Segregation, precipitation, andα−α′phase separation in Fe-Cr alloys

Abstract

Iron-chromium alloys, the base components of various stainless steel grades, have numerous technologically and scientifically interesting properties. However, these features are not yet sufficiently understood to allow their full exploitation in technological applications. In this work, we investigate segregation, precipitation, and phase separation in Fe-Cr systems analyzing the physical mechanisms behind the observed phenomena. To get a comprehensive picture of Fe-Cr alloys as a function of composition, temperature, and time the present investigation combines Monte Carlo simulations using semiempirical interatomic potential, first-principles total energy calculations, and experimental spectroscopy. In order to obtain a general picture of the relation of the atomic interactions and properties of Fe-Cr alloys in bulk, surface, and interface regions several complementary methods have to be used. Using the exact muffin-tin orbitals method with the coherent potential approximation (CPA-EMTO) the effective chemical potential as a function of Cr content (0-15 at. % Cr) is calculated for a surface, second atomic layer, and bulk. At ∼10 at. % Cr in the alloy the reversal of the driving force of a Cr atom to occupy either bulk or surface sites is obtained. The Cr-containing surfaces are expected when the Cr content exceeds ∼10 at. %. The second atomic layer forms about a 0.3 eV barrier for the migration of Cr atoms between the bulk and surface atomic layer. To get information on Fe-Cr in larger scales we use semiempirical methods. However, for Cr concentration regions less than 10 at. %, the ab initio (CPA-EMTO) result of the important role of the second atomic layer to the surface is not reproducible from the large-scale Monte Carlo molecular dynamics (MCMD) simulation. On the other hand, for the nominal concentration of Cr larger than 10 at. % the MCMD simulations show the precipitation of Cr into isolated pockets in bulk Fe-Cr and the existence of the upper limit of the solubility of Cr into Fe layers in Fe/Cr layer systems. For high Cr concentration alloys the performed spectroscopic measurements support the MCMD simulations. Hard x-ray photoelectron spectroscopy and Auger electron spectroscopy investigations were carried out to explore Cr segregation and precipitation in the Fe/Cr double layer and Fe 0.95 Cr 0.05 and Fe 0.85 Cr 0.15 alloys. Initial oxidation of Fe-Cr was investigated experimentally at 10 −8 Torr pressure of the spectrometers showing intense Cr 2 O 3 signal. Cr segregation and the formation of Cr-rich precipitates were traced by analyzing the experimental atomic concentrations and chemical shifts with respect to annealing time, Cr content, and kinetic energy of the exited electron.

Mössbauer Spectroscopy Study of Laboratory Produced ODS Steels

“…The reason for absence of the iron rich phase and chromium rich α phase in the case of KOT 18 and KOT 17 samples could be due to elevated amount of Cr content which is in the case of both samples around 14.7 wt% [11,12], despite of the fact that those two materials are containing Al which is known as alloying elements encouraging of α-Fe formation [13]. The paramagnetic singlet significantly contributes to the spectrum of ODS steel containing 16 wt% of Cr (see Fig.…”

Кинетика Преципитации В Бинарных Сплавах Вблизи Границ Зерен

“…Львов, В.В. Светухин мацией кристаллической решетки [12], либо с изменением энергии парного взаимодействия между компонентами раствора [4][5][6]. Как правило, считается, что параметры взаимодействия изменяются в достаточно узком слое, характерная толщина которого δ составляет несколько периодов решетки (см.…”

“…Как правило, считается, что параметры взаимодействия изменяются в достаточно узком слое, характерная толщина которого δ составляет несколько периодов решетки (см. например, [5,6]). В качестве модельной аппроксимации данной зависимости удобно использовать степенные или экспоненциальные функции координат [10], отражающие плавное изменение параметра взаимодействия от 0 до GB на характерной толщине зернограничного слоя δ. В более общем случае, когда требуется рассматривать более двух зерен, а также учитывать движение самих границ (см.…”

“…состоят из большого числа зерен, представляющих собой фрагменты с однородной кристаллической структурой [1][2][3]. Области сопряжения (границы) соседних зерен, имеющих различную ориентацию, представляют собой дефекты кристаллической структуры и могут приводить к изменению энергии взаимодействия между компонентами твердого раствора [4][5][6]. В результате границы зерен могут оказывать существенное влияние на различные характеристики материалов, например, на их прочностные свойства [2], распределение фаз [2,3], скорости диффузии компонентов [2,[6][7][8][9] и др.…”

First-principles localized cluster expansion study of the kinetics of hydrogen diffusion in homogeneous and heterogeneous Fe-Cr alloys

“…They are also promising structural materials for Generation IV reactors [19]. The bcc Fe-Cr binary alloy represents a model system for ferritic steels and for binary alloys in general and has therefore been the subject of various recent studies [20][21][22][23][24][25].…”