R. Banerjee

Self-lubricating carbon nanotube reinforced nickel matrix composites

Nickel (Ni)—multiwalled carbon nanotube (CNT) composites have been processed in a monolithic form using the laser-engineered net shape (LENS™) processing technique. Auger electron spectroscopy maps determined that the nanotubes were well dispersed and bonded in the nickel matrix and no interfacial chemical reaction products were determined in the as-synthesized composites. Mechanisms of solid lubrication have been investigated by micro-Raman spectroscopy spatial mapping of the worn surfaces to determine the formation of tribochemical products. The Ni-CNT composites exhibit a self-lubricating behavior, forming an in situ, low interfacial shear strength graphitic film during sliding, resulting in a decrease in friction coefficient compared to pure Ni.

Elemental partitioning between α and β phases in the Ti–5Al–5Mo–5V–3Cr–0.5Fe (Ti-5553) alloy

Using atom probe tomography, the partitioning of alloying elements between α and β in the alloy Ti metal-5553 (Ti–5Al–5Mo–5V–3Cr–0.5Fe) has been investigated as a function of heat-treatment. It has been shown that β-solutionizing followed by step-quenching to a higher temperature (700°C) or slow-cooling leads to substantial partitioning of the alloying elements, including an enrichment of slow-diffusing Mo at the α/β interfaces. In contrast, it was found that the combination of β-solutionizing, quenching to room temperature and aging at 400°C leads to rather limited partitioning of these alloying elements.

Selection of α variants during microstructural evolution in α/β titanium alloys

The solid-state β→β + α transformation in titanium alloys leads to complex microstructures with feature spanning across a range of length scales. In order to develop a better understanding of the microstructural evolution process, a detailed characterization of the crystallography of α laths formed from the β phase in a candidate α/β Ti alloy, Timetal 550, has been carried out. Specifically, the influence of the orientation relationship (OR) between the grain boundary α (GB α) and the adjacent β grains on the microstructural evolution has been investigated in this alloy employing orientation imaging microscopy (OIM) studies in a high-resolution SEM. The results indicate that the colony microstructure (clustering of α laths belonging to the same variant) tends to develop in the β grain that exhibits the Burgers OR with the GB α allotriomorph, whereas the basketweave microstructure (clustering of multiple variants) develops in the adjacent β grain. Additionally, the multiple variants of α laths forming the basketweave microstructure appear to be related by certain selection criteria.

Evolution of a honeycomb network of precipitates in a hot-rolled commercial Mg–Y–Nd–Zr alloy

Coupled processes of dynamic recovery and precipitation, occurring during hot-rolling and subsequent aging, lead to the formation of a unique honeycomb network of precipitates in commercial Mg–Y–Nd–Zr or WE43 alloy. The honeycomb network is developed on the (0 0 0 1)Mg basal planes and consisted of fine Nd-rich β 1 platelets lying on all three planes presumably decorating recovery-generated dislocation subcell boundaries. Three variants of β 1 platelets are connected by Y-rich precipitates at the nodes of the hexagonal honeycomb network.

Understanding the Origins of Intergranular Corrosion in Copper-Containing Al-Mg-Si Alloys

A definitive understanding of the mechanism of intergranular corrosion (IGC) in under-aged (UA) Cu-containing Al-Mg-Si alloys has not been clear to date. The grain boundary microstructure and chemistry in an UA Cu-containing Al-Mg-Si alloy were characterized by coupling atom probe tomography and scanning transmission electron microscopy. The rapid formation of an ultra-thin wetting Cu layer and discrete Q-phase (Al4Cu2Mg8Si7) precipitates along the grain boundaries, and a precipitate-free zone adjacent to the grain boundaries in the UA condition contribute to IGC.

Probing the crystallography of ordered Phases by coupling of orientation microscopy with atom probe tomography

The determination of atomic scale structural and compositional information using atom probe tomography is currently limited to elemental solids and dilute alloys. In the present article, a unique coupling of orientation microscopy and atom probe tomography successfully facilitates the crystallographic study of non-dilute alloy systems, with high evaporation fields. This reproducible methodology affords a new perspective to the conventional atom probe tomography of ordered precipitate strengthened superalloys. The high accuracy in crystallographic site-specific sample preparation results in high spatial resolution in APT, which has been demonstrated in Co-base superalloys. The practical applications of this technique can be extended to accurately characterize the nature of buried order/disorder interfaces at the atomic scale, as well as the site occupancies associated with different solute atoms in multi-component superalloys.

The effect of boron on the grain size and texture in additively manufactured β-Ti alloys

AbstractOne of the critical microstructural attributes affecting the properties of additively manufactured (AM) alloys is the growth of large columnar grains along the build direction. While most of the work in the reported literature is focused on Ti–6Al–4V and other α/β alloys, there are rather limited investigations on grain growth and texture development in AM β-Ti alloys. The addition of trace amounts of boron to these AM β-Ti alloys resulted in significant changes in the microstructure. Depending on the alloy system, a grain refinement of 50–100 times was noted. The change in the grain size has been attributed to a combined effect of constitutional supercooling, caused by boron rejection from the growing β grains, and the growth restriction factor (Q) of the grains caused by the solute elements. The addition of boron also changed the morphology of the grains from being columnar to more equiaxed, a much more pronounced change than observed in traditional α/β alloys such as Ti–6Al–4V. A change in texture of the β grains along the build direction was also noted, wherein the addition of boron randomized the texture from the typically observed strong (001)β oriented grains in AM Ti alloys. Finally, the addition of boron changed the morphology of the α precipitates in the Ti–Mo system from lath-like to more equiaxed, while preserving the Burgers orientation relationship between the α and β phases.

Laser additive processing of functionally-graded Fe–Si–B–Cu–Nb soft magnetic materials

ABSTRACT Laser additive manufacturing is a novel tool for processing compositionally-graded alloys that are challenging to process via a conventional route. This article discusses a novel combinatorial approach for assessing composition–microstructure–magnetic property relationships, using laser deposited compositionally-graded Fe–Si–B–Nb–Cu alloys (by changing the silicon to boron ratios). The microstructure of Fe–Si–B–Nb–Cu alloys with a lower Si to B ratio consists of dendritic α-Fe3Si grains, with B and Nb partitioning to the inter-dendritic regions, resulting in the formation of Fe3B grains. As the Si/B ratio increases, the (Fe, Nb) enriched eutectic phase was observed along with α-Fe3Si grains; and no Fe3B was observed. These microstructural changes with varying Si/B ratios significantly affect the magnetic properties of these laser-deposited soft magnetic alloys.

Effect of β-stabilizer elements on stacking faults energies and ductility of α-titanium using first-principles calculations

The effect of W, Mo, V, Ta, and Nb, five common β-stabilizing substitutional elements, on α-Ti stacking fault energy has been studied using first principle calculations. The generalized stacking fault energy (GSFE) curves have been determined for different concentrations of β-stabilizers at the fault plane using supercells with up to 360 atoms. Both basal and prismatic slip systems with the stable (γSF) and unstable (γUSF) stacking faults and twinning fault energies were determined. All the alloying elements reduce the stacking fault energy for Ti for both basal and prismatic slip. At higher concentration of 25 at. % of V, Ta, and Nb at the slip plane, the basal slip becomes more favorable than the prismatic slip in Ti. Ti-Mo and Ti-W systems also show a significant shift in the GSFE curve towards a higher shear deformation strain along 〈 011¯0〉 due to the change in bond character between Ti and those two elements. Using Rice criterion, which employs γS/γUSF ratio to estimate ductility, we show that all t...

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.