Michael S. Titus

Transmission scanning electron microscopy: Defect observations and image simulations

The new capabilities of a FEG scanning electron microscope (SEM) equipped with a scanning transmission electron microscopy (STEM) detector for defect characterization have been studied in parallel with transmission electron microscopy (TEM) imaging. Stacking faults and dislocations have been characterized in strontium titanate, a polycrystalline nickel-base superalloy and a single crystal cobalt-base material. Imaging modes that are similar to conventional TEM (CTEM) bright field (BF) and dark field (DF) and STEM are explored, and some of the differences due to the different accelerating voltages highlighted. Defect images have been simulated for the transmission scanning electron microscopy (TSEM) configuration using a scattering matrix formulation, and diffraction contrast in the SEM is discussed in comparison to TEM. Interference effects associated with conventional TEM, such as thickness fringes and bending contours are significantly reduced in TSEM by using a convergent probe, similar to a STEM imaging modality, enabling individual defects to be imaged clearly even in high dislocation density regions. Beyond this, TSEM provides significant advantages for high throughput and dynamic in-situ characterization.

Solute segregation and deviation from bulk thermodynamics at nanoscale crystalline defects

Atomistic processes governing the high-temperature strength of structural materials were accurately characterized and modeled. It has long been known that solute segregation at crystalline defects can have profound effects on material properties. Nevertheless, quantifying the extent of solute segregation at nanoscale defects has proven challenging due to experimental limitations. A combined experimental and first-principles approach has been used to study solute segregation at extended intermetallic phases ranging from 4 to 35 atomic layers in thickness. Chemical mapping by both atom probe tomography and high-resolution scanning transmission electron microscopy demonstrates a markedly different composition for the 4–atomic-layer–thick phase, where segregation has occurred, compared to the approximately 35–atomic-layer–thick bulk phase of the same crystal structure. First-principles predictions of bulk free energies in conjunction with direct atomistic simulations of the intermetallic structure and chemistry demonstrate the breakdown of bulk thermodynamics at nanometer dimensions and highlight the importance of symmetry breaking due to the proximity of interfaces in determining equilibrium properties.

Dislocation injection in strontium titanate by femtosecond laser pulses

Femtosecond laser ablation is used in applications which require low damage surface treatments, such as serial sectioning, spectroscopy, and micromachining. However, dislocations are generated by femtosecond laser-induced shockwaves and consequently have been studied in strontium titanate (STO) using transmission electron microscopy (TEM) and electron backscatter diffraction analysis. The laser ablated surfaces in STO exhibit dislocation structures that are indicative of those produced by uniaxial compressive loading. TEM analyses of dislocations present just below the ablated surface confirm the presence of ⟨110⟩ dislocations that are of approximately 35° mixed character. The penetration depth of the dislocations varied with grain orientation relative to the surface normal, with a maximum depth of 1.5 μm. Based on the critical resolved shear stress of STO crystals, the approximate shockwave pressures experienced beneath the laser irradiated surface are reported.

EBSD Imaging of Femtosecond Laser Ablated Surfaces Using the TriBeam System

A TriBeam system has been developed to enable the femtosecond laser ablation of materials in situ in a dualbeam microscope. Material can be removed at rates that are 5-6 orders of magnitude faster than a conventional gallium source FIB, enabling the gathering of tomographic sectioned datasets with volumes approaching mm. Femtosecond lasers have been well characterized in the literature as having low collateral (dislocation) damage [1] and a negligible heat affected zone [2], making them well suited to material removal for tomographic sectioning. In a tomography experiment using the TriBeam, millions of tightly focused pulses are used to ablate the surface of a material. In order to characterize surface modification produced by the laser ablation events, a series of TEM foils have been cross-sectioned from the imaged surfaces from the last slice from tomography experiments.

Femtosecond Laser Damage in Metals and Semiconductors During TriBeam Tomography

The incorporation of ultrashort pulsed lasers into a dualbeam electron microscope, otherwise known as the TriBeam, has enabled applications such as the fast removal of material for tomography [1], micromachining, plasma based chemical diagnostics [2], and beam chemistry [3]. The unique low damage properties of commercial femtosecond lasers arise from the large impulse of energy imparted into the electronic structure of a material over time periods typically between 20-500 femtoseconds yielding complicated thermo-mechanical loading.

In situ Femtosecond Laser and Argon Ion Beams for 3D Microanalysis using the TriBeam

Advanced engineering materials require microstructural characterization in 3D across lengthscales, motivating the development of new tomography techniques and coupling with existing capabilities. The acquisition of 3D datasets with structural and chemical information at lengthscales between those accessible using Ga and Xeon FIB SEMs and those of X-ray tomography techniques is still challenging, particularly for dense multiphase materials. Femtosecond lasers have been employed for low damage [1,2] material removal, in tomography applications, over mm regions in situ in a FIB SEM as shown in Figure 1. FIB cross sections investigated by TEM have shown that dislocations can be injected to microns in depth in some materials [3], but are primarily confined to less than 100s of nanometers of the surface in the low fluence ablation regime. Parametric studies of laser fluence and beam scanning conditions in silicon in the TriBeam show that, when the propagating laser beam is scanned parallel with the sample surface, the damage is exclusively limited to that of the low fluence ablation regime. Laser ablation studies have also shown the ability to resolve surface sensitive EBSD maps from the ablated surface of many metals and/or alloys primarily containing magnesium, titanium, nickel, steel, copper, tungsten, tin and niobium.

Sub-nanometer Resolution Chemi-STEM EDS Mapping of Superlattice Intrinsic Stacking Faults in Co-based Superalloys

With the recent discovery of a ternary γ’-Co3(Al,W) phase of L12 crystal structure, new precipitation strengthened Co-base alloys have been investigated for potential use as gas turbine blades for energy and aero applications [1]. A prevalent deformation mode for turbine blades includes time-dependent plastic deformation, known as creep, which is caused by the centrifugal forces exerted on the blades from the rotating rotor. TEM analysis of interrupted creep specimens of various single crystal Co-based superalloys reveal the presence of numerous superlattice intrinsic stacking faults (SISFs) in the γ’ phase that are remnants of a dislocation shearing events. HRSTEM analysis confirms the nature of the SISFs, as well as compositional variation across the SISFs that must be included in creep deformation models, as shown in Figures 1 and 2.