Morphology of ordering Fe-Si alloys

The microstructure of some Fe-Si alloys water-quenched from 700–1200°C and aged at 550–675°C for various times has been studied by means of transmission electron microscopy (TEM). It has been shown that at temperatures of 700°C and above the microstructure of the alloy is a solid solution with two-dimensional particles of a B2 ordered phase. These particles have a FeSi composition and lie along (100) planes of the matrix. At temperatures of 675°C and below a modulated microstructure is formed; in its enriched modulations the composition of the Fe₃Si phase is attained and D0₃ ordering occurs. Phase composition of some high-silicon alloys has been determined by X-ray diffraction phase analysis. It corresponds to + B2 or + D0₃. These results are plotted on the accepted Fe-Si phase diagram.     

High-temperature creep and structure investigation of nearly stoichiometric Fe3Si

Single crystals of nearly stoichiometric Fe₃Si were creep-deformed at temperatureT = 450 to 850° C and applied stress=40 to 250 MPa. While the temperature dependence of the steady-state creep rate of crystals with less than 25 at% Si can be described by an exponential function exp (–H _exp/kT), the Fe-26 at% Si samples show an exponential dependence only below 500° C and above 600° C. At intermediate temperatures the dependence is weak. It is suggested that in this intermediate range two phases exist. The experimental results are consistent with the assumption that the phase boundaries do not hinder dislocation movement, and that the disocation velocity in the two phases is different.     

Effect of carbon on lattice strain and hole mobility in Si1-x Ge x alloys

Pseudomorphic Si_1-x Ge _x and partially strain compensated layers with different Ge and C fractions have been grown at 500 °C by ultra high vacuum chemical vapor deposition on Si (100) substrates. The degree of strain compensation of the layers has been investigated by high resolution X-ray diffraction and simple application of the linear elasticity theory. The surface morphology of the layers has been characterized by atomic force microscopy. The dependence of Si–Si Raman mode vibrations on strain and composition of binary and ternary alloys have been explained with experimental and theoretically calculated results. The Hall hole mobility is found to increase with decreasing compressive strain or effective Ge content in the layer throughout the temperature range of 120–300 K.     

High-temperature strength of bulk single crystals of III-V nitrides

A Vickers indentation method was used to determine the hardness of AlN and GaN, grown by the hydride vapor phase epitaxy technique, in the temperature range 20–1400 °C. At room temperature, the hardnesses of GaN and AlN are 10.2 and 17.7 GPa, respectively. The hardness of GaN and AlN shows a gradual decrease from RT and then a steep decrease from around 1000 °C. AlN is harder than GaN but softer than SiC. The steep decrease of the hardness means the beginning of macroscopic dislocation motion and plastic deformation. The mechanical strength of bulk single-crystal GaN is investigated at elevated temperatures directly by means of compressive deformation. The yield stress of GaN in the temperature range 900–1000 °C is around 100–200 MPa, i.e., similar to that of 6H-SiC and much higher than those of Si, Ge, GaAs.     

Steady-state creep behaviour of Ti3Al-base intermetallics

The steady-state creep behaviour of Ti₃Al and Ti₃Al+10 wt% Nb was studied in the temperature range 550 to 825° C and in the stress range 69 to 312 MN m^–2. The temperature and stress dependences of the steady-state creep rates were determined for both intermetallics, and the activation energy and stress-exponent were measured. At temperatures above 700° C, the stress dependence of the steady-state creep rate indicated two distinct creep regimes: at stresses above 138 MN m^–2, the creep was controlled most probably by dislocation climb; at stresses below 138 MN m^–2, a transition regime with a lower stress-exponent value was obtained.     

Glass Formation in the GeS2–EuS System

Crystalline and glassy alloys in one of the highest temperature glass-forming systems GeS₂–EuS are studied by differential thermal analysis, x-ray diffraction, chemical analysis, and IR spectroscopy. The glass-transition, crystallization, and melting temperatures of the alloys are determined with a high accuracy. The results indicate that, with increasing EuS content, the glass-transition temperature decreases from 492 to 448°C. The most homogeneous glasses can be prepared at EuS contents below 15 mol %. To obtain homogeneous glasses in the range 25–30 mol % EuS, an increased cooling rate is needed: 20–30°C/s.     

Damping behaviour and mechanical properties of rapidly solidified Al–Fe–Mo–Si/Al alloys

In order to develop a new high damping aluminium alloy with strength and toughness for advanced aircraft structure application, rapidly solidified (RS) Al–Fe–Mo–Si/Al alloys were synthesized. The damping behaviour, mechanical properties and microstructures of the alloys were studied. Results showed that the damping capacities of RS Al–Fe–Mo–Si/10–15% Al alloys are stable between 7.0–10.0×10-3 at room temperature, which almost reach the high damping threshold, 10.0×10-3. At lower frequency (0.1–10 Hz) the damping capacity is decidely frequency and temperture dependent above 50°C, with lowest frequency and highest temperature resulting in the highest less factor. It was noted that mechanical properties of the Al–Fe–Mo–Si/10–15% Al alloys are both excellent at room temperature (b=536–564 MPa, =7.2–11.4%) and at elevated temperature (250°C: b=295–324 MPa). Analysis of microstructures reveal that the damping capacity arises from deformation of the pure Al areas, and strength at elevated temperature from the dispersion strengthening of intermetallic phase. © 1998 Chapman & Hall     

Mechanical properties of vanadium beryllide,VBe12

The mechanical properties of VBe₁₂, both at room and elevated temperatures (up to 1200°C), have been measured. Room-temperature properties, including Young's modulus, flexural strength, and fracture toughness are reported. The material behaved elastically at room temperature but became plastic at temperatures above 1000°C. Creep properties of VBe₁₂ were also studied in temperature ranges from 1000–1200°C and applied stress ranges from 33–58 MPa. At low strain rates (approximately < 10^–5s^–1), the stress exponent was about 4, suggesting deformation was controlled by dislocation climb. Microstructural examination indicated that fracture was initiated from grain boundaries subjected to tensile stresses. The creep behaviour of VBe₁₂ is briefly compared with that of other intermetallics.     

Investigation of phase formation of RFe11V1 ? xTix (R = Pr, Ce) compounds

Phase formation of PrFe₁₁V_{1 – x}Ti_x and CeFe₁₁V_{1 – x}Ti_x compounds with the ThMn₁₂-type structure has been investigated by x-ray diffraction (XRD), differential thermal analysis (DTA) and ac initial susceptibility. The stable temperature range of the 1-12 phase for PrFe₁₁V_{1 – x}Ti_x alloys has been constructed as a function of Ti content. It is found that the PrFe₁₁V compound with the ThMn₁₂-type structure do not form. The PrFe₁₁Ti compound with the ThMn₁₂-type structure can only be obtained by annealing at a narrow temperature range between 1030 °C and 1113 °C. Furthermore, 1-12 phase can be obtained at lower temperature and wider temperature range with decreasing Ti content x(0.2 x 1). The CeFe₁₁V and CeFe₁₁Ti compound with ThMn₁₂-type structure can be synthesized, and it is more favorable to synthesis the 1-12 single phase for the pseudotenary CeFe₁₁V_{1 – x}Ti_x compounds. For PrFe₁₁V_{1 – x}Ti_x compounds with x = 0.2–1, the lattice parameters a and c do not vary very much: around 0.860 and 0.479 nm, and Curie temperature T_c is 268 °C–329 °C. For CeFe₁₁V_{1 – x}Ti_x compounds the lattice parameters a and c are around 0.857 and 0.478 nm, and Curie temperature T_c is 211 °C–252 °C.     

Effect of Fe and Al additions on nitridation of silicon

The effect of Fe and AI on the nitridation of Si was investigated under controlled O₂ partial pressures in the range 1.2×10^–12 to 1.2×10^–15 atm at 1420° C. The volatilization of Si during nitridation is attributed to SiO gas formation. Addition of Fe promoted the nitridation to phase at lower temperatures than the melting point of Si, whereas addition of Al increased the amount of phase. The stabilization of the structure is explained by Al₂O₃ formation and the dissolution of it in Si₃N₄ to form (Si, AL)₃ (N, O)₄ (sialon).     

Thermal stability in the Al-Al3Ti system

Directionally solidified samples of an Al-2 wt% Ti alloy were annealed at temperatures between 435° C and 660° C to investigate the thermal stability of phases formed during an incomplete peritectic transformation. The proportion of Al₃Ti present in the assolidified alloy is less than equilibrium up to about 480° C, and more than equilibrium at higher temperatures. Hence, Al₃Ti particles will be stable up to about 500° C and will tend to dissolve at higher temperatures. Diffusion due to non-equilibrium composition of the phase continues at all temperatures but is sluggish up to about 600° C. The diffusion coefficient of Ti in Al at 635° C is estimated to be 2×10^–11 cm² sec^–1.     

Mechanical properties and dislocation mobility in barium fluoride single crystals

Conclusions The elastic limit temperature dependence between 50 and 1000°C consists of three regions differing in the mechanisms governing dislocation motion. The general form of the dependence, however, is similar to that for metals. Schmid's law is not obeyed between 50 and 250°C for compressional deformations along the [110] and [112] directions. The increased elastic limit for the [110] orientation is explained by an additional interaction between dislocations lying in the same slip plane but pertaining to different systems.The dislocation velocity dependence on applied shear stress is rapid at 150°C (m-12–18) but decreases quite strongly as the impurity content changes. According to calculations the activation area is (100–200)b² for all the studied BaF₂ crystals. This indicates that dislocation motion is not limited by Peierls' barriers in the stress region in question, but by barriers connected with point defects.Chernogolovka, Leningrad. Institute of Solid-State Physics, Academy of Sciences of the USSR, Moscow. S. I. Vavilov State Optical Institute. Translated from Problemy Prochnosti, No. 6, pp. 10–16, June, 1970.     

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.     

Mechanical properties of Ni3(Si,Ti) polycrystals alloyed with substitutional additions

The mechanical properties of the L1₂-type Ni₃(Si, Ti) polycrystals, which were alloyed with 1–2 at% of various transition metals and also doped with boron, were investigated over a wide range of temperatures. The addition of Hf enhanced the levels of yield stress whereas the addition of Cr, Mn and Fe reduced the levels of the yield stress over a wide range of temperatures. Ni₃(Si, Ti) alloyed with Cr, Mn and Fe showed a shallow minimum in the yield stress-temperature curves. This result was correlated with the modification of the micro-cross-slip process by the additives. At low temperatures, the addition of Hf and Nb slightly reduced the elongation, while the addition of Cr, Mn and Fe improved elongation. This elongation behaviour was interpreted as the alloying effect on the intergranular cohesive strength of L1₂ ordered alloys. At high temperatures, the elongation of Ni₃(Si, Ti) alloyed with Hf showed a particularly high value. This elongation behaviour is discussed based on the alloying effect on the competition between dynamic recrystallization and cavitation at grain boundaries. The fracture surfaces exhibited a variety of fracture patterns, depending on temperature and the alloy, and were primarily well correlated with the elongation behaviour. The ductilities of most of the alloys at high temperatures were reduced by the tests in air.     

Production of Al-(12–25) wt% Si alloys by rapid solidification: melt spinning versus centrifugal atomization

Al-Si-x(x=Cu, Mg, Ni, Co or Sr) alloys with Si content in the range 12–25 wt% were quenched from the liquid state using two methods: melt spinning and centrifugal atomization. The powders obtained were degassed followed by hot extrusion. Effects of chemical composition, quenching conditions, hot extrusion and heat treatment on the variation in the microstructure were examined. The present results show the necessary conditions for supersaturated solid solution, and those required for mechanism of solute trapping by moving the solid/liquid interface. Also, the mechanical properties of the products obtained were evaluated. It is observed that melt spun ribbons with Si concentrations of more than 12% possess high yield strength with low ductility. These materials undergo softening on ageing at temperatures above 150 °C. The properties of extruded alloy powders are markedly improved as compared to those made by ingot metallurgy. This effect is mainly due to the silicon particle refinement brought about by rapid solidification with cooling rates higher than 10⁵ Ks^–1.     

Microplasticity in Elgiloy

The room temperature microstrain characteristics of Elgiloy (composition in wt%, 40 Co, 20 Cr, 15 Ni, 7 Mo, 2 Mn, 0.1 C, balance Fe) have been determined for a range of microstructures corresponding to various alloy treatments reported previously. The friction stress was found to be constant ( _F=12±3 MN m^–2) for the solution treated (after prestrain), cold-worked and cold-worked plus aged at 500° C conditions, but decreased to _f=5±3 MN m^–2 after ageing at 800° C. The microscopic yield stress (MYS) increased with cold-working and subsequent ageing at 500° C to a maximum value of 210 MN m^–2 reflecting the increase in the long range internal stress field. An increase in the MYS of the solution treated strip was noted with ageing at 800° C, whereas the MYS of the cold-worked samples was decreased by this treatment. The solution treated and aged at 800° C samples exhibited a two-stage stress-plastic microstrain curve, whereas the cold-worked and cold-worked plus aged at 500° C conditions showed a three-stage stress-plastic microstrain behaviour.     

The temperature and strain rate dependence of the flow stress of tantalum

The temperature and strain rate dependence of the flow stress of tantalum was studied between 78 to 800 K at strain rates from 10^–5 to 2×10⁴ sec^–1. The effect of temperature and strain rate on the lower yield stress can be explained by a model incorporating the combined operation of the Peierls mechanism and dislocation drag processes. The general behaviour of the stress—strain curve at various strain rates and temperatures is analysed in terms of a rate—temperature parameter.     

Low-temperature fracture toughness study of Fe-7Al-27Mn-C alloys

This research studied the fracture toughness of the Fe-7Al-27Mn alloys with increasing carbon contents: 0.5% C, Fl alloy: 0.7% C, F2 alloy (with 4.0% Cr); and 1.0% C, F3 alloy. Fracture toughness experiments were conducted at temperatures of 25, – 50, – 100 and – 150 °C. It was found that plane-stress,K _C, values as measured by the R-curve method, decreased as the temperature dropped. F1 alloy possessed the highestK _C value at all temperatures among the three alloys. TheK _C values of the F2 and F3 alloys were similar at ambient temperatures, but F3 maintained the toughness property and ductility better at sub-zero temperatures. Quantitatively,K _IC values of the F2 alloy at – 150 °C were ca, 60% less than at 25 °C, but F1 and F3 alloys dropped by only ca. 30%. Using a compact-tension specimen, 20.0 mm thick, at –150°C only alloy F2 satisfied the requirement of plane-strain fracture toughness with aK _C value of 106 MPa m^1/2. The existence of Cr (4.0%) and the formation of a ferrite phase in an austenite matrix was responsible for the low toughness value observed.     

Magnetic resonance characterization of solid-state intermediates in the generation of ceramics by pyrolysis of hydridopolysilazane

Chemical intermediates produced from the pyrolysis of hydridopolysilazane (HPZ) were studied in the solid state by multinuclear nuclear magnetic resonance and electron spin resonance. When pyrolysed at temperatures of 1200°C, uncured HPZ forms a ceramic material with a composition of Si_2.2N_2.2C_1.0. A series of HPZ-derived ceramics was produced using a number of different heat-treatment temperatures, varying between 300 and 1200°C. Solid-state magnetic resonance data generated from this set of HPZ-derived ceramics elucidate important features of this complex transformation. Silicon atoms initially exist in two types of sites in the polymer,Si(Me)₃ and ()₃SiH sites. Upon pyrolysis between 300 and 400°C, the silazane cyclizes and cross-links, forming an intractable, insoluble solid. Increasing the pyrolysis temperature to between 400 and 600°C creates a matrix that is partially inorganic; at heat-treatment temperatures in this range, many of the C-H bonds of the starting polymer are cleaved. Elevating the heat-treatment temperature to between 600 and 1200°C generates a series of chemical structures with silicon in a tetrahedral site of the general form SiN_4–xC_x, where x=0, 1, 2, 3, 4. No crystalline forms of Si₃N₄ or SiC were detected in the material prepared at even the highest heat-treatment temperature of 1200 °C.     

The microstructure of experimentally deformed limestones

The paper presents preliminary results of TEM studies of the microstructure of fine-grained Solnhofen limestone which was deformed in compression experiments at various strain-rates, temperatures and confining pressures. The aims of the tests were to study changes in the patterns of preferred orientation and possible changes in boundaries of the stability fields of the phases, induced by non-hydrostatic stress or large shearing strains. The TEM work on sputter-etched specimens followed extensive investigations by X-ray methods.Low-temperature deformation (200 °C, 26% strain, =10^–4% sec^–1) produces heavily cold-worked structures, with many sub-cells containing unresolvable defects. At 300 °C the microstructure is slightly less confused: some grains contain only tangles of dislocations while others exhibit ragged deformation bands. At higher temperatures but low strain-rates (600 °C, 36% strain, =10^–6% sec^–1) and within the aragonite stability field, the microstructure is more inhomogeneous, with slip and extensive faulting in both the aragonite and the residual calcite grains. Preferred orientation is indicated by significant alignment between fault structures in neighbouring grains. At yet higher temperatures (900 °C), twinning still occurs in the calcite but it is less profuse; dislocation densities are generally lower and preferred orientation is weaker, as shown by X-ray results.