Room-temperature deformation and stress- induced phase Transformation of Laves Phases in Fe-10 At. Pct Zr Alloy

The microstructure within particles of the Laves phase Fe₂Zr in a two-phase Fe-10 at. pct Zr alloy after compression was investigated using X-ray diffraction and electron microscopy. Stress- induced phase transformation between C36 and C15 structures was found to be a major defor- mation mode for the Laves phase. Twinning and stacking faults were also found within C15 regions. Phase transformation models based on partial dislocations are discussed.     

Phase field and room-temperature mechanical properties of the C15 laves phase in the Zr-Ta-Cr alloy system

The phase field of the C15 Laves phase in the Zr-Ta-Cr alloy system at 1273 K exhibited a continuous solid solution along the ZrCr₂-TaCr₂ pseudobinary line and also a broad off-stoichiometric range on the (Zr,Ta)-rich side. The defect structures in the C15 Laves phase were evaluated by lattice parameter and density measurements. The room-temperature fracture toughness was assessed by Vickers indentation testing. Alloying, off-stoichiometry, and the related defect structures were shown to be beneficial for improving the room-temperature deformability of the C15 Laves phase in the Zr-Ta-Cr alloy system. The results are discussed in terms of the constituent atomic sizes on the most closely packed planes in the C15 Laves phase where shear deformation takes place. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Dynamic Precipitation of Laves Phase in FeCrAl Alloy with Zr Addition

FeCrAl alloy is one of potential candidates for accident-tolerant-fuel (ATF)-cladding materials due to its excellent oxidation and corrosion resistance at accident temperature, combining good mechanical properties at service temperature. Alloying strategy is an important way for improving comprehensive properties of FeCrAl alloy through the precipitation of fine Laves phase. Zr alloying can stabilize the Laves phase due to its lower diffusion coefficient and solubility in body-centered-cubic ferrite matrix. Herein, it is found that Zr addition changes the dynamic precipitation features of Laves phase in FeCrAl alloy during high-temperature deformation, from only one type of Fe₂M (M = Nb, Mo, Ta) Laves phase to Fe₂Zr combining Fe₂M-type Laves phase. The Fe₂Zr-type Laves phase precipitates dynamically first, and the interface precipitates between which with ferrite matrix creates more nucleation sites for subsequent precipitation of Fe₂M Laves phase. The results can be possibly applied for alloy design and microstructure tailoring in series of FeCrAl alloys used for ATF cladding in the near future.     

Microstructural and crystallographic relationships in directionally solidified NbCr2Nb and CrCr2Nb eutectics

The present paper describes processing, microstructures and phase relationships in directionally solidified Cr₂NbNb and Cr₂NbCr eutectics. Both of these eutectics, and the stoichiometric Laves phase Cr₂Nb, were directionally solidified using cold crucible Czochralski crystal growth with growth rates from 1 to 15 mm/min. Cr₂NbNb had a rod/ribbon-type structure and Cr₂NbCr had a lamellar structure. The crystallographic oreintation relationships between both Cr and Nb and the Cr₂Nb Laves phase in the individual eutectics were characterized using transmission electron microscopy. Inter-phase orientation relationships in these eutectics were complicated by the transformation of Cr₂Nb from the C14 to the C15 crystal structure on post-solidification cooling. Twins were observed in the C15 Cr₂Nb phase in both the single phase and eutectic samples. The relationship between the twins, the C14–C15 transformation, and the eutectic morphology is also discussed.     

Defect structures and room-temperature mechanical properties of C15 laves phases in Zr-Nb-Cr and Zr-Hf-Cr alloy systems

The effect of composition on the pseudobinary ZrCr₂-NbCr₂ and ZrCr₂-HfCr₂ C15 Laves phase alloys were investigated, focusing upon defect structures and room-temperature fracture toughness. The defect structures were evaluated by lattice parameter and density measurement. The room-temperature fracture toughness was assessed by Vickers indentation test. Alloying, off-stoichiometry, and related defect structures were shown to be generally beneficial for improving the room-temperature fracture toughness of the C15 Laves phases. The results were discussed, based on the local atomic size argument on the most closely packed planes in the C15 Laves phases where shear deformation takes place.     

Examination of Multiphase (Zr,Ti)(V,Cr,Mn,Ni)<Subscript>2</Subscript> Ni-MH Electrode Alloys: Part I. Dendritic Solidification Structure

The solidification microstructures of three nine-element Zr-Ni–based AB₂ type C14/C15 Laves hydrogen storage alloys are determined. The selected compositions represent a class of alloys being examined for usage as an MH electrode in nickel metal-hydride batteries that often have their best properties in the cast state. Solidification is accomplished by dendritic growth of hexagonal C14 Laves phase, peritectic solidification of cubic C15 Laves phase, and formation of cubic B2 phase in the interdendritic regions. The B2 phase decomposes in the solid state into a complex multivariate platelike structure containing Zr-Ni–rich intermetallics. The observed sequence C14/C15 upon solidification agrees with predictions using effective compositions and thermodynamic assessments of the ternary systems, Ni-Cr-Zr and Cr-Ti-Zr. Experimentally, the closeness of the compositions of the C14 and C15 phases required the use of compositional mapping with an energy dispersive detector capable of processing a very high X-ray flux to locate regions in the microstructure for quantitative composition measurement and transmission electron microscope examination.     

Microstructure and phase identification in type 304 stainless steel-zirconium alloys

Stainless steel-zirconium alloys have been developed at Argonne National Laboratory to contain radioactive metal isotopes isolated from spent nuclear fuel. This article discusses the various phases that are formed in as-cast alloys of type 304 stainless steel and zirconium that contain up to 92 wt pct Zr. Microstructural characterization was performed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), and crystal structure information was obtained by X-ray diffraction. Type 304SS-Zr alloys with 5 and 10 wt pct Zr have a three-phase microstructure—austenite, ferrite, and the Laves intermetallic, Zr(Fe,Cr,Ni)_2+x. whereas alloys with 15, 20, and 30 wt pct Zr contain only two phases—ferrite and Zr(Fe,Cr,Ni)_2+x. Alloys with 45 to 67 wt pct Zr contain a mixture of Zr(Fe,Cr,Ni)_2+x and Zr₂(Ni,Fe), whereas alloys with 83 and 92 wt pct Zr contain three phases—α-Zr, Zr₂(Ni,Fe), and Zr(Fe,Cr,Ni)_2+x. Fe₃Zr-type and Zr₃Fe-type phases were not observed in the type 304SS-Zr alloys. The changes in alloy microstructure with zirconium content have been correlated to the Fe-Zr binary phase diagram.     

Features of hydrogen-induced amorphization of the Er(Co,Mn)2 pseudo-binary Laves compound

Hydrogen-induced amorphization behavior of Er(Co_1−xMn_x)₂ pseudo-binary Laves compound is investigated by means of X-ray diffractometry and transmission electron microscopy. The substitution of Mn for Co results in a homogenous single phase within the whole composition range. Up to x = 0.8, C15 cubic phase is obtained, but C14 structure is found at higher Mn composition. At all compositions except for the binary ErMn₂, the amorphization phenomena are observed. Thus it is suggested that amorphization of the Laves phases by the hydrogen absorption is not concerned with the crystal structure but the chemical bonding between the unlike atoms causing the kinetic constraints of the motion of the metal atoms. By the Mn addition into ErCo₂, the amorphization temperature, which is defined as the temperature amenable to the full amorphization reaction in a fixed times of 24 h, gradually decreases from 350°C for ErCo₂ to 230°C for Er(Co_0.2Mn_0.8)₂ within the C15 region. This behavior is explained by the variation of the formation enthalpy (ΔH_f) and the bulk modulus of the compound with the Mn content. A compound with a high negative ΔH_f and high modulus is amorphized at higher temperature range due to the larger activation energy barrier for the motion of the metal atoms than other compounds with relatively lower heat of mixing and modulus values.     

P92耐热钢物理化学相分析

采用物理化学相分析方法研究了P92耐热钢的回火及时效后碳化物和金属间化合物的析出行为,并通过实验确定了P92钢中析出相的萃取和分离方法、钢中析出相的类型、含量及组成结构式等。研究结果表明,回火态试样中含有M₂₃C₆与M(CN)两种析出相,时效100 h后,X-ray分析结果出现Laves相。随着时效时间的增加,Laves相也明显增加,且W、Mo含量高的试样在时效后Laves相析出量增加较多, M₂₃C₆的析出量增加较少, M(CN)析出相含量变化不大。P92钢在短时间内析出的 Laves相有显著的强化效果,在1 000 h内增加Laves相析出量能有效提高P92钢室温及600 ℃下强度。     

Heterogeneous Creep Deformations and Correlation to Microstructures in Fe-30Cr-3Al Alloys Strengthened by an Fe<Subscript>2</Subscript>Nb Laves Phase

A new Fe-Cr-Al (FCA) alloy system has been developed with good oxidation resistance and creep strength at high temperature. The alloy system is a candidate for use in future fossil-fueled power plants. The creep strength of these alloys at 973 K (700 °C) was found to be comparable with traditional 9 pct Cr ferritic–martensitic steels. A few FCA alloys with general composition of Fe-30Cr-3Al-.2Si-xNb (x = 0, 1, or 2) with a ferrite matrix and Fe₂Nb-type Laves precipitates were prepared. The detailed microstructural characterization of samples, before and after creep rupture testing, indicated precipitation of the Laves phase within the matrix, Laves phase at the grain boundaries, and a 0.5 to 1.5 μm wide precipitate-free zone (PFZ) parallel to all the grain boundaries. In these alloys, the areal fraction of grain boundary Laves phase and the width of the PFZ controlled the cavitation nucleation and eventual grain boundary ductile failure. A phenomenological model was used to compare the creep strain rates controlled by the effects of the particles on the dislocations within the grain and at grain boundaries. (The research sponsored by US-DOE, Office of Fossil Energy, the Crosscutting Research Program).     

Precipitation of Fe2W laves phase and modeling of its direct influence on the strength of a 12Cr-2W steel

Precipitation of Fe₂W Laves phase in a 12Cr-2W power plant steel is investigated by using transmission electron microscopy (TEM). Fe₂W Laves phase is found to be coherent with the matrix and has a stacking fault structure. The influence of formation of Fe₂W Laves phase on the yield strength of the steel is quantitatively evaluated. The modeling result indicates that the strengthening effect from the formation of Laves phase particles is diminished by the loss of solid solution strengthening of alloying elements, and, as a result, the strength of the steel remains similar. The effect of clustering and coarsening of Laves precipitates on the strength of the steel is also studied. It is showed that clustering and coarsening decreases the strengthening effect of Laves phase. The limitation of the modeling approach currently adopted is also discussed.     

Microstructural characterization of multicomponent Nb-Ti-Si-Cr-Al-X alloys

Recently, alloys based on the Nb-Ti-Si system have become of interest for high-temperature structural applications. In the present work, the microstructure of multicomponent Nb-30Ti-8Si-10Cr-10Al-X (in at. pct) alloys in the as-cast and heat-treated conditions was studied using X-ray diffraction, electron probe microanalysis, scanning electron microscopy and transmission electron microscopy. The effect of temperature and time on phase evolution was examined in detail. The as-cast microstructure was found to be composed of three phases:β (bcc), silicides (M₅Si₃ type), and a Cr-rich Laves phase. Theβ phase was found to display B2-type ordering. The silicides in these alloys were generally quite stable during heat treatment, whereas the Cr-rich Laves phase was observed to dissolve on solutionization at temperatures above 1300 °C. Aging of the solutionized materials between 900 °C and 1100 °C led to the precipitation of fine particles of another Laves phase in theβ matrix. In addition, theβ matrix revealed a tendency toward phase separation into Nb-rich (β ₁) and Ti-rich (β ₂) regions. The volume percentage and chemical composition of each phase has been determined as a function of time and temperature and the changes in microstructure have been rationalized in terms of the distribution of elements in various phases. The role of different alloying elements on the formation of these phases has also been critically examined.     

Precipitation-strengthened austenitic Fe−Mn−Ti alloys

The precipitation of intermetallic compounds in the Fe−20Mn−2Ti and Fe−28Mn−2Ti alloy systems has been investigated over the temperature range 700 to 900°C by hardness measurements, optical and scanning electron microscopy, and X-ray diffraction. In both systems only the equilibrium Laves phase was observed. The precipitate was identified as C14(MgZn₂) type hexagonal Laves phase with a chemical composition close to Fe₂ (Ti, Mn). In an as-annealed sample precipitation occurred in a heterogeneous manner, predominantly along grain boundaries. The effect of a cold deformation between the solution annealing and aging processes was also investigated. In addition to a high density of dislocations, martensitic phases were induced by deformation: a γ→∈ transformation occurred in the Fe−28Mn−2Ti alloy while a γ→α′ transformation was predominant in the Fe−20Mn−2Ti alloy. Subsequent aging was conducted at temperatures above theA _f . A large number of very fine precipitates formed randomly in the matrix after a short aging period. This cold work plus aging treatment resulted in an increase in yield strength. The enhancement of mechanical properties is due to the randomly distributed precipitates combined with the high defect density and fine substructure.     

The Computational Design of W and Co-Containing Creep-Resistant Steels with Barely Coarsening Laves Phase and M23C6 as the Strengthening Precipitates

Generally, Laves phase and M₂₃C₆ are regarded as undesirable phases in creep-resistant steels due to their very high-coarsening rates and the resulting depletion of beneficial alloying elements from the matrix. In this study, a computational alloy design approach is presented to develop martensitic steels strengthened by Laves phase and/or M₂₃C₆, for which the coarsening rates are tailored such that they are at least one order of magnitude lower than those in existing alloys. Their volume fractions are optimized by tuning the chemical composition in parallel. The composition domain covering 10 alloying elements at realistic levels is searched by a genetic algorithm to explore the full potential of simultaneous maximization of the volume fraction and minimization of the precipitates coarsening rate. The calculations show that Co and W can drastically reduce the coarsening rate of Laves and M₂₃C₆ and yield high-volume fractions of precipitates. Mo on the other hand was shown to have a minimal effect on coarsening. The strengthening effects of Laves phase and M₂₃C₆ in the newly designed alloys are compared to existing counterparts, showing substantially higher precipitation-strengthening contributions especially after a long service time. New alloys were designed in which both Laves phase and M₂₃C₆ precipitates act as strengthening precipitates. Successfully combining MX and M₂₃C₆ was found to be impossible.     

Defect structures and nonbasal slip of C36 laves phase MgNi2 in a two-phase alloy

The microstructure of the C36 Laves phase MgNi₂ in a two-phase Ni-15 at. pct Mg alloy was studied using electron microscopy. Both intrinsic and extrinsic stacking faults were found on the basal plane. The displacement vectors of the faults and Burgers vectors of partial dislocations either lying on the basal faults or bounding each side of the faults were analyzed using high-resolution transmission electron microscopy (TEM). Nonbasal faults were found on the pyramidal planes {1011} and {1012} and the prismatic plane {1010}, all connected to basal faults. The displacement vector of the pyramidal faults was determined to be 1/12 〈4043〉. The microstructure was also studied after room-temperature compression, and nonbasal slip was found to be the major deformation mode.     

Structure and elevated temperature properties of carbon-free ferritic alloys strengthened by a laves phase

A Laves phase, Fe₂Ta, was utilized to obtain good elevated temperature properties in a carbon-free iron alloy containing 1 at. pct Ta and 7 at. pct Cr. Room temperature embrittlement resulting from the precipitation of the Laves phase at grain boundaries was overcome by spheroidizing the precipitate. This was accomplished by thermally cycling the alloys through theα →γ transformation. The short-time yield strength of the alloys decreased very slowly with increase in test temperature up to 600°C, but above this temperature, the strength decreased rapidly. Results of constant load creep and stress rupture tests conducted at several temperatures and stresses indicated that the rupture and creep strengths of spheroidized 1 Ta−7 Cr alloy were higher than those of several commercial steels containing chromium and/or molybdenum carbides but lower than those of steels containing substantial amounts of tungsten and vanadium. When molybdenum was added to the base FeTa-Cr alloy, the rupture and creep strengths were considerably increased. Formerly with Lawrence Berkeley Laboratory.     

Hydrides based on phases in the Ti2Ni-Zr2Ni section of the Ti-Ni-Zr system

A study has been made on the interaction of hydrogen with a phase based on the intermetallide Zr₂Ni and with the ternary Laves phase occurring in the Ti-Ni-Zr system in the temperature range between room temperature and 700°C. The Laves phase and the phase based on Zr₂Ni form two hydrides: a low-temperature unstable one, which decomposes in air, and a high-temperature one that gives up hydrogen on heating above 400°C. The hydrogen capacities of the two phases can be used to store hydrogen.Materials Science Institute, Ukrainian National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1/2(377), pp. 81–84, January–February, 1995.     

低放氢压的Ti-Mn基Laves相贮氢合金

研究了Ti/Zr比与Mn/Cr比变化对Ti Mn基Laves相贮氢合金贮氢性能的影响 ,发现随Ti/Zr比降低 ,合金放氢压降低 ,贮氢量略微增加 ,平台坡度变陡 ;随Mn/Cr比降低 ,合金放氢压降低 ,平台区变短 ,但合金的贮氢量变化不大。Ti0 .85Zr0 .15Mn1.6-yCryV0 .32 Fe0 .0 8系列合金当 0 1≤y≤ 0 2时比较适宜作为小型燃料电池的氢源。     

Structure and elevated temperature properties of carbon-free ferritic alloys strengthened by a laves phase

A Laves phase, Fe₂Ta, was utilized to obtain good elevated temperature properties in a carbon-free iron alloy containing 1 at. pct Ta and 7 at. pct Cr. Room temperature embrittlement resulting from the precipitation of the Laves phase at grain boundaries was overcome by spheroidizing the precipitate. This was accomplished by thermally cycling the alloys through the α→γ transformation. The short-time yield strength of the alloys decreased very slowly with increase in test temperature up to 600°C, but above this temperature, the strength decreased rapidly. Results of constant load creep and stress rupture tests conducted at several temperatures and stresses indicated that the rupture and creep strengths of spheroidized 1 Ta-7 Cr alloy were higher than those of several commercial steels containing chromium and/or molybdenum carbides but lower than those of steels containing substantial amounts of tungsten and vanadium. When molybdenum was added to the base Fe-Ta-Cr alloy, the rupture and creep strengths were considerably increased. M. Dilip Bhandarkar, formerly with Lawrence Berkeley Laboratory.