arXiv subjects

Michael J. Mills

Publications and source records attributed to Michael J. Mills.

6 recordsLinked to original sources

arXiv2026

Stacking faults from a different angle - Overcoming the edge-on limit in high-resolution defect analysis

The nature of stacking faults (SF) - whether intrinsic or extrinsic - plays a pivotal role in defect-mediated processes in crystalline materials. Yet, transmission electron microscopy (TEM) techniques for their reliable analysis remain limited to either conventional fringe-contrast imaging of inclined faults or atomic-resolution imaging of edge-on configurations. Here, we overcome this long-standing geometric constraint by introducing a high-resolution scanning TEM method that enables full structural discrimination of inclined SFs, as demonstrated in fcc, $L1_2$, and sphalerite crystals. This approach complements edge-on analysis and provides access to SFs on all glide planes along commonly utilized zone axes. We demonstrate the method's robustness in a CoNi-based superalloy by discriminating fault types even in overlapping configurations and for foil thicknesses exceeding 100 nm, and extend its application to analyzing bounding partial dislocations in inclined geometries to reveal the fault formation mechanism. Probe propagation simulations reveal that fault-induced de-channeling is key to contrast formation and is strongly governed by the fault's depth within the sample. Leveraging this effect, we further establish a route to artificially generate ultrathin TEM lamellae - bounded by the SF itself - enhancing contrast for atomic-scale studies of long-range ordering and compositional fluctuations.

cond-mat.mtrl-sci↗

arXiv2021

Shearing Mechanisms of Co-Precipitates in IN718

The Ni-base superalloy 718 is the most widely used material for turbomachinery in the aerospace industry and land-based turbines. Although the relationship between processing and the resulting properties is well known, an understanding of the specific deformation mechanisms activated across its application temperature range is required to create more mechanistically accurate property models. Direct atomic-scale imaging observations with high angle annular dark-field scanning transmission electron microscopy, complemented by phase-field modeling informed by generalized stacking fault surface calculations using density functional theory, were employed to understand the shear process of $γ''$ and $γ'/γ''$ co-precipitates after 1 \% macroscopic strain at lower temperature (ambient and $427 °C$). Experimentally, intrinsic stacking faults were observed in the $γ''$, whereas the $γ'$ was found to exhibit anti-phase boundaries or superlattice intrinsic stacking faults. Additionally, the atomically flat $γ'/γ''$ interfaces in the co-precipitates were found to exhibit offsets after shearing, which can be used as tracers for the deformation events. Phase-field modeling shows that the developing fault-structure is dependent on the direction of the Burgers vector of the $a/2 \langle110\rangle$ matrix dislocation (or dislocation group) due to the lower crystal symmetry of the ${γ''}$ phase. The interplay between $γ'$ and $γ''$ phases results in unique deformation pathways of the co-precipitate and increases the shear resistance. Consistent with the experimental observations, the simulation results indicate that complex shearing mechanisms are active in the low-temperature deformation regime and that multiple $a/2 \langle110\rangle$ dislocations of non-parallel Burgers vectors may be active on the same slip plane.

cond-mat.mtrl-sci↗

arXiv2020

Heat treatment - microstructure - hardness relationships of new nickel-rich nickel-titanium-hafnium alloys developed for tribological applications

The effects of various heat treatments on the microstructure and hardness of new Ni56Ti41Hf3 and Ni56Ti36Hf8 (atomic %) alloys were studied to evaluate the suitability of these materials for tribological applications. A solid-solution strengthening effect due to Hf atoms was observed for the solution annealed (SA) Ni56Ti36Hf8 alloy (716 HV), resulting in a comparable hardness to the Ni56Ti41Hf3 alloy containing 54 vol.% of Ni4Ti3 precipitates (707 HV). In the Ni56Ti41Hf3 alloy, the maximum hardness (752 HV), achieved after aging at 300C for 12 h, was attributed to dense, semi-coherent precipitation of the Ni4Ti3 phase. Unlike the lenticular morphology usually observed within binary NiTi alloys, a blocky Ni4Ti3 morphology formed within Ni56Ti36Hf3 due to a smaller lattice mismatch in the direction normal to the habit plane at the precipitate/matrix interface. The maximum hardness for Ni56Ti36Hf8 (769 HV) was obtained after applying an intermediate aging step (300C for 12 h) followed by normal aging (550C for 4 h). This two-step aging treatment induces dense nanoscale precipitation of two interspersed precipitate phases, namely H-phase and a new cubic Ni-rich precipitate phase, resulting in the highest hardness exhibited yet by this family alloys. The composition of cubic Ni-rich precipitates was measured using atom probe tomography to be approximately Ni61.5Ti31Hf7.5, while HAADF-STEM revealed a 54 atom motif cubic structure (a= 8.87 Angstroms), and electron diffraction showed that the structure belongs to the pm-3m (No. 221) space group.

cond-mat.mtrl-sci↗

arXiv2019

Knowledge of Process-Structure-Property Relationships to Engineer Better Heat Treatments for Laser Powder Bed Fusion Additive Manufactured Inconel 718

Dislocation structures, chemical segregation, {γ^{\prime}, {γ^{\prime \prime}}, δ precipitates and Laves phase were quantified within the microstructures of Inconel 718 (IN718) produced by laser powder bed fusion additive manufacturing (AM) and subjected to standard, direct aging, and modified multi-step heat treatments. Additionally, heat-treated samples still attached to the build plates vs. those removed were also documented for a standard heat treatment. The effects of the different resulting microstructures on room temperature strengths and elongations to failure is revealed. Knowledge derived from these process structure property relationships was used to engineer a super solvus solution anneal at 1020 degC for 15 minutes, followed by aging at 720 degC for 24 hours heat treatment for AM-IN718 that eliminates Laves and δ phases, preserves AM specific dislocation cells that are shown to be stabilized by MC carbide particles, and precipitates dense {γ^{\prime} and {γ^{\prime \prime}} nanoparticle populations. This 'optimized for AM-IN718 heat treatment' results in superior properties relative to wrought/additively manufactured, then industry standard heat treated IN718: relative increases of 7/10 percent in yield strength, 2/7 percent in ultimate strength, and 23/57 percent in elongation to failure are realized, respectively, regardless of as-built vs. machined surface finishes.

physics.app-ph↗

arXiv2018

Three-dimensional in situ characterization of phase transformation induced austenite grain refinement in nickel-titanium

Near-field and far-field high-energy diffraction microscopy and microcomputed tomography X-ray techniques were used to study a bulk single crystal nickel-titanium shape memory alloy sample subjected to thermal cycling under a constant applied load. Three-dimensional in situ reconstructions of the austenite microstructure are presented, including the structure and distribution of emergent grain boundaries. After one cycle, the subgrain structure is significantly refined, and heterogeneous Σ3 and Σ9 grain boundaries emerge. The low volume and uneven dispersion of the emergent Σ boundaries across the volume show why previous transmission electron microscopy investigations of Σ grain boundary formation were inconsistent.

cond-mat.mtrl-sci↗

arXiv2018

Ferroelastic twin reorientation mechanisms in shape memory alloys elucidated with 3D X-ray microscopy

Three-dimensional (3D) X-ray diffraction methods were used to analyze the evolution of the load-induced rearrangements of monoclinic twin microstructures within bulk nickel-titanium specimens in 3D and across six orders of magnitude in length scales: changes in lattice plane spacings and orientations at the nanoscale, growth and nucleation of martensite twin variants at the microscale, and localization of plastic strain into deformation bands at the macroscale. Portions of the localized deformation bands were reconstructed in situ and in 3D. Analyses of the data elucidate the sequence of twin rearrangement mechanisms that occur within the propagating localized deformation bands, connect these mechanisms to the texture evolution, and reveal the effects of geometrically necessary lattice curvature across the band interfaces. The similarities between shear bands and localized deformation bands in twin reorientation are also discussed. These findings will guide future researchers in employing twin rearrangement in novel multiferroic technologies, and they demonstrate the strength of 3D, multiscale, in situ experiments to improve our understanding of complicated material behaviors and to provide opportunities to advance our abilities to model them.

cond-mat.mtrl-sci↗