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High Entropy Alloys (HEA) can be classified in three domains according to their e/a and r values, with e/a, the number of itinerant valence electrons and r the average radius for a 12 nearest atoms neighborhood. The phase composition, thermal stability and possible phase transformations of a series of HEA alloys, CoCrzFeNi-XY (with X and Y = Al, Cu, Pd, Ru, Ti and z = 0 or 1), selected according to their e/a ratio were investigated in cast conditions (T0), after 3 h homogenization at 1100 °C (T1) and after 3 h annealing at 700 °C (T3). When observing the behavior of the different Domains of HEAs as classified by electronic structure it is observed that for the alloys from Domain I which contain fcc structures, the microstructure transforms from multi-to almost single-phase under homogenization (T1). In Domain III alloys containing cubic (bcc and/or B2) structures, very small multi-structural changes are observed. Alloys in Domain II have a mixed structure, i.e. several different structures in the diffraction pattern, which changes during heat treatments.

The microstructures and mechanical properties of Zr64 − xNi10Al7Cu19Cox (x = 0–16 at.%) bulk metallic glasses (BMGs) were investigated as a function of cobalt content. Due to negative mixing enthalpy between cobalt and zirconium, the atomic packing structure of studied BMGs changes with the addition of cobalt while the positive mixing enthalpy between cobalt and copper leads to phase separation and the formation of nanoglass with glassy grain sizes of about 5–10 nm. With the unique topological structure, the stored recoverable elastic energy in these studied BMGs increases dramatically with the increasing cobalt content. The Zr55.7Ni10Al7Cu19Co8.3 BMG exhibits excellent mechanical properties at ambient temperature.

The temperature dependent viscosities of stable and undercooled liquid zirconium (Zr) have been studied by the electromagnetic levitation (EML) experiments on the International Space Station ISS together with molecular dynamics (MD) simulations. Oxygen additions of about 1.2 ± 0.5 at.% in EML experiments increases the viscosity of undercooled liquid Zr. MD simulations reveal a Vogel-Fulcher-Tammann-type temperature dependent viscosity for pure Zr in the temperature range of 1600–2700 K. The evolution of the internal energy and the nearest neighboring coordination numbers show crossovers at about 2000 K, indicating a local structural change when stable liquid Zr is cooled into undercooled liquid state. This local structural change is reflected by a slight drop of fraction of the cluster, and the nonlinear temperature dependent evolution of icosahedral-like, bcc-like and tetrahedral-like short-range order. The temperature dependent fraction of mobile atoms with high potential energy exhibits a strong anti-correlation with the temperature dependent viscosity. The decrease of these mobile atoms reduces the potential energy and increases the high-coordinated clusters with strong spatial correlations upon cooling, resulting in an increase of the local stability and viscosity of stable and undercooled liquid Zr. All results obtained here will promote the research activities of the thermophysical properties and structural evolution of metallic liquids.

The viscosities of Cu50Zr50 melt from 1250 K to 1435 K were measured in a containerless electromagnetic levitator under microgravity. The viscosity in the investigated temperature range can be expressed by an Arrhenius temperature dependence η(T) = η0 exp(EA/kBT), with η0 = (0.08 ± 0.02) mPa·s and EA = (0.58 ± 0.03) eV. We find a correlation between the fraction of closer neighboring atoms, f, and viscosity, which is described by η = expn f m, where n = 1.746/Δpec-4.478, m = −0.374/Δpec + 0.440, and Δpec is the excess potential energy, indicating that the temperature dependent viscosity during cooling is controlled by the fraction of closer neighboring atoms.

The systems containing highly reactive element such as Ti are the most difficult to be determined experimentally and therefore, it is often necessary to estimate the missing values by theoretical models. The thermodynamic data of the Al–Ti system are scarce, its phase diagram is still incomplete and there are very few data on the thermophysical properties of Al–Ti melts. The study on surface, dynamic and static structural properties of liquid Al–Ti alloys has been carried out within the framework of the Compound Formation Model. In spite of the experimental difficulties, the surface tension of liquid Al–2 at.%Ti alloy has been measured over a temperature range by the pinned drop method.

Atomic force acoustic microscopy was used to image bulk metallic glasses of composition Zr63−xCu24AlxNi10Co3 with 8

Mechanical properties of the two-phase TiAl(Nb) alloys have been studied after being subjected to mechanical as well as electrolytic polishing. Indentation tests at different maximum loads using different instrumentation allow probing the overall mechanical behavior of the alloys. Nanoindentation experiments have been carried out to study the local hardening of each phase during mechanical polishing. Since the deformation of γ-TiAl is facilitated by twin formation, the mechanical polishing associated with considerable frictional forces causes a significant work hardening of the γ-TiAl phase resulting in an enhancement of the overall surface hardness of the alloy. A similar hardening effect has been observed during a scratch test. The influence of local hardening owing to different crystal structures on the overall mechanical behavior in the mechanically polished alloys has also been investigated.

The Poisson’s ratio of Zr-based bulk metallic glasses in the system Zr63−xCu24AlxNi10Co3 was found to exhibit a non-monotonous behavior as a function of x when measured with ultrasound by the pulse–echo technique. In addition, from wave propagation velocity measurements at different frequencies, i.e. f = 2.25 MHz and f = 10 MHz, a composition-dependent anelastic behavior as a function of x is found, exhibiting a similar non-monotonous behavior. In this work we further investigated the plastic deformation and the creep properties of this glass system in compression tests and by nanoindentation. The plastic strain and the measured creep deformation show correlations with the Poisson’s ratio. We then discuss the anelastic behavior observed while measuring the sound-wave propagation velocity in the frame of the thermoelastic damping and the bond reorientation as proposed by Egami. Finally we discuss these effects with regard to X-ray diffraction analysis.

We report composition optimization, thermal and physical properties of new La-based bulk metallic glasses with high glass forming ability (GFA) based on a ternary La62Al14Cu24 alloy. By refining the (Cu, Ag)/(Ni, Co) and La/(Cu, Ag) ratios in the La–Al–(Cu,Ag)–(Ni, Co) pseudo-quaternary alloy, the formation of 30 mm diameter of La65Al14(Cu5/6Ag1/6)11(Ni1/2Co1/2)10 bulk metallic glass (BMG) alloy is achieved using water quenching. The origin of the high GFA was investigated from the kinetic, structural and thermodynamic points of view, and was found to be due to the smaller difference in Gibbs free-energy between the amorphous and crystalline phases in the pseudo-quaternary alloy. These alloys exhibit low glass transition temperatures, below 430 K, and relatively wide supercooled liquid regions of 40–60 K. Mechanical tests on these alloys show a fracture strength of 650 GPa, Vicker’s hardness 200 kg mm−2, Young’s modulus 35 GPa, shear modulus 13 GPa and Poisson ratio 0.356. The La-based BMGs are useful for both scientific and engineering applications.

The cavitation erosion (CE) behaviour of four Zr-based bulk metallic glasses (BMGs) of general composition Zr–Al–CuNi(Co) was investigated by means of an ultrasonic vibration device and compared with that of S30431 austenitic stainless steel. The results show that the CE resistance of Zr-based BMGs, in terms of incubation time and maximum rate of erosion, is approximately 10 times higher than that of S30431 austenitic stainless steel. It was also found that a small addition of Co (3 at.%) decreases significantly the maximum rate of erosion of the Zr-based BMG investigated. Initial hardness of the Zr-based BMGs was found to be in a good correlation with their maximum cavitation erosion rates.

Thermodynamic properties, enthalpy relaxation and fragility of a Pd43Ni10Cu27P20 bulk metallic glass-forming liquid have been investigated and the thermodynamic functions been calculated. The enthalpy relaxation from the amorphous state into the equilibrium supercooled liquid state was found to follow a stretched exponential function with the relaxation time obeying an Arrhenius law. The fragility index, calculated from the heating rate dependence of Tg, yields a value of 65. The excellent glass-forming ability of this alloy is attributed to a smaller thermodynamic driving force and larger diffusion length for crystallization than in other bulk metallic glass-forming alloys.

The influence of annealing regarding the structure, phase composition and microhardness of nanostructured carbon steel produced by high pressure torsion (HPT) was examined. This material which is far from thermodynamic equilibrium exhibits enhanced thermal stability and a more sluggish mechanical softening than conventional martensite obtained by quenching from elevated temperatures.

We present the formation of ceramic/metallic glass composite powders by mechanical alloying of an elemental powder mixture with the composition Zr65Al7.5Ni10Cu17.5 together with SiC particles. The effect of the added ceramic particles on glass formation and thermal stability was investigated with structural and thermal analysis. During the milling process an amorphous matrix with a homogeneous dispersion of SiC particles develops. This composite material reveals a distinct glass transition followed by crystallization. The onset temperatures for these transformations are found to be increased by about 10 K in comparison to the amorphous Zr-based alloy without SiC. This observation may be explained by small compositional changes in the amorphous matrix during the milling process. No indication for SiC dispersoids acting as heterogeneous nucleation sites was found. The kinetics of primary crystallization was found to be delayed. The microhardness of the ceramic/metallic glass composites was increased by about 25% compared to the value found for the single amorphous powder. As such, mechanical alloying represents a useful method for the preparation of ceramic/metallic glass composite powders combining the mechanical properties of the amorphous state and the ceramic particle dispersions.

The synthesis of amorphous alloys by non-equilibrium techniques such as rapid quenching or ball milling has become an active subject during the past decade. Recently, a new group of Zr-based multicomponent metallic alloys with large glass forming ability have been found and are characterized by an extended supercooled liquid region and a glass transition before crystallization. Due to their low critical cooling rates, these alloys also offer the possibility to produce bulk metallic glasses by mechanical alloying and subsequent warm consolidation of the obtained amorphous powders above the glass transition temperature. Although amorphous phase formation by ball milling of elemental or intermetallic powders was studied extensively on several binary alloys systems such as Zr combined with late transition metals, the driving forces and the mechanisms of amorphization are still the subject of many investigations and discussions. In this regard, in order to allow for a meaningful comparison of the thermal stability and compositional homogeneity of mechanical alloyed amorphous powders with that of metallic glasses of the same average composition prepared by rapid liquid quenching, it is essential to develop a clear understanding of the microstructural evolution and phase formation during ball milling. This issue is addressed in the present workmore » by showing detailed structural investigations on the early stages of phase formation during mechanical alloying of an elemental Zr{sub 60}Al{sub 10}Ni{sub 9}Cu{sub 18}Co{sub 3} powder mixture. The results reveal that formation of strongly supersaturated solid solutions and fast grain refinement are the prior steps before the onset of amorphization.« less

Glass formation by ball milling of a mixture of intermetallic phases with composition Zr 60 Al 10 Ni 9 Cu 18 Co 3 was investigated by X-ray diffraction and thermal analysis and compared with glassy phases produced by liquid undercooling and ball milling of an elemental powder mixture of same composition. The so obtained amorphous phase reveals a distinct glass transition and a large stability range of the highly undercooled melt of 95 K. It is believed that this is by far the widest temperature range of a highly undercooled melt found for ball milled amorphous materials.

The specific heat and the resulting entropy, enthalpy and Gibbs free energy functions of highly undercooled liquids are essential in the analysis of the nucleation kinetics of crystallization and glass formation. However, for metallic systems the measured data are very scarce and in general limited to low melting point metals and alloys. For alloys which exhibit higher melting points such data generally do not exist. In order to measure the specific heat of metals and alloys at elevated temperatures a new modulation method is being developed. Experimental results for solid niobium samples demonstrate the feasibility of this approach.

The specific heat and the related thermodynamic functions of highly undercooled liquids are essential in analyzing the nucleation kinetics during crystallization and glass formation from the liquid or solid state. However, for metals and alloys the measured data are very scarce and in general limited to low-melting point systems. For alloys which exhibit higher melting points, such data generally do not exist. Thus, in order to measure specific heat of metals and alloys at elevated temperatures a new modulation method is being developed. The basic principle for this measurement and preliminary data on solid samples are discussed.

Isotope selective effects in resonance ionization mass spectrometry (RIMS) pose a potentially serious limitation to the application of this technique to the precise and reproducible measurement of isotope ratios. In order to identify some of the underlying causes of isotope selectivity in RIMS and to establish procedures for minimizing these effects, we investigated laser-induced isotope selectivity in the resonance ionization of Os. A single-color, one-photon resonant ionization scheme was used for several different transitions to produce Os photoions from a thermal atomization source. Variations in Os isotope ratios were studied as a function of laser parameters such as wavelength, bandwidth, power and polarization state. Isotope selectivity is strongly dependent on laser power and wavelength, even when the bandwidth of the laser radiation is much larger than the optical isotope shift. Variations in the ^(190)Os/^(188)Os ratio of ≈20% for a detuning of 0.8 cm^(−1) were observed on a transition with a small oscillator strength. Large even—odd isotope selectivity with a 13% depletion of ^(189)Os was observed on a ΔJ = +1 transition at low laser intensity; the odd mass Os isotopes are systematically depleted. For ΔJ = −1 and 0 transitions the isotope selectivity was reduced by polarization scrambling and for strongly saturating conditions. A technique employing the wavelength dependence of even—even isotope selectivity as an internal wavelength standard was developed to permit accurate and reproducible wavelength adjustment of the laser radiation. This technique provides control over laser-induced isotope selectivity for single-color ionization and enabled us to obtain reproducible measurements of ^(192)Os/^(188)Os and ^(189)Os/^(190)Os ratios in the saturation regime for a ΔJ = +1 transition with a precision of better than 0.5%. The application of this wavelength-tuning procedure should significantly improve the quality of RIMS isotope ratio data for many elements.

Saturation of the photoionisation of Ga atoms by resonance ionisation is demonstrated. The photoionisation cross section of the excited 4s 2 (1S) 3/2 state is measured to be 3 × 10 −18 cm 2 . Saturation (100% ionisation) of a given samnple is achieved at power levels of the order of 100 MW/cm 2 .