Mechanical Behavior of Three-Dimensional Braided Nickel-Based Superalloys Synthesized via Pack Cementation

Braided tubes of Ni-based superalloys are fabricated via three-dimensional (3-D) braiding of ductile Ni-20Cr (wt pct) wires followed by post-textile gas-phase alloying with Al and Ti to create, after homogenization and aging, γ/γ′ strengthened lightweight, porous structures. Tensile tests reveal an increase in strength by 100 MPa compared to as-braided Ni-20Cr (wt pct). An interrupted tensile test, combined with X-ray tomographic scans between each step, sheds light on the failure behavior of the braided superalloy tubes

Experimental and Modeling Study of Compressive Creep In 3D-Woven Ni-Based Superalloys

Micro-architectured Ni-based superalloy structures, with Ni-20Cr-3Ti-2Al (wt.%) composition and γ/γ′-microstructure, are created by a multi-step process: (i) non-crimp orthogonal 3D-weaving of ductile, 202 μm diameter Ni-20%Cr wires, (ii) gas-phase alloying with Al and Ti, (iii) simultaneous transient-liquid phase (TLP) bonding between wires and homogenization within wires via interdiffusion, (iv) solutionizing to create a single-phase solid solution, and (v) aging to precipitate the γ′ phase. The creep behavior of these 3D-woven γ/γ′ nickel-based superalloys is studied under uniaxial compression via experiments at 825 °C and via finite element (FE) analysis, using a 3D model of the woven structures obtained through X-ray micro-tomography. The creep strain rate for the woven Ni-based superalloy is higher than that for the bulk superalloy due to the lower solid volume fraction of the woven structure, while the creep exponents are identical. The compressive creep behavior is sensitive to the geometry of the woven structures: fewer wires perpendicular to the load and fewer bonds between wires cause lower creep resistance of the woven structure, due to a reduction in load transfer from the longitudinal wires (which are primarily load-bearing) and the perpendicular wires. Creep buckling of longitudinal wires drastically reduces creep resistance of the woven structure, confirming the importance of maintaining longitudinal wires vertical and parallel to the uniaxial compression direction. Finally, reducing wire cross-section, e.g., via oxidation, reduces creep resistance. The oxidation kinetics of the wire structures at 750, 825, and 900 °C displayed parabolic rate constants comparable to commercial Ni-based superalloys, but indicates that up to 35% of the wire cross-section is oxidized after 7 days at 825 °C, such that oxidation-resistant coatings are needed for long-term use in oxidative environment

Effects of Nb and Ta additions on the strength and coarsening resistance of precipitation-strengthened Al-Zr-Sc-Er-Si alloys

A dilute Al-0.07Zr-0.02Sc-0.005Er-0.06Si (at.%) alloy was microalloyed with 0.08 at.% Nb or Ta. Atom-probe tomography reveals that, upon aging, Nb and Ta partition to the coherent L12-Al3(Zr,Sc,Er) nanoprecipitates (with average concentrations of 0.2 and 0.08 at.%, respectively), with both segregating at the matrix/nanoprecipitate heterophase interface. This is consistent with the Nb- and Ta-modified alloys exhibiting, as compared to the unmodified alloy: (i) higher peak microhardness, from a higher nanoprecipitate volume fraction and/or lattice parameter mismatch; and (ii) improved aging resistance, from slower nanoprecipitate coarsening due to the small diffusivities of niobium and tantalum in aluminum. Analogous results were previously reported for a V-modified alloy

Effect of Diffusion Distance on Evolution of Kirkendall Pores in Titanium-Coated Nickel Wires

Microtubes of near-equiatomic nickel-titanium (NiTi) alloys can be created via the Kirkendall effect during NiTi interdiffusion, when nickel wires are surface-coated with titanium via pack cementation and subsequently homogenized. This study explores the effect of diffusion distance upon Kirkendall microtube formation in NiTi by considering a range of Ni wire diameters. For Ni wire diameters of 25, 50 and 100 μm, titanized at 925 °C for 0.5, 2, and 8 h to achieve average NiTi composition, partial interdiffusion occurs concurrently with Ti surface deposition, resulting in concentric shells of NiTi2, NiTi and Ni3Ti around a Ni core, with some Kirkendall porosity created within the wires. Upon subsequent homogenization at 925 °C, near-single-phase NiTi wires are created and the Kirkendall porosity increases, leading to a variety of pore/channel structures: (i) for 25 μm Ni wires where diffusion distances and times are short, a high volume fraction of micropores is created near the final NiTi wire surface, with 1–2 larger pores near its core; (ii) for 50 μm Ni wires, a single, ∼20 μm diameter pore is created near the NiTi wire center, transforming the wires into microtubes, and; (iii) for 100 μm Ni wires, a ∼50 μm diameter irregular pore is formed near the NiTi wire center, along with an eccentric crescent-shaped pore of similar cross-section, resulting from interruption of a single diffusion path, due to the longer diffusion distances and times

Structure and Growth of Core–shell Nanoprecipitates in Al–Er–Sc–Zr–V–Si High-temperature Alloys

Lightweight Sc-containing aluminum alloys exhibit superior mechanical performance at high temperatures due to core–shell, L12-ordered trialuminide nanoprecipitates. In this study, the structure of these nanoprecipitates was studied, using different transmission electron microscopy (TEM) techniques, for an Al–Er– Sc–Zr–V–Si alloy that was subjected to a two-stage overaging heat treatment. Energy-dispersive X-ray spectroscopy of the spherical Al3(Sc, Zr, Er ,V) nanoprecipitates revealed a core–shell structure with an Sc- and Er-enriched core and a Zr-enriched shell, without a clear V outer shell. This structure is stable up to 72% of the absolute melting temperature of Al for extended periods of time. High-angle annular dark-field scanning TEM was used to image the {100} planes of the nanoprecipitates, demonstrating a homogeneous L12-ordered superlattice structure for the entire nanoprecipitates, despite the variations in the concentrations of solute atoms within the unit cells. A possible growth path and compositional trajectory for these nanoprecipitates was proposed using high-resolution TEM observations, where different rod-like structural defects were detected, which are considered to be precursors to the spherical L12-ordered nanoprecipitates. It is also hypothesized that the structural defects could consist of segregated Si; however, this was not possible to verify with HAADF-STEM because of the small differences in Al and Si atomic numbers. The results herein allow a better understanding of how the Al–Sc alloys’ core–shell nanoprecipitates form and evolve temporally, thereby providing a better physical picture for future atomistic structural mappings and simulations

Processing and Characterization of Liquid-Phase Sintered NiTi Woven Structures

Porous NiTi is of interest for bone implants because of its unique combination of biocompatibility (encouraging osseointegration), high strength (to prevent fracture), low stiffness (to reduce stress shielding), and shape memory or superelasticity (to deploy an implant). A promising method for creating NiTi structures with regular open channels is via 3D weaving of NiTi wires. This paper presents a processing method to bond woven NiTi wire structures at contact points between wires to achieve structural integrity: (i) a slurry consisting of a blend of NiTi and Nb powders is deposited on the surface of the NiTi wires after the weaving operation; (ii) the powders are melted to create a eutectic liquid phase which collects at contact points; and (iii) the liquid is solidified and binds the NiTi woven structures. The bonded NiTi wire structures exhibited lower transformation temperatures compared to the as-woven NiTi wires because of Nb diffusion into the NiTi wires. A bonded woven sample was deformed in bending and showed near-complete recovery up to 6% strain and recovered nearly half of the deformation up to 19% strain

Equal Channel Angular Pressing of a Newly Developed Precipitation Hardenable Scandium Containing Aluminum Alloy

Precipitation hardenable aluminum alloys are well-known for their high strength-to-weight ratio, good thermal stability, electrical conductivity, and low cost. Equal channel angular pressing (ECAP) is proven to further improve the mechanical properties of metallic alloys through microstructure modification. In this work, ECAP of a recently developed, precipitation hardenable, cast Al–Er–Sc–Zr–V–Si alloy in peak-aged condition by route 4Bc was carried out to create an alloy with ultra-fine grain structure. The combined effect of grain refinement and precipitation on the tensile behavior and thermal stability of the ECAPed alloy is reported here. Improvement in yield strength and lack of strain hardening in ECAPed alloy were as expected. Microhardness contour plots with a narrower spread indicated enhancement in microstructural homogeneity after four ECAP passes as compared to the peak-aged condition. The variations in microhardness after annealing heat treatments at different temperatures highlighted the important role precipitates play in maintaining microstructure stability up to 250 °C in the ECAPed material

Fabrication, characterization and applications of novel nanoheater structures

Nanoheaters are reactive nanostructures that can generate localized heat through controlled ignition. Besides the widely used nanofoil structure with multiple alternative Al-Ni layers, various new nanostructures have been fabricated in the last several years, including consolidated films, bimetallic nanoparticles and nanowires, and ball milled micro/nano powders. In this paper, we demonstrate the (1) Synthesis of Al-Ni bimetallic nanoparticles by a galvanic replacement reaction method using Al nanoparticle templates; (2) Fabrication of Al-Ni nanowire structures by a two-step process involving electrodeposition and thermal evaporation; (3) Fabrication of Al-Ni composites by a novel ultrasonic powder consolidation method, using Al and Ni nanoparticles as source materials and (4) Synthesis of nanostructured Al-Ni powders by low energy ball milling with microscale Al and Ni powders. The structure and compositions of the nanoheater structures have been characterized by scanning electron microscopy and transmission electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction. The thermal characteristics of the samples were studied using differential scanning calorimetry. These novel nanoheater structures have great potential to be used in micro-joining, microelectronics assembly, and flexible electronics bonding. © 2012 Elsevier B.V