Substitutional Alloying Elements Research Articles

Modeling solute drag during austenite–ferrite transformation with ab initio binding energies

The solute drag effect can have a considerable impact on the austenite to ferrite transformation. Most models describing the solute drag effect require knowledge of solute binding energies, which due to lack of accurate data typically are treated as fit parameters. In this study we use density functional theory (DFT) calculations to estimate the effective binding energies of the substitutional alloying elements in an ultra low carbon steel. For the first time these energies are used to calculate the solute drag effect considering site competition for the austenite to ferrite phase transformation. The depletion of elements in solid solution as a result from precipitation is computed and the solute drag contribution of each element is calculated. Comparison of the binding energies with the resulting solute drag effect to literature data shows reasonable agreement. The outlined approach points the way to future alloy development based on interface-controlled integrated computational materials engineering. Another field of application is given by the circular economy driven transition in the steel industry from the BF/BOF- to the EAF-based production route with increased utilization of scrap material introducing new tramp elements.

Understanding fusion zone hardness in resistance spot welds for advanced high strength steels: Strengthening mechanisms and data-driven modeling

The fusion zone (FZ) hardness of resistance spot welds is a crucial factor affecting the performance and durability of the welds. Failure mode transition, tendency to fail in interfacial mode, interfacial failure load, and the impact of liquid metal embrittlement cracks on the weld strength are influenced by the FZ hardness. Therefore, accurately predicting the FZ hardness in resistance spot welds made on automotive steels is essential. A simple thermal model is used to calculate the time required for the temperature to drop from 800 °C to 500 °C (Δt85). With the aid of continuous cooling transformation (CCT) diagrams and experimental confirmation, it is shown that in most automotive steels, the FZ exhibits an almost full martensitic microstructure. Generally, the FZ hardness tends to increase in the order of interstitial-free (IF), drawing quality specially killed (DQSK), high strength low alloy (HSLA), ferrite-martensite dual-phase steels, transformation induced plasticity (TRIP), and quench & partitioning (Q&P) steels. To predict the FZ hardness, data-driven regression-based models have been developed based on carbon content, carbon equivalent concept, and the strengthening mechanisms of the martensite. Among these models, the model based on martensite strengthening mechanisms showed the best performance and robustness in estimating the FZ hardness. The assessment of the strengthening mechanism of the FZ showed that most important factors determining the FZ hardness are dislocation hardening due to carbon atoms, interface hardening due to block boundaries, and solid solution hardening due to substitutional alloying elements, respectively. A simple relation is proposed to estimate the load-bearing capacity of automotive steel spot welds during interfacial failure based on the FZ hardness.

Redistribution of substitutional alloying elements between α-Fe matrix and cementite after long-term tempering in a low alloying steel

ABSTRACT The partitioning behaviour of non-carbide forming elements (Si, Cu and Ni) and carbide forming elements (Mn and Mo) between α-Fe matrix and cementite (θ) after long-term tempering a quenched reactor pressure vessel (RPV) model steel has been studied by atom probe tomography (APT). It has demonstrated that a non-carbide forming element enriched layer near the interface of α/θ in the α-Fe matrix side is a common feature for tempered steels containing non-carbide forming element, which is widely accepted as the mechanism by which substitutional elements retard the coarsening of cementite. In addition, a carbide forming element enriched layer near the interface of α/θ in the cementite side, Si, Cu and Ni enriched clusters formed in the cementite and Mo2C carbide formed beside cementite were observed, and all this is taken as valuable mechanisms by which addition of substitutional alloying elements can increase tempering resistance of alloying steels.

Novel insights on the near atomic scale spatial distributions of substitutional alloying and interstitial impurity elements in Ti-6Al-4V alloy

The present study explores the near atomic scale spatial distributions of substitutional alloying elements (aluminium and vanadium) and interstitial impurities (oxygen and carbon) in the microstructures of Ti-6Al-4V and its boron modified counterpart (Ti-6Al-4V-0.1B) alloys using transmission electron microscopy (TEM) and atom probe tomography (APT) techniques. The latter alloy possesses zero solubility of boron in α-Ti and high oxygen concentration around TiB particles. The results suggest that aluminium and vanadium atoms enrich α and β phases, respectively due to strong elemental partitioning between these phases, to an extent that the atomic scale concentrations of various elements considerably differ from the bulk compositions of the two alloys. Furthermore, oxygen atoms are preferentially distributed within the α phase where other interstitial impurities (carbon) also segregate. Substitutional aluminium atoms share an inverse interrelation with interstitial oxygen atoms while vanadium atoms reside besides the impurity-rich regions for both the alloys. The size differences for these elements (substitutional vs. interstitial) with host titanium atoms and related lattice distortion is (partially) held responsible for the atomic scale elemental distributions and the observed interrelations within the α phase. These arguments are further substantiated from DFT results previously obtained for Ti-6Al-4V alloy.

An investigation of precipitation strengthened Inconel 718 superalloy after triode plasma nitriding

In this study, we investigated the microstructural evolution, surface hardening and general corrosion properties of a precipitation-strengthened Inconel 718 Ni-superalloy after triode-plasma nitriding (TPN) at low treatment temperatures of 400–450 °C (i.e. thermodynamic paraequilibrium conditions) and a high treatment temperature of 700 °C. At low treatment temperatures, apart from the formation of nitrogen-expanded austenite (γN) from the high-Cr γ matrix, the pre-existing γ′ and γ″ intermetallic nano-precipitates appear to exhibit different nitriding responses. The spheroidal N-modified γ′ (or γ′N) precipitates were ‘slightly-expanded’, leading to slightly shifted XRD peaks, i.e. 2-theta angles of ~0.2° from γsubstrate(111) and ~0.5° from γsubstrate(200). In contrast, N-modified γ″ (or γ″N) could experience substantial lattice expansion close to that of the γN matrix. With increasing treatment temperature, nitride formation started as additional nano-sized precipitates (e.g. ~3–6 nm diameter as observed at 450 °C) and grew into laths (e.g. ~5–10 nm thick and ~15–30 nm wide as observed at 700 °C).Without changing core microstructure/properties, surface nitrogen modification and hardening were obtained on alloy 718 after TPN (e.g. from ~486 HV0.025 to ~1212 HV0.025 after TPN at 400 °C). No degradation of corrosion performance was observed for the nitrogen-supersaturated surface after TPN at 400 °C. However, the 450 °C TPN-treated surface showed a slightly increased current density in the anodic region, which can be associated with early-stage nitride formation. The significantly deteriorated corrosion performance after TPN treatment at 700 °C is due to pronounced nitride formation and segregation of substitutional alloying elements.

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.

Martensitic αʺ-Fe16N2-Type Phase of Non-Stoichiometric Composition: Current Status of Research and Microscopic Statistical-Thermodynamic Model

The literature (experimental and theoretical) data on the tetragonality of martensite with interstitial–substitutional alloying elements and vacancies are reviewed and analysed. Special attention is paid to the studying the martensitic αʺ-Fe16N2-type phase with unique and promising magnetic properties as an alternative to the rare-earth intermetallics or permendur on the world market of the production of permanent magnets. The period since its discovery to the current status of research is covered. A statistical-thermodynamic model of ‘hybrid’ interstitial–substitutional solid solution based on a b.c.t. crystal lattice, where the alloying non-metal constituents (impurity atoms) can occupy both interstices and vacant sites of the host b.c.c.(t.)-lattice, is elaborated. The discrete (atomic-crystalline) lattice structure, the anisotropy of elasticity, and the ‘blocking’ and strain-induced (including ‘size’) effects in the interatomic interactions are taken into account. The model is adapted for the non-stoichiometric phase of Fe–N martensite maximally ordered by analogy with αʺ-Fe16N2, where nitrogen atoms are in the interstices and at the sites of b.c.t. iron above the Curie point. It is stressed an importance of adequate data on the available (in the literature) temperature- and concentration-dependent microscopic energy parameters of the interactions of atoms and vacancies. The features of varying (viz. non-monotonic decreasing with increasing temperature) the relative concentration of N atoms in the octahedral interstices of b.c.t. Fe, and therefore, the degree of its tetragonality (correlating with this concentration) are elucidated. Within the wide range of varying the total content of introduced N atoms, the ratio of the equilibrium concentration of residual site vacancies to the concentration of thermally activated vacancies in a pure b.c.c. Fe is demonstrated at a fixed temperature.

Understanding the interactions between interstitial and substitutional solutes in refractory alloys: The case of Ti-Al-O

Refractory metals such as titanium can dissolve significant quantities of interstitial oxygen. The interactions between dissolved interstitial species and substitutional alloying elements remain poorly understood in refractory metals, but are increasingly recognized as being of importance, especially in high entropy refractory alloys. We performed a first-principles statistical mechanics study of the interactions between substitutionally dissolved Al and interstitial O in hcp Ti. A systematic analysis of phase stability in the Ti-Al-O composition space predicts the low temperature stability of a new ternary Ti6Al2O compound. In spite of a well-known affinity between Al and O, extensive first-principles calculations using density functional theory predict a strong nearest-neighbor repulsion between Al and O when both are dissolved in hcp Ti. The repulsion has its origin in the unique electronic structure of the Ti host. The combination of a cluster expansion Hamiltonian and Monte Carlo simulations shows that the strong short-range interaction between dissolved Al and O has important consequences for phase stability and the degree of order at elevated temperature in Ti-Al-O alloys. Monte Carlo simulations predict that an increase in the dissolved oxygen concentration leads to a reduction of the order-disorder transition temperature of α-Ti3Al. The results of this study provide guidance as to how the solubility and degree of ordering of interstitial species in refractory metals can be tailored by different alloying strategies.

Pearlite in Multicomponent Steels: Phenomenological Steady-State Modeling

A steady-state model for austenite-to-pearlite transformation in multicomponent steel is presented, including Fe, C, and eight more elements. The model considers not only classic ingredients (formation of ferrite–cementite interface, volume diffusion, boundary diffusion, and optimization of lamellar spacing) but also finite austenite–pearlite interfacial mobility that resolves some previous difficulties. A non-Arrhenius behavior of interfacial mobility is revealed from growth rate and lamellar spacing data. A smooth and physical transition between orthopearlite and parapearlite is realized by optimizing the partitioning of substitutional alloying elements between ferrite and cementite to maximize growth rate or dissipation rate while keeping carbon at equilibrium. Solute drag effect is included, which accounts for the bay in growth rate curves. Grain boundary nucleation rate is modeled as a function of chemical composition, driving force, and temperature, with consideration of grain boundary equilibrium segregation. Overall transformation kinetics is obtained from growth rate and grain boundary nucleation rate, assuming pearlite colonies only nucleate on austenite grain boundaries. Further theoretical and experimental work are suggested for generalization and improvements.

Extremely rapid age hardening behavior in solution treated Ti-5Al-2Fe-3Mo

To grasp age hardening and phase transformation behaviors in a β rich α+β type titanium alloy, Ti-5Al-2Fe-3Mo, during aging at 300-500°C after the solution treatment at high α+β temperature. Vickers hardness and microstructure changes during aging were closely investigated using XDR and TEM/EDS. Vickers hardness rapidly increased with increasing holding time and reached about 440HV by aging at 450°C for only 5 min. It further increased to 510HV in 8h of aging time. The initial stage of age hardening is extremely fast compared to that in other conventional α+β and β type titanium alloys. After aging for only 5min, extremely fine acicular products of about 2 to 10 nm in width were formed in the transformed β phase. TEM/EDS analysis revealed that all substitutional alloying elements, Al, Fe and Mo, homogeneously distributed after the aging, indicating that the transformation is diffusionless as far as substitutional elements are concerned just like martensite transformation although it has time dependency. To explain the mechanism of this unique phase transformation having features of isothermal martensite transformation, we propose bainitic transformation where interstitials such as O diffuse without conspicuous diffusion of substitutional elements.

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.

Ferritic-martensitic structure in steel hardenable by copper precipitation

The copper precipitation offers possibility of effective and easily achievable ferrite strengthening. There is reported yield point rise by 150 MPa thanks to formation of nanometric copper precipitates in ferrite for Cu content 1.5 wt.%. Such rise of yield strength is most significant for soft and ductile materials, containing free ferrite. Ferritic-martensitic dual steels are example of such material. They are widely used for their favourable combination of strength and ductility. Their yield strength is defined by the soft phase, the ferrite. Ferrite strengthening in these steels is achieved mainly by grain refinement and by solution strengthening caused by substitutional alloying elements. This article describes preparation of dual phase structure in 0.2% C steel alloyed by Cu, Mn, Ti and B. Dual phase structure was prepared by controlled rolling with rolling finishing temperatures in the intercritical region. Samples were analysed by light and scanning electron microscopy. Hardness was measured and tensile tests were performed.

Atom Probe Compositional Analysis of Interphase Precipitated Nano-Sized Alloy Carbide in Multiple Microalloyed Low-Carbon Steels.

The composition of nano-sized alloy carbides formed by interphase precipitation in V-Nb and V-Ti multiple microalloyed low-carbon steels is analyzed by using three-dimensional atom probe. Carbide-forming alloying elements including V, Nb, and Ti, are simultaneously precipitated from the early stage of isothermal treatment, whose atoms are uniformly distributed in the carbide particles, even after prolonged holding. Cluster analysis by the maximum separation method, with parameters optimized using different methods, is carried out to extract alloy carbides from matrix. The composition of alloy carbides evaluated by site fraction of substitutional carbide-forming alloying elements indicates that at the early stage of their formation, Nb and Ti are more strongly enriched than V.

Carbon Redistribution in Martensite in High-C Steel: Atomic-Scale Characterization and Modelling

The microstructure of the as-quenched plate martensite in a high-C steel 100Cr6 was characterized by means of electron microscopy and atom probe tomography. The carbon redistribution behavior was investigated at the atomic scale, which revealed the nature of the transformation dynamics influenced by carbon and other substitutional alloying elements. A model was proposed to predict the carbon redistribution at twins and dislocations in martensite, which was based on their spatial arrangements.

A Novel Method to Calculate the Carbides Fraction from Dilatometric Measurements During Cooling in Hot-Work Tool Steel

Dilatometry is a useful technique to obtain experimental data concerning transformation. In this paper, a dilation conversional model was established to calculate carbides fraction in AISI H13 hot-work tool steel based on the measured length changes. After carbides precipitation, the alloy contents in the matrix changed. In the usual models, the content of carbon atoms after precipitation is considered as the only element that affects the lattice constant and the content of the alloy elements such as Cr, Mo, Mn, V are often ignored. In the model introduced in this paper, the alloying elements (Cr, Mo, Mn, V) changes caused by carbides precipitation are incorporated. The carbides were identified using scanning electron microscope and transmission electron microscope. The relationship between lattice constant of carbides and temperature are measured by high-temperature X-ray diffraction. The results indicate that the carbides observed in all specimens cooled at different rates are V-rich MC and Cr-rich M23C6, and most of them are V-rich MC, only very few are Cr-rich M23C6. The model including the effects of substitutional alloying elements shows a good improvement on carbides fraction predictions. In addition, lower cooling rate advances the carbides precipitation for AISI H13 specimens. The results between experiments and mathematical model agree well.

Distribution of Alloying Elements over the Phase Constituents in Nb–Si in situ Composites

The distribution of alloying elements over the phases in a eutectic Nb–Nb5Si3 composite material during directional solidification is studied. The influence of substitutional alloying elements on the type of structure in the reinforcing phase of silicide is analyzed. The crystal structures of the hexagonal and tetragonal modifications of the Nb5Si3 silicide are characterized and compared. The dependence of the creep resistance on the type of silicide structure is explained.

First-principles simulations of binding energies of alloying elements to the ferrite-austenite interface in iron

The kinetics of the ferrite-austenite (bcc-fcc) phase transformation in steels are markedly affected by substitutional alloying elements. However, the detailed mechanisms of their interaction with the bcc-fcc interfaces are not fully understood. In this study, the effects of common alloying elements (e.g., Nb, Mo, Mn, Si, Cr, and Ni) on the structure, segregation, and magnetic properties of bcc-fcc interfaces in Fe are systematically investigated using spin-polarized Density Functional calculations within a generalized gradient approximation to the exchange correlation potential and a super cell approach with a Kurdjumov-Sachs orientation relationship between bcc and fcc. The calculation results are in semi-quantitative agreement with the experimental results, i.e., Nb has the largest binding energy to the bcc-fcc interface in Fe followed by Mo.

Bainite Transformation During Continuous Cooling: Analysis of Dilatation Data

The amount of athermal martensite as a function of undercooling below the martensite start temperature was quantified by analyzing the dilatation data using a novel method, and the results are compared with existing empirical equations. The discrepancy between the two results was attributed to the difference in the concentration ranges of the alloying elements considered. The importance of including the effect of substitutional elements on the lattice parameters of martensite for accurate quantitative interpretation of dilatation data was highlighted. Equations that include the effect of substitutional alloying elements were proposed to calculate martensite lattice parameters. It is further shown that it is possible to calculate the lattice parameter coefficient of a substitutional alloying element directly from the dilatation curve. It was used to estimate, for the first time, the lattice parameter coefficient of aluminum (Al) in ferrite/martensite from the dilatation curves of the two alloy steels studied in the current work. To corroborate the value of the lattice parameter coefficient of Al estimated from the dilatation data, the Bain model was also used to calculate the lattice parameter coefficient of Al independently and a good match was obtained. The lattice parameter coefficient value of Al in ferrite/martensite calculated by both these methods follows the overall trend shown by other substitutional alloying elements. The equations proposed for the lattice parameters of martensite were validated by Rietveld analysis of the X-ray diffraction (XRD) patterns.

Elucidating the effect of Mn partitioning on interface migration and carbon partitioning during Quenching and Partitioning of the Fe-C-Mn-Si steels: Modeling and experiments

A fundamental understanding of carbon partitioning and the martensite/austenite interface migration during the Quenching & Partitioning (Q&P) process is essential for tailoring the microstructures of the advanced Q&P steels. In this study, two Q&P models (QP-LE and QP-PE), in which effects of substitutional alloying elements have been considered by assuming Local Equilibrium (LE) or Paraequilibrium (PE) at the interface, are proposed to simulate the kinetics of martensite/austenite interface migration and carbon partitioning in the Fe-C-Mn-Si steels. The QP-LE and QP-PE models predict that whether the interface migrates or not and its moving direction are dependent on quenching/partitioning temperature (QT/PT) and alloy composition. A careful comparison between experiments and model predictions indicates that the dependence of interface migration behavior on QT/PT indirectly measured by experiments during the Q&P process could be explained reasonably well by the QP-LE model, rather than the Constrain Carbon Equilibrium (CCE) model and the QP-PE model. The carbon partitioning behavior is also well predicted by the QP-LE model while the QP-PE model predictions deviate significantly from experiments. The current study suggests that interfacial partitioning of Mn plays a significant role in carbon partitioning and interface migration during the Q&P process.