Stability Of Laves Phase Research Articles

Additively manufactured ultrastrong Al-containing austenitic stainless steels via engineering cellular structure with multi-nanoprecipitates

High-performance austenitic stainless steel with ultrahigh strength is in strong demand across various industries. However, traditional austenitic stainless steels generally exhibit lower strength, particularly at elevated temperatures. In this contribution, an austenitic stainless steel with ultrahigh strength from ambient temperature to elevated temperature was successfully fabricated via laser additive manufacturing. This achievement is realized by engineering incoherent and coherent multi-nanoprecipitates via cellular structure in austenite, including the co-precipitation of incoherent precipitates of Laves phase, B2 phase and Cr-rich α′ phase at the cellular walls and coherent L12 phase within the cellular structure. The cellular dislocation networks in austenite introduce high density incoherent Laves phase within grains by providing massive nucleation sites, followed by the successive formation of B2 phase and Cr-rich α′ phase around the Laves phase. The high density incoherent precipitates within austenite grains not only substantially strengthen the matrix at ambient temperature, but also contribute to the superior strength at elevated temperature significantly through interacting with dislocations and nano SFs. In addition, the co-precipitation structure substantially improves thermal stability of Laves phase. This work provides a methodology for engineering multi-nanoprecipitates via cellular structure to fabricate austenitic stainless steel with superior performance.

High-temperature strengthening mechanism and thermal stability of Laves phase in ferritic matrix

The high-temperature creep strengthening mechanism and thermal stability of Laves phase are the key factors that affect its strengthening property in ferritic matrix. Therefore, in this study, two types of C14 Laves phase alloys, namely, Fe2Nb Laves phase Alloy S1 (Fe–9Cr–4Al–2Nb) (wt.%) and Fe2W Laves phase Alloy S2 (Fe–9Cr–4Al–2W) (wt.%) were prepared to investigate the strengthening mechanism and strengthening effect of Laves phase with different compositions in ferritic matrix. The creep properties and microstructure characteristics of the two alloys, in particular, the interfacial structure characteristics between Laves phases (Fe2Nb and Fe2W) and ferritic matrix were analyzed. The true stress exponent n for both Alloys S1 and S2 is 5, and the true activation energies Q of Alloys S1 and S2 are 372.47 and 365.82 kJ mol−1, respectively. The creep mechanisms of Alloys S1 and Alloy S2 are both dislocation climbing-dominated processes reinforced with precipitated phases. Alloy S1 exhibits higher high-temperature creep resistance than that of Alloy S2. The grain size of Alloy S1 is smaller than that of Alloy S2. Both the precipitates Fe2Nb in Alloy S1 and Fe2W in Alloy S2 are mostly located at the grain boundary. The density of dislocation in Alloy S1 is higher than that in Alloy S2, and the bending degree of dislocation lines caused by precipitate hindrance is also higher than that in Alloy S2. The orientation relationships between the Laves phase Fe2Nb and ferritic matrix in Alloy S1 are [011]α-Fe//[5 1‾9‾4‾]Fe2Nb, (011‾)α-Fe//(03 3‾ 1)Fe2Nb, with the atomic interface mismatch of 1.6%; the orientation relationships between the Laves phase Fe2W and ferritic matrix in Alloy S2 are [001]α-Fe//[01 1‾ 2]Fe2W, (1 1‾ 0)α-Fe//(01 1‾1‾) Fe2W, with the atomic interface mismatch of 1.9%. The results indicate that the Fe2Nb-type Laves phase may be more suitable to candidate for the strengthener in the ferritic matrix compared with Fe2W-type.

Atomic site occupancy of alloying elements and Laves phase stability in γ-γ′ Co-base superalloys

The atomic site occupancy of alloying elements in the ZrCo2 and HfCo2 based Laves phases formed in the Co-9Al-9W-2Zr and Co-9Al-9W-2Hf alloys was experimentally determined by the combined techniques of energy-dispersive X-ray spectroscopy mapping with atomic resolution and the atom location by channeling enhanced microanalysis (ALCHEMI) method. The results show that both Al and W occupy the Zr site in the C15 and the Hf site in the C36 crystal structure. The atomic locations of Al and W were further confirmed by comparing calculated inelastic cross-sections with experimental ALCHEMI results. Using the determined atomic site occupancies of alloying elements, special quasirandom structure solid solution models with 192 atoms were constructed and employed in first-principles calculations. It is found theoretically that the energy of formation of the C15 structure is always lower than that of the C36 structure at 0 K in both ZrCo2 and HfCo2 phases, no matter whether Al and W are incorporated or not. However, further ab initio molecular dynamics simulations suggest that lattice vibration at finite temperature contributes significantly to the phase stability, stabilizing the C36 structure compared to the C15 type for the (Hf, Al, W)Co2 phase at finite temperature, which fits well with the experimental findings.

The effect of Laves phase on heavy-ion radiation response of Nb-containing FeCrAl alloy for accident-tolerant fuel cladding

As a promising candidate material for the accident tolerant fuel cladding in light water reactors, the Nb-containing FeCrAl alloy has shown outstanding out-of-pile service performance due to the Laves phase precipitation. In this work, the radiation response in FeCrAl alloys with gradient Nb content under heavy ion radiation has been investigated. The focus is on the effect of the Laves phase on irradiation-induced defects and hardening. We found that the phase boundary between the matrix and Laves phase can play a critical role in capturing radiation defects, as verified by in-situ heavy-ion radiation experiments and molecular dynamic simulations. Additionally, the evolution of Laves phase under radiation is analyzed. Radiation-induced amorphization and segregations observed at high radiation doses will deepen the fundamental understanding of the stability of Laves phases in the radiation environment.

Effect of Zr addition on the stability of precipitated Laves phase and mechanical properties of Fe–Cr–Al-based alloys at high temperatures

The effect of a small amount of Zr addition on the temperature-dependent stability of Laves phase particles and mechanical properties of Fe-13.5Cr-4.73Al-2.07Mo-(0.34–0.5)Nb-(0.65–0.98)Ta-(0–0.33)Zr (wt. %) ferritic alloys was investigated in the present study. The designed alloy ingots were hot-rolled, aged at 1073 ​K for 24 ​h, and then re-treated at 1273 ​K, 1323 ​K, 1373 ​K, and 1473 ​K for 1 ​h, respectively. It was found that the Zr addition could not only stabilize the Fe2M Laves phase (M ​= ​Mo,Nb,Ta,Zr) to a much higher temperature, but also induce the formation of stable Fe23Zr6 phase. The high-temperature (HT) microstructural stability of the alloys significantly was improved, as evidenced by the fact that a certain amount (0.66–1.19%) of precipitates (Fe2M, Fe23Zr6, and core(Fe23Zr6)-shell(Fe2M)-structured particles) with an appropriate size (∼1.0 ​μm) uniformly distributed in the ferritic matrix even after being re-treated at 1473 ​K. Particularly, the formation of core-shell-structured particles at HTs was studied from the viewpoint of both solid solubility and diffusion coefficient of M in the matrix. All these aged alloys exhibited prominent mechanical properties at both room and elevated temperatures, showing high yield strength with σYS ​= ​490–560 ​MPa at room-temperature and σYS ​= ​80–85 ​MPa at 1073 ​K. The strengthening effect was further discussed in light of various strengthening mechanisms, and the calculated strength are in good agreement with the experimental results.

Significant Grain Refinement in the Simulated Heat-Affected Zone (HAZ) of Ferritic Stainless Steels by Alloying with Tungsten

We elucidate here the effect of tungsten on grain refinement in the HAZ of ferritic stainless steels (FSSs) at 1200 °C and 1350 °C. The average simulated HAZ grain size of W-containing FSS (1.5 wt. pct W) was reduced by ~ 50 pct compared to W-free FSS at 1350 °C. The replacement of Mo with W in FSSs enhanced the thermal stability of Laves phase at high temperatures and contributed to fine grain size in the HAZ via pinning effect.

Formation and stability of C14 type Laves phase in multi component high-entropy alloys

The present investigation reports the formation and stability of multi-component AB2-type Laves phase in Ti–Zr–Ni (V, Cr, Fe, Mn, Co)ternary to octonary alloys system. To probe the formation and stability of AB2-type Laves phase in high-entropy alloys, a series of multi-component alloys have been synthesized via melting under protective Ar atmosphere and subsequent annealing at 525 K for 40 h. Systematic investigation has been carried out to understand phase stability using both thermodynamic and parametric approaches. X-ray diffraction measurement and Rietveld analysis indicate the formation of C14 Laves phase in multi-component alloys. The enthalpy of mixing (ΔHmix), size difference (δ) and valence electron concentrations (VEC) have been found to play stellar role in controlling the formation of multi-component Laves phase in these alloys. The valence electron concentrations and electrons per atom ratio (e/a) dictate the choice of elements for the stabilization of Laves phase in these high-entropy alloys.

Validation of an alloy design strategy for stable Fe–Cr–Al–Nb-X ferritic alloys using electron microscopy and atom probe tomography

The Fe–Cr–Al–Nb ferritic alloys strengthened by A2B Laves phases, with superior oxidation and corrosion resistance, are being considered for high temperature operation within fossil-fired steam power plants to increase process efficiency and reduce CO2 emissions. In this study, new sets of alloys based on Fe–30Cr–3Al–1Nb (in weight percent) were designed with (i) a high microstructural stability of Laves phase precipitates in a BCC-Fe matrix and (ii) reduced precipitate free zones along the grain boundaries, targeting an operating temperature of 700 °C. Two alloys with titanium or tungsten additions were down-selected through thermodynamic calculations with a design strategy to maximize the amount of Laves phase at 700 °C and lower the BCC solvus temperature. To validate this design strategy, the candidate alloys were cast, heat treated and aged at 700 °C, and the resulting microstructure was characterized using scanning electron microscopy and atom probe tomography. The alloys with titanium addition (1 wt%) showed monotonic precipitate coarsening. On the other hand, the alloys with tungsten addition (6 wt%) showed a reduced coarsening rate of Laves phase precipitates in the matrix up to 1008 h aging, whereas an acceleration of the precipitate coarsening after that. Atom probe tomography of this alloy revealed the formation of a core-shell precipitate structure with a Nb/Si rich core and W-rich shell, while the alloy with Ti-addition did not show the core-shell structure. The detailed characterization results revealed that the core-shell structure was strongly correlated with the observed two-step coarsening mode in the tungsten containing alloy.

Influence of Atomic Size Factors on the Phase Stability of Laves Phase in Nb-Cr-Ni-Al and Nb-V-Ni-Al Phase Diagrams

Influence of Atomic Size Factors on the Phase Stability of Laves Phase in Nb-Cr-Ni-Al and Nb-V-Ni-Al Phase Diagrams

P64] Stability of Laves Phase in the Mn–Ta System

Remodeling of liquid phase Mn-Ta, calculation of energy of formation of Laves phase Mn2Ta and comparison with recent experimental phase equilibrium data were presented.

Laves phases in RE(Mg,Cd) 2 (RE = rare earth) pseudo-binary alloys

Trends in the stability of Laves phases in the pseudo-binary alloy systems RE(Mg,Cd) 2 (RE: rare earth element) are studied experimentally. Alloys are synthesized with various compositional ratios between Mg and Cd and with different REs. For most REs except Yb, a Laves phase forms only in the Mg-rich part of the phase diagram including the binary limit REMg 2, while for Yb a Laves phase is maintained throughout the whole compositional range between YbMg 2 and YbCd 2. A thorough analysis on the compositions of the Laves phases in the Mg-rich part is performed, which reveals a systematic tendency towards off-stoichiometry with Mg-enrichment of about 5–10 atomic %. The phenomenon can be understood as a consequence of an unbalanced mixing of atomic species between two kinds of atomic sites. Rietveld refinements of the crystal structures reveal that a better fit is achieved by allowing the RE-positions to be replaced partially by Mg, which provides a supporting evidence for the atomistic model of off-stoichiometry.

Thermodynamic modeling of Laves phases in the Cr–Hf and Cr–Ti systems: Reassessment using first-principles results

The Cr–Hf and Cr–Ti belong to interesting systems exhibiting the existence of all polytypes of Laves phases, i.e. lower-temperature cubic C15 and higher-temperature hexagonal C14 and C36, although in the Cr–Hf phase diagram only C14 and C15 phases occur. Comparison of total energies of these structures calculated from first principles with the total energy of the ideal mixture of elemental constituents reveals the relative stability of Laves phases in these systems. The effect of magnetic order in the Laves phases is also briefly discussed. The calculated total energies of formation of all the three polytypes are employed in two- and three-sublattice models to revise the thermodynamic description of both the systems published recently. New remodeled Gibbs energies of Laves phases require less fitting parameters than those obtained in previous treatments and corresponding phase diagrams provide an excellent agreement with the experimental phase data found in the literature. The proposed procedure allows us to compare the optimised heat capacity differences with those determined experimentally or theoretically and to use them in phase diagram calculations.

Effect of electron concentration on the Laves phase stability of NbCr2–Ni produced by powder metallurgy

The powder metallurgy technology of mechanical alloying followed by hot-pressing consolidation has been used to obtain NbCr2–Ni Laves phases. High-purity Nb, Cr, and Ni crystalline powders were milled for periods up to 35 h. These powders were vacuum sintered at 1250°C for 0.5 h under 45 MPa. The phase transformations of NbCr2–Ni Laves phases have been investigated by X-ray diffraction. The grain size decreases and the microstrain increases with increased Ni content. Different phase transformations were observed from NbCr2–Ni intermetallics. It is found that the electron concentration and phase stability correlation is obeyed in the Nb–Cr–Ni systems. However, the electron concentration corresponding to the C15/C14 phase transition is found to move to a higher value (i.e. 6.103) than the predicted one (i.e. 5.88) in the literature.

Stability of Laves phases in the Cr–Zr system

The total energies of Laves phases with various occupations of inequivalent lattice sites in all three structural forms C14, C15 and C36 have been calculated ab initio by a pseudopotential VASP code with a complete relaxation of all structural parameters. The relative stability of Cr 2Zr polytypes is discussed. The calculated values were used in two-sublattice and three-sublattice models for the re-modeling of Gibbs energies of Laves phases and subsequently for the calculation of phase diagram of the Cr–Zr system by the CALPHAD method. A comparison of phase diagrams obtained by our model using first-principles results with previous treatments using an empirical approach as well as with experimental phase equilibrium data is presented. An application of the structural energy differences (lattice stabilities) calculated ab initio provides a similarly good description of the phase diagram of the Cr–Zr system as previous studies, but much fewer adjustable parameters are needed for a thermodynamic description of Laves phases.

Structure and stability of Laves phases part II—structure type variations in binary and ternary systems

In Part I, various models for the prediction of the occurrence and stability of Laves phases have been discussed. In the present Part II paper, an overview is given on the various types of manifestation of Laves phases in intermetallic alloy systems with emphasis on transition metal systems. Temperature- and composition-dependent as well as stress-induced phase transformations between the cubic C15 and the hexagonal C14 and C36 structural polytypes of Laves phases were observed in various binary and ternary systems. The phase fields of the different polytypes are generally separated by small two-phase fields. Some general rules for the occurrence of the different Laves phase polytypes are derived from a study of the results of experimental phase diagram investigations of various binary and ternary systems. The solubilities and site occupancies of ternary additions in the different Laves phase polytypes and the mutual miscibility of different binary Laves phases are discussed. The need for careful experimental investigations of phase equilibria is demonstrated. Existing models discussed in the preceding Part I paper are shown to fail to predict the structure and stability of the presented Laves phases in many cases.

Structure and stability of Laves phases. Part I. Critical assessment of factors controlling Laves phase stability

Laves phases form the largest group of intermetallic phases. Although they are well known since long, there are still unsolved problems concerning the stability of the respective crystal structures. The Laves phases crystallize with a cubic MgCu 2- or a hexagonal MgZn 2- or MgNi 2-type structure which differ only by the particular stacking of the same four-layered structural units. It is still not possible to predict which of the structure types is the stable one for a Laves phase compound AB 2. Phase transformations from a cubic low-temperature structure to a hexagonal high-temperature structure were observed as well as stress-induced transformations from the hexagonal structure to the cubic one. In addition, deviations from the stoichiometric composition were reported to result in a change of the stable polytype in various systems. In this first of two consecutive papers dealing with fundamental aspects of the stability of Laves phases, some factors which are known to affect the occurrence and structure type of Laves phases are discussed and it is shown that, at least up to now, the existing models and calculations are not well suited to give a general description of the stability of Laves phases.

Preparation and structural analysis of Fe 2+ xTi 1− x thin films in the C14 Laves phase stability range

We report the epitaxial growth of single phase (0 0 1)-oriented thin films of Fe 2+ x Ti 1− x in the C14 Laves phase stability range using molecular beam epitaxy (MBE). The growth was studied by reflection high-energy electron diffraction (RHEED) and X-ray diffraction (XRD). The onset temperature for epitaxial growth and the temperature range for improved crystalline coherence were identified. From X-ray reflectometry analysis the rms roughness was estimated to 0.5 nm for typical film thicknesses of 22 nm. As revealed by scanning tunneling microscopy (STM), this roughness is discrete and due to step edges corresponding to the full c-axis length of Fe 2Ti. The epitaxial growth implies an extended range of homogeneity as compared to bulk samples. A strong dependence of the growth behavior on the composition within the range of homogeneity was observed exhibiting increased crystalline coherence at higher iron concentrations. Furthermore, the volume increase of the unit cell with the Ti concentration is compatible to Vegard's law. Evidence for an iron-induced surface reconstruction was found in RHEED investigations.

Analysis of stacking faults in the Hf-V-Nb cubic laves phase

It has been shown that increasing the Nb concentration within the cubic laves phase of a Hf-V-Nb alloy such that the composition is closer to a region of hexagonal laves phase stability increases the occurrence of stacking defects in the material. The stacking faults were shown to have intrinsic character by HREM analysis in contrast to alloys of lower Nb content. Although no direct measurement was made, the data are consistent with a reduced stacking fault energy of the cubic laves phase. Although a lower SFE in elemental fcc materials can increase deformation twinning, the implications for twinning in cubic laves phases (complex fcc) is unclear since intergrowths of hexagonal material may compromise polycrystalline ductility.

Direct measurement of carbon solubility in the intermetallic (Fe,Cr) 2(Mo,W) Laves phase using atom-probe field-ion microscopy

The (Fe,Cr){sub 2}(Mo,W) Laves phase found in the studied 10.5% chromium steel, which had been creep tested for nearly 1,500 h at 600 C, contained 0.8 at.% carbon. Thus, the solubility of carbon in this phase cannot be considered to be negligible. When compared with matrix analyses of the same material in the as-tempered state, the matrix surrounding the Laves phase in the creep tested material showed a slight enrichment in carbon. Besides carbon, Laves phase was enriched in silicon (3.7 at.%) and phosphorus (0.7 at.%). Further studies of the role of these elements on the nucleation and stability of Laves phase in 9%--12% chromium steels should be performed.