P. Wen

Signature of viscous flow units in apparent elastic regime of metallic glasses

We characterize and identify the flow units in two typical metallic glasses (MGs), which have markedly different β-relaxation behaviors and mechanical properties. The viscoelastic hysteresis loops are found in the cyclic deformation in the nominal elastic regime of the metallic glasses. We show that the hysteresis loops are related to the activation of the flow units in metallic glasses, and a model is proposed to describe the flow units. We demonstrate that the flow units are both the deformation units of the anelastic and plastic deformation behaviors and the structural origin of the β-relaxation in metallic glasses.

A highly glass-forming alloy with low glass transition temperature

A rare-earth Pr-based bulk metallic glass (BMG) is obtained in the shape of rod up to 5 mm in diameter by die cast. Unlike other rare-earth-based BMGs, it exhibits a distinct glass transition, the low glass transition temperature (Tg=409 K), a large and stable supercooled liquid region, and paramagnetic property. The glass transition as well as its kinetic nature and the fragility parameters of the BMG have been studied. The BMG offers an ideal model to investigate the nature of glass transition as well as the relaxation and nucleation with a large experimentally accessible time and temperature window at low temperatures.

Pronounced slow β-relaxation in La-based bulk metallic glasses.

Most metallic glasses (MGs) exhibit weak evidence of slow β-relaxation in their dynamic mechanical spectroscopy spectra. In contrast to other MGs, the La-based MGs we report here show a distinct slow β-relaxation peak in the mechanical relaxation measurements. We find that the slow β-relaxation behavior can be tuned by modification of the chemical composition and fragility. The structural origin of the slow β-relaxation and correlation between β-relaxation and α-relaxation in the MGs are also discussed. MGs with pronounced slow β-relaxation and tunable properties might provide a model system to investigate some long-standing issues in the glass field.

Stability of ZrTiCuNiBe bulk metallic glass upon isothermal annealing near the glass transition temperature

The stability of Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass (BMG) upon isothermal annealing near the glass transition temperature has been investigated by using x-ray diffraction, differential scanning calorimetry, and the pulse echo overlap method. The density, elastic constants, and thermodynamic parameters as well as their annealing time dependence have been determined. The microstructural and properties changes of the annealed BMG were checked by acoustic measurement. Obvious structural and property changes were observed with prolonged annealing of the BMG near the glass transition temperature.

Characterization of flow units in metallic glass through structural relaxations

We characterize the evolution of flow units associated with the flow “defects” in metallic glass by monitoring the fictive temperature change of a typical metallic glass upon isothermal annealing below its glass transition temperature. The correlations between the fictive temperature Tf, enthalpy change, and the concentration of flow units have been obtained. Such correlations help in understanding the evolution process of flow units, structural feature, and structural relaxation behaviors in metallic glasses, and can rationalize effects of the cooling rate, aging, and annealing on properties and structure of metallic glasses.

Compositional origin of unusual β-relaxation properties in La-Ni-Al metallic glasses

The β-relaxation of metallic glasses (MGs) bears nontrivial connections to their microscopic and macroscopic properties. In an effort to elucidate the mechanism of the β-relaxation, we studied by dynamical mechanical measurements the change of its properties on varying the composition of La60Ni15Al25 in various ways. The properties of the β-relaxation turn out to be very sensitive to the composition. It is found that the isochronal loss peak temperature of β-relaxation, Tβ,peak, is effectively determined by the total (La + Ni) content. When Cu is added into the alloy to replace either La, Ni, or Al, the Tβ,peak increases with decrease of the (La + Ni) content. The trend is in accordance with data of binary and ternary MGs formed from La, Ni, Al, and Cu. Binary La-Ni MGs have pronounced β-relaxation loss peaks, well separated from the α-relaxation. In contrast, the β-relaxation is not resolved in La-Al and La-Cu MGs, showing up as an excess wing. For the ternary La-Ni-Al MGs, increase of La or Ni content is crucial to lower the Tβ,peak. Keeping the Al content fixed, increase of La content lowers the Tβ,peak further, indicating the more important role La plays in lowering Tβ,peak than Ni. The observed effects on changing the composition of La60Ni15Al25 lead to the conclusion that the properties of the β-relaxation are mainly determined by the interaction between the largest solvent element, La, and the smallest element, Ni. From our data, it is further deduced that La and Ni have high mobility in the MGs, and this explains why the β-relaxation in this La-based MGs is prominent and well resolved from the α-relaxation as opposed to Pd- and Zr-based MGs where the solvent and largest atoms, Pd and Zr, are the least mobile.

Crossover from stochastic activation to cooperative motions of shear transformation zones in metallic glasses

We observe that the stress relaxation of metallic glasses below glass transition temperatures presents a universal double power-law decay behavior, indicating that relaxation dynamics transforms from a fast mode to a slow mode. This is attributed to a crossover from the stochastic activation to the self-organized cooperative motion of localized shear transformation zones. The phenomenon is further confirmed via strain recovery experiments and illustrated by a stochastic model. The results demonstrate that the plastic deformation exhibits the hallmarks of critical phenomenon, and offer a picture on the onset of deformation and evolution of relaxations in metallic glasses.

Correlation between dynamic flow and thermodynamic glass transition in metallic glasses

We report the values of steps of heat capacity (ΔCp) during the glass transition in a variety of metallic glasses (MGs). It is found that ΔCp is around 13.69 J mol−1 K−1 and almost invariable for the MGs. Based on the Eyring’s theory [N. Hirai and H. Eyring, J. Polym. Sci. 37, 51 (1959)], the phenomenon corresponds to a critical reduced free volume value. This exhibits that the glass transition takes place when the reduced free volume approaches to ∼2.35% in the MG systems. The value, consistent with that of the yielding of MGs, confirms that temperature and stress are equivalent for fluidizing MGs. Our results give an implication to understanding the glass transition in MGs as a Lindemann-type melting behavior [F. A. Lindemann, Z. Phys. 11, 609 (1910)].

Nucleation and growth in bulk metallic glass under high pressure investigated using in situ x-ray diffraction

The effects of pressure up to 8.3 GPa on nucleation and growth in a Zr57Nb5Cu15.5Ni12.5Al10 bulk glass-forming alloy during heating, isothermal annealing, and cooling processes are investigated using in situ x-ray diffraction with synchrotron radiation. The metallic glass shows markedly different crystallization behaviors for different heat treatments under high pressure. Heating under high pressure can stabilize the supercooled liquid state. Isothermal high pressure annealing leads to nanocrystallization. The glass forming ability of the alloy can be enhanced by high pressure during cooling process. The effects of pressure on the nucleation and growth are discussed based on the nucleation theory.

Investigation of InGaN/GaN laser degradation based on luminescence properties

Degradation of InGaN/GaN laser diode (LD) is investigated based on the luminescence properties. Gradual degradation of the LD is presented with the threshold current increase and the slope efficiency decrease. The cathodoluminescence and photoluminescence characterizations of the LD show a dislocation independent degradation of the active region under the ridge. Detailed studies on the temperature-dependent micro-photoluminescence and the electroluminescence indicate that the degradation of the LD is attributed to the generation of non-radiative recombination centers in the local multiple quantum well regions with lower indium content. The activation energy of the non-radiative recombination centers is about 10.2 meV.