L12 Type Research Articles

Elemental partitioning, lattice misfit and creep behaviour of Cr containing γ′ strengthened Co base superalloys

Novel Cr containing Co-Al-W base superalloys were studied by atom probe tomography and neutron diffraction. Cr is found to predominantly partition to the γ matrix and decrease partitioning of W to γ′. Furthermore, Cr significantly enhances the γ′ volume fraction, decreases the γ/γ′ lattice misfit and deteriorates the creep resistance. Addition of Ni to the Cr containing alloys affects partitioning of W and Al, further decreases the lattice misfit and results in the formation of irregularly shaped precipitates. Al, W and Cr tend to occupy the ‘B'sublattice in the γ′-A3B phase (L12 type), while Co and Ni reside in the ‘A' sublattice.

Electronic Properties of Cu‐Pd Ordered Alloys

SummaryThe Cu‐Pd alloy, for Pd concentration greater than 50 at% has a distorted FCC structure, while for Pd less than 18 at%, has a L12 type structure. The electronic structure of Cu3Pd and CuPd3, which are the ordered alloys of the Cu‐Pd system are investigated by the FP‐LAPW method based on the density functional theory. The temperature dependence of the electrical resistivity and Seebeck coefficients has been interpreted in terms of band structures and density of states.

Elastocaloric effect in Ni50Fe19Ga27Co4 single crystals

The elastocaloric effect of the [001]P and [111]P orientated (subscript P represents the parent phase) Ni50Fe19Ga27Co4 (at.%) single crystals exhibiting first-order martensitic transformations have been studied at temperatures of 298–448K under different compressive stresses. The adiabatic temperature change ΔTadi shows a significant dependence on the crystal orientation. The temperature decrease caused by adiabatic removal of stress 300MPa reaches to about 9–10K in a temperature range 328–398K for [001]P specimen and 3K at temperature near Af for [111]P one. The large elastocaloric effect is essentially attributed to the entropy change during the stress-induced martensitic transformation. The accumulated defect such as the a/2〈112〉 L10 type dislocation is the main reason for the attenuation of the elastocaloric effect in a fatigue test up to 3000 cycles. Large specific adiabatic temperature (|ΔTadi/Δσ|≈35K/GPa), wide specific effective temperature window (|ΔTwin/Δσ|≈320K/GPa) and high coefficient of performance (COP≈14) in a temperature range as wide as 50K during the unloading process enable the [001]P orientated Ni50Fe19Ga27Co4 single crystal as a potential elastocaloric material.

The structure and thermal parameters of ordered Cu65Fe10Pd25 ternary alloy

Structural and thermal parameters have been studied in Cu65Fe10Pd25 alloy during order–disorder (O–D) transformation using differential scanning calorimetry (DSC) and high temperature X-ray diffraction (HTXRD). The results reveal that the Cu65Fe10Pd25 alloy undergoes an O–D transformation at Tc=797K. The alloy shows L12 type ordering below Tc and has disordered face centered cubic (fcc) structure above Tc. The lattice parameter shows a positive deviation from Vegard’s rule which may be related to the weakening of interatomic forces by the addition of Fe. The integrated intensity data obtained from the diffraction experiments was utilized to determine the coefficient of thermal expansion (α(T)), mean square amplitude of vibration (u2¯(T)) and Debye temperatures (ΘD) during the O–D transformation. The abrupt change in the value of lattice parameter and coefficient of thermal expansion at Tc shows that the nature of O–D transition is first order. These results have been discussed by comparing them to those for Cu3Pd alloy.

Structural Characterization and Enhanced Oxygen Reduction Activity of Chemically-Ordered Intermetallic PtFeNi Catalysts

The polymer electrolyte fuel cells (PEFCs) are excellent candidates as next-generation electric generators for vehicles, residential uses, laptop, etc. However, there are many issues to be solved to enlarge commercially uses. One of issues is related to Pt catalysts for oxygen reduction reaction (ORR) on a cathode electrode. Pt catalysts show low ORR activity and low durability, leading to lower performance in PEFCs. Our earlier studies found the chemically ordered face centered tetragonal (fct) intermetallic Pt-alloy catalysts (L10 type super-lattice structure) can satisfy both of higher ORR activity and long-term durability, whereas many of the previously-reported Pt-alloys cannot. It is considered that the high performance of fct-Pt alloy catalysts would result from the chemically-ordered structures, but it is unclear how they work to enhance ORR activity and durability. Thus, in this presentation, we will discuss the relationship between the ORR activities and surface structures in fct-Pt alloys by the detailed characterization of the catalyst’s surface structures.The fct-PtFeNi catalysts with different atomic compositions of Fe and Ni (Pt50Fe35Ni15, Pt50Fe25Ni25, Pt50Fe15Ni35) were prepared using a simple solid-state impregnation method; the mixture of metal (Pt, Fe, Ni)-salts and carbon blacks were annealed under H2/N2 at a temperature of 800°C. The prepared fct-PtFeNi catalysts exhibit about 3 times higher ORR mass activity, compared with the commercial Pt catalyst. Especially, the mass activity of Pt50Fe35Ni15 is about 0.62 A/mgPt; it is higher than that of fct-Pt50Fe25Ni25 and fct-Pt50Fe15Ni35 (about 0.5 A/mgPt). To discuss more about the enhancement of the ORR activities in the fct-PtFeNi alloys, the surface structures of the catalysts were examined by in-situ XAFS measurements. The EXAFS analysis of Pt-L3 edge reveals that the distance of Pt-Pt bonds in the fct-PtFeNi catalysts is about 2.72 nm, which is shorter than that of commercial Pt catalyst. Interestingly, the coordination number of Pt oxides (Pt-O or Pt-OH) in the fct-Pt50Fe35Ni15 is more increased at high potential of 0.8~1.2V, compared with other fct-PtFeNi catalysts with the lower content of Fe. The tendency of the formation of Pt oxides is in accord with that of the ORR activities in the fct-PtFeNi with the different compositions of Fe and Ni. Therefore, it is inferred that fct-PtFeNi with higher content of Fe would form more Pt oxides, leading to a faster reaction rate of oxygen reduction. This study brought out new insights; the surface structures of chemically-ordered fct-Pt alloys, such as the Pt-Pt bond distance and the formation of Pt oxides, strongly affect the ORR activities, as those of the previously-reported disordered Pt-alloys do as well. It is believed that our findings can aid in developing fct-Pt alloys with more enhanced catalytic activity and durability for PEFCs.

Elemental partitioning and mechanical properties of Ti- and Ta-containing Co–Al–W-base superalloys studied by atom probe tomography and nanoindentation

Elemental partitioning and hardness in Ti- and Ta-containing Co-base superalloys, strengthened by γ′-Co3(Al,W) precipitates, have been studied by local measurements. Using atom probe tomography, we detect strong partitioning of W (partitioning coefficients from 2.4 to 3.4) and only slight partitioning of Al (partitioning coefficients ⩽1.1) to the γ′-Co3(Al,W) phase. Al segregates to the γ/γ′ phase boundaries, whereas W is depleted at the γ side of the boundaries after aging at 900°C and slow air cooling. This kind of Al segregation and W depletion is much less pronounced when water quenching is applied. As a result, these effects are considered to be absent at high temperatures and therefore should not influence the creep properties. Ti and Ta additions are found to strongly partition to the γ′ phase and greatly increase the γ′ volume fraction. Our results indicate that the alloying elements Al, W, Ti and Ta all occupy the B sublattice of the A3B structure (L12 type) and affect the partitioning behavior of each other. Nanoindentation measurements show that Ta also increases the hardness of the γ′ phase, while the hardness of the γ channels remains nearly constant in all alloys. The change in hardness of the γ′ phase can be ascribed to the substitution of Al and W atoms by Ti and/or Ta.

Dislocation slip and twinning in Ni-based L12 type alloys

We report theoretical results on dislocation slip and twinning in Ni3 (Al, Ti, Ta, Hf) compositions with L12 crystal structures utilizing first-principles simulations. The lattice parameters of Ni3Al, Ni3Al0.75Ta0.25, Ni3Al0.5Ta0.5, Ni3Ta, Ni3Ti and Ni3Al0.75Hf0.25 are calculated, and the crystal structures with lower structural energies are determined. We established the Generalized Stacking Fault Energy (GSFE) and Generalized Planar Fault Energy (GPFE), and calculated stacking fault energies APB (anti-phase boundary) and CSF (complex stacking fault) matched other calculations and experiments. Based on the extended Peierls–Nabarro model for slip and the proposed twin nucleation model, we predict slip and twinning stress and the results show a general agreement with available experimental data. The results show that in the studied intermetallic alloys, twinning stress is lower than slip stress; Ta and Hf ternary addition are substantial to increase flow stress in Ni3Al. The models proposed in the paper provide quantitative understanding and guidelines for selecting optimal precipitate chemistry and composition to obtain higher mechanical strength in Shape Memory Alloys.

Magnetic and Martensitic Transformations in Ni<sub>46</sub>Mn<sub>41.5-x</sub>Fe<sub>x</sub>Sn<sub>12.5</sub> Melt Spun Ribbons

Four alloys with nominal compositions Ni46Mn41.5-xFexSn12.5 (x=0, 2, 4, 6 at.%) were cast in an induction vacuum furnace and homogenized. Then they were melted in quartz tubes and ejected onto a rotating copper wheel to produce ribbons. The X-Ray phase analyses of as melt spun ribbons have shown that in both, the ternary as well as in the quaternary alloys a single phase of the Heusler L21 type ordered structure was found. The characteristic temperatures of magnetic (TC) and martensitic (Ms) transformations were determined by a vibrating sample magnetometer (VSM). Both the Ms and TC increase with the increase of Fe content in all alloys, which is in accordance with the theory of valence electron concentration (e/a) influence on Ms. The phase structures, chemical compositions, grains sizes and type of microsegregation were characterized by transmission electron microscope (TEM). The equi-axed grains of size from 0.95 to 1.7 μm were observed in all ribbons. The grains posses the L21 structure at room temperature, however in the alloys with higher Fe content the different type of martensite was observed at the grain boundaries of L21 phase. Appearance of this martensite was explained in relation to microsegregation of particular elements during melt spinning process and simultaneous change in the e/a ratio.

Mössbauer Study of Cubic Phase in the Mn-Sb System

One of the methods to produce novel materials is high temperature and high pressure treatment. In such a manner in the Mn Sb system the cubic modi cation of Mn3Sb could be produced [1, 2]. This compound has the L12-type of crystal structure (Cu3Au) with the lattice parameter a = 0.400 nm. The Mn atoms occupy 3c positions with the coordinates (0, 1/2, 1/2), the Sb atoms the positions 1a (0, 0, 0), Fig. 1. In normal conditions, the compound exists in a thermodynamically non-equilibrium state and decomposes at 420 K on the Mn2Sb + Mn phases. The magnetic measurements of the Mn3Sb were performed by the Faraday method in the eld 8.6 kOe and the temperature region from the liquid nitrogen to the temperature of phase decomposition. The derived value of speci c magnetization is very small, ≈ 1.0 A m/kg [2]. The neutron di raction analysis for the Mn3Sb was performed as well [3]. The data obtained were interpreted in the model of the ferrimagnetic ordering of Mn magnetic moments. The magnetic moments of Mn atoms are supposed to be antiparallel and have di erent values initially taking into account the similar local environment of Mn atoms in both tetragonal Mn2Sb and cubic Mn3Sb. However, it is possible to interpret the neutron di raction data in a di erent way, using the model of triangular magnetic ordering of Mn magnetic moments. In contrast to previous one, in this model the Mn atoms are considered to have equal magnetic moments ordered in the (111) plane and forming an equilateral triangle with each moment pointing to the center of the triangle. The similar model was used in [4] to describe the magnetic ordering of Mn in Mn3Pt with the same L12 type of structure below 400 K. Thus the model of the magnetic ordering of Mn atoms in the cubic Mn3Sb is ambiguous. Therefore the purpose of this study was to get Mossbauer information on the number of magnetically nonequivalent states of Mn atoms and to clarify a type of the magnetic ordering in Mn3Sb.

Synthesis and characterization of Co2FeAl Heusler alloy nanoparticles

Heusler alloy Co2FeAl (CFA) nanoparticles have been synthesized by reducing a mixture of the precursors: CoCl2·6H2O, Fe(NO3)3·9H2O and AlCl3·6H2O under H2 atmosphere. XRD, SEM and TEM techniques have been used for the characterization of the prepared material. XRD and SAED data from TEM show the formation of mixed phases of L21, B2 and A2 type crystal structure of the alloy. The estimated particle size from XRD data and TEM micrograph has been found in the range of 10 nm to 50 nm. The saturation magnetization has been found of 115 emu/g from M-H characteristics which is close to its bulk value of saturation magnetization. Chemical composition of the elements has also been estimated from EDAX, which shows a ratio of Co:Fe:Al as 2.12:1.06:0.81.

Room temperature magneto-structural transition in Al for Sn substituted Ni–Mn–Sn melt spun ribbons

Martensitic and magnetic transformations in Ni48Mn39.5Sn12.5−xAlx (x=0, 1, 2, 3) Heusler alloy ribbons were investigated. It is demonstrated that both magnetic and structural transformations occur in all of the studied samples. It is also shown that substitution of Sn with Al causes the martensitic transformation (MT) and the reverse martensitic transformation (RMT) temperatures to increase to room temperature (ΔTMT=49K; ΔTRMT=43K), whereas the Curie temperature of martensite TCM decreases (ΔT=36K) and the Curie temperature of austenite TCA remains practically insensitive to Al introduction. This then allows to tune TCA and the MT temperature leading to their coincidence at ambient temperature. The austenite phase with the L21 type structure has been identified to exist in all the samples regardless of composition. On the other hand the structure of martensite has been shown to be sensitive to composition. It has been determined as the 10M martensite with (32¯) stacking sequence in Al free samples and the 4O martensite with the stacking periodicity (31¯) in Al containing samples. In addition, the splitting of the field cooling (FC) and the field heating (FH) thermo-magnetic curves at low (50Oe) magnetic field and below the TCM has been attributed to intermartensitic transition. The application of large magnetic field (50kOe) has shown the existence of two distinct ferromagnetic states with a considerable hysteresis loop. The properties of these materials make them promising for magnetocaloric applications.

Structure and magnetic properties of nanostructured MnNi alloys fabricated by mechanical alloying and annealing treatments

A nearly equiatomic MnNi alloy was fabricated from the elemental powders by means of mechanical alloying in a planetary ball milling apparatus. X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and measurements of magnetization were conducted to identify the structural states and properties of the prepared alloys. After ball milling for 20 h, a disordered face-centered cubic (f.c.c.) solid solution was formed which increased in lattice parameter by further milling up to 50 h. An exothermic reaction took place at around 300–400°C during continuous heating of the disordered f.c.c. solid solution. This reaction is attributed to a structural ordering leading to the formation of a face-centered tetragonal (f.c.t.) phase with L10 type ordering. Examination of the magnetic properties indicated that the structural ordering increases remnant magnetization and decreases coerecivity.

Electronic structure and vibrational properties in cobalt-based full-Heusler compounds: A first principle study of Co2MnX (X = Si, Ge, Al, Ga)

First-principles self-consistent pseudopotential plane wave calculations based on density functional theory were performed in order to study magnetic moments, density of states and half-metallicity of L21 type full Heusler alloys with formula Co2MnX (X=Si, Ge, Al, Ga). Half-metallicity in terms of total spin-moments was discussed since perfect half-metals show Slater–Pauling (SP) behavior. The effects of the atomic number on the lattice constants, the bulk moduli and the Curie temperatures were shown. The magnetic moments were calculated, while slight deviations of about 0.06–0.16μB were found for Co2MnAl and Co2MnGa, the Co2MnSi and Co2MnGe have been found to be half metals. Mechanical stability of these compounds has been analyzed in terms of their elastic constants. The size of the gap in the minority states and the position of EF inside the gap was also discussed as it is an important factor for the application of half-metallic ferromagnetic alloys. Finally, phonon dispersion curves and the density of states were calculated by employing the density-functional perturbation theory and discussed.

Site-preference of Mo/Ta/W alloying additions in NixAl1-x(x=0.25,0.5,0.75)

The characteristic properties including equilibrium lattice constants, elastic constants, cohesive energies and alloy formation energies, of stoichiometric L12 type NiAl3, L12 type Ni3Al, L10 type NiAl and B2 type NiAl, are calculated using a constructed n-body potential model combined with molecular dynamics. Furthermore, the types of point defects in the ordered NixAl1-x (x=0.25, 0.5, 0.75) systems in the nonstoichiometric case are discussed, to validate the model. The present calculation shows that the point defects are the antisites. It is in good agreement with other investigations. Then the n-body potential calculations are performed to investigate the site preference of ternary elements Mo/Ta/W alloying additions in the NixAl1-x systems. The occupation site of the ternary elements are analyzed based on the occupational energy minimizing. And the results show obviously that the occupational sites of Mo/Ta/W are the Al site.

Crystal Structures of Long-Period Stacking-Ordered Phases in the Mg-TM-RE Ternary Systems

ABSTRACTCrystal structures of long-period stacking-ordered (LPSO) phases in the Mg-TM (transition-metal)-RE(rare-earth) systems were investigated by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The 18R-type LPSO phase is constructed by stacking 6-layer structural blocks, each of which contains four consecutive close-packed planes enriched with TM and RE atoms. Formation of the TM6RE8 clusters with the L12 type atomic arrangement is commonly observed in both Mg-Al-Gd and Mg-Zn-Y LPSO phases. The difference between the crystal structures of Mg-Al-Gd and Mg-Zn-Y LPSO phases can be interpreted as the difference in the in-plane ordering of the TM6RE8 clusters in the structural block. The Mg-Al-Gd LPSO phase exhibits a long-range in-plane ordering of Gd and Al, which can be described by the periodic arrangement of the Al6Gd8 clusters with the L12 type atomic arrangement on lattice points of a two-dimensional 2$\sqrt 3 $aMg × 2$\sqrt 3 $aMg primitive hexagonal lattice, although the LPSO phase in the Zn/Y-poor Mg-Zn-Y alloys exhibits a shortrange in-plane ordering of the Zn6Y8 clusters.

Solid-state synthesis and atomic ordering in thin Cu/Au films (atomic ratio, Cu : Au = 3 : 1)

In situ transmission electron microscopy investigations of the processes of solid-state synthesis and atomic ordering in bilayer Cu/Au nanofilms (atomic ratio, Cu: Au = 3: 1) are conducted. It is found that solid-state synthesis starts at 170°C. A Cu3Au atomic-disordered structure (Fm3m space group; lattice constant, a = 3.76 ± 0.01 A) forms at 280°C. Annealing the film for 1 hour at 380°C produced a Cu3AuI (L12 type) atomic-ordered superstructure, a Pm-3m space group, and lattice constant, a = 3.76 ± 0.01 A in the bulk of the film.

Martensitic transformations and magnetocaloric effect in Sn-doped NiMnIn shape memory alloy

The structural, magnetic and magnetocaloric properties of the Ni50Mn34Sn8In8 alloy have been presented. It was observed to form in single phase L21 type cubic structure. The two step martensitic transformations were observed and both martensite and austenite phases were observed to be ferromagnetic in nature. Magnetocaloric effect was evaluated both around the structural and magnetic transitions. Double peak behavior was observed in the magnetic entropy change which has widened the working temperature range.

Pressure effects on elastic and thermodynamic properties of Zr3Al intermetallic compound

The elastic and thermodynamic properties of the L12 type (Cu3Au) structure Zr3Al intermetallic compound under high pressure and temperature are investigated by using ab initio plane-wave pseudopotential density functional theory (DFT) within the generalized gradient approximation (GGA). The result of the heat of formation of Zr3Al crystal investigated is in consistent with those by others. The elastic properties of the cubic Zr3Al under high pressure are studied for the first time, and we found its elastic modulus, compressional and shear wave velocities are increasing monotonically with increasing pressure. Finally, the thermodynamic properties of the cubic Zr3Al are predicted by using the quasi-harmonic Debye model. The Debye temperature Θ, the bulk modulus B, the heat capacity Cv, and the thermal expansion α are calculated as a function of the pressure and temperature in the ranges of 0–55GPa and 0–1800K.

Magnetocrystalline anisotropy in FePt with L1 ordering and tetragonal distortion

We evaluate the magnetocrystalline anisotropy energy of L10 type FePt alloys with the lattice distortion and atomic disorder by using the first-principles calculation, which adopts the tight-binding linear muffin-tin orbital method in conjunction with the coherent potential approximation techniques. The calculated result indicates that the magnetocrystalline anisotropy energy is quite sensitive to the mentioned factors. In particular, it is drastically decreased with the degree of ordering compared with the expected value from the completely ordered structure. We will suggest that the improvement of the chemical ordering of the L10 crystal is one of the significant points to obtain a large magnetocrystalline anisotropy from FePt compounds.

Optical and magnetooptical properties of Heusler alloys XMnSb (X = Co, Pt)

The ellipsometry method (with photon energies ħω= 0.1–5.0 eV) has been used to study the optical properties of the CoMnSb phase with a structure of the L21 type. Infrared-absorption features indicating the existence of a pseudogap in the density of states at the Fermi level N(E F) have been revealed. The transverse Kerr effect in CoMnSb and PtMnSb has been measured at photon energies ħω = 0.4–4.5 eV. It has been shown that the observed peaks of magnetooptical activity in CoMnSb are due to the off-diagonal component ɛ ij of the tensor of the complex dielectric constant \(\hat \varepsilon\). Spectral dependences of the magnetooptical effect δ q quadratic in magnetization M have been studied for the first time for PtMnSb for the s and p polarizations of the light wave. In the region where a giant Kerr effect (ħω ∼ 1.7 eV) is observed in PtMnSb, a sharp increase in the δ q effect has been revealed caused by the change in the diagonal component ɛ ii of the tensor] of the dielectric constant in an external magnetic field. Based on the δ q spectra for the p polarization of the light wave, additions have been calculated to the real and imaginary parts of the diagonal component of the \(\hat \varepsilon\) tensor in the magnetic field (Re(Δɛ ij ) p and (Δɛ) ij ) p , respectively). The spectrum of Im(Δe ij ) p has been analyzed in terms of the existing concepts on the electron structure of PtMnSb.