Miscibility gap due to ordering in the BCC Fe-Ge system

Phase equilibria in the bcc Fe-Ge system have been investigated by means of electron probe microanalysis and transmission electron microscopy. Two kinds of ordered phases, B2 and DO₃ types, have been detected, the B2 phase being formed from the disordered A2 phase by a 2nd order transition. The DO₃ phase is also formed from the B2 phase by a 2nd or higher order transition at higher temperatures. Below about 820 °C, however, the transition becomes a 1st order, and a miscibility gap between these ordered phases occurs in the composition range from 13 to 16 at. pct Ge. It has been also confirmed that the solubility of Ge in the bec phase shows a retrogression due to the formation of DO₃ phase. On the basis of these experimental results, the phase diagram for the Fe-Ge system has been partly revised.     

The influence of critical points and structure and microstructural evolution in iron rich FeSi alloys

The evolution of microstructure in Fe-8.7 to 14.2 at.% Si alloys in the neighbourhood of higher order transition lines has been investigated. Systematics of various features of the FeSi phase diagram, highlighting the multicritical points, have been presented. The possible influence of these critical points on the kinetic pathways for the microstructural evolution has been discussed and compared with experimental results. It is shown that below the metastable tricritical point the alloy undergoes continuous ordering and a conditional spinodal followed by the renucleation of B2 phase in the DO₃ matrix. The latter process leads to the anomalous contrast in the B2 superlattice darkfield images. It is also shown that above the metastable (extrapolated) higher order B2 → DO₃ transition line a small domain exists in the two phase field where decomposition to two phase B2 + DO₃ takes place by classical nucleation of fine DO₃ domains in B2 matrix. No evidence for DO₃ ordering could be obtained in alloy containing less than 11 at.% Si.     

Deformation behavior of a Ni3Al(B,Zr) alloy during cold rolling: Part I. changes in order and structure

A hypostoichiometric Ni₃Al(B,Zr) alloy was homogenized and cold rolled by amounts ranging from 25 to 73 pct. The alloy consisted of two phases—a partially ordered γ′ phase (L1₂) and a Ni-rich fcc solid solution (γ). On deforming the alloy by rolling at room temperature, the order parameter showed a gradual change. In fact, between 35 and 45 pct deformation, the order characteristic of the L1₂ structure changed into that of a DO₂₂ structure. The possibility of transition from L1₂ to DO₂₂ structure is also corroborated from strain parameter, microhardness, and detailed x-ray diffraction (XRD) measurements. This structural transformation is accompanied by a change in the deformation mode (from slip to twinning), as is evident from the relevant microstructures.     

Strong decrease of the activation energy as a function of A1 content in FeAlx alloys (x ≦ 30 at.%) deduced from kinetic measurements of ordering

The kinetics of short and long-range order in the FeAl_x alloys have been systematically studied. The results were obtained in the A₂ solid solution (x = 0.10and0.17), in the DO₃ ordered phase (x = 0.23, 0.27, 0.30) and in the A₂ + DO₃ two-phase region (x = 0.19), using 4K or in situ resistometry. The kinetic curves are well described by a classical model with a lognormal distribution of the relaxation times whose center varies with the temperature according to an Arrhenius law. An important point of this work is the strong decrease of the activation energy with the aluminium content and with the appearance of the DO₃ ordered phase, as well as quite short relaxation times. These results show an abnormally high atomic mobility that can be attributed to a decrease of the migration energy through a lattice dynamics effect, as recently proposed in models which explain the enhanced diffusion observed in some b.c.c. transition metals and alloys.     

Phase Structure and High-Temperature Mechanical Properties of Two-Phase Fe-25Al-xZr Alloys Compared to Three-Phase Fe-30Al-xZr Alloys

The structure and high-temperature mechanical properties of Fe-30 at. pct Al and Fe-25 at. pct Al alloys with various Zr contents are compared. The scanning electron microscope images in chemical contrast mode (R-BSE) as well as EDS, EBSD, and X-ray diffraction were used to determine the structure and phase composition. The as-cast alloys (both Fe-30Al and Fe-25Al) were observed to be two-phase DO₃/B2 + Laves phase λ ₁ (Fe,Al)₂Zr alloys with typical fine lamellar eutectic areas. During the heat treatment of the Fe-25Al alloys, their structure transformed from a DO₃/B2 matrix with fine lamellar eutectic into λ ₁ globular particles situated in a DO₃/B2 matrix. The same structure of Fe-30Al alloys decomposed into three phases: λ ₁ and τ ₁ Zr(Fe,Al)₁₂ particles in a DO₃/B2 matrix. The hardening in both groups of alloys (Fe-25Al and Fe-30Al) due to the presence of Zr-containing λ ₁ and τ ₁ phases is compared.     

Study of Microstructural Changes in an Fe-Mn-Al-Si-C Alloy

The microstructural changes in a 0.05C-26.3Mn-4.88Si-4.13Al-Fe alloy were studied. The two constituting phases were austenite and ordered DO₃. The morphology of austenite phase was found to be different under various conditions. The DO₃ phase had a modulated microstructure, and secondary austenite flakes precipitated within the DO₃ matrix when both cast and hot-rolled alloys were heat treated at 1073 K (800 °C). An attempt was made to explain the secondary austenite precipitation in view of a schematic phase diagram.     

Microstructure of a pressure-cast Fe3AI intermetallic alloy composite reinforced with zirconia-toughened alumina fibers

A composite of Fe-28Al-2Cr-lTi (at. pct) reinforced with 20-@#@ μm diameter zirconia-toughened alumina fiber, PRD-166, was pressure cast and examined by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). A new phase, tentatively identified as Fe₂AlZr, with an fcc crystal structure and a lattice parameter of 1.18 nm was occasionally found at fiber/ matrix interfaces. It was proposed that the phase formed by the eutectic reaction L → Fe(Al) + Fe₂AlZr. The Zr in the compound became available as a result of the dissolution of ZrO₂ from the fiber into the molten alloy. The matrix contained a high density of dislocations resulting from a difference in the coefficients of thermal expansion between the matrix and fiber. It was proposed that dislocations which formed at high temperatures in either A2 or B2 states were incompatible with the low-temperature DO₃ state. Geometrically necessary antiphase boundaries have been proposed to provide compatibility between dislocations formed in either the A2 or B2 states and the DO₃ state.     

Ordering reactions in Ni-Al-Mo-Ta and Ni-Al-Mo-W superalloys

The ordered structures formed in two experimental nickel base superalloys have been determined using selected area electron diffraction. Upon quenching from 1300 °C, the alloys contained ordered γ′ precipitates (L1₂ structure) and the matrix exhibited diffuse intensity at {1 1/2 0} positions, indicating the presence of short range order. The high refractory metal content of the alloys caused the D1a, DO₂₂, and Pt₂Mo prototype structures to form in the matrix following aging at 600, 700, and 800 °C. The detailed structural effects of the Ta and W quaternary additions are similar to those observed in Ni₃(Mo, Al), Ni₃(Mo,Ta), and Ni₃(Mo, W) ternary alloys. The decomposition products observed in the quaternary alloys studied can be explained by considering the partitioning of solutes between the γ′ and the matrix.     

Phase transformations and modulated microstructures in Ti-Al-Nb alloys

The decomposition of alloys roughly based on the composition Ti₃Al with ternary additions of Nb has been studied through the use of transmission electron microscopy and selected area electron diffraction. It has been shown that a wide variety of transformation behavior in the metastable high temperature β phase can be produced through varying composition, cooling rates, and heat treatment. Transformations observed during quenching from the β phase field include the formation of hcp martensite in alloys with low Nb contents, the occurrence of B2(CsCl)-type ordering over a wide composition range in alloys richer in Nb, and the formation of an “ω-type≓ phase in the parent matrix subsequent to B2 ordering. In many of the as-quenched alloys a “tweed-like≓ or modulated micro-structure is observed with accompanying elaborate networks of rel rods and local diffuse maxima in electron diffraction patterns. During aging the decomposition proceeds through a complex sequence of reactions the nature of which will be discussed below. Many of the anomalies observed in these alloys are quite similar to behavior observed during decomposition of other ordered β(B2, DO₃)-type phases in a wide variety of other alloy systems. This paper is based on a presentation made in the symposium “Pre-transformation Behavior Related to Displacive Transformations in Alloys≓ presented at the 1986 annual AIME meeting in New Orleans, March 2–6, 1986, under the auspices of the ASM-MSD Structures Committee.     

DO3-B2-α phase relations in Fe-Al-Ti alloys

A transmission electron microscopic investigation has been conducted to determine the phase relations, transformation temperatures, and microstructures in Fe-Al-5 at. pct Ti alloys with Al content varying from 18 to 25 at. pct. As compared to the binary Fe-Al alloys, the effects of Ti addition are (i) a significant expansion of the (α + DO₃) phase field, (ii) increased transition temperatures, and (iii) a tendency of the DO₃ thermal anti-phase domain boundaries to exhibit anisotropy with a preference to lie on {100} planes.     

Structure,tensile deformation,and fracture of a Ti3Al-Nb alloy

The development of Ti₃Al-Nb alloys is an excellent example of the recent resurgence of interest in the use of intermetallics for high-temperature applications. We examine, in this contribution, the structure of a typical alloy Ti-24A1-11Nb and show it to consist primarily of the ordered α₂ phase (based on Ti₃Al, DO₁₉) and β_o, (based on Ti₂NbAl, B2) phases, with small amounts of a third phase, which is distorted slightly to an orthorhombic symmetry from the D0₁₉ (hexagonal) structure. Tensile properties have been examined on samples heat-treated to vary the size, shape, and volume fraction of α₂ phase and the deformation and fracture behavior of the ordered, two-phase mixture established. The tensile ductility is seen to maximize at intermediate volume fractions of the α₂ and β_o phases (∼30 pct) at values of 6 to 10 pct elongation to fracture, depending on the grain size of the β_o phase. A rationale incorporating the failure modes of the two phases—cleavage of α₂ and slipband decohesion of β_o—has been evolved to explain the trends in ductility with heat treatment.     

Some Effects of Parent Phase Aging on the Martensitic Transformation in a Cu- AI- Ni Shape Memory Alloy

Transmission electron microscopy observations have been carried out for a Cu-14 pct Al-4 pct Ni (wt pct) alloy aged in the thin foil state in an electron microscope. It was found that large cuboidal precipitates of theγ ₂ phase and many small domains of a highly ordered phase form in the DO₃ matrix during aging. The small ordered domains form preferentially on matrix antiphase boundaries as well as within the antiphase domains. The formation ofγ ₂ and the highly ordered phase, both of which are rich in alloy content, depletes the matrix of solute and thus raises the transformation temperaturesM _s andM _f. The small domains of the highly ordered phase prevent the propagation and reversion of martensite plates, leading to higherM _s-M_f andA _fins-A_f temperature intervals.     

Phase selection in undercooled ti3sn melts

The solidification of undercooled Ti₃Sn melts was investigated using electromagnetic levitation and electrohydrodynamic atomization experiments followed by extensive microstructural char- acterization. The study was motivated by several reports on the kinetic preference for the body- centered cubic (bcc) phase over more closely packed disordered and ordered structures during competitive crystallization from undercooled melts. At low undercoolings, Ti₃Sn melts yield the equilibrium ordered hexagonal DO₁₉ structure, which is retained without change upon cool- ing. Undercoolings between ~100 and ~300 K yield primary dendrites with hexagonal sym- metry but a final microstructure which is clearly martensitic in origin. Two previously unknown metastable forms of Ti₃Sn were identified: an ordered base-centered orthorhombic derived from the α martensite and an ordered monoclinic phase related to the face-centered orthorhombic martensite observed in the Ti-V system. Both phases are believed to evolve from the solid state transformation of a high temperature β phase, but the dendritic structure clearly indicates the formation of a hexagonal phase different from DO₁₉,i.e., α. The latter forms in preference to β, which has a larger driving force in at least part of the undercooling regime studied. It is proposed that the primary α transforms to β as a consequence of recalescence, which subse- quently transforms martensitically and orders to yield the observed metastable forms of Ti₃Sn.     

The states of order and the phase diagram of Fe1−xSix, 0.06 ⪕ x ⪕ 0.20, investigated by neutron scattering

The statesof order of iron-silicon solid solutions with atomic fractions c of silicon between 0.06 and 0.2 have been studied in neutron scattering experiments. Up to 873 K, Fe-0.06Si is short-range ordered. The structures of the other alloys can be described as follows. They are DO₃ long-range ordered at far lower c and higher temperatures than indicated in the current phase diagram: e.g. Fe-0.099Si up to at least 900 K and Fe-0.197Si up to at least 1473 K. The DO₃-order parameter as well as the size of the antiphase domains decrease as the temperature is raised. Some phase boundaries shown in the current phase diagram turned out to be no phase boundaries at all: the respective lines have bearing only on the size of antiphase domains. Revisions of the current FeSi phase diagram are proposed.     

Optimization of Process Conditions for Ultralightweight Steel with More Than 13 wt% of Al and 5 wt% of Cr

An ultralightweight Fe–30Mn–13.2Al–1.6C–5Cr steel, which contains more than 13 wt% of Al and thereby reduces the density by 20%, is developed. The ultralightweight steel, which is very brittle due to high Al content, is fabricated by optimizing hot rolling and heat treatment conditions. Hot rolling is conducted after soaking at the temperature range of 1100–1200 °C for 2 h. The ultralightweight steel is hot-rolled successfully after soaking at 1100 °C, whereas specimens soaked at 1150 and 1200 °C are intergranularly cracked after hot rolling, resulting from coarse grain and a large fraction and size of ferrite, which is transformed to ordered DO₃ phase during cooling, at the grain boundaries. In homogenization heat treatment, water quenching and air cooling are performed, respectively, after holding at 1050 °C for 2 h. The air-cooled steel has inferior tensile property due to the formation of brittle ordered DO₃ phase at grain boundaries. Meanwhile, the water-quenched steel shows an excellent tensile property, which is attributed to a uniform microstructure comprising austenite, fine κ-carbide in austenite, and a very small fraction of the ordered DO₃ phase.     

Modeling of the atomic ordering processes in Fe3Al intermetallics by the monte carlo simulation method combined with electronic theory of alloys

The evolution of atomic ordering processes in Fe₃Al has been modeled by the Monte Carlo (MC) simulation method combined with the electronic theory of alloys in pseudopotential approximation. The magnitude of atomic ordering energies of atomic pairs in the Fe₃Al system has been calculated by means of electronic theory in pseudopotential approximation up to sixth coordination spheres and subsequently used as input data for MC simulation for more detailed analysis for the first time. The Bragg-Williams long-range order (LRO) and Cowley-Warren short-range order (SRO) parameters have been calculated from the equilibrium configurations attained at the end of MC simulation for each predefined temperature and Al concentration levels, which reveal the evolution of the system from DO₃→B2→disordered state as temperature increases. The variation of ordering parameters with temperature has identified the transition temperature from DO₃→B2 type superlattice, and from B2→disordered (α) solid solution at about 540 °C and >900 °C, respectively, showing good qualitative agreement with experimental results. The results of the present study imply that combination of electronic theory of alloys in pseudopotential approximation with MC simulation can be successfully applied for qualitative or semiquantitative analysis of energetical and structural characteristics of atomic ordering processes in Fe₃Al intermetallics.     

Order-disorder phenomena in the platinum rich part of the PtV phase diagram

The Pt-rich side of the PtV phase diagram is re-examined using electron diffraction and high resolution electron microscopy. An ordered Pt₈V structure isomorphous with Pt₈Ti is discovered and two long period antiphase boundary structures are found in Pt₃V on annealing in the temperature interval between 900 and 1000°C. A formation mechanism for the stable DO₂₂ phase is presented and some structural defects are discussed. Microdomains corresponding to the ordering relations extracted from the diffuse intensity distribution in reciprocal space are revealed using the high resolution technique.     

Modeling of the atomic ordering processes in Fe<Subscript>3</Subscript>Al intermetallics by the monte carlo simulation method combined with electronic theory of alloys

The evolution of atomic ordering processes in Fe₃Al has been modeled by the Monte Carlo (MC) simulation method combined with the electronic theory of alloys in pseudopotential approximation. The magnitude of atomic ordering energies of atomic pairs in the Fe₃Al system has been calculated by means of electronic theory in pseudopotential approximation up to sixth coordination spheres and subsequently used as input data for MC simulation for more detailed analysis for the first time. The Bragg-Williams long-range order (LRO) and Cowley-Warren, short-range order (SRO) parameters have been calculated from the equilibrium configurations attained at the end of MC simulation for each predefined temperature and Al concentration levels, which reveal the evolution of the system from DO₃→B2→ disordered state as temperature, increases. The variation of ordering parameters with temperature has identified the transition temperature from DO₃→B2 type superlattice, and from B2→disordered (α) solid solution at about 540°C and >900 °C, respectively, showing good qualitative agreement with experimental results. The results of the present study imply that combination of electronic theory of alloys in pseudopotential approximation with MC simulation can be successfully applied for qualitative or semiquantitative analysis of energetical and structural characteristics of atomic ordering processes in Fe₃Al intermetallics.     

Transition metal stannides with MgAgAs and MnCu2Al type structure

The ternary phases ZrNiSn and ZrNi₂Sn were found to crystallize with the MgAgAs (ordered CaF₂) and MnCu₂Al (ordered BiF₃, Heusler) type structure, respectively. The atomic order in these phases has been established from quantitative X-ray data. Although the ZrNi₂Sn structure may be described as a filled up ZrNiSn structure, the two phases do not form a continuous range of solid solutions. The phases NbCoSn, NbRhSn, TiNiSn, ZrNiSn, HfNiSn, and NrPdSn are isotypic with MgAgAs. The phases ZrCo₂Sn, HfCo₂Sn, NbCo₂Sn, ZrNi₂Sn, HfNi₂Sn and NbNi₂Sn were found to crystallize with the MnCu₂Al type structure. The previously reported phases TiCo₂Sn and TiNi₂Sn are confirmed. Lattice constants for the phases are reported. The three phases TiCo₂Sn ZrCo₂Sn, and HfCo₂Sn were the only ones found to be ferromagnetic. All known transition metal stannides with the MgAgAs type structure have an average valence electron concentration of 8/3.