D03 Structure Research Articles

Phase transition induced anelasticity in Fe–Ga alloys with 25 and 27%Ga

Neutron diffraction and mechanical spectroscopy techniques were applied to study phase transitions in Fe–Ga alloys with 25 and 27 at.% Ga. The following sequences of phase transitions at continuous heating and subsequent cooling in the 20–900 °C temperature range were recorded: D03 → L12 (limited amount) → A2(B2) was recorded at heating and A2(B2) → D03 at cooling for Fe-24.8Ga alloy, and the D03 → L12 → D019 → A2(B2) was recorded at heating and A2(B2) → L12 at cooling for Fe-27.4Ga alloy. Thus, the difference in 2.6 at.%Ga between two studied compositions with D03 structure leads to their different structures after heating to 900 °C. These transition sequences determine different temperature dependencies of elastic and anelastic properties. The D03 → A2(B2) transition (in Fe-25Ga) does not lead to a well-pronounced anelastic effect, in contrast the D03 → L12 transition (in Fe-27Ga) generates internal stresses due to a different rate of an increase in the lattice parameter with temperature and leads to a well-pronounced transient internal friction effect.

Magnetic and structural properties of Mn-Ga thin films

A systematic experimental work has been conducted to understand the magnetic properties of Mn-Ga thin films. Multilayer structured thin films of [MnGa 2 nm/Mn x nm]×25 (x = 0.2∼3.5, which corresponds to Mn at%56∼86) were sputter-deposited onto silica glass substrates, followed by annealing in vacuum. It is found that the magnetic properties strongly depend on x. For x = 0.5, the high magnetization values are found, where the nanocrystalline L10 structure is present. The samples with x = 2.0-3.0 exhibit the coercivity Hc higher than 10 kOe at room temperature where the nanocrystalline D022 structures are found to form. The correlation between the magnetic anisotropy constant K and saturation magnetization Ms is also discussed. The nth power dependence of magnetic anisotropy constant K on Ms is found, where the values of n are 7.8 and 1.9 for x = 0.5 and 2.5, respectively. The present result of the power dependence of n equals about 8 for the L10 MnGa suggests that the magnetic anisotropy in a nanocrystalline L10 MnGa phase is much different from the ordered FePt phase. On the other hand, the power dependence of the D022 nanocrystalline phase suggests the two-ion mechanism.

Structure and properties of the Fe3Al-type intermetallic alloy fabricated by laser engineered net shaping (LENS)

The microstructure and mechanical properties of the Fe3Al–0.35Zr–0.1B alloy fabricated by the Laser Engineered Net Shaping (LENS) technique are presented. Proper technological parameters of the LENS manufacturing process were selected to obtain elements with a minimal porosity and a satisfactory shape congruency. The Fe3Al based alloy samples fabricated by the LENS technique are characterized by a homogenous chemical composition and diversified grain morphology with columnar grains at the bottom and equiaxed grains at the center and edge in the analyzed sample. Phase analysis showed the presence of B2 ordered Fe3Al intermetallic and (Fe, Al)2Zr Laves phases. An isothermal annealing carried out at 450°C for 50h led to a phase transformation from a B2 to an equilibrium D03 phase. The Fe3Al based alloy samples with the B2 structure, directly after the LENS process and with the D03 structure after annealing, exhibited similar yield strength and ultimate tensile strength. However, the presence of the D03 structure caused a noticeable increase of elongation, especially at elevated temperatures.

LONG-PERIOD STRUCTURES IN ALLOYS OF METALLIC SYSTEMS BASED ON Cu – Pd – Me AND Ti – Al – Me

Abstract. Based on the systematization of state diagrams of the systems on the basis of Cu – Pd – Me (where Me – Ag, Au, Ni, Pt, Ga) it has been shown that in the areas of stoichiometric compositions of CuPd and Cu3Pd the alloying with gold (aurum) atoms, as well as palladium atoms can lead to the formation of vast areas of phase homogeneity with long-period structures after the appropriate heat treatment. It has been established that in the systems based on Ti – Al – Me (where Me – V, Cr, Mn, Mo, Nb, Cu) one can observe the decrease of the sizes of homogeneity areas of long-period phases based on Al3(Ti, Mе) with D022 structure in the row of the following alloying elements: V → Mo → Nb → Cr → Mn → Cu.

Magnetic anisotropy of single-crystalline Mn3Sn in triangular and helix-phase states

The magnetic structure and phase transitions in single-crystalline Mn3Sn with the D019 structure have been investigated by torque measurements and magnetic measurements. The results show that a triangular antiferromagnet (AFM) and a weak ferromagnet coexist above 270 K. At these temperatures, the torque measurements indicate a six-fold symmetry and another two-fold symmetry of the magnetic anisotropy. The [112¯0] direction is the lowest-energy state in the triangular configuration. At low temperatures, two AFM helix-phase states are found in an applied magnetic field (H): In one of them, the [112¯0] direction is the easiest deviation direction from the AFM state. In the other, it is the [0001] direction. The variation of the two states with temperature (T) and magnetic field is presented in an H-T phase diagram.

Long-period structure in Cu–Pd–Me and Ti–Al–Me metallic alloys

Analysis of the phase diagrams of Cu–Pd–Me systems (Me = Ag, Au, Ni, Pt, Ga) shows that introducing gold and palladium atoms in alloys of stoichiometric composition CuPd and Cu3Pd may give rise to extensive regions of homogeneity of phases with long-period structures after the appropriate heat treatment. In Ti–Al–Me systems (Me = V, Cr, Mn, Mo, Nb, Cu), the regions of homogeneity of Al3(Ti, Me) phases with long-period structures with D022 structure become smaller in the following sequence: V → Mo → Nb → Cr → Mn → Cu.

Shape memory behavior in Fe3Al-modeling and experiments

The Fe3Al alloy with D03 structure exhibits large recoverable strains due to reversible slips. Tension and compression experiments were conducted on single crystals of Fe3Al, and the onset of slip in forward and reverse directions were obtained utilizing high-resolution digital image correlation technique. The back stress provides the driving force for reversal of deformation upon unloading, resulting in a superelastic phenomenon as in shape memory alloys. Using density functional theory simulations, we obtain the energy barriers (GSFE – generalized stacking fault energy) for {1 1 0}〈1 1 1〉 and {1 1 2}〈1 1 1〉 slips in D03 Fe3Al and the elastic moduli tensor, and undertake anisotropic continuum calculations to obtain the back stress and the frictional stress responsible for reversible slip. We compare the theoretically obtained slip stress magnitudes (friction and back stress) with the experimental measurements disclosing excellent agreement.

Tunnel magnetoresistance in full-epitaxial magnetic tunnel junctions with a top electrode consisting of a perpendicularly magnetized D022-Mn3Ge film

We grew a magnetic tunnel junction (MTJ) with a top electrode consisting of a Mn3Ge film using a thin Co–Fe alloy film as a seed layer. X-ray diffraction showed that the Mn3Ge had (001)-oriented D022 structure epitaxially grown on an MgO(001) substrate. Magnetic hysteresis loops suggested that the D022-Mn3Ge film possessed perpendicular magnetic anisotropy. A magnetoresistance (MR) ratio of 11.3% was observed in the microfabricated MTJ at room temperature. The resistance–field curve suggested that the top-Co–Fe and D022-Mn3Ge layer are weakly coupled antiferromagnetically. The optimization of top-Co–Fe composition would improve MR ratio.

Tem Observation Of Age-Hardening Precipitation In Mg-Gd-Y Alloys As Different Gd/Y Ratio

Abstract In this study, the early stage of aging in Mg-Gd-Y alloys has been observed by transmission electron microscopy (TEM), high angle annular dark field – scanning transmission electron microscopy (HAADF-STEM) and calculations of images and electron density and bond overlap population (BOP) by first principal to understand the origin of precipitation in this alloy. The small hexagon of 0.37 nm is the first precipitate in this alloy, and this is the evidence of short range ordering of D019 structure. This is referred as the pre β”-phase. In the peak aged condition, β’ phase with bco structure was mainly observed.

Structural and magnetic properties of ferromagnetic Fe1−xSix (0.18 ≤ x ≤ 0.33) films formed by rapid thermal annealing on silicon-on-insulator substrates

We systematically investigate the structural and magnetic properties of ferromagnetic Fe1−xSix (0.18 ≤ x ≤ 0.33) films formed by rapid thermal annealing (RTA) on silicon-on-insulator (SOI) substrates. During RTA of an Fe film deposited on a SOI substrate (consisting of a top Si layer, a buried oxide SiO2 layer, and a Si substrate), an Fe1−xSix film is synthesized by the thermal reaction of the deposited Fe film and the top Si layer, but the reaction is limited by the buried oxide layer in the SOI substrate, thus the Si concentration x in Fe1−xSix can be controlled by both the initial thicknesses of the Fe film and the top Si layer. A variety of characteristics show that single-phase Fe1−xSix (x = 0.18, 0.22, and 0.25) films with D03 + B2 structure are successfully obtained by choosing the optimum annealing temperature and time. Furthermore, the ordering fraction of D03 and B2 structures in these films is found to be more than 87%, indicating that the crystalline quality of these films is comparable to that of bulk Fe1−xSix materials reported so far. On the other hand, it is found that the Fe1−xSix (x = 0.33) film has Fe3Si and FeSi phases as in the case of bulk Fe1−xSix with x = 0.33. The film production technique and the quality of the ferromagnetic Fe1−xSix presented in this study are very attractive and useful for silicon-based spintronic devices which are compatible with the complementary metal-oxide-semiconductor technology.

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.

Forward scattering in hard X-ray photoelectron spectroscopy: Structural investigation of buried Mn–Ga films

X-ray photoelectron diffraction (XPD) in combination with hard X-ray photoelectron spectroscopy (HAXPES) has been used to study the structure of buried layers in thin multilayer films. A detailed layer-by-layer investigation was performed using the element-specific, local-probe character of XPD. In the present work, angular-resolved HAXPES at a photon energy of 7.94 keV photon energy was used to investigate a Cr/Mn62Ga38/Mg/MgO multilayer system. Differences in the angular distributions of electrons emitted from Mn and Ga atoms revealed that the structure of Mn62Ga38 changes from L10 towards D022 for increasing annealing temperatures. A c/a ratio of 1.81 ± 0.06 was determined for the buried Mn62Ga38 layer in a D022 structure from the XPD experiment. The improvement of the structural order of the Mn62Ga38 layer is accompanied by an improvement of the structure of the overlying MgO layer.

Ab-initio study of the stability of the D8m-Nb5Sn2Ga and D8m-Ta5SnGa2 compounds

First principles calculations have been performed in the T–Sn–Ga (T=V, Nb, Ta) systems along the section xT=0.625. The enthalpies of formation of the binary and ternary D8m, D81, and D88 structures have been calculated. In the V–Sn–Ga system, no ternary structure is stable in the section. In the Nb–Sn–Ga system, the ternary compound D8m-Nb5Sn2Ga is stable. In the Ta–Sn–Ga system, a combination of the ab-initio calculations and Gibbs energy calculations using the sublattice model allows the show that the phase D8m-Ta5(Sn,Ga)2Ga with a mixed occupancy of the 8h sites of the structure by Ga and Sn atoms is stable at high temperature due to the configurational entropy. These results are in agreement with the experimental determinations previously published in the literature.

Microstructure evolution and hardness change in ordered Ni3V intermetallic alloy by energetic ion irradiation

Ni3V bulk intermetallic compounds with ordered D022 structure were irradiated with 16MeV Au ions at room temperature. The irradiation induced phase transformation was examined by means of the transmission electron microscope (TEM), the extended X-ray absorption fine structure measurement (EXAFS) and the X-ray diffraction (XRD). We also measured the Vickers hardness for unirradiated and irradiated specimens. The TEM observation shows that by the Au irradiation, the lamellar microstructures and the super lattice spot in diffraction pattern for the unirradiated specimen disappeared. This TEM result as well as the result of XRD and EXAFS measurements means that the intrinsic D022 structure of Ni3V changes into the A1 (fcc) structure which is the lattice structure just below the melting point in the thermal equilibrium phase diagram. The lattice structure change from D022 to A1 (fcc) accompanies a remarkable decrease in Vickers microhardness. The change in crystal structure was discussed in terms of the thermal spike and the sequential atomic displacements induced by the energetic heavy ion irradiation.

First Principles Study of Structural Stability and Magnetic Property of Non-equilibrium Co–Mo Alloys

First principles calculations are carried out to investigate the structural stability of several non-equilibrium intermetallic phases in the cobalt (Co)–Mo system using spin polarized projected augmented-wave potentials. It is revealed that the Co3Mo, CoMo, and CoMo3 alloys are energetically favored to be in D019, B11, and A15 structures, respectively, and that the magnetic moments of Co atoms would decrease rapidly with an increasing percentage of Mo content and would most probably disappear when the content of Mo is no less than 50 at%. Generally, the calculated results in the present work match well with the available experimental observations.

Phase transformation and decomposition mechanisms of the βo(ω) phase in cast high Nb containing TiAl alloy

High Nb containing TiAl (Nb–TiAl) alloys are thought to be used at higher temperatures than the conventional TiAl alloys, typically up to 900°C. However, the βo and ω-related phases are usually induced by Nb-segregation in cast alloys. It is important to reveal the transformation mechanisms of the βo and ω-related phases at elevated temperatures for better design of the alloy and optimizing the heat treatment process. In this study, the interconversion mechanism between the ordered ω and βo phases and the decomposition process of the βo phase in the as-cast Ti–45Al–8.5Nb–0.2W–0.2B–0.02Y (at.%) alloy are studied. The ωo particles grow up by richening Nb and rejecting W to the surrounding βo matrix. The βo and ωo phase are separated after annealing at 900°C, whereas the ωo phase dissolves into the βo matrix after annealing at 950°C. The βo and ωo phases undergo βo→γ and ωo (B82 structure)→D88-ω (D88 structure)→γ transformation respectively during annealing, and the βo→γ transformation can be achieved in several ways. The ωo particles with a high Nb content act as pinning points at the βo/γ boundaries during the βo→γ transformation. The corresponding mechanisms of the transformations mentioned above are discussed.

ChemInform Abstract: New Permanent Magnets; Manganese Compounds

The exponential growth of maximum energy product that prevailed in the 20th century has stalled, leaving a market dominated by two permanent magnet materials, Nd2Fe14B and Ba(Sr)Fe12O19, for which the maximum theoretical energy products differ by an order of magnitude (515 kJ m−3 and 45 kJ m−3, respectively). Rather than seeking to improve on optimized Nd–Fe–B, it is suggested that some research efforts should be devoted to developing appropriately priced alternatives with energy products in the range 100–300 kJ m−3. The prospects for Mn-based hard magnetic materials are discussed, based on known Mn-based compounds with the tetragonal L10 or D022 structure or the hexagonal B81 structure.

New permanent magnets; manganese compounds

The exponential growth of maximum energy product that prevailed in the 20th century has stalled, leaving a market dominated by two permanent magnet materials, Nd2Fe14B and Ba(Sr)Fe12O19, for which the maximum theoretical energy products differ by an order of magnitude (515 kJ m−3 and 45 kJ m−3, respectively). Rather than seeking to improve on optimized Nd–Fe–B, it is suggested that some research efforts should be devoted to developing appropriately priced alternatives with energy products in the range 100–300 kJ m−3. The prospects for Mn-based hard magnetic materials are discussed, based on known Mn-based compounds with the tetragonal L10 or D022 structure or the hexagonal B81 structure.

Precipitation Sequence in the Mg–Gd–Y System Investigated by HRTEM and HAADF-STEM

In this work, the early stage of precipitation have been investigated in a Mg–2.9 at% Gd–0.8 at% Y by high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and high resolution transmission electron microscopy (HRTEM).At the underaged condition, precipitates observed by HRTEM were classified as follows; mono-layer, a part of β″, β′. By HAADF-STEM, zig-zag structure, small hexagonal network, and β′ could be recognized. The small “super hexagons” are the first precipitates in this alloy. This structure, referred to as the pre β″-phase, displays a short range similar to the one present in the D019 structure. This phase is formed during quenching and prevails over the zig-zag structure at the beginning of aging. The zig-zag contrast is one of the variations of this small hexagonal network or heterogeneous nucleation during aging. Finally, we concluded that the proposed precipitation sequence is as follows: SSSS → pre β″-phase having D019-SRO → β″-phase → β′-phase. The comparison of HRTEM, HAADF-STEM and simulated images have helped to clarify the meaning of HRTEM features.

<i>In Situ</i> Observation of Pseudoelasticity in Fe<sub>3</sub>Al Single Crystals with D0<sub>3</sub> Structure

In order to clarify the mechanism of pseudoelasticity in Fe­23.0Al (at%) single crystals with the D03 structure, in situ observation of tensile deformation was performed at room temperature using an optical microscope (OM), an electron back-scatter diffraction (EBSD) device and an X-ray Laue back-reflection (XLBR) machine. In the present study, neither martensite nor twin was observed during the pseudoelasticity. In particular, the whole of the tensile specimen could be indexed with respect to the D03 structure by in situ EBSD measurement. (��), (� 101) and (� 211) slips were confirmed to occur depending on the loading axis by in situ OM observation. When (�) [111] slip was activated, coarse slip bands parallel to (� 101) slip plane were formed during loading. The area fraction of the slip bands increased linearly with increasing plastic strain up to 0.066. During unloading, the slip bands disappeared, resulting in pseudoelasticity. From in situ XLBR tests, the crystal rotation of the loading axis toward [111] slip direction took place during loading. The rotation angle and the plastic strain satisfied the Schmid-Boas relationship at small plastic strains, especially for (��) and (� 211) slips. These results suggest that the reversible motion of 1=4h111i superpartial dislocation dragging an antiphase boundary (APB) was responsible for the pseudoelasticity in Fe­23.0Al single crystals. [doi:10.2320/matertrans.M2013461]