Arda Genc

Direct laser deposition of in situ Ti–6Al–4V–TiB composites

Abstract Ti–6Al–4V–TiB composites have been in situ deposited from powder feedstocks consisting of a blend of pre-alloyed Ti–6Al–4V and elemental boron using the Laser Engineered Net-Shaping (LENS™) process. The microstructure of the as-deposited composites has been characterized in detail using scanning (SEM) and transmission electron microscopy (TEM). A homogeneous refined dispersion of TiB precipitates is formed within the Ti–6Al–4V α/β matrix. The scale of the microstructure is substantially refined as compared with composites produced using other recently developed powder processing techniques. In addition to the refined dispersion of the reinforcing phase, the influence of the TiB precipitation on the solid-state β→β+α transformation that takes place in the matrix has been examined using SEM and TEM. Finally, heat-treatments carried out post-LENS™ deposition, suggest that LENS™ fabricated composites are thermodynamically stable, exhibiting limited TiB coarsening.

Size induced metal–insulator transition in nanostructured niobium thin films: intra-granular and inter-granular contributions

With a reduction in the average grain size in nanostructured films of elemental Nb, we observe a systematic crossover from metallic to weakly insulating behaviour. An analysis of the temperature dependence of the resistivity in the insulating phase clearly indicates the existence of two distinct activation energies corresponding to inter-granular and intra-granular mechanisms of transport. While the high temperature behaviour is dominated by grain boundary scattering of the conduction electrons, the effect of discretization of energy levels due to quantum confinement shows up at low temperatures. We show that the energy barrier at the grain boundary is proportional to the width of the largely disordered inter-granular region, which increases with a decrease in the grain size. For a metal–insulator transition to occur in nano-Nb due to the opening up of an energy gap at the grain boundary, the critical grain size is ≈8 nm and the corresponding grain boundary width is ≈1.1 nm.

Heterotwin formation during growth of nanolayered Al-TiN composites

High stacking fault energy (SFE) materials such as Al do not form twins easily. Here, the authors report, through high-resolution transmission electron microscopy, that Al layers in an alternating Al/TiN composite grow in a twin relationship to both the TiN and the underlying Al layers. Density functional theory based ab initio modeling reveals that nitrogen termination in the {111} growth plane of the TiN layers greatly favors the growth of twin oriented Al layers on them. This finding provides a definite way of creating a twin-modulated structure in high SFE materials.

Complementary techniques for the characterization of thin film Ti/Nb multilayers.

An aberration corrector on the probe-forming lens of a scanning TEM (STEM) equipped with an electron energy-loss spectrometer (EELS) and X-ray energy-dispersive spectrometer (XEDS) has been employed to investigate the compositional variations as a function of length scale in nanoscale Ti/Nb metallic multilayers. The composition profiles of EELS and XEDS were compared with the profiles obtained from the complementary technique of 3D atom probe tomography. At large layer widths (h > or = 7 nm, where h is the layer width) of Ti and Nb, XEDS composition profiles of Ti/Nb metallic multilayers are in good agreement with the EELS results. However, at reduced layer widths (h approximately 2 nm), profiles of EELS and atom probe exhibited similar compositional variations, whereas XEDS results have shown a marked difference. This difference in the composition profiling of the layers has been addressed with reference to the effects of beam broadening and the origin of the signals collected in these techniques. The advantage of using EELS over XEDS for these nanoscaled multilayered materials is demonstrated.

The microstructure and electrical transport properties of immiscible copper-niobium alloy thin films

Mutually immiscible in the solid state, copper and niobium exhibit a relatively strong clustering (phase separating) tendency in the liquid state and can therefore only be alloyed in a highly metastable form: for example, by vapor quenching. We have deposited metastable Cu–Nb alloy thin films with nominal compositions ranging from 5 to 90u2002at.u2009% Nb by magnetron cosputtering. The microstructure of these films depends strongly on the composition and ranges from coarse-grained solid solutions for Cu-rich and Nb-rich compositions to phase-separated amorphous mixtures when the two elements are in comparable amounts. The crystalline Cu- or Nb-rich compositions exhibit positive temperature coefficients of resistivity (TCR) with the Cu–90u2002at.u2009% Nb film exhibiting a superconducting transition with (TC)onset∼4.5u2002K. The amorphous films show high room temperature resistivity, a negative TCR, and composition dependent superconducting transitions. We investigate the relation between the microstructure, phase stability, ...

Correlation between TEM Imaging and Microanalysis for Atom Probe Reconstruction Verification

The atom probe instrument field evaporates atoms from a specimen of interest and these atoms (now ions) are collected on a position-sensitive, mass-spectrum detector. By reconstructing the trajectory path and impact position of each ion from the field evaporation event, a volumetric reconstructed rendering of the material is generated with near atomic precision for each individual atom. The reconstruction method of an atom probe volume is dependent upon a constant evaporation field [1]. When the evaporation process proceeds through an interface of two different phases, the field can change resulting in aberrations in the atom probe reconstruction. These aberrations typically appear as density variations across interfaces and/or incorrect morphologies of precipitates within the matrix [2]. To help validate the atom probe reconstructions, TEM imaging and microanalysis can be employed. This proceeding addresses specific examples where the coupling of TEM can assist in the validation of the atom probe reconstruction. In addition, the proceeding addresses some experimental difficulties in bridging the two microscopy techniques.

A Small Spot, Inert Gas, Ion Milling Process as a Complementary Technique to Focused Ion Beam Specimen Preparation

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.

Ga+ and Xe+ FIB Milling and Measurement of FIB Damage in Aluminum

S/TEM sample preparation of aluminium and aluminium alloys to characterize grain boundary phases by focused ion beam (FIB) continues to be a major interest in metallurgical analysis because of FIB’s ability to prepare site specific specimens and eliminating damage from mechanical polishing or electro-polishing [1]. Recent instrumentation using plasma FIB (PFIB) technology and Xe ions offer increased milling rates because of its ability to deliver 30 – 40 times more current compared to Ga FIBs. While the measured sputter rate of aluminum using Ga and Xe differs by about 25% (0.31 uf06dm/nC [Ga] and 0.41 uf06dm/nC [Xe]), the ability to use more current for micromachining will allow users to increase throughput significantly and prepare much larger cross-sections for S/TEM sample preparation if PFIB is employed. Therefore, it is of interest to understand the amount of FIB damage introduced into the sidewall of a thin section of aluminum by FIB. 30 kV FIB damage employing a different preparation method has been measured to be ~ 4 nm [2].

Use Electrons Sparingly but Efficiently, the Battle to get All the Required Information Needed While Minimizing Dose and Maximizing Data Collection at the Highest Resolution

Since aberration correction has been applied on modern electron microscope systems, there has been a need to demonstrate the benefits of this capability and in some respect, to justify the cost of these complex systems. Sometimes the justification of such a system is by the presentation of a colorful elemental map which correlates with the atomic periodicity in the sample. This type of visualization, while artistic, may not be sufficient to characterize materials at the levels proclaimed, since there are many events happening which are difficult to place with atomic certainty. There are many benefits of the correctors on the imaging side of the sample by removing delocalization and improving the image interpretability for phase contrast imaging, however there is a strict requirement on the sample both in terms of cleanliness, thickness and damage layers. In the case of correctors on the condenser or probe forming part of the microscope, this requirement is only amplified especially when the imaging techniques are combined with various spectroscopies.

Xe+ FIB Milling and Measurement of Amorphous Damage in Diamond

Microand nanomachining of diamond using focused ion beam (FIB) continues to generate interest in applications such as diamond anvil cells, photonic devices, micro-cantilevers and tools for imprinting applications [1,2]. However, the milling rate of diamond by FIB is approximate 4X slower when compared to silicon using 30 kV Ga FIB [3]. Recent instrumentation using PFIB technology and Xe ions offer increased milling rates because of their ability to deliver up to 30X more current compared to Ga FIBs. While the sputter rate of diamond using Ga and Xe differs only slightly (0.07 μm/nC [Ga] and 0.09 μm/nC [Xe]), the ability to use more current for micromachining will allow users to increase throughput significantly. Therefore, it is of interest to understand the amount of amorphous damage introduced into a sidewall of diamond. Previous results indicate that for a glancing angle ~0 degrees, up to 35 nm of amorphous damage is introduced by Ga FIB in single crystal diamond [4].