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The effects of a combined 1.0 at.% W and 1.0 at.% Re addition to a model Ni-10.0Al-8.5Cr at.% superalloy aged at 1073 K (800 °C) for aging times up to 256 h are investigated using atom-probe tomography (APT) and Vickers microhardness measurements. The nanoscale properties, including volume fraction, mean-volume equivalent-radius and number density of the γ′(L12)-precipitates, the chemical compositions of the γ(f.c.c.)-matrix and γ′(L12)-precipitates, the concentration profiles and partitioning ratios between the two phases, γ(f.c.c.)/γ′(L12), hetero-interfacial concentration widths, and microhardness are measured as function of increasing aging time. Localized excesses of the solvent Ni at the γ(f.c.c.)/γ′(L12) interfaces persist at aging times as long as 256 h. Transient diffusional concentration gradients associated with solute-atoms during diffusion-limited growth exist in either the γ(f.c.c.)-matrix or γ′(L12)-precipitates for the as-quenched state; these gradients disappear after 0.25 h for Al and Cr, and after 1 h for W and Re. Tungsten partitions to the γ′(L12)-precipitates and Re partitions to the γ(f.c.c.)-matrix, respectively, and this partitioning behavior enhances the partitioning of Al to the γ′(L12)-precipitates and Cr to the γ(f.c.c.)-matrix. The partitioning behavior indicates that W and Re evolve temporally more slowly than Al and Cr due to their small diffusivities in Ni. The interfacial concentration widths in the Ni-Al-Cr-W-Re alloy decrease with increasing aging time and increasing mean precipitate-radius. Adding W and Re to the base Ni-10.0Al-8.5Cr at.% alloy increases the γ′(L12)-precipitate volume fraction and the microhardness due to the increased volume-fraction and solid-solution strengthening provided by W and Re.

The temporal evolution of ordered γ′(L12)-precipitates precipitating in a disordered γ(f.c.c.) matrix is studied in extensive detail for a Ni-12.5 Al at.% alloy aged at 823 K (550 °C), for times ranging from 0.08 to 4096 h. Three-dimensional atom-probe tomography (3-D APT) results are compared to monovacancy-mediated lattice-kinetic Monte Carlo (LKMC1) simulations on a rigid lattice, which include monovacancy-solute binding energies through 4th nearest-neighbor distances, for the same mean composition and aging temperature. The temporal evolution of the measured values of the mean radius, 〈 R ( t ) 〉 , number density, aluminum supersaturations, and volume fraction of the γ′(L12)-precipitates are compared to the predictions of a modified version of the Lifshitz-Slyozov diffusion-limited coarsening model due to Calderon, Voorhees et al. The resulting experimental rate constants are used to calculate the Gibbs interfacial free-energy between the γ(f.c.c.)- and γ′(L12)-phases, which enter the model, using data from two thermodynamic databases, and its value is compared to all exiting values. The diffusion coefficient for coarsening is calculated utilizing the same rate-constants and compared to all archival diffusivities, not determined from coarsening experiments, and it is demonstrated to be the inter-diffusivity, D ˜ , of Ni and Al. The monovacancy-mediated LKMC1 simulation results are in good agreement with our 3-D APT data. The compositional interfacial width, for the {100}-interface, between the γ(f.c.c.)- and γ’(L12)-phases, decreases continuously with increasing aging time and 〈 R ( t ) 〉 , both for the 3-D APT results and the monovacancy-mediated LKMC1 simulations, in disagreement with an ansatz intrinsic to the trans-interface diffusion-controlled coarsening model, which assumes the exact opposite trend for binary alloys.

The temporal evolution of a γ(f.c.c.)/γ’ (L1 2 ) Co-8.8Al-7.3 W superalloy aged at 650 °C (10 min–4096 h) and 750 °C (10 min–256 h) is studied utilizing atom-probe tomography (APT), scanning electron microscopy, and Vickers microhardness testing. The evolution of the phase compositions, γ’ (L1 2 ) volume fraction, and mean precipitate radius, 2-D (t)>, are determined. Coarsening rate constants and temporal exponents are calculated for -D (t)> of the γ’ (L1 2 )-nanoprecipitates. The temporal exponents are found to be generally close to 1/p = 1/3 as required for diffusion-limited coarsening. Tungsten solid-solubility is significantly reduced in the γ(f.c.c.)-matrix at 650 °C (0.54 ± 0.04 at. %) and 750 °C (1.35 ± 0.06 at. %) when compared with aging at 900 °C (5.5 at. %). The value of 2D (t)> of the γ’ (L1 2 )-nanoprecipitates increases with increasing aging time corresponding to an increase in the Vickers microhardness; the peak strength was not, however, achieved for the aging times investigated. The morphology of the γ’ (L1 2 )-nanoprecipitates begins as spheroids but transitions to cuboids at longer aging times, with final the γ’ (L1 2 ) volume fractions for aging at 650 °C and 750 °C being ϕ = 53% and 54%, respectively. The effect of quench-rate (either furnace-cooled, air-cooled, oil quenched, or water quenched) from a supersolvus temperature of 1050 °C on the microstructure of the alloy is also investigated. Slow cooling (furnace and air-cooling) is shown to result in a uniform distribution of nanometer sized γ’ (L1 2 )-nanoprecipitates, unlike Ni-based superalloys in which the γ’ (L1 2 )-nanoprecipitates form in a non-uniform or multimodal distribution.

Metal amorphous nanocomposite (MANC) alloys are an emerging class of soft magnetic materials showing promise for a range of inductive components targeted for higher power density and higher efficiency power conversion applications including inductors, transformers, and rotating electrical machinery. Magnetization reversal mechanisms within these alloys are typically determined by composition optimization as well as controlled annealing treatments to generate a nanocomposite structure composed of nanocrystals embedded in an amorphous precursor. Here we demonstrate the concept of spatially varying the permeability within a given component for optimization of performance by using the strain annealing process. The concept is realized experimentally through the smoothing of the flux profile from the inner to outer core radius achieved by a monotonic variation in tension during the strain annealing process. Great potential exists for an extension of this concept to a wide range of other power magnetic components and more complex spatially varying permeability profiles through advances in strain annealing techniques and controls.

A Ni50.3Ti41.7Hf8 alloy was studied after two-step aging treatments consisting of 300 °C for 12 h followed by 550 °C for different times. An anomalous change in transformation temperatures was observed as the second aging time was increased from 7.5 to 13.5 h. Initially with increased aging time (0.5–7.5 h) at 550 °C, coherency strain fields about H-phase precipitates increased. The corresponding backstress favored martensite formation, hence an increase in transformation temperatures. However, a point was eventually reached where misfit dislocations relaxed those strain fields and the effect was reduced, resulting in a decrease in transformation temperatures.

Substituting Ni with Au in NiTi leads to dramatic increases in transformation temperatures, meeting one of the requirements for a viable high temperature actuator material. Consequently, four alloys containing between 49 and 51 at.% Ti, a fixed 40 at.% Au, and balance Ni, were prepared and investigated in detail using load-biased thermal cycling (LBTC), scanning electron microscopy (SEM), aberration corrected scanning transmission electron microscopy (STEM), and X-ray energy dispersive spectroscopy (XEDS). LBTC experiments demonstrated work output well above 400 °C, with full recovery up to 100 MPa. The alloys exhibit minimal variation in shape memory properties despite the relatively large composition range from Ti-lean to Ti-rich, in stark contrast to most other NiTi-based systems, which demonstrate extreme compositional sensitivity. Electron beam analysis revealed the presence of two types of secondary phases present in all compositions, which are subsequently characterized. Differences in secondary phase content as a function of alloy composition is shown to have a moderating effect on the transforming matrix composition - an important asset for this alloy system - potentially easing processing requirements and opening up shape memory alloys to new fabrication techniques. Unrecovered strain during cycling at higher loads is analyzed from a theoretical perspective to gain insight into the mechanisms of defect formation responsible for functional fatigue.

The effects of various heat treatments on the microstructure and hardness of new Ni56Ti41Hf3 and Ni56Ti36Hf8 (atomic %) alloys were studied to evaluate the suitability of these materials for tribological applications. A solid-solution strengthening effect due to Hf atoms was observed for the solution annealed (SA) Ni56Ti36Hf8 alloy (716 HV), resulting in a comparable hardness to the Ni56Ti41Hf3 alloy containing 54 vol.% of Ni4Ti3 precipitates (707 HV). In the Ni56Ti41Hf3 alloy, the maximum hardness (752 HV), achieved after aging at 300°C for 12 h, was attributed to dense, semi-coherent precipitation of the Ni4Ti3 phase. Unlike the lenticular morphology usually observed within binary NiTi alloys, a “blocky” Ni4Ti3 morphology formed within Ni56Ti36Hf3 due to a smaller lattice mismatch in the direction normal to the habit plane at the precipitate/matrix interface. The maximum hardness for Ni56Ti36Hf8 (769 HV) was obtained after applying an intermediate aging step (300 °C for 12 h) followed by normal aging (550 °C for 4 h). This two-step aging treatment induces dense nanoscale precipitation of two interspersed precipitate phases, namely H-phase and a new cubic Ni-rich precipitate phase, resulting in the highest hardness exhibited yet by this family of alloys. The composition and structure of this new precipitate phase was characterized using atom probe tomography and transmission electron microscopy techniques to be cubic with a lattice parameter of a = 8.87 A and symmetry belonging to one of the primitive subgroups of m 3 ¯ m with approximately Ni61.5Ti31Hf7.5 composition .

The deformation mechanisms that dictate the tribological performances of heat treated Ni55Ti45, Ni54Ti45Hf1 and Ni56Ti36Hf8 alloys are revealed through rolling contact fatigue (RCF) testing and transmission electron microscopy (TEM) analyses. Analysis of worn samples that passed a 5×108 cycle RCF runout condition shows that damage is primarily confined to deformation bands that propagate several hundred nanometers – to – several microns beneath the surface. These bands nucleate via localization of dislocation slip within the B2 austenite phase of the alloys. For the Ni55Ti45 and Ni54Ti45Hf1 samples, further damage and eventual spall failures occur by shearing and dissolution of the strengthening Ni4Ti3 nanoprecipitates within the deformation bands, followed by nanocrystallization that sometimes includes stress-induced nucleation of B19′ nanocrystals. Eventually, complete amorphization occurs prior to fracture. The relative 15 – 25% increase in RCF contact stress performance of the Ni56Ti36Hf8 alloy correlates with a more limited depth of damaged material beneath the wear surface; heavily damaged material beneath spall failure sites only extends 1.5 µm into the sample, compared to > 6 µm for Ni55Ti45 and Ni54Ti45Hf1 alloys. This superior RCF damage resistance of the Ni56Ti36Hf8 alloy results from a lower fraction of B2 matrix phase (≤ 13 %) that is highly confined by both cubic Ni-rich NiTiHf and H-phase nanoprecipitates. Specifically, this microstructure limits the widths of deformation bands within the B2 austenite phase to less than the size of the strengthening nanoprecipitates, which in turn inhibits precipitate shearing and dissolution processes that precede nanocrystallization and amorphization.

The effects of refractory ( R ) elements ( R = Re, Ru, W, or Ta) on a base Ni-Al-Cr alloy are studied, for an aging temperature of 1073 K (800 °C) and an aging time of 256 h, employing atom-probe tomography (APT) and first-principles calculations. We find that there are strong attractive chemical binding energies between R -elements and solute ( S ) atoms ( S = Al, Cr) in Ni-Al-Cr based alloys utilizing experimental partial radial distribution function (RDF) results, and first-principles calculations performed at 0 K. We demonstrate that correlated R - S binding energies play a key role in the observed Ni retention-excesses at γ(f.c.c.)/γ′(L1 2 -structure) interfaces at aging times as long as 256 h. The total reduction of the γ(f.c.c.)/γ′(L1 2 ) interfacial energy, as a result of Ni interfacial-excesses in both γ(f.c.c.)-matrix and γ′(L1 2 )-precipitates, lies between −0.16 ± 0.06 mJ m −2 and −0.05 ± 0.02 mJ m −2 . The R - S binding energies cause changes in the compositional diffusion flux-vectors in and out of γ′(L1 2 )-precipitates, which result in larger solvent Ni retention-excesses and wider interfacial compositional widths at 256 h, when compared with the base Ni-Al-Cr alloy. Refractory elements are slow diffusers in nickel and the attractive R -Cr binding energies decelerate the solute diffusional fluxes, which results in a decrease of the Ni diffusivity, which in turn hinders the flux of Ni atoms away from the γ(f.c.c.)/γ′(L1 2 ) interfaces.

The effects of heat treatment on the shape memory characteristics of a polycrystalline Ni50.3Ti29.7Hf20 alloy were studied via thermal cycling under stress and isothermal stress cycling experiments in tension. It was revealed that transformation temperatures could be increased above 100 °C with aging at temperature above 500 °C and in particular were stabilized against stress-free thermal cycling after aging at 500 °C. Recoverable strain of ~ 5% was observed for the as-extruded samples and decreased to ~ 4% after aging due to the formation of non-transformable precipitates. The aged alloys demonstrated near perfect shape memory effect under tensile stresses as high as 700 MPa and perfect superelasticity at temperatures up to 230 °C. Finally, the tension-compression asymmetry observed in NiTiHf alloys was discussed.

The relationship between precipitate evolution, martensitic transformation temperatures, hardness, and functional load-bias behavior has been analyzed for a Ni50.8Ti34.2Zr15 (at.%) alloy. In the solutionized condition, the alloy was fully austenitic and no transformation (at least to −90 °C) was observed. Upon aging at 550 °C, the onset of a martensitic transformation (Ms), as determined by differential scanning calorimetry, was observed at approximately −10 °C and 75 °C after 24 and 300 h, respectively. Electron diffraction identified the precipitation of the orthorhombic H-phase within the B2 matrix. When the inter-precipitate spacing was ~12 nm, a greater undercooling was necessary to initiate the martensitic transformation due to overlapping strain fields of the precipitates. As the precipitates coarsened with aging time, a corresponding increase in the inter-precipitate spacing occurred and the chemical partitioning effects between the matrix and precipitate, as determined by atom probe tomography, began to dominate the transformation behavior resulting in an increase in transformation temperatures. For selected aging conditions, the load-biased shape memory behavior was determined under compressive and tensile loading using uniaxial constant-force thermal cycling experiments. A tension-compression asymmetry was noted with larger transformation strains in tension than compression at constant stresses up to 400 MPa. A recoverable transformation strain of 3% was observed in the sample aged for 4 h at 550 °C under a tensile stress of 400 MPa, which is the largest recoverable strain currently reported for a precipitation-strengthened NiTiZr alloy.

A "cubic Ni3Ti2" precipitate phase recently reported to exist within NiTi and NiTiNb alloys is documented to also form within a Ni56Ti36Hf8 alloy upon aging. Using transmission electron microscopy and ab intio density functional theory techniques, the phase is more precisely identified to be Ni16Ti11 with symmetry of the pm3m space group and a lattice parameter a = 8.816 A. In this structure, nickel atoms preferentially segregate to the perimeter lattice sites of the 54-atom unit cell.

The effect of Hf (0–1 at.%) additions in a Ni-Ti-Pd alloy on P-phase precipitation and martensitic transformations was studied. The addition of hafnium resulted in the refinement of precipitates with an increase in number density. The overlapping strain fields created due to the decrease in inter-precipitate spacing are suspected to reduce the matrix volume to be less than the critical free volume size needed for the martensitic transformation over the temperature range studied (183–573 K). Hafnium was also found to delay the aging time to achieve peak hardness, suggesting a reduction in growth and coarsening kinetics.

The temporal evolution of a Ni–10.0Al–8.5Cr–2.0Ru (at.%) alloy aged at 1073 K was investigated using transmission electron microscopy (TEM) and atom-probe tomography. The γ′(L12)-precipitate morphology is spheroidal through 256 h of ageing as a result of adding Ru, which decreases the lattice parameter misfit between the γ′(L12)- and γ(f.c.c.)-phases. The addition of Ru accelerates the compositional evolution of the γ′(L12)- and γ(f.c.c.)-phases, which achieve their equilibrium compositions after 0.25 h. Initially, Ru accelerates the partitioning of Ni and Cr to the γ(f.c.c.)-phase, and the partitioning of Al to the γ′(L12)-phase, but after 0.25 h, Ru, which partitions to the γ(f.c.c.)-phase, decreases the partitioning of Ni and increases the partitioning of Al and Cr. The temporal evolution of the average radius, ⟨R(t)⟩, number density, volume fraction of the γ′(L12)-precipitates, and the supersaturations of Ni, Al, Cr, and Ru in the γ(f.c.c.)- and γ′(L12)-phases are compared in detail with predictions o...

The microstructure and properties of a precipitation-hardenable Ni-48Ti-25Pd (at. pct) shape memory alloy have been investigated as a function of various aging conditions. Both the hardness and martensitic transformation temperatures increased with increasing aging time up to 100 hours at 673 K (400 °C), while no discernable differences were observed after heat treatment at 823 K (550 °C), except for a slight decrease in hardness. For aging at 673 K (400 °C), these effects were attributed to the formation of nano-scale precipitates, while precipitation was absent in the 823 K (550 °C) heat-treated specimens. The precipitation-strengthened alloy exhibited stable pseudoelastic behavior and load-biased-shape memory response with little or no residual strains. The precipitates had a monoclinic base-centered structure, which is the same structure as the P-phase recently reported in Ni(Pt)-rich NiTiPt alloys. 3D atom probe analysis revealed that the precipitates were slightly enriched in Ni and deficient in Pd and Ti as compared with the bulk alloy. The increase in martensitic transformation temperatures and the superior dimensional stability during shape memory and pseudoelastic testing are attributed to the fine precipitate phase and its effect on matrix chemistry, local stress state because of the coherent interface, and the ability to effectively strengthen the alloy against slip.

A stable Ni-rich Ni-29.7Ti-20Hf (at. pct) shape memory alloy, with relatively high transformation temperatures, was shown to exhibit promising properties at lower raw material cost when compared to typical NiTi-X (X = Pt, Pd, Au) high-temperature shape memory alloys (HTSMAs). The excellent dimensional stability and high work output for this alloy were attributed to a coherent, nanometer size precipitate phase observed using transmission electron microscopy. To establish an understanding of the role of these precipitates on the microstructure and ensuing stability of the NiTiHf alloy, a detailed study of the micromechanical and microstructural behaviors was performed. In-situ neutron diffraction at stress and temperature was used to obtain quantitative information on phase-specific internal strain, texture, and phase volume fractions during both isothermal and isobaric testing of the alloy. During isothermal testing, the alloy exhibited low isothermal strains due to limited detwinning, consistent with direct measurements of the bulk texture through neutron diffraction. This limited detwinning was attributed to the pinning of twin and variant boundaries by the dispersion of fine precipitates. During isobaric thermal cycling at 400 MPa, the high work output and near-perfect dimensional stability was attributed to the presence of the precipitates that act as homogeneous sources for the nucleation of martensite throughout the material, while providing resistance to irrecoverable processes such as plastic deformation.

The microstructure of 46Ti–37.5Ni–16.5Pd (at.%) alloy was investigated by electron diffraction and high-resolution scanning transmission electron microscopy. The phase content and stability were determined at several different temperatures and times. Aging at 400 °C for 1 h results in a new phase (P1-phase), which is consumed by P-phase at longer aging times. At 450 °C, the P1-phase appears first, and then coexists with P-phase. At 500 °C, the entire alloy transforms into the P1-phase. At 550 °C, Ti3(Ni,Pd)4 phase begins to form.

While NiTiPd alloys have been extensively studied for proposed use in high-temperature shape-memory applications, little is known about the shape-memory response of these materials under stress. Consequently, the isobaric thermal cyclic responses of five (Ni,Pd)49.5Ti50.5 alloys with constant stoichiometry and Pd contents ranging from 15 to 46 at. pct were investigated. From these tests, transformation temperatures, transformation strain (which is proportional to work output), and unrecovered strain per cycle (a measure of dimensional instability) were determined as a function of stress for each alloy. It was found that increasing the Pd content over this range resulted in a linear increase in transformation temperature, as expected. At a given stress level, work output decreased while the amount of unrecovered strain produced during each load-biased thermal cycle increased with increasing Pd content, during the initial thermal cycles. However, continued thermal cycling at constant stress resulted in a saturation of the work output and nearly eliminated further unrecovered strain under certain conditions, resulting in stable behavior amenable to many actuator applications.

The phase separation of a Ni–6.5 Al–9.5 Cr at. % alloy aged at 873 K was studied by atom-probe tomography and compared to the predictions of classical precipitation models. Phase separation in this alloy occurs in four distinct regimes: (i) quasi-stationary-state γ′(L12)-precipitate nucleation; (ii) concomitant precipitate nucleation, growth and coagulation and coalescence; (iii) concurrent growth and coarsening, wherein coarsening occurs via both γ′-precipitate coagulation and coalescence and by the classical evaporation–condensation mechanism; and (iv) quasi-stationary-state coarsening of γ′-precipitates, once the equilibrium volume fraction of precipitates is achieved. The predictions of classical nucleation and growth models are not validated experimentally, likely due to the complexity of the atomistic kinetic pathways involved in precipitation. During coarsening, the temporal evolution of the γ′-precipitate average radius, number density and the γ(fcc)-matrix and γ′-precipitate supersaturations foll...

Shape memory alloys (SMAs) have been used in various applications since their discovery. However, their use as actuation devices in high-temperature environments has been limited due to the temperature constraints of commercially available materials. Recently, SMAs that produce good work characteristics at elevated temperatures have been developed at NASA’s Glenn Research Center. One such alloy, Ni19.5Ti50.5Pd25Pt 5, has shown repeatable strain recovery on the order of 2.5% in the presence of an externally applied stress at temperatures greater than 250°C. Based on these findings, potential applications for this alloy are being explored and further work is being done to assess the use of this alloy in various structural forms. In this article, the characterization of Ni 19.5Ti50.5Pd25Pt5 helical actuators is reported, including their mechanical responses and how variations in their responses correlate to changes in geometric parameters and training loads. Finally, implementation of previously published SMA spring design methodology in future SMA helical actuator development is considered through comparison of the observed and predicted responses.

The effects of a 2.0 at.% addition of Ta to a model Ni-10.0Al-8.5Cr (at.%) superalloy aged at 1073 K are assessed using scanning electron microscopy and atom-probe tomography. The gamma'(Ll2)-precipitate morphology that develops as a result of gamma-(fcc)matrix phase decomposition is found to evolve from a bimodal distribution of spheroidal precipitates, to {001}-faceted cuboids and parallelepipeds aligned along the elastically soft {001}-type directions. The phase compositions and the widths of the gamma'-precipitate/gamma-matrix heterophase interfaces evolve temporally as the Ni-Al-Cr-Ta alloy undergoes quasi-stationary state coarsening after 1 h of aging. Tantalum is observed to partition preferentially to the gamma'-precipitate phase, and suppresses the mobility of Ni in the gamma-matrix sufficiently to cause an accumulation of Ni on the gamma-matrix side of the gamma'/gamma interface. Additionally, computational modeling, employing Thermo-Calc, Dictra and PrecipiCalc, is employed to elucidate the kinetic pathways that lead to phase decomposition in this concentrated Ni-Al-Cr-Ta alloy.

The temporal evolution of the nanostructure and chemistry of a model Ni–8.5 at.% Cr–10 at.% Al alloy with the addition of 2 at.% Re was studied using transmission electron microscopy and atom-probe tomography in order to measure the number density and mean radius of the γ ′ (L1 2 ) precipitates and the chemistry of the γ ′-precipitates and the γ (fcc)-matrix. In this article, the coarsening behavior of the γ ′-precipitates is discussed in detail and compared with the Umantsev–Olson model for multi-component alloys. In addition, the experimental results are evaluated with PrecipiCalc™ simulations. The results show that the diffusivities of the solute elements play a major role in the coarsening behavior of the γ ′-precipitates and that the addition of Re retards the coarsening kinetics and stabilizes the spheroidal morphology of the precipitates by reducing the interfacial energy.

The temporal evolution of the nanostructure and chemistry of a model Ni–8.5 at.% Cr–10 at.% Al alloy, with the addition of 2 at.% Re, aged at 1073 K from 0.25 to 264 h, was studied. Transmission electron microscopy and atom-probe tomography were used to measure the number density and mean radius of the γ ′ (L1 2 structure)-precipitates and the chemistry of the γ ′-precipitates and the γ (face-centered cubic)-matrix, including the partitioning behavior of all alloying elements between the γ - and γ ′-phases and the segregation behavior at γ / γ ′ interfaces. The precipitates remained spheroidal for an aging time of up to 264 h and, unlike commercial nickel-based superalloys containing Re, there was not confined (nonmonotonic) Re segregation at the γ / γ ′ interfaces.

For a Ni–5.2Al–14.2Cr at.% alloy with moderate solute supersaturations, the compositional pathways, as measured with atom-probe tomography, during early to later stage γ′(L12)-precipitation (R = 0.45–10 nm), aged at 873 K, are discussed in light of a multi-component coarsening model. Employing nondilute thermodynamics, detailed model analyses during quasi-stationary coarsening of the experimental data establish that the γ/γ′ interfacial free-energy is 22–23 ± 7 mJ m−2. Additionally, solute diffusivities are significantly slower than model estimates. Strong quantitative evidence indicates that an observed γ′-supersaturation of Al results from the Gibbs–Thomson effect, providing the first experimental verification of this phenomenon. The Gibbs–Thomson relationship for a ternary system, as well as differences in measured phase equilibria with CALPHAD assessments, are considered in great detail.

In a Ni-5.2 Al-14.2 Cr at.% alloy with moderate solute supersaturations and a very small gamma/gamma prime lattice parameter misfit, the nanostructural and compositional pathways during gamma prime(L12) precipitation at 873 K are investigated using atom-probe tomography, conventional transmission electron microscopy, and hardness measurements. Nucleation of high number densities (N(sub v) greater than 10(sup 23) per cubic meters) of solute-rich precipitates (mean radius = [R] = 0.75 nm), with a critical nucleus composition of Ni-18.3 plus or minus 0.9 Al-9.3 plus or minus 0.7 Cr at.%, initiates between 0.0833 and 0.167 h. With increasing aging time (a) the solute concentrations decay in spheroidal precipitates ([R] less than 10 nm); (b) the observed early-stage coalescence peaks at maximum N(sub v) in coincidence with the smallest interprecipitate spacing; and (c) the reaction enters a quasi-stationary regime where growth and coarsening operate concomitantly. During this quasi-stationary regime, the c (face-centered cubic)-matrix solute supersaturations decay with a power-law dependence of about -1/3, while the dependencies of [R] and N(sub v) are 0.29 plus or minus 0.05 and -0.64 plus or minus 0.06 at a coarsening rate slower than model predications. Coarsening models allow both equilibrium phase compositions to be determined from the compositional measurements. The observed early-stage coalescence is discussed in further detail.

The temporal evolution of γ'(L1 2 )-precipitates in Ni-5.2 Al-14.2 Cr at.% and Ni-9.5 Al-8.6 Cr-2.0 Re at.% alloys are studied utilizing three-dimensional atom-probe microscopy. For the times investigated (0.0833 to 1024 h), these alloys exhibit a high number density (10 2 2 -10 2 4 m - 3 ) of spheroidal y'-precipitates, 0.45-30 nm in radius. For the Ni 5.2 Al-14.2 Cr at.% alloy, nucleation, growth, and coarsening of the y'-precipitates is observed during the isothermal transformation at 873 K. Precipitate coalescence is present, and its influence on the power-law dependencies of the number density and mean radius during coarsening is discussed and compared to classical models. For Ni-9.5 Al-8.6 Cr-2.0 Re at.%, phase separation occurs during the initial quench leading to a highly interconnected '-nanostructure. The temporal evolution of the elemental partitioning behavior at 1073 K between the γ-matrix and y'-precipitates is studied in detail.

An analysis of the site subsitution behavior of Pd in NiTi was performed using the BFS method for alloys. Through a combination of Monte Carlo simulations and detailed atom-by-atom energetic analyses of various computational cells, representing compositions of NiTi with up to 10 at% Pd, a detailed understanding of site occupancy of Pd in NiTi was revealed. Pd subsituted at the expense of Ni in a NiTi alloy will prefer the Ni-sites. Pd subsituted at the expense of Ti shows a very weak preference for Ti-sites that diminishes as the amount of Pd in the alloy increases and as the temperature increases.

The influence of W on the temporal evolution of γ′ precipitation toward equilibrium in a model Ni-Al-Cr alloy is investigated by three-dimensional atom-probe (3DAP) microscopy and transmission electron microscopy (TEM). We report on the alloys Ni-10 Al-8.5 Cr (at.%) and Ni-10 Al-8.5 Cr-2 W (at.%), which were aged isothermally in the γ + γ′ two-phase field at 1073 K, for times ranging from 0.25 to 264 h. Spheroidal-shaped γ′ precipitates, 5–15 nm diameter, form during quenching from above the solvus temperature in both alloys at a high number density (∼1023m−3). As γ′ precipitates grow with aging at 1073 K, a transition from spheriodal- to cuboidal-shaped precipitates is observed in both alloys. The elemental partitioning and spatially resolved concentration profiles across the γ′ precipitates are obtained as a function of aging time from three-dimensional atom-by-atom reconstructions. Proximity histogram concentration profiles (Hellman et al., 2000) of the quaternary alloy demonstrate that W concentration gradients exist in γ′ precipitates in the as-quenched and 0.25-h aging states, which disappear after 1 h of aging. The diffusion coefficient of W in γ′ is estimated to be 6.2 × 10−20m2s−1at 1073 K. The W addition decreases the coarsening rate constant, and leads to stronger partitioning of Al to γ′ and Cr to γ.

The proximity histogram (or proxigram for short) is used for analyzing data collected by a three-dimensional atom probe microscope. The interfacial excess of Re (2.41 +/- 0.68 atoms/sq nm) is calculated by employing a proxigram in a completely geometrically independent way for gamma/gamma' interfaces in Rene N6, a third-generation single-crystal Ni-based superalloy. A possible dependence of interfacial excess on the variation of the threshold value of an isoconcentration surface is investigated using the data collected for Rene N6 alloy. It is demonstrated that the dependence of the interfacial excess value on the threshold value of the isoconcentration surface is weak.

Ideally, computational modelling techniques for nanoscopic physics would be able to perform free of limitations on the type and number of elements, while providing comparable accuracy when dealing with bulk or surface problems. Computational efficiency is also desirable, if not mandatory, for properly dealing with the complexity of typical nanostructured systems. A quantum approximate technique, the Bozzolo–Ferrante–Smith method for alloys, which attempts to meet these demands, is introduced for calculation of the energetics of nanostructures. The versatility of the technique is demonstrated through analysis of diverse systems, including multiphase precipitation in a five-element Ni–Al–Ti–Cr–Cu alloy and the formation of mixed composition Co–Cu islands on a metallic Cu(111) substrate.

Atom-probe field-ion microscopy (APFIM) is used to study partitioning of the alloying elements between the γ (FCC) and γ′ (L12) phases and their segregation behavior at γ/γ′ interfaces of a Rene N6 nickel-based superalloy. The atomic-scale resolution and real space reconstruction capability for elemental chemical mapping makes three-dimensional atom-probe microscopy especially suitable for subnanoscale investigations of complex multicomponent superalloys. Concentration profiles of this alloy, obtained from an atom probe analysis, reveal the partitioning behavior of the alloying elements in Rene N6. As anticipated, the matrix strengtheners, such as Mo and W, are partitioned to the γ (FCC) matrix, while Re segregates at the γ/γ′ interfaces; the Gibbsian interfacial excess of Re is determined by both one-dimensional (2.32 atoms nm−2) and three-dimensional atom-probe microscopies (3.92 atoms nm−2) and the values obtained are in reasonable agreement.

Three NiAl-based alloys containing 3Nb–10Mo, 5Nb–10Mo or 13.6Nb–18Mo (at%) were directionally solidified to develop three dimensional Mo-based dendrite networks. Examination of the alloys indicated that the desired chemistry was achieved for the 3Nb–10Mo and 5Nb–10Mo versions but the composition of the highly alloyed ingot was NiAl–14.6Nb–13.2Mo. The as-grown structure for all three materials consisted of three major phases: essentially unalloyed B2 crystal structure NiAl, Laves NiAlNb phase alloyed with ∼8.5 Mo, and a bcc metallic Mo solid solution containing 27Nb–7Ni–7Al. Compressive properties were measured between 1200 to 1400 K in air under constant velocity and constant load creep conditions with strain rates ranging from ∼10−4 to ∼10−8 s−1. The flow strengths of the two alloys with 10Mo were nearly identical and much weaker than those for NiAl–14.6Nb–13.2Mo under all conditions. Comparison the properties of this latter alloy with other directionally solidified NiAl-based eutectics revealed that it was the strongest material under lower temperature/fast deformation conditions, but this advantage was lost at higher temperatures and/or slower strain rates.

The study of surface segregation, although of great technological importance, has been largely restricted to experimental work due to limitations associated with theoretical methods. However, recent improvements in both first-particle and semi-empirical methods are opening, the doors to an array of new possibilities for surface scientists. We apply one of these techniques, the Bozzolo, Ferrante and Smith (BFS) method for alloys, which is particularly suitable for complex systems, to several aspects of the computational modeling of surfaces and segregation, including alloy surface segregation, structure and composition of alloy surfaces, and the formation of surface alloys. We conclude with the study of complex NiAl-based binary, ternary and quaternary thin films (with Ti, Cr and Cu additions to NiAl). Differences and similarities between bulk and surface compositions are discussed, illustrated by the results of Monte Carlo simulations. For some binary and ternary cases, the theoretical predictions are compared to experimental results, highlighting the accuracy and value of this developing theoretical tool.

We introduce the Bozzolo-Ferrante-Smith (BFS) method for alloys as a computationally efficient tool for aiding in the process of alloy design. An intuitive description of the BFS method is provided, followed by a formal discussion of its implementation. The method is applied to the study of the defect structure of NiAl binary alloys. The groundwork is laid for a detailed progression to higher order NiAl-based alloys linking theoretical calculations and computer simulations based on the BFS method and experimental work validating each step of the alloy design process.

The Bozzolo-Ferrante-Smith (BFS) method for alloy energetics is applied to the study of ternary additions to NiAl. A description of the method and its application to alloy design is given. Two different approaches are used in the analysis of the effect of Ti additions to NiAl. First, a thorough analytical study is performed, where the energy of formation, lattice parameter and bulk modulus are calculated for a large number of possible atomic distributions of Ni, Al and Ti. Substitutional site preference schemes and formation of precipitates are thus predicted and analyzed. The second approach used consists of the determination of temperature effects on the final results, as obtained by performing a number of large scale numerical simulations using the Monte Carlo-Metropolis procedure and BFS for the calculation of the energy at every step in the simulation. The results indicate a sharp preference of Ti for Al sites in Ni-rich NiAl alloys and the formation of ternary Heusler precipitates beyond the predicted solubility limit of 5 at. % Ti. Experimental analysis of three Ni-Al-Ti alloys confirms the theoretical predictions.

The effect of a 0.1 at. % alloying addition of Zr on the low-cycle fatigue behavior of polycrystalline NiAl was determined at 1000 K and compared to that of binary NiAl. Samples of binary NiAl and the Zr-doped alloy were processed by either HIP consolidation or extrusion of prealloyed intermetallic powders. The cyclic stress response, cyclic stress-strain behavior, and strain-life relationships were all significantly influenced by the microalloying addition of Zr, regardless of the processing technique. A detailed examination of the post-tested low-cycle fatigue (LCF) samples was conducted by optical and electron microscopy to determine variations in fracture and deformation modes and to characterize any microstructural changes that occurred during LCF testing. Differences in LCF behavior due to the Zr addition are attributed to the strong effect that Zr has on modifying the deformation behavior of the intermetallic.

In spite of the advance in theoretical modelling of surfaces, there are a limited number of investigations on the characteristics of multicomponent alloy surfaces. Moreover, the corresponding experimental work does not compare in scope and depth with the work done on binary alloy surfaces. Recent developments in computational modelling using semiempirical methods both in the fields of alloy design as well as surface analysis, hint to the possibility of exploiting the predictive power of these methods towards a comprehensive analysis of multicomponent alloy surfaces. In this work, we apply the BFS (Bozzolo-Ferrante-Smith) method for alloys to determine the essential features of multicomponent alloy surfaces. Several issues will be illustrated, ranging from the influence of ternary additions to the surface composition of (110) surfaces of binary alloys (Ti or Cr additions to NiAl), the composition and energetics of ternary (NiAlCr and NiAlTi alloys) alloy surfaces and quaternary (NiAlTiCr and NiAlTiCu) alloy surfaces.

Systematic characterization of the shape memory properties of a quaternary Ni-45.3-Ti-29.7 Hf-20-Pd-5 (at.%) polycrystalline alloy was performed in compression after selected aging treatments. Precipitation characteristics were revealed by transmission electron microscopy. The effects of aging temperature and time on transformation temperatures, recoverable and residual strains, and temperature and stress hystereses were determined by differential scanning calorimetry, constant-load thermal cycling experiments and isothermal strain cycling (superelasticity) tests. The crystal structure and lattice parameters of the transforming phases were determined from X-ray diffraction analysis. It was revealed that precipitation hardening significantly improved the shape memory properties of the NiTiHfPd alloy. Under optimum aging conditions, shape memory strains of up to 4% under 1 GPa were possible, and superelasticity experiments resulted in full strain recovery without any plastic deformation, even at stress levels as high as 2 GPa. The NiTiHfPd polycrystalline alloy exhibited very high damping capacity/absorbed energy (30-34 J cm(-3)) and work output (30-35 J cm(-3)), which were attributed to the ability to operate at high stress levels without significant plastic deformation and to a high mechanical hysteresis (>900 MPa) at temperatures ranging from 20 degrees C to 80 degrees C. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Shape memory properties of a Ni50.3Ti29.7Hf20 (at.%) polycrystalline alloy were characterized after selected heat treatments. The effects of heat treatment temperature and time on the transformation temperatures (TTs) and temperature hysteresis were determined by differential scanning calorimetry. Thermal cycling under constant compressive stress was carried out to reveal the changes in transformation strain, temperature hysteresis, and TT as a function of stress. Isothermal stress cycling experiments were conducted to reveal the critical stresses, transformation strain, and stress hysteresis as a function of temperature. The crystal structure and lattice parameters of the transforming phases were determined by X-ray diffraction at selected temperatures. Precipitate characteristics and martensite morphology were revealed by transmission electron microscopy. Precipitation was found to alter the martensite morphology and significantly improve the shape memory properties of the Ni-rich NiTiHf alloy. For the peak aged condition shape memory strains of up to 3.6%, the lowest hysteresis, and a fully reversible superelastic response were observed at temperatures up to 240 °C. In general, the nickel-rich NiTiHf polycrystalline alloy exhibited a higher work output (≈16.5 J cm−3) than other NiTi-based high temperature alloys.

Major advances in developing the next generation high-temperature, high pressure turbine blades and vanes will be accomplished by coordinated efforts involving innovative component design and advanced materials and processing. Furthermore, the greatest gains in performance will arise through revolutionary changes in materials as opposed to evolutionary and incremental improvements in the properties of current materials. The primary objectives are to reduce blade cooling flow while increasing use temperature and at the same time reduce overall engine weight. This will be accomplished through novel component designs and the replacement of Ni-based superalloy with ordered intermetallic NiAl alloys. However, the current materials development approach, no matter how successful, has been strictly empirical. The advancement and implementation of theoretical alloy design procedures would significantly decrease the time and expense for screening various alloy compositions. Consequently, with this much larger goal in mind, this paper contributes to the improvement and testing of one such analytical technique and at the same time provides additional insight to the structure of ordered alloys. The authors apply the recently developed BFS semiempirical method for alloys to model non-stoichiometric {beta}-NiAl at zero temperature, in order to provide theoretical support to the current understanding of the defect arrangement in suchmore » alloys. A more ambitious project, that of the study of ternary additions to Ni-Al alloys and their effect on properties, could only be attempted once the basic Ni-Al system is adequately described by this theoretical tool in light of the current experimental evidence.« less

Shape memory properties and microstructure of four Ni-rich NiTiHf alloys (Ni50.3Ti29.7Hf20, Ni50.7Ti29.3Hf20, Ni51.2Ti28.8Hf20, and Ni52Ti28Hf20 (at.%)) were systematically characterized in the furnace cooled condition. H-phase precipitates were formed during furnace cooling in compositions with greater than 50.3Ni and the driving force for nucleation increased with Ni content. Alloy strength increased while recoverable strain decreased with increasing Ni content due to changes in precipitate characteristics. When the precipitates were small (~5–15 nm), they were readily absorbed by martensite plates, which resulted in maximum recoverable strain of 2% in Ni50.7Ti29.3Hf20. With increasing Ni content, the size (>100 nm) and volume fraction of precipitates increased and the growth of martensite plates was constrained between the precipitates when the Ni concentration was greater than 50.7 at.%. Near perfect dimensional stability with negligible irrecoverable strain was observed at stress levels as high as 2 GPa in the Ni52Ti28Hf20 alloy, though the recoverable strain was rather small. In general, strong local stress fields were created at precipitate/matrix interphases, which lead to high stored elastic energy during the martensitic transformation.

The yield stress, σy, strain to fracture, ef, and thickness-compensated fracture load of monocrystalline NiAl were investigated as functions of purity, annealing treatment and cooling rates. A miniaturized disk bend test was employed to test disks 3 mm in diameter and approximately 250 μm thick. Specimens in the soft (110) orientation from commercial purity (CP) and high-purity (HP) alloys were tested. The annealing treatments affected σy more strongly than the other mechanical properties. The yield stresses of both of the as-received alloys increased significantly after annealing at 1300 °C followed by furnace cooling. Subsequent annealing at 400 °C for 2 h resulted in a reduction of σy. This behavior is attributed to the role of excess vacancies retained during cooling, which annealed out at the lower temperature. The mean values of the yield strength also tended to decrease with increasing cooling rate from 400 °C, but the effect was small. A substantial increase in σy of the CP alloy was found on prolonged aging at 400 °C, whereas σy of the HP alloy was unaffected. We attribute this behavior to the precipitation of very small particles or solute atom clusters in the CP alloy. The ductility of the CP alloy, and the thickness-compensated fracture loads of both alloys, were relatively insensitive to the heat treatments.

Three B2 structure aluminide sheet materials produced by hot-rolling of alloys from compacted powders were evaluated for crystallographic texture. Two essentially binary NiAl alloys showed strong components in the plane of the sheet. A NiFeAl alloy had dramatically different texture which consisted of a strong components in the plane of the sheet. In addition to microstructures, measured pole figures and the orientation distribution function are reported and compared with textures produced in similar materials via extrusion.

In the work to be presented, vacuum plasma spray forming has been used as a process to deposit and consolidate prealloyed NiTi and NiTiPd powders into near net shape actuators. Testing showed that excellent shape memory behavior could be developed in the deposited materials and the investigation proved that VPS forming could be a means to directly form a wide range of shape memory alloy components. The results of DSC characterization and actual actuation test results will be presented demonstrating the behavior of a Nitinol 55 alloy and a higher transition temperature NiTiPd alloy in the form of torque tube actuators that could be used in aircraft and aerospace controls.

Potential applications involving high-temperature shape memory alloys have been growing in recent years. Even in those cases where promising new alloys have been identified, the knowledge base for such materials contains gaps crucial to their maturation and implementation in actuator and other applications. We begin to address this issue by characterizing the mechanical behavior of a Ni 19.5 Pd 30 Ti 50.5 high-temperature shape memory alloy in both uniaxial tension and compression at various temperatures. Differences in the isothermal uniaxial deformation behavior were most notable at test temperatures below the martensite finish temperature. The elastic modulus of the material was very dependent on strain level; therefore, dynamic Young's Modulus was determined as a function of temperature by an impulse excitation technique. More importantly, the performance of a thermally activated actuator material is dependent on the work output of the alloy. Consequently, the strain-temperature response of the Ni 19.5 Pd 30 Ti 50.5 alloy under various loads was determined in both tension and compression and the specific work output calculated and compared in both loading conditions. It was found that the transformation strain and thus, the specific work output were similar regardless of the loading condition. Also, in both tension and compression, the strain-temperature loops determined under constant load conditions did not close due to the fact that the transformation strain during cooling was always larger than the transformation strain during heating. This was apparently the result of permanent plastic deformation of the martensite phase with each cycle. Consequently, before this alloy can be used under cyclic actuation conditions, modification of the microstructure or composition would be required to increase the resistance of the alloy to plastic deformation by slip.

Pt additions substituted for Ni in NiTi alloys are known to increase the transformation temperature of the alloy but only at fairly high Pt levels. However, until now only ternary compositions with a very specific stoichiometry, Ni50-xPtxTi50, have been investigated and then only to very limited extent. In order to learn about this potential high-temperature shape memory alloy system, a series of over twenty alloys along and on either side of a line of constant stoichiometry between NiTi and TiPt were arc melted, homogenized, and characterized in terms of their microstructure, transformation temperatures, and hardness. The resulting microstructures were examined by scanning electron microscopy and the phase compositions quantified by energy dispersive spectroscopy."Stoichiometric" compositions along a line of constant stoichiometry between NiTi to TiPt were essentially single phase but by any deviations from a stoichiometry of (Ni,Pt)50Ti50 resulted in the presence of at least two different intermetallic phases, depending on the overall composition of the alloy. Essentially all alloys, whether single or two-phase, still under went a martensitic transformation. It was found that the transformation temperatures were depressed with initial Pt additions but at levels greater than 10 at.% the transformation temperature increased linearly with Pt content. Also, the transformation temperatures were relatively insensitive to alloy stoichiometry within the range of alloys examined. Finally, the dependence of hardness on Pt content for a series of Ni50-xPtxTi50 alloys showed solution softening at low Pt levels, while hardening was observed in ternary alloys containing more than about 10 at.% Pt. On either side of these "stoichiometric" compositions, hardness was also found to increase significantly.

The microstructure, transformation temperatures, basic tensile properties, shape memory behavior, and work output for two (Ni,Ti)Pt high-temperature shape memory alloys have been characterized. One was a Ni30Pt20Ti50 alloy (referred to as 20Pt) with transformation temperatures above 230 C and the other was a Ni20Pt30Ti50 alloy (30Pt) with transformation temperatures about 530 C. Both materials displayed shape memory behavior and were capable of 100% (no-load) strain recovery for strain levels up to their fracture limit (3-4%) when deformed at room temperature. For the 20Pt alloy, the tensile strength, modulus, and ductility dramatically increased when the material was tested just about the austenite finish (A(sub f)) temperature. For the 30Pt alloy, a similar change in yield behavior at temperatures above the A(sub f) was not observed. In this case the strength of the austentite phase was at best comparable and generally much weaker than the martensite phase. A ductility minimum was also observed just below the A(sub s) temperature in this alloy. As a result of these differences in tensile behavior, the two alloys performed completely different when thermally cycled under constant load. The 20Pt alloy behaved similar to conventional binary NiTi alloys with work output due to the martensite-to-austenite transformation initially increasing with applied stress. The maximum work output measured in the 20Pt alloy was nearly 9 J/cu cm and was limited by the tensile ductility of the material. In contrast, the martensite-to-austenite transformation in the 30Pt alloy was not capable of performing work against any bias load. The reason for this behavior was traced back to its basic mechanical properties, where the yield strength of the austenite phase was similar to or lower than that of the martensite phase, depending on temperature. Hence, the recovery or transformation strain for the 30Pt alloy under load was essentially zero, resulting in zero work output.

The chemical pathways leading to gamma-prime(L1_2)-nucleation from nondilute Ni-5.2 Al-14.2 Cr at.%, gamma(f.c.c.), at 873 K are followed with radial distribution functions and isoconcentration surface analyses of direct-space atom-probe tomographic images. Although Cr atoms initially are randomly distributed, a distribution of congruent Ni3Al short-range order domains (SRO), =0.6 nm, results from Al diffusion during quenching. Domain site occupancy develops as their number density increases leading to Al-rich phase separation by gamma-prime-nucleation, =0.75 nm, after SRO occurs., Comment: 5 pages, 4 figures