Antiferromagnetic order and the structural order–disorder transition in the Cd6Ho quasicrystal approximant

It has generally been accepted that the orientational ordering of the Cd₄ tetrahedron within the Cd₆ R quasicrystal approximants is kinetically inhibited for R?=?Ho, Er, Tm and Lu by steric constraints. Our high-resolution X-ray scattering measurements of the Cd₆Ho quasicrystal approximant, however, reveal an abrupt (first-order) transition to a monoclinic structure below T _S?=?178?K for samples that have ‘aged’ at room temperature for approximately one year, reopening this question. Using X-ray resonant magnetic scattering at the Ho L ₃-edge we have elucidated the nature of the antiferromagnetic ordering below T _N?=?8.5?K in Cd₆Ho. The magnetic Bragg peaks are found at the charge forbidden H?+?K?+?L?=?2n?+?1 positions, referenced to the high-temperature body-centred cubic structure. In general terms, this corresponds to antiferromagnetic arrangements of the Ho moments on adjacent clusters in the unit cell as previously found for Cd₆Tb.     

Experimental determination and molecular dynamics simulation of specific heat for high temperature undercooled liquid

The accurate data of specific heat are of great significance in the thermodynamic analysis of phase transformation. Here, we report the experimental and calculated specific heat of liquid Ni_90.1Si_9.9 alloy by electromagnetic levitation drop calorimeter and molecular dynamics calculation, respectively. The experimental result, 39.54 J mol^?1 K^?1, is in good agreement with the calculated value of 38.03 J mol^?1 K^?1, and there exists only a little difference between them. The calculated undercooling range of 0 ～ 623 K is much wider than the experimental range of 0 ～ 218 K. The undercooled liquid structure of this alloy is analyzed according to the total and partial pair distribution functions, which show that the in-order degree of atom distribution enhances from normal liquid to undercooled liquid. The Si atoms are homogeneously dispersed among the Ni atoms, whereas the average distance among Ni atoms changes little.     

Thermophysical properties of substantially undercooled liquid Ti-Al-Nb ternary alloy measured by electromagnetic levitation

The thermophysical properties of undercooled liquid alloys at high temperature are usually difficult to measure by experiment. Here, we report the specific heat of liquid Ti₄₅Al₄₅Nb₁₀ ternary alloy in the undercooled state. By using electromagnetic levitation technique, a maximum undercooling of 287?K (0.15 T _L) is achieved for this alloy. Its specific heat is determined to be 32.72?±?2.51?J?mol^?1 K^?1 over a broad temperature range of 1578–2010?K.     

Microstructural evolutions and hardness during heat treatment of Al64Cu20Fe12Si4 quasicrystal alloy

The microhardness and microstructural characteristics and subsequent heat treatment of conventionally solidified Al₆₄Cu₂₀Fe₁₂Si₄ quasicrystal were investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM) together with energy dispersive spectroscopy (EDS), differential thermal analysis (DTA), and Vickers microhardness tester. XRD analysis indicated that the conventionally solidified samples showed a quasicrystalline icosahedral phase (i-phase) together with cubic β-AlFe, tetragonal θ-Al₂Cu, and monoclinic λ-A₁₃Fe₄ crystal phases. However, the i-phase together with cubic β-AlFe and monoclinic λ-A₁₃Fe₄ phases observed heat threaded samples. As-cast and subsequently heat-treated quasicrystal samples were measured using a microhardness test device. Vickers microindentation tests were carried out on the heat-treated quasicrystal samples with the load ranging from 1 to 500?mN at room temperature. The melting point of the i-phase was determined as 900°C by DTA examinations.     

Effect of cooling rate on size-dependent atomic ordering of CoPt nanoparticles

We report on the effect of cooling rate on the size-dependent atomic ordering of CoPt nanoparticles using aberration corrected high-resolution transmission electron microscopy. It was found that cooling rate plays a crucial role in promoting atomic ordering during the cooling process after annealing. Nanoparticles of ≈3?nm in diameter show the A1-disordered phase after annealing at 873?K for 1?h followed by rapid cooling (110?K/min), while the L1₀-ordered phase is obtained when the cooling rate is slow (1.5?K/min). The disordered phase is also obtained by rapid cooling after annealing at 973?K for 1?h. These results unambiguously indicate that the order–disorder transformation temperature is reduced to a temperature at least lower than 873?K for CoPt nanoparticles smaller than 3?nm in diameter. The slow cooling process promotes the atomic ordering, which resulted in an enhancement of magnetic coercivity as high as 2200?Oe. This study demonstrates that hard magnetic properties of the CoPt nanoparticles can be improved by controlling the cooling rate after heat treatments.     

Specific heat,enthalpy, and density of undercooled liquid Fe–Si–Sn alloy

The specific heat of liquid Fe₉₀Si_6.95Sn_3.05 alloy has been investigated by electromagnetic levitation drop calorimetery in the temperature range of 1390–2140 K. The enthalpy of this liquid alloy increases linearly with the rise of temperature. Furthermore, the enthalpy obtained from molecular dynamics calculation shows a similar trend to the experimental results in a broader temperature range of 1000–2200 K. The calculated specific heat is 39.7 J mol^?1 K^?1, which agrees well with the experimental result of 39.9 J mol^?1 K^?1. The density of this liquid alloy decreases as a quadratic function of temperature.     

Surface tension of substantially undercooled liquid Ti–Al alloy

It is usually difficult to undercool Ti–Al alloys on account of their high reactivity in the liquid state. This results in a serious scarcity of information on their thermophysical properties in the metastable state. Here, we report on the surface tension of a liquid Ti–Al alloy under high undercooling condition. By using the electromagnetic levitation technique, a maximum undercooling of 324 K (0.19 T _L) was achieved for liquid Ti-51 at.% Al alloy. The surface tension of this alloy, which was determined over a broad temperature range 1429–2040 K, increases linearly with the enhancement of undercooling. The experimental value of the surface tension at the liquidus temperature of 1753 K is 1.094 N m^?1 and its temperature coefficient is ?1.422 × 10^?4 N m^?1 K^?1. The viscosity, solute diffusion coefficient and Marangoni number of this liquid Ti–Al alloy are also derived from the measured surface tension.     

Formation of metastable phase and its influence on the solidification microstructure of highly undercooled eutectic Fe40Ni40B20 alloy

Solidification of a highly undercooled eutectic Fe₄₀Ni₄₀B₂₀ alloy melt has been studied by high-speed video in combination with an analysis of the temperature history. The metastable phase with a colony shape structure solidified primarily from the melt. DSC analysis confirmed the formation of the metastable phase. The metastable phase was re-melted, transformed or decomposed into stable phases during further solidification or cooling processes, and gave birth to the final as-solidified structures. On this basis, the effects of the metastable phase formation and transformation on evolution of the as-solidified structure is described.     

Synthesis of a La-based bulk metallic glass matrix composite

A bulk metallic glass matrix composite, that is a crystalline phase in an amorphous matrix, has been successfully synthesized by chill casting a La-based quaternary La₆₆Al₁₄Cu₁₀Ni₁₀ alloy. A composite rod as large as 12?mm in diameter was obtained. The reinforcing crystalline phase was identified as α-La, which was uniformly distributed and well developed throughout the sample. The critical cooling rate for the formation of the composite was determined by the Bridgman solidification process to be below 15?K?s^?1. Increased thermal stability was observed for the composite compared with the fully glassy rods, the reason for which remains unclear.     

Icosahedral quasicrystal in annealed ZnMgSc alloys

A new icosahedral quasicrystal has been found in Zn 85-x Mg x Sc 15 ( x = 3,4,5) alloys annealed at 922K for 5-100h. Electron diffraction and powder X-ray diffraction experiments indicate that this quasicrystal has a primitive icosahedral quasilattice (P type) and a six-dimensional lattice parameter a 6D =0.7115nm. The stoichiometric composition of the icosahedral quasicrystal was estimated to be close to Zn 80 Mg 5 Sc 15, since the icosahedral quasicrystal coexists in the x = 5 samples with small amounts of a MgZn 2 -type Laves phase and a Zn 17 Sc 3 -type cubic phase with lattice parameter a =1.3854nm. The latter is interpreted to be a 1/1-type approximant crystal of the icosahedral quasicrystal. The atomic cluster included in the Zn 17 Sc 3 -type cubic phase indicates a structural similarity between the Zn-Mg-Sc quasicrystal and the Cd-Yb (and Cd-Ca) icosahedral quasicrystals recently reported.     

Electronic and vibrational properties of an Al-Cu-Ru icosahedral quasicrystal deduced from heat capacities

Heat capacity measurements have been made in the temperature range 1-310 K for a phasonless perfect quasicrystal Al65Cu20Ru15, an Al65Cu20Ru15 quasicrystal with phason disorder, and a crystal approximant Al70Cu20Ru10. The electronic heat capacity of the perfect quasicrystal was found to be essentially zero. The existence of some excitation associated with an energy of about 1.6 meV is proposed. The vibrational properties of the quasicrystal are distinctly different from those of the crystal approximant in the region T < ThetaD/10, indicating that the densities of states are significantly different at the corresponding energies.     

Surface tension measurement of undercooled liquid Ni-based multicomponent alloys

The surface tensions of liquid ternary Ni–5%Cu–5%Fe, quaternary Ni–5%Cu–5%Fe–5%Sn and quinary Ni–5%Cu–5%Fe–5%Sn–5%Ge alloys were determined as a function of temperature by the electromagnetic levitation oscillating drop method. The maximum undercoolings obtained in the experiments are 272 (0.15T _L), 349 (0.21T _L) and 363?K (0.22T _L), respectively. For all the three alloys, the surface tension decreases linearly with the rise of temperature. The surface tension values are 1.799, 1.546 and 1.357?N/m at their liquidus temperatures of 1719, 1644 and 1641?K. Their temperature coefficients are ?4.972?×?10^–4, ?5.057?×?10^?4 and ?5.385?×?10^?4?N/m/K. It is revealed that Sn and Ge are much more efficient than Cu and Fe in reducing the surface tension of Ni-based alloys. The addition of Sn can significantly enlarge the maximum undercooling at the same experimental condition. The viscosity of the three undercooled liquid alloys was also derived from the surface tension data.     

Interplay of diffusion and phase transformations in microstructural evolution of the interaction zone between U–9 wt.% Mo and Zr–1 wt.% Nb alloys

Metallurgical interaction between U–9?wt.% Mo metallic fuel alloy and Zr–1?wt.% Nb clad material has been assessed. Interdiffusion of constituent elements across their interface, together with the phase reactions occurring at high temperature and during subsequent cooling, resulted in development of a layered interaction zone where coexistence of a bcc solid solution phase with varying compositions, along with α-U, α-Zr and Mo₂Zr phases could be noticed.     

Synthesis,transformation and superconductivity of dual phase In–Sn alloy nanoparticles embedded in an Al matrix

We report the synthesis of nanoembedded biphasic alloy particles of In–Sn near eutectic alloy compositions embedded in an aluminium matrix by rapid solidification processing. Detailed transmission electron microscopy indicates that the two phases present at room temperature in as-synthesized samples are β and γ phases with tetragonal and hexagonal crystal structures, respectively. These co-exist with a small amount of single phase In or Sn particles with sizes less than 10?nm. Low temperature magnetization measurements indicate a superconducting transition temperature of 5?K, suggesting complete decomposition of the β-phase at small sizes and at low temperature. The small particles show type II behavior with a critical field Hc₁≈44?G and two values for Hc₂ of 250 and 1000?G, respectively. These values are considerably lower than those observed in bulk In–Sn alloys.     

Giant electrocaloric effect of highly (1 0 0)-oriented 0.68PbMg1/3Nb2/3O3–0.32PbTiO3 thin film

PbZr_0.95Ti_0.05O₃ thin film has the highest electrocaloric properties of all the oxide thin films (0.48?K?V^?1). Here, it is shown giant electrocaloric properties in 200?nm (1?0?0)-oriented PMN–PT 68/32 film near the ferroelectric Curie temperature of 146?°C. The results indicate the significance of this system to achieve electrocaloric entropy change and temperature change, up to 32.21?J/kg?°C and 14?K, respectively, in 12?V (i.e. 1.155?K?V^?1) near the Curie point. This exceeds the previous best results obtained in PbZr_0.95Ti_0.05O₃ thin film.     

Pressure dependence of the electrical resistivity of natural cassiterite SnO2 and of undoped and Co-doped nanocrystalline SnO2

The pressure dependence of the electrical resistivity of three different samples of cassiterite, namely natural cassiterite SnO₂, synthetic nanocrystalline SnO₂ (with crystallite size 46?nm) and nanocrystalline Co-doped SnO₂ (with crystallite size 32?nm), has been measured up to 7?GPa at room temperature. The resistivity of natural cassiterite SnO₂ decreases from 2.5?×?10⁴?Ωm at normal pressure and temperature to 1.7?×?10⁴?Ωm at 7.0?GPa. The nanocrystalline SnO₂ has a high resistivity 6.0?×?10⁵?Ωm at normal pressure and temperature and decreases with pressure reaching a value of 2.98?×?10⁵?Ωm at 7?GPa. The activation energy of the electrical conduction of the studied samples were found to be 0.32?eV for the natural SnO₂, 0.40?eV for the nanocrystalline SnO₂ sample and 0.28?eV for the nanocrystalline Co-doped SnO₂. Measurements of the pressure dependence of the electrical resistivity of the Co-doped SnO₂ showed a decrease from 3.60?×?10⁵ to 5.4?×?10⁴?Ωm at 7.0?GPa. We did not observe any pressure-induced phase transition in SnO₂ up to 7?GPa. This study of the high-pressure phase stability of cassiterite corroborates the experimental findings of SnO₂ nanoinclusions in diamonds.     

Absorption and photoluminescence spectroscopy of Er3+-doped SrAl2O4 ceramic phosphors

A spectroscopic characterization of Er³⁺-doped SrAl₂O₄ phosphor materials synthesized by a solid-state reaction method with Er concentrations varying from 0.1 to 1?mol% has been performed by studying photoluminescence (PL) in the temperature range 10 to 360?K and absorption spectra. PL signals containing five emission bands at 1492, 1529, 1541, 1558, and 1600?nm, respectively, have been observed at room temperature for Er³⁺ transitions in the near infrared region. The samples exhibit a main luminescence peak at 1.54?µm, which is assigned to recombination via an intra-4f Er³⁺ transition. Sharp bands centered at around 378, 488, 521, 651, 980, 1492, and 1538?nm in the absorption spectra can be associated with transitions from ⁴I_15/2 level to ²H_9/2, ⁴F_7/2, ²H_11/2, ⁴F_9/2, ⁴I_11/2, ²H_11/2, and ⁴I_13/2 levels, respectively. The sharp emission peaks and excellent luminescence properties show that SrAl₂O₄ is a suitable host for rare-earth-doped phosphors, which may be suitable for optical applications.     

Importance of chamber gas pressure on processing of Al-based metallic glasses during melt spinning

The effect of chamber gas pressure on the amorphicity of Al₈₅Ni₅Y₁₀ alloy was studied for the melt-spinning process. The amorphicity of as-quenched ribbons was characterized by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The chamber atmosphere pressure is crucial to the cooling rate of melt spinning. At high vacuum, at pressure less than 0.001?atm, fully crystalline fragments are obtained. Monolithic amorphous ribbons were only obtained at a gas pressure of 0.1?atm, 0.2?atm or higher. The extended contact length between ribbons and the copper wheel contributes to the high cooling rate of melt spinning in Al-based glass forming alloys; that is supported by images recorded by a high-speed camera. Higher chamber pressure increases contact length between ribbons and the wheel, which is qualitatively elucidated by Bernoulli's equation.     

Nucleation of He bubbles in amorphous FeBSi alloy irradiated with He ions

It is interesting to investigate the formation of He bubbles in amorphous alloys because point defects do not exist in amorphous materials. In the present study, the microstructural evolution of amorphous Fe₇₉B₁₆Si₅ alloy, either irradiated with 5?keV He⁺ ions or implanted with 150?eV He⁺ ions without causing displacement damage, and then annealed at a high temperature, was investigated using transmission electron microscopy (TEM). Vacancy-type defects were formed in the amorphous alloy after irradiation with 5?keV He⁺ ions, and He bubbles formed during annealing the irradiated samples at high temperature. On the other hand, for samples implanted with 150?eV He⁺ ions, although He atoms are also trapped in the free volume, no He bubbles were observed during annealing the samples even up to 873?K. In conclusion, the formation of He bubbles is related to the formation and migration of vacancy-type defects even in amorphous alloys.     

Reaction of a Ni-coated Al nanoparticle to form B2-NiAl: A molecular dynamics study

The kinetic reaction in a Ni-coated Al nanoparticle with equi-atomic fractions and diameter of approximately 4.5 nm is studied by means of molecular dynamics simulation, using a potential of the embedded atom type to model the interatomic interactions. First, the large driving force for the alloying of Ni and Al initiates solid state amorphization of the nanoparticle with the formation of Ni₅₀Al₅₀ amorphous alloy. Amorphization makes intermixing of the components much easier compared to the crystalline state. The average rate of penetration of Ni atoms can be estimated to be about two times higher than Al atoms, whilst the total rate of inter-penetration can be estimated to be of the order of 10^?2 m/s. The heat of the intermixing with the formation of Ni₅₀Al₅₀ amorphous alloy can be estimated at approximately ?0.34 eV/at. Next, the crystallization of the Ni₅₀Al₅₀ amorphous alloy into B2-NiAl ordered crystal structure is observed. The heat of the crystallization can be estimated as approximately ?0.08 eV/at. Then, the B2-NiAl ordered nanoparticle melts at a temperature of approximately 1500 K. It is shown that, for the alloying reaction in the initial Ni-coated Al nanoparticle, the ignition temperature can be as low as approximately 200 K, while the adiabatic temperature for the reaction is below the melting temperature of the nanoparticle with the B2-NiAl ordered structure.