Kinetics of Oxidation of Fe–6Si

Surface oxidation of Fe–6Si during annealing in low-pressure air (～10⁴ Pa) in the temperature range 500–550 °C was investigated using resistivity measurements, Mössbauer spectroscopy, X-ray diffraction and scanning-electron microscopy (SEM). The time dependence of the resistivity exhibits an increase in two steps, which indicates changes in the structure and/or phase composition of the alloy. Structure and phase investigations show that the first step can be explained as formation of hematite (α-Fe₂O₃) and the second step is due to transformation of the hematite to magnetite (Fe₃O₄). The kinetics of the transformations were derived from the resistivity data. The activation energies (estimated from Arrhenius plots) of 194 kJ/mol and 165 kJ/mol were obtained for the formation of hematite and transformation of hematite to magnetite, respectively.     

Observation of Multi-layer Film on Si Substrate

ＯｂｓｅｒｖａｔｉｏｎｏｆＭｕｌｔｉ－ｌａｙｅｒＦｉｌｍｏｎＳｉＳｕｂｓｔｒａｔｅＬｉｕＡｎｓｈｅｎｇ；ＡｎＳｈｅｎｇ；ＳｈａｏＢｅｉｌｉｎｇ；ＷａｎｇＪｉｎｇ；ＳｈｅｎＨｕｉｚｈｕ（刘安生）（安生）（邵贝羚）（王敬）（沈惠珠）（ＧｅｎｅｒａｌＲｅｓ...     

Strengthening mechanisms of MgSi alloys

The origins of high room temperature strength of rapidly solidified () Mg-high Si alloys have been analysed using various strengthening mechanism models. The analysis revealed that the high strength was attributed to the fine-grain strengthening mechanism as well as the strengthening mechanisms due to Mg₂Si particles. A large contribution of the fine-grain strengthening mechanism results from the strong dependence of strength on grain size for h.c.p. metals. However, the strength of the alloys containing small particles (?1 μm) rapidly decreased at 473 K. This is explained by the critical particle size concept.     

Effect of strontium on crystallization of Mg2Si phase in Al—Si—Mg casting alloys

Otical microscope and SEM were used to observe the changes of the microstructure of Al-11.6%Si-0.4% Mg alloys with varying strontium additions and the effect of strontium on the crystallization of Mg2Si phase was discussed.It is found that Mg2Si phase nucleastes on the the surfaces of the eutectic silicon flakes in the unfully modified alloys,growing as meshwork or bamboo-shoot shape,however,very few and fine Mg2Si particles phase are isolated at the boundaries of the eurectic cells in the fully modified alloys.Strontium has an important influence on the crystallization of Mg2Si phase in the Al-Si-Mg casting alloys and it is thought to be related to the increase of the amunt of dendritic αphase and the modifying degree of eutectic silicon phase.     

Effect of Si on the reversibility of stress-induced martensite in Fe–Mn–Si shape memory alloys

Fe–Mn–Si is a well-characterized ternary shape memory alloy. Research on this alloy has consistently shown that the addition of 5–6 wt.% Si is desirable to enhance the reversibility of stress-induced martensite vis-à-vis shape memory. This paper examines the effect of Si on the morphology and the crystallography of the martensite in the Fe–Mn–Si system. It is concluded that the addition of Si increases the c/a ratio of the martensite, reduces the transformation volume change and decreases the atomic spacing difference between the parallel close-packed directions in the austenite–martensite interface (habit) plane. It is proposed that, in addition to austenite strengthening, Si enhances reversibility by reducing the volume change and the interfacial atomic mismatch between the martensite and the austenite. Although shape memory is improved, transformation reversibility remains limited by the necessary misfit dislocations that accommodate the atomic spacing differences in the interface.     

On the physicochemical mechanism of modification of Al−Si alloys

Conclusions  

Thermodynamic modeling of the Nb–Si system

Optimized coefficients of the Gibbs free energy expression for each stable phase of the binary Nb–Si have been obtained, using the Thermo-Calc program for this purpose. The Nb₃Si, αNb₅Si₃, NbSi₂ and Diamond (Si) have been modeled as stoichiometric phases and the liquid L, BCC (Nb) and the βNb₅Si₃ phases as solutions, with the excess term described using the Redlich–Kister formalism. The Si solubility in the βNb₅Si₃ phase has been modeled according to two possibilities: (i) Si substituting for Nb and (ii) vacancies in the Nb positions. The calculated diagrams compare well with the experimental information taken from the literature.     

Non-equilibrium solidification of undercooled Co–Si melts

The effect of melt undercooling on solidification modes of peritectic Co–Si alloys has been studied by electromagnetic levitation. For Co_82.5Si_17.5, Co₈₁Si₁₉ and Co₇₅Si₂₅ alloys there are transitions towards direct crystallization of peritectic ε-Co and a metastable Co₃Si phase, respectively. For Co₇₃Si₂₇ a morphology change of the equilibrium α-Co₂Si phase was observed.     

Influence of vibrational entropy on structural stability of Nb–Si and Mo–Si systems at elevated temperatures

The heats of formation of stable and metastable phases of the Nb–Si and Mo–Si systems were studied using density functional theory (DFT). The high-temperature behavior of the competing phases was studied by performing additional phonon calculations. Our theoretical results rationalize the major differences observed in the behavior of the Nb–Si and Mo–Si systems: Nb₃Si is only stable at temperatures above 2043 K, whereas Mo₃Si is always stable; Nb₅Si₃ and MoSi₂ undergo phase changes at elevated temperatures, in contrast to Mo₅Si₃ and NbSi₂. These differences are qualitatively explained by including the vibrational entropy to the free energies within the harmonic approximation. In particular, the softer shear moduli of the Nb₅Si₃ and MoSi₂ βphases cause their stabilities over the α phases at elevated temperature.     

Effects of boron on eutectic modification of hypoeutectic Al–Si alloys

The effects of boron on the eutectic modification and solidification mode of hypoeutectic Al–Si alloys have been studied adding different boride phases. The results show that boron does not cause modification of the eutectic silicon. Boron-containing samples display eutectic nucleation and growth characteristics similar to that of unmodified alloys.     

Effect of pressure on solidification cracking susceptibility of Al–Si alloys

ABSTRACT

An approach was developed to calculate the crack susceptibility under various levels of pressure, and the corresponding numerical method was presented. The binary Al–Si alloy system was selected for study because the effect of high pressure on its phase diagram has been reported. The results showed a higher pressure can lead to a higher crack susceptibility and shift the most crack susceptible composition to higher solute contents. It was found a higher pressure can increase the effect of back diffusion on the solidification path and hence the crack susceptibility. This study provides a new understanding of the effect of pressure on solidification cracking susceptibility and can be a relevant starting point for studying solidification cracking under high pressures.     

The influence of Si as reactive bonding agent in the electrophoretic coatings of HA–Si–MWCNTs on NiTi alloys

In this study, different composite coatings with 20 wt.% silicon and 1 wt.% multi-walled carbon nanotubes of hydroxyapatite were developed on NiTi substrate using a combination of electrophoretic deposition and reactive bonding during the sintering. Silicon was used as reactive bonding agent. During electrophoretic deposition, the constant voltage of 30 V was applied for 60 s. After deposition, samples were dried and then sintered at 850 °C for 1 h in a vacuum furnace. SEM, XRD and EDX were used to characterize the microstructure, phase and elemental identification of coatings, respectively. The SEM images of the coatings reveal a uniform and compact structure for HA–Si and HA–Si–MWCNTs. The presence of silicon as a reactive bonding agent as well as formation of new phases such as SiO₂, CaSiO₃ and Ca₃SiO₅ during the sintering process results in compact coatings and consumes produced phases from HA decomposition. Formation of the mentioned phases was confirmed using XRD analysis. The EDX elemental maps show a homogeneous distribution of silicon all over the composite coatings. Also, the bonding strength of HA–Si–MWCNTs coating is found to be 27.47 ± 1 MPa.     

Tensile creep of Mo–Si–B alloys

Multiphase Mo–Si–B alloys containing a Mo solid solution matrix and brittle Mo₃Si and Mo₅SiB₂ (T2) intermetallic phases are candidates for ultra-high-temperature applications_. The elevated temperature uniaxial tensile response at a nominal strain rate of 10^?4 s^–1 and the tensile creep response at constant load between 1000 °C and 1300 °C of a (i) single phase solid solution (Mo–3.0Si–1.3B in at.%), (ii) two-phase alloy containing ～35 vol.% T2 phase (Mo–6Si–8B in at.%) and (iii) three-phase alloy with ～50 vol.% T2 + Mo₃Si phases (Mo–8.6Si–8.7B in at.%) were evaluated. The results confirm that Si in solid solution significantly enhances both the yield strength and the creep resistance of these materials. A Larson–Miller plot of the creep data showed improved creep resistance of the two- and three-phase alloys in comparison with Ni-based superalloys. The extent of Si dissolved in the solid solution phase varied in these three alloys and Si appeared to segregate to dislocations and grain boundaries. A stress exponent of ～5 for the solid solution alloy and ～7 at 1200 °C for the two multiphase alloys suggested dislocation climb to be the controlling mechanism. Grain boundary precipitation of the T2 phase during creep deformation was observed and the precipitation kinetics appear to be affected by the test temperature and applied stress.     

Microstructures of unidirectionally solidified Al-rich CuSi alloys

Aluminum-copper-silicon alloys lying in the monovariant through between the binary eutectic (Al)CuAl₂ and the ternary eutectic (Al)CuAl₂(Si) have been frozen unidirectionally over a range of rates from ca. 0.1 to 10 cm/hr and with imposed temperature gradients ranging from 150 to ca. 300°C/cm. The shapes of the growth fronts can be rationalized in terms of the ratio thermal-gradient/rate-of-solidification and the composition, as well as in binary alloys. The detailed morphologies of silicon are described for the different shapes of the solidification profiles, the six types of microstructures which appear in the AlSi alloys being formed, i.e., massive crystals, longitudinal fibers with or without lateral plates, complex-regular structures, complex-irregular structures, and polyhedral crystals.With regard to the CuAl₂ phase, the growth process is cooperative for planar and dendritic fronts. During cellular growth it appears as a semicontinuous matrix in which (Al) fibers are embedded. The conditions for existence of the observed microstructures are discussed with reference to the binary AlSi and AlCu systems.     

Modification of primary Mg2Si in Mg-5Si alloys with Y2O3

The Y2O3 addition to Mg-5Si alloys has a good modification effect on the primary Mg2Si. With 0.05% or 0.1% （mass fraction） Y203 additions, the primary Mg2Si begins to change from coarse dendritic shape （about 100 μm） into small polyhedral shape and therefore the alloys exhibits sub-modified microstructure. With 0.2% Y2O3 addition, most of the primary Mg2Si becomes polyhedral shape and its average size is only 25 μm or less. The Mg-5Si alloy exhibits modified microstructure. In addition, the experiments show that the reaction between Mg and Y2O3 cannot occur in the sintered Mg-6Y2O3 compact; however, the reaction among Mg, Si and Y2O3 can occur in the sintered Mg-5Si-6Y2O3 compact. Apart from the adsorption and poisoning manners, other mechanisms may exist in the modification of Y2O3 addition on the primary Mg2Si.     

Mechanical alloying of Mo—Si—Fe powders

Mechanical milling behavior of Mo-Si-Fe powders was investigated u sing XRD, SEM and TEM techniques. The mixtures of elemental molybdenum (>99%), s ilicon (>99%) and iron (>98%) powders with a stoichiometry of Mo5-xFe xSi3 (x=0.5, 1, 2) were milled in a planetary mill for up to 195 h. For all three powder mixt ures, high-energy milling of 60h led to formation of the Mo(Fe, Si) supers aturated solid solution (Moss); and to a remarkable expansion of the solub ility of Fe, Si in molybdenum. The transformation of Moss to an amorphous phase was identified after longer time milling. In the milling process, the grain size of Mo (Fe, Si) decreased gradually and the internal stress increased linearly. After 40 h milling, the grain size was reduced to about 11 nm. SEM analysis of milled powders showed that the particle size increased initially with milling time. After 195 h milling, particles exhibited a spherical morphology and the particle size were reduced to about 100 nm.