Phase Transformation Paths Research Articles

A study of phase transformation in a TiAlNb alloy and the effect of Cr addition

Abstract The phase transformation paths, transformation temperatures and phase equilibria of Ti–45Al–27Nb and Ti–45Al–22Nb–5Cr (at%) alloys were evaluated over a temperature range from 865 °C to 1600 °C. Both alloys solidified as single β-phase and transformed to γ + σ phases upon slow cooling. The addition of Cr did not affect the β → γ transformation temperature upon slow cooling. In contrast, the temperature, at which the σ-phase formed, was reduced noticeably. Upon heating, the temperature at which the β-phase evolves from the γ + σ microstructure was found to decrease significantly with the addition of Cr. In the ternary alloy the formation of the γ-phase could not be retarded on quenching, however, the substitution of Nb with Cr allowed for the retainment of the β-phase to room temperature. These results are explained by the partitioning of Cr into the β-phase, which in addition to thermodynamic stability reduces the kinetics of transformations at lower temperatures.

Modifications in the melted surface layer under low energy high current pulsed electron beam (LEHCPEB) treatment

This paper gives a detailed analysis of the rapidly solidified layers formed by low energy high current pulsed electron beam (LEHCPEB) on a D2 steel. The results of electron backscattering diffraction (EBSD) measurements revealed that the microstructures and phase components of the steel vary dramatically through the melted layer and the heat affected zone. The exact nature of the graded structure also changes with the number of pulses. The phase transformation paths, which vary with depth and in different areas (close to or far away from the carbides) of the melted layer, are determined by local composition gradients induced by the LEHCPEB treatment.

Cross-sectional analysis of the graded microstructure in an AISI D2-steel treated with low energy high-current pulsed electron beam

A detailed analysis from cross-section of the rapidly solidified layer of a steel formed by low energy high-current pulsed electron beam (LEHCPEB) treatment has been successfully carried out. The results of electron backscattering diffraction (EBSD) measurements revealed that the microstructures and phase components vary dramatically through the melted layer and the heat-affected zone. The exact nature of such a graded structure also changes with the number of pulses. The phase transformation paths vary at different depth and different areas (close to or far away from the carbides). They are determined both by (i) the temperature gradients and (ii) the composition gradients in the surface layers induced by the LEHCPEB treatment.

Simulation of the precipitation process of ordered intermetallic compounds in binary and ternary Ni–Al-based alloys by the phase-field model

With the microscopic phase-field model, atomic-scale computer simulation programs for the precipitation mechanism of the ordered intermetallic compound γ′ in binary Ni–15.5 at.%Al alloy, θ and γ′ in ternary Ni 75Al x V 25− x alloys were worked out based on the microscopic diffusion equation and non-equilibrium free energy. The simulation can be applied to the whole precipitation process and composition range. A prior assumptions on the new phase structure or transformation path was unnecessary, the possible non-equilibrium phases, atomic clustering and ordering could be described automatically, and atomic images, order parameters and volume fractions of precipitates were obtained. Computer simulation was performed systematically on the precipitation mechanism, precipitation sequence of θ and γ′ in complicated system with ordering and clustering simultaneously. Through the simulated atomic images and chemical order parameters of precipitates, we can explain the complex precipitation mechanisms of θ (Ni 3V) and γ′ (Ni 3Al) ordered phases. For the binary alloy, the precipitation mechanism of γ′ phase has the characteristic of both non-classical nucleation and growth (NCNG) and congruent ordering and spinodal decomposition (COSD). For the ternary alloys, the precipitation characteristic of γ′ phase transforms from NCNG to COSD gradually, otherwise, the precipitation characteristic of θ phase transforms from COSD to NCNG mechanism gradually.

Crystallization behavior in a highly driven marginal glass forming alloy

Al 90Sm 10, a marginal glass former, was rapidly solidified using Cu-block single roller melt spinning at wheel speeds of 30 and 40 m/s. The product phases of rapid solidification were identified and analyzed using high energy synchrotron X-ray diffraction (HEXRD), high resolution transmission electron microscopy, and atom probe tomography. The as-quenched structure consists of a saturated amorphous phase and nanocrystalline Al with typical length scale of about 5 nm. The appearance of a pre-peak on HEXRD diffraction patterns and a low activation energy for first crystallization as determined using the Kissinger and Ozawa methods indicate some local ordering in the amorphous phase. The devitrification phase transformation path was determined using in situ high energy synchrotron radiation. Three phases, MS1, H1, and Al 4Sm, were identified during decomposition of the amorphous phase. MS1, H1 and Al 4Sm are cubic, hexagonal and orthorhombic metastable phases, respectively.

Thermal decomposition of AlH 3 studied by in situ synchrotron X-ray diffraction and thermal desorption spectroscopy

The thermal decomposition of alane was investigated by application of synchrotron X-ray diffraction (SR-XRD) and thermal desorption spectroscopy (TDS). Two polymorphs were studied, α- and γ-AlH 3. Activation energies, anisotropic volume expansions, and phase transformation paths were found. In addition, the crystal structure data, including structure of hydrogen sublattice, and small charge transfer from the aluminium towards the hydrogen sites were observed during a high-resolution SR-XRD study of α-AlH 3.

Effects of Long-Term Service on The Aging Behavior of A Water-Quenched U6Nb Alloy

AbstractMicrohardness testing and transmission electron microscopy are used to study the effects of long-term service on the aging behavior of a water-quenched U-6wt.% Nb alloy when subjected to isothermal aging at 200°XC. The original α''c phase in the WQ-U6Nb alloy is found to become partially ordered over 18 years of aging at ambient temperatures, i.e., natural aging, forming a microstructure that is featured by antiphase domain boundaries (APBs). When subsequently aged at 200 °C, an ordered phase U3Nb is precipitated through a nucleation-and-growth mechanism, suppressing spinodal decomposition that occurs when the water-quenched alloy is artificially aged at the same temperature. The different phase transformation paths lead to different microhardness changes during artificial aging: the naturally aged alloy is more slowly hardened, but to a greater microhardness peak value.

Ab initiostudy of phase transformations in boron nitride

The structural properties and phase stability of the four common polytypes of boron nitride, cubic zinc blende (c-BN), hexagonal (h-BN), wurtzite (w-BN) and rhombohedral (r-BN), are studied by ab initio calculations. Electronic energies are calculated using an ultra soft pseudopotential method under the density-functional theory, and phonon dispersions are calculated using the first-principles force-constant method. The $p\ensuremath{-}T$ phase diagrams of these four boron nitride phases are constructed with the quasiharmonic approximation. Direct compression simulations are then performed to find probable phase transformation paths among these polytypes, with additional energy calculations of plausible transition structures. The c-BN phase is the most thermodynamically stable in ambient conditions among these four polytypes, and the transformation between r-BN and c-BN has the smallest energy barrier. Direct transformation between h-BN and c-BN is far less favorable than indirect transformation, with w-BN or r-BN as an intermediate. The presence of structural defects is a key attribute in reducing the energy barrier of phase transformation. The results in this work offer theoretical clues to experimental data on c-BN film growth, particularly the absence of w-BN.

Instability of higher-energy phases in simple and transition metals

We perform full-potential first-principles total energy calculations for several simple and transition metals along three distinct phase transformation paths, i.e. the tetragonal, trigonal and hexagonal paths. Our results show that higher-energy phases, such as the bcc structure for Al, Cu and Ti and the fcc structure for Nb, Mo, Ta and W, are always unstable with respect to one or more of transformation modes. Some local minima along the total energy profiles are found to correspond to the structures not dictated by the symmetry. We discuss the most interesting problem why a higher-energy phase may be stabilized in a pseudomorphic film.

Effect of the powder precursor characteristics in the reaction sintering of aluminum titanate

Aluminum titanate precursor powders, containing 2.5 wt.% MgO additive, were prepared by two powder processing methods, the hydrolysis of alkoxides and coprecipitation of inorganic salts. The hydrolysis-derived precursor powder showed a higher chemical homogeneity than the coprecipitated-derived precursor powder. Dilatometric analysis, accomplished with XRD and microscopy, was a very good method to study both phase transformation and sintering processes. Different phase transformation paths of TiO 2 and Al 2O 3 were observed in the two powders. The formation of aluminum titanate solid solution began at ∼1220°C in both, but in the hydrolysis-derived powder compact the aluminum titanate grains grew faster, leading to incomplete reaction and a non-uniform microstructure of large grains with small residual inclusions.

Ordering and decomposition of the β phase in melt-spun TiAl 1 − x V x alloys

Ordering and decomposition of the β phase in melt-spun and aged TiAl 1 − x V x ribbons were studied using calculated phase equilibria and electron microscopy. The phase transformation paths were dramatically changed by rapid quenching from the melt. In the melt-spun ribbons, γ nucleation in the β/B2 matrix and α → α 2 ordering were completely suppressed. The β → B2 ordering temperature T C was modelled. The results support the predicted decrease in T C with substitution of Al with V. The effective β → B2 ordering temperature was lowered by high cooling rates. The formation of the ω phase in the melt-spun ribbons and of ω and α 2 phases in the aged ribbons is also discussed.

A transmission electron microscope study of the effect of cyclohexane adsorption on the crystallization of Pd 80Si 20 metallic glass

The strong effect of the adsorption of cyclohexane on the crystallization of metastable Pd 80Si 20 metallic glass was studied in situ via a high voltage electron microscope equipped with a hot stage and an environmental cell. The crystallization temperature of the Pd 80Si 20 glass was lowered by 200 °C in 20 Torr of cyclohexane as compared to 10 −6 Torr vacuum. The path of the solid state phase transformation in the bulk glass was also altered, since a different crystalline second phase formed in cyclohexane as compared to vacuum.

Theoretical stability of a perfect thorium crystal subjected to multidirectional stresses

A theoretical study is made of the lattice stability or theoretical strength of thorium subjected to various kinds of simple stresses. Calculations are carried out with the use of generalized Morse potential and the stability criterion of Born. The region of $G$, $M$, and $S$ stability of thorium in the ${a}_{2}={a}_{3}$ plane of the ${a}_{1}$, ${a}_{2}$, ${a}_{3}$ space is investigated. Many simple cases of lattice deformation arise as specific cases of the generalized calculations presented here. A wealth of data for various elastic moduli, internal energy, and stresses throughout a wide range of lattice deformation is presented. This study indicates the presence of a stress-free bcc phase of thorium, which has a cell length of 4.04 \AA{}, in close agreement with the experimental observation. We have found two internal-energy minima corresponding to the stress-free fcc and bcc phases. The path of phase transformation (fcc\ensuremath{\rightarrow}bcc) in thorium is predicted on the basis of the present study.