This paper reports on the substantial improvement of specimen quality by use of a low voltage (0.05 to ~1 keV), small diameter (~1 μm), argon ion beam following initial preparation using conventional broad-beam ion milling or focused ion beam. The specimens show significant reductions in the amorphous layer thickness and implanted artifacts. The targeted ion milling controls the specimen thickness according to the needs of advanced aberration-corrected and/or analytical transmission electron microscopy applications.

The role of slip transfer processes across the heterophase interfaces in two-phase TiAl intermetallics has been studied. Polysynthetically twinned crystals of TiAl (PST-TiAl) have been used as model systems for individual grains in technologically relevant polycrystalline lamellar TiAl alloys. Compressive plastic loads have been applied for orientations of the lamellar interfaces parallel and perpendicular to the loading directions to produce hard mode slip activity. Transmission electron microscopy has been used to determine the active deformation modes in the constituent phases and to study the slip transfer across heterophase interfaces. The results are discussed with respect to the mechanical behavior of PST-TiAl and lamellar TiAl alloys, which is of relevance to in-service performance and metallurgical processing operations.

Nb-Si “in-situ” metal matrix composites consist of Nb3Si and Nb5Si3 intermetallic phases in a body centered cubic Nb solid solution, and show promising potential for elevated temperature structural applications. Cr and Ti have been shown to increase the oxidation resistance and metal loss rate at elevated temperatures compared to the binary Nb-Si system. In this study, the LENS™ (Laser Engineered Net Shaping) process is being implemented to construct the Nb-Ti-Cr-Si alloy system from elemental powder blends. Fast cooling rates associated with LENS™ processing yield a reduction in microstructural scale over conventional alloy processes such as directional solidification. Other advantages of LENS™ processing include the ability to produce near net shaped components with graded compositions as well as a more uniform microstructure resulting from the negative enthalpy of mixing associated with the silicide phases. Processing parameters can also be varied, resulting in distinct microstructural differences. Deposits were made with varying compositions of Nb, Ti, Cr and Si. The as-deposited as well as heat treated microstructures were examined using SEM and TEM techniques. The influence of composition and subsequent heat treatment on microstructure will be discussed.

The role of slip transfer processes across the heterophase interfaces in two-phase TiAl intermetallics has been studied. Polysynthetically twinned crystals of TiAl (PST-TiAl) have been used as model systems for individual grains in technologically relevant polycrystalline lamellar TiAl alloys. Compressive plastic loads have been applied for orientations of the lamellar interfaces parallel and perpendicular to the loading directions to produce hard mode slip activity. Transmission electron microscopy has been used to determine the active deformation modes in the constituent phases and to study the slip transfer across heterophase interfaces. The results are discussed with respect to the mechanical behavior of PST-TiAl and lamellar TiAl alloys, which is of relevance to in-service performance and metallurgical processing operations.

The crystallographic texture, orientation relationships, coherency stress, and thermal stability of sputter-deposited Ni/Ni3Al multilayered thin films were studied as a function of bilayer period (Λ) as well as processing parameters such as substrate type, deposition temperature, and prebake conditions. Deposition onto oxidized Si or single-crystal Cu(001), NaCl(001), or KBr(001) substrates near room temperature produces multilayers with a [111] crystallographic texture along the Ni/Ni3Al interface normal and a disordered face-centered cubic structure for the Ni3Al phase. In contrast, deposition at 673 K onto NaCl(001) or KBr(001) substrates that are prebaked in vacuum at 693 K produces a chemically ordered L12 structure for the Ni3Al phase and (001) epitaxial growth. X-ray diffraction measurements of (001) multilayers with equal volume fraction of Ni and Ni3Al reveals a transition from a nearly incoherent state at Λ=40 nm to a semicoherent one at Λ 40 nm. Remarkably, (001) multilayers were observed to solutionize at 1373 K, which is approximately 100 K below that predicted by the Ni–Al phase diagram.

Nb-Ti-Si “in-situ” metal ceramic composites consist of Nb3Si and Nb5Si3 intermetallic phases in a body centered cubic Nb solid solution, and show promising potential for elevated temperature structural applications. The addition of Cr has also been shown to increase the oxidation resistance at high temperatures. In this study, the LENS™ (Laser Engineered Net Shaping) process is being implemented to construct the Nb-Ti-Cr alloy system from elemental powder blends. Advantages of the LENS™ process include the ability to produce near net shaped components with graded compositions as well as a more uniform microstructure resulting from the negative enthalpy of mixing associated with the silicide phases. This study focuses on characterization of the microstructure of the Nb-27Ti-5Si-10Cr (at%) system using SEM and TEM analysis.

The interactions of dislocations asscociated with deformation modes active in hard orientation compressed polysynthetically-twinned (PST) TiAl with lamellar boundaries have been studied by transmission electron microscopy (TEM). The deformation modes observed in the γ-phase involved soft superdislocation and hard ordinary dislocation slip, and hard twin systems. The transfer of these latter hard twin shears occurred across all types of lamellar (γ/γ)- and frequently across (γ/α2)-boundaries. Slip of superdislocations in the minority U2-phase lamellae has been shown to be associated with both direct transfer of piled-up twinning dislocation shears across the lamellar boundaries and the activation of interfacial sources due to pile-up stresses. The active deformation modes were consistent with macroscopic shape changes of the PST-TiAI when the various defect interactions with the lamellar boundaries and the proposed shear trasnfer processes were considered.