DO22 Phase Research Articles

Ab initio comparison of spin-transport properties in MgO-spaced ferrimagnetic tunnel junctions based on Mn3Ga and Mn3Al

We report on first-principles spin-polarised quantum transport calculations (from NEGF + DFT) in MgO-spaced magnetic tunnel junctions (MTJs) based on two different Mn-based Heusler ferrimagnetic metals, namely Mn3Al and Mn3Ga in their tetragonal DO22 phase. The former is a fully compensated half-metallic ferrimagnet, while the latter is a low-moment high-spin-polarisation ferrimagnet, both with a small lattice mismatch from MgO. In identical symmetric and asymmetric interface reconstructions across a 3-monolayer thick MgO barrier for both ferrimagets, the linear response (low-voltage) spin-transfer torque (STT) and tunneling magneto-resistance (TMR) effects are evaluated. A larger staggered in-plane STT is found in the Mn3Ga case, while the STT in Mn3Al vanishes quickly away from the interface (similarly to STT in ferromagnetic MTJs). The roles are reversed for the TMR, which is practically 100% in the half-metallic Mn3Al-based MTJs (using the conservative definition) as opposed to 60% in the Mn3Ga case. The weak dependence on the exact interface reconstruction would suggest Mn3Ga–Mn3Al solid solutions as a possible route towards optimal trade-off of STT and TMR in the low-bias, low-temperature transport regime.

Revealing the Effect of Phase Composition and Transformation on the Mechanical Properties of a Cu-6Ni-6Sn-0.6Si Alloy.

In the present study, a Cu–6Ni–6Sn–0.6Si alloy is fabricated through frequency induction melting, then subjected to solution treatment, rolling, and annealing. The phase composition, microstructure evolution, and transition mechanism of the Cu–6Ni–6Sn–0.6Si alloy are researched systematically through simulation calculation and experimental characterization. The ultimate as-annealed sample simultaneously performs with high strength and good ductility according to the uniaxial tensile test results at room temperature. There are amounts of precipitates generated, which are identified as belonging to the DO22 and L12 phases through the transmission electron microscope (TEM) analysis. The DO22 and L12 phase precipitates have a significant strengthening effect. Meanwhile, the generation of the common discontinuous precipitation of the γ phase, which is harmful to the mechanical properties of the copper–nickel–tin alloy, is inhibited mightily during the annealing process, possibly due to the existence of the Ni5Si2 primary phase. Therefore, the as-annealed sample of the Cu–6Ni–6Sn–0.6Si alloy possesses high tensile strength and elongation, which are 967 MPa and 12%, respectively.

Formation and magnetic properties of hexagonal DO19 phase in Fe2MnGa: An experimental and theoretical investigation

The formation and magnetic properties of Fe2MnGa hexagonal DO19 phase were investigated in detail. Hexagonal Fe2MnGa can be stabilized by furnace cooling, while the FCC phase can be obtained by quenching from high temperature. Strong ferromagnetic character is identified in Fe2MnGa DO19 phase. Its saturation magnetic moment Ms at 5 K is as large as 4.40 μB/f.u. and Curie temperature TC is 563 K. First-principles calculations indicate that Fe and Mn atoms tend to occupy the 6h sites randomly in the DO19 lattice. The DO19 type Fe2MnGa shows a normal ferromagnetic metal character in its electronic structure. Similar to the experimental results, the calculated total magnetic moment is also quite large and mainly comes from the contributions of Fe and Mn spin moments. Both of them are larger than 1.8 μB and are parallel coupled. These properties are quite different from the antiferromagnetic character in FCC Fe2MnGa.

Phase stability and magnetic properties of hexagonal and cubic Fe2MnGe

Abstract The electronic structure, phase stability and magnetic properties of cubic and hexagonal type Fe2MnGe were investigated and compared with the well-known Heusler alloy Fe2MnSi. Three different structures L21, DO3 and DO19 were considered for them. In both alloys the cubic phase (L21 and DO3) has a lower total energy comparing with the hexagonal DO19 phase at 0 K. But in Fe2MnGe the energy difference between the DO19 and L21 phases is much smaller than Fe2MnSi. The DO3 disorder can lower the stability of the cubic phase further and make the total energies of hexagonal and cubic phases closer, this can help the DO19 phase gain stability. Finally, in Fe2MnGe, the energy difference is 0.12 eV/f.u., much smaller than the 0.41 eV/f.u. in Fe2MnSi. That is the possible reason why Fe2MnGe DO19 phase can be synthesized experimentally and Fe2MnSi cannot. L21 type Fe2MnGe is a half-metal with 100% spin polarization and has a total spin moment of 3μB/f.u. The spin polarization ratio of DO3 phase is also high. But for the DO19 hexagonal phase, a normal ferromagnetic metal character is observed. The calculated total moment for DO19 type Fe2MnGe is larger than 6μB/f.u. This total moment comes mainly from the large and parallel coupled partial spin moments of Fe and Mn.

Generation of high-performance Ni-Cr-Mo-based superalloys via γ to DO22 superlattice ordered phase transformation upon addition of trace alloying elements

The mechanical properties of Ni-Cr-Mo-based alloys with tailored ordered phases were investigated. Introduction of niobium induced the formation of a DO22 superlattice and stabilized this structure by avoiding its precipitation in ternary Ni-Cr-Mo. The tetragonal structure and high thermal stability of DO22 resulted in alloys with higher hardness (13 GPa) and the Young′s modulus (225 GPa). Moreover, the sample containing the DO22 phase showed low creep strains and creep strain rates at holding time.

Fe2MnGe: A hexagonal Heusler analogue

We synthesized bulk polycrystalline samples of Fe2MnGe using arc-melting and vacuum annealing. Our experimental investigations revealed that Fe2MnGe crystallizes in a hexagonal DO19 phase at room temperature, in analogy with its “parent” compound Fe3Ge. We find a large saturation magnetization, even larger than that predicted for the tetragonal phase, of approximately 5μB/f.u. at T = 5 K, with a Curie temperature of Tc∼505 K. The large moment and magnetocrystalline anisotropy, along with the possibility of half-metallicity presents Fe2MnGe as a strong candidate for spin-transfer-torque RAM and other spintronic applications. We do not find evidence for the cubic or tetragonal phases predicted by first-principles theory during these investigations. However, our neutron diffraction investigations were suggestive of a low temperature structural transformation.

Microstructure and Mechanical Properties of a Ag Micro-Alloyed Mg-5Sn Alloy

Effects of 1.5 wt.% Ag addition and solid solution + artificial ageing at 160 °C on the microstructure and mechanical properties of a Mg-5Sn alloy have been studied. The results show that Ag addition has significantly hardened the solution-treated Mg-5Sn alloy. During the ageing process, the hardness increase rate and the strength and ductility of the Mg-5Sn alloy at each state are also improved by Ag addition. The improved strengthening behavior is primarily attributed to the refinement distribution of the Mg2Sn precipitates, the enhanced precipitation process, and the synergistic strengthening effect of Mg2Sn and a metastable plate DO19 phase formed at lower ageing temperature. For each solution-treated alloy, the strength and ductility are higher than the corresponding cast ones. Ageing further enhances the yield strength, and the ductility of the Mg-5Sn-1.5Ag alloy is also increased after ageing. The fracture surfaces of the both peak-aged alloys exhibit the characteristic of a mixture of quasi-cleavage and ductile fracture.

The effect of Ni3(Cr0.2W0.4Ti0.4) particles with DO22 structure on the deformation mode and mechanical properties of the aged Ni-Cr-W-Ti alloy

It has been found that the DO22 Ni3(Cr0.2W0.4Ti0.4) particles play a dominant role in improving the strength of the aged Ni-Cr-W-Ti superalloy. The precipitation of DO22 phase also changed the preferred deformation mode from slip in the as-solutionized alloy into twinning in the aged alloy. Strengthening of the aged superalloy has been determined to be based on shearing mechanism to achieve high cutting stress which is required for partial dislocations to cut through coherent DO22 precipitates and thus produce plastic deformation, resulting in a dramatic increase in yield strength of the aged superalloy.

Phase dynamics, structural, and magnetic properties of a Mn2.6Ga1−xSnx alloy series

Although Mn-Ga alloys are discussed as novel permanent magnetic materials, there are several drawbacks, including the high price of Ga and the yet too small saturation magnetization to name a few. We used several guidelines to motivate the replacement of Ga by Sn to tackle these issues and explored the phase formation, structural and magnetic properties of a Mn2.6Ga1−xSnx alloy series. The structure of Sn-rich samples is dominated by the corresponding hexagonal and orthorhombic phases, while the structure of Ga-rich samples shows additional contributions from the tetragonal DO22 structure. However, formation of the tetragonal phase happens only upon an appropriate annealing procedure. In contrast to the discussion in the literature, our experiments do not hint on the tetragonal DO22 structure being metastable, but rather indicate the DO22 structure being an equilibrium phase. In general, the experimentally found phase fractions impact on the magnetic properties and hard magnetic properties are downgraded very rapidly even if only a small amount of Ga is substituted by Sn. This indicates that the DO22 phase found only in the Ga-rich samples is indeed the one which is most relevant for realization of high magnetic anisotropy.

The effect of Ti on precipitation of fully coherent DO22 superlattice in an Ni-Cr-W-based superalloy

The ordering reaction in the newly developed Ni-Cr-W-Ti alloy aged at 700°C has been investigated using atom probe tomography and transmission electron microscopy. The results reveal that the existence of titanium in the superalloy induces and stabilizes the DO22 superlattices, which cannot precipitate in the ternary Ni-Cr-W superalloy. The stoichiometry of the DO22 phase has been determined to be Ni3(Cr0.2W0.4Ti0.4), and the interfaces between DO22 precipitates and matrix have been revealed to be fully coherent at the atomic scale.

Ordering Transformation and Age Hardening in a Ni-Cr-W Superalloy

The microstructural changes occurring in a Ni-Cr-W superalloy during prolonged exposure to proper temperature have been investigated using transmission electron microscopy. It is demonstrated that nanometer-sized C11b (Pt2Mo-type) and DO22 superlattices can precipitate in the Ni-Cr-W alloy by means of a simple aging treatment at temperatures varying in the range of 773 K to 973 K (500 °C to 700 °C). The mechanism of transformation to long-range order has been revealed to accord with continuous mode based on transmission electron microscopy results and variation trend in Vickers microhardness. No signs of overaging and coarsening of C11b and DO22 phases with further aging have been found, which indicates that both of them have a high-thermal stability. The orientation relationships and interfaces between C11b/DO22 precipitates and Ni-based matrix have been investigated by high-resolution transmission electron microscopy, and the results reveal that the interfaces between C11b/DO22 precipitates and surrounding matrix are coherent at the atomic scale. Because of the high-density nanometer-sized C11b/DO22 precipitates, the microhardness of the alloy is improved remarkably.

Microcalorimetry: A powerful tool for quantitative analysis of aging hardening response of Cu-Ni-Sn alloys

The method for the deconvolution of overlapping DSC peaks here proposed has been used by the first time for the quantitative determination of the enthalpies associated to the phase transitions undergone during the aging of an alloy. They have been determined the enthalpies evolved along the first and the second overlapping DSC traces of Cu-10 wt%Ni-5.5 wt%Sn alloy, which are associated, respectively, to the spinodal decomposition of the alloy and the segregation of a DO22 (CuxNi1-x)3Sn tetragonal phase. The fraction of the DO22 phase (responsible of the aging hardening of this alloy) has been successfully determined from DSC as a function of the annealing treatment, while TEM and XRD failed for this purpose. It has been demonstrated that a threshold higher than 50% of crystallization of the DO22 phase is required for achieving a significant increase of the hardness as a function of the crystallization percentage. These results suggest that microcalorimetric measurement can be a powerful tool to establish quantitative relationships between the mechanical, electrical or functional properties of alloys and their structural changes undergone by aging.

The calorimetric analysis as a tool for studying the aging hardening mechanism of a Cu-10wt%Ni-5.5wt%Sn alloy

The transformations of a Cu-10wt%Ni-5.5wt%Sn alloy as a function of the aging time in the range from room temperature up to 600 °C have been followed by Differential Scanning Calorimetry (DSC). The results obtained have shown that this alloy undergone two overlapping exothermic phase transitions with DSC peaks at 208 °C and 305 °C, respectively, followed by an endothermic phase transformation with a DSC peak at 526 °C. The structural analysis by TEM, ED, EDX and XRD of the intermediates phases previously discriminated by DSC suggests that the first exothermic peak is associated to the spinodal decomposition of the sample, while the second one is associated to the segregation of a DO22 (CuxNi1−x)3Sn tetragonal phase coherent with the α-Cu structure of the starting alloy. The endothermic peak has been associated to the precipitation of cubic DO3 nanocrystals from the DO22 phase previously formed. The microhardness measurements carried out in combination with the structural characterization demonstrate that the aging hardening of the alloy under study is exclusively due to the formation of the coherent DO22 phase. The DO22 → DO3 transition leads to a dramatic drop of the hardness of the alloy.

Microstructure evolution during the precipitation and growth of fully coherent DO22 superlattice in an Ni-Cr-W alloy

The ordering transformation occurring in a model Ni-Cr-W superalloy during prolonged exposure to proper temperature has been investigated systematically. It is demonstrated that nanometer-sized precipitates with a DO22 structure can precipitate in the Ni-Cr-W alloy by means of simple aging treatment at 650–700°C. The mechanism of transformation to DO22 superlattice has been determined to be continuous ordering based on the results of high resolution transmission electron microscopy investigation and variation trend in Vickers microhardness. Different variants of DO22 phase can coexist in the matrix with no signs of overaging as aging time increases, indicating it has a high thermal stability. The precipitates of DO22 superlattice has been found to be of ellipsoidal shape which results in the greatest reduction of strain energy. The interfaces between DO22 precipitates and matrix have been revealed to be coherent at the atomic scale, resulting in considerable coherency strain attributing to the lattice misfit between DO22 particle and matrix. Because of the high-density nanometer-sized DO22 phase, the microhardness of the alloy has been improved remarkably after aging treatment.

Site occupation evolution of alloying elements in Ni3V phase during phase transformation in Ni75Al4.2V20.8

Based on the microscopic phase-field model, the correlation between site occupation evolution of alloying elements in Ni3V-DO22 phase and growth of Ni3Al-L12 phase was studied during the phase transformation of Ni75Al4.2V20.8. The results demonstrate that the growth of L12 phase can be divided into two stages: at the early stage, the composition of alloying elements in DO22 phase almost remains unchanged; at the late stage, the compositions of Ni and Al decrease while V increases in DO22 phase. Part of alloying elements for L12 phase growth are supplied from the site occupation evolution of alloying elements on three kinds of sublattices in DO22 phase. Ni is mainly supplied from V sublattice, and part of Al is supplied from Niı and V sites at the centre of DO22 phase. The excessive V from the decreasing DO22 phase migrates into the centre of DO22 phase and mainly occupies V and NiII sites. It is the site occupation evolution of antisite atoms and ternary additions in DO22 phase that controls the growth rate of L12 phase at the late stage.

Microscopic Phase Field Study on the Kinetics of Order-Disorder Transition of APDBs Formed between DO<sub>22</sub> Phases during Stress Aging

Kinetics of order-disorder transition at antiphase domain boundary (APDB) formed between DO22 (Ni3V) phases during stress aging was investigated using microscopic phase field model. The results demonstrated that whether order-disorder transition happens or not depends on the atomic structure of the APDB. Accompanied with the depletion of V and enrichment of Ni and Al, order-disorder transition happened at the APDB (001)//(002). Whereas at the APDB {100}·1⁄2[100], which remains ordered with temporal evolution, Ni and Al enrich and V depletes. Composition evolution of APDB with order-disorder transition favors the nucleation of the L12 and disordered phase. Some of the grains grew bigger while the others disappeared, accompanying the formation of disordered phase layer during order-disorder transition of APDBs, and the order-disorder transition of APDBs can be considered as accompanying process of coarsening of ordered domain phases and growth of disordered phases.

Structural properties and large coercivity of bulk Mn3−xGa (0 ≤ x ≤ 1.15)

The tetragonal DO22 phase of the Mn3−xGa compounds, with x varying from 0 to 1.15, has been successfully synthesized by annealing the ingots with the cubic γ-phase. It is found that the lattice parameter a of the tetragonal cell remains almost constant, while the lattice parameter c increases significantly as x varies from 0 to 1.15. In the meantime, the magnetization of Mn3−xGa (at 7 T) increases dramatically with increasing x. A substitution model, which is different to the model with manganese vacancies, is proposed to explain these changes. According to this model, the increase of the lattice parameter c and the magnetization with x can be explained by an assumption that Mn atoms at 2b sites are preferentially substituted by larger and nonmagnetic Ga atoms. A coercivity which is higher than that of other bulk Mn3−xGa alloys ever reported is achieved. Coercivities as large as iHc = 21.4 kOe and 18.2 kOe are obtained for Mn3.0Ga at 5 K and 300 K, respectively.

Phase-field study the effects of elastic strain energy on the occupation probability of Cr atom in Ni–Al–Cr alloy

The occupation probability (OP) of Cr atom at α sublattices and β sublattices of DO22–Ni3Cr and L12–Ni3Al in Ni–Al–Cr alloy was studied by using phase-field microelasticity model. The elastic strain energy (ESE) arised by a coherent misfit has little effect on the precipitation sequence of alloy while it changes the behavior of the temporal evolution OP of Cr atom in DO22 and L12 phases. With the ESE increasing, the OP of Cr atom at both α sublattices and β sublattices in DO22 phase increases, the OP of Cr atom at β sublattices in L12 phase decreases, and the OP of Cr atom at α sublattices in L12 phase increases. Eventually, the ESE leads to directional coarsening of coherent microstructure.

Microscopic Phase-Field Simulation of the Structure Transformation from DO22 to L10(M=1) Phase in Ni64Al21V15 Alloy

Abstract The structure transformation from DO22 to L10 (M=1) phase in Ni64Al21V15 alloy at 1150 K was studied by the microscopic phase-field kinetic model. The results show that L10 (M=1) structure precipitates at interphase during the middle stage of the aging process with three types of the formation mechanism. Firstly, the L10 (M=1) structure which aligns along [001] direction and nucleates at the interphase of L12 and DO22 grows towards DO22 phase. Secondly, the L10 (M=1) structure which aligns along [001] direction nucleates at 90° ordered domain boundary of DO22 phase and expands to the [001] direction of DO22 phase. Thirdly, the L10 (M=1) structure which aligns along [100] direction nucleates at the interphase of two [100] aligned DO22 structure, and expands towards DO22 phase. The phase transformation from DO22 structure to L10 (M=1) structure is accomplished by atomic diffusion. In a perfect DO22 phase, the β site is mainly occupied by V atoms, while the α2 and α3 sites by Ni atoms and the α1 site by Al atoms. As the aging process proceeds, V atoms which migrate to α2 site, leading to the atomic ordering on (002) plane takes the place of both β and α2 sites ultimately, and the Ni atoms migrate from α2 to α1 site and occupy both α1 and α3 sites lastly, while Al atoms diffuse towards outside of DO22 structure facilitating the formation of highly ordered L10 (M=1) structure finally.

Site occupation evolution of alloying elements in L12 phase during phase transformation in Ni75Al7.5V17.5

Correlation between site occupation evolution of alloying elements in L12 phase and growth of DO22 phase in Ni75Al7.5V17.5 was studied using microscopic phase field model. The results demonstrate that the growing process of DO22 phase can be divided into two stages. At the early stage, composition in the centre part of L12 phase almost remains unchanged, and the nucleation and growth of DO22 phase is controlled by the decrease of interface between L12 phases. At the late stage, part of V for growth of DO22 phase is supplied from the centre part of L12 phase and mainly comes from Al sublattice, the excess Ni spared from the decreasing L12 phase migrates into the centre part of L12 phase and occupies the Ni sublattices exclusively, while the excess Al mainly occupies the Al sublattice. At the late stage, the growth of DO22 phase is controlled by the evolution of antisite atoms and ternary additions in the centre part of L12 phase.