Michael Francis Xavier Gigliotti

Rapidly solidified Al 3 Ti-base alloys containing Ni

Two Ni-modified Al 3 Ti alloys (Al 65 Ni 10 Ti 25 and Al 62 Ni 8 Ti 30 ) were rapidly solidified by melt spinning. The resulting microstructure was studied using light microscopy and analytical electron microscopy. Significant variations in the microstructure and phases were observed between the two ribbons and through the thickness of each ribbon.A single-phase γ-TiAl structure was seen near the wheel side of the Al 62 Ni 8 Ti 30 ribbon, having microcrystalline grains ∼ 100 nm in diameter. Second-phase particles of Λ-AlNiTi were found in the remaining regions of that ribbon as the structure became columnar due to reduced rates of cooling. The Al 65 Ni 10 Ti 25 alloy exhibited a primary phase of π-Al 6.5 NiTi 2.5 . A second phase of μ-Al 2 NiTi formed with morphology and distribution varying through thickness. Microchemistry measurements on the phases indicated substantial deviations (up to 14 at. %) from the stoichiometric compositions. Further, the π, γ, and μ are low-temperature phases that do not form by solidification under equilibrium conditions. The observation of these phases thus suggests significant undercoolings achieved during the melt-spinning processing of the present alloys. Both ribbons are brittle as spun.

Superplastic roll forming of Ti alloys

Abstract The high cost of aerospace alloys and their components makes them prime candidates for net-shape manufacturing techniques. Conventional processes for manufacturing disk components include hammer, hot die, and isothermal forging. This paper will examine the potential of a revolutionary approach for the manufacture of aircraft engine disks, superplastic roll forming. The process of superplastic roll forming, developed at the Institute for Metals Superplasticity Problems, Ufa, employs pairs of small opposed rollers to shape a cylindrical workpiece into a complex axisymmetric shape by simultaneously adjusting the roll gap and by moving the rolls radially outward on the workpiece while it is rotated about its axis of symmetry. Both the workpiece and the rolls are maintained at temperatures close to the beta transus. This paper will describe metallurgical evaluations of superplastically roll formed disks of alloy VT25. The evaluations of the disks included microstructure, crystallographic texture, heat treatment response, tensile strength, stress rupture resistance, and ultrasonic characteristics. The disk microstructures were found to be uniform and without any strongly textured colonies. Mechanical properties of the roll formed VT 25 were compared with those of Ti-6242S, IMI834, and conventionally forged VT25. The RF VT25 disk was found to possess low ultrasonic noise and high inspectability, which provided an increase in signal to noise for synthetic flaws.

Effect of dopants on grain boundary decohesion of Ni: A first-principles study

First-principles density functional theory (DFT) calculations were used to determine decohesion properties of Σ5(012) grain boundary of Ni with dopants B, C, S, Cr, and Hf. The relative stability of sites was evaluated and cleavage energies were calculated. Electronic structure was used to understand these properties in terms of changes in bonding with addition of dopants. It was found that strengthening of the Ni grain boundary results from Hf, B, and Cr doping. In contrast, the grain boundary weakens with S and C doping. These results should be useful in the design of next-generation nanostructured Ni-based alloys with improved mechanical behavior.

Effect of deformation conditions on grain size and microstructure homogeneity of β-rich titanium alloys

The mechanical behavior and microstructure evolution of a stable β, metastable β, and β-rich titanium alloys during hot deformation in both β and α/β phase fields were studied. The effects of thermomechanical processing and alloy content on final grain size and microstructure homogeneity in the alloys are given. The processing windows in both β and α/β phase fields for the formation of homogeneous microstructures with grain sizes down to submicron values are discussed. Isothermal multiple-step forging was used to produce the billets of a β-rich alloy with homogeneous fine-grained microstructure and low ultrasonic noise.

Dissolution rate measurements of TiN in Ti-6242

Abstract This paper describes measurements of the dissolution rate of nitrided Ti sponge and monolithic TiN rod in molten Ti-6242. The dissolution rate is described in terms of an interface recession rate that was 2.2 μm/s for a Ti-6242 temperature of 1725°C and dissolution times between 1 and 100 min. Similar dissolution rates were measured for nitrided sponge and monolithic rod. This report also describes the microstructural and chemical interdiffusion phenomena that occur during dissolution of solid δTiN in molten Ti-6242. There is a N-containing solid αTi layer and a N-solidified βTi layer between the solid δTiN and liquid Ti-6242 during dissolution. Microprobe measurements indicate that diffusion of Al, Zr, Sn and Mo into δTiN did not occur. Steep N concentration profiles were observed in the αTi layer. Al, Zr, Sn and Mo were observed in the N-solidified βTi layer, but their concentrations were depleted with respect to Ti-6242. The N-solidified βTi layer contained

Microstructure and sound velocity of Ti-N-O synthetic inclusions in Ti-6Al-4V

The ultrasonic properties of titanium-nitrogen-oxygen inclusions within Ti-6A1-4V (Ti64) blocks were measured and related to inclusion chemistry. Sound velocities were measured on Ti-N-O alloy samples that had been prepared by powder metallurgy and ingot-melting techniques. The contributions to sound velocity from oxygen and nitrogen contents were determined. Then, Ti64 blocks were hot isostatic pressing (HIP) bonded to contain inclusions of the Ti-N-O alloys. The signal-to-noise ratios of reflections from uncracked inclusions were found to be an increasing function of inclusion interstitial content and were related to changes in sound velocity with inclusion chemistry. Measurements were made of the reflectance of titanium-nitrogen inclusions in titanium and Ti64.

Furnace atmosphere effects on casting of eutectic superalloys

Control of furnace atmosphere is a key factor in the use of silica-bonded alumina shell molds for the directional solidification of eutectic superalloys reinforced with tantalum monocarbide whiskers. The use of a furnace atmosphere which is simultaneously oxidizing to aluminum in the eutectic alloy and reducing to silica phases in the mold results in the formation of an alumina barrier layerin situ at the metal/mold interface and an absence of silica phases in the mold region adjacent to this barrier layer. The presence of this microstructure permits castings of eutectics at metal temperatures up to 1750°C.

Superplasticity of nickel-based alloys with micro- and sub-microcrystalline structures

This paper describes the generation of micro- and sub-microcrystalline structures in two Ni-based alloys that are typically strengthened by phases, such as {gamma}{prime} and {gamma}{double_prime}+{delta}. The relationship between the superplastic behavior and microstructure is discussed. High strain deformation processing in the temperature range of 0.9T{sub m} to 0.6T{sub m} results in reduction of the initial coarse-grained structure (> 100 {micro}m) to a range of structures including microcrystalline (MC) (grain size < 10 {micro}m) and sub-microcrystalline (SMC) (grain size < 1 {micro}m) with increasing deformation. The influence of alloy chemistry and constituent phases on dynamic and static recrystallization is considered, and their effect on grain refinement is described. Low-temperature and high strain rate superplasticity can be observed in dispersion-strengthened alloys with SMC structures. it was established that in dispersion-hardened Ni alloys with SMC structures, superplasticity can be observed at temperatures 200--250 C lower than in alloys with MC structure.

Design and Fabrication of Forged Ti-6Al-4V Blocks with Synthetic Ti-N Inclusions for Estimation of Detectability by Ultrasonic Signal-To-Noise

In the work described subsequently, synthetic “hard alpha” inclusions have been fabricated within Ti-6A1-4V (Ti64) forgings. Several compositions of synthetic hard alpha were made by arc melting Ti sponge and TiN powder. Small solid cylinders of the titanium-nitrogen alloys were made by electro-discharge machining (e. d. m.) the arc-melted ingots to diameters of 0.031, 0.047, 0.062, and 0.078 inches, respectively, with heights equal to the respective diameter. Sets of eight or sixteen each identical cylinders were hot isostatic press (HIP) bonded within forged Ti64 blocks to yield uncracked inclusions with sharp interfaces with the Ti64 matrix. Detectability of the uncracked hard alpha was estimated as a function of flaw size, orientation, and nitrogen content from ultrasonic signal to noise ratios determined from C-Scan images of the blocks. Relationships of detectability to physical properties of hard alpha, and methodologies of signal to noise determinations are discussed.

Multi-phase functionally graded materials for thermal barrier systems

Jet engine and gas turbine hot section components can be protected from the 1,350--1,650 C combustion gases by thermal barrier coatings (TBCs). Metallic candidates for functionally graded material (FGM) coatings have been evaluated for potential use in bonding zirconia to a single crystal superalloy. Properties for four materials were studied for the low-expansion layer adjacent to the ceramic. Ingots were produced for these materials, and oxidation, expansion and modulus were determined. A finite element model was used to study effects of varying the FGM layers. Elastic modulus dominated stress generation, and a 20--25% reduction in thermal stress generated within the zirconia layer may be possible.