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 ($R\overline{3}m$ 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.     

Wear behaviour of an FeAl intermetallic alloy containing carbon and titanium

FeAl based alloys with carbon and titanium additions were prepared using arc induction melting and their effect on wear behaviour was investigated using ball-on-disk technique. The experimental results showed that carbon addition to FeAl alloys results in formation of perovskite-type Fe₃AlC_0.5 carbide phase and graphite. Addition of Ti promotes the formation of TiC and Fe₃AlC_0.5 and prevents the formation of graphite in the alloy. Hardness and wear resistance of FeAl based alloys increase with increase in the volume fraction of carbides. The FeAl alloys containing Ti exhibited low wear rate and coefficient of friction. Examination of wear tracks revealed micro ploughing at a lower load of 5N. Thin surface flakes with traces of their detachment were observed at a higher load of 10N. It was also observed that presence of graphite in localized regions reduce the wear resistance of the alloy. The results are correlated with observed microstructure and hardness.     

Obtaining of a fine near-lamellar microstructure in TiAl alloys by Spark Plasma Sintering

This work presents a study of the Spark Plasma Sintering of a boron and tungsten containing alloy (Ti_49,92Al₄₈W₂B_0,08, called IRIS) as a function of the sintering temperature. Microstructures of sintered alloys are analyzed by scanning and transmission electron microscopies. Investigations mainly focus on a fine near-lamellar microstructure. Attention is paid to both characteristic dimensions of this microstructure and orientation relationships between various phases.The fine near-lamellar microstructure is formed by lamellar grains surrounded by extended γ zones containing β₀ precipitates. The size of lamellar grains ranges from 35 to 45 μm while the width of the borders remains between 5 and 10 μm. Effects of both boron addition and sintering temperature are studied. Orientation relationships between γ lamellae and γ grains in the borders, as well as between the γ matrix and the β₀ precipitates are investigated. As a result of these investigations, a formation mechanism of this microstructure is proposed and discussed. The origin of the grain growth limitation during the SPS processing is particularly analyzed.     

Pre-martensitic phenomena in Ti40.7Hf9.5Ni44.8Cu5 shape memory alloy

Pre-martensitic phenomena such as abnormal resistivity growth, diffusion scattering, “tweed” contrast and internal friction peak were observed in Ti_40.7Hf_9.5Ni_44.8Cu₅ alloy prior to the forward martensitic transformation on cooling. It was shown that all the observed phenomena were due to the formation of quasi-static strain nanodomains in the B2 phase prior to the forward martensitic transformation. This led to accumulation of the elastic energy before the phase transition and resulted in the variation in thermodynamic balance for the forward martensitic transformation and, as a result, influenced the parameters of the phase transition. The appearance of elastic energy prior to the forward transformation caused a decrease in the forward and reverse martensitic transformations' start temperatures, a widening of the temperature range of the reverse transformation and an increase in the hysteresis of the transformation.     

Effects of microstructure and C and Si additions on elevated temperature creep and fatigue of gamma TiAl alloys

The creep properties of K5 (Ti-46Al-3Nb-2Cr-0.2W) based alloys were analyzed in wrought processed microstructure forms. The brittle–ductile-transition-temperature (BDTT) depends distinctly on microstructure as well as strain rate, with the minimum value for each microstructure achieved at ∼10⁻⁶/s being about 680 °C and 780 °C, respectively. The greatest creep resistance is achieved in coarse-grained fully lamellar (FL) material and is related to the strong anisotropy of lath structure, large grain size and consequently high BDTT. Additional significant resistance improvement is realized with additions or increases of refractory elements (Nb or W) and decrease in Al content. The most remarkable improvements in primary as well as the minimum creep resistance are realized when small amounts of C or C + Si are added to generate incoherent (to gamma) carbide and silicide particles along γ/γ_T interfaces. The significance of primary creep is assessed for controlling subsequent creep behavior and discussed for its crucial role in satisfying the stringent design creep requirements for advanced rotational components. The accelerated or tertiary creep is used to explain the high temperature (870 °C) high cycle fatigue deformation that exhibits two-stage SN curves with the rapidly softening second stage.     

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.     

Structural stability and the alloying effect of TiB polymorphs in TiAl alloys

Borides have been widely used in cast TiAl alloy for grain refinement and a variety of stoichiometry and crystal structure of borides were reported. Here the effects of alloying elements Nb, Ta, and Mn on the structural stability of fine boride precipitates in TiAl alloys have been studied combining transmission electron microscopy (TEM) and first-principles calculations. The results show that most boride particles have the TiB stoichiometry. In the alloy containing Nb and Mn, all the TiB particles have the B27 structure and are highly enriched with Nb but depleted with Mn. In the alloy containing Nb and Ta, however, the intergrowth of B_f and B27 structure has been observed, and the TiB particles are enriched with both Nb and Ta. First-principles calculations reveal different effects of Nb, Ta, and Mn on the structural stability of TiB polymorphs. Nb stabilizes B27 but destabilizes B_f. Ta strongly stabilizes both B27 and B_f structures. Mn strongly destabilizes both B27 and B_f structures.     

Effect of Mn addition on the thermal stability and magnetic properties of rapidly-quenched L10 FePt alloys

Nano-composite magnets with L1₀ structure derived from binary FePt alloys and prepared as melt-spun ribbons are of current interest due to their higher operating temperature and the ability to be cast as a two-phase magnet with exchange spring magnetic properties, as both soft and hard magnetic phase may emerge from the same metastable precursor, i.e. the disordered cubic A1 phase. The present paper studies the effect of Mn addition on the thermal stability and phase structure, on the abundance of the hard magnetic phase and relative proportion of the soft ones, on the microstructure of the alloy as a function of temperature and on the overall magnetic properties. The interplay of the various magnetic sublattices in the ordering of the L1₀ phases as a consequence of introducing antiferromagnetically coupled Mn atoms in the alloy composition is discussed and interpreted in terms of microstructural changes induced by this addition as revealed by high resolution transmission electron microscopy and X-ray diffraction. The temperature evolution of the phase composition and structural parameters is monitored using synchrotron radiation powder diffraction, while the compositional aspects are investigated using proton-induced X-ray emission and energy dispersive X-ray spectroscopy. Magnetic measurements reveal the magnetic parameters of interest (coercivity, remanence, Curie temperature, saturation magnetization), as well as the exchange-coupled two-phase nature of these magnets and provide information that hints at possible spin reorientation transitions in the Mn-containing planes of the L1₀ superlattice.     

Structural investigation upon the high glass-forming ability in Fe-doped ZrCuAl multicomponent alloys

Understanding the underlying mechanism of microalloying affecting the glass formation in multicomponent alloys is scientifically required, while addressing this issue remains a challenge. In this work, the structural mechanisms of extraordinarily high glass-forming ability (GFA) caused by Fe doping in ZrCuAl bulk metallic glasses was investigated via synchrotron radiation techniques combined with simulations. It is revealed that when minor Fe (5 at.%) are doped, the local structures (clusters) have relatively high values of some structural parameters, in terms of fivefold symmetry, geometrical regularity, atomic packing, and chemical interaction between heterogeneous atoms. The combination of these structural factors leads to stabilized amorphous structure and enhanced GFA. This work may extend our understanding on the sensitive dependence of GFA on the concentration of doping atoms in multicomponent metallic glasses.     

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.     

Fe3Nb3N precipitates of the Fe3W3C type in Nb stabilized ferritic stainless steel

A Nb stabilized ferritic stainless steel with 0.45 wt.%Nb, 82 ppm C and 170 ppm N is investigated to reveal the nature of the precipitates present at 950 °C. In particular, Fe₃Nb₃X precipitates of the Fe₃W₃C type are analyzed with WDS and EELS to determine the light elements X stabilizing this phase in the steel. According to WDS on large precipitates after 500 h at 950 °C, the Fe₃Nb₃X phase contains 10.4 at.% N, 1.2 at.% O and 1.0 at.% C. Auger Electron Spectroscopy on the same precipitates confirms the presence of N. In addition, it is revealed that the C and O peaks observed with WDS result from surface contamination as they disappear after Ar sputtering. The presence of a N peak in the EELS spectra of small Fe₃Nb₃X precipitates which have formed after 6 min at 950 °C indicate that N stabilizes this phase already from the initial precipitation stage. With this analysis it is demonstrated that N is an effective stabilizer of Fe₃Nb₃X precipitates in ferritic stainless steels. The formation of this phase should therefore be considered when predicting the precipitation behavior of Nb in industrial Nb stabilized ferritic stainless steels containing residual N.     

Characterization of silicide phases formed during pack siliconizing coating on the Nb-1Zr-0.1C alloy

The present paper describes the morphology, chemistry and crystallography of the phases observed in the silicide coatings produced by pack cementation technique on Nb based alloys. Cross-sectional microstructures examined by transmission electron microscopy and scanning electron microscopy techniques have shown that the coating has two silicide layers: NbSi₂ and Nb₅Si₃. NbSi₂ formed at the surface of the sample and Nb₅Si₃ formed in between the substrate (Nb alloy) and NbSi₂ coating layer. Electron diffraction analyses revealed that NbSi₂ has hexagonal crystal structure with lattice parameters as a = 0.48 nm and c = 0.66 nm and Nb₅Si₃ has tetragonal crystal structure with lattice parameters as a = 0.65 nm and c = 1.19 nm. Nb₅Si₃ showed fine equiaxed grains, whereas, NbSi₂ exhibited duplex morphology having columnar grain morphology near to the Nb₅Si₃ layer and large equiaxed grains at the surface of the coating sample. The presence of duplex morphology was explained by estimating diffusion of various species and it was shown that columnar morphology of grains could be attributed to outward diffusion of Nb and equiaxed grains to inward diffusion of Si. In the case of Nb₅Si₃, growth takes place due to single element Si diffusion, leading to development of single equiaxed grain morphology of the Nb₅Si₃ 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.     

添加钛、铌、锰、硫及镁对亚共晶灰铸铁初生奥氏体形核及生长的影响

共晶铸铁在共晶凝固时,熔体中碳的状态,即“C的微小集合体”已经被深入研究.然而时于与初生奥氏体密切相关的铁原子的状态,即“铁的微小集合体”却缺乏研究.本文作者提出,在亚共晶灰铸铁熔体中,初晶γ的形核受“Fe的富集部”的影响,而“Fe的富集部”的形成及消失与“C的微小集合体”有关.研究表明,初生奥氏体的形核过冷度、形核数和晶核生长形貌随工艺条件如过热度、保温时间和冷却速率而变化.本文讨论了在熔体的过热度、保温时间和冷却速率固定的条件下,钛、铌、锰、硫和镁这五种添加元素对亚共晶灰铸铁初晶γ的凝固过程的影响.