M. X. Pan

Magnetocaloric effect in Gd-based bulk metallic glasses

Magnetocaloric effect and refrigerant capacity of Gd-based Gd53Al24Co20Zr3 and Gd33Er22Al25Co20 bulk metallic glasses are investigated. It is found that the magnetic entropy changes compare favorably with that of Gd and are slightly larger than that of the known crystalline magnetic refrigerant compound Gd5Si2Ge1.9Fe0.1. Their good refrigerant efficiency combining with high electrical resistivity, high thermal stability, outstanding mechanical properties, tunable nature, and sufficiently soft magnetic property make them an attractive candidate for magnetic refrigerants in the temperature range of 10–100K.

“Soft” bulk metallic glasses based on cerium

CeAlNiCu alloys can be readily cast into glassy rods with up to 5mm in diameter. The Ce-based bulk metallic glasses (BMGs) exhibit a wide supercooled region up to 78K, very low glass transition temperature (Tg=359K), melting temperature (Tm=637K), and Debye temperature (θD=144K). Ultrasonic measurements demonstrate that these Ce-based BMGs are very soft, having the lowest elastic moduli in known BMGs. These features suggest that the “soft” BMGs are an ideal model system for investigating physical problems in glass transition, supercooled liquid and melt states, and have potential applications as a functional material as well.

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.

Magnetocaloric effect of Ho-, Dy-, and Er-based bulk metallic glasses in helium and hydrogen liquefaction temperature range

The authors study the magnetocaloric effect and refrigerant capacity of Ho30Y26Al24Co20, Dy50Gd7Al23Co20, and Er50Al24Co20Y6 bulk metallic glasses. Their magnetic entropy changes associated with spin glass to paramagnetic transition are larger than those of Gd, Gd5Si2Ge1.9Fe0.1, and many other intermetallic compounds reported in the same temperature range. The good refrigerant efficiency combined with their high electrical resistivity, high thermal stability, outstanding mechanical properties, and tunable nature makes these glassy materials be attractive candidates for magnetic refrigerants in helium and hydrogen liquefaction temperature range of 2–50K.

Elastic constants and their pressure dependence of Zr41Ti14Cu12.5Ni9Be22.5C1 bulk metallic glass

The acoustic velocities and their pressure dependence of bulk Zr41Ti14Cu12.5Ni9Be22.5C1 metallic glass (MG) have been measured up to 0.5 GPa by using a pulse echo overlap method. The elastic constants and thermodynamic parameters as well as their pressure dependence of the MG have been determined. The obtained elastic constants were compared to that of other kinds of glasses. More information about the microstructure, elastic properties, and glass forming ability of the MG was obtained.

Evolution of nanoscale morphology on fracture surface of brittle metallic glass

The authors report the observations of periodic morphology evolution on fracture surface of a brittle metallic glassy ribbon, suggesting a wavy local stress intensity factor along the crack propagation. The authors find that the formation of nanoscale damage cavity structure is a common characteristic morphology on the fracture surfaces. On the surface of the hackle zone, these cavities assemble and generate the nanoscale swirling periodic corrugations. The elastic waves interfering with the plastic process zone on the crack front is proposed to explain such dynamic crack instability. The authors’ observations support the notion of an impinging effect of elastic waves on propagating crack front.

Glass forming properties of Zr-based bulk metallic alloys

The compositions of Zr41Ti14Cu12Ni10Be23 and Zr55Ni10Cu20Al15 bulk metallic glasses, were modified by the addition of other elements, such as Nb, Fe, Mg, Y, Ta, and C. The modified alloys also exhibit excellent glass forming ability. The glass transition temperature (Tg), crystallization temperature (Tx), and offset melting temperature (Tl) of the composition modified Zr-based alloys were determined by differential temperature analysis. The results show that the Tg, Tx, and Tl are all sensitive to the composition. The undercooled temperature from Tl to Tx,ΔTl defined by ΔTl=Tl−Tx, has a stronger correlation with the reduced glass transition temperature Trg (Trg=Tg/Tl) than that of ΔTx (ΔTx=Tx−Tg).

Relationship between glass transition temperature and Debye temperature in bulk metallic glasses

The Debye temperature and glass transition temperature of a variety of bulk metallic glasses (BMGs) were determined by acoustic measurement and differential scanning calorimetry, respectively. The relationship between the Debye temperature and glass transition temperature of these BMGs was analyzed, and their observed correlation was interpreted in terms of the characteristics of the glass transition in BMGs.

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.

Periodic corrugation on dynamic fracture surface in brittle bulk metallic glass

Dynamic crack propagation in a model brittle bulk metallic glass (BMG) is studied. Contrary to other brittle glassy materials, the authors find nanometer scale out-of-plane periodic corrugations along the crack surface of the BMG. The nanoscale periodicity remains nearly constant at different loading rates. An interpretation is presented to explain the evolution and the periodic coalescence of the nanometer scale cavities along the crack surface. The observation sheds light on the origin of dynamic fracture surface roughening in brittle materials and could be generally applicable to brittle materials.