Formation Of Fcc Research Articles

Applied stress anisotropy effect on melting of tungsten: molecular dynamics study

Atomic-level insight into pre- and post- melting phenomena under complex stresses spanning compression, tension, and shearing in refractory metals is critical. The exploration of high temperature and melting behavior in tungsten under magnitude- and orientation-dependent uniaxial and biaxial compression and tension, approaching pure shear stresses was thoroughly investigated by molecular dynamics using extended Finnis-Sinclair potential. Using equilibrium solid-liquid coexistence simulations, we demonstrated the enhancement (reduction) of the melting point temperature Tm taking place with the increase of applied compression (tension) stress magnitudes, reaching a minimum under pure shear stress. Explored heating and melting behaviors under various stress types, magnitudes, and orientations were well supported by the corresponding trends in radial distribution function g(r) and Lindemann index δ. The highest resistance to melting was found under uniaxial compression likely due to bonds stiffening. On the other hand, the highest compliance to melting was revealed under pure shear stress, triggered by the accelerated vibrational instability and destruction of the bcc crystalline order driven by simultaneous shear deformation and formation of fcc and hcp phases.

Learning phase selection and assemblages in High-Entropy Alloys through a stochastic ensemble-averaging model

Efficient alloy design requires a knowledge of phase selection, phase-fractions and microstructure of the material. Prediction of the phase equilibria, though well-established for traditional alloys, is still an open challenge for novel materials and concentrated multicomponent systems such as the High-Entropy Alloys (HEAs). In this paper, we present a novel data-driven approach for learning the phase-equilibria in HEAs through the use of a stochastic ensemble averaging method. The proposed model employs a model-averaging technique on an ensemble of 150 artificial neural networks that have been trained on a 323 alloy dataset. A seven-label classification is presented using a three-element vector description of phases. This allows us to fit the classification boundaries to a relatively larger number of phase combination labels while using only three target parameters for training thereby improving accuracy. The phase prediction capabilities (i.e., formation of FCC, BCC, intermetallics or different combinations of these phases) were tested on 320 alloys outside of the training dataset. Additionally, quantitative estimate of the phase equilibria, i.e., relative phase-fractions, were estimated and compared with experimental measurements and CALPHAD predictions in three different high-entropy systems, viz., Fex-Niy-(AlCoCr0.5)1-x-y, Alx-Tiy-(CrFeNi)1-x-y and Crx-Moy-(VNbTi)1-x-y. The model’s capability of going a step beyond current state-of-the-art model allows greater insights into target composition spaces for the alloy designer and establishes the first such approach for HEAs.

Influence of Al content on high temperature oxidation behavior of AlxCoCrFeNiTi0.5 high entropy alloys

Studies on the oxidation behavior of high entropy alloys (HEAs) from the AlxCoCrFeNiTi0.5 (x = 0.5, 1.0, 1.5) metallic systems are conducted. In this work, the alloys are synthesized using arc-melting method, and the microstructure, anti-oxidant properties and oxidation products of the alloys at 700 °C, 900 °C and 1100 °C up to 100 h have been studied. The main substrate phases constitution in the arc-melted AlxCoCrFeNiTi0.5 HEAs was inter-dendritic FeCr-rich BCC1 and dendritic FeCr-poor BCC2 phases. With the increase of Al content, the fraction of the BCC2 phase increases while the BCC1 phase decreases, meanwhile the formation of FCC and σ inter-metallic phases are suppressed. At 700 °C −1100 °C, all alloys exhibit initial transient oxidation followed by various degrees of parabolic oxide growth; the Al1.5CoCrFeNiTi0.5 alloy exhibits excellent oxidation resistance because a continuous compact layer of (Al0.9Cr0.1)2O3 oxide films are produced on the alloy surface. The Kp values for the Al1.5CoCrFeNiTi0.5 alloy oxidized at 1100 °C for 100 h can be up to about 1.03×10−12 g2cm−4s−1.

Precipitation, Recrystallization, and Evolution of Annealing Twins in a Cu-Cr-Zr Alloy

In this paper, the precipitation, recrystallization, and evolution of twins in Cu-Cr-Zr alloy strips were investigated. Tensile specimens were aged at three different temperatures for various times so as to bring the strips into every possible aging condition. The results show that the appropriate aging parameter for the 70% reduced cold-rolled alloy strips is 723 K for 240 min, with a tensile strength of 536 MPa and an electrical conductivity of 85.3% International Annealed Copper Standards (IACS) at the peak aged condition. The formation of fcc (face-centered cubic) ordered Cr-rich precipitates (β′) is an important factor influencing the significant improvement of properties near the peak aged condition. In terms of crystallographic orientation relationships, there are basically two types of β′ precipitates in the alloy. Beyond the Cr-rich precipitates (β′(I)) formed during the early aging stages, which mimic a cube-on-cube orientation relationship (OR) with the matrix, another Cr-rich precipitate (β′(II)) is observed in the peak aged condition. β′(II) is coherent with the matrix, with the following ORs: [111]β′(II)//[100]Cu, {02-2}β′(II)//{02-2}Cu and [011]β′(II)//[211]Cu, {200}β′(II)//{-111}Cu. These precipitates have a strong dislocation and grain boundary pinning effect, which hinder the dislocation movement and crystal boundary migration, and eventually delay recrystallization and enhance the recrystallization resistance of the peak aged strips. During the subsequent annealing process, the transition phase β′ gradually loses the coherence mismatch and grows into a larger equilibrium phase of chromium with a bcc (body-centered cubic) structure (β), resulting in the reduction of the pinning effect to dislocations and sub-grains, so that recrystallization occurs. Annealing twins are formed during the recrystallization process to release the deformation energy and to reduce the drive force for interface migration, eventually hindering grain growth.

Effect of Ta Underlayer on Thickness-Dependent Magnetic Properties of Ni–Fe Films

We report the effect of Ta underlayer on the structural and the magnetic properties of Permalloy (Ni81Fe19) thin films over a wide range of film thicknesses ( t Ni–Fe = 10–200 nm). For this purpose, Ta (5 nm)/Ni–Fe ( t Ni–Fe nm)/Ta (2 nm) films were grown on thermally oxidized Si substrates under the presence and the absence of biasing magnetic field (1000 Oe). The Ni–Fe films grown under the presence and the absence of biasing magnetic fields are referred to Ta/Ni–Fe(M) and Ta/Ni–Fe(NM), respectively. X-ray diffraction studies revealed that the Ta underlayer plays an important role in the formation of fcc-(111) Ni–Fe texture in both the Ta/Ni–Fe(M) and Ta/Ni–Fe(NM) films. In addition, the values of average crystallite size depend on the film thickness, biasing field, and Ta underlayer. Thickness-dependent magnetic hysteresis ( M–H ) loops demonstrate a strong in-plane uniaxial anisotropy in both the Ta/Ni–Fe(NM) and Ta/Ni–Fe(M) films. The uniaxial anisotropy constant ( Ku ) depends not only on the film thickness, but also strongly on the biasing magnetic field. Coercivity decreases with increasing film thickness for both the Ta/Ni–Fe(NM) and Ta/Ni–Fe(M) films, and approaches to a minimum of $\sim$ 0.4 Oe even at t Ni–Fe = 200 nm. Furthermore, the observed magnetic domain patterns of Ni–Fe films show their dependence on the Ta underlayer, biasing field, and Ni–Fe thickness. The effects of the Ta underlayer, the Ni–Fe film thickness, and the biasing-field driven uniaxial anisotropy on the structural and the magnetic properties are discussed on the basis of reduction in strain at the Ta/Ni–Fe interface.

Formation of cubic structured (Al1 − xCrx)2 + δO3 and its dynamic transition to corundum phase during cathodic arc evaporation

Mixed aluminum–chromium oxide coatings in the form of (Al,Cr)2O3 solid solutions have attracted extensive research interest during the past years due to their successful use for challenging wear applications and the comparative ease of their moderate-temperature deposition by physical vapor deposition techniques. During our research into the reactive cathodic arc deposition of this type of coating, we found a previously unobserved transition between two crystalline aluminum–chromium oxide structures. During the early growth stage, films arc-deposited from Al0.55Cr0.45 targets form a first zone, that was found to contain exclusively the metastable cubic fcc-(Al1−xCrx)2+δO3 phase. This kinetically favored phase is reproducibly followed by the growth of a second zone made of the initially expected corundum phase, α-(Al1−xCrx)2+δO3, as observed by TEM. This dynamic transition has a significant effect on the film properties. XPS studies and structural data show that the formation of fcc-(Al1−xCrx)2+δO3 with a (200) preferred orientation arises from the initial presence of a metastable monoxide (M1−xO) film, which is stabilized by the incorporation of metal vacancies (31%) in the B1 structure. However, as the thickness of coating increases, the thermodynamic aspect becomes more important as compared to kinetics and leads to a loss of structural stability in the cubic layer, which is a kinetically favored phase. As a result, the system will transform into the metastable corundum α-(Al1−xCrx)2+δO3, which is thermodynamically more stable than the cubic phase. In this paper, formation of fcc-(Al1−xCrx)2+δO3 and its transformation to corundum phase are discussed in detail with respect to the structural and electronic properties of the different phases.

Microstructural and magnetic characterization of iron precipitation in Ni–Fe–Al alloys

The influence of annealing on the microstructural evolution and magnetic properties of Ni{sub 50}Fe{sub x}Al{sub 50-x} alloys for x = 20, 25, and 30 has been investigated. Solidification microstructures of as-cast alloys reveal coarse grains of a single B2 type {beta}-phase and typical off eutectic microstructure consisting of proeutectic B2 type {beta} dendrites and interdendritic eutectic for x = 20 and x > 20 at.% Fe respectively. However, annealing at 1073 K results in the formation of FCC {gamma}-phase particles along the grain boundaries as well as grain interior in x = 20 at.% Fe alloy. The volume fraction of interdentritic eutectic regions tend to decrease and their morphologies start to degenerate by forming FCC {gamma}-phase for x > 20 at.% Fe alloys with increasing annealing temperatures. Increasing Fe content of alloys induce an enhancement in magnetization and a rise in the Curie transition temperature (T{sub C}). Temperature scan magnetic measurements and transmission electron microscopy reveal that a transient rise in the magnetization at temperatures well above the T{sub C} of the alloys would be attributed to the precipitation of a nano-scale ferromagnetic BCC {alpha}-Fe phase. Retained magnetization above the Curie transition temperature of alloy matrix, together with enhanced roommore » temperature saturation magnetization of alloys annealed at favorable temperatures support the presence of ferromagnetic precipitates. These nano-scale precipitates are shown to induce significant precipitation hardening of the {beta}-phase in conjunction with enhanced room temperature saturation magnetization in particular when an annealing temperature of 673 K is used. - Research Highlights: {yields} Evolution of microstructure and magnetic properties with varying Fe content. {yields} Transient rise in magnetization via the formation of ferromagnetic phase. {yields} Enhancements in saturation magnetization owing to precipitated ferromagnetic phase. {yields} Nanoscale precipitation of ferromagnetic BCC {alpha}-Fe confirmed by TEM. {yields} Hardness of the {beta}-phase correlates with room temperature saturation magnetization.« less

The research of the structure of amorphous metals by molecular dynamics method

The paper is concerned with the research of the dynamics of amorphous Ni structure by molecular-dynamics method. Ni was obtained from the melt state by a superrapid cooling. It was shown that the structure of amorphous metals could have at least two variants of the structure, depending on the velocity of cooling. The structure contained conjugated and variously deformed tetrahedrons at the cooling using velocities of 1014-1015 K/s. Franck-Casper phase consisting of conjugated and variously deformed icosahedrons formed at the velocity of nearly 1013 K/s. The formation of FCC and HCP nanocrystals competing with Franck-Casper phase took place at the above mentioned velocity of cooling.

Phase behavior and microstructural length scales of a diblock copolymer in the presence of a selective solvent

We employ self-consistent mean-field (SCMF) theory in studying the phase behavior as well as the microstructural domain sizes for a diblock copolymer in the presence of a selective solvent. First we examine the effects of solvent addition on the formation of fcc and bcc packed spheres. As has been found in experiments, the so-called “normal” spheres, i.e., formed by the minority blocks, tend to pack into the bcc array, while the “inverted” spheres formed by the majority blocks favor the fcc packing. Upon increasing the solvent selectivity and/or solvent amount, the formed inverted spheres tend to pack from bcc to fcc. This thermotropic transition of bcc → fcc upon increasing the solvent selectivity is induced by the fact that the intermicellar interactions vary from long-range to short-range via a combination of the solvent exclusion from the cores and an increase in the aggregation number. In analyzing the effects of solvent addition on the microstructural sizes, the SCMF results have successfully captured the crossover behavior of characteristic domain spacing from decreasing with added solvent to increasing by increasing the solvent selectivity. Further, the variation of the characteristic domain spacing when the systems transform to a more curved structure changes from a discontinuous decreasing behavior to even a discontinuous increasing behavior upon increasing the solvent selectivity and/or the formation of inverted structures.

Phase formation during the devitrification of Al-rich melt-spun Al?8.5Ni?5.0Y?3.0(Co,Fe) alloys

Abstract DSC data from melt-spun ribbons of two Al–Ni–Y–Co–Fe alloys have revealed three distinct stages in the devitrification. TEM studies show that these stages correspond to the formation of fcc α-Al, Al9(Ni,Co,Fe)2, and a mixture of Al19(Ni,Co,Fe)5Y3 + Al23(Ni,Co,Fe)6Y4, respectively.

Structure, composition and microhardness of (Ti,Zr)N and (Ti,Al)N coatings prepared by DC magnetron sputtering

The composition, structure and microhardness of (Ti,Zr)N and (Ti,Al)N thin films deposited by DC reactive magnetron sputtering technique have been studied. The films were synthesized with a single magnetron source by co-sputtering route using constituent elemental targets under different conditions of nitrogen gas flow and substrate bias. The X-ray diffraction (XRD) analysis indicated the formation of fcc (Ti,Zr)N and (Ti,Al)N phases. The atomic composition of films, determined by backscattering techniques, varied with deposition conditions. The microhardness of the films estimated by J-H model is closely related to the atomic composition of the films, particularly to their nitrogen content.

Oxidation-induced degradation of strength of tungsten carbide-cobalt

Mixed aluminum–chromium oxide coatings in the form of (Al,Cr)2O3 solid solutions have attracted extensive research interest during the past years due to their successful use for challenging wear applications and the comparative ease of their moderate-temperature deposition by physical vapor deposition techniques. During our research into the reactive cathodic arc deposition of this type of coating, we found a previously unobserved transition between two crystalline aluminum–chromium oxide structures. During the early growth stage, films arc-deposited from Al0.55Cr0.45 targets form a first zone, that was found to contain exclusively the metastable cubic fcc-(Al1 − xCrx)2 + δO3 phase. This kinetically favored phase is reproducibly followed by the growth of a second zone made of the initially expected corundum phase, α-(Al1 − xCrx)2 + δO3, as observed by TEM. This dynamic transition has a significant effect on the film properties. XPS studies and structural data show that the formation of fcc-(Al1 − xCrx)2 + δO3 with a (200) preferred orientation arises from the initial presence of a metastable monoxide (M1 − xO) film, which is stabilized by the incorporation of metal vacancies (31%) in the B1 structure. However, as the thickness of coating increases, the thermodynamic aspect becomes more important as compared to kinetics and leads to a loss of structural stability in the cubic layer, which is a kinetically favored phase. As a result, the system will transform into the metastable corundum α-(Al1 − xCrx)2 + δO3, which is thermodynamically more stable than the cubic phase. In this paper, formation of fcc-(Al1 − xCrx)2 + δO3 and its transformation to corundum phase are discussed in detail with respect to the structural and electronic properties of the different phases.

Structure determination of metastable cobalt films deposited on GaAs

The local structure of epitaxially grown thin films of cobalt deposited on GaAs(110) have been unequivocally determined to be the body‐centered phase, a metastable phase, by the use of conversion electron x‐ray absorption fine structure (conversion electron EXAFS). A detailed comparison of the data for a 350 Å Co film to similar measurements for polycrystalline bcc Fe, hcp Co, and fcc Cu concludes that the Co/GaAs thin films possess the metastable bcc structure with an extracted lattice constant of 2.82±0.01 Å. An observed reduction in the amplitude of the fine structure for the Co films is only partly due to distortion within these films, which is determined to be equivalent to an increase in a Debye–Waller factor of 0.0011 Å2. Replacing the GaAs surface with an epitaxially grown ZnSe surface, which maintains nearly the same lattice spacing, results in the formation of fcc and/or hcp Co. This first application of conversion electron EXAFS to single crystal epitaxial films shows it to be a most powerful procedure for establishing their crystal structure.