Two-phase Alloys Research Articles

Thin film combinatorial sputtering of Al-Ce alloys: Investigating the phase separation of as-deposited solid solutions and determining the coefficient of thermal expansion

AlxCe100−x thin films with a composition range of ~75.0 < x < 99.5 at% (36.5 < x < 97.5 wt%) were synthesized via combinatorial co-sputtering from an Al and an Al50Ce50 target. The crystal structure, phase fraction, film morphology, electrical resistivity, and temperature-dependent coefficients of thermal expansion (CTE) are all correlated to the AlxCe100−x composition. The as-deposited films form a metastable solid-solution, and annealing leads to the formation of the thermodynamically stable two-phase system of Al and the α-Al11Ce3 intermetallic. Temperature dependent x-ray diffraction (XRD) reveals that the two phases expand independently of one another, and the thin film Al temperature-dependent CTE is similar to bulk Al. The thin film Al11Ce3 intermetallic phase has a nearly constant CTE of ~1.5 × 10−5/°C within the temperature range studied (25–550 °C). To confirm the thin film Al11Ce3 results, bulk stoichiometric Al11Ce3 and +/- 1 wt% Ce samples were prepared and the CTE of each was measured with the same conditions. A Rietveld analysis of the bulk data enabled an estimation of the CTE in each of the 3 orthorhombic lattice parameters, which displayed anisotropic behavior. The thin film and bulk CTE measurements were in very good agreement. Estimations of the temperature dependent CTE of the two-phase alloys are made via the Reuss and Voigt models. By demonstrating the efficacy of the approach, more complex multi-component rapid materials discovery of low CTE Al-alloys can be pursued via the combinatorial thin film synthesis and XRD measurement.

Potential kink band formation on [formula omitted] two-phase Ti-10Cr alloy under compressive condition

The plastic deformation of an α/β two-phase Ti-10Cr alloy with fine lamella microstructure was investigated under uniaxial compression. The material was composed of coarse equiaxed β grains with {332}〈113〉 twins introduced by cold rolling and α precipitates after aging. The crystallographic relationship between the matrix, the twin, and the precipitates was analyzed. A combination of optical microscopy, scanning electron microscopy, and electron backscatter diffraction with energy-dispersive X-ray spectroscopy analyses was carried out to investigate the microstructure and plastic deformation behavior of the Ti-10Cr alloy. During uniaxial compression tests, {110}β - rotation-type kinks were located in the Ti-10Cr alloy. The crystallographic and kinematical relationships among the formed deformation bands were clarified with the rank-1 connection theory.

An efficient and quantitative phase-field model for elastically heterogeneous two-phase solids based on a partial rank-one homogenization scheme

This paper presents an efficient and quantitative phase-field model for elastically heterogeneous alloys that ensures the two mechanical compatibilities—static and kinematic, in conjunction with chemical equilibrium within the interfacial region. Our model contrasts with existing phase-field models that either violate static compatibility or interfacial chemical equilibrium or are computationally costly. For computational efficiency, the partial rank-one homogenization (PRH) scheme is employed to enforce both static and kinematic compatibilities at the interface. Moreover, interfacial chemical equilibrium is ensured by replacing the composition field with diffusion potential field as the independent variable of the model. Its performance is demonstrated by simulating four single-particle and one multi-particle cases for two binary two-phase alloys: Ni–Al γ′/γ and UO2/void. Its accuracy is then investigated against analytical solutions. For the single-particle γ′/γ alloy, we find that the accuracy of the phase-field results remain unaffected for both planar and non-planar geometries, when the PRH scheme is employed. Fortuitously, in the UO2/void simulations, despite a strong elastic heterogeneity – the ratio of Young’s modulus of the void phase to that of the UO2 phase is 10−4 – we find that the PRH scheme shows significantly better convergence compared to the Voigt–Taylor scheme (VTS) for both planar and non-planar geometries. Nevertheless, for the same interface width range as in the γ′/γ case, the interface migration in these simulations shows dependence on interface width. Contrary to the γ′/γ simulations, we also find that the simulated elastic fields show deviations from the analytical solution in the non-planar UO2/void case using the PRH scheme.

Development and characterization of a two-phase cerium yttrium alloy: Effect of heat treatment and composition on microstructure in Ce–Y alloys

A two-phase cerium yttrium alloy was developed to understand the effect of varying phase fraction on quasi-static and dynamic properties in comparison to unalloyed cerium. The goal of the current work is to discuss the processing pathway as related to the resulting microstructure and phase fraction and eventually to material properties. Test samples are characterized using a combination of metallography, X-ray diffraction, and inductively coupled plasma atomic emission spectroscopy (ICP AES) to identify the influence of the two process variables, heat treatment and starting composition, on microstructure and final phase composition. Through the study, the team developed a process to reproducibly fabricate a two-phase cerium-yttrium alloy with a composition of 5.75 and 6.5 wt percent yttrium heat treated to 450 °C to be mechanically and dynamically tested in future work.

A Microstructure Based Elasto-Plastic Polygonal FEM and CDM Approach to Evaluate LCF Life in Titanium Alloys

This work presents the influence of microstructure on the LCF life of two-phase titanium alloys. An elasto-plastic polygonal FEM and continuum damage mechanics approach has been used in this work. Damage coupled Chaboche-Lemaitre combined isotropic-kinematic hardening model is implemented to accurately capture the effects of deformation inhomogeneity and material plasticity. A new strain-based damage evolution law is proposed to predict the effect of mean grain size on the LCF life. The effect of different microstructural parameters i.e., topological variation, elastic anisotropy, mean grain size and shape, volume fraction of phases and initial voids is statistically analyzed on the LCF life.

Effects of laser shock processing on θ-Al2O3 to α-Al2O3 transformation and oxide scale morphology evolution in (γ’+β) two-phase Ni-34Al-0.1Dy alloys

• The oxidation resistance of (γ’+β) two-phase NiAlDy alloy is optimized by LSP. • LSP reduces scale growth rate due to its impact on initial oxidation behavior. • LSP promotes θ-α transition by increasing dislocation density and surface roughness. • LSP leads to distinction in oxide scale morphology on both the γ’ and β phase. • LSP facilitates the formation of large Al 2 O 3 particles (2–3 μm) on the β phase. (γ’+β) two-phase Ni-Al is a promising high-temperature protective coating material used on Ni-base superalloys since it has good interfacial compatibility with superalloys due to the low Al content compared to single-phase β-NiAl. In this paper, we aim to improve the oxidation resistance, whereby Ni-34Al-0.1Dy, a (γ’+β) two-phase Ni-Al alloy, was treated by laser shock processing (LSP) and the oxidation behavior at 1150 °C was investigated. The results showed that after oxidation, Al 2 O 3 scale formed on the original β phase of the untreated alloy with a small grain size (200–800 nm), while for the LSP-treated samples, the scale grown on the original β phase was dominantly composed of larger Al 2 O 3 grains with a size of 2–3 μm. The distinction was attributed to the promotion of θ-Al 2 O 3 to α-Al 2 O 3 transformation induced by the LSP, because the dislocation density, as well as surface roughness, were increased during LSP treatment which can provide heterogeneous nucleation sites for α-Al 2 O 3 . In addition, the larger-size Al 2 O 3 particles, derived from the direct conversion of needle-like θ-Al 2 O 3 in the initial oxidation stage, could rapidly overspread the whole β phase surface thus reducing the scale growth rate.

Deformation Mechanism and Structural Changes in the Globular Ti-6Al-4V Alloy under Quasi-Static and Dynamic Compression: To the Question of the Controlling Phase in the Deformation of α+β Titanium Alloys

The deformation mechanism of the Ti-6Al-4V (wt.%) alloy with globular structure was studied under conditions of quasi-static and high-strain compression with rates 10−3 s−1 and 2.1–3.3 × 103 s−1, respectively. High-strain compression was conducted using a Split Hopkinson Pressure Bar (SHPB). The details of the deformation mechanism were evaluated based on the analysis of the deformation hardening curves using the strain hardening exponent concept developed for titanium alloys in tension conditions. The used approach allowed us to identify the stages of plastic deformation observed and the controlling phase in deformation of two-phase alloy through the assessment of the strengthening index, n. It has been found that three deformation stages can be identified in quasi-static conditions. However, when the alloy is compressed at a high strain rate, the third deformation stage does not develop due to the high process rate. Further analysis of deformation curves reveals the leading role of the β-phase under the quasi-static conditions and the essential contribution of the second, α-phase, at a high compression rate. The findings on the deformation mechanism based on the analysis of hardening curves were supported by a detailed structural study.

Evolution of microstructure and crystallographic texture during cold rolling of liquid phase sintered tungsten heavy alloy

In this study, the evolution of microstructure and crystallographic texture of two-phase tungsten heavy alloy (almost pure tungsten embedded in nickel‑tungsten‑iron matrix) during cold rolling was investigated systematically. Electron back scatter diffraction studies reflected that the matrix phase accommodates more strain in the early stages of deformation and that as deformation progresses, the matrix phase exhausts its work hardening ability, resulting in the onset of micro-shear band formation. The tungsten phase on the other hand showed the presence of large orientation gradients accompanied with significant flattening and elongation of grains. At higher rolling reductions (effective true strain of −2.66), extensive shear bands spanning across several tungsten particles and the matrix were observed in the microstructure, which in turn had implications on the evolution of the crystallographic texture. It was observed that weak αb. c. c. (<110>∥RD) and γ fiber (<111>∥ND) emerge in tungsten at low to intermediate rolling reductions, and that αb. c. c. fiber strengthens much more than γ fiber at higher rolling reductions with prominent intensity at rotated cube position. For the matrix phase, a weak beta fiber (fiber connecting the brass {011}<211>, copper {112}<111>, and S {123}<634> orientations in the Euler space) was observed at low to intermediate rolling reductions, followed by strengthening of Goss and Brass orientations at higher rolling reductions.

α-Mg/LPSO (Long-Period Stacking Ordered) phase interfaces as obstacles against dislocation slip in as-cast Mg-Zn-Y alloys

The influence of α-Mg/LPSO (Long-Period Stacking Ordered) phase interfaces on the flow stress of as-cast Mg-Zn-Y alloys was experimentally and numerically evaluated by compressive loading samples with different volume fractions of LPSO phase. The experimental results showed that the flow stresses of Mg-Zn-Y alloys with LPSO volume fractions ranging from 40 to 85% are clearly higher than those of single phase α-Mg or LPSO alloys. Comparison of deformation behavior between Mg-Zn-Y alloys and Mg-Zn-Y-Zr alloys, where Zr effectively decreases grain size of α-Mg phase, showed that the flow stresses of Mg-Zn-Y alloys with 25 and 40% of LPSO phase were insensitive to grain size whereas the flow stress of α-Mg single-phase alloy showed clear grain size dependence. By including the dependence of critical resolved shear stress (CRSS) on the average interface spacing, crystal plasticity analysis successfully reproduces the observed experimental stress-strain behavior, which suggests that α-Mg/LPSO phase interfaces are effective obstacles against dislocation slip in two-phase Mg alloys containing LPSO phase.

Effect of grain size on damage and failure in two-phase materials: Homogenized CuPb

It is well known that spall failure strongly depends on the microstructure of a material. There have been numerous studies to study the effect of grain size on the overall spall strength and the total amount of damage in single element metals like copper. However, such systematic studies remain rare in two-phase materials and alloys. In this work, two incipient spall experiments were performed on a Cu–1%Pb alloy to understand the effect of grain size on the damage and failure in a two-phase material. Overall, these results showed that even though the spall strength did not change as a function of grain size, there were significant differences in the total amount of damage as a function of grain size. A clear increase in the total damage present in the material was seen as the grain size was increased from 32 to either 70 or 75 μm in either of the experiments. This difference was attributed to variations in the void growth rate as the grain size was increased.

Evolution of the Microstructure during Hot Working of Gamma-Based TiAl Alloys

Abstract Cast products of two-phase titanium aluminides suffer from microsegregation of the alloying elements, which arises from the peritectic solidification pathway. The resulting variation in composition and microstructure occurs on a length scale in the range of 1 mm and, thus, cannot be tolerated for the anticipated applications of the materials. A significant structural and chemical consolidation can be achieved by homogenization and hot-working procedures, however, the processing experience is limited. This problem is addressed in the present paper in that the evolution of the microstructure of two-phase alloys after different stages of wrought processing is characterized by structural and chemical microanalysis involving scanning electron microscopy and energy dispersive X-ray analysis. The investigations involve different two-phase alloys in order to assess the effects of ternary and higher alloying elements on the finally obtained microstructure.

The influence of Hf content on oxide scale microstructure and HfO2 formation mechanisms in two-phase (γ' + β) Ni-Al alloys

Hafnium is often doped into alumina-forming alloys to enhance their oxidation resistance, though the effect varies with the doping amount. In this paper, two-phase (γ' + β) Ni-30Al alloys with different Hf contents were prepared and the cyclic oxidation behaviour at 1100 °C was investigated. The oxidation rate increased remarkably with the increase of Hf content, because excessive Hf leads to severe internal oxidation. And it is interesting to note that oxide scales that formed on the alloys exhibited very different microstructures. On the Ni-30Al-0.1Hf alloy the scale mainly consisted of single α-Al2O3. But on the Hf over-doped alloys (Hf > 0.1 at.%), the scales microstructure varied with locations. The scale in-situ formed on β phase consisted of an outer spinel layer and an inner α-Al2O3 layer; while, the one formed on γ' phase consisted of an outmost NiO layer, a middle spinel layer dispersed with fine HfO2 particles and an inner α-Al2O3 layer. Moreover, Hf-rich oxide pegs were prone to be formed at scale/alloy interface on original γ' phase. However, when Hf content reached to 2 at.%, the pegs were also found at scale/alloy interface on original β phase. The different oxidation behaviour and peg formation mechanisms of the alloys are discussed.

A two-phase ultrafine-grained NbMoTaWV refractory high entropy alloy with prominent compressive properties

A NbMoTaWV high entropy alloy (HEA) was fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). Characterization of milled powders indicated that only a single body-centered cubic (BCC) solid solution phase was generated in the MA process. After SPS, a tetragonal (Ta,V)O2 oxide phase precipitated from the primary BCC phase. The NbMoTaWV alloy exhibited a compressive yielding strength (σy) of 2785 MPa, compressive strength (σmax) of 3661 MPa and strain to failure (εf) of 16.2% at room temperature. The compressive yielding strength of the sintered alloy was dramatically higher compared with most previously reported HEAs. The compressive strength of NbMoTaWV alloy at 1000 °C arrived at 1978 MPa. The excellent strength at room and elevated temperatures are mostly ascribed to the fine (Ta,V)O2 particles dispersed in the BCC matrix.

Microstructural stability of a two-phase (O + B2) alloy of the Ti-25Al-25Nb system (at.%) during thermal cycling in a hydrogen atmosphere

&lt;abstract&gt; &lt;p&gt;In this work, the stability of the microstructure of the experimentally obtained two-phase (O + B2) alloy of the Ti–25Al–25Nb (at.%) system were studied during thermal cycling in a hydrogen atmosphere. It was found that the two-phase structure (O + B2) of the alloy of the Ti–Al–Nb system shows high thermodynamic stability. In this case, phase transformations of secondary phases (α2, AlNb&lt;sub&gt;2&lt;/sub&gt;) are observed in the microstructure of the alloy, the volumetric content of which at all stages of testing does not exceed 2%. Thus, after the first cycle of high-temperature exposure, single inclusions of the α2 phase precipitate, while in the areas enriched in Ti and Al, due to the redistribution of Nb, a new colony of the α2 phase is observed. After five test cycles, it was found that large accumulations of the α2 colony, due to the α2 → B2 phase transformations, form new micron-sized grains of the B2 phase. A volumetric accumulation of nanosized precipitates of the AlNb&lt;sub&gt;2&lt;/sub&gt; phase was found near the triple joints of the grain boundaries of the B2 phase after 10 cycles of thermal exposure, which is caused by the supersaturation of B2 grains with niobium.&lt;/p&gt; &lt;/abstract&gt;

Ударная вязкость титанового сплава ВТ6 с ультрамелкозернистой структурой, полученной методом равноканального углового прессования

The wide use of two-phase titanium alloys in aircraft engine building, as well as the intense development of this industry, stipulate more and more stringent requirements to structural materials and the enhancement of their reliability, strength and performance characteristics. The formation of an ultrafine-grained (UFG) state in metals and alloys using severe plastic deformation (SPD) processing enables achieving high strength properties. However, an important aspect of UFG materials is their structural and textural effects which may lead to a strong anisotropy of their properties. In this respect, the authors studied the effect of microstructural features on the mechanical properties and impact toughness of the VT6 alloy after equal-channel angular pressing (ECAP) and subsequent deformation by upsetting, imitating die forging. The study showed that the formation of a UFG structure in the VT6 titanium alloy with a grain size of about 0.4 µm allows increasing the ultimate tensile strength up to 1250 MPa. The additional upsetting of the UFG alloy at T=750 °C leads to grain growth up to 0.5–1 µm and a decline in strength to 1090 MPa as a result of the recovery and recrystallization processes. Impact toughness tests were conducted on specimens with a V-shaped stress raiser at room temperature, showing that the impact toughness of the UFG VT6 alloy was 0.41 MJ/m2. The tests revealed the anisotropy of impact toughness in the UFG VT6 alloy after equal-channel angular pressing and additional upsetting due to the metallographic and crystallographic texture formed as the result of deformation treatment. In test direction No. 1, the impact toughness value is the lowest and equals 0.31 MJ/m2.

The Constant-Stress, Constant-Heating-Rate Test: A Novel Method for Characterizing Transient Mechanical Behavior of Metallic Materials

A novel mechanical test for characterizing the transient, on-heating mechanical behavior of metallic structural materials was developed. It comprises the continuous heating of a specimen at a constant rate while loaded under a constant true stress. Key features of the approach consist of quantifying the thermal expansion (“thermal compliance”) and mechanical compliance of the load train, modeling the plastic flow and onset of localized necking of the test specimen, and the simulation of concurrent phase changes for multi-phase alloys. From such experiments and analyses, the instantaneous, true plastic strain and strain rate as a function of temperature can readily be determined and used to establish constitutive parameters (such as the apparent activation energy, Qapp, for plastic flow) and temperatures at which rapid deformation may occur. The test technique was validated for three alloys, Ti–7Al, Ti–6Al–2Sn–4Zr–2Mo–0.1Si (Ti6242S), and superalloy 718. For the single-phase-α titanium alloy, Ti–7Al, semi-log plots of strain rate vs inverse temperature (ln $$\dot{\varepsilon }$$ vs 1/T) were linear, and yielded a value of Qapp comparable to that for self diffusion of hcp titanium. The corresponding plots for Ti6242S with either a fully equiaxed or duplex (equiaxed/colony α) microstructure were also linear, but exhibited higher values of Qapp, a finding rationalized using simulations of the kinetics of dissolution of equiaxed α as a function of temperature and the so-called “mechanical contribution” to the activation energy of two-phase alloy systems. Last, ln $$\dot{\varepsilon }$$ -vs-1/T plots of 718 were either linear (in the solution-treated condition) or approximately bilinear (in the solution-treated-and-aged condition). These observations were interpreted with the aid of simulations of precipitation and dissolution of γ′′ and γ′ during continuous heating and the retarding effect on straining of a threshold (back) stress associated with the precipitates.

Evolution of coordination between α and β phases for two-phase titanium alloy during hot working

The evolution of coordination between α and β phases for a two-phase titanium alloy was investigated. For this purpose, hot compression and heat treatment under different conditions were carried out. The results show that the ability of coordinated deformation between α and β phases can influence uniformity of microstructure evolution. Specifically, α phase maintains the lamellar structure and β phase has a low degree of recrystallization when the ability of coordinated deformation is good. In this case, α and β phases still maintain the BOR (Burgers orientation relationship), and their interface relationship is not destroyed even at large deformation. Both of the deformation extent of α lamellae and recrystallization degree of β phase increase with the decline of ability of coordinated deformation. The α phase only maintains the BOR with β phase on one side, while the uncoordinated rotation with the β phase on the other side occurs within 10°. The α and β phases rotate asynchronously when ability of coordinated deformation is poor. The degree of interface dislocation increases, and α and β phases deviate from the BOR.

Characterization of primary, secondary and tertiary {20[formula omitted]1} successive internal twins in the D019 ordered hexagonal α2-Ti3Al phase

Due to the lack of independent slip systems and the difficulty in mechanical twinning, D019 ordered α2-Ti3Al phase is known to be of substantial mechanical anisotropy, which partly accounts for the low ductility of two-phase (α2+γ) TiAl alloys. However, recent studies have shown that in Al-rich α2 phase compression deformation twins can be activated, which could be beneficial for the ductility of the alloys. In the present study, for the first time, we have revealed a novel internal twinning mechanism of α2 phase in a TiAl alloy deformed at high temperature. Three twin generations of a common type {202¯1}<1¯014> were revealed: Secondary twins formed within the primary twin, and tertiary twins formed in the secondary twin. It is important to note that this internal twin structure can only be observed in the <21¯1¯6> direction but not the commonly used <112¯0> direction. The shear plane P of all twin variants is of type {12¯10}. Due to the variation of the twinning elements between the twin variants, the twinning shear is blunted. The interfaces between the twin variants are characterized by high-resolution electron microscopy. Special attention was paid to the structure of the coherent and incoherent twin boundaries. The effect of chemical ordering is discussed. Our results also shed light on the complex twinning mechanisms in hexagonal structures.

Electrochemical Recovery of Rare-Earth Elements Using Liquid Metals in Molten Salts

This work presents electrodeposition of rare-earth elements (Nd and Gd) in molten salts using liquid metal cathodes (Bi and Sn) at 773–973 K with an aim enhance the recovery of rare-earth elements. Recovery of rare-earth elements from molten salts is technically challenging due to high reactivity of rare-earth metals, multivalency of rare-earth cations in electrolytes that can results in a disproportionation reaction (e.g., Nd + 2NdCl3 → 3NdCl2), and dissolution of rare-earth metals in molten salts (e.g., 9 mol% La metal in LaCl3 at 1099 K). Improved rare-earth recovery is discussed based on round-trip coulombic efficiencies using liquid metal electrodes in molten salts and the electromotive force (emf) measurements of rare-earth alloys (e.g., Nd-Bi, Nd-Sn, and Gd-Bi). The rare-earth alloys are also characterized using X-ray diffraction, electron microscopy, and differential scanning calorimetry to determine their phase behavior during electrodeposition processes. The use of two-phase rare-earth alloys, e.g., Nd-Sn alloy (10-90 mol%), as a stable reference electrode is discussed within the context of reliable electrochemical characterization of rare-earth alloys.

Effect of Mn Content on the Passivation and Corrosion of Al0.3Cr0.5Fe2MnxMo0.15Ni1.5Ti0.3 Compositionally Complex Face-Centered Cubic Alloys

Al0.3Cr0.5Fe2MnxMo0.15Ni1.5Ti0.3 (x = 0, 0.25, 0.5, 1) compositionally complex alloys are synthesized and annealed at 1,070°C to form two-phase alloys with a face-centered cubic (fcc) matrix and a second phases enriched in Al, Ti, and Ni with slightly reduced density, raw element costs, and passivating elements distributed across both phases. The global corrosion resistance is evaluated in 0.01 M and 0.1 M NaCl at both natural pH and pH 4. Overall corrosion resistance is suggested to be optimized at Mn concentrations of 5.0 at%, indicated by pitting potentials comparable to or exceeding those of 316L stainless steel. Improvements in corrosion resistance and optimization of Mn concentration are further assessed by polarization, impedance, and gravimetric analysis after extended aqueous exposure. The fate of individual elements during the dissolution and passivation processes is evaluated with in situ atomic emission spectroelectrochemistry and ex situ x-ray photoelectron spectroscopy. Passivity was derived from combinations of Ti4+, Cr3+, and Al3+ oxides in an undetermined solid solution or complex oxide. Enhanced corrosion resistance is attributed to the improved chemical homogeneity of passivating elements within the two-phase microstructure, while the decreased corrosion resistance of alloys with higher Mn concentrations is attributed to high Mn dissolution rates and/or destabilization of the passive films. The underlying determinants of the role of Mn in the design of corrosion-resistant lightweight compositionally complex alloys are discussed.