Study of Ordering in Fe—25%Al—Cr Alloys by Dilatometry, Heat Flow and Mechanical Spectroscopy

It is known that the addition of Cr to the Fe₃Al-based alloys leads to an improvement in their ductility and other properties. In a given work, dilatometric, differential scanning calorimetric (DSC) and internal friction tests are performed to study the ordering process in stoichiometric and overstoichiometric Fe₃Al alloys with Cr addition at a constant heating rate from room temperature up to 1000°C. The results indicate that the addition of Cr slows the D0₃-type ordering process; this can be confirmed by obtained values of activation energy. Cr addition has also an effect on the mechanical properties of studied alloys annealed in D0₃-type order domain. The heights and broadening of three peaks (of X-, Zener-, and Snoek-type) can be changed at the presence of Cr. X-ray diffraction (XRD) and TEM studies are carried out at different temperatures chosen from the obtained DSC curves and affirmed that a rapid quenching just after heat treatment from highest temperature cannot suppress the formation of B2 ordered particles in overstoichiometric Fe₃Al alloys.Відомо, що додавання Cr до стопів на основі Fe₃Al спричиняє поліпшення їх пластичности та інших властивостей. У даній статті виконано дилатометричні випробування та випробування методами диференційної сканівної калориметрії (ДСК) і внутрішнього тертя з метою вивчення процесу впорядкування в стехіометричних та надстехіометричних стопах Fe₃Al з додаванням Cr при сталій швидкості нагрівання від кімнатної температури до 1000°C. Додавання Cr сповільнює процес упорядкування за надструктурним типом D0₃, що підтверджується одержаними значеннями енергії активації. Додавання хрому впливає також на механічні властивості досліджених стопів, відпалених в області D0₃-порядку. Висоти та розширення трьох піків (Зінерового, Снукового та X-типу) можуть змінюватися в присутності Cr. Рентґенодифракційна аналіза та трансмісійна електронна мікроскопія, виконані за різних температур, обраних на підставі одержаних ДСК-кривих, підтвердили, що швидке гартування від самої високої температури зразу після теплового оброблення не може пригнітити формування впорядкованих частинок типу B2 у надстехіометричних стопах Fe₃Al.Известно, что добавление Cr к сплавам на основе Fe₃Al приводит к улучшению их пластичности и других свойств. В данной статье выполнены дилатометрические испытания и испытания методами дифференциальной сканирующей калориметрии (ДСК) и внутреннего трения с целью изучения процесса упорядочения в стехиометрических и сверхстехиометрических сплавах Fe₃Al с добавлением Cr при постоянной скорости нагрева от комнатной температуры до 1000°C. Добавление Cr замедляет процесс упорядочения по сверхструктурному типу D0₃, что подтверждается полученными значениями энергии активации. Добавление хрома влияет также на механические свойства исследуемых сплавов, отжигаемых в области D0₃-порядка. Высоты и уширение трёх пиков (зинеровского, снуковского и X-типа) могут изменяться в присутствии Cr. Рентгенодифракционный анализ и просвечивающая электронная микроскопия, выполненные при различных температурах, выбранных на основании полученных ДСК-кривых, подтвердили, что быстрая закалка от самой высокой температуры сразу после тепловой обработки не может подавить формирование упорядоченных частиц типа B2 в сверхстехиометрических сплавах Fe₃Al

Superplastic deformation behaviour and microstructure evolution of near-α Ti-Al-Mn alloy

Superplastic deformation behaviour of conventional sheets of a near-α titanium alloy (Ti-2.5Al-1.8Mn) was studied by a step-by-step decrease of the strain rate and constant strain rate tests in a temperature range of 790–915 °C. The research found that superplastic deformation is possible in a temperature range of 815–890 °С and a constant strain rate range of 2 × 10−4 to 1 × 10−3 s−1 with elongation above 300% and m-index above 0.4. Also, the research identified the optimum superplastic temperature range of 815–850 °C and constant strain rate of 4 × 10−4 s−1 which provide a maximum elongation of 600–650%. Strain hardening is accelerated by dynamic grain growth at high temperatures of 865 and 890 °С. High dislocation activity is observed at superplastic flow in α-phase. Constitutive modelling of superplastic deformation behaviour is performed, and possible deformation mechanisms are discussed. It is suggested that grain boundary sliding between the α-grains is accommodated by a dislocation slip/creep mechanism

The Dislocation-Enhanced Finkelshtain-Rosin Effect (DEFRE) in Austenitic Steels

A systematic study of the influence of plastic deformation and low temperature ageing on the carbon Dislocation - Enhanced Finkelshtain - Rosin Effect (DEFRE/C) in austenitic alloys of Fe-Ni, Fe-Ni-Mo, Fe-Mn-Al and Fe-Cr-Ni systems with different carbon content is reported. The experiments prove that after plastic deformation at room temperature the FR effect is transformed into DEFRE/C, with both phenomena occurring at the same temperature. The influence of plastic deformation, carbon and alloying elements concentration on DEFRE/C in deformed austenitic steels is reported

Damping in some Metallic Cellular Metals due to Microplasticity

The damping capacities of a few porous structures are studied. Damping in porous structures is enhanced by localised stresses in comparison with the corresponding dense materials. A model for mechanical damping in porous materials is suggested for the "microplastic" range of deformation on the basis of statistical mechanics of micro-heterogeneous materials. Two types of heterogeneity are discussed: one-level porosity (direct by sintered powder steels) and two-level porosity (larger pores in carcass with smaller pores). (C) 2003 Elsevier B.V. All rights reserved

The Hydrogen Influence upon Oxygen Snoek Relaxation in Nb-Ti Alloys

The oxygen Snoek peak was measured at frequency f = 1..10 Hz for Nb-0.05 wt.%O and 48wt.%Nb - 52wt.%Ti - 0.1wt.%O alloys. The alloys hydrogenizing leads to the decrease of height and to the shift of the temperature of oxygen Snoek peak for Nb-O and Nb-Ti-O alloys both. The new approach for computer simulation of temperature dependent internal friction spectrum has been carried out for the study of the hydrogen influence mechanism. The following pair interactions have been taken into account : H-O, H-Ti, O-Ti, H-H, O-O. The long-ranged strain-induced (elastic) interaction model, supplemented by short-range Coulomb repulsion for H-H, H-O, O-O and "chemical" interaction for H-substitutional and O-substitutional atom was used for simulation. The influence of the interaction on atoms arrangement (short-range order), the activation energy and the pre-exponential factor of oxygen relaxation time was taken into account. It is shown that the influence of hydrogen on oxygen Snoek peak can be explained by the influence of hydrogen on energy of oxygen atoms in solid solution and therefore on activation energy of diffusion

The Study of Microplasticity Mechanism in Ti-50 wt.%Nb Alloy with High Hydrogen Content

The upper yield point (≈700MPa) appears at the compression test curves (ε=0.024 sec-1) of b.c.c. Nb-50wt.%Ti due to the increase of hydrogen content from 0 to 0.2 wt.% and more and leads to the non monotonous increase in compressive lower yield stress from 400 to 550 MPa. Taking into account close connection between macro- and microplasticity of metallic materials the low frequency (≈2Hz) amplitude dependent internal friction (ADIF) spectrum (γ=1 .. 60.10-5) in hydrogenized Nb-50 wt.% Ti and Nb samples are studied. The ADIF investigation of the closed hysteresis loop "loading-unloading" shows the dependence of its width from the hydrogen content which evidences the fact of dislocation unpinning from hydrogen atmospheres in the 1/2 cycle of loading. The study of ADIF spectrum for samples with different hydrogen content before and after torsion deformation (γ≈2%) shows the sharp increase of IF level at γ=1 .. 10.10-5 after ≈1 hour of natural ageing. At that time the ADIF curves change its shape from Γ-shape to U-shape. The amplitude range of the IF increase depends on the hydrogen content. It is the interaction of hydrogen atoms with dislocations that caused the above mentioned effect which has not been observed in hydrogen free samples. The time estimation for the formation of thermodynamically stable hydrogen atmospheres on dislocations shows that hydrogen atmospheres could not follow the dislocation during compressive tests and that leads to the upper yield point appearance