Jaimie Tiley

Processing, microstructure, and properties of β titanium alloys modified with boron

The development of next-generation βTi alloys is expected to involve very attractive combinations of strength-toughness-fatigue resistance at large cross sections, improved and affordable thermomechanical processing, and enhanced elevated temperature capability. This article describes the development of βTi alloys that are modified with small boron (B) additions to achieve these goals. Two important aerospace alloys, Ti-15Mo-2.6Nb-3Al-0.2Si and Ti-5Al-5V-5Mo-3Cr microalloyed (0.1%) with B were considered. Ingots that were 70 mm in diameter and 500 mm in length were cast using induction skull melting. A detailed microstructural characterization and tensile property evaluation were conducted. Microalloying with B refines the cast grain size to about 50 µm, which enhances strength and ductility. The effect of B additions on the microstructural stability and properties in the as-cast condition was established. The implications of B additions on the microstructural evolution and affordability of subsequent processing is also discussed.

A novel method for acquiring large‐scale automated scanning electron microscope data

Recent software and hardware advances in the field of electron backscatter diffraction have led to an increase in the rate of data acquisition. Combining automated stage movements with conventional beam control have allowed researchers to collect data from significantly larger areas of samples than was previously possible. This paper describes a LabVIEW™ and AutoIT© code which allows for increased flexibility compared to commercially available software. The source code for this software has been made available in the online version of this paper.

Novel automatic electrochemical–mechanical polishing (ECMP) of metals for scanning electron microscopy

A low-stress automated polishing device was developed for preparing titanium and nickel alloys for scanning electron microscopy imaging. The system used pulsed electrochemical reactions within an alkaline electrolyte to generate a thin passivation layer on the surface of the sample, which was removed by the mechanical vibration of the system. The passivation layer development and removal were documented for Ti-6Al-4V and IN718 samples subjected to varying electrical potential cycles and polishing times. Results indicated that the applied cyclic potentials removed material faster than typical removal techniques. In addition, electron back scatter diffraction data showed a decrease in subsurface damage using the developed electrochemical-mechanical process compared to standard mechanical polishing techniques.

Site preference and interaction energies of Co and Cr in gamma prime Ni3Al: a first-principles study

Nickel-based superalloys are critical for aerospace and power applications due to excellent high-temperature properties. These high-temperature properties are attributed to the coherently precipitated gamma prime phase in the gamma matrix. The segregation of alloying elements between the matrix and the gamma prime phase drives precipitate misfit strains and impacts material strength. This study aims at understanding the site preference of Co and Cr within the ordered gamma prime phase. The study also calculates the interaction energy between alloying additions within the ternary systems Ni–Al–Cr and Ni–Al–Co, and the quaternary system Ni–Al–Cr–Co. It is found that Co has mixed substitution behavior between the Al and Ni sites in the gamma prime phase. The results from the Ni–Al–Cr ternary system show that two Cr atoms prefer being close to each other, with the most stable configuration of the first nearest neighbors of the Al–Al site. The interaction energies calculated from the Ni–Al–Co system show that the initial distance between two Co atoms will decide whether the two Co atoms prefer Ni–Ni or Ni–Al configuration. The study on the quaternary system Ni–Al–Cr–Co reveals that the initial configuration of Cr and Co atoms will affect the final most stable configuration. The results are found to be consistent with our previous findings.

Site occupancy of chromium in the γ′-Ni3Al phase of nickel-based superalloys: a combined 3D atom probe and first-principles study

Transition-metal dopants play a critical role in the high-temperature mechanical strength and corrosion resistance of nickel-based superalloys. In this article, the site occupancy behavior of chromium in γ′-Ni3Al has been investigated by combining three-dimensional (3D) atom probe and high-resolution transmission electron microscopy characterizations with ab initio density functional theory (DFT) calculations. The 3D atom probe data show a clear preference of chromium on the aluminum sublattice over the nickel sublattice in Rene88 super alloys. First-principles DFT total-energy calculations were performed to understand the site occupancy of chromium in the L12 structured γ-Ni3Al. The obtained chromium site preference energies have been compared using the anti-site and vacancy-based substitution formation mechanism, as well as using the standard defect formation formalism. It was found that chromium prefers aluminum site, consistent with the 3D atom probe result. In addition, interaction energies between two chromium atoms have also been determined from first-principles calculations. Our results show that chromium atoms prefer to be close by on either nickel or aluminum sublattices or on a nickel–aluminum mixed lattice, suggesting a potential tendency of chromium segregation in the γ′ phase.

3D reconstruction of prior β grains in friction stir-processed Ti-6Al-4V.

The prior β grain structure and orientations in the central stir zone of friction stir–processed Ti–6Al–4V were reconstructed from measured α phase orientations obtained by three‐dimensional serial sectioning in a dual‐beam focused ion beam scanning electron microscope. The data were processed to obtain the α colony and β grain size distributions in the volume. Several β grains were individually analysed to determine the total number of unique α variants and the respective volume fractions of each. The analysis revealed that some grains experienced overwhelming variant selection (i.e. one variant dominated) whereas other β grains contained a more evenly distributed mixture of all 12 variants.

Spark Plasma Sintering (SPS) of Carbon Nanotube (CNT) / Graphene Nanoplatelet (GNP)-Nickel Nanocomposites: Structure Property Analysis

Carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) are attractive reinforcements for lightweight and high strength metal matrix composites due to their excellent mechanical and physical properties. The CNT/Ni (DM) nanocomposites exhibiting a tensile yield strength of 350 MPa (about two times that of nickel ∼ 160 MPa) and an elongation to failure ∼ 30%. In contrast, CNT/Ni (MLM) exhibited substantially higher tensile yield strength (∼ 690 MPa) but limited ductility with an elongation to failure ∼ 8%. GNP/Nickel nanocomposites were also processed via DM followed by SPS consolidation. The Ni-1vol%GNP nanocomposite exhibited the best balance of properties in terms of strength and ductility. The enhancement in the tensile strength (i.e. 370 MPa) and substantial ductility (∼ 40%) of Ni-1vol%GNP nanocomposites was achieved due to the combined effects of grain refinement, homogeneous dispersion of GNPs in the nickel matrix, and well-bonded Ni-GNP interface effectively transfers stress across metal-GNP interface during tensile deformation.

Foreword: Special Topic on “Neutron and X-Ray Diffraction Studies of Advanced Materials IV”

The X-ray diffuse scattering from nanoscale precipitates in Ni-Al-Si alloys was used to explore coupling between the stress annealing and the precipitate strength and shape in the article by Barabash et al. From the analysis of intensity distributions around the superstructure and fundamental reflections, the authors obtain the information about strain fields in the matrix surrounding precipitates and suggest how the application of external strains may enhance the precipitation-induced strengthening.