Dhriti Bhattacharyya

Refractory high‐entropy alloys (RHEAs) show potential for use in extreme environments, such as advanced nuclear reactors, owing to their high melting temperature, and often outstanding combinations of mechanical properties, corrosion resistance, and irradiation‐damage tolerance. This study evaluates the fracture toughness of a TiZrNbHfTa RHEA across different scales and microstructures, with a focus on the impact of He²⁺‐ion irradiation. Micro‐ and millimeter‐scale specimens with nanocrystalline (NC) microstructures are compared to existing ASTM standard sized coarse‐grained (CG) specimen data, with critical dimensions spanning over three orders of magnitude, from 10 μm to 12 mm. The ASTM standard sized CG specimens exhibit a fracture toughness 41‐fold greater than their NC microscale counterparts (210–5.1 MPa m^1/2), while NC millimeter‐scale specimens show a 7.5‐fold higher fracture toughness than NC microscale specimens (38.1–5.1 MPa m^1/2). He²⁺‐ion irradiation leads to a 27% decrease in fracture toughness in the NC microscale specimens. The results highlight the impact of sample dimensional scale, microstructure, and ion irradiation on the fracture toughness of the RHEA, indicating a need for thorough examination of such factors when investigating the mechanical properties of these materials.

The rapid progress in space exploration, mining, and tourism has been fuelled by both public and private sector investments. The latter has led to the need to reduce manufacturing and launch cost of space hardware to create a competitive and sustainable space economy. A major step in making space accessible is to develop affordable power systems for “commercial space” use. Photovoltaics has in the past and will in the future be a key component. Metal halide perovskite solar cells show the greatest potential of all emerging technologies for low‐cost space photovoltaics. They have demonstrated the highest rate of power conversion efficiency improvement. Compared to the triple junction III–V compound semiconductor cells commonly used for space applications, perovskite cells have a higher power to weight ratio and are significantly cheaper to be manufactured. They have high radiation tolerance and can be fabricated onto flexible substrates for expand‐on‐demand solar panels. This paper outlines the major space markets for photovoltaics, and research and development opportunities for perovskite space solar cells in the context of their recent progress.

Nanoindentation is a commonly used method to measure the hardness of surfaces with thin layers, and is especially useful in studying the change in mechanical properties of ion irradiated materials. This research compares two different methods of nanoindentation to study the changes in hardness resulting from ion irradiation of SS316 alloy. The samples were irradiated by He2+ ions at beam energies of 1, 2, and 3 MeV, respectively. The first method involves the indentation of the irradiated surface perpendicular to it using the continuous stiffness mode (CSM), while the second applies the indents on an oblique surface, accessing an inclined cross-section of the irradiated material. Finite element modelling has been used to further illuminate the deformation processes below the indents in the two methods. The hardness profiles obtained from the two nanoindentation methods reveal the differences in the outcomes and advantages of the respective procedures, and provide a useful guideline for their applicability to various experimental conditions. It is shown through an in depth analysis of the results that the ‘top-down’ method is preferable in the case when the ion irradiation energy, or, equivalently, the irradiated depth is small, due to its greater spatial resolution. However, the oblique cross section method is more suitable when the ion irradiation energy is >1 MeV, since it allows a more faithful measurement of hardness as a function of dose, as the plastic field is much smaller and more sensitive to local hardness values.

This paper reports a study of local orientation change occurring within micro-scale tensile samples as a function of strain. These samples were fabricated from a thin film of single crystal bcc Fe and deformed in tension using an in situ micro-mechanical testing device inside a scanning electron microscope. Samples were loaded along the <110> direction parallel to the specimen axis, strained to different levels, and then subjected to electron backscatter diffraction scans over the entire area of the gauge section. Analysis of the surface orientation data shows that, within a necked zone of the micro-sample gauge section, there are two distinct regions of significant orientation change, in which local crystal rotations occur in opposite directions. These two regions are separated by an intermediate band that shows minimal misorientation from the original state. Crystal rotations within the two regions that develop opposite orientations are found to be consistent with classic single crystal slip, where the slip direction rotates toward the tensile axis. It is shown that increasing tensile strain causes an increasing degree of rotation away from the starting orientation. The tests also illustrate the occurrence of slip on at least two different slip systems, based on the slip traces and orientation change.

Magnetron sputtered thin films of Cu, Nb, and Cu-Nb multilayers with 2.5 and 5 nm nominal layer thickness were deposited on Si and implanted with ⁴He⁺ and ³He⁺ ions. Secondary ion mass spectroscopy and nuclear reaction analysis, respectively, were used to measure the ⁴He⁺ and ³He⁺ concentration profile with depth inside the films. Cross-sectional transmission electron microscopy was used to characterize the helium bubbles. Analysis of the contrast from helium bubbles in defocused transmission electron microscope images showed a minimum bubble diameter of 1.25 nm. While pure Cu and Nb films showed bubble contrast over the entire range of helium implantation, the multilayers exhibited bubbles only above a critical He concentration that increased almost linearly with decreasing layer thickness. The work shows that large amounts of helium can be trapped at incoherent interfaces in the form of stable, nanometer-size bubbles.

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.

Recent studies have shown that two-phase nanocomposite materials with semicoherent interfaces exhibit enhanced strength, deformability, and radiation damage resistance. The remarkable behavior exhibited by these materials has been attributed to the atomistic structure of the bimetal interface that results in interfaces with low shear strength and hence, strong barriers for slip transmission due to dislocation core spreading along the weak interfaces. In this work, the low interfacial shear strength of Cu/Nb nanoscale multilayers dictates a new mechanism for shear banding and strain softening during micropillar compression. Our findings, supported by molecular dynamics simulations, provide insight on the design of nanocomposites with tailored interface structures and geometry to obtain a combination of high strength and deformability. High strength is derived from the ability of the interfaces to trap dislocations through relative ease of interfacial shear, while deformability can be maximized by controlling the effects of loading geometry on shear band formation.

Atomistic modeling shows that Cu–Nb and Cu–V interfaces contain high excess atomic volume due to constitutional vacancy concentrations of ∼5 at. % and ∼0.8 at. %., respectively. This finding is supported by experiments demonstrating that an approximately fivefold higher He concentration is required to observe He bubbles via through-focus transmission electron microscopy at Cu–Nb interfaces than in Cu–V interfaces. Interfaces with structures tailored to minimize precipitation and growth of He bubbles may be used to design damage-resistant composites for fusion reactors.

In recent years, the high strength of nanomaterials has gathered much interest in the materials community. Nanomaterials (polycrystalline and composites) have already been used, largely by the semiconductor community, as critical length scales for chip design have decreased to tens of nanometers. However, to ensure reliability of nanomaterials, the mechanisms underlying their structural integrity must be well understood. For these materials to be put into service, not only should their strength be considered, but also ductility, toughness, formability, and fatigue resistance. While some progress has been made into constructing models for the deformation mechanisms governing these behaviors, the body of experimental knowledge is still limited, especially for length scales below 10 nanometers. The results described here show stress-strain curves for nanolaminate composites with individual layer thickness of 40 nm and 5 nm. Nanolaminate composites fabricated via magnetron sputtering comprised of alternating 5 nm thick Cu and Nb multilayers (two relatively soft metals) exhibit strengths on par with hardened tool steel and deformability in compression in excess of 25% [1]. The deformability of nanoscale composites is found to be limited by the onset of geometric instability.

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

Nanoscale metallic multilayers, comprising two sets of materials—Cu/Nb and Cu/Ni—were deposited in two different layer thicknesses—nominally 20 and 5 nm. These multilayer samples were indented, and the microstructural changes under the indent tips were studied by extracting samples from underneath the indents using the focused ion beam (FIB) technique and by examining them under a transmission electron microscope (TEM). The deformation behavior underneath the indents, manifested in the bending of layers, reduction in layer thickness, shear band formation, dislocation crossing of interfaces, and orientation change of grains, has been characterized and interpreted in terms of the known deformation mechanisms of nanoscale multilayers.

In this work, micropillar compression testing has been used to obtain stress-strain curves for sputter-deposited Cu–Nb nanolaminate composites with nominal bilayer thickness of 10nm. In addition to the extremely high flow strength of 2.4GPa, the 5nm Cu∕5nm Nb nanolaminate exhibits significant ductility, in excess of 25% true strain.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.