Thermal expansion of semiconducting silicides β-FeSi2 and Mg2Si

The thermal expansion of β-FeSi₂ and Mg₂Si was investigated at high temperatures (ranging from 300 to 1173 K for β-FeSi₂ and from 293 to 873 K for Mg₂Si) using powder X-ray diffraction. The linear thermal expansion coefficients α_L for the three lattice parameters of β-FeSi₂ range from 10.6(2) to 11.8(4) × 10⁻⁶ K⁻¹, which indicates small anisotropy, which is in contrast to the large anisotropy reported previously. The volumetric thermal expansion coefficient α_V for β-FeSi₂ is relatively large among the transition-metal disilicides. α_L for Mg₂Si can be expressed by the linear expression of T: α_L = 11(1) × 10⁻⁶ + 6.9(2) × 10⁻⁹T K⁻¹. α_V for Mg₂Si is larger than that of the transition-metal disilicides, including β-FeSi₂. Based on a comparison of α_L among Mg₂Si, several metals and silicides, the candidates for electrode materials are discussed. In particular, temperature dependence and value of α_L for Ni is close to those for Mg₂Si, which suggests that Ni is a good candidate electrode material with respect to thermal expansion.     

Influence of alloy composition and thermal history on carbide precipitation in γ-based TiAl alloys

Carbide precipitation in TiAl alloys with different alloying element additions and thermal history was investigated by transmission electron microscopy and high energy X-ray diffraction. The results reveal that Nb addition in TiAl alloys does not increase the carbon solubility in the γ matrix significantly. With increasing carbon concentration in the alloys, a splitting of the P-type carbides takes place earlier in the course of ageing at 800 °C and the formation of H-type carbides is promoted. For a nearly single γ phase alloy Ti51Al5Nb0.5C, H-type carbides are the thermodynamically stable phase. However, with decreasing Al concentration, P-type carbides become thermodynamically stable at low carbon concentration (below 1%) in Ti45Al and Ti45Al5Nb alloys. It is feasible to achieve high thermodynamical stability of the P-type carbides in TiAl alloys through controlling alloying elements.     

Influence of Y and Al co-doping on the crystal structure and magnetic properties of Nd2−xYxFe17−yAly

The synergic effects of Y and Al co-doping on the structural and magnetic properties of Nd_2−xY_xFe_17−yAl_y compounds (0 ≤ x ≤ 1.5, 0 ≤ y ≤ 3.0) have been comprehensively investigated by means of X-ray diffraction, neutron diffraction and magnetic measurement. Rietveld refinements indicate that all the prepared samples crystallize in Th₂Zn₁₇-type structure ( space group). For a given Al content (y), the lattice parameter a, c and unit cell volume V of Nd_2−xY_xFe_17−yAl_y all decrease linearly with increasing Y concentration while the c/a ratio increases. For a given Y content (x), the lattice parameter a, c and unit cell volume V of Nd_2−xY_xFe_17−yAl_y all increase linearly with increasing Al content. Al atoms prefer to take 18h sites and completely avoid the 9d sites. For a given Al content, M_s of Nd_2−xY_xFe_17−yAl_y first increases to a maximum and then decreases with increasing Y content. This is quite different from what's observed in other mixed rare earth systems. For a given Y content, T_C of Nd_2−xY_xFe_17−yAl_y first increases rapidly and then increases slowly with increasing Al content but M_s first increases and then decreases. This can be attributed to the competition between the positive effect resulting from the optimization of bond lengths and the negative effect caused by magnetic dilution of nonmagnetic substituent.     

Characterization of complex planar faults in FeAl(B) alloys

Complex planar faults were observed by diffraction contrast transmission electron microscopy in a B2 FeAl based alloy containing Ni and B. The comparison of experimental images to simulated ones revealed the detailed structure of these faults that lie on {001} planes with both in-plane and out-of-plane components of the displacement vector. It was deduced that these defects form by segregation of (B-Al vacancies) complexes on a<100> dislocations, leading to various defect structures depending on the screw or edge character of the dislocation.     

Correlation between the synthesis conditions and the compositional and magnetic properties of Co2(Cr1−xFex)Al Heusler alloys

In this study we give evidence for the strong dependence of the compositional and magnetic properties on the synthesis conditions of polycrystalline Co₂(Cr_1−xFe_x)Al Heusler alloys (0 ≤ x ≤ 1) by comparing the properties of as-grown and annealed compounds. Strong chemical inhomogeneities are found at the micrometric level depending on the compound and the synthesis method. Moreover, we find that the Co content is homogeneous at the micrometric level in all the studied samples in sharp contrast with significant inhomogeneous distribution of (Fe/Cr) and Al at the micrometric level, especially for Cr-rich compounds (x ≤ 0.4). We have found that the magnetic properties (the Curie temperature and the saturation magnetization) are strongly depressed in the annealed compounds with respect to the corresponding as-grown compounds. For the as-grown compounds the saturation magnetization is close to the theoretically predicted one for x ≥ 0.7 whereas it is lower than the theoretically predicted one for x ≤ 0.4, which correlates with the observed chemical inhomogeneity.     

Mixed site occupancies in the μ phase

The μ phase has been studied in different systems (Ta–Ni, Mo–Co, Nb–Ni) as a function of the composition. In addition, its stability in the Mn–Si system has been investigated. The atom distribution on the five different sites of the crystal structure has been obtained from Rietveld refinement of X-ray powder diffraction data. These experimental data are compared to the values computed from first principles results and from existing Calphad assessments of the different systems. Conclusions are drawn concerning the model to be chosen for describing this phase.     

Microstructure of pulsed laser deposited FePd thin films on amorphous and crystalline substrates

Equiatomic FePd thin films have been deposited at room temperature by pulsed laser deposition (PLD) on amorphous, Si₃N₄ and SiO₂, and crystalline, (100)-NaCl, substrates. The resulting FePd film microstructures have been characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). We observed a complex orientation relationship between grains in nanocrystalline thin films of FCC FePd and the single crystal NaCl substrates. FePd films obtained under identical deposition conditions using the amorphous substrates also exhibited nanocrystalline morphology but with a fiber texture and consisted of a phase mixture of FCC FePd and the tetragonal ordered L1₀-FePd phase.     

Al–Ti–C–Sr master alloy—A melt inoculant for simultaneous grain refinement and modification of hypoeutectic Al–Si alloys

Present article is focused on the microstructural features of Al–Ti–C–Sr master alloy, an inoculant for simultaneous grain refinement and modification of hypoeutectic Al–Si alloys. This master alloy is basically a metal matrix composite consisting of TiC and Al₄Sr phases formed in situ in the Al-matrix. TiC particles initiate the refinement of primary α-Al through heterogeneous nucleation in molten hypoeutectic Al–Si alloy, while Al₄Sr phase dissolves in molten Al–7Si alloy enriching the melt with Sr, which eventually leads to modification of eutectic silicon during solidification of the Al–7Si alloy casting. Thus present master alloy serves in both ways, as a grain refiner and a modifier for hypoeutectic Al–Si alloys.     

Flow characteristics and constitutive modeling for elevated temperature deformation of a high Nb containing TiAl alloy

The hot deformation behavior of a high Nb containing TiAl alloy with a nominal composition of Ti–42Al–8Nb–(W, B, Y) was investigated at temperatures ranging from 1000 °C to 1150 °C and strain rates from 10⁻³ s⁻¹ to 0.5 s⁻¹ on a Gleeble thermo-simulation machine. The work hardening regime and flow softening behavior of the alloy were analyzed in detail. The results revealed that the onset of dynamic recrystallization (DRX) was quite easy for the present alloy, whereas the dynamic recovery (DRV) was impeded during the hot deformation. The DRX kinetics was studied by Avrami-type equation. The low Avrami exponents of the proposed equation indicate a lower recrystallization rate compared to ordinary metals and alloys. Based on the classical hyperbolic-sine law and the kinematics of DRX, the constitutive equations of the work hardening-recovery period (i.e. flow stress before the peak) and flow softening process (i.e. flow curve after the peak stress) were established for the present alloy, respectively. Comparisons between the experimental and calculated results revealed that except the severely cracked specimens, the stress–strain curves predicted by the established model are in good agreement with experimental results.     

Microstructure and chemical characterization of the intermetallic phases in Cu/(Sn,Ni) diffusion couples with various Ni additions

The paper presents new results concerning the influence of nickel addition (1 and 5 at.%) into tin on the development of the Cu/(Sn,Ni) interface area in diffusion couple experiment. The morphology and chemical composition of the intermetallic phases growing in the Cu/(Sn,Ni) diffusion couples were examined by means of the scanning (SEM) and transmission (TEM) electron microscopy after annealing at 215 °C in vacuum for different period time.It was shown that even 1 at.% of nickel addition into tin resulted in formation of intermetallics of complex microstructure. The presence of (Cu_1−xNi_x)₆Sn₅ in two morphological and compositional variants was noted. The discontinuous layer consisting up to 7.2 at.% of Ni closer to copper end-member coexisted with needle-like and faceted precipitates with even 22.3 at.% of Ni, which intensively detached from the interface. At the Cu/(Cu_1−xNi_x)₆Sn₅ interface the formation of Cu₃Sn wavy layer compound was observed in all examined diffusion couples which became thicker with time. The porosity within the both formed intermetallic phases existed irrespective of the amount of added nickel.     

Experimental study of phase transformations in 3003 aluminium alloys during heating by in situ high energy X-ray synchrotron radiation

This paper aims to study microstructural evolutions in the 3003 aluminium alloys during the heating part of the homogenization treatment from the as-cast state. After solidification, both the solid solution and primary precipitates are far from equilibrium. During a subsequent heating, an eutectoid transformation of the primary eutectic particles and a fine precipitation of dispersoids occur. The evolution of the precipitation amounts are obtained by high energy synchrotron radiation diffraction and microstructural observations (TEM and SEM) combined with image analysis. The analysis of these different results lead to the volume fraction evolutions of dispersoids and primary particles with temperature. Moreover, the different evolutions are compared to in situ electrical resistivity leading to propose the main causes of the variations observed.     

Optimisation of precipitation for controlling recrystallisation of wrought Fe3Al based alloys

A Fe–26Al–5Cr (at.%) single-phase (:A2/B2/D0₃) alloy and two-phase (+TiC) alloys with different amounts of TiC particles have been hot rolled at 800 °C and the kinetics of static recrystallisation have been studied. In the alloys with a high amount of TiC, needle-like TiC of more than 1 μm in length formed during cooling after homogenisation in the single-phase region and coarsened during hot rolling. The large particles cause particle stimulated nucleation (PSN) and hence accelerate recrystallisation. In order to accomplish both strengthening by precipitates and inhibition of recrystallisation that deteriorates room-temperature ductility, a thermo-mechanical treatment consisting of hot deformation with a low amount of precipitates and a subsequent heat treatment for further precipitation is proposed. This process is difficult to carry out in the (Fe–26Al–5Cr)–TiC system due to the high precipitation temperature of TiC. The precipitation temperature is significantly decreased by replacing TiC by VC or MoC.     

Local chemical and topological order in Al-Tb and its role in controlling nanocrystal formation

How the chemical and topological short- to medium-range order develops in Al-Tb glass and its ultimate effect on the control of the high number density of face-centered-cubic-Al (fcc-Al) nuclei during devitrification are described. A combined study using high-energy X-ray diffraction (HEXRD), atom probe tomography (APT), transmission electron microscopy and fluctuation electron microscopy (FEM) was conducted in order to resolve the local structure in amorphous Al₉₀Tb₁₀. Reverse Monte Carlo simulations and Voronoi tessellation analysis based on HEXRD experiments revealed a high coordination of Al around Tb atoms in both liquid and amorphous states. APT results show Al-rich and Al-depleted regions within the as-quenched alloy. A network structure of Tb-rich clusters divides the matrix into nanoscale regions where Al-rich clusters are isolated. It is this finely divided network which allows the amorphous structure to form. Al-rich regions are the locus for fcc-Al crystallization, which occurs before the intermetallic crystallization. FEM reveals medium-range ordered regions ∼2 nm in diameter, consistent with fcc-Al and trigonal-like Al₃Tb crystal structures. We propose that the high coordination of Al around Tb limits diffusion in the intermetallic network, allowing for the isolated Al-rich regions to form at high density. These regions are responsible for the extremely high density of Al nanocrystal nuclei.     

Evolution and stability of phases in a high temperature shape memory alloy Ni49.4Ti38.6Hf12

Ni_49.4Ti_38.6Hf₁₂ shape memory alloy has been characterized for structure, microstructure and transformation temperatures. The microstructure of the as-cast sample consists of B19′ and R-phases, and (Ti,Hf)₂Ni precipitate phase along the grain boundaries in the form of dendrites. The microstructure of the solution treated sample contains only B19′ martensite phase, whereas a second heat treatment after solutionizing results in reappearance of the R-phase and the (Ti,Hf)₂Ni grain boundary precipitate phase in the microstructure. A detailed microstructural examination shows the presence of precipitates having both coherent and incoherent interface with the matrix, the type of interface being dictated by the crystallographic orientation of the matrix phase. The present study shows that the (Ti,Hf)₂Ni precipitates having coherent interface with the matrix, drive the formation of the R-phase in the microstructure.     

Role of thermal stability and vapor pressure of bis(2,2,6,6-tetramethyl-3,5-heptanedionato)magnesium(II) and its triamine adduct in producing magnesium oxide thin film using a plasma-assisted LICVD process

Thin films of magnesia were deposited on various substrates using plasma-assisted liquid injection chemical vapor deposition with volatile Mg(tmhd)₂·2H₂O (1) (tmhd = 2,2,6,6-tetramethyl-3,5-heptanedione). The precursor complexes, Mg₂(tmhd)₄·(2), and Mg(tmhd)₂·pmdien (3) (pmdien; N,N,N′,N″,N″-pentamethyldiethylenetriamine) were prepared from Mg(tmhd)₂·2H₂O (1). The temperature dependence equilibrium vapor pressure (p_e)_T data yielded a straight line when log p_e was plotted against reciprocal temperature in the range of 360–475 K, leading to standard enthalpy of vaporization (Δ_vapH°) values of 59 ± 1 and 67 ± 2 kJ mol^− 1 for (2) and (3) respectively. Thin films of magnesium oxide were grown at 773 K using complex (1) on various substrate materials. These films were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray for their composition and morphology.     

Modelling and experimental study of impedance spectra of electron beam physical vapour deposition thermal barrier coatings

A 2-dimensional finite element model has been developed to calculate the impedance spectra of electron beam physical vapour deposition (EB-PVD) thermal barrier coatings (TBCs). The model has been used to examine the effect of thermally grown oxide (TGO) growth and the TGO conductivity change on impedance spectra of TBCs. According to modelling, different spectra were generated due to the TGO growth and TGO conductivity change. Impedance measurements have been carried out on both as-deposited and thermally treated TBCs where the thermal treatments lead to the TGO growth. In addition, the thermal treatment of TBCs at different temperatures produced TGO with different compositions, probably leading to different electrical conductivities of TGO. Measured impedance spectra of TBCs with different TGO thicknesses and TGO compositions agree with modelled spectra of TBCs with different TGO thicknesses and conductivities.     

Corrosion of aluminium and aluminium alloy in ethylene glycol–water mixtures

Samples of pure aluminium and aluminium alloy were heated at 150 kPa and 130 °C in ethylene glycol (EG)–water mixture (volume ratio 1:1) for 172 h (8–12 h heating, 12–16 h pause). The corrosion of the metals was characterised by the transfer of Al into solution, changes in the surface morphology of the samples and their electrochemical behaviour. Formation of a white crystalline aluminium-organic precipitate was found during the heating of metals and some of its characteristics were determined. The effect of the addition of glycolic, citric and sebacic acid to the EG–water mixture on the metal corrosion behaviour was investigated. A significant corrosive action of glycolic acid and a protective effect of sebacic acid were found.     

Preparation of cross sections of thermal spray coatings for TEM investigation

A technique for the preparation of cross sections for transmission electron microscopy (TEM) of thermal spray coatings has been developed. The procedure is designed to minimize specimen damage during mechanical thinning and to reduce the effect of differential thinning during ion milling. Specimens were made by two different coating systems— WC- Co coating produced by the FARE Gun process on a mild steel substrate and Tribaloy T- 800 sprayed by the HVOF process on a nickel- base superalloy. These specimens have large areas that are electron transparent on either side of the interface, and the results have shown the atomic scale microstructure of the interface between the thermal spray coating and the substrate.