H.S. Chen

A method for evaluating viscosities of metallic glasses from the rates of thermal transformations

The apparent activation energies E(T) for the glass transition and crystallization in Pd77.6ue5f8Cu6Si16.5 and Pd48Ni32P20 glass are seen to coincide with those for the viscous flow. This implies that both the rates of glass transition and crystallization in metallic glasses scale as the viscosity. Based on this proposition, the viscosity of a Pt45Ni30P25 glass, for example, has been evaluated, by means of thermal measurements, from the glass transition far into the crystallization temperatures. The viscosity η decreases rapidly from 1013 P at 480 K to 107 P at 580 K and is described by a Fulchers expression as η = 10−3 exp[5950/(T − 320)].

Elastic constants, hardness and their implications to flow properties of metallic glasses

The longitudinal and transverse sound velocities and Vickers hardness of metallic glasses (Pd1 − xNix)0.80P0.20, (Pd1 − xFex)0.80P0.20 and (Pt1 − xNix)0.75P0.25 have been measured. The elastic constants at room temperature exhibit a positive deviation with composition χ from linearity whereas the hardness shows a negative deviation. The increase in elastic constants has been attributed to a denser packing of the alloys on mixing. The reduced hardness Hrue5fcH/μ versus χ exhibits a remarkable similarity to a Tg versus χ relationship. This seems to indicate that flow mechanisms involved in metallic glasses above and below the glass-transition temperature are of similar origins. It is the excess entropy of disorder associated with alloying which lowers the hardness as well as the viscosity of metallic glasses. The metallic glasses possess in general a relatively high Poissons ratio ν ≈ 0.40 and a shear modulus approaching that of the noble metals Cu, Ag and Au. Among the metallic glasses observed, the Ptue5f8P glasses exhibit the highest ν = 0.42, whereas the glasses containing Fe tend to have lower values. The phenomenon that the conduction electrons in the glassy alloys behave as in the noble metals may be partly attributed to the filling of d shell orbitals of the transition metals in the Ptue5f8P, Pdue5f8P and Niue5f8P alloys. The high ν of metallic glasses is believed to be responsible for the ductile behavior of these glasses. Poissons ratio ν of metallic glasses was observed to decrease with decreasing temperature. It is suggested that the decreasing ν with falling temperature causes the rapid increase in the fracture strength of Fe-based glasses.

On mechanisms of structural relaxation in a Pd48Ni32P20 glass

Abstract Structural relaxation processes are investigated calorimetrically for a pre-conditioned Pd48Ni32P20 glass over a wide temperature range from well below to just above the glass transition. The low temperature anneals further stabilize the glassy structure. Upon heating, the annealed sample shows an excess endothermic specific heat ΔCp above the annealing temperature and completely recovers the initial enthalpy before any manifestation of glass transition, Tg. Significantly the ΔCp peak evolves in a continuous manner with annealing time. A physically reasonable activation energy spectrum N 0 ∗ (Q) is obtained with the proper choice of coupling constants which are dependent on annealing temperature. Results suggest the existence of localized relaxation modes which do not contribute to macroscopic flow. A concept of distribution in glass transition temperatures H(Tg,m) is conceived to account for the reversible relaxation with temperature. A model glass transition based on percolation theory is proposed and is found to reproduce the calorimetric relaxation phenomena well.

Thermal expansion and density of glassy PdNiP and PtNiP alloys

Abstract Thermal expansion and density of (Pd1−xNix)0.80P0.20 and (Pt1−yNiy)0.75P0.25 alloys in their various states have been measured from room temperature to the glass transition temperature Tg. The thermal expansion of the glassy alloys at room temperature varies linearly with x and y and is 10 to 20% higher than that of corresponding pure metals. The thermal expansion of the undercooled alloy liquids near Tg as well as the molar volume v deduced from the density of glasses in contrast exhibits a negative deviation with composition x and y. This behavior is in line with the previously reported negative deviation of the glass transition temperature of these glassy alloys with metal content and may be explained in terms of excess volume associated with a mixture of hard spheres.

Alloying effect on the viscous flow in metallic glasses

Abstract The temperature dependence of viscosities near the glass transition is measured from the rates of thermal transformation for metallic glasses Ptue5f8Niue5f8P, Pdue5f8Niue5f8P, NiPBA1 and (Fe, Co)PBA1. Alloying among metallic elements which lowers the glass transition temperature Tg lowers the ideal glass transition temperature T0, but raises the residual configurational entropy Sg and the activation energies for “diffusive” rearrangement, Δμ ∗ , of the alloying glasses, while compositional ordering associated with the addition of metalloids raises the Tg and T0 and lowers the Sg and Δμ ∗ . Results are correlated to the atomic ordering and stability of the glasses. The extracted free volume and the critical diffusive volume are much smaller, by a factor of 4, for metallic glasses than for many other glasses.

Young's modulus of Fe-, Co-, Pd- and Pt-based amorphous wires produced by the in-rotating-water spinning method

This paper presents the Youngs modulus of Fe100−x−ySixBy, Fe100−x−yPxCy, Co100−x−ySixBy, Pd77.5Cu6Si16.5, Pd48Ni32P20 and Pt60Ni15P25 amorphous wires determined from the Youngs modulus sound velocity measurement. With increasing metalloid content, the Youngs modulus increases from 1.58×1011 to 1.87×1011 N m−2 for Fe-Si-B, from 1.40×1011 to 1.52×1011 N m−2 for Fe-P-C and from 1.73×1011 to 1.75×1011 N m−2 for Co-Si-B systems. The increase in Youngs modulus with the amount of metalloid elements is the largest for B, followed by Si, C and then P. The Youngs modulus of Fe- and Co-Si-B amorphous wires increases significantly with the replacement of iron or cobalt by IV–VII group transition metals. It was recognized that there existed a strong correlation between Youngs modulus (E) and tensile fracture strength (σf); the ratio of σf to E is approximated to be 0.02 for all the amorphous wires investigated. These results imply that the Youngs modulus is dominated mainly by the structural and compositional short-range orderings due to the strong interaction between metal and metalloid atoms which hinders the internal displacements. The existence of a constant ratio for σf/E was interpreted to originate from a common mechanism for plastic flow of the amorphous wires. Further, it was noted that the Youngs modulus of the Fe- and Co-based amorphous wires with diameters of ≃ 100 to 120 Μm was slightly lower than that of the amorphous ribbons with thicknesses of ≃ 20 to 25 Μm. This difference was attributed to the difference in structural ordering due to the differences in the solidification processes.

Amorphous structural information from Mossbauer Zeeman spectra: iron-metalloid systems

The authors demonstrate that if one is willing to focus only on the coarser aspects of p(H) (the detailed shape of the hyperfine field distribution), such as its mean and its variance, then the Mossbauer spectra can be analysed consistently in a manner which retains all relevant parameter correlations even if no analytic relationships exist between the parameters. Measurements of room-temperature Mossbauer Zeeman spectra are reported for six different boron and phosphorus metalloid glasses and are analysed in this fashion. Complete results for mean values and variances of hyperfine fields, isomer shifts and pure quadrupole distributions are tabulated, the last two variances being reported for the first time in a ferromagnetic phase.

Structural investigation of amorphous FeZr, CoZr and NiZr alloys with low zirconium concentration

Abstract A structural investigation has been made of alloy glasses a low concentration of zirconium: compositions M 100− x Zr x with M = Fe, Co and Ni, and x = 9 at.% using X-ray diffraction. The characteristic second peak splitting in the radial distribution function is found for all samples presently investigated. The partial radial distribution functions of amorphous Feue5f8Zr and Coue5f8Zr alloys were derived from the measured total distribution function data by applying the concentration method with the anomalous scattering technique. The amplitude in the oscillation of the radial distribution function for M - M and M -Zr pairs is more enhanced in comparison with that of glasses with high Zr content, x = 40–45 at.%. The estimated coordination number of nearest neighbor Fe atoms (11.6 ± 0.5) for iron in the Fe 84 Zr 16 glass is larger than that (10.7 ± 0.5) found in the Fe 83 B 17 glass. This is consistent with the measurements of magnetic properties of these glasses with low zirconium concentration. The possible structural features of intertransition metal alloy glasses with low zirconium concentration is also discussed based on the present experimental data.

Entropy model for flow behavior in metallic glasses

Abstract A model based on configurational entropy is proposed to interpret phenomenologically the flow behavior in metallic glasses. From available thermal and viscosity measurements, the effect of temperature, strain rate, pressure and composition on flow stress of metallic glasses have been evaluated. The values calculated agree fairly well with experimental data. The failure of atomic rearrangements to relieve the imposed strain results in stress concentrations and local structural disorder at deformed regions. This leads to inhomogeneous flow and softening of the metallic glasses.

Local structure of amorphous MO50Ni50 determined by anomalous x-ray scattering using synchroton radiation

Abstract Anomalous (resonance) x-ray scattering technique using synchrotron radiation was applied to determine the compositionally resolved local structure of sputter deposited amorphous Mo50Ni50. The local environments of Mo atoms and Ni atoms were found to be significantly different from each other, but similar to the corresponding local environments in crystalline MoNi. The results compare favorably with those of the EXAFS measurement.