Special features of the α → γ transformation in powder carbon steels

The structural state of powder steels after sintering depends on the technological regimes of their production and can markedly affect the transformations in subsequent heat treatment. The effect of the technological regimes on the nature of the transformation is investigated.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 14–16, March, 1996.     

Effect of alloying elements on the γ to α transformation in steel. I

A newly constructed computer program was used to simulate partitionless growth of α in γ of Fe-Ni-C alloys, taking into account so-called solute drag by evaluating the dissipation of Gibbs energy due to diffusion inside the interface and in the nickel spike being pushed in front of it. It could be shown how the conditions at the α/γ interface vary with the velocity. A continuous change from paraequilibrium to quasi-paraconditions could be illustrated in the phase diagram. By combination with an approximate analytical growth equation, it was possible to derive the thickness of α as function of time. The growth velocity was assumed to start at very high values but decreased due to the pile-up of carbon. For alloy compositions outside the limit for quasi-paraconditions and just inside it the growth was predicted to stop suddenly when critical conditions are approached during the reaction. For alloy compositions further inside the limit, there was no such stop, except by the action of impingement of diffusion fields for carbon. Somewhere between the lines for paraequilibrium and quasi-paraconditions there is a rather rapid drop of the final thickness of α. In order to explain experimental information on this drop one must accept an appreciable tendency of segregation of nickel to the α/γ interface.     

Phase-field simulations of α → γ precipitations and transition to massive transformation in the Ti–Al alloy

A phase-field model for the solid–solid α → γ transition of Ti–Al binary alloys is presented based on analytical Gibbs free energies and couplings to the thermodynamical database ThermoCalc. The equilibrium values recover the α + γ phase boundaries. Morphological transitions from diffusive to massive (partitionless) growth are observed on increasing the initial mole fraction of aluminum. Temporal evolution of the interface shows a $\sqrt{t}$ behavior for diffusive and a linear behavior for massive growth, which is in accordance with theoretical predictions. An estimate of the interfacial mobility of Ti–Al based on the Burke–Turnbull equation is calculated. The expression of the mobility follows an Arrhenius law. Using the derived interfacial mobility, the calculated interfacial velocities of the massive transformation are in quantitative agreement with those observed in experiments.     

Analytical modeling of stress-induced martensitic transformation in the crack tip region of nickel–titanium alloys

A novel analytical method, which predicts the extent of the stress-induced martensitic transformation (SIM) in the crack tip vicinity of nickel–titanium (NiTi)-based shape memory alloys, as well as describing the stress distribution in both transformed and untransformed regions, is presented. The method has been validated by comparisons with results from finite-element simulations with good agreement. Furthermore, the method has been used to analyze the effects of various thermomechanical parameters on the extent of the transformation region near the crack tip. Finally, the effects of several thermomechanical loading conditions, in terms of both applied stress and temperature, on the crack tip transformation behavior have been analyzed. The results highlight a marked effect of temperature on the extent of the transformation region and, consequently, on the crack tip stress distribution. As a consequence, temperature plays a role in the fracture process of NiTi alloys.     

Kinetics of the δ to γ zirconium hydride transformation in Zr-2.5Nb

When the concentration of hydrogen exceeds the solubility limit in a metal matrix, metal hydrides may appear as precipitates that degrade the performance of the material. Neutron diffraction was combined with microscopy to study the δ to γ phase transformation of zirconium hydride precipitates in Zr-2.5 wt.% Nb. Specimens were heated to dissolve all hydrides, then cooled to holding temperatures ranging from 17–100 °C, to investigate the kinetics of transformation from the high-temperature δ-hydride to the low-temperature γ-hydride. The δ to γ transformation proceeds over a period of many hours, with a rate that increases as the holding temperature is decreased. Transmission Electron Microscopy images indicate that the boundary regions of hydride precipitates transform to the γ-phase, leaving a shrinking core of the δ-phase. The crystallographic orientations of the hydrides appear to be determined by the texture of the α-Zr matrix, even after complete dissolution and re-precipitation.     

Microstructural evolution during the α2→α2+O transformation in Ti–Al–Nb alloys: phase-field simulation and experimental validation

The microstructural development during precipitation of a coherent orthorhombic phase (O-phase) from an α₂ matrix (DO₁₉) in a Ti–Al–Nb system is investigated through computer simulations using the phase-field approach and through experimental observations using transmission electron microscopy (TEM). Two compositions were considered in the simulations in order to examine the influence of volume fraction of the O-phase on the microstructure. It is found that in the alloy with higher volume fraction of the O-phase, the precipitates have either square or rectangular shapes on (0001)_α2 or (001)_O planes. All the particles are interconnected by sharing their corners. In the alloy with lower volume fraction, the dominant morphology for the precipitates is thin plate. The spatial distribution of precipitates is highly non-uniform with the precipitates aggregating together to form various unique patterns to accommodate the elastic energy arising from the lattice misfit between the α₂ and O-phase. All the interfaces between the α₂ and O-phase are found to be undistorted habit planes of the type {470)_O, and the domain boundaries between different orientation variants of the O-phase are twin boundaries which are the strain-free planes {110)_O or {130)_O. The simulation predictions agree remarkably well with existing experimental observations and the concurrent TEM study.     

Active Eutectoid Decomposition of α → γ + τ _1 and the Morphological Evolution in a Ru-Containing TiAl Alloy

The active eutectoid decomposition and its morphological evolution during slow cooling and isothermal holding were investigated in a Ru-containing Ti Al alloy. The fine τ 1/γ active eutectoid nodules precipitated at α grain boundary and interior during water quenching. The active eutectoid microstructure evolved from sheaf-like, granular to bulky net-like sluggish eutectoid morphology gradually with the decrease in quenching/holding temperatures. The active eutectoid reaction occurs from 1220 to 1290 ℃, while the beginning temperature of sluggish eutectoid locates at 1305 ± 5 ℃. The combination of the intact τ_1 phase and immature nano-sized γ laths suggests a short incubation period of τ_1 phase in the active type. Furtherly, the semiquantitative estimation shows that the growth velocity of active eutectoid is about ninety times higher than sluggish type. In addition, a representative feature of γ phase abruptly ripening along {111} crystallographic plane was also observed in the active eutectoid.     

Model of the B2 → R martensitic transformation in alloys with a B2 superstructure

Various B2 ?? R structural transformations that can occur through an intermediate R ₀ structure belonging to the space group D _3d ³ have been considered. Subgroups with a hexagonal lattice (group D _3d ³ ) related to the symmetry breakdown of the R ₀ structure due to atomic displacements with wave vectors belonging to the two-arm star $K_{13}^H = \left\{ {k_1^H = \frac{1} {3}\left( {b_1^H + b_2^H } \right),k_2^H = - k_1^H } \right\}$ , where b ₁ ^H and b ₂ ^H are the reciprocal-lattice vectors of the hexagonal system, have been calculated. A crystallographic model of the possible transformations of the B2 structure into the R martensite, which has a single (common for all transformations) order parameter, has been suggested.     

Kinetics and thermodynamics of the α→γm massive transformation in a Ti–47.5 at.% Al alloy

Continuous cooling experiments, utilizing in situ, high-speed computer-controlled temperature and electrical resistivity measurements, were performed to study the kinetics and thermodynamics of the α→γ_m massive transformation in a Ti–47.5 at.% Al alloy. Samples of the alloy were heated by controlled direct resistance heating in vacuum to the α-phase field and cooled at various rates either in vacuum or by controlling the flow of a helium jet quench. The results have shown that the α→γ transformation is sensitive to cooling rate and is accompanied by large discontinuous changes in resistivity and thermal arrest. Lamellar, feathery and massive γ_m are observed with increasing cooling rate. Correlation of the resistivity–temperature–time data with microstructure allowed the determination of the cooling velocity range in which the massive transformation takes place, the reaction temperatures of this transformation as a function of cooling rate, and the growth rate of the massive γ_m phase as a function of undercooling below T₀. The enthalpy and driving force associated with the formation of the massive γ_m phase were determined and compared with those in other alloy systems; the experimental values of the driving force were also found to compare well with theoretically calculated values. Comparison of estimates of the activation enthalpy for boundary diffusion obtained with values for bulk diffusion established that the α→γ_m massive transformation is controlled by interfacial rather than volume diffusion. Possible growth mechanisms of the massive phase, whether by a ledge process or by continuous growth, were examined and are discussed.     

An X-ray microtomographic and finite element modeling approach for the prediction of semi-solid deformation behaviour in Al–Cu alloys

There is a dearth of published experimental measurements of flow stress behaviour of semi-solids, yet it is critical for simulating phenomena ranging from the processing of metals to the flow of magma. In this paper, a method for calculating flow stress behaviour of semi-solids was developed using a combination of high-temperature compression testing, X-ray microtomography (XMT) imaging and direct finite element modeling (DFEM). This novel methodology was applied to columnar dendritic structures in semi-solid Al–Cu alloys via first quantifying the complex geometry of the semi-solid using XMT. Then these three-dimensional datasets were meshed and their behaviour was simulated using DFEM to derive the stress–strain relationship with a fraction solid (f_S) dependency term. The mechanical behaviour of the solid dendrites near the liquidus temperature was not available in the literature; therefore, samples were fabricated and compression tested using a Gleeble 3500 thermomechanical simulator. The resulting XMT–DFEM-derived constitutive equation predicts the flow stress behaviour of semi-solid in the range of f_S equal to 0.1–0.9, showing good correlation to prior experimental data for both other aluminium and ferrous alloys.     

Nucleation kinetics of the α→γM massive transformation in a Ti-47.5 at.% Al alloy

Continuous cooling experiments, utilizing in situ, high-speed computer-controlled temperature and electrical resistivity measurements and quantitative stereological analysis of microstructures, coupled with calculations based on the classical theory of nucleation, were performed to study the nucleation kinetics of the α→γ_M massive transformation in a Ti-47.5 at.% Al alloy. Using previously determined thermodynamic data on the reaction temperature, time, undercooling and driving force for the massive transformation, the free energies for critical nucleus formation and the nucleation rates were computed for different nucleus shape models, including incoherent grain face, edge and corner nuclei and faceted grain face nuclei, and compared with experimentally determined nucleation rates. The results indicate that nucleation of singly faceted grain face nuclei and incoherent grain corner and grain edge nuclei are highly likely. Good agreement was also obtained between the calculated and experimental nucleation rates at these various sites. Based on these results, possible nucleation mechanisms during the massive transformation are discussed in light of current thinking on the nature of this transformation.     

Elasticity-based model of the variant selection observed in the β to α phase transformation of a Zircalloy-4 sample

The β→α texture inheritance of a Zircalloy-4 sample has been investigated after an α→β→α transformation cycle. The final inherited α texture has been determined from a crystal orientation map determined by electron back-scattering diffraction, whereas the texture of the high temperature β phase has been reconstructed by a method analysing the orientations and misorientations of α variants. The comparison of the α texture calculated from the parent β texture without variant selection with the experimental sharp α texture shows differences due to a strong variant selection mechanism occurring during the phase transformation at cooling.A model of a variant selection mechanism based on the elastic anisotropy of the parent β phase leads to a simulated inherited α texture with the main characteristics of the experimental texture.     

Effect of carbon,chromium, and nickel on the α ⇄ γ transformation and properties of Cr - Ni - Mo - V steels

The aim of the present investigation is to determine some regular features of alloying Cr - Ni - Mo - V steels for critical parts with allowance for the structure, hardenability, level of properties, and semibrittleness temperature. For this purpose the concentration of carbon in Cr - Ni - Mo - V steels was varied from 0.03 to 0.47%, chromium from 0 to 4%, and nickel from 0 to 5% at a constant concentration of the other elements (0.5% Mo, 0.6% Mn, 0.25% Si, -0. 1% V, 0.015% S, 0.012% P), and the cooling rate from the austenitization temperature was varied from 25 to 1000°C/h.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 3, pp. 7 – 9, March, 1996.     

Order–disorder transition of vacancies from the full- to the half-Heusler structure in Ni2−xMnSb alloys

The thermal transformations and crystal structures in the ordered bcc phases appearing in Ni_2−xMnSb alloys have been investigated. Thermo-analyses on NiMnSb(x = 1) show three λ-shaped peaks suggesting a magnetic and chemical ordering transitions at approximately 450, 865 and 965 °C. With decreasing x, the first and second peak temperatures gradually decrease, while the third one increases. By Z-contrast imaging technique, it was confirmed that the second peak corresponds to the order-disorder transition of vacancies from the L2₁ to the C1_b structure.     

Characterization of γ plate-shaped precipitates in an Al–4.2 at.% Ag alloy — growth kinetics,solute field,composition and modeling

The lengthening and thickening rates of γplate-shaped precipitates in an Al–4.2 at.% Ag alloy were determined by transmission electron microscopy (TEM) for aging times of 120–720 s at 400°C. The compositions of the γ plates and the solute fields surrounding them were measured quantitatively using energy-dispersive X-ray spectroscopy (EDS) and revealed qualitatively by energy-filtering TEM (EFTEM). The shape of the solute field around the plates was also revealed by employing a double-step aging treatment to produce GP zones in the supersaturated α matrix around the precipitates, but not in the solute-depleted zone surrounding the plates. The combined data obtained from these measurement were then compared with the Ham, Horvay and Cahn (HHC) model for precipitate growth. The results show that: (1) the lengthening rate of the plates ranged from 77 nm/s at 120 s aging to 15 nm/s at 720 s aging, the thickening rate ranged from 0.2 nm/s at 120 s aging to 0.09 nm/s at 720 s aging, and the aspect ratio increased rapidly with time; (2) the solute gradient around the plates is approximately two plate thicknesses wide and has a faceted-elliptical shape for all aging times examined; (3) the solute concentration in the matrix immediately adjacent to the faces of the plates is higher than adjacent to the edges, indicating inhibited growth at the faces, (4) the Ag concentration in the γ plates is significantly lower than that given by the equilibrium Al–Ag phase diagram; and (5) the HHC model for the growth of an oblate spheroidal precipitate predicted the lengthening and thickening rates of the plates reasonably well when the time dependence of the aspect ratio was included in the calculations.     

The thermodynamics of and strengthening due to co-clusters: General theory and application to the case of Al–Cu–Mg alloys

Co-clusters in ternary or higher order metallic alloys are metastable structures involving two or more distinct alloying atoms that retain the structure of the host lattice. A thermodynamic model based on a single interaction energy of dissimilar nearest neighbour interaction energy is presented, and a model for the strengthening due to these co-cluster dimers is derived. The model includes a new treatment of (short-range) order strengthening relevant to these co-clusters and further encompasses modulus hardening and chemical hardening. The models are tested against data on a wide range of Al–Cu–Mg alloys treated at temperatures between 20 and 220 °C. Both quantitative calorimetry data on the enthalpy change due to co-cluster formation and strengthening due to co-clusters is predicted well. It is shown that in general (short-range) order strengthening will be the main strengthening mechanism.     

The Effect of Niobium-Ion Implantation on the Oxidation Behavior of γ -TiAl Alloys in Static and Flowing Air

-TiAl (Ti–50Al at.%) alloys were implanted with Nb ions at an acceleration energy of 50 keV, at a dose of 1.2×10¹⁷ ions/cm². The cyclic-oxidation behavior of the unimplanted and Nb⁺-implanted TiAl specimens was investigated at 850°C in static air and in air with a flow velocity of 12.0 m/s (1000 ml/min). In static air, the unimplanted TiAl specimen showed rapid oxidation during a transition period of about 80 hr, after which partial scale spallation occurred and a net mass loss was observed. In flowing air, the whole scale spalled off after each cycle. On the other hand, Nb-ion implantation led to the formation of an adherent protective Al₂O₃ scale during oxidation in both static and flowing air, thereby significantly improving the cyclic-oxidation resistance of -TiAl alloys. A remarkable deference in the initial-oxidation behavior between unimplanted and Nb⁺-implanted specimens was also observed. A mixed TiO₂/A₂O₃ scale on the unimplanted specimen developed at a high growth rate during the very initial stage of oxidation. In contrast, the initial scale growth rate was significantly decreased by Nb-ion implantation and an Al₂O₃-rich layer was found present as the inner part of the initial scale on Nb⁺-implanted TiAl. Flowing air appeared to cause severe scale spallation during oxidation of unimplanted TiAl, but not to have any influence on the adhesion of the scale on Nb⁺-implanted TiAl.     

Effect of Nb and Sn on the transformation of α-Ti to β-Ti in Ti-35 Nb−2.5Sn nanostructure alloys using mechanical alloying

This study was conducted to observe the phase transformation of Ti-35 %Nb-2.5 %Sn during milling for various milling times using a mixing machine and a high-energy ball milling machine (HEBM). In this work, niobium was chosen as the β stabilizer and tin as the reducer of the elastic modulus. The starting materials were blended and milled in a mixing machine (24 h) and a high-energy ball mill machine (1 h, 4 h, and 12 h). The particle sizes and phases of the produced powders were analyzed using XRD, SEM, TEM, and PSD; the titanium was found to be fully transformed from the α phase to Ti the β phase Ti after 12 h of milling.     

HREM study and structure modeling of the η′ phase,the hardening precipitates in commercial Al–Zn–Mg alloys

The structure of the η′ phase, one of the most important age-hardening precipitates in commercial Al–Zn–Mg alloys, has been studied at the atomic level by means of high-resolution electron microscopy (HREM). A structural model of the η′ phase has been constructed on the basis of the structural characteristics in the observed images and the structure of the η-MgZn₂ phase. Image simulation of this model shows a good agreement between calculated and experimental images. Comparison of this model with the early existing model on the basis of the X-ray diffraction is also given.