model of growth-controlled kinetics that takes into account breakdown of the Stokes-Einstein relationship. Upon cooling, the quench rate required to suppress crystallization of the melt is much higher than for bulk samples. This reduced asymmetry in critical heating and cooling rates compared to bulk materials suggests that crystallization of the thin-film metallic glass is controlled by heterogeneous nucleation.

Vitrification from physical vapor deposition is known to be an efficient way for tuning the kinetic and thermodynamic stability of glasses and significantly improve their properties. There is a general consensus that preparing stable glasses requires the use of high substrate temperatures close to the glass transition one, Tg. Here, we challenge this empirical rule by showing the formation of Zr-based ultrastable metallic glasses (MGs) at room temperature, i.e., with a substrate temperature of only 0.43Tg. By carefully controlling the deposition rate, we can improve the stability of the obtained glasses to higher values. In contrast to conventional quenched glasses, the ultrastable MGs exhibit a large increase of Tg of ∼60 K, stronger resistance against crystallization, and more homogeneous structure with less order at longer distances. Our study circumvents the limitation of substrate temperature for developing ultrastable glasses, and provides deeper insight into glasses stability and their surface dynamics., Producing ultrastable metallic glasses has always been associated with substrates heated close to the glass transition temperature. Here, the authors show that reducing the deposition rate of the metallic glass on a cold substrate produces ultrastable metallic glasses with remarkably improved stability.

In order to stabilize the supercooled liquid and enhance the glass forming ability of Al-based glass forming alloy, the alloying design based on minor Ca substitution for Al was examined to retard the precipitation of face-centered-cubic (FCC) Al. Calcium was selected as the alloying element, because it has a large negative mixing heating with Al but a positive value with the other components. This is anticipated to facilitate formation of Al–Ca clusters and then inhibit the diffusion of Al atoms which is thought to be the dominant reason that limits the glass forming ability (GFA) of Al-based alloys. The proposed strategy was confirmed in experiments by the observation of a greatly increased reduced glass transition temperature, T rg = T g / T l ( T g the glass transition temperature and T l the liquidus temperature), the increased the activation energy for crystallization, and the increased activation energy for diffusion. The Al 85− x Y 8 Ni 5 Co 2 Ca x ( x = 0.5–5) system is found to possess extraordinarily high T rg = 0.613 far exceeding that for any known Al-based glass forming alloys.

We perform molecular dynamics (MD) simulations of the crystallization process in binary Lennard-Jones systems during heating and cooling to investigate atomic-scale crystallization kinetics in glass-forming materials. For the cooling protocol, we prepared equilibrated liquids above the liquidus temperature $T_l$ and cooled each sample to zero temperature at rate $R_c$. For the heating protocol, we first cooled equilibrated liquids to zero temperature at rate $R_p$ and then heated the samples to temperature $T > T_l$ at rate $R_h$. We measured the critical heating and cooling rates $R_h^*$ and $R_c^*$, below which the systems begin to form a substantial fraction of crystalline clusters during the heating and cooling protocols. We show that $R_h^* > R_c^*$, and that the asymmetry ratio $R_h^*/R_c^*$ includes an intrinsic contribution that increases with the glass-forming ability (GFA) of the system and a preparation-rate dependent contribution that increases strongly as $R_p \rightarrow R_c^*$ from above. We also show that the predictions from classical nucleation theory (CNT) can qualitatively describe the dependence of the asymmetry ratio on the GFA and preparation rate $R_p$ from the MD simulations and results for the asymmetry ratio measured in Zr- and Au-based bulk metallic glasses (BMG). This work emphasizes the need for and benefits of an improved understanding of crystallization processes in BMGs and other glass-forming systems., 8 pages, 7 figures

A bulk metallic glass forming alloy is subjected to shear flow in its supercooled state by compression of a short rod to produce a flat disc. The resulting material exhibits enhanced crystallization kinetics during isothermal annealing as reflected in the decrease of the crystallization time relative to the non-deformed case. The transition from quiescent to shear-accelerated crystallization is linked to strain accumulated during shear flow above a critical shear rate $\dot\gamma_c\approx 0.3$ s$^{-1}$ which corresponds to P\'{e}clet number, $Pe\sim\mathcal{O}(1)$. The observation of shear accelerated crystallization in an atomic system at modest shear rates is uncommon. It is made possible here by the substantial viscosity of the supercooled liquid which increases strongly with temperature in the approach to the glass transition. We may therefore anticipate the encounter of non-trivial shear-related effects during thermoplastic deformation of similar systems., Comment: 7 pages, 7 figures. To appear in Phys. Rev. E

Bulk metallic glasses (BMGs) are produced by rapidly thermally quenching supercooled liquid metal alloys below the glass transition temperature at rates much faster than the critical cooling rate R_c below which crystallization occurs. The glass-forming ability of BMGs increases with decreasing R_c, and thus good glass-formers possess small values of R_c. We perform molecular dynamics simulations of binary Lennard-Jones (LJ) mixtures to quantify how key parameters, such as the stoichiometry, particle size difference, attraction strength, and heat of mixing, influence the glass-formability of model BMGs. For binary LJ mixtures, we find that the best glass-forming mixtures possess atomic size ratios (small to large) less than 0.92 and stoichiometries near 50:50 by number. In addition, weaker attractive interactions between the smaller atoms facilitate glass formation, whereas negative heats of mixing (in the experimentally relevant regime) do not change R_c significantly. These studies represent a first step in the development of computational methods for quantitatively predicting glass-formability., Comment: 8 pages, 9 figures

We explore finite size effects in the crystallization of a bulk metallic glass with nm-scale dimensions. Nanorods of Pt57.5Cu14.7Ni5.3P22.5 are produced by thermoplastic extrusion of supercooled liquid through a nanoporous template. The nanorods exhibit remarkable differences in their crystallization behavior above the glass transition. Crystallization for 100 and 200 nm diameter nanorods occurred at 6 and 24 °C lower, respectively, than the nominal crystallization temperature for bulk material while the glass transition temperatures were unchanged from the bulk value. Size dependent crystallization kinetics is discussed within a framework of classical nucleation theory, as well as possible shear and surface-induced effects.