Si Phase Diagram Research Articles

Phase diagram of the Nb–Al–Si ternary system

A high-efficiency diffusion-multiple approach was employed to map the phase diagram of the Nb–Al–Si ternary system which is very valuable for the design of niobium silicide-based composites. These composites have high potential as a replacement for Ni-base superalloys for jet engine applications. Aluminum is an alloying element for these composites, thus the Nb–Al–Si phase diagram, especially solubility of Al in Nb 5Si 3, is important information for the composite design. An isothermal section at 1000 °C was constructed from the results obtained from a diffusion multiple using scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). A ternary phase Nb 3Si 5Al 2 was observed. The solubility data of Al in αNb 5Si 3 and NbSi 2 as well as Si solubility in Nb 3Al, Nb 2Al and NbAl 3 were obtained. The new isothermal section helps to judge the reliability of the existing literature results and to add new data to the Nb–Al–Si phase equilibria.

Cerium—Silicon System.

AbstractFor Abstract see ChemInform Abstract in Full Text.

X‐Ray Investigation of the Gd—Zn—Si System.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Isothermal section at 1673 K of the Mo–Ru–Si diagram and crystallographic structures of ternary phases

Efforts to improve the high temperature behavior of MoSi2 in oxidizing environments led to the investigation of the Mo–Ru–Si phase diagram. The isothermal section at 1673 K was determined by X-ray diffraction, optical and scanning electron microscopies and EPMA. Five new silicides were identified and their crystallographic structure was characterized using conventional and synchrotron X-ray as well as neutron powder diffraction. Mo15Ru35Si50, denoted α-phase, is of FeSi-type structure, space group P213, a=4.7535 (5) Å, Dx=7.90 g. cm−3, Bragg R=7.13. Mo60Ru30Si10 is the ordered extension of the Mo70Ru30 σ-phase with space group P42/mnm, a=9.45940(8) Å, c=4.94273(5) Å, Dx=6.14 g. cm−3, Bragg R=5.75.

The Al–Ni–Si phase diagram between 0 and 33.3 at.% Ni

The ternary Al–Ni–Si phase diagram between 0 and 33.3 at.% Ni was investigated by a combination of differential thermal analysis (DTA), powder X-ray diffraction (XRD), metallography and electron probe microanalysis (EPMA). Ternary phase equilibria and accurate phase compositions of the respective equilibrium phases were determined within the isothermal section at 550 °C by means of EPMA. Al 3Ni 2 and NiSi 2 were found to form extended ternary solid solutions while Al 3Ni and the ternary phase Al 6Ni 3Si have only small homogeneity ranges within the ternary system. The variation of lattice parameters with the composition was studied for all solid solution phases. DTA was used to investigate ternary phase reactions. A total number of five invariant reactions could be derived from DTA data, i.e. one ternary eutectic reaction, three transition reactions and one ternary peritectic reaction. All experimental data were combined to yield a ternary reaction scheme (Scheil diagram) of the investigated composition range and a liquidus surface projection was constructed based on DTA results. The melting behavior of the extended solid solution phase NiSi 2− x Al x was studied in detail by DTA measurements at low heating rates. The melting reaction was found to change from peritectic decomposition in the binary Ni–Si system to congruent melting at approximately 20 at.% Al.

Crystallographic investigation of the ternary compounds in the Zr–Li–Si system

By means of X-ray structural analysis the isothermal section of the Zr–Li–Si phase diagram at 470 K has been built. Four new ZrLi 2Si, ZrLiSi, Zr 4Li 1.38Si 4 and Zr 2− x Li x+ y Si 1− y ( x=0.17, y=0.12) ternary compounds have been found in this system. The ZrLi 2Si compound crystallizes in the ZnLi 2Si structure type (space group P 3m1 ), ZrLiSi compound crystallizes in the CdJ 2 structure type (space group P 3m1 ). Crystal structure of the Zr 4Li 1.38Si 4 (new structure type, space group Cmcm) and Zr 2− x Li x+ y Si 1− y ( x=0.17, y=0.12) compounds were investigated by the X-ray single crystal methods. The Zr 2− x Li x+ y Si 1− y ( x=0.17, y=0.12) new ternary compound is a antitype to the CaSb 2 structure type (space group P2 1/ m).

X-ray investigation of the Gd–Zn–Si system

The isothermal section of the Gd–Zn–Si phase diagram at 600°C over the whole concentration region has been constructed using X-ray phase analysis and in part microprobe analysis (CAMEBAX-device). The formation of the previously reported phase GdZnSi (AlB2 structure type) was confirmed, and one new ternary compound was observed. The crystal structure of the GdZnxSi2−x (x=0.48–0.54) (α-ThSi2 structure type) compound was determined using X-ray powder diffraction data. An investigation of the GdSi2−x-phase also was performed.

Cerium–silicon system

The Ce–Si phase diagram was constructed by means of differential thermal, X-ray and metallography examination of 15 as-cast key alloys. Ce 5Si 3, Ce 3Si 2, Ce 5Si 4, CeSi were confirmed. Two cerium disilicides were found, CeSi 2− a1 (α-GdSi 2 structure type) and CeSi 2− a2 (α-ThSi 2 structure type). The two-phase region CeSi 2− a1 +CeSi 2− a2 is very narrow. CeSi and CeSi 2− a1 melt congruently at 1630 and 1725 °C, respectively. The intermetallics Ce 5Si 3, Ce 3Si 2, Ce 5Si 4, CeSi 2− a2 form in peritectic reactions at 1260, 1335, 1500 and ∼1575 °C, respectively. Three eutectics occur at 635, 1532±12 and 1198±2 °C and the compositions 10.5, 55 and 82 at% Si, respectively.

RuSi 2: evidence of a new binary phase in the ruthenium–silicon system

Ru 2Si 3 single crystals have been grown by the zone melting technique with radiation heating. These crystals contain inclusions, about 500 nm in size, which consist of monocrystalline ruthenium disilicide. Endothermic peaks detected at T=962°C in the DTA traces of RuSi x are interpreted as decomposition of RuSi 2 to Ru 2Si 3 and Si. This enabled us to update the Si-rich part of the Ru–Si phase diagram.

Lanthanum–silicon system

By using differential thermal analysis, metallography and X-ray examination, the La–Si phase diagram was constructed. La 5Si 3, La 3Si 2, La 5Si 4, LaSi were confirmed. Two lanthanum disilicides were found — LaSi 2-a1 (structure type α-GdSi 2) and LaSi 2-a2 (structure type α-ThSi 2), the two-phase region LaSi 2-a1+LaSi 2-a2 being very narrow. The LaSi and LaSi 2-a1 intermetallics melt congruently at 1620 and 1730°C, respectively. La 5Si 3, La 3Si 2, La 5Si 4 and LaSi 2-a2 form in peritectic reactions at 1260, 1470, 1570 and ∼1600°C, respectively. Three eutectics occur at 722±9 (11 at.% Si), 1528±9 (55.5 at.% Si) and 1205±7°C (82 at.% Si).

Isothermal cross-section of the Ce–Pd–Si phase diagram at 600°C

The interaction of the components in the Ce–Pd–Si system at 600°C was investigated over the whole concentration range by X-ray powder and single crystal diffraction, DTA, and X-ray microprobe analysis. The formation of the two previously reported phases, CePd 2Si and Ce 2Pd 3Si 5, was not confirmed. There are ten ternary compounds in the Ce–Pd–Si system. The crystal structures of seven of them were determined.

Comparative Study of Novel Structures in Silicon and Germanium

We report the results of a first-principles theoretical study of the high-pressure phase diagram of Si and Ge, with a particular emphasis on the Cmca and r8 phases which have been recently discovered in Si. Our results for these phases are in excellent agreement with the available experimental observations in the case of Si and we predict the formation of similar phases in Ge.

Effect of antimony on the phase equilibrium of binary AlSi alloys

Small additions of antimony to eutectic AlSi alloys cause an increase in eutectic undercooling. The undercooling results in a eutectic with primary αAl in its microstructure. The metastable AlSi phase diagram is constructed in order to explain the undercooling and the existence of αAl in the microstructure. A method of calculating the undercooling, Δ T, from f α, the volume fraction of αAl in the microstructure, is given. The ternary AlSiSb phase diagram is calculated. From the ternary system it is possible to predict overmodification of AlSi alloys is caused by the formation of the compound AlSb.

Constitution and crystal growth of RE2TMSi 3 intermetallic compounds

Rare earth ( RE)-transition metal ( TM)-silicides RE 2 TMSi 3 ( RE=Ce, Tb, Dy; TM=Ni, Pd, Co) exhibit a layered hexagonal AlB 2 structure. RE– TM–Si phase diagram sections relevant for crystallization have been revealed by differential thermal analysis and microstructure investigations. Bulk single crystals of selected compounds were grown by the r.f. floating zone method in order to elucidate the anisotropy of low temperature magnetic ordering phenomena, metamagnetic transitions and magnetoresistive properties. The inferior element segregation along the rod axis is consistent with congruent melting of the RE 2 TMSi 3 intermetallic compounds, which exhibit melting points of 1500 to 1700°C. This conclusion is also supported by in situ tracking of the zone temperature during the growth process. Precipitates of RESi or RESi 2 within the single crystalline matrix formed during post-annealing processes could partially be dissolved by annealing up to 1200°C and subsequent quenching.

Thermodynamic calculated and TEM observed microstructure of Al–Li–Mg–Si alloys

Abstract The microstructures of two Al–Li–Mg–Si alloys have been studied by thermodynamic calculation and TEM observation in this paper. The results show that the addition of lithium changes the precipitation behavior of Al–Mg–Si alloys. The calculated Al–Li–Mg–Si phase diagrams are used to investigate the equilibrium phase distribution as a function of temperature for the Al–1.8Li–1.8Mg–0.8Si alloy and Al–1.8Li–2.8Mg–0.8Si alloy. According to calculation, δ′ phase (Al 3 Li) becomes the mayor precipitate in Al–1.8Li–1.8Mg–0.8Si alloy while a few amount of β′(Mg 2 Si) phase and AlLiSi phase precipitate. TEM observation show that δ′ phase and AlLiSi phase are precipitated in Al–1.8Li–1.8Mg–0.8Si and heterogeneous rod Mg 2 Si phase is also observed. Calculation on Al–1.8Li–2.8Mg–0.8Si show that both δ′ phase and Mg 2 Si phase precipitate on aging temperature. Experimental results are in agreement with calculation.

Thermodynamic Analysis and Experimental Study of Composite Structure Formation in Eutectic Alloys α -Al + Mg2Si

The Al ― Mg ― Si phase diagram was studied and the formation conditions for a regular two-phase eutectic structure (α-Al + Mg2Si) were established. Concentration limits were found for optimum alloy compositions with maximum melting point (∼595°C) and a narrow (or zero) melting (crystallization) range (less than 5°C). The structures of these alloys are formed by Mg2Si monocrystal fibers and lamellae with a high degree of ordering, located in the aluminum matrix.

Self-Propagating Reactions in the Ti–Si System: A SHS-MASHS Comparative Study

In this work, we report on the self-propagating reaction in Ti–Si blends, observed by SHS and MASHS (mechanical activated SHS) techniques. In spite of the differences between the two reacting methods, correlations were found between the key parameters of the two modes of activation. Moreover, this comparative study enabled us to gain some hints on the reaction mechanism. The combustive behavior of powder mixtures with stoichiometries corresponding to the intermetallics present in the Ti–Si phase diagram (TiSi2, TiSi, Ti5Si4, and Ti5Si3) was studied. The SHS characteristics, such as combustion temperature, propagation rate, and ignition temperature was strongly dependent on both the initial stoichiometry and milling time. Particular attention was paid to the influence of the initial stoichiometry and milling conditions on the reaction mechanism. A single-step dissolution-precipitation mechanism was found for the composition Ti : Si = 5 : 3. On the other hand, at the composition Ti : Si = 1 : 2, the mechanism shows two steps, the first, active at the leading front of the combustion front, involving only solid phases, and the second, active in the afterburn region, involving solid–liquid interaction.

Phase equilibria in the Nb–Fe–Cr–Si System

Efforts to improve the CVD process used to elaborate protective coatings against oxidation for niobium alloys have needed a good knowledge of the quaternary Nb–Fe–Cr–Si phase diagram. Many phase equilibria have been established at 1473 K and a new quaternary, Nb 6.6Fe 1.4Cr 4Si 8, phase has been identified and studied using single X-ray diffraction data. The crystal structure is of a new type, derived from the Cr 11Ge 8-type structure (space group Pnma, Z=4, a=13.649(2) Å, b=4.953(1) Å, c=16.386(3) Å, R= R w=5.7%). Higher temperatures stabilize the ternary Nb 2Cr 3Si 3 silicide in the Mn 5Si 3-type structure (space group P6 3/mcm, Z=2, a=7.190(2) Å, c=4.860(1) Å, R=3.2%, R w=2.7%). The two structures are characterized by presence of Si octahedra sharing faces and edges.

Study of a reaction at the solid Cu/ -SiC interface

A solid-state wetting technique has been used to investigate the interface between solid Cu and α-SiC at high temperatures. An intermediate phase is found to form as an interfacial reaction product between Cu and SiC at high temperature, and to thicken upon further heating at 1123 K. The interfacial phase has an approximate composition of Cu–19 at%Si–5 at%C and displays an f.c.c. structure. Such a phase is not present in the published Cu–Si phase diagram, and forms under conditions which cannot be explained from that phase diagram. It is postulated that this phase represents a previously unreported silicide of Cu which may be stabilized by the presence of C in solution. © 1998 Chapman & Hall

New silicides for new niobium protective coatings

Efforts to improve at high temperature the oxidation resistance of pure niobium or commercial niobium alloys have led to the development of a pack cementation process for the co-deposition of Si, Ti, Cr and Fe. Owing to the knowledge of the quaternary Nb(Ti)–T–Cr–Si phase diagrams (T=Fe or Co or Ni) and of the crystallographic features of phases present in the silicide coatings, new protective coatings have been applied on pure niobium and Cb752 alloy. The results of the crystallographic study of three new silicides isostructural with Nb 3Fe 3CrSi 6, in which Nb is substituted by Ti and Fe by Co or Ni are reported. The oxidation performances of two new coatings mainly consisting of such a silicide are also outlined.