Tetragonal D022 Structure Research Articles

Lightweight Al3Ti-based medium-entropy alloys with well-balanced strength and ductility

The lightweight Al3Ti intermetallic compound shows superior properties at elevated temperatures but its room-temperature brittleness largely limits its usefulness. We propose Al3Ti-based lightweight medium-entropy alloys (MEAs) with selected alloying elements, which change Al3Ti from a tetragonal D022 structure to a ductile cubic L12 one with embedded hard B2 and D8a phases. A combination of improved ductility and strength can be achieved. Four lightweight AlTiCrMnFeCu MEAs (ρ=3.65∼4.27 g/cm3) with tunable balance between hardness and ductility were fabricated. Ti and Fe promoted the formation of hard B2 phase, while Cr and Mn favored the formation of D8a phase which strengthened the alloy at the minor expense of ductility. The compressive yield and peak strengths of the MEAs e.g., Al55Ti20Cr5Mn10Fe5Cu5, can reach as high as 650 MPa and 1366 MPa, respectively, with its fracture strain around 17 %. This study provides useful information for designing Al3Ti-based lightweight MEAs with desirable properties.

Niobium nanoparticle-enabled grain refinement of a crack-free high strength Al-Zn-Mg-Cu alloy manufactured by selective laser melting

Grain refinement is an effective way to avoid hot cracking of non-weldable high strength aluminum alloys during additive manufacturing. In the present work, primary Al3Nb with the tetragonal D022 structure was discovered to be an extremely effective inoculant for Al-Zn-Mg-Cu alloys manufactured by selective laser melting (SLM). The columnar crystals were transformed into fine equiaxed crystals with an average grain size of ~1.9 µm through adding 1.5 wt% Nb nanoparticles, eliminating the defects such as cracks and porosity. The high number density of primary Al3Nb and the coherent interface with high lattice matching between the primary Al3Nb and the Al matrix are the main reasons for grain refinement. The Time-Temperature-Transformation (TTT) curves of the precipitation kinetics curves of primary Al3X (XNb, Zr, Ti, Sc) show that primary Al3Nb has the advantage in grain refinement compared with Al3Sc and Al3Ti. Due to grain refinement and defect elimination, the 1.5 wt% Nb-modified Al-Zn-Mg-Cu alloy after T6 heat treatment exhibits excellent mechanical properties, with a tensile strength of ~ 505 ± 12 MPa, and an elongation after fracture of ~ 12.3 ± 1.3%.

WITHDRAWN: Review on nucleation catalysis of tetragonal D022 Al3Ti and L12 modified (Al1−xMex)3Ti intermetallic compounds for solidification of alpha-Al grains

Abstract Heterogeneous nucleation could be considered as a surface catalyzed nucleation process. It is known that Al3Ti phase in Al-Ti refiner is an effective catalytic substrate for solidification of alpha-Al. However, since the Al3Ti phase has a tetragonal D022 structure with lattice constants of a = 0.3851 nm and c = 0.8608 nm, different planes of Al3Ti phase have different lattice misfit between Al. Moreover, it is reported that the shape of Al3Ti in Al-Ti refiner is platelet and wide plane of the platelet has the relatively large lattice misfit between Al. Therefore, the dominant face of Al3Ti platelets in Al-Ti refiner is not the best plane for heterogeneous nucleation. To overcome these shortcomings, in our previous studies, high-symmetry L12 modified (Al1-xMex)3Ti phases with a lattice constant of a = 0.3927 nm for Me = Cu, a = 0.393 nm for Me = Fe, a = 0.394 nm for Me = Ni were used as catalytic substrate. In this paper, the role of lattice misfit on heterogeneous nucleation of Al will be discussed.

Review, Role of L12 Modified (Al1−xMex)3Ti Intermetallic Compounds on Heterogeneous Nucleation of Alpha Aluminum Grains

Al3Ti intermetallic compound with the tetragonal D022 structure undergoes a phase transformation to the high-symmetry L12 cubic structure by addition of third elements, Me. The lattice constants of some L12 modified (Al1−xMex)3Ti intermetallic compounds are closed to that of alpha aluminum. Therefore, it is expected that the addition of L12 modified (Al1−xMex)3Ti intermetallic compound particles show good grain refining performance of cast aluminum. In this paper, our recent results on novel refines containing heterogeneous nucleation site particles of L12 modified (Al1−xMex)3Ti intermetallic compounds have been reviewed.

Al substitution effects on phase stabilities and magnetic properties of tetragonal Mn3Ge

We have investigated Al substitution effects on phase stabilities and magnetic properties of Mn3Ge with tetragonal D022 structure. The lattice constant a is expanded, and the c is contracted by substituting Al for Ge. Consequently, the unit cell volume slightly increases with the increasing Al content, although Al atom has smaller ionic radius than Ge atom. These composition dependencies of the structural parameters are influenced by the number of the valence electrons rather than the ionic radius. Thermal stability of the D022 structure is enhanced by the Al substitution. The magnetic curves of the Al-substituted system mostly unchanged, suggesting that the structural properties such as the unit cell volume and the atomic distance between the Mn atoms play an important role for the magnetic properties of the alloy.

Mn2CrGa-based Heusler alloys with low net moment and high spin polarization

Mn2CrGa-based Heusler compounds combining small magnetization with high spin polarization, fabricated by Fe and Pt substitution for Cr, are investigated experimentally and discussed in terms of sublattice site occupancies. X-ray diffraction and electron diffraction indicate that the parent alloy and Mn2Cr0.6Fe0.4Ga are normal cubic Heusler alloys with substantial chemical disorder, whereas Mn2Cr0.6Pt0.4Ga crystallizes in the tetragonal D022 structure. Magnetization measurements indicate that the spin structure of the parent alloy is essentially ferrimagnetic, but alloying with Fe or Pt reduces the net magnetization per formula unit from 0.40 µB in the parent alloy to nearly zero in the Fe case. The Curie temperature, the magnetoresistance, and the Andreev-reflection transport-spin polarization are less affected by the substitutions, the last changing from 65% in Mn2CrGa to 60% and 61% in the Fe- and Pt-substituted alloys, respectively. The properties of these alloys are favorable for spintronics applications.

Heteroepitaxial growth of tetragonal Mn2.7−xFexGa1.3 (0 ≤ x ≤ 1.2) Heusler films with perpendicular magnetic anisotropy

This work reports on the structural and magnetic properties of Mn2.7−xFexGa1.3 Heusler films with different Fe content x (0 ≤ x ≤ 1.2). The films were deposited heteroepitaxially on MgO single crystal substrates, by magnetron sputtering. Mn2.7−xFexGa1.3 films with the thickness of 35 nm were crystallized in a tetragonal D022 structure with (001) preferred orientation. Tunable magnetic properties were achieved by changing the Fe content x. Mn2.7−xFexGa1.3 thin films exhibit high uniaxial anisotropy Ku ≥ 1.4 MJ/m3, coercivity from 0.95 to 0.31 T, and saturation magnetization from 290 to 570 kA/m. The film with Mn1.6Fe1.1Ga1.3 composition shows high Ku of 1.47 MJ/m3 and energy product (BH)max of 37 kJ/m3 at room temperature. These findings demonstrate that Mn2.7−xFexGa1.3 films have promising properties for mid-range permanent magnet and spintronic applications.

Isotropic, high coercive field in melt-spun tetragonal Heusler Mn3Ge

Nanostructured Mn3Ge ribbons with a composition ranging from 77 to 74 at.% Mn were prepared using induction melting, melt-spinning, and subsequent heat treatment. The hard magnetic properties of the ribbons originate from the highly anisotropic tetragonal D022 structure of Mn3Ge. Depending on the composition and the amount of ferrimagnetic Mn5Ge2 as a secondary phase, a coercivity of up to μ0HC = 2.62 T was obtained for the Mn3Ge ribbons. Microstructure investigations by transmission electron microscopy confirmed the formation of the secondary phase. All samples show isotropic coercive fields, i.e., independent of the direction of the applied magnetic field in contrast to already known epitaxial thin films. The Curie temperature was found to be higher than 800 K, which is the temperature of the phase transition from the tetragonal D022 structure to the hexagonal D019 structure.

Magnetic properties of ultrathin tetragonal Heusler D22-Mn3Ge perpendicular-magnetized films

We investigated the crystal structure and magnetic properties of Manganese-germanium (Mn3Ge) films having the tetragonal D022 structure, with varied thicknesses (5–130 nm) prepared on chromium (Cr)-buffered single crystal MgO(001) substrates. A crystal lattice elongation in the in-plane direction, induced by the lattice mismatch between the D022-Mn3Ge and the Cr buffer layer, increased with decreasing thickness of the D022-Mn3Ge layer. The films exhibited clear magnetic hysteresis loops with a squareness ratio close to unity, and a step-like magnetization reversal even at a 5-nm thickness under an external field perpendicular to the film's plane. The uniaxial magnetic anisotropy constant of the films showed a reduction to less than 10 Merg/cm3 in the small thickness range (≤20 nm), likely due to the crystal lattice elongation in the in-plane direction.

Magnetic and Structural Properties of Rapidly Quenched Tetragonal Mn$_{3 - {\rm x}}$Ga Nanostructures

Nanostructured Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3-x</sub> Ga ribbons with x = 0, 0.4, 0.9 and 1.1 were prepared using arc-melting, melt-spinning and annealing. As-spun samples crystallized into hexagonal D0 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sub> and cubic L2 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> Heusler crystal structures based on the concentration of Mn in Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3-x</sub> Ga. Upon vacuum-annealing the samples at 450 °C for about 50 hours, both the hexagonal and cubic structures transformed into a tetragonal D0 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sub> structure. High-temperature x-ray diffraction and high-temperature magnetometry showed that the samples with low Mn content (Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.9</sub> Ga and Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2.1</sub> Ga) retain their tetragonal structure up to 850 K but the samples with high Mn concentrations (Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2.6</sub> Ga and Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3.0</sub> Ga) undergo a structural phase transition from tetragonal to hexagonal phases around 800 K. The magnetic properties of Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3-x</sub> Ga ribbons were very sensitive to Mn concentration, where the magnetization and anisotropy energy increased and the coercivity decreased as x increased from 0 to 1.1. Although the Curie temperatures of Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2.6</sub> Ga and Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3.0</sub> Ga samples could not be determined because of the structural phase transition, the Curie temperature decreased with increasing x in Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3-x</sub> Ga. The maximum magnetization of 57 emu/g (300 emu/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) and the coercivity of 6.5 kOe were measured in the Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.9</sub> Ga and Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3.0</sub> Ga ribbons, respectively.

Magnetic and electronic properties of D22-Mn3Ge (001) films

Oriented thin films of Mn3Ge with the tetragonal D022 structure, grown on strontium titanate substrates, exhibit a low magnetization Ms = 73 kA m−1 combined with high uniaxial anisotropy Ku = 0.91 MJ m−3 at 300 K, making them potentially useful for thermally stable sub-10 nm spin torque memory elements. The highly ordered epitaxial Mn3Ge (001) films show 46% diffusive spin polarization at the Fermi level, measured by point contact Andreev reflection. Density functional calculations show that the compounds are ferrimagnetic, with anisotropic spin polarization at the Fermi level.

Formation of ODS L12–(Al,Cr)3Ti by mechanical alloying

Abstract Recently, it has been shown that the addition of a certain amount of ternary alloying elements like Cr transforms the tetragonal D022-structure of Al3Ti to the cubic L12-structure with higher crystal symmetry. However, this attempt of overcoming its brittleness at ambient temperature results in relatively poor creep strength at high temperatures. Therefore, the methods of mechanical alloying and high energy ball milling were used to prepare the ternary (Al,Cr)3Ti L12-intermetallic compound additionally strengthened by small volume fractions of incoherent Y2O3 dispersoids. The development of the microstructure and the phase formation were investigated by X-ray diffraction and transmission electron microscopy (TEM). First results on consolidated bulk samples are presented, indicating the possibility of producing oxide dispersion strengthened (ODS) intermetallics from mechanically milled powders.

Nanoanalysis of precipitation in a modified Inconel 718 alloy

The nickel-base superalloy Inconel 718 (IN718) is strengthened primarily by precipitation of two types of intermetallic phases in the austenite (γ) matrix: metastable γ' (cubic Ll2 structure) and γ" (tetragonal D022 structure) precipitates. Minor changes of the chemistry of the IN718 alloy can significantly affect its mechanical properties at high temperatures. It has been found that the microstructural stability of the precipitates and the high temperature performance of IN718 depend critically on the (Ti+Al)/Nb and Al/Ti ratios. The small sizes of these intermetallic phases and the fact that γ' precipitates are frequently associated with γ" precipitates in modified IN718 make it difficult to determine their composition. In this study, nanometer resolution analytical electron microscopy techniques have been used to characterize the structure, the morphology and the composition of the precipitated phases.The fabrication and the heat treatment of the modified IN718 alloy and its high temperature mechanical properties have been reported elsewhere.

First Principles Study of Phase Stability in The Al-Ti System

Due to its technological importance, the Al-Ti system has received much attention lately. For example, TiAl3 (as well as TiAl) has a low density and very high elevatedtemperature strength. However, this compound forms in the tetragonal D022 structure which has low ductility. Recent studies [1,2] have tried to understand the role of tetragonal distortion and the effect of ternary additions [3] in stablizing the D022 relative to the L12.

Characterization of {001} stacking faults in Al67Ni8Ti25

Abstract Intermetallic alloys with compositions near Al67Ni8Ti25 have the cubic L12 crystal structure, in contrast with Al3 Ti which has the tetragonal D022 structure. Features exhibiting stacking-fault-like contrast were observed on {001} planes in an Al67Ni8Ti25 intermetallic alloy after annealing at 1050°C. Characterization of the features using transmission electron microscopy revealed that they are intrinsic stacking faults on {001} planes, which result from dissociation of a0 {001} dislocations. The displacement vector associated with the faults was determined to be approximately 0.4a0 . Computer-simulated electron micrographs were instrumental in identifying the fault characteristics. The fault characteristics reveal that they are sessile and thus do not directly participate in deformation processes in this intermetallic alloy.