Orwik

Heat-Mass Transfer and Related Microstructures in TiAl-Based Alloys Directionally Solidifying in the Earth Gravity and Microgravity

The first results are presented of ESA MAXUS 8 sounding rocket benchmark experiment on γ-TiAl(Nb,B) intermetallics solidification, that was a part of the IMPRESS EU FP6 project. Having the aim to investigate the columnar and equiaxed primary microstr... expand abstractuctures formation at diffusion controlled melt growth, an experiment was designed applying the power-down directional solidification technique with fixing quench in automatic spaceborne furnace module TEM03-5M. Two related alloys were tested: one inoculated with boron grain refiner Ti-44Al-7.5Nb-2.7B (at.%); and the other Ti-45.5Al-8Nb (at.%) without grain refinement. The series of terrestrial reference processes has been performed accordingly in counter-gravity solidification direction. The numerical studies of heat-mass transfer, melt hydrodynamics and temporal solidification dynamics of these alloys have been implemented under the earth- and zero-gravity approximations using the GIGAN software package (IPPE). The comparison is performed of numerical model findings with the real microstructure and composition of samples, whose analyses were carried out by backscattered scanning electron microscopy (SEM) and X-ray microprobe (EDX) techniques. The convection-induced peritectic macrosegregation effect appearance and development in ground-based reference ingots is analytically observed and numerically studied. The achieved microstructure formation in space benchmarks is discussed. collapse abstract

On the Primary Phase Microstructure of Solidifying Ti-46Al-8Nb Refractory Intermetallic Alloy

The reported contradictory data on microstructure formation of the refractory intermetallic Ti–46Al–8Nb (at.%) alloy and on the high-temperature phase transformations proceeding within the Ti–Al–Nb phase diagram are analysed and clarified experimenta... expand abstractlly. To determine the primary solidifying phase, a set of experiments is performed on melting of the alloy specimens with low oxygen contamination in a high purity argon atmosphere using crucibles made of oxygen-free ceramics (99.99% AlN), and subsequent rapid solidification. Volumetrically-isothermal cooling from 1943 K at rates of 5, 10, and 20 K/s and following quench of mini-ingots from 1763 K are used. Specimens were studied by scanning electron microscopy (SEM) in backscattered electron (BSE) mode. SEM-BSE micrographs demonstrate contrasting shadow regions of non-uniform niobium segregation, which are fixed by quench and decorate the primary polycrystalline microstructure formed within the temperature range between 1843 (liquidus) and 1773 K (solidus). The primary crystallizing phase is proven to be represented by β(Ti) dendrites, which have clearly pronounced fourfold (bcc-lattice) symmetry being formed with secondary dendrite arms development. The solidification path is shown to be described with single-phase scheme L→L+β(Ti)→β(Ti); no peritectic β(Ti)→α(Ti) bcc-hcp phase transformation revealed within the mushy state of alloy. collapse abstract

Bench for High-Temperature Material Testing and Its Application in Metallurgy of Refractory Intermetallides

A simple laboratory technological bench (low-inertial high-temperature electric furnace) with a resistance plane heater based on SiC is produced. The bench makes it possible to carry out the spatial isothermal thermal treatment of samples with a vol... expand abstractume of up to 8 cm3, which are placed in the heater, with a controlled thermal cycle at temperatures of up to 2400°C within the inertial gas flow, including their hardening at the final stage of the process at a rate of up to 20°C/s. The photography and the video filming of the sample under test together with optical pyrometric control of its temperature can be carried out by means of a removable quartz or sapphire body of a reactor. Possible fields of application of the bench and the experimental technological and materials science problems are considered. Examples of the comparative investigation of the chemical compatibility of intermetal refractory alloy TiAl–Nb with crucible oxygenfree special ceramics-AlN and BN during melting, superheating, fixed isochronousisothermal holding of the melt at 1670°C, andits abrupt crystallization within test cymbiform crucibles in high-purity Ar are presented. collapse abstract

Numerical study of convection-induced peritectic macro-segregation effect at the directional counter-gravity solidification of Ti-46Al-8Nb alloy

Unidirectional counter-gravity (upward) melt solidification of Ti-46Al-8Nb (at%) intermetallic alloy was performed in the three-zone resistive tube electro-furnace with power-down thermal profile operation. The laboratory refinement of cylindrical ... expand abstractingots growth technique was developed in course of terrestrial preparation experiments in the facility specially designed for a sounding rocket flight. Despite Ti-46Al-8Nb is nominally slightly pro-peritectic composition, an axial elongated channel-like area with peritectically-transformed microstructure was observed in solidified ingots, where Al content locally exceeds 47 at%. For revealing the reasons of this microstructural inhomogeneity formation, the numerical modeling was applied. The real-time-scale 2D temperature field mapping, macro-scale study of melt hydrodynamics, heat-mass transfer, segregation effects, mushy zone evolution and solidification dynamics of TiAleNb melt/solid system have been performed accordingly. It was found that appearance of peritectic axial “spindle” in the solid is induced by the joint action of two factors in the melt: (i) rejection of Al solute ahead of the concave growth interface (dendrite tips front); (ii) development of weak laminar thermo-gravitational convective flow that picks-up, pulls upward and stratifies an Al-enriched stream along the axis of melt column. The driving force of convection is a radial thermal- gradient that depends on the furnace operational thermal conditions. To prevent the segregation, it was shown numerically that single regular convective cell could be broken into several cells by the appropriate variation of power-down cooling rate. The resulting uniform as-solidified microstructure of alloy obtained confirms the modeling findings. collapse abstract

Review of the Maxus 8 Sounding Rocket Experiment to Investigate Solidification in a Ti-Al-Nb Alloy

A review of the MAXUS 8 sounding rocket microgravity experiment to investigate solidification structures in a Ti-Al-Nb intermetallic alloy is presented. The experiment was part of the Intermetallic Materials Processing in Relation to Earth and Space ... expand abstractSolidification (IMPRESS) EU FP6 project. Key objectives were to investigate columnar and equiaxed solidification, and to achieve Columnar-to-Equiaxed Transition (CET) in the alloy. A microgravity experiment was designed to achieve this using a controlled power-down method. Two alloys were tested: one inoculated with a grain refiner and the other without grain refinement. Unrefined samples displayed axial and radial columnar growth. Boride inoculated samples displayed an equiaxed structure. No clear CET was achieved. The design, details, and results of the experiment are presented. collapse abstract

Convection-induced peritectic macro-segregation proceeding at the directional solidification of Ti–46Al–8Nb intermetallic alloy

Authors extended the research on TiAl–Nb solidification/modeling reported in Kartavykh et al. (2010) [1]. Unidirectional counter-gravity (upward) melt solidification of Ti–46Al–8Nb (at.%) intermetallic alloy was performed in the three-zone resistiv... expand abstracte tube electro-furnace TEM 01-3M with power-down thermal profile operation. The laboratory refinement of this cylindrical ingot growth technique was developed in course of terrestrial preparation experiments in the furnace facility specially designed for a sounding rocket flight. Although Ti–46Al–8Nb is nominally slightly pro-peritectic composition, an axial elongated channel-like area with peritectically transformed microstructure was firstly observed in solidified ingots by SEM-BSE and EDX analyses, where Al content locally exceeds 47 at.%. For revealing the reasons of this microstructural inhomogeneity formation, the numerical modeling was applied. The real-time-scale 2D temperature field mapping, macro-scale study of melt hydrodynamics, heat-mass transfer, segregation effects, mushy zone evolution and solidification dynamics of TiAl–Nb melt/solid system have been performed accordingly. It was found that appearance of peritectic axial “spindle” in the solid is induced by the joint action of two factors in the melt: (i) rejection of Al solute ahead of the concave growth interface (dendrite tips front); (ii) development of weak laminar thermo-gravitational convective flow that picks up, pulls upward and stratifies an Al-enriched stream along the axis of melt column. The driving force of convection is a radial thermal gradient that depends on the furnace operational thermal conditions. To prevent the segregation, it was shown numerically that single regular convective cell could be broken into several cells by the appropriate variation of power-down cooling rate. The resulting uniform assolidified microstructure of alloy produced confirms the modeling findings. Valuable data are obtained for the optimization of cast processes of TiAl-intermetallics. collapse abstract

Электронная микроскопия первичной микроструктуры быстро закристаллизованного сплава Ti-46Al-8Nb

Electron microscopy of the primary microstructure of rapidly solidified Ti-46Al-8Nb alloy

The contradictory data on the formation of the microstructure of a refractory intermetallic Ti–46 at % Al–8 at % Nb alloy and on the phase transformations shown in the Ti–Al–Nb phase diagram are analyzed and improved experimentally. To determine the ... expand abstractprimary solidified phase, a set of experiments is performed on melting of the alloy, which is synthesized in a high-purity argon atmosphere using crucibles made of an oxygen-free ceramics (99.99% AlN), and subsequent rapid volumetric isothermal solidification. Cooling from 1943 K at rates of 5, 10, and 20 K/s and subsequent quenching from 1763 K are used. Polished sections of ingots are studied by scanning electron microscopy using backscattered electron mode. Scanning electron microscopy micrographs demonstrate contrast regions of nonuniform niobium segregation, which are fixed by quenching and decorate the primary polycrystalline microstructure formed in the temperature range from 1843 (liquidus) to 1773 K (solidus). The primary crystalline phase is shown to be represented by β(Ti) dendrites, which have clearly pronounced fourfold symmetry that form during the development of secondary arms. collapse abstract

Математическая модель и метод расчёта кристаллизации двухкомпонентных расплавов

Numerical study of convection-induced peritectic macro-segregation effect at the directional counter-gravity solidification of Ti-46Al-8Nb intermetallic alloy

The specific effect of centerline macro-segregation is firstly experimentally observed, analytically confirmed, numerically studied and modeled in the directionally solidified (DS) cylindrical Ti-46Al-8Nb (at%) samples. The segregation effect is ex... expand abstractplained from the positions of hydrodynamics, heat- and mass transfer of rejected Al solute in the solidifying vertical melt column. The convection-induced nature of primary segregation is demonstrated, followed by the fundamental peritectic transformation of local elongated channel-like Al-enriched area in the solid according to the phase diagram. It is shown that slightly pro-peritectic composition of the alloy is highly sensitive with this respect to the melt hydrodynamics and solidification regimes. The importance of special melt flow engineering is underlined at the industrial-scale solidification of Ti-46Al-8Nb for the manufacturing of structurally-uniform cast items. collapse abstract

Наноструктурированный жаропрочный интерметаллический сплав Ti-46Al-8Nb и проблемы его технологической разработки

Стенд для высокотемпературного тестирования материалов и его применение в металлургии жаропрочных интерметаллидов

Melt structural self-organization and viscosity within the transient layer during a single crystal growth in microgravity

A brief review is given of the results obtained and published in 2003–2007 by IChPM and IPPE during their joint study and modeling of Ge:Ga, Ge:Sb, GaSb:Te, InP:S single crystal growth from stoichiometric and non-stoichiometric melts on board the Pho... expand abstractton satellite series. The use of microgravity is shown to be justified and holding promise for research into the structural selforganization processes (cluster forming) taking place within the transient layer of the melt during the solidification. The mathematical model of convective heat and mass transfer taking into account the dual-phase character of matter in the boundary layers near the interface has been created and used as an independent tool for the study of such processes. Prospects are discussed for this new area of space material science. collapse abstract

The Development of a Microgravity Experiment involving Columnar to Equiaxed Transition for Solidification of a Ti-Al based Alloy

The authors are members of the integrated project Intermetallic Materials Processing in Relation to Earth and Space Solidification (IMPRESS), funded within the European Framework (FP6). One of the aims of IMPRESS is to develop new alloys and proces... expand abstractses for the casting of TiAlbased turbine blades for the next generation of aero and industrial gas turbine engines. Within IMPRESS, two related issues have been identified during the primary solidification stage, namely, segregation and the columnar-to-equiaxed transition (CET). The authors have set out to isolate the effects of thermo-solutal convection, by designing a microgravity experiment to be performed on a European Space Agency platform. This experiment will investigate the CET formation during solidification. It is planned to use a sounding rocket providing a microgravity time of approximately twelve minutes. The results of this microgravity solidification experiment will be used as unique benchmark data for development and validation of new computational models of TiAl solidification. This in turn will produce accurate models and ultimately new robust industrial processes by project partners in the aerospace industry. The evolution of the design of the microgravity experiment is discussed and the results of preliminary ground reference experiments are presented. Future plans and objectives for the project are also highlighted. collapse abstract

Numerical simulation of TiAl-Nb alloy solidification experiment in TEM 01-3M facility aboard MAXUS 8

The problem of numerical modeling of directional solidification of TiAl refractory intermetallics aboard the MAXUS 8 sounding rocket is considered. The research is of relevance to the FP6 Integrated project IMPRESS (Intermetallic Materials Processi... expand abstractng in Relation to Earth and Space Solidification). Attention is paid to columnar-to-equiaxed microstructure transition (CET) phenomenon and mushy zone evolution in Ti-45.9Al-8Nb (at %) alloy being processed in TEM 01- 3M high-temperature (up to 1700 degr.C) furnace. In this three-zone resistive furnace the “bent” temperature profile is applied with two strongly different axial thermal gradients, presumably allowing the achieving of CET conditions along the sample of 160 mm length. Temperature profile evolution is defined by power-down furnace operation. 2D-numerical study of heat transfer and realtime-scale solidification dynamics of TiAl-Nb under zero gravity approximation is performed. The approaches used for solution of
avier-Stokes equations and phase transition (Stefan) problem are briefly described. The solidification time is shown to be satisfying the 12-minute microgravity limit aboard a MAXUS. The position and the time at which CET may be triggered are predicted and confirmed in line with the Hunt diagram. The comparison is performed of model predictions with the real microstructure of TiAl-Nb reference sample solidified on-ground in TEM 01-3M facility. collapse abstract

TiAl–Nb melt interaction with pyrolytic boron nitride crucibles

This paper studies the chemical interaction on contact, and the related evolution of composition and microstructure of cast TiAl-based alloy, when using refractory oxygen-free pyrolytic boron nitride (pBN) crucibles/moulds instead of traditional ox... expand abstractide ceramic ones. We extend the research begun with AlN crucibles and reported in [4]. Three melting tests were performed in resistive SiC electro-furnace with Ti–46Al–8Nb (at%) alloy in 6N-purity pBN boat crucibles with fixed melt superheating times of 5, 12 and 25 min at 1670 ◦C and subsequent quenching with high-purity Ar gas flow. As-cast samples were examined by optical microscopy/metallography, X-ray diffractometry, SEM-BSE, EDX and EBSD microanalyses, and compared to the initial material. Unlike the AlN it was shown that dissolved BN reacts with the melt, forming Ti2AlN and (Ti,Nb)B micro-precipitates at the expense of selective destruction of the 2-Ti3Al major intermetallic phase in the solidified alloy: Ti3Al + (Nb)+BN→Ti2AlN + (Ti,Nb)B. The key features and temporal dynamics of this interaction are studied. collapse abstract

Сопротивление растекания и компенсация новителей заряда в ферромагнитном кремнии, имплантированном марганцем

Spreading resistance and compensation of charge carriers in ferromagnetic silicon implanted with manganese

Profiles of impurity distribution and spreading resistance have been studied in the layers of ferro- magnetic silicon obtained by implantation of Mn (or Co). Standard wafers of n- and p-Si with a high or low electrical conductivity were implanted w... expand abstractith Mn ions with the dose (1–5) × 10^16 cm^–2. It is found that, as a result of postimplantation annealing in vacuum for 5 min at 850°C, Mn manifests itself as an amphoteric impurity and compensates acceptors in high-resistivity p-Si and donors in low-resistivity n-Si. It is shown that only an insignificant fraction of Mn ions (1–2%) is electrically active and is involved in compensation. The magnitude of compensation is used to determine energies of the levels Ec – 0.12 eV for n-Si and Ev + 0.32 eV for p-Si; these levels are attributed to Mn ions at interstitial sites in the silicon crystal lattice, i.e., (Mni)–/0 and (Mni)+/++, respectively. collapse abstract

Structure, electrical and magnetic properties, and the origin of the room temperature ferromagnetism in Mn-implanted Si

The structure and the electrical and magnetic properties of Mn-implanted Si, which exhibits ferromagnetic ordering at room temperature, are studied. Single-crystal n- and p-type Si wafers with high and low electrical resistivities are implanted by ma... expand abstractnganese ions to a dose of 5 × 10^16 cm^−2. After implantation and subsequent vacuum annealing at 850°C, the implanted samples are examined by various methods. The Mn impurity that exhibits an electric activity and is incorporated into the Si lattice in interstitial sites is found to account for only a few percent of the total Mn content. The main part of Mn is fixed in Mn15Si26 nanoprecipitates in the Si matrix. The magnetization of implanted Si is found to be independent of the electrical resistivity and the conductivity type of silicon and the type of implanted impurity. The magnetization of implanted Si increases slightly upon short-term postimplantation annealing and disappears completely upon vacuum annealing at 1000°C for 5 h. The Mn impurity in Si is shown to have no significant magnetic moment at room temperature. These results indicate that the room temperature ferromagnetism in Mn-implanted Si is likely to be caused by implantation-induced defects in the silicon lattice rather than by a Mn impurity. collapse abstract

Структура, электрические и магнитные свойства и природа ферромагнетизма при комнатной температуре в кремнии, имплантированном марганцем

TiAl-Nb melt interaction with AlN refractory crucibles

The paper considers contamination of cast TiAl-basedalloy, relatedmicrostructure evolution and chemical contact interaction when using refractory aluminium nitride crucibles/moulds as an alternative to the traditional Al2O3-, ZrO2- and Y2O3-based o... expand abstractxide ceramics. A series of melting tests has been performed in resistive SiC electro-furnace with Ti–46Al–8Nb (at%) alloy in 99.99% purity AlN boat crucibles with fixed melt superheating times 5, 12 and 25 min at 1670 ◦C and consequent quenching with high-purity Ar gas flow. As-cast samples were examined by X-ray diffractometry, SEM, SIMS, EDX, EBSD and vacuum fusion analysis of O, N, C and S interstitials content with respect to the initial material. The key features of TiAl–Nb melt interaction with AlN ceramics are revealed. As a result of slow thermal dissociation AlN→Al +N, and the reaction of nascent nitrogen with the melt, a solid continuous TiN-based reaction layer is formed up to 6.4m in thickness, together with an enriched Al liquid film between it and the cruciblewall. It causes perfectwetting of crucible with the melt and easy removal of the solidified sample. The partial suitability of AlN crucible is restricted by 12 min of the melt superheating from the point of view of invariable (2 +)-microstructure and reasonable contamination of the as-cast samples. collapse abstract

Способ получения слитков и литых изделий из интерметаллических сплавов

Изобретение относится к изготовлению слитков и литых изделий из интерметаллических сплавов на основе титана и алюминия. Способ включает плавление исходной шихты в тигле и последующую кристаллизацию расплава в форме, выполненных из нитрида алюминия и... expand abstractли футерованных изнутри нитридом алюминия, причем кристаллизацию расплава осуществляют в атмосфере инертного газа, например аргона с точкой росы не выше минус 70°С. Повышается качество слитков и изделий за счет снижения общего содержания кислорода и снижаются расходы на их производство. FIELD: technological processes. SUBSTANCE: invention is related to manufacturing of ingots and cast articles out of intermetallic alloys based on titanium and aluminium. Method includes melting of initial charge in crucible and further crystallisation of melt in mold, made of aluminium nitride or lined with aluminium nitride inside, at that melt crystallisation is carried out in atmosphere of inertial gas, for instance argon with dew point of not more than minus 70° C. EFFECT: improved quality of ingots and articles due to reduction of common oxygen content, and lower production costs. collapse abstract

Charge Carriers Compensation in a Ferromagnetic Mn-Implanted Si

Secondary ions mass-spectrometry and spreading resistance profiles in the layers of a ferromagnetic Si implanted with Mn has been studied. Czochralski Si wafers both n- and p-type, of high- and low-resistivity, as well as a float zone Si were implant... expand abstracted with impurity fluencies of (1 - 5) x 1016 cm-2. The Mn impurity was found to compensate acceptors in a high-resistivity p-Si and donors in a low-resistivity n-Si. Only the small part of Mn ions in Si apparently incorporates into the Si crystal lattice, occupies the interstitial sites and the appropriate energy levels (Mni)-/0 and (Mni)+/++ equal to Ec – 0.12 eV for n-type Si and Ev + 0.32 eV for p-type Si, respectively, are activated after vacuum annealing. collapse abstract

Структурная самоорганизация расплавов в переходном слое при росте кристаллов в условиях микрогравитации

Кластерная гидродинамическая модель тепломассопереноса при росте кристаллов и её значение для космического материаловедения