Study of Structure and Deformation Pathways in Ti-7Al Using Atomistic Simulations,Experiments, and Characterization

Ti-7Al is a good model material for mimicking the α phase response of near-α and α+β phases of many widely used titanium-based engineering alloys, including Ti-6Al-4V. In this study, three model structures of Ti-7Al are investigated using atomistic simulations by varying the Ti and Al atom positions within the crystalline lattice. These atomic arrangements are based on transmission electron microscopy observations of short-range order. The elastic constants of the three model structures considered are calculated using molecular dynamics simulations. Resonant ultrasound spectroscopy experiments are conducted to obtain the elastic constants at room temperature and a good agreement is found between the simulation and experimental results, providing confidence that the model structures are reasonable. Additionally, energy barriers for crystalline slip are established for these structures by means of calculating the γ-surfaces for different slip systems. Finally, the positions of Al atoms in regards to solid solution strengthening are studied using density functional theory simulations, which demonstrate a higher energy barrier for slip when the Al solute atom is closer to (or at) the fault plane. These results provide quantitative insights into the deformation mechanisms of this alloy.     

Elastic Properties,Thermal Expansion Coefficients,and Electronic Structures of Mg and Mg-Based Alloys

Ab-initio density functional theory (DFT) calculations were performed to study alloying effects on hcp Mg. The alloy solid solution strengthening represented by bond strength enhancement in alloys, elastic properties, thermal expansion coefficients, and electronic structures of Mg-based alloys was investigated. Results show that alloying additions with sp-metal Al and rare earth (RE) Y are capable of increasing the bond strength, with the addition of Y achieving a better effect. The bond strength enhancement due to an RE Y addition is associated with a hybridization between the d-orbital of Y and the p-orbital of the Mg atoms near the Fermi energy, and this was consistent with the electron localized function (ELF) evaluations showing that more localized and stronger covalent bonds are formed between Y and Mg atoms. It is also found that alloying additions of Al, Zn, and Y are not capable of increasing elastic coefficients and moduli, indicating that bond strength enhancement could play a major role in alloy solid solution strengthening in Mg-based alloys. Possible reasons for the elastic properties accompanying the alloying addition are given from the electronic point of view. Furthermore, from the calculated negative Cauchy pressure (C ₁₃ – C ₄₄ < 0), it is concluded that the chemical bonds between Y and Mg atoms show angular characteristics.     

Synergetic effect in modifying with master alloys having an aluminide cubic structure

Experimental data on the preparation of test master alloys Al–Sc–(Zr, Ti, Y), Al–Zr–(Ti, Y), and Al–Ti–Y, which contain two transition metals and are characterized by the formation of aluminides with the L1₂ cubic lattice (which is identical to the crystal lattice of an aluminum-alloy matrix), are presented. The growth forms of aluminides in alloys of various compositions are demonstrated. Using Al–4% Cu model alloys (experiments were carried out with 15 and 200 g samples cooled at different cooling rates), the modifying ability of the test ternary master alloys and industrial binary master alloys (used for comparison) has been estimated. Synergetic effects of two transition metals, which consist in grain refining in Al–4% Cu alloys, and a substantial difference in the modifying effects of the binary and ternary master alloys have been shown.     

Density and electrical conductivity of synthetic slags added to the ladle–furnace unit

The properties of a series of synthetic slag melts for steel refining in the ladle–furnace unit are determined in the range 1430–1645°C: the slag density ρ on the basis of the maximum pressure in a gas (argon) bubble; and the electrical conductivity æ by the double-contact ac volt–ampere method (at 2 and 5 kHz). On the basis of the measurements, the following characteristics are calculated: the bulk expansion coefficient β and actual molar volume V _M (both referred to 1600°C), the relative excess molar volume δV _M , the activation energy E _æ of the electrical conductivity, and the preexponential factor æ in the Arrhenius equation that approximates the influence of the temperature on æ. The dependence of these variables on the degree of replacement N of the polymer-forming oxides Al₂O₃ and SiO₂ in the slag batch by sodium monoxide is analyzed. With increase in N, it is found that ρ, δV _M , E _æ, and æ₀ decline monotonically, while æ increases. The dependence of β on N is more complex. The behavior of β and δV _M is discussed in terms of the polymer theory of slags.     

Suppression of Martensitic Transformation in Co<Subscript>2</Subscript>Cr(Ga,Si) Heusler Alloys by Thermal Cycling

We have investigated the influence of thermal cycles on martensitic transformation of a Co₂Cr(Ga,Si) ferromagnetic Heusler alloy. The as-quenched specimen exhibits successive L2₁(L)–D0₂₂–L2₁(H) martensitic transformation in the cooling process, which is known as reentrant martensitic transformation. However, heating to 800 K (527 °C) for reverse D0₂₂–L2₁ transformation with a rate of 10 K/min (10 °C/min) stabilizes the parent phase, meaning that the martensitic transformation is suppressed by the thermal cycles. We found precipitate after thermal cycles, and it will be the reason for the stabilization of parent phase.     

Enthalpy of mixing of liquid Cu-Ti-Zr alloys

The enthalpy of mixing of liquid Cu-Ti-Zr ternary alloys is studied by high-temperature isoperibolic calorimetry at 1873 K along three ray sections characterized by the ratios x _Zr: x _Cu = 3: 7, x _Ti: x _Cu = 3: 7, and x _Zr: x _Ti = 1 at x _Cu = 1?0.4. The isotherm of the integral enthalpy of mixing of these melts is described in terms of the Redlich-Kister-Muggianu model. Along with the substantial contributions of binary copper-titanium and copper-zirconium interactions, the contribution of a ternary interaction to the enthalpy of mixing of liquid Cu-Ti-Zr alloys also exists. The first partial enthalpies of mixing of Ni, Al, Si, Sn, and Y with the melts are studied to determine the character of the interaction between the ternary Cu-Ti-Zr melts and metal additions that facilitate amorphization upon melt quenching. The introduction of these metals into the ternary melts is shown to increase their thermodynamic stability.     

Interfacial Tension in the CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript>-(MgO) Liquid Slag–Solid Oxide Systems

Interfacial phenomenon is critical in metal processing and refining. While it is known to be important, there are little data available for key oxide systems in the literature. In this study, the interfacial tension (σ _LS) of liquid slag on solid oxides (alumina, spinel, and calcium aluminate), for a range of slags in the CaO-Al₂O₃-SiO₂-(MgO) system at 1773 K (1500 °C), has been evaluated. The results show that basic ladle-type slags exhibit lower σ _LS with oxide phases examined compared to that of acid tundish-type slags. Also, within the slag types (acid and base), σ _LS was observed to decrease with increasing slag basicity. A correlation between σ _LS and slag structure was observed, i.e., σ _LS was found to decrease linearly with increasing of slag optical basicity (Λ) and decrease logarithmically with decreasing of slag viscosity from acid to base slags. This indicated a higher σ _LS as the ions in the slag become larger and more complex. Through a work of adhesion (W) analysis, it was shown that basic ladle slags with lower σ _LS result in a greater W, i.e., form a stronger bond with the solid oxide phases examined. This indicates that all other factors being equal, the efficiency of inclusion removal from steel of inclusions of similar phase to these solid oxides would be greater.     

Structural heat-resistant β-NiAl + γ′-Ni<Subscript>3</Subscript>Al alloys of the Ni–Al–Co system: I. Solidification and structure

When analyzing the ternary Ni–Al–M phase diagrams, where M is a group VI–VIII transition metal, we chose the Ni–Al–Co system, where the γ′ and γ phases are in equilibrium with the β phase, as a base for designing alloys with the following physicochemical properties: a moderate density (≤7.2 g/cm³) and satisfactory heat resistance at temperatures up to 1300°C. The structure formation in heterophase β + γ′ alloys during directional solidification is studied. It is found that, in contrast to cobalt-free β + γ′ alloys (where the γ′-Ni₃Al aluminide forms according to the peritectic reaction L + β ? γ′), the alloys with 8–10 at % Co studied in this work during directional solidification at 1370°C contain the degenerate eutectic L ? β + γ. The transition from the β + γ field to the β + γ′ + γ field occurs in the temperature range 1323–1334°C, and the γ′ phase then forms according to the reaction β + γ ? γ′.     

Solidification of the eutectic Sn–43 mol % Bi alloy

The processes of melting and solidification of the eutectic Sn–43 mol % Bi alloy are studied by cyclic thermal analysis. It is found that this alloy melts at a temperature T _L = 412 K (which corresponds to the reference melting temperature of the eutectic) upon heating and solidifies isothermally at a temperature T _S = 394 K upon cooling; that is, the temperature difference is ΔT _LS ^? = 18 K. A comparison of temperatures T _L and T _S reveals a temperature hysteresis (TH). The activities and the activity coefficients of tin and bismuth in the eutectic are calculated at temperatures T _L and T _S . The enthalpies of melting at T _L and solidification at TS are measured. The ways of changing the Gibbs energy during TH are determined.     

Rare-earth metals (REMs) in nickel aluminide-based alloys: I. Physicochemical laws of interaction in the Ni-Al-REM and Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>-REM-AE (alloying element) systems

The data on the Ni-Al-R (R = REM Sc, Y, La, lanthanides) binary and ternary systems and the interactions of three rare-earth metals (yttrium, lanthanum, cerium) with the main alloying elements (Ti (Zr, Hf), Cr (Mo, W) that are introduced into Ni₃Al-based VKNA alloys are analyzed. The binary aluminides of REMs in the Ni-Al-R ternary systems are shown to be in equilibrium with neither NiAl nor Ni₃Al. The solid solution of aluminum in RNi₅, which penetrates deep into these ternary systems, is the most stable phase in equilibrium with Ni₃Al. In the NiAl (Ni₃Al)-AE-R systems, REM precipitation (segregation) on various defects and interfaces in nickel aluminides is likely to be the most probable, and REMs are thought to interact with the most active impurities in real alloys (C, O, N), since REMs have a large atomic radius and, thus, are virtually undissolved in nickel, aluminum, and nickel aluminides.     

Liquid Inclusions in Heat-Resistant Steel Containing Rare Earth Elements

Abundant thermodynamic data of pure substances were incorporated in the coupled thermodynamic model of inclusion precipitation and solute micro-segregation during the solidification of heat-resistant steel containing rare earth elements. The liquid inclusions Ce_2x Al_2y Si_1?x?y O _z (0 < x < 1, 0 < y < x and z = 1 ? x ? y) were first introduced to ensure the model more accurately. And the computational method for generation Gibbs free energy of liquid inclusions in molten steel was given. The accuracy of accomplished model was validated through plant trials, lab-scale experiments, and the data published in the literature. The comparisons of results calculated by FactSage with the model were also discussed. Finally, the stable area of liquid inclusions was predicted and the liquid inclusions with larger size were found in the preliminary experiments.     

Influence of Grain Coarsening on the Creep Parameters During the Superplastic Deformation of a Severely Friction Stir Processed Al-Zn-Mg-Cu Alloy

During grain boundary sliding in ultrafine-grain materials at intermediate temperatures and high strain rates (~10^?2 s^?1), apparent creep parameters usually deviate from the theoretical values, due to microstructural coarsening. An analysis has been carried out in a severely friction stir processed (FSP) 7075 alloy with three different ultra-fine grain sizes (L), obtaining explicit grain size dependence of the creep parameters n _ap = n _ap(L) and Q _ap = Q _ap(L), confirming the validity of the theoretical values of these parameters in the constitutive equation.     

Columnar-to-equiaxed transition during the solidification of a Ti–10V–2Fe–3Al alloy

The columnar-to-equiaxed transition in the solidification of a Ti–10V–2Fe–3Al alloy is considered during vacuum arc remelting. A GV curve is plotted using a linear model of a multicomponent alloy along with modified Hunt (to determine temperature gradient G as a function of supercooling ΔT) and Ivantsov (to determine the concentrations at the dendrite tip versus liquidus isotherm velocity V) models. The unknown parameters of the models are determined by comparing the results of model calculations with experimental data, which are also obtained in this work. The plotted GV curve is compared with the GV curve of a VT3-1 alloy.     

Effect of the severe plastic deformation temperature on the diffusion properties of the grain boundaries in ultrafine-grained metals

A model is proposed to explain the effect of the severe plastic deformation (SPD) temperature on the diffusion properties of the grain boundaries in ultrafine-grained (UFG) metals and alloys. It is shown that an increase in the SPD temperature in UFG metals leads to an increase in the activation energy of grainboundary diffusion from (3–5)k _B T _m, which corresponds to the diffusion parameters of nonequilibrium grain boundaries, to (8–10)k _B T _m, which corresponds to the diffusion parameters of equilibrium grain boundaries (k _B is the Boltzmann constant, T _m is the melting temperature). The dependence of the activation energy of grain-boundary diffusion on the SPD temperature is found to be determined by the kinetics of the competing processes of defect accumulation at grain boundaries and the diffusion accommodation of defects.     

Efficiency of the Inertia Friction Welding Process and Its Dependence on Process Parameters

It has been widely assumed, but never proven, that the efficiency of the inertia friction welding (IFW) process is independent of process parameters and is relatively high, i.e., ~70 to 95 pct. In the present work, the effect of IFW parameters on process efficiency was established. For this purpose, a series of IFW trials was conducted for the solid-state joining of two dissimilar nickel-base superalloys (LSHR and Mar-M247) using various combinations of initial kinetic energy (i.e., the total weld energy, E _o), initial flywheel angular velocity (ω _o), flywheel moment of inertia (I), and axial compression force (P). The kinetics of the conversion of the welding energy to heating of the faying sample surfaces (i.e., the sample energy) vs parasitic losses to the welding machine itself were determined by measuring the friction torque on the sample surfaces (M _S) and in the machine bearings (M _M). It was found that the rotating parts of the welding machine can consume a significant fraction of the total energy. Specifically, the parasitic losses ranged from 28 to 80 pct of the total weld energy. The losses increased (and the corresponding IFW process efficiency decreased) as P increased (at constant I and E _o), I decreased (at constant P and E _o), and E _o (or ω _o) increased (at constant P and I). The results of this work thus provide guidelines for selecting process parameters which minimize energy losses and increase process efficiency during IFW.     

Investigation of the End-Point Temperature Control Based on the Critical Temperature of Vanadium Oxidation During the Vanadium Extraction Process in BOF

To solve the problem related to the end-point temperature control during the vanadium extraction, industrial experiments and thermodynamic analyses were implemented to find out and verify the foundation for the new standard. The transition temperatures of carbon-vanadium oxidation (T _tr ) and the molten bath temperature measured on the industrial experiments showed a negative temperature difference after smelting for 3.5 min. However, for about 30 wt% vanadium was removed when the molten bath temperature exceeded T _tr . T _tr was only suitable for achieving the goal of “keeping carbon while extracting vanadium” in the early and medium period. The critical temperature of vanadium oxidation (T _cr ) stood for the thermodynamic equilibrium between the oxidation and reduction of vanadium in the later period. It served as the basis to control the end-point temperature to satisfy the demand of “deep vanadium extraction”. As the molten bath temperature increased above T _cr , the residual vanadium neither decreased further nor increased, but showed an equilibrium state.     

Effect of Continuous Galvanizing Heat Treatments on the Microstructure and Mechanical Properties of High Al-Low Si Transformation Induced Plasticity Steels

Heat treatments were performed using an isothermal bainitic transformation (IBT) temperature compatible with continuous hot-dip galvanizing on two high Al–low Si transformation induced plasticity (TRIP)-assisted steels. Both steels had 0.2 wt pct C and 1.5 wt pct Mn; one had 1.5 wt pct Al and the other had 1 wt pct Al and 0.5 wt pct Si. Two different intercritical annealing (IA) temperatures were used, resulting in intercritical microstructures of 50 pct ferrite (α)-50 pct austenite (γ) and 65 pct α-35 pct γ. Using the IBT temperature of 465 °C, five IBT times were tested: 4, 30, 60, 90, and 120 seconds. Increasing the IBT time resulted in a decrease in the ultimate tensile strength (UTS) and an increase in the uniform elongation, yield strength, and yield point elongation. The uniform elongation was higher when using the 50 pct α-50 pct γ IA temperature when compared to the 65 pct α-35 pct γ IA temperature. The best combinations of strength and ductility and their corresponding heat treatments were as follows: a tensile strength of 895 MPa and uniform elongation of 0.26 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 90-second IBT time; a tensile strength of 880 MPa and uniform elongation of 0.27 for the 1.5 pct Al TRIP steel at the 50 pct γ IA temperature and 120-second IBT time; and a tensile strength of 1009 MPa and uniform elongation of 0.22 for the 1 pct Al-0.5 pct Si TRIP steel at the 50 pct γ IA temperature and 120-second IBT time.     

Effect of Recovery and Recrystallization on the Hall–Petch Relation Parameters in Submicrocrystalline Metals: I. Experimental Studies

Yield strength σy, macroelastic limit σ₀, and effective grain-boundary hardening coefficient K_eff in the Hall–Petch relation (\({\sigma _y} = {\sigma _0} + {K_{eff}}/\sqrt d \)) in the submicrocrystalline (SMC) materials produced by equalchannel angular pressing are experimentally studied. It is shown that, as compared to parameter σ₀ and K in the Hall–Petch relation for coarse-grained metals, the SMC metals are characterized by higher values of σ0 and lower values of K_eff. The critical grain size (d₁) at which Keff in the σ_y–d^–1/2 relations of SMC materials changes falls in the range 0.2–0.5 μm. The dependences of macroelastic limit σ₀ and coefficient K_eff on the annealing temperature are found to be determined by recrystallization. If abnormal grain growth develops in annealing of SMC metals, anomalous hardening is detected and a nonmonotonic temperature dependence of coefficient K_eff takes place. In the case of conventional recrystallization at a high annealing temperature, SMC metals exhibit a smooth decrease in σ₀ and an increase in K_eff to the values of K characteristic of coarsegrained materials.     

The effect of the aging period on the martensitic transformation and kinetic characteristic of at % Cu<Subscript>68.09</Subscript>Al<Subscript>26.1</Subscript>Ni<Subscript>1.54</Subscript>Мn<Subscript>4.27</Subscript> shape memory alloy

In this work, the effect of aging period on the characteristic transformation temperatures, thermodynamic parameters and structural variations of CuAlNiMn shape memory alloys were investigated. Aging was performed at above the austenite finish temperature of the un-aged specimen (120°C) for six different retention times, namely 1h, 2h, 3h, 4h, 5h and 6h. The changes in the transformation temperatures were examined by differential scanning calorimetry at different heating/cooling rates. The aging period was found to have an effect on the characteristic austenite and martensite transformation temperatures and thermodynamic parameters such as the enthalpy and entropy of alloys. High-temperature order-disorder phase transitions were determined using a differential thermal analysis, which showed that all the un-aged and aged specimens had an A2 → B2, B2 → L2₁ and an L2₁ → 9R, 18R transition. The structural analysis of the un-aged and aged specimens was performed through X-ray diffraction measurements at room temperature. The intensities of the diffraction peaks varied according to the aging time.     

Thermodynamic laws of the oxygen solubility in liquid metals (Ni,Co, Fe,Mn, Cr) and the formation of oxygen-containing solutions in the alloys based on them

The solubility of oxygen in liquid Ni, Co, Fe, Mn, and Cr metals (Group IV in the periodic table) has been found for the first time. Linear dependences of the oxygen solubility on the standard Gibbs energy for the oxidation reaction of a liquid metal with gaseous oxygen are found. The revealed function of oxygen solubility is of scientific importance and allows one to develop a theory of oxygen solutions in liquid metals and liquid multicomponent metallic compositions and to calculate the energies of mixing of liquid metals with oxygen from ΔG _MO ^° for metal oxidation reactions with allowance for pseudoregular-solution model equations. Using the energies of mixing and metal compositions, we calculated the equilibrium oxygen concentrations in a metal molten pool at the end of oxidation stage of melting 08Kh18N10T steel in an arc furnace. This fact indicates practical importance of the found function of the oxygen solubility in metals.