Ordering of Fe-Si phases

The microstructure of Fe-Si alloys containing 8 and 15.5 at % Si and heat-treated between 550 and 1200°C is studied by transmission electron microscopy, and the phase composition of alloys containing 19 and 23 at % Si is determined by x-ray diffraction. The Fe-15.5 at % Si alloy heat-treated above 700°C is found to consist of a disordered solid solution and B2 phase. The B2 particles can be thought of as portions of {100} layers consisting entirely of Si atoms and sandwiched between {100} layers of Fe atoms, that is, as a two dimensional phase. At t 675°C, a compositionally modulated microstructure develops in which the Si-enriched zones have the Fe₃ Si stoichiometry and DO₃ structure. At high temperatures, the Fe-19 at % Si alloy consists of the and B2 phases, and the Fe-23 at % Si alloy consists of the and DO₃ phases. These findings are at variance with the generally accepted Fe-Si phase diagram.Translated from Neorganicheskie Materialy, Vol. 41, No. 1, 2005, pp. 28–35.Original Russian Text Copyright © 2005 by Ustinovshchikov, Sapegina.     

Polymorphism of the Ag8S4O4 and Ag6S3O4 compounds

Experimental results of the reaction occurring between aqueous solutions of AgNO₃ and Na₂S₂O₃ were presented in this work. It has been demonstrated that these reagents form two phases to which the summary formulas: -Ag₈S₄O₄ and -Ag₆S₃O₄ were ascribed. The -Ag₈S₄O₄ phase crystallizes in a tetragonal system and has the following parameters of the unit cell: a = b = 0.72052 nm, c = 0.51140 nm, V = 0.26550 nm³, Z = 1, d_x = 6.60 g/cm³. At 223°C -Ag₈S₄O₄ undergoes an irreversible, endothermic polymorphic transition. The heat of this transition amounts 8.03 × 10^–3 J/mol. A high-temperature polymorphic form -Ag₈S₄O₄ melts at 400°C. The -Ag₆S₃O₄ phase crystallizes in a monoclinic system and has the following parameters of the unit cell:a = 2.09616 nm, b = 0.53118 nm, c = 1.49885 nm, = 102.78°, V = 1.62753 nm³, Z = 8, d_x = 6.59 g/cm³. -Ag₆S₃O₄ also undergoes an irreversible, endothermic polymorphic transition at 221°C. The heat of this transition amounts 7.65 × 10^–3 J/mol. A high-temperature form, -Ag₆S₃O₄, melts at 390°C.     

Phase decompositions of Fe-Si-Al ordered alloys

Stable structures of Fe-Si-Al ternary alloys and Fe-Si and Fe-Al binary alloys containing up to about 40 at% solute atoms were investigated by means of transmission electron microscopy. The following results were obtained. Two types of phase separation, B2+DO₃ and + DO₃ were observed in the alloys whose compositions lie in a narrow band connecting Fe-10 to 14 at% Si with Fe-20 to 25 at % Al and also in the neighbourhood of a Fe-30 at%Al-3 at% Si alloy. Such compositions of the alloys are located in the phase boundary of B2 and DO₃ single phases or and DO₃ single phases. The phase separation in the Fe-Si-Al and Fe-Si alloys produce the 100 modulated structure which differs from the morphology formed by the phase separation of the Fe-Al system.     

Atomic ordering and α′-Cr phase precipitation in long-term aged Ni3Cr and Ni2Cr alloys

Phase instabilities of binary Ni₂Cr and Ni₃Cr alloys after long-term exposure at temperatures between 450 and 600 °C were studied by monitoring lattice parameter, electrical resistivity and microhardness variations and by analysing the microstructural evolution of the alloys at interrupted annealing times. Both materials undergo two metallurgical changes: atomic ordering, based on the Ni₂Cr superlattice, and the precipitation of the -Cr phase. Short-range order develops in both alloys during the first's hours of ageing. The degree of order and kinetics of ordering transformation depend on the alloy composition, time and temperature of ageing. In Ni₂Cr, the short-range ordered structure transforms to long-range order after ageing for 100–1000 h below 525 °C, but in Ni₃Cr the transformation occurs after 30 000 h. The embrittling -Cr phase precipitates at grain boundaries in both alloys after long-term ageing, mainly at 550–600 °C. The amount and size of particles increase with time and temperature of ageing.     

The second phases in a burn resistant stable beta titanium alloy—Ti40

The second phases in Ti40 (Ti25V-15Cr-0.2Si) burn resistant titanium alloy are studied. The higher solution temperature, the more second phases in Ti40 alloy. There are not second phases if the solution temperature is below 850°C, while there are rod-like and little Ti₅Si₃ phases if temperature is over 850°C. , Ti₅Si₃ and oblique phases emerge after Ti40 solution at 910°C followed by aging at 600°C, while only Ti₅Si₃ phases emerge after solution at 860°C followed by aging at 600°C. The effect of the second phases on properties at RT are not obvious. There are different shapes of and Ti₅Si₃ precipitates after Ti40 alloy exposure at 540°C for 100 h, which decreases the thermal stability.     

Equilibrium phases surrounding the αZr solid solution in the Zr-Sn-O system

Phases in equilibrium with the Zr solid solution from 1014–1314 °C (1287–1587 K) in zirconium-tin-oxygen alloys were characterized and their compositions were determined. The experimental results provide information on the equilibria of the Zr solid solution in order to propose tentative boundaries at isothermal sections of the phase diagram.     

Anomalous thermal expansion of silver chlorate in the tetragonal and cubic phases

Silver chlorate (AgClO₃) undergoes a phase transition from a tetragonal to a cubic phase at 139° C. The temperature dependence of lattice parameters and the coefficient of thermal expansion in the tetragonal phase between 23° C and 130° C and in the cubic phase between 140° C and 184° C have been investigated. The lattice parameters in the tetragonal phase and the lattice parameter in the cubic phase increase with temperature. In the tetragonal phase the coefficient of expansion ( _c) increases with temperature whereas the coefficient of expansion along a perpendicular direction ( _a) decreases with increasing temperature. In the cubic phase the coefficient of expansion shows a pronounced decrease with increasing temperature. These results have been discussed in the light of the known structure.     

Effect of α-SiC on the microstructure and toughening of hot-pressed SiC-AlN solid solutions

The Effect of -SiC on the microstructure and toughening ofSiC-AlN solid solutions from powder mixtures of -SiC and AlNby hot-pressing were studied in the 1870 to 2030°Ctemperature range. The reaction of AlN and -SiC(3C) powderscausing transformation to the 2H(wurtzite) structure appeared todepend on hot-pressing temperatures and an additive of -SiC.For the composition of 49 wt% AlN/49 wt% SiC with 2 wt%-SiC and 47.5 wt% AlN/47.5 wt% SiC with 5 wt%-SiC at 2030°C for 1 h, the complete solid solutionswith a single phase of 2H could be obtained. The appreciable amountof -SiC could develop the columnar inter-grains of 4H phaseand the stable 2H phase with the relatively uniform composition andgrain size distributions. The effect of -SiC on the phasespresent and compositional microstructures with columnar inter-grainswas investigated using X-ray diffraction, scanning electronmicroscopy and transmission electron microscopy. The fracturetoughness and Vickers hardness of the hot-pressed solid solutionswere examined by the indentation-fracture-test method.     

Effects of grain-boundary configuration on the high-temperature creep strength of cobalt-base L-605 alloys

The effects of grain-boundary configuration on the high-temperature creep strength are investigated using commercial cobalt-base L-605 alloys with low carbon content in the temperature range 816 to 1038° C (1500 to 1900° F). Serrated grain boundaries are formed principally by the precipitation of tungsten-rich b c c phase (the same as ₂ phase found in Ni-20Cr-20W alloys) on grain boundaries by a relatively simple heat treatment in these alloys. The creep rupture properties are improved by strengthening of grain boundaries by the precipitation of tungsten-rich bcc (₂) phase. The specimens with serrated grain boundaries have longer rupture lives and higher ductility than those with normal straight grain boundaries under low stress and high-temperature creep conditions, while the rupture lives and the creep ductility of both specimens are almost the same under high stresses below 927° C. The matrix of the alloys is strengthened by the precipitation of carbides at temperatures below 927° C and by the precipitation of tungsten-rich ₂ phase at 1038° C during creep. It is found that there is an orientation relationship between tungsten-rich a2 phase particles and-Co matrix, such that (0 1 1)₂ ¶ (1 1 1) _-Co and [1 1]₂ ¶ [1 0] _-Co. The fracture surface of specimens with serrated grain boundaries is a ductile grain-boundary fracture surface, while typical grain-boundary facets prevailed in specimens with straight grain boundaries.     

The structure of a rapidly solidified Al-Fe-Ti-C alloy

The microstructures of melt-spun Al-2.03 Fe-0.46 Ti-0.35 C (at %) superheated to 1523 K (ribbon I), and 1673 K (ribbon II), respectively, before quenching, have been characterized using analytical electron microscopy and X-ray diffraction. A duplex microstructure has been observed for ribbon I, consisting of a microcellular region, across a sharp transition, followed by a coarser cellular or dendritic structure. The intercellular phases consisted mostly of Al₆Fe (few of Al₃Fe) and the dispersed TiC particles distributed in the -Al matrix with an exact orientation relationship. However, the microstructure of ribbon II comprised uniform, fine-scale dispersions of Al₆Fe phase in -Al grains, and larger size, elongated amorphous phase particles located along the grain boundaries, and approximately 0.46 at % Ti and 0.35 at % C dissolved in the -Al matrix. During the annealing of ribbon II, the amorphous phase transformed to _T-AlFeSi phase, the Al₆Fe dispersoids grew upwards and Al₃Fe, TiC particles precipitated in the -Al matrix. TiC phase formed both in ribbon I and in annealed ribbon II all had an atomic composition of TiC_0.79 (the nominal atomic percent ratio for the alloy was 0.74) and a lattice parameter of 0.424 nm. Moreover, there is a cube-cube orientation relationship between TiC and -Al matrix with a disregistry =0.049. In addition, the solidification characteristics of rapid solidification processing (RSP) Al-Fe-Ti-C alloy and mechanism of TiC formation have been discussed.     

The occurrence of stress-induced α′ martensite in an (α+γ) Fe-Cr-Ni alloy

The interaction of an applied stress with the displacive shear during the martensitic transformation determined theK-S variants which formed in three types of tensile specimens, with tensile directions of 0°, 45° and 90° to a rolling direction respectively. The 3–6, 4–5 and 1–3 variants in a 0°-specimen, 1–5 and 4–5 variants in a 45°-specimen and 4-4 and 3–6 variants in a 90°-specimen are chosen asK-S variants which have the maximum value ofU/ in respective tensile directions. These variants are related to the occurrence of martensite with particular orientations as (100), (110) and (211).     

Anisotropic grain growth of R-α-sialon (R = Nd and Er)

Two R--sialon (R_0.6Si_9.3Al_2.7O_0.9N_15.1, R=Nd and Er) compositions were first fired at 1750°C/25 min and 1650°C/2 h respectively for completion of the phase transformation. Elongated -sialon grain morphology was developed in both samples after being re-fired at 1800°C for different periods of time. The growth in width of R--sialon grains is controlled by diffusion in the liquid, while the length growth tends to be interfacial reaction controlled. The anisotropic growth of R--sialon is attributed to the large difference in the growth rate constant between the length and the width directions of the grain.     

The determination of the diffusion constants of oxygen in nickel and α-iron by an internal oxidation method

The diffusion constants of oxygen in nickel and -iron are calculated from experimentally determined internal oxidation rates in dilute silicon-bearing alloys. A single crystal alloy of composition 0.058 wt % silicon in nickel and polycrystalline alloys of composition 0.48 wt % silicon in nickel and 0.072 wt % silicon in iron were oxidised and allowance is made for the absorption of oxygen in the reaction with silicon. For nickel the values of activation energy Q=73.9 kcal/g atom in the range 800 to 1200° C and Q=74.4 kcal/g atom in the range 900 to 1300° C are in good agreement with a previous result. The activation energy required for oxygen diffusion in -iron was found to be 39.9 kcal/g atom in the range 700 to 850° C.     

A Mössbauer study of the oxidation of Fe-Ni alloys at 535 and 635°C

Fe⁵⁷ transmission Mössbauer spectroscopy, supported by metallography, SEM and X-ray diffraction analysis, has been employed to study the oxidation of Fe-Ni alloys at 535 and 635° C in 1 atm. of air. With increasing Ni content of the alloy, the composition of the scale changed and the oxidation rate decreased. For an alloy containing 0.9% Ni, the oxide scale produced at 535° C was Fe₃O₄ covered by a thin outer layer of-Fe₂O₃, while at 635° C FeO was additionally present as a major phase. The scale formed on a 10% Ni alloy at both 535 and 635° C was similar to that observed for the 0.9% Ni alloy oxidized at 535° C (i.e. of Fe₃O₄ and-Fe₂O₃), although the-Fe₂O₃ layer tended to be relatively thicker. For a 49% Ni alloy, the scale at both 535 and 635° C comprised an inner layer of Ni _x Fe_3–x O₄ (withx0.5, on average) and an outer layer of-Fe₂O₃, of similar thickness. Finally, on an 83% Ni alloy oxidized at 635° C, the scale consisted of roughly equally thick layers of NiO (next to the metal) and NiFe₂O₄, and a thin outer covering of-Fe₂O₃. The decrease in oxidation rate with increasing Ni content of the alloy is discussed briefly in relation to the changing composition of the scale and diffusion in the alloy.     

Solid state phase transformation of BaB2O4 during the isothermal annealing process

Solid-state phase transformation of BaB₂O₄ during the isothermal annealing process for both to and to were investigated using a platinum crucible. For the -phase crystal at the -phase stable temperature (> 925 °C), the phase transforms to the phase perfectly below the melting temperature of 1100 °C. Meanwhile, for the -phase crystal at the -phase stable temperature (< 925 °C), the phase transforms to the phase perfectly above 800 °C. There is some difference in phase transformation behaviour between bulk-shape crystals and the powder, caused by thermal stress.     

Microstructural characterization of γ-TiAl base alloy by electron probe x-ray microanalysis and electron backscatter diffraction

Electron backscatter diffraction (EBSD) and electron probe x-ray microanalysis (EPMA) in combination with x-ray diffraction (XRD) were applied for phase identification of the ternary precipitates and accompanying phases in Ti-49.6Al-1.9Fe alloy after heat treatment at 1400°C followed by furnace cooling. The heat treatment resulted in formation of the duplex structure consisting of equiaxed grains of the phase (AuCu type) and lamellae of the and ₂ (Ni₃Sn type). The ternary ₂ (Mn₂₃Th₆ type) phase, containing 21–22 at. % Fe, was revealed on the grain boundaries of the -matrix and lamellae, and is accompanied by ₂ precipitates. Different morphologies of the ₂ + ₂ colonies were found to differ in chemical composition, coarse particles being depleted in titanium, and the fine particles enriched in it. The combination of EPMA and EBSD in scanning electron microscopy proved to be very effective in local phase identification of specimens with fine multiphase structure.     

Mechanical properties of β-Si3N4 whisker reinforced α′-SiAlON ceramics

The effects of -Si₃N₄ whisker additions on the mechanical properties of -SiAlON ceramics were studied. The room temperature fracture toughness and fracture strength of the composites increased with increasing whisker content, and were 6.5 MPa m^1/2 and 900 MPa, respectively, for the addition of 30 vol% whiskers. Although creep resistance of the composites was not enhanced at 1200°C, the whisker additions were observed to be beneficial in reducing the oxidation induced slow crack growth of -SiAlON that occurred at 1300 °C, and thereby, improved the creep resistance of the composites at 1300°C.ORNL Postdoctoral Fellow, Oak Ridge Institute of Science and Technology, Oak Ridge Associated University.     

The morphology of α-Fe crystals which grow in the amorphous Fe80(C1−xBx)20 alloys

Crystallization behaviour of amorphous Fe₈₀(C_1–x B _x )₂₀ alloys, obtained by splat-cooling technique, for x values ranging from zero to unity has been investigated mainly by transmission electron microscopy. The crystalline phase which first appeared in the amorphous matrix was -Fe for all alloys studied. However, the morphology of -Fe phase changed from a spherical shape for low x values to a watch-glass shape for intermediate x values and to dendritic for large x values. The nucleation of -Fe crystals was homogeneous for low x samples while preferred nucleation on edges and surfaces was noted for samples with higher x values. The final volume fraction of -Fe phase before the appearance of the second crystalline phase increased with the increase in x.     

Structure and properties of rapidly solidified Al rich Al-Mn-Si alloys Part I Melt spun ribbons

The evolution of microstructure as-spun and during subsequent heat treatment at 200 to 500°C for up to 1000 h has been studied for Al-6.3 Mn-3.3 Si, Al-8.3 Mn-3.7 Si and Al-14.5 Mn-5.8 Si (wt %) alloys, containing 17, 26 and 48 vol % AlMnSi at equilibrium respectively. Microstructure as-spun ranged from primary icosahedral phase nucleating radial cellular Al arrays to less regular duplex arrays of Al and AlMnSi with decreasing alloy content and decreased section thickness or reduced distance from the chill surface. Heat treatment in the range 200 to 500°C transformed any icosahedral phase present to AlMnSi along with spheroidization and coarsening/coalescence of AlMnSi, to produce isolated spheroids when volume fraction f was lower and very stable interlinked chains at higher f. Measured coarsening rates of AlMnSi were a factor of 10 below predictions of LSW theory at lower f but were within a factor of 2 of prediction at highest f. Hardness was governed by a combination of Hall-Petch and matrix solid solution hardening as-spun supplanted by particle-radius dependent Orowan combined with matrix Hall-Petch hardening for the evolution of hardness during prior long term heat treatment at 425°C.     

In situ enhancement of toughness of SiC—TiB2 composites

A process based on liquid phase sintering and subsequent annealing for grain growth is presented to obtain the in situ enhancement of toughness of SiC–30 wt%, 50 wt%, and 70 wt% TiB2 composites. Its microstructures consist of uniformly distributed elongated -SiC grains, relatively equiaxed TiB2 grains, and yttrium aluminium garnet (YAG) as a grain boundary phase. The composites were fabricated from -SiC and TiB2 powders with the liquid forming additives of Al2O3 and Y2O3 by hot-pressing at 1850°C and subsequent annealing at 1950°C. The annealing led to the in situ growth of elongated -SiC grains, due to the phase transformation of SiC, and the coarsening of TiB2 grains. The fracture toughness of the SiC–50 wt% TiB2 composites after 6 h annealing was 7.3 MPa m1/2, approximately 60% higher than that of as-hot-pressed composites (4.5 MPa m1/2). Bridging and crack deflection by the elongated -SiC grains and coarse TiB2 grains appear to account for the increased toughness of the composites.