Wha-Nam Myung

Glass transition behaviour of Zr- and Ti-based binary amorphous alloys

Abstract A study has been made to investigate the effects of free-volume fraction, steady state viscous flow temperature and viscosity on the glass transition and viscous flow behaviour of Cu x Zr 100− x ( x = 40, 50 and 60), Ni x Zr 100− x ( x = 40 and 50) and Cu x Ti 100− x ( x = 40 and 50) amorphous alloys. It was observed that the displacement at T x increases in the order Cu 40 Zr 60 > Ni 40 Zr 60 > Cu 40 Ti 60 . We found that this order can be explained by the free-volume fraction and maximum displacement rate at T vs . The relationship between viscosity and free-volume fraction below T x is consistent with Doolittles empirical formula for the CuZr, CuTi and NiZr systems. It is presumed that the kinetics of free-volume production and annihilation are involved in the thermal stability and glass transition temperature.

Glass transition and viscous flow behavior of amorphous FeMP (M = Cr, V or Mo) alloys

Abstract The glass transition and viscous flow behavior of Fe 83− x M x P 17 (M = Cr, V or Mo) amorphous alloys was examined using DSC and TMA to clarify the effect of M-substitution on the ideal glass transition temperature ( T gi ), onset temperature of crystallization, viscosity and displacement during a TMA test under an applied stress of 2.5–20 MPa. All the samples exhibit minimum viscosity at the steady state viscous flow temperature. The higher the stress level, the lower the T gi . The decrease in T gi seems to be attributed to a decrease in the free volume and defects in the amorphous alloys. We observed mid-contraction phenomena due to the structural relaxation effected by an external applied stress. The stress level at which the viscous flow occurs increases in the order: chromium, vanadium, molybdenum.

Abrasive Wear Resistance of Bulk Metallic Glasses

This paper reviews work on the wear of metallic glasses in general, as well as reporting recent results on the abrasive wear of bulk metallic glasses. The distinctive mechanical properties of metallic glasses make their wear resistance of fundamental interest. Metallic glasses, and the partially or fully crystalline materials derived from them, can have very good resistance to sliding and abrasive wear. Standard wear laws are followed, with behaviour similar to that of conventional hardened alloys. The microhardness and abrasive wear resistance are measured for four bulk metallic glasses (based on La, Mg, Pd or Zr). The hardness and wear resistance correlate well with the Youngs modulus of the glass.

Kinetic and thermodynamic aspects of crystallization in CuTiNi and CuTiAl metallic glasses

Abstract Cu 50 Ti 40 Ni 10 and Cu 50 Ti 40 Al 10 alloys were amorphized by rapid solidification. Their crystallization kinetics was studied by differential scanning calorimetry (DSC) and X-ray diffraction. Examples were found for different mechanisms according to the alloy composition and temperature range of the experiments. Cu 50 Ti 40 Ni 10 crystallizes to nanocrystalline (Cu, Ni) 3 Ti 2 . In the neighbourhood of T g the activation energy is 236 kJ mol −1 . Substantial incubation times are found and the Avrami coefficient always exceeds 4, indicating an increasing nucleation rate. Cu 50 Ti 40 Al 10 crystallizes predominantly to a phase isomorphous with γ-CuTi with an activation energy of 328 kJ mol −1 . The Avrami coefficient is around 3 and the incubation time is low. The process appears to be controlled by crystal growth on a steady state population of nuclei. A thermodynamic discussion of glass formation and crystallization is given with the help of data on the melting behaviour of the alloys obtained by high temperature DSC.

Characterization of the non-isothermal viscous flow and thermal stability of amorphous Zr-Al-Ni-Cu alloys with a wide supercooled liquid region

Abstract The thermal stability and viscous flow behavior of amorphous Zr 65 Al 8 Ni 27− x Cu x (x= 12, 17 and 22) ribbons were examined by using differential scanning calorimetry and thermomechanical analysis. It was observed that the viscosity decreases with increasing temperature until the first steady-state viscous flow temperature is reached and above this temperature the viscosity depends on both the composition and applied stress.

Internal friction and structural relaxation of amorphous FeMnP alloys

Abstract Measurements of internal friction and dynamic modulus have been carried out on amorphous Fe 83− x Mn x P 17 ( x =9,12, or 15) alloys in the temperature range 300–800 K, as a function of applied frequency, by a forced oscillation method. The characteristic transition temperatures and activation energies for crystallization were measured by differential scanning calorimetry (DSC). The viscous flow of the sample was measured using a thermomechanical analyzer (TMA) under continuous heating conditions. Internal friction increases rapidly with temperature starting from 450 K for 0.1 Hz, 474 K for 0.5 Hz, 486 K for 1.0 Hz, 511 K for 5 Hz, and 525 K for 10 Hz for the Fe 74 Mn 9 P 17 specimen. It was found that the internal friction peak temperature was very closely the same as the steady state viscous flow temperature from the TMA curves for Fe 74 Mn 9 P 17 and Fe 68 Mn 15 P 17 but not for Fe 71 Mn 12 P 17 . It is assumed that the internal friction peak does not correspond to the glass transition temperature or crystallization temperature for these alloy systems. Two maxima in the free-volume fraction and two minima in the viscosity were also found for the samples which exhibit a ‘mid-contraction’ in their TMA displacement curves. The internal friction peak occurred at the steady state viscous flow temperature. This occurrence means that the origin of the internal friction peak is related to the viscous flow behavior and free-volume fluctuation for this alloy system.

Crystallization characteristics and viscous flow behavior of WC/amorphous Ni73Si10B17 metal matrix composites

Abstract The crystallization and viscous flow behavior of WC/amorphous Ni 73 Si 10 B 17 metal matrix composites was examined by using X-ray diffraction, differential scanning calorimetry and thermochemical analysis. The progress of crystallization is faster in the tungsten carbide (WC)-dispersed composites than in the single-phase amorphous Ni 73 Si 10 B 17 , and the enthalpy of crystallization decreases with increasing volume fraction of the WC particles. It was observed that the activation energy of crystallization of WC-containing samples increased rapidly at almost the same peak temperature relative to the amorphous Ni 73 Si 10 B 17 alloy. For every composition studied, the viscosity decreased with increasing volume fraction of WC particles; hence the yield stress was raised by increasing the volume fraction of WC particles. T gi can be controlled by changing the volume fraction of WC particles.

The influence of annealing atmosphere on the formation of nanocrystals from devitrification of a Ti38.5Cu32Co14Al10Zr5.5 amorphous alloy

Abstract The kinetics of the polymorphic transformation of a Ti 38.5 Cu 32 Co 14 Al 10 Zr 5.5 glassy alloy to an ordered bcc phase was determined by means of combined DSC, XRD and TEM experiments. Samples annealed isothermally under Ar at 723 K for 5, 10, 15, 20 min contain crystals up to 20 nm in size. After the quick formation of nanocrystals, growth is almost blocked. Samples contained in Al pans sealed in a glove box, gave DSC signals significantly displaced at longer times showing an effect of the annealing atmosphere on the crystallization kinetics. This was confirmed by annealing other samples in vacuum. The size of the crystals after these longer annealings is still in the nanometer range. The size distribution, determined on TEM micrographs is compatible with nucleation on homogeneously distributed impurities.

Structural relaxation and crystallization of amorphous Ti60Cu30Ni10 alloy

Abstract Structural relaxation and crystallization have been followed in a Ti 60 Cu 30 Ni 10 metallic glass by a combination of experimental techniques. At a frequency of 0.1 Hz, the internal friction shows two distinct peaks at 662.5 ± 1.6 and 722.2 ± 1.7 K, and the positions of the peaks correspond approximately to the peak temperatures observed in a differential scanning calorimetry measurement. The origin of the internal friction peak for Ti 60 Cu 30 Ni 10 metallic glass is related to the amorphous-crystalline transition. The onset temperature for viscous flow occurs at a temperature close to that at which the internal friction starts to increase. It is thought that structural relaxation in the low-temperature range is strongly related to the viscous-flow-induced internal friction for this alloy system.

Viscous-flow behavior of amorphous Co69Fe4.5Nb1.5Si10B15 alloys

Abstract The viscous-flow behavior of amorphous Co 69 Fe 4.5 Nb 1.5 Si 10 B 15 alloys has been investigated. Thermomechanical analysis experiments showed effective viscosity decreasing monotonically with increasing temperature until steady-state viscous-flow temperature ( T vs ), where the effective viscosity of the ribbon sample showed a minimum but increased after passing through that point. Annealing-induced changes of the free-volume fraction decreased with increasing temperature. We found that the effective linear expansion coefficient and the free-volume fraction are sensitive to annealing, and the secondary working point in vicinity of the T vs was confirmed by experiment. We confirmed that relative displacement under tensile stress depends strongly upon the state of amorphicity of the sample. We think that applied stress level and the values of the T vs temperature at each stress level are very useful information for work with consolidated powders.