Single Crystalline Alloys Research Articles

The Brittle-to-Ductile Transition Behaviour in Fe-Al Single Crystalline Alloys

The morphology of deformation twinning, which influences a brittle fracture at low temperatures, was investigated in Fe-8mass%Al. Tensile tests were performed at 129K and room temperature. The specimen tested at room temperature showed yielding and kept deformed by usual slip while the specimen tested at 129K fractured in a brittle manner in an elastic regime with a number of straight markings due to deformation twinning. Detail analysis of those deformation twins suggests that the collision of deformation twinning is the initiation site of the brittle fracture.

Aging Response to Microstructure and Properties of Ferromagnetic Shape Memory Alloys

Ni-Mn-Ga magnetic shape memory alloys are employed for applications in actuators and sensing devices. Ni-Mn-Ga single crystalline alloys exhibit ferromagnetic shape memory effect with large reproducible strains in moderate magnetic fields. The cost for producing single crystals is high and there is a requirement to investigate the polycrystalline Ni-Mn-Ga alloys for similar applications. This work presents a study of the effect of composition and heat treatment on the microstructure, in polycrystalline off-stoichiometric compositions of high Ni, Ni-Mn-Ga alloys. Cast polycrystalline alloys were homogenized and analysed using optical microscopy, X-ray diffraction, and thermal analysis. Stability of the martensitic transformation temperature was studied by aging the alloys at different temperatures. Martensitic structure was found in both the alloys (~ 54at% and 58 at%). The alloy with high Ni~58 at% content was found to be having a dual phase structure (martensite and FCC γ). Single phase Ni-Mn-Ga alloy has shown transformation at temperature >400K while the dual phase alloy with Ni ~58at% has transformed at temperature >700K thus making it suitable for high temperature applications. Martensitic stabilization effect was observed in alloy with Ni ~54 at% after aging treatment while it was absent in alloy with ~58at% Ni.

Compositional and Stress-Sensitive Factors during the Thermopower Characterization of Single Phase Crystalline and Glassy Alloys

For the study of stress state in engineering alloys, the thermopower (S) measurement could be acceptable method after a suitable calibration (fitting the S value to the change of well known other physical parameters like the magnetic coercive force, hardness or strength increase during various thermomechanical treatments). For this purpose a simple equipment is constructed and built by which the S shift can be measured versus the increasing mechanical tensile load. The first results will be published in this paper, obtained on various metallic glass ribbons.

Nanoporous Membranes Produced from Polycrystalline Ni-Based Superalloys

Porous metal membranes are produced from a two phase system in which the discrete cubic gamma'-precipitates connect during self assembly. In the so called rafting process the cubic particles start to coarsen and finally create a network within the gamma-matrix. In a following electrochemical leaching process one of the phases can be removed leaving the nanoporous membrane. So far, single crystalline alloys have been used for producing thin nanoporous membranes. Now research is in progress to produce the nanoporous membranes from polycrystalline alloys in a creep process. A modification of the commercially available alloy Nimonic 115 was used for these membranes. The permeability of these membranes was proven in a gas-flow test.

Magnetostrictive and elastic properties of Fe100−xMox (2<x<12) single crystals

In this paper we investigate the magnetostrictive [(3/2)λ100 and (3/2)λ111] and elastic (c′ and c44) behavior of single crystalline alloys Fe100−xMox for 2<x<12; the magnetoelastic coupling constants (−b1 and −b2) are computed from the measurements. Similar to other Fe–X (X=Al, Ga, and Ge) alloys, the tetragonal magnetostriction (3/2)λ100 increases monotonically from ∼70×10−6 at ∼2.5 at. % Mo to a maximum of either ∼100×10−6 at ∼8 at. % Mo for the slow cooled crystals or ∼125×10−6 at ∼11 at. % Mo for quenched crystals. A sharp decrease after the peak is observed for the slow cooled crystals due to the formation of a second phase. The rhombohedral magnetostriction (3/2)λ111 of the Fe–Mo alloys is found to be insensitive to the Mo content. This behavior is distinctly different from other Fe–X (X=Al, Ga, and Ge) alloys where a slight decrease in magnitude and a sign reversal upon chemical ordering was observed for (3/2)λ111. Both shear elastic constants (c′ and c44) for Fe–Mo are remarkably insensitive to the Mo content, which is also distinct from the other Fe-based alloys used in the comparison. The two magnetoelastic coupling constants −b1=3λ100c′ (with values from 7.15 to 9.77 MJ/m3) and −b2=3λ111c44 (with values from −4.96 to −5.81 MJ/m3) were calculated and compared with those of other Fe–X (X=Al, Ga, and Ge) alloys.

Effect of atomic order on the martensitic and magnetic transformations in Ni–Mn–Gaferromagnetic shape memory alloys

The influence of long-range L21 atomic order on the martensitic and magnetic transformations of Ni–Mn–Ga shape memoryalloys has been investigated. In order to correlate the structural and magnetic transformationtemperatures with the atomic order, calorimetric, magnetic and neutron diffractionmeasurements have been performed on polycrystalline and single-crystalline alloys subjected todifferent thermal treatments. It is found that both transformation temperatures increasewith increasing atomic order, showing exactly the same linear dependence on the degree ofL21 atomic order. A quantitative correlation between atomic order and transformationtemperatures has been established, from which the effect of atomic order on the relativestability between the structural phases has been quantified. On the other hand, the kineticsof the post-quench ordering process taking place in these alloys has been studied. It isshown that the activation energy of the ordering process agrees quite well with theactivation energy of the Mn self-diffusion process.

Influence of Micro-Structure on Oxidation Behavior of a Ni-Based Superalloy at 1000°C

The isothermal and cyclic oxidation behaviors of a Ni-based superalloy with singlecrystalline (SC), polycrystalline and nanocrystalline (NC) structures were studied at 1000°C. Results indicated that a uniform oxides scale consisted of external Cr2O3 with little TiO2 and internal continuous Al2O3 formed on SC alloy. A non-uniform external oxide of which some locations were nodule-like scale was formed on surface of cast alloy. The nodule-like parts consisted of TiO2, Cr2O3 and serious internal oxidation of Al, and rest flat surface was a Cr2O3 and Al2O3 layer. A continuous Al2O3 layer formed on the sputtered NC coating. The micro-structure influenced the oxidation mechanism and resulted in different oxide scale formed on three materials, which greatly influenced materials’ oxidation and cyclic-oxidation resistance.

The effect of micro-structure on the oxidation behavior of a Ni-based superalloy in water vapor plus oxygen

The oxidation behavior of a Ni-based superalloy with polycrystalline, single-crystalline (SC) and nanocrystalline (NC) structures was studied at 1000 °C in water vapor (20.12 vol.%) plus oxygen. The oxidation behavior of the SC alloy was similar to that of the cast alloy, while nanocrystallization increased the corrosion resistance. The oxides scale on the SC alloy and cast alloy consisted of external TiO 2, Cr 2O 3 and internal Al 2O 3, while only external Al 2O 3 scale formed on the NC coating in water vapor plus oxygen. Meanwhile, the morphologies of oxides scale on three samples are significantly different from one another. The effect of micro-structures on the oxidation behavior of this Ni-based superalloy is discussed.

Determination of the non-chemical energy terms and the equilibrium transformation temperature ( T0) in CuAlNi poly- and single crystalline alloys

Results of the analysis, developed in the author's group and applied in poly- and single crystalline Cu–20 at.%Al–2.2 at.%Ni–0.5 at.%B alloy, are reviewed and the dependence of the derivatives of the elastic and dissipative energy terms on the transformed fraction are presented. The T 0( σ) functions ( T 0 is the equilibrium transformation temperature and σ is the uni-axial tensile stress, respectively), calculated on the basis of the method, by uniform heating and cooling of the specimen, are also presented. Similarly, the stress dependences of the non-chemical contributions to the phase transformation are determined. It is shown that integrals of the differential values (the derivatives of the energy contributions by the transformed fraction) give self-consistent results with the (integral) quantities directly measured in differential scanning calorimetry or obtained from the area of the hysteresis loops.

Lattice dynamics of Ni–Mn–Al Heusler alloys

We have studied the lattice dynamics of a Ni 54Mn 23Al 23 (at.%) Heusler single-crystalline alloy by means of neutron scattering and ultrasonic techniques. Results show the existence of a number of precursor phenomena. We have found an anomaly (dip) in the low TA 2 phonon branch at the wave number ξ 0 ≈ 0.33 (in reciprocal lattice units) that becomes more pronounced (phonon softening) with decreasing temperature. We have also observed softening of the associated shear elastic constant ( C ′ ) with decreasing temperature. Ultrasonic measurements under applied magnetic field, both isothermally and varying the temperature show that the values of elastic constants depend on magnetic order thus evidencing magnetoelastic coupling.

Determination of the equilibrium transformation temperature ( T0) and analysis of the non-chemical energy terms in a CuAlNi single crystalline shape memory alloy

A method for the determination of the equilibrium transformation temperature ( T 0) in CuAlNi single crystalline alloys, by traditional uniform heating and cooling of the specimen under constant uniaxial applied stress, σ, is presented and the T 0( σ) functions are constructed. Above a certain stress level the phase transformations, even in a multiple interface mode, can be driven in such a way that the thermal hysteresis loops have a rectangular part, from which T 0 can be determined via the well-known relation: T 0 = ( M s + A f)/2, where M s and A f are the martensite start and austenite finish temperatures, respectively. At low stress values the heating up branch of the hysteresis is different; it starts with a vertical part showing that the beginning of the austenite formation is free of the release of the elastic energy (this takes place only in the second part of this branch). Here the T 0 temperature can be determined as the arithmetic mean of M s and the austenite start temperature, A s. Using the experimentally determined stress dependence of the transformation strain, the T 0 vs. σ function was also constructed from the Clausius–Clapeyron relation and this curve fitted very well to the points obtained from the above relationships at high and low stress levels, respectively. The stress dependence of the non-chemical energy contributions to the phase transformation are also determined. It is shown that integrals of the differential values (the derivatives of the energy contributions by the transformed fraction) give self-consistent results with the (integral) quantities directly measured in differential scanning calorimetry (DSC) experiments or obtained from the area of the hysteresis loops.

Thermoelectric properties of Bi‐Sb samples grown by mechanical alloy

Seebeck coefficient S(T), electric resistivity ϱ(T) and thermal conductivity κ(T) measurements are reported on polycrystalline Bi0.88Sb0.12 samples grown by mechanical alloy (MA) by using different milling time. The electrical and thermal characterization was carried out in the temperature range between 77 K and 300 K. The Seebeck coefficient is negative in whole measurement range and its magnitude increases with the temperature, S(T) exhibit an enhancement with the milling time up to reach values close to –300 µV/K in the samples milled through 45 hours. The electrical resistivity shows a weak semiconductor behavior with the temperature, ϱ(T) was increasing with the processing time, but in all cases it shown values close to 10–3 Ω cm. The thermal conductivity measurements were carried out by using a steady-state heat-flow method. κ(T) show a linear behavior with the temperature and its magnitude decreases with processing time up to values smaller than 1 W/mK. By using SEM analysis the behavior of the granularity of the samples was studied as a function of the milling time. The dimensionless thermoelectric figure of merit ZT, reaches values close to 0.3, which are characteristic values for single crystalline alloys of these kind compounds. These results show that the mechanical alloy could be a useful technique in the production of thermoelectric materials. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Assessment of a powder metallurgical processing route for refractory metal silicide alloys

A powder metallurgical (PM) processing route for the manufacturing of two different refractory metal silicide alloys comprising inert gas atomization of presintered bars, hot isostatic pressing, and hot extrusion (reduction in cross section of 6:1) was established. The mechanical properties between room temperature and 1200 °C of the PM-processed Mo-3Si-1B and Nb-24Ti-20Si-5Cr-3Hf-2Al alloys (in wt pct) were assessed with tensile tests vs a state-of-the-art Ni-base single crystalline alloy (CMSX 4) and a directionally solidified (MASC) niobium-base silicide alloy, respectively. The microstructural characterization of both the hot-isostatically pressed and extruded materials was carried out applying scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), and X-ray diffraction (XRD) analysis. The Mo-Si-B alloy is characterized by an intermetallic matrix surrounding globular Mo particles in the hot isostatic press and a nearly continuous molybdenum solid solution matrix with dispersed intermetallic particles in the hot-extruded condition. Hot extrusion results in a substantial reduction of the DBTT of about 200 °C and tensile strengths superior to CMSX 4 at temperatures above 1000 °C. In the case of the Nb-base silicide alloy, a niobium solid solution surrounding intermetallic particles with Nb5Si3-type structure characterizes the final alloy. In the intermediate temperature range of 500 °C to 816 °C, a strength level equivalent to the directionally solidified MASC alloy was observed.

Compositional dependence of structure, magnetization and magnetic anisotropy in Ni–Mn–Ga magnetic shape memory alloys

Abstract Magnetic and structural properties of Ni–Mn–Ga polycrystalline and single crystalline alloys were investigated in wide composition range. Depending on the average electron concentration, the structure of room temperature martensite can be five-layered, seven-layered and nonmodulated. Low-temperature saturation magnetization decreased linearly from the peak at stoichiometry due to antiferromagnetic arrangement of excess Mn atoms regardless of the martensitic structure. Magnetic anisotropy of five-layered martensite increases with increasing Mn content.

Structure and magnetic property of Ni-Cu alloy nanowires electrodeposited into the pores of anodic alumina membranes

Ordered Ni-Cu alloy nanowire arrays embedded in the hexagonally ordered nanochannels of anodic alumina membranes (AAMs) were fabricated by electrodeposition. The x-ray diffraction, x-ray energy dispersion spectrum, scanning and transmission electron microscopy were utilized to characterize the Ni-Cu alloy nanowire arrays. The results reveal that single crystalline Ni-Cu alloy nanowires are single crystal and that the atomic composition of Ni and Cu is very close to a 1 : 1 stoichiometry. Magnetic hysteresis loop shows that the Ni-Cu alloy nanowire arrays embedded in AAMs have perpendicular magnetic anisotropy, indicating their potential applications in perpendicular magnetic recording.

Ni tracer diffusion in the B2-compound NiAl: influence of temperature and composition

The effect of composition and temperature on Ni bulk self-diffusion is investigated for nine different single crystalline NiAl alloys with well-defined compositions between 46.8 and 56.6 at.% Ni in the temperature range from 1050 to 1630K. The diffusion penetration profiles of Ni in NiAl were determined by applying two different techniques of profile detection. Radiotracer experiments have been carried out using the 63Ni tracer, a serial sectioning technique, and sensitive liquid scintillation counting for the high temperature measurements, while at lower temperatures the diffusion profiles were analyzed by secondary ion mass spectrometry (SIMS) using the highly enriched stable isotope 64Ni. In contrast to the literature data on Ni self-diffusion in NiAl alloys by Hancock and McDonnell [Phys. stat. sol. A4, (1971) 143], the present measurements show an unexpected concentration dependence of the Ni diffusion coefficients D with nearly constant diffusivities for stoichiometric and Al-rich alloys and increasing D values with increasing Ni content on the Ni-rich side of the NiAl composition range. The effective diffusion activation enthalpy Q is equal to about 3.0±0.07 eV for the Al-rich, stoichiometric, and slightly Ni-rich NiAl alloys, while for the compositions with larger Ni content a decrease of Q was observed with increasing Ni content, for example, Q=2.39 eV for the Ni 56.6Al 43.4 alloy. The present experimental results imply that mainly the same diffusion mechanism operates on both sides of stoichiometry in NiAl. This mechanism is identified with the triple defect mechanism. Its contribution is compositionally independent. The activation energy of Q=3.18 eV was calculated for the triple defect mechanism using empirical EAM potentials in agreement with the experimental data. The decrease of Q at large Ni concentrations on the Ni-rich side is explained by an additional contribution of the anti-structure bridge mechanism with the activation energy of Q=1.73 eV.

Shape of precipitates in Ni-Al-Mo single crystals

A method is presented to determine the shape of the average precipitate from anisotropic small-angle scattering patterns from single crystalline alloys during decomposition. The form factor squared of an average precipitate is described by an analytical expression with a single parameter, the morphology parameter α, to account for changes from spherical to cuboidal shape. The method is applied to microstructural states of Ni-12 at.% Al-2 at.% Mo and Ni10 at.% Al-5 at.% Mo after aging treatments of up to 10 h at 970 or 1070 K. The value of α increases with aging time and smaller Mo content, in good agreement with the average shape of precipitates obtained from transmission electron microscopy.

Characterization of the phase transformations in shape-memory alloys by modulated differential scanning calorimetry

Abstract Modulated differential scanning calorimetry (MDSC) is a recently developed calorimetric technique, which has demonstrated some significant advantages over the conventional differential scanning calorimetry (DSC). By separating the reversing quantity from the non-reversing component in the total thermal events, it provides some new information that can not be obtained from the conventional DSC. The technique has been applied to various polycrystalline and single crystalline shape-memory alloys, including Cu-Zn-Al, Cu-Al-Ni, Ti-Ni(Cu), Ni-Mn-Ga and Fe-Mn-Si, to characterize the martensitic transformations, bainitic transformation, chemical and magnetic ordering transitions, atomic reordering and other kinetic relaxation processes in the alloys. The preliminary results of the MDSC measurements are summarized and the interpretation of the MDSC results and some factors affecting the results are discussed.

Processing and high temperature properties of refractory alloy single crystals

Abstract Creep behavior of molybdenum-based alloy single crystals at an elevated temperature (∼0.6Tm) was examined in this study. Grain boundary sliding is absent in solution strengthened single crystalline alloys, resulting in significantly reduced creep rates compared to their polycrystalline counterparts. Hafnium (Hf) exhibits better strengthening effects in molybdenum (Mo) relative to niobium (Nb), as a result of the larger solute–solvent atomic size misfit in Mo–Hf compared to than in Mo–Nb. Dislocation climb and viscous drag are two competing processes during creep. The dominant creep mechanism depends on solute type, concentration and temperature. Results from this study confirm that the creep behavior in solution strengthened Mo single crystals corresponds to the class II alloys, as defined by Sherby and Burke. In addition, the creep response of these single crystals was found to be governed by the average nearest distance between the strengthening solute atoms in the lattice. Creep data show that the activation energy for creep contains two contributions: the normal diffusion energy and an additional solute-related energy which is proportional to the product (ne*c1/3) where n is the stress exponent, e* is the atomic misfit factor and c is the atomic solute concentration. Results also indicate that for Mo–Nb, there is a transition from class II to class I behavior at an Nb concentration of about three atomic percent.

Modeling of the reorientation behavior of a single crystalline shape-memory alloy by a micromechanical approach

A Helmholtz free energy for a martensitic transformation of a single crystalline shape-memory alloy is obtained by a micromechanical approach. 24 variants of the single crystal are taken into account. In the framework of irreversible thermodynamics, a kinetic relation, a martensitic nucleation criterion and the reorientation criterion of martensitic variants are obtained. These relations are valid for a three-dimensional proportional or non-proportional mechanical loading or a combination of mechanical and thermal loading. Reorientation behavior of a single crystalline shape-memory alloy CuZnAl is simulated. When a tensile load is applied to a thermally-induced martensite, 24 self-accommodated martensitic variants are reoriented to the most favorable variant. In the following unloading and compression load, the most favorable variant in the tensile load is reoriented to the most favorable variant in this loading condition.