W. L. Johnson

A highly processable metallic glass: Zr41.2Ti13.8Cu12.5Ni10.0Be22.5

We report on the properties of one example of a new family of metallic alloys which exhibit excellent glass forming ability. The critical cooling rate to retain the glassy phase is of the order of 10 K/s or less. Large samples in the form of rods ranging up to 14 mm in diameter have been prepared by casting in silica containers. The undercooled liquid alloy has been studied over a wide range of temperatures between the glass transition temperature and the thermodynamic melting point of the equilibrium crystalline alloy using scanning calorimetry. Crystallization of the material has been studied. Some characteristic properties of the new material are presented. The origins of exceptional glass forming ability of these new alloys are discussed.We report on the properties of one example of a new family of metallic alloys which exhibit excellent glass forming ability. The critical cooling rate to retain the glassy phase is of the order of 10 K/s or less. Large samples in the form of rods ranging up to 14 mm in diameter have been prepared by casting in silica containers. The undercooled liquid alloy has been studied over a wide range of temperatures between the glass transition temperature and the thermodynamic melting point of the equilibrium crystalline alloy using scanning calorimetry. Crystallization of the material has been studied. Some characteristic properties of the new material are presented. The origins of exceptional glass forming ability of these new alloys are discussed.

Quasi-static constitutive behavior of Zr41.25Ti13.75Ni10Cu12.5Be22.5 bulk amorphous alloys

Abstract Mechanical tests have been performed on bulk amorphous metal alloys to determine their constitutive behavior. Based on the experimental results, it appears that amorphous metal alloys obey a Von Mises yield criterion. This result has implications in determining the micromechanisms of plastic deformation in these materials.

Mechanically driven alloying and grain size changes in nanocrystalline Fe‐Cu powders

Highly supersaturated nanocrystalline FexCu100−x alloys (10≤x≤95) have been prepared by mechanical alloying of elemental crystalline powders. The development of the microstructure is investigated by x‐ray diffraction, differential scanning calorimetry, and transmission electron microscopy. The results are compared with data for ball‐milled elemental Fe and Cu powders, samples prepared by inert gas condensation, and sputtered films. The deformation during milling reduces the grain size of the alloys to 6–20 nm. The final grain size of the powders depends on the composition of the material. Single‐phase fcc alloys with x≤60 and single‐phase bcc alloys with x≥80 are formed even though the Fe‐Cu system exhibits vanishingly small solid solubilities under equilibrium conditions. For 60≤x≤80, fcc and bcc solid solutions coexist. The alloy formation is discussed with respect to the thermodynamic conditions of the material. The role of the large volume fraction of grain boundaries between the nanometer‐sized cryst...

Thermal stability and grain growth behavior of mechanically alloyed nanocrystalline Fe-Cu alloys

X-ray diffraction, transmission electron microscopy, and differential scanning calorimetry were used to study the thermal stability of highly supersaturated nanocrystalline FexCu100−x alloys (10 ~80. For 60<=x<=80 fcc and bcc phases coexist. Heating to elevated temperatures leads to structural relaxation, phase separation, and grain growth of the metastable nanocrystalline solid solutions. Single-phase fcc and bcc alloys undergo significant strain release but no appreciable grain growth prior to phase separation. After phase separation pronounced grain growth sets in. In contrast, samples in the two-phase region show some grain growth and significant chemical redistribution even at low temperatures. The phase separation of single-phase fcc and bcc alloys proceeds via different mechanisms: fcc solid solutions decompose by forming small Fe precipitates, while demixing in bcc alloys starts by segregation of Cu atoms to bcc grain boundaries before nucleation of Cu precipitates. These results show that the stability and grain growth behavior of nanocrystalline alloys is strongly affected by the microstructure of the material.

Metallic glass formation in highly undercooled Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 during containerless electrostatic levitation processing

Various sample sizes of Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 with masses up to 80 mg were undercooled below Tg (the glass transition temperature) while electrostatically levitated. The final solidification product of the sample was determined by x-ray diffraction to have an amorphous phase. Differential scanning calorimetry was used to confirm the absence of crystallinity in the processes sample. The amorphous phase could be formed only after heating the samples above the melting temperature for extended periods of time in order to break down and dissolve oxides or other contaminants which would otherwise initiate heterogeneous nucleation of crystals. Noncontact pyrometry was used to monitor the sample temperature throughout processing. The critical cooling rate required to avoid crystallization during solidification of the Zr41.2Ti13.8Cu12.5Ni10.0Be22.5 alloy fell between 0.9 and 1.2 K/s.

Time-temperature-transformation diagram of a highly processable metallic glass

Abstract Recently, the present authors found a highly processable metallic glass with a low critical cooling rate of 10 K s −1 or less. This glassy alloy showed several interesting features such as high thermal stability in the supercooled liquid region, and a strong dependence of the crystallization temperature on the heating rate as well as on the environment. In order to process these materials in the supercooled liquid region, time-temperature-transformation (TTT) diagrams are very useful. We report the calculation of the TTT diagram as well as its comparison with experimental results. From the constructed TTT diagram, it is concluded that these excellent glass-forming alloys which require very low critical cooling rates also tend to have a strong dependence of the crystallization temperature on the heating rate. It is also proposed that heterogeneous nucleation is an important factor in the crystallization of these alloys.

Precursors of amorphization in supersaturated Nb--Pd solid solutions

The possibility that crystal-to-amorphous phase transformations can be induced by one or more underlying instabilities of the crystalline phase has been investigated in highly supersaturated solid solutions of Nb--Pd. Several unusual properties were discovered that may be identified as precursor effects of the collapse of the bcc [alpha]--Nb terminal solution to the amorphous phase. Elastic neutron diffraction measurements of [alpha]--Nb solutions found, with increasing Pd concentration, an anomalously large increase of the average atomic root-mean-square displacement to about half of the value at which the Lindemann criterion predicts the lattice should melt. Low-temperature heat capacity measurements yielded a concomitant decrease in the Debye temperature, suggesting that supersaturation causes an elastic modulus to soften. Single crystals of [alpha]--Nb solutions at high supersaturations have a highly anisotropic structure that is visible in transmission electron microscopy images; it is consistent with the development of a soft phonon mode leading to a bcc-to-[omega] phase transformation. Considered together with the results of other recent experiments, these findings suggest that shear instability of the crystalline phase plays an important role in the crystal-to-amorphous transformation and that the average static mean-square displacement of atoms in the lattice acts as a useful parameter for the stability of themorexa0» crystal with respect to amorphization.«xa0less

Gibbs free-energy difference between the glass and crystalline phases of a Ni-Zr alloy

The heats of eutectic melting and devitrification, and the specific heats of the crystalline, glass, and liquid phases have been measured for a Ni24Zr76 alloy. The data are used to calculate the Gibbs free-energy difference, DeltaGAC, between the real glass and the crystal on an assumption that the liquid-glass transition is second order. The result shows that DeltaGAC continuously increases as the temperature decreases in contrast to the ideal glass case where DeltaGAC is assumed to be independent of temperature.

Static mean-square displacement as an indicator of the crystal-to-amorphous transformation

Abstract Catastrophic amorphization of crystalline alloys supersaturated beyond a critical concentration has been postulated by Fecht, Desre and Johnson [Philos. Mag. B59 (1989) 577]. Nbue5f8Pd is a good candidate system for testing their ideas because of its plunging T 0 line on the Nb-rich side of the phase diagram. Signs of instability in bcc Nbue5f8Pd solutions were looked for by elastic neutron diffraction measurements of atomic mean-square displacements (MSDs) between 12 K and room temperature. Thermally induced MSD values have been estimated from low-temperature heat capacity measurements and subtracted from the total MSD. The remaining static MSD increases rapidly with Pd concentration and is much larger than expected from the size mismatch of Nb and Pd atoms. At 42 at.% Pd, the rms static disorder reaches approximately half the value at which the Lindemann criterion predicts that the lattice should melt. Supersaturation also causes the Debye temperature to decrease, suggesting that an elastic modulus softens. In light of these results, the usefulness of static atomic disorder as a measure of the stability of a crystal against amorphization is discussed.