Structural evolution during mechanical milling and subsequent annealing of Cu–Ni–Al–Co–Cr–Fe–Ti alloys

This study reports the structural evolution of high-entropy alloys from elemental materials to amorphous phases during mechanical alloying, and further, to equilibrium phases during subsequent thermal annealing. Four alloys from quaternary Cu_0.5NiAlCo to septenary Cu_0.5NiAlCoCrFeTi were analyzed. Microstructure examinations reveal that during mechanical alloying, Cu and Ni first formed a solid solution, and then other elements gradually dissolved into the solid solution which was finally transformed into amorphous structures after prolonged milling. During thermal annealing, recovery of the amorphous powders begins at 100 °C, crystallization occurs at 250–280 °C, and precipitation and grain growth of equilibrium phases occur at higher temperatures. The glass transition temperature usually observed in bulk amorphous alloys was not observed in the present amorphous phases. These structural evolution reveal three physical significances for high-entropy alloys: (1) the annealed state of amorphous powders produces simple equilibrium solid solution phases instead of complex phases, confirming the high-entropy effect; (2) amorphization caused by mechanical milling still meets the minimum criterion for amorphization based on topological instability proposed by Egami; and (3) the nonexistence of a glass transition temperature suggests that Inoue's rules for bulk amorphous alloys are still crucial for the existence of glass transition for a high-entropy amorphous alloy.     

Electrical Resistance Measurement of Glass Transition and Crystallization Characteristics of Zr—Al—Cu—Ni Metallic Glasses

In this paper,glass transition and thermal stability of the Zr-Al-Cu-Ni metallic glasses were investigated by using electrical resistance measurement(ERM),DSC and X-ray diffraction techniques.The experimental results show that the ERM is capable of detecting the glass transition of the amorphous alloys and can help to distinguish the crystallization products of the Zr-Al-Cu-Ni metallic glasses owing to the difference of the electrical resistivity between the precipitation phases.     

Cast bulk ZrTiAlCuNi amorphous alloys

Cylindrical bulk amorphous samples with diameters up to 10 mm have been prepared by casting ZrTiAlCuNi alloys in a copper mould. In order to rank glass-forming ability as a function of alloy composition, alloys were also cast into wedge-shaped moulds; to a first approximation, the thickness of the amorphous region obtained can be taken as an indication of glass-forming ability. The compositions which lead to the production of bulk glasses all have reduced glass transition temperatures in excess of 0.65 and the extremely high glass-forming ability of these compositions is discussed. We suggest that both the Al and Ti contents are determining factors for the production of bulk amorphous samples and these are believed to reduce the driving force for, and hence the rate of, crystallisation. These amorphous alloys have been found to display high thermal stability and can be annealed for several minutes in the supercooled liquid region. They are ductile at room temperature and have a high value of yield stress.     

In Situ Diffraction Experiments for the Investigation of Phase Fractions and Ordering Temperatures in Ti‐44 at% Al‐(3‐7) at% Mo Alloys

Being a strong β stabilizer, Mo has gained importance as an alloying element for so‐called β/γ‐TiAl alloys. Intermetallic TiAl‐based alloys which contain a significant volume fraction of the body‐centered cubic β‐phase at elevated temperatures have proven to exhibit good processing characteristics during hot‐working. Unfortunately, the effect of Mo on the appearing phases and their temperature dependence is not well known. In this work, sections of the Ti‐Al‐Mo ternary phase diagram derived from thermodynamic calculations as well as experimental data are presented. The phase transition temperatures stated in these phase diagrams are compared with the results of high‐temperature diffraction studies using high‐energy synchrotron radiation. Additionally, the disordering temperature of the β_o‐phase is determined.     

Effect of crystalline precipitations on the mechanical behavior of bulk glass forming Zr-based alloys

Production of bulk amorphous Zr-based alloys with a significant supercooled liquid region was carried out by die casting into a copper mold. Fully amorphous and partially crystalline samples were prepared. The microstructure was analyzed by X-ray diffraction (XRD), scanning (SEM) and transmission electron microscopy (TEM), and chemical analysis with special emphasis on the size and composition of the crystallites. The mechanical behavior was investigated by constant compression rate tests at strain rates from 1 x 10⁻⁴ s⁻¹ to 3 x 10⁻⁴ s⁻¹ at room temperature using cylindrical specimens of 3 mm diameter and 6 mm height. Independent of the chosen composition the samples exhibit relatively low Young's moduli of about 70 GPa, flow stresses around 2 GPa, and elastic strains of up to 3%. Specimens with a high volume fraction of crystalline phases are extremely brittle. In contrast, almost fully amorphous samples show microplasticity up to 1% strain without significant work hardening. A first explanation is given for the influence of crystalline phases upon crack initiation and propagation.     

Influence of the substrate temperature on the structure and surface roughness of Cd0.18Sb0.64Te0.18 films

Using a radiofrequency sputtering deposition technique, ternary Cd0.18Sb0.64Te0.18 thin films have been grown on glass substrates at several substrate temperatures (50–250°C). The samples have an Sb content of about 63 at %, as measured by Auger spectroscopy. The surface roughness, the structural and the electrical properties of the films were studied as a function of substrate temperature. X-ray diffraction (XRD) measurements showed that the structure of the films changes from an amorphous phase, when deposited at lower substrate temperatures, to a mixture of two crystalline phases (CdTe and Sb) for higher substrate temperatures. Atomic force microscopy shows an increase in the surface roughness with an increase in the substrate temperature, clearly showing the formation of crystalline phases with microcrystallite sizes in good agreement with those determined from XRD measurements. The amorphous-to crystalline transition is accompanied by an abrupt increase in the room temperature electrical conductivity of the films. This increase in the conductivity as well as its temperature dependence in the range of room temperature to 150°C can be understood in terms of an electrical percolation process through the conducting Sb crystallites.     

Review: A decade of quenching from the melt

The technique of rapidly quenching metals and alloys from the melt developed in 1960 by Duwez and his collaborators has gained wide popularity. The extremely high cooling rates (10⁶ to 10⁸° C/sec) attainable with these techniques have yielded supersaturated solid solutions, non-equilibrium crystalline phases and also amorphous alloys. The fascinating results obtained so far, particularly the unusual structures and properties of the new products, are extensively reviewed in this article.     

Anomalous thermal effects in crystallization of hydrogenated amorphous alloys of the TiNi-TiCu system

Several anomalous effects not reported previously for the interaction of hydrogen with amorphous alloys of the TiNi-TiCu system have been observed, including (i) the passage from one-to multistage mechanism of the transition from the amorphous to crystalline state in hydrogenated alloys, (ii) a manifold increase in the thermal effect of glass transition in hydrogen-saturated compositions, and (iii) the appearance of an endothermal peak in the differential scanning calorimetry curves of hydrogenated alloys, which is related to the decomposition of finely dispersed hydride phases.     

Reentrant glass transition leading to ultrastable metallic glass

Polyamorphs are often observed in amorphous matters, and a representative example is the reentrant glass transition in colloid systems. For metallic amorphous alloys, however, the cases reported so far are limited to metallic glasses (MGs) that undergo electronic transitions under gigapascal applied pressure, or the presence of two liquids at the same composition. Here we report the first observation of a reentrant glass transition in MGs. This unusual reentrant glass transition transforms an MG from its as-quenched state (Glass I) to an ultrastable state (Glass II), mediated by the supercooled liquid of Glass I. Specifically, upon heating to above its glass transition temperature under ambient pressure, Glass I first transitions into its supercooled liquid, which then transforms into a new Glass II, accompanied by an exothermic peak in calorimetric scan, together with a precipitous drop in volume, electrical resistance and specific heat, as well as clear evidence of local structural ordering on the short-to-medium-range scale revealed via in-situ synchrotron X-ray scattering. Atomistic simulations indicate enhanced ordering of locally favored motifs to establish correlations in the medium range that resemble those in equilibrium crystalline compounds. The resulting lower-energy Glass II has its own glass transition temperature higher than that of Glass I by as much as 50 degrees. This route thus delivers a thermodynamically and kinetically ultrastable MG that can be easily retained to ambient conditions.     

Inverse ductile–brittle transition in metallic glasses?

There is evidence that metallic glasses can show increased plasticity as the temperature is lowered. This behaviour is the opposite to what would be expected from phenomena such as the ductile–brittle transition in conventional alloys. Data collected for the plasticity of different metallic–glass compositions tested at room temperature and below, and at strain rates from rate 10^?5 to 10³ s^?1, are reviewed. The analogous effects of low temperature and high strain rate, as observed in conventional alloys, are examined for metallic glasses. The relevant plastic flow in metallic glasses is inhomogeneous, sharply localised in thin shear bands. The enhanced plasticity at lower temperature is attributed principally to a transition from shear on a single dominant band to shear on multiple bands. The origins of this transition and its links to shear bands operating ‘hot’ or ‘cold’ are explored. The stress drop on a shear band after initial yielding is found to be a useful parameter for analysing mechanical behaviour. Schematic failure mode maps are proposed for metallic glasses under compression and tension. Outstanding issues are identified, and design rules are considered for metallic glasses of improved plasticity.     

Microstructure and superconductivity in annealed Cu-Nb-(Ti,Zr, Hf) ternary amorphous alloys obtained by liquid quenching

Melt-quenched Cu-Nb-(Ti, Zr, Hf) ternary alloys have been found to be amorphous possessing high strength and good bend ductility. The niobium content in the amorphous alloys was limited to less than 35 at % and the titanium, zirconium or hafnium contents from 25 to 50 at %. The Cu₄₀Nb₃₀(Ti, Hf)₃₀ alloys showed a superconducting transition above the liquid helium temperature (4.2 K) after annealing at appropriate temperatures. The highest transition temperatures attained were 5.6 K for the Cu₄₀Nb₃₀Ti₃₀ alloy annealed for 1 h at 873 K and 8.4 K for the Cu₄₀Nb₃₀Hf₃₀ alloy annealed for 1 h at 1073 K. In addition, these alloys exhibited upper critical magnetic fields of 1.8 to 2.3×10⁶ Am^–1 at 4.2 K and critical current densities of 2×10³ to 1×10⁴ A cm^–2 at zero applied field and 4.2 K. Since the structure of the superconducting samples consisted of ordered phases based on a b c c lattice with a lattice parameter of 0.31 nm, it was concluded that the superconductivity in the Cu₄₀Nb₃₀Ti₃₀ and Cu₄₀Nb₃₀Hf₃₀ alloys was due to the precipitation of the metastable ordered b c c phases.     

Thermophysical Properties of Five Binary Copper–Nickel Alloys

Thermophysical properties (e.g., specific enthalpy, heat of fusion, electrical resistivity, thermal volume expansion) are measured in the liquid phase up to very high temperatures by an extreme fast pulse-heating method. Heating rates of about 10⁸ K · s^?1 are applied by self-heating of wire-shaped metallic specimens with a current of approximately 10,000 A. Pure elements seem to be still close to thermal equilibrium as the obtained results are in good agreement with those obtained by static methods. However, this situation might be different for alloys. The rapid volume heating can shift diffusion-controlled phase transitions at heating to higher temperatures or even make them not noticeable anymore. The simple binary Cu–Ni system was chosen to test the heating rate dependence; this system is well known and shows complete miscibility in the liquid and solid ranges of interest. This study is a further step to test the performance of the fast pulse-heating method being applied to simple and more complex alloys. Measured results of enthalpy, heat of fusion, heat capacity, and electrical resistivity in the vicinity of the melting range are presented. The results of enthalpy and heat capacity agree with simple mixing rules. The measured electrical resistivity of different compositions is compared to results obtained by electromagnetic levitation measurements.     

Effect of Rare Earth and Transition Metal Elements on the Glass Forming Ability of Mechanical Alloyed Al–TM–RE Based Amorphous Alloys

The present work aims to compare the amorphous phase forming ability of ternary and quaternary Al based alloys(Al86Ni8Y6, Al86Ni6Y6Co2, Al86Ni8La6 and Al86Ni8Y4.5La1.5) synthesized via mechanical alloying by varying the composition, i.e. fully or partially replacing rare earth(RE) and transition metal(TM) elements based on similar atomic radii and coordination number. X-ray diffraction and high resolution transmission electron microscopy study revealed that the amorphization process occurred through formation of various intermetallic phases and nanocrystalline FCC Al. Fully amorphous phase was obtained for the alloys not containing lanthanum, whereas the other alloys containing La showed partial amorphization with reappearance of intermetallic phases attributed to mechanical crystallization. Differential scanning calorimetry study confirmed better thermal stability with wider transformation temperature for the alloys without La.     

Magnetothermal Analysis of Nanocrystallization of Amorphous and Relaxations of Nanocrystalline Materials

For a few years it has been realized that nanocrystalline phases can be formed during crystallization of amorphous alloys annealed isothermally below the crystallization temperature of usual heating experiments. Data of this transformation monitored by the measurement of magnetic susceptibility are presented. A method using a magnetic balance with electronic stabilisation and combined computer facilities is applied. Constant heating and cooling rates as well as isothermal heat treatments are used. Magnetic measurements are able to detect the onset of the transformation of amorphous Ni-P alloys much earlier than was possible with differential scanning calorimetry. The transformation kinetics can be analyzed by means of the Avrami plot based on the Johnson-Mehl-Avrami equation.The kinetics of solid state reactions in the nanostructured material can be investigated similarly. Formation of a Ni-phase in a nanostructured Hf-Ni alloy could be detected in a very early stage, where calorimetric methods are not sensitive. Segregation phenomena could be detected from the experiments even after long time. The sensitivity of the applied method is not dependent on the heating rate as the sensitivity of scanning calorimetry is     

Superconducting properties of amorphous Zr-Ge binary alloys

A new type of refractory metal-metalloid amorphous alloys exhibiting superconductivity has been found in a binary Zr-Ge system by a modified melt-spinning technique. Specimens are in the form of continuous ribbons 1 to 2 mm wide and 0.02 to 0.03 mm thick. The germanium content in the amorphous alloys is limited to the range of 13 to 21 at%. These amorphous alloys are so ductile that no cracks are observed even after closely contacted bending test. The Vickers hardness and crystallization temperature increase from 435 to 530 DPN and from 628 to 707 K, respectively, with germanium content, and the tensile fracture strength is about 1460 MPa. Furthermore, the amorphous alloys exhibit a superconducting transition which occurs very sharply. The superconducting transition temperature (T _c) increases with decreasing germanium content and reaches a maximum value of 2.88 K for Zr₈₇Ge₁₃. The upper critical magnetic field for Zr₈₇Ge₁₃ alloy was of the order of 21.8 kOe at 2.0 K and the critical current density for Zr₈₅Ge₁₅ alloy was about 175 A cm^–2 at 1.70 K in the absence of an applied field. The upper critical field gradient atT _c and the electrical resistivity at 4.2 K increase significantly from 24.6 to 31.5 kOe K^–1 and from 235 to 310cm, respectively, with the amount of germanium. The Ginzburg-Landau (GL) parameter and the GL coherence length §_GL (0) were estimated to be 72 to 111 and about 7.9 nm, respectively, from these experimental values by using the Ginzburg-Landau-Abrikosov-Gorkov (GLAG) theory and hence it is concluded that the Zr-Ge amorphous alloys are extremely soft type-II superconductor with high degree of dirtiness which possesses theT _c values higher than zirconium metal, in addition to high strength combined with good ductility.     

High resolution electron microscopy of alloys

High resolution electron microscopy (HREM) has emerged as a very powerful tool for probing the structure of metals and alloys. It has not only helped in unravelling the structure of materials which have been at the forefront of novel materials development such as quasicrystalline phases and high temperature superconducting compounds, but also is fast becoming a technique for solving some outstanding issues in the case of the commercial alloys thereby helping alloy development. In addition to the determination of the structures of phases, this tool is used for obtaining a first hand information of the arrangement of atoms around the various types of crystallographic defects and interphase interfaces. This mode of microscopy allows direct observation of orientation relationships between two phases across interfaces. HREM can be used for the direct examination of the prenucleation process. Initial stages of nucleation can also be studied readily in amorphous alloys, precipitation hardening alloys like maraging steels and in those systems where the formation of the omega phase occurs. This presentation describes some results of HREM studies on various alloys, commercial as well as alloys of scientific interest, where some of the aforementioned aspects have been examined. The specific examples cited pertain to metallic glasses, NiTi shape memory alloys, Ni-Mo, Zr-Nb and Ti-Al alloys.     

Study on the Crystallization of Amorphous Cr-Si-Ni Thin Films Using in situ X-ray Diffraction

Crystallization behavior of amorphous Cr-Si-Ni thin films was investigated by means of high temperature in situ X-ray diffraction measurements. The diffraction spectra were recorded isothermally at temperature between 250 and 750 degreesC. The in situ testing of crystallization enables the direct observation of structure evolution which is dependent on heat treatment. Based on the testing results, the grain sizes of the crystalline phases were compared and phase transition tendency was understood. In the mean time, electrical properties of the films as functions of annealing temperature and time have been studied. The increase of volume fraction of CrSi2 crystalline phases in the Cr-Si-Ni films leads to the decrease in conductivity of the films. The annealing behavior of temperature coefficient of resistance (TCR) is a result of competition between a negative contribution caused by the weak localization effects in amorphous region and a positive contribution caused by CrSi2 grains. Thus the proper mixture of amorphous and crystalline constituents could result in a final zero TCR.     

On the structure and transformation of the orange form of Sb2S3 layers

X-ray diffraction study showed that the mange modification of antimony trisulphide can neither be considered as amorphous material nor does it change into the black modification upon grinding. Thin vacuum-deposited layers, prepared by conventional thermal evaporation of the bulk material in 5 × 10^–6 torr on an amorphous substrate at room temperature, were crystalline. The behaviour of the electrical conductivity with temperature in the range 25 to 190° C indicated transition points at 80, 108, 135 and 175° C.     

宽过冷液相区铁基非晶合金的形成和磁性

用熔体急冷法制备出具有明显的玻璃转变和较宽的过冷液相区的Fe-Co-(Nb)-Zr-B非晶合金,研究了热稳定性和软磁性能。结果表明,在Fe-Co-Zr-B四元合金中添加适量的Nb可以显著扩大过冷液相区,提高合金的热稳定性。当冷却速率降低时,急冷合金具有非晶和纳米晶的复相结构。非晶合金的饱和磁化强度随Nb含量的增加而减小。不同Nb含量的非晶合金的饱和磁致伸缩系数均较低。在低于晶化温度的温度下退火可以有效地降低矫顽力,改善软磁性能。晶化导致软磁性能降低。     

ESR study of the equimolar PbO-V2O5 system

The semiconducting system 50 mol% PbO-V₂O₅ has been studied by electron spin resonance (ESR) in the temperature range 150 to 675 K. Depending on the cooling rate, this system can be obtained from the melt as an amorphous phase, as polycrystalline leadmetavanadate (PbV₂O₆), or as two different mixtures containing polycrystalline and amorphous phases. In all these materials the ratio V⁴⁺/V_tot has been measured by wet analysis showing that the V⁴⁺ concentration is nearly constant. It has been possible to show that the appearance of very different ESR line-shapes for the various materials is due to the difference in the hopping rate of the charge carriers. In particular the presence of a well-defined hyperfine structure is characteristic of the amorphous phase, in which the mobility of the charge carriers is lowered by the presence of a termW _D in the expression of the hopping rate. The lead-metavanadate ESR spectrum shows the presence of two lines of different widths; this fact has been explained assuming the presence of two distinct jumping rates. No temperature variation of the spectra has been observed at low temperatures. The high temperature spectra have permitted us to follow the thermal evolution of the system.