High-temperature Partitioning Research Articles

Thermodynamic Study of the High-Temperature Partitioning Process of Carbon Elements in Intercritical Quenching and Partitioning High-Strength Steels

Thermodynamic Study of the High-Temperature Partitioning Process of Carbon Elements in Intercritical Quenching and Partitioning High-Strength Steels

Heterogeneous Multiphase Microstructure Formation through Partial Recrystallization of a Warm-Deformed Medium Mn Steel during High-Temperature Partitioning.

A novel processing route is proposed to create a heterogeneous, multiphase structure in a medium Mn steel by incorporating partial quenching above the ambient, warm deformation, and partial recrystallization at high partitioning temperatures. The processing schedule was implemented in a Gleeble thermomechanical simulator and microstructures were examined by electron microscopy and X-ray diffraction. The hardness of the structures was measured as the preliminary mechanical property. Quenching of the reaustenitized sample to 120 °C provided a microstructure consisting of 73% martensite and balance (27%) untransformed austenite. Subsequent warm deformation at 500 °C enabled partially recrystallized ferrite and retained austenite during subsequent partitioning at 650 °C. The final microstructure consisted of a heterogeneous mixture of several phases and morphologies including lath-tempered martensite, partially recrystallized ferrite, lath and equiaxed austenite, and carbides. The volume fraction of retained austenite was 29% with a grain size of 200–300 nm and an estimated average stacking fault energy of 45 mJ/m2. The study indicates that desired novel microstructures can be imparted in these steels through suitable process design, whereby various hardening mechanisms, such as transformation-induced plasticity, bimodal grain size, phase boundary, strain partitioning, and precipitation hardening can be activated, resulting presumably in enhanced mechanical properties.

Heterogeneous Multiphase Microstructure Formation Through Partial Recrystallization of a Warm-Deformed Medium Mn Steel During High Temperature Partitioning

Heterogeneous Multiphase Microstructure Formation Through Partial Recrystallization of a Warm-Deformed Medium Mn Steel During High Temperature Partitioning

Characterization of untransformed ferrite in 10Cr and 12Cr ODS steels

Two new ferrito-martensitic oxide dispersion strengthened (ODS) steels reinforced with (Y, Ti, O) nanoparticles were elaborated using a high-energy attritor. The milled powder was consolidated by hot extrusion at 1050 °C. The two types of ODS steels differ by chromium content, with 10 wt% Cr and 12 wt% Cr respectively. According to thermodynamic calculations, those grades are supposed to exhibit an austenitic transformation at high temperatures. X-ray diffraction (XRD) above austenitic temperature transformation reveals the presence of both ferrite and austenite phase. This unexpected ferrite phase is assumed to be untransformed low temperature ferrite. The α→γ phase transformation specific enthalpy is monitored by differential scanning calorimetry (DSC). The untransformed ferrite fraction is calculated using dilatometric data and confirmed by electron backscatter diffraction (EBSD) microstructural analysis. The quenched samples from the austenitic domain give an image of the high-temperature partitioning. EBSD maps reveal two distinct elementary microstructures, one martensitic inherited from austenite and the other corresponds to the untransformed ferrite. This untransformed ferrite keeps the crystallographic α-fiber conferred by hot-extrusion. The 10 Cr ODS has equiaxed untransformed ferrite areas. In contrast, the untransformed ferrite into 12 Cr ODS is distributed as elongated areas, parallel to the hot-extrusion direction. Moreover, electron probe micro analyzer (EPMA) mapping exhibits chromium content gradients, consistent with phase partitioning at high temperatures. Creep properties are evaluated at 650 °C for both grades. Small-angle X-rays scattering (SAXS) shows a similar size and distribution of the oxide particles in both grades.

Microstructural Impact of Si and Ni During High Temperature Quenching and Partitioning Process in Medium-Mn Steels

Austenite stabilization through carbon partitioning from martensite into austenite is an essential aspect of the quenching and partitioning (Q&P) process. Substitutional alloying elements are often included in the chemical composition of Q&P steels to further control the microstructure development by inhibiting carbide precipitation (silicon) and further stabilize austenite (manganese and nickel). However, these elements can interfere in the microstructure development, especially when high partitioning temperatures are considered. In this study, the microstructural development during the Q&P process of four low-carbon, medium-manganese steels with varying contents of silicon and nickel is investigated. During partitioning at 400 °C, silicon hinders cementite precipitation in primary martensite thereby assisting carbon partitioning from martensite to austenite. During partitioning at temperatures of 500 °C and 600 °C, presence of nickel inhibits pearlite formation and promotes austenite reversion, respectively. It is observed that the stabilization of austenite is significantly enhanced through the addition of nickel by slowing down the kinetics of competitive reactions that are stimulated during the partitioning stage. Results of this study provide an understanding of the interplay among carbon, silicon and nickel during Q&P processing that will allow the development of new design strategies to tailor the microstructure of this family of alloys.

Evaluation of Equivalent Cladding Reacted parameters of Cr-coated claddings oxidized in steam at 1200 °C in relation with oxygen diffusion/partitioning and post-quench ductility

Cr-coated M5Framatome1 cladding materials are studied and developed within the CEA-Framatome-EDF French nuclear fuel joint program as Enhanced Accident Tolerant Fuel claddings for Light Water Reactors. The objective of this paper is to bring some insights into the relationship between Equivalent Cladding Reacted (ECR) parameters, oxygen diffusion/partitioning and Post-Quench (PQ) ductility of Cr-coated M5Framatome fuel claddings oxidized in steam at 1200 °C.The physical meaning of the ECR parameter, evaluated experimentally from the measured Weight Gain (WG) or calculated using time and temperature correlations such as the Baker-Just (BJ) or Cathcart-Pawel (CP) kinetics correlations, is discussed in the light of the benefit brought by Cr coating to oxidation resistance of cladding. As shown in this article, when applied to the Cr-coated M5Framatome materials, the “experimental” ECR derived from WG does not have the same physical meaning than for the uncoated cladding materials. As discussed in the paper, this is fundamentally due to the use of the ECR as a surrogate for retained ductility for uncoated claddings, and to the differences between uncoated and Cr-coated cladding in the high temperature (HT) steam oxidation processes and partitioning of the oxygen between the different layers of the oxidized cladding.It is shown in this article that Cr-coated M5Framatome cladding brings significant additional time-at-temperature before full embrittlement of the cladding after one-sided oxidation at 1200 °C and quenching, compared to uncoated materials. The oxidation times and associated Baker-Just ECR (BJ-ECR) values, above which the cladding becomes brittle after low temperature quenching, are respectively ten times and three times higher than the ones for the uncoated reference cladding. When analyzing the PQ ductility of the Cr-coated M5Framatome cladding using a similar methodology as the one used to derive the ECR criterion for uncoated cladding, the 1–2% ductility limit corresponds to a BJ-ECR of about 50% or higher, for a 12-15 μm-thick Cr-coated cladding tested herein.

Microstructure evolution during high-temperature partitioning of a medium-Mn quenching and partitioning steel

Medium-Mn Quenching & Partitioning (Q&P) steels have been recently considered as potential candidates for the 3rd generation advanced high-strength steels. The processing of these steels aims to induce the partitioning of substitutional alloying elements from martensite to austenite during an isothermal treatment at high temperature, where the diffusivity of substitutional alloying elements is sufficiently high. In this way, austenite increases its concentration of austenite-stabilising elements and thus its thermal stability. The present study aims to investigate the microstructural evolution during high temperature partitioning treatments in a medium-Mn steel and the possible occurrence of additional phase transformations that may compete with the process of atomic partitioning between martensite and austenite. Q&P routes in which the partitioning steps take place in the range of 400 °C–600 °C for times up to 3600 s were investigated. The final microstructures display an increased fraction of retained austenite with increasing holding times during partitioning at 400 °C, while at higher partitioning temperatures, 450 °C–600 °C, leads to cementite precipitation in austenite films and pearlite formation in blocky austenite, resulting in a decrease of the fraction of retained austenite with the holding time. This observation is supported with theoretical calculations of the volume change, suggesting that for maximising the fraction of retained austenite, short holding times are preferred during partitioning at high temperatures. Observations from the current study reveal that the successful application of high-temperature partitioning treatments in medium-Mn steels requires microstructure design strategies to minimize or suppress competitive reactions.

Microstructure Evolution During High-Temperature Partitioning of a Medium-Mn Quenching and Partitioning Steel

Medium-Mn Quenching & Partitioning (Q&P) steels have been recently considered as potential candidates for the 3rd generation advanced high-strength steels. The processing of these steels aims to induce the partitioning of substitutional alloying elements from martensite to austenite during an isothermal treatment at high temperature, where the diffusivity of substitutional alloying elements is sufficiently high. In this way, austenite increases its concentration of austenite-stabilising elements and thus its thermal stability. The present study aims to investigate the microstructural evolution during high temperature partitioning treatments in a medium-Mn steel and the possible occurrence of additional phase transformations that may compete with the process of atomic partitioning between martensite and austenite. Q&P routes in which the partitioning steps take place in the range of 400 °C - 600 °C for times up to 3600 s were investigated. The final microstructures display an increased fraction of retained austenite with increasing holding times during partitioning at 400 °C, while at higher partitioning temperatures, 450 °C - 600 °C, leads to cementite precipitation in austenite films and pearlite formation in blocky austenite, resulting in a decrease of the fraction of retained austenite with the holding time. This observation is supported with theoretical calculations of the volume change, suggesting that for maximising the fraction of retained austenite, short holding times are preferred during partitioning at high temperatures. Observations from the current study reveal that the successful application of high-temperature partitioning treatments in medium-Mn steels requires microstructure design strategies to minimize or suppress competitive reactions.

Octahedral cation partitioning in Mg,Fe2+-olivine. Mössbauer spectroscopic study of synthetic (Mg0.5 Fe2+ 0.5)2SiO4 (Fa50)

The high-temperature partitioning of Fe2+ and Mg between the two nonequivalent octahedral M1 and M2 sites in synthetic olivine (Fa50) was studied by Mossbauer spectroscopy. Powder samples have been equilibrated in annealing experiments performed under reducing oxygen fugacity at temperatures between 500 and 800°C followed by rapid quenching in order to prevent redistribution of cations. M-site ordering with Fe2+ preferring M1, Mg preferring M2 sites increases continuously with rising equilibrium temperature. K D values increase from 1.21 at 500°C to 1.48 at 750°C. The results are consistent with both room temperature as well as in situ high temperature single crystal X-ray diffraction experiments of Heinemann et al. [8, 9].

Octahedral cation partitioning in Mg,Fe2+-olivine. Mossbauer spectroscopic study of synthetic (Mg0.5 Fe2+0.5)2SiO4(Fa50)

The high-temperature partitioning of Fe 2+ and Mg between the two non-equivalent octahedral M1 and M2 sites in synthetic olivine (Fa 50 ) was studied by Mossbauer spectroscopy. Powder samples have been equilibrated in annealing experiments performed under reducing oxygen fugacity at temperatures between 500°C and 800°C followed by rapid quenching in order to prevent redistribution of cations. M-site ordering with Fe 2+ preferring M1, Mg preferring M2 sites increases continuously with rising equilibrium temperature. K D values increase from 1.21 at 500°C to 1.48 at 750°C. The results are consistent with both room temperature as well as in situ high temperature single crystal X-ray diffraction experiments of Heinemann et al. (2003a and b).

Partitioning of trace elements between crystals and melts

Advances in analytical geochemistry have made it possible to determine precisely the concentration of many trace elements and their isotopes in rocks. These data provide the cornerstone for geochemical models of the Earth and terrestrial planets. However, our understanding of how trace elements behave has not kept pace with the analytical advances. As a result, geochemists are often hampered in their interpretation of geochemical data by an incomplete knowledge of trace element partitioning under the conditions of interest. Through advances in trace element microbeam analysis it is now possible to determine partition coefficients experimentally under important conditions, such as during melting of the crust and mantle. This large body of experimental data can be used to investigate the fundamental controls on element partitioning. Simple continuum theories of elastic strain and point charges in crystal lattices, that account, respectively, for mismatch in ionic radius and ionic charge between the substituent trace ion and the lattice site on which it is accommodated, provide a very useful theoretical framework. This approach can be used as the basis for quantitative models of trace element partitioning, in terms of pressure, temperature, redox state and composition, and as a means of predicting partition coefficients for elements not routinely analysed. Experimental studies of partitioning are supported by atomistic computer simulations at zero K. Developments in computational techniques that enable direct simulation of high temperature and pressure mineral-melt partitioning will revolutionise the field in the near future. Use of novel, spectroscopic techniques to probe the structural environment of trace elements in crystals and glasses will provide valuable new data for computational and theoretical models. Extension of high temperature partitioning theory to ambient conditions is an essential step in understanding current climate change proxies, and tackling a host of environmental problems.

High-temperature partitioning of 181Hf-probe impurities between Li and group-V sites in LiNbO3 and LiTaO3.

Perturbed-angular-correlation (PAC) spectroscopy was used to measure nuclear-electric-quadrupole interactions at high temperatures in ceramic ternary metal oxides, ${\mathrm{LiNbO}}_{3}$ and ${\mathrm{LiTaO}}_{3}$. In these ferroelectric ceramics, the $^{18}\ensuremath{\rightarrow}^{181}$Ta PAC probe was carried by approximately 0.03 at. % Hf as an impurity dopant, and the PAC measurements were made over a temperature range from approximately 1300 to 1700 K, which included the ferroelectric-to-paraelectric transition for ${\mathrm{LiNbO}}_{3}$. As we recently reported, at temperatures below \ensuremath{\approxeq}1100 K, the probe substitutes primarily into the Li sites in both compounds. At higher temperatures, we find that the $^{181}\ensuremath{\rightarrow}^{181}$Ta probe partitions between the Li and group-V sites. At these higher temperatures, the Li-site electric-field gradients (efg's) are characterized by high frequencies that are similar to those observed at lower temperatures. The group-V site efg's are characterized by frequencies that are approximately one tenth the magnitude of those corresponding to the Li sites. The occupancy of the Li site decreases and that of the group-V sites increases monotonically with increasing temperature. This partitioning is an equilibrium, reversible process that can be described by a simple thermodynamic model of second-order displacement. The temperature dependence of the associated equilibrium constant indicates enthalpies of several eV for the process.

Partitioning behaviour of oxygen in steam-oxidized Zircaloy

A model has been developed which describes the high temperature partitioning behaviour of oxygen in Zircaloy. The growth kinetics of the oxide/α and α/β layers are derived from a consideration of the material continuity requirements at the respective phase boundaries. It is shown that the rate of advance of the α/β interface in specimens of finite thickness will deviate from parabolic behaviour, a departure attributable to the progressive approach of the total oxygen content in the β phase towards its saturation value. This conclusion is significant since determination of the oxygen profile in the β phase is dependent on reliable evaluation of the α/β interface advance kinetics. The validity of the model is tested by comparing the observed oxide/α and α/β interface growth rates with those derived using basic oxygen diffusion and solubility data in conjunction with total oxygen uptake kinetics.