DO22 Phase Research Articles

The L12 ↔ DO19 transformation in the intermetallic compound Fe3Ge

From a crystallographic point of view, the transition from the L12 to DO19 phase is an ordered version of the widely investigated fcc to hcp transformation. In the present study, the transformation kinetics of the forward and backward reactions of the L12↔ DO19transitions in Fe3Ge have been studied and it is shown that a large hysteresis exists between the forward and backward reactions. The detailed microstructural changes that accompany the L12 ↔ DO19 transition have been characterized and these explain the observed transformation hysteresis.

FIM observation of ordered Phase in Ti-Al alloy

We have observed Ti-42at%Al alloy using FIM. FIM images showed that the strength for filed evaporation are differenced the Ll0 phase and the DO19 phase.

Evolution of the two-phase microstructure L12 + DO22 in near-eutectoid Ni3(Al,V) alloy

Transmission electron microscopy and powder x-ray diffraction methods have been used to investigate the evolution of two-phase (L12 + DO22) microstructures from the quenched fcc phase of the Ni-5Al-20V (at. %) alloy. The microstructure after annealing in a temperature range from 650 to 900 °C differs from the eutectoid structure which might be expected for the alloy according to the eutectoid-type phase diagram of the Ni3Al-Ni3V section. This structure results from fast kinetics of ordering in the fcc → L12 and fcc → DO22 phase transitions. Four main stages in the microstructural evolution were observed. Stage I is the formation of spheroidal coherent L12 clusters in a disordered fcc matrix. During stage II the L12 clusters transform into cuboidal precipitates, and the fcc matrix orders into three DO22 variants (which may have interfaces that are wetted by thin fcc layers). In stage III accommodation of misfit (elastic energy) between different phases and variants occurs by formation of (110) twins or a single variant of the DO22 phase and tetragonally strained lamellae of the L12 phase. Stage IV is a discontinuous coarsening process in which a coarse incoherent two-phase structure replaces the fine coherent one. Grains of the coarse structure are nucleated on high-angle boundaries of primary fcc or other surfaces. Many of the grains are found twinned.

Transformation of BCC and B2 High Temperature Phases to HCP and Orthorhombic Structures in the Ti-Al-Nb System. Part II: Experimental TEM Study of Microstructures.

Possible transformation paths that involve no long range diffusion and their corresponding microstructural details were predicted by Bendersky, Roytburd, and Boettinger [J. Res. Natl. Inst. Stand. Technol. 98, 561 (1993)] for Ti-Al-Nb alloys cooled from the high temperature BCC/B2 phase field into close-packed orthorhombic or hexagonal phase fields. These predictions were based on structural and symmetry relations between the known phases. In the present paper experimental TEM results show that two of the predicted transformation paths are indeed followed for different alloy compositions. For Ti-25Al-12.5Nb (at%), the path includes the formation of intermediate hexagonal phases, A3 and DO19, and subsequent formation of a metastable domain structure of the low-temperature O phase. For alloys close to Ti-25Al-25Nb (at%), the path involves an intermediate B19 structure and subsequent formation of a translational domain structure of the O phase. The path selection depends on whether B2 order forms in the high temperature cubic phase prior to transformation to the close-packed structure. The paper also analyzes the formation of a two-phase modulated microstructure during long term annealing at 700 °C. The structure forms by congruent ordering of the DO19 phase to the O phase, and then reprecipitation of the DO19 phase, possibly by a spinodal mechanism. The thermodynamics underlying the path selection and the two-phase formation are also discussed.

DO22 to L12 transition in intermetallic systems

Ternary additions of metals such as chromium, manganese, iron, cobalt, nickel, copper and zinc to tetragonal (DO22) Al3Ti are known to lead to stabilization of the cubic (L12) structure. This DO22 to L12 transition has been studied in the Al-Ti-Ni system using X-ray diffraction and scanning electron microscopy. The results show that nickel substitution has no significant effect on the lattice parameter (and therefore on the tetragonality) of the DO22 phase and that the solid solubility of nickel in the DO22 phase is very limited. The L12 phase precipitates out on addition of nickel to Al3Ti, its amount increasing with increasing nickel content of the alloy. The compositions of the DO22 and L12 phases do not change significantly with the alloy composition. These results are discussed in terms of theories of structural transitions in ordered alloys. Similar transitions have been reported in transition metal-based systems. An analysis of the transition in intermetallic systems is presented.

ELECTRONIC STRUCTURE AND CRYSTAL STRUCTURE IN Al3Y

The electronic structures of Al3Y in the three (DO19, Ll2, and DO22) phases were investigated using the allelectron total energy self-consistent LMTO method. The calculated results for total energy showed that the hexagonal DO19 phase is the most stable structure among these three phases, this is in agreement with experiment. Furthermore, it is found that the stability depends strongly upon the band filling of the bonding states and exhibits a inverse relationship with the density of states at EF.

Structural Energy Differences in Al3Ti: The Role of Tetragonal Distortion in APB and Twin Energies

ABSTRACTAt stoichiometry, DO22 is the observed ground state of Al2Ti and has a C/A ratio of 2.23, but as a function of both concentration and temperature other ordered arrangements of APB's are observed. These phase transitions have sparked many studies in which the energy has been treated as that of chemical rearrangement on an fcc lattice. We have found that at the ideal C/A ratio, the Ll2 structure is lower in energy, but as the tetragonal distortion increases the DO22 energy drops below that of Ll2. The critical role played by the tetragonal distortion in the balance between Ll2 and DO22 energies precludes the use of any model based on the undistorted lattice.The major impediment to the development of Al3Ti as a high-temperature material is its lack of ductility. The standard approach is to make alloy additions which transform the structure to Ll2. An alternate approach is to work toward the enhancement of ductility in the DO22 phase. As a first step we have calculated the twinning energy in Al3Ti.

Electronic Structure vs Phase in Al3V

AbstractThe electronic structure of Al3V vs its two different crystal structures (DO22 and Ll2) were investigated using local density total energy approach. The calculated results of the total energy showed that in Al3V the tetragonal DO22 phase is energetically favored as compared to the cubic Ll2 phase, the total energy in the former case is about 60 mRy/F.U. lower than that in the later case. The calculated lattice constant (a=3.72 Å, c=8.20 Å) is in fairly good agreement with experiment (a=3.778 Å, c=8.326 Å),and the bulk modulus (1.3 Mbar) is comparable with the experimental Young modulus (150 GPa) for Al3Ti. Furthermore, it is interesting to note that the density of states at EF in the tetragonal DO22 phase (0.14 states/eV-F.U.) is about one order magnitude smaller than that in the Ll2 phase (2.89 states/eV-F.U.). The electronic structure of Al3V seems to be fairly satisfactory in explaining its phase stability.

An investigation of the ordered DO19 phase formation in the Ti-Al system

For the Ti-rich, Ti-AI system, theα-β andα-α2 phase boundaries have been obtained through differential thermal analysis for alloy compositions ranging from 10.2 to 25.2 at% Al (6 to 16 wt% Al). It has been shown that in the above mentioned composition range, a metastable disorderedα-phase can be quenched in. This metastableα then transforms to the orderedα2(DO)19 phase upon heating and/or isothermal ageing. It has also been observed that the kinetics of this ordering phenomenon is composition dependent.

Electron Microscopy Study of Structural Changes During Isothermal Annealing of a Ni4Mo-Based Alloy Containing Chromium

ABSTRACTTransmission electron microscopy has been used to study phase transformations in a Ni4Mo alloy containing 2.08 wt pct chromium following isothermal annealing at 850, 800 and 700°C. At these temperatgres, the DO22 phase forms initially, which then transforms to Ni3Mo. At 700°C, the DO22 phase forms from bands composed of Ni4Mo and Ni2Mo aligned parallel to {111} fcc planes, and is relatively stable. A second mechanism for the formation of Ni3Mo occurs at 700°C. Large Ni4Mo domains form at grain boundaries. These su~sequently transform to Ni3Mo, with the simultaneous presence of stacking faults on (010) Ni3Mo planes.These observations are correlated to the optical microstructure. Grain size influences the type, volume, and distribution of second phases. Sequences and mechanisms of transformations are related to the experimental observations.

THE COMPLEMENTARY USE OF ATOM PROBE FIELD ION MICROSCOPY AND ANALYTICAL TRANSMISSION ELECTRON MICROSCOPY FOR THE STUDY OF A Ni-BASE SUPERALLOY

Compositions of the γ, γ' and DO22 phases in the nickel-base superalloy RSR 143 (76Ni-13A1-9Mo-2Ta, at. %) have been determined via atom probe field-ion microscopy (APFIM) as 76Ni-6A1-18Mo, 67Ni-25A1-4Mo-4Ta and be 73Ni-4A1-20Mo-3Ta respectively. Using energy dispersive x-ray spectroscopy in the analytical transmission electron microscopy (TEM), the composition of the γ' phase (69Ni-22A1-4Mo-5Ta) was found to be similar to that determined by APFIM. The deviation of the DO22 composition from Ni3Mo is in agreement with the prediction of a recent TEM study (Martin et al, 1980, 1982).

The Disordering Phase Transformation in (Ni70Fe30)3(V98-xAlxTi2) Alloys with 0 < × < 80

ABSTRACTThe sequence of disordering transformation processes in the A3B type alloy series (Ni70Fe30)3(V98-xAlxTi2), currently under development for high-temperature structural applications, was studied by differential scanning calorimetry (DSC), x-ray diffraction, optical microscopy, and transmission electron microscopy (TEM). Results of DSC show that in all alloys there are two endothermic stages of phase transformation from the ordered to the disordered state. With increasing x, the disordering transition temperature, Tc, reaches a maximum ∼1000°C at x ≈ 50, and then decreases. Interrupted heating, followed by water quenching, was used to characterize the crystal structure and the microstructure of the intermediate phases. For the x + 20 alloy, TEM observations showed ordered regions of DO22 phase in a matrix of disordered fcc (Al) phase at intermediate temperatures. The ordered domains transformed morphologically into cuboid like regions at higher temperatures. From a combined study by all the techniques, we conclude that in alloys with x between 0 and 20, the sequence of phase transformations upon heating is: DO22 → DO22 + Al → Al, whereas in alloys with x > 40, the major sequence is Ll2 + B2 → Ll2 + Al → Al.

Precipitate Anisotropy in RSR 197 Superalloy

With the possible exception of Au-Cu alloys, no ordering system has been explored as intimately as the Ni-rich Ni-Mo system. The ordered intermediate metastable phases that form in this binary have been extensively reviewed. One binary intermediate, the Ni3Mo metastable DO22 phase, also appears in RSR 197 alloy (Al-13, Mo-9, Ta-2, Ni at.%) and in many other Ni-Mo-Al-X alloys as well. The most obvious difference (beyond compositional differences) between Ni-Mo binaries and the higher order systems is the presence of a second phase, γ', Ni, Al, a Dl2 superlattice, in addition to the disordered γFCC phase. This stable Dl2 phase is present before the intermediate metastable DO22 phase and has an due to the high ratio of Mo partitioned to the γ phase, in contrast to most γ/γ' superalloys where . The presence of this Dl2 phase has interesting effects on the precipitation morphology and crystallography of the subsequently formed DO22 phase.

New Ll<SUB>2</SUB> Ordered Alloys having the Positive Temperature Dependence of Strength

The positive temperature dependence of strength was confirmed by means of compression tests in Ll2 ordered alloys which had been newly added to the group of those exhibiting a positive strength behavior in hot hardness tests in previous work. Those were Pt3In, Pt3Sn, Pd3Pb, Fe3Ga, Pt3Ti, Pt3Cr, Pd4Ti, and Zr3In. Moreover, Pt3Al, Pt3Ga and Pt4Sb were added to the group of anomalous compouds in this study. The activation constants were derived from a modified model based on the thermally activated model proposed by Liang and Pope.From the relationships among the derived activation constant, the apparent atomic radius ratio and the electron concentration for platinum- and nickel-based alloys, it was deduced that the Ll2 phase tends to be destabilized with respect to the DO22 phase before the DO19 phase becomes stable as the relative atomic radius of the B-subgroup element minority component in A3B alloys is increased. It was also shown that 4B-subgroup elements make the Ll2 phase of A3B alloys less stable than 3B-subgroup elements as one of the components. It was confirmed that the lower phase stability of the Ll2 phase with respect to the DO22 phase is responsible for the anomalous strength behavior. This conclusion is consistent with the thermally activated cross slip model.From this result it was proposed that a systematic variation in phase stability can be achieved by the partial replacement of one of the elements in an alloy in either direction, according to whether the atomic radius or electron concentration is to be increased or decreased.

Initial stages of decomposition in Cu–9Ni–6Sn

The early stages of the decomposition process in the ternary Cu-9 wt.% Ni-6 wt.% Sn alloy have been studied by transmission electron microscopy. The supersaturated solid solution was found to decompose through the spinodal mechanism into periodic and aligned regions which are Sn-rich and Sn-lean. It was also found that in addition to this clustering reaction, an ordering reaction also progressed with the ageing treatment to produce a metastable phase with the DO22 (Al3Ti type) structure. However, on prolonged ageing, a discontinuous reaction started at the grain boundaries and consumed the matrix leaving behind colonies of a pearlitic product composed of the equilibrium α (A1 structure) and the γ (DO3 structure) phases. This paper discusses the mechanism of formation of the DO22 phase in the alloy.

Coherency strengthening in Ni base alloys hardened by DO22 γ′ precipitates

The strengthening phase in nickel-base Alloy 718 is a metastable DO22 precipitate termed γ′ which is based upon Ni3Nb. Since the coherent γ′ particles produce a tetragonal distortion of the matrix, the specific variants of γ′ present can be controlled by the application of stress during aging. The resultant variation in strength is appreciable. A detailed comparison with likely strengthening mechanisms indicates that hardening is primarily due to the coherency strains arising from the γ′ precipitate. Implications with regard to design of alloys strengthened by the DO22 phase are considered.