Rohit Sarkar

Electron Beam Induced Artifacts During in situ TEM Deformation of Nanostructured Metals.

A critical assumption underlying in situ transmission electron microscopy studies is that the electron beam (e-beam) exposure does not fundamentally alter the intrinsic deformation behavior of the materials being probed. Here, we show that e-beam exposure causes increased dislocation activation and marked stress relaxation in aluminum and gold films spanning a range of thicknesses (80–400 nanometers) and grain sizes (50–220 nanometers). Furthermore, the e-beam induces anomalous sample necking, which unusually depends more on the e-beam diameter than intensity. Notably, the stress relaxation in both aluminum and gold occurs at beam energies well below their damage thresholds. More remarkably, the stress relaxation and/or sample necking is significantly more pronounced at lower accelerating voltages (120 kV versus 200 kV) in both the metals. These observations in aluminum and gold, two metals with highly dissimilar atomic weights and properties, indicate that e-beam exposure can cause anomalous behavior in a broad spectrum of nanostructured materials, and simultaneously suggest a strategy to minimize such artifacts.

Local, atomic-level elastic strain measurements of metallic glass thin films by electron diffraction.

A novel technique is used to measure the atomic-level elastic strain tensor of amorphous materials by tracking geometric changes of the first diffuse ring of selected area electron diffraction patterns (SAD). An automatic procedure, which includes locating the centre and fitting an ellipse to the diffuse ring with sub-pixel precision is developed for extracting the 2-dimensional strain tensor from the SAD patterns. Using this technique, atomic-level principal strains from micrometre-sized regions of freestanding amorphous Ti0.45Al0.55 thin films were measured during in-situ TEM tensile deformation. The thin films were deformed using MEMS based testing stages that allow simultaneous measurement of the macroscopic stress and strain. The calculated atomic-level principal strains show a linear dependence on the applied stress, and good correspondence with the measured macroscopic strains. The calculated Poissons ratio of 0.23 is reasonable for brittle metallic glasses. The technique yields a strain accuracy of about 1×10(-4) and shows the potential to obtain localized strain profiles/maps of amorphous thin film samples.

Revealing anelasticity and structural rearrangements in nanoscale metallic glass films using in situ TEM diffraction

ABSTRACT We used a novel diffraction-based method to extract the local, atomic-level elastic strain in nanoscale amorphous TiAl films during in situ transmission electron microscopy deformation, while simultaneously measuring the macroscopic strain. The complementary strain measurements revealed significant anelastic deformation, which was independently confirmed by strain rate experiments. Furthermore, the distribution of first nearest-neighbor distances became narrower during loading and permanent changes were observed in the atomic structure upon unloading, even in the absence of macroscopic plasticity. The results demonstrate the capability of in situ electron diffraction to probe structural rearrangements and decouple elastic and anelastic deformation in metallic glasses. GRAPHICAL ABSTRACT IMPACT STATEMENT This paper employs a novel in situ electron diffraction technique to reveal deformation-induced structural rearrangements, and decouple atomic-level elastic strain from larger scale anelastic strain in metallic glasses.

Synthesis of thin films with highly tailored microstructures

ABSTRACT We report a new methodology to synthesize thin films with exceptional microstructural control via systematic, in-situ seeding of nanocrystals into amorphous precursor films. When the amorphous films are subsequently crystallized by thermal annealing, the nanocrystals serve as preferential grain nucleation sites and control their micro/nanostructure. We demonstrate the capability of this methodology by precisely tailoring the size, geometry and spatial distribution of nanostructured grains in structural (TiAl) as well as functional (TiNi) thin films. This synthesis methodology is likely to be applicable to other amorphously grown materials and enables explicit microstructural control in a wide spectrum of thin films/coatings. IMPACT STATEMENT This paper describes a novel synthesis methodology to precisely tailor the microstructure of thin films and reveals the mechanisms underlying this methodology using in-situ TEM annealing experiments. GRAPHICAL ABSTRACT

In-situ Deformation of Various Micro/Nanoscaled Samples in the Transmission Electron Microscope: Experimental Results and Pitfalls

Properties including mechanical properties of micro/nanoscaled devices are of great technological importance and scientific interest. In order to obtain simultaneously both mechanical information and structural changes on atomic and nano scale, in-situ experiments in the transmission electron microscope (TEM) are supposed to be most appropriate. In the following paper we present (i) experimental TEM results obtained by in-situ tensile tests of nanocrystalline metals and metallic glasses and (ii) point to possible pitfalls leading to artifacts.

De-coupling Anelastic and Elastic Deformation in Metallic Glass Thin Films via Measurement of Micro Strain Tensors Using in situ Electron Diffraction

Several techniques have been recently developed to measure the lattice strain of crystalline materials using the TEM [1]. The methods employ either (i) High resolution TEM [2] and dark filed electron holography [3] or (ii) nano beam electron diffraction [4] and convergent beam electron diffraction [5]. However, though metallic glasses have been studied extensively using the TEM, measurements of micro-strains in these materials have not been carried out. Micro strains in metallic glasses has only been evaluated by techniques that employ high energy x-ray and neutron diffraction[6]. These techniques involve calculating the micro strain tensors by measuring the relative shift of diffraction peaks in reciprocal space during straining. Such techniques are limited to bulk specimen and lack the ability to monitor changes in the microstructure using high resolution imaging. In our work we use a novel in situ TEM technique described in [7] to measure local micro strain tensors in amorphous TiAl thin films.

Anomalous Beam Effects during in situ Transmission Electron Microscopy Deformation of Nanocrystalline and Ultrafine-grained Metals

It is well established that the TEM e-beam can cause radiation damage to specimen. Radiolysis, knockon damage and specimen heating are known to be the most common damage mechanisms [1]. Apart from these conventional damage mechanisms, the e-beam has also been recently shown to alter the deformation mechanisms in nanomaterials. The e-beam can activate dislocations, and defects generated by e-beam exposure can lead to additional deformation processes. For example, low to moderate intensity e-beam has been used to induce superplastic deformation in nanoscale silica particles and nanowires that are usually brittle at low temperatures [2].