Z. P. Lu

A new glass-forming ability criterion for bulk metallic glasses

A new indicator of glass-forming ability (GFA) for bulk metallic glasses (BMGs) is proposed based on crystallization processes during cooling and reheating of the supercooled liquid. The interrelationship between this new parameter and the critical cooling rate or critical section thickness is elaborated and discussed in comparison with two other representatives, i.e. reduced glass transition temperature Trg (=Tg/Tl, where Tg and Tl are the glass transition temperature and liquidus temperature, respectively) and supercooled liquid range ΔTxg (=Tx−Tg, where Tx is the onset crystallization temperature and Tg the glass transition temperature). Our results have shown that ΔTxg alone cannot infer relative GFA for BMGs while the new parameter γ, defined as Tx/(Tg+Tl), has a much better interrelationship with GFA than Trg. An approximation of the critical cooling rate and critical section thickness for glass formation in bulk metallic glasses is also formulated and evaluated.

Role of yttrium in glass formation of Fe-based bulk metallic glasses

In this study, we discovered that a small addition of Y is very effective in improving glass-forming ability of Fe-based alloys. As-cast bulk amorphous alloys containing 2 at. % Y showed large thermal stability, with glass transition temperatures above 900 K and supercooled liquid regions above 55 K, and high strength, with Vickers hardnesses larger than HV 1200. The beneficial effect of Y on glass formation is twofold: (1) Y adjusted the compositions closer to the eutectic and thus lowered their liquidus temperatures, and (2) Y improved the manufacturability of these alloys by scavenging the oxygen impurity from it via the formation of innocuous yttrium oxides.

New criterion of glass forming ability for bulk metallic glasses

It has been confirmed that glass-forming ability (GFA) is related to not only liquid phase stability but also the crystallization resistance. In this study, it was found the liquidus temperature Tl and supercooled liquid region Tx−Tg could reflect the stability of glass-forming liquids at the equilibrium and undercooled state, respectively, while the onset crystallization temperature Tx could indicate the crystallization resistance during glass formation. Thus, a modified γ parameter, defined as γm=(2Tx-Tg)∕Tl, has been established. This parameter shows an excellent correlation with the GFA of bulk metallic glasses, with the statistical correlation factor of R2=0.931.

Glass-forming tendency of bulk La–Al–Ni–Cu–(Co) metallic glass-forming liquids

The excess Gibbs free-energy difference, ΔG1−s, between the undercooled liquid and the crystalline solid for La–Al–Ni–Cu–(Co) alloys was calculated by using directly measured heat capacity data. The glass-forming tendency of these liquids correlated well with the computed Gibbs free-energy difference. The small value of Gibbs free energy of the undercooled liquid with respect to the crystalline solid is a contributing factor to the high glass-forming ability in this system. The kinetics of bulk La–Al–Ni–Cu–(Co) glass-forming liquids were also investigated using differential scanning calorimetry. The fragility plot of the viscosities indicate that these bulk metallic glass formers are strong liquids. The glass-forming ability of these alloys is also explained in the light of this fragility concept.

Oxygen effects on plastic deformation of a Zr-based bulk metallic glass

Starting with Zr of two different purities, Zr-based bulk metallic glasses (BMGs) with a nominal composition of Zr62Cu15.5Ni12.5Al10 were prepared having oxygen concentrations of about 3900 and 920at.ppm, respectively. Uniaxial compression tests showed that the BMG containing the higher level of oxygen has a higher yield strength and is capable of undergoing much less plastic deformation than that containing the lower level of oxygen. It appears that oxygen suppresses the formation of multiple shear bands but leads to an inability to sustain shear-band propagation, thus, changing the failure mode from relatively ductile to catastrophic brittle fracture.

Glass-forming ability enhanced by proper additions of oxygen in a Fe-based bulk metallic glass

Effects of oxygen on glass formation in the Fe73Mo3.0C7.0Si3.3B5.0P8.7 bulk metallic glass were studied in detail. Surprisingly, it was found that a critical level of oxygen is effective in enhancing the glass-forming ability of the current Fe-based bulk metallic glass due to the facts that (i) oxygen can increase the crystallization resistance via suppressing the precipitation of the competitive primary phase and (ii) oxygen can stabilize the glass-forming liquid as manifested by the decrease in the liquidus temperature. However, excess oxygen additions will destabilize the liquid and induce the formation of more stable oxygen-containing crystalline phase, thus deteriorating the glass-forming ability.

Effects of nanocrystal formation on the soft magnetic properties of Fe-based bulk metallic glasses

We developed several Fe-based bulk metallic glasses with a unique combination of large glass-forming ability and excellent soft magnetic properties by minor doping of Cu in the Fe76C7.0Si3.3B5.0P8.7 alloy. Proper additions of the non-magnetic copper element which has a positive heat of mixing with Fe, coupled with adequate annealing, can stimulate formation of ∼5 nm αFe ferromagnetic nanocrystals, which results in the increment in the saturation magnetization. Over-annealing which induced coarsening of the α-Fe nanocrystals reduces the ferromagnetic exchange interaction between the nanosized α-Fe crystals and increases the effective magneto-crystalline anisotropy, thereby deteriorating the soft magnetic properties.

Enhancing glass-forming ability via frustration of nano-clustering in alloys with a high solvent content

The glass-forming ability (GFA) of alloys with a high-solvent content such as soft magnetic Fe-based and Al-based alloys is usually limited due to strong formation of the solvent-based solid solution phase. Herein, we report that the GFA of soft magnetic Fe-based alloys (with >70 at.% Fe to ensure large saturation magnetization) could be dramatically improved by doping with only 0.3 at.% Cu which has a positive enthalpy of mixing with Fe. It was found that an appropriate Cu addition could enhance the liquid phase stability and crystallization resistance by destabilizing the α-Fe nano-clusters due to the necessity to redistribute the Cu atoms. However, excessive Cu doping would stimulate nucleation of the α-Fe nano-clusters due to the repulsive nature between the Fe and Cu atoms, thus deteriorating the GFA. Our findings provide new insights into understanding of glass formation in general.

A quantitative link between microplastic instability and macroscopic deformation behaviors in metallic glasses

Based on mechanical instability of individual shear transformation zones (STZs), a quantitative link between the microplastic instability and macroscopic deformation behavior of metallic glasses was proposed. Our analysis confirms that macroscopic metallic glasses comprise a statistical distribution of STZ embryos with distributed values of activation energy, and the microplastic instability of all the individual STZs dictates the macroscopic deformation behavior of amorphous solids. The statistical model presented in this paper can successfully reproduce the macroscopic stress-strain curves determined experimentally and readily be used to predict strain-rate effects on the macroscopic responses with the availability of the material parameters at a certain strain rate, which offer new insights into understanding the actual deformation mechanism in amorphous solids.

Evaluation Of Glass-Forming Ability

Bulk Metallic Glasses explores an emerging field of materials known as bulk metallic glasses. It summarizes the rapid development of these materials over the last decade and includes documentation on diverse applications of bulk metallic glasses; from structural applications to microcomponents. Some of the applications covered are pressure sensors, microgears for motors, magnetic cores for power supplies, and nano-dies for replacing next generation DVDs. The chapters cover current theories and recent research including an atomistic theory of local topological fluctuations, atomistic simulations, and unique microstructures of these amorphous materials. Other topics include glass formation, glass forming ability, and the underlying mechanisms and physical insights of these criteria. The mechanical deformation of bulk metallic glasses, fatigue, fracture, and corrosion behaviors of these materials are also reviewed.