Peter Maxwell Sarosi

Anatase assemblies from algae: coupling biological self-assembly of 3-D nanoparticle structures with synthetic reaction chemistry

The shape-preserving conversion of biologically self-assembled 3-D nanoparticle structures (SiO(2)-based diatom frustules) into a new nanocrystalline material (anatase TiO(2))via a halide gas/solid displacement reaction route is demonstrated.

Microtwinning during intermediate temperature creep of polycrystalline Ni-based superalloys: mechanisms and modelling

Deformation mechanisms, operative during intermediate temperature creep of Ni-based polycrystalline superalloys, are poorly understood. The creep deformation substructure has been characterized in Renè 88DT following rapid cooling from the super-solvus temperature, yielding a fine γ′-precipitate microstructure. After creep to modest strain levels (up to 0.5% strain) at 650°C and an applied tensile stress of 838 MPa, microtwinning is found to be the predominant deformation mode. This surprising result has been confirmed using diffraction contrast and high-resolution transmission electron microscopy. Microtwinning occurs via the sequential movement of identical 1/6[11–2] Shockley partials on successive (111) planes. This mechanism necessitates reordering within the γ′ precipitates in the wake of the twinning partials, so that the L12 structure can be restored. A quantitative model for creep rate has been derived on the basis that the reordering process is rate-limiting. The model is in reasonable agreement with experimental data. The results are also discussed in relation to previous studies under similar deformation conditions.

Effects of Cooling Rate on the Microstructure of a Commercial Ni-Based Superalloy Using Atom Probe Tomography

Nickel-base superalloys are widely used for high temperature structural materials such as hot sections of jet turbine engines because they possess the ability to retain excellent creep and yield strengths at high temperatures (700 o C). These strengths are derived from the distribution in the microstructure of the main constituent phases: (Ni-FCC) and (L12-ordered structure based on Cu3Au). More specifically, a bi-modal distribution of provides the best combination of mechanical properties for commercial turbine disk alloys such as Rene 88 DT, and Rene 104 the alloy in this investigation [1]. The importance of this type of microstructure was demonstrated [2] for a similar alloy, showing that just subtle changes in the smaller (tertiary) distribution, resulting from different heat treatments, can profoundly improve the creep strength of disk alloys by inducing a remarkably sluggish creep deformation mechanism known as microtwinning [3]. This paper presents an examination of the effects of cooling rate on this and other microstructural parameters such as / interface widths, element partitioning phase composition, crystal ordering and precipitate size that also greatly impact the superalloy’s creep strength using Atom Probe Tomography. The microstructures, of which a typical atom probe reconstruction is shown in Figure 1, from samples linearly cooled at different cooling rates were examined and compared. The / interfacial widths, phase composition and element partitioning were obtained from proximity histograms [4] determined using Apex software [5].

Lightweight thermal energy recovery system based on shape memory alloys: a DOE ARPA-E initiative

Over 60% of energy that is generated is lost as waste heat with close to 90% of this waste heat being classified as low grade being at temperatures less than 200°C. Many technologies such as thermoelectrics have been proposed as means for harvesting this lost thermal energy. Among them, that of SMA (shape memory alloy) heat engines appears to be a strong candidate for converting this low grade thermal output to useful mechanical work. Unfortunately, though proposed initially in the late 60s and the subject of significant development work in the 70s, significant technical roadblocks have existed preventing this technology from moving from a scientific curiosity to a practical reality. This paper/presentation provides an overview of the work performed on SMA heat engines under the US DOE (Department of Energy) ARPA-E (Advanced Research Projects Agency - Energy) initiative. It begins with a review of the previous art, covers the identified technical roadblocks to past advancement, presents the solution path taken to remove these roadblocks, and describes significant breakthroughs during the project. The presentation concludes with details of the functioning prototypes developed, which, being able to operate in air as well as fluids, dramatically expand the operational envelop and make significant strides towards the ultimate goal of commercial viability.

Microstructures of LENS™ Deposited Nb-Si Alloys

Nb-Si “in-situ” metal matrix composites consist of Nb 3 Si and Nb 5 Si 3 intermetallic phases in a body centered cubic Nb solid solution, and show promising potential for elevated temperature structural applications. Cr and Ti have been shown to increase the oxidation resistance and metal loss rate at elevated temperatures compared to the binary Nb-Si system. In this study, the LENS™ (Laser Engineered Net Shaping) process is being implemented to construct the Nb-Ti-Cr-Si alloy system from elemental powder blends. Fast cooling rates associated with LENS™ processing yield a reduction in microstructural scale over conventional alloy processes such as directional solidification. Other advantages of LENS™ processing include the ability to produce near net shaped components with graded compositions as well as a more uniform microstructure resulting from the negative enthalpy of mixing associated with the silicide phases. Processing parameters can also be varied, resulting in distinct microstructural differences. Deposits were made with varying compositions of Nb, Ti, Cr and Si. The as-deposited as well as heat treated microstructures were examined using SEM and TEM techniques. The influence of composition and subsequent heat treatment on microstructure will be discussed.

Microstructural Evaluation of LENS™ Deposited Nb-Ti-Si-Cr Alloys

Nb-Ti-Si “in-situ” metal ceramic composites consist of Nb 3 Si and Nb 5 Si 3 intermetallic phases in a body centered cubic Nb solid solution, and show promising potential for elevated temperature structural applications. The addition of Cr has also been shown to increase the oxidation resistance at high temperatures. In this study, the LENS™ (Laser Engineered Net Shaping) process is being implemented to construct the Nb-Ti-Cr alloy system from elemental powder blends. Advantages of the LENS™ process include the ability to produce near net shaped components with graded compositions as well as a more uniform microstructure resulting from the negative enthalpy of mixing associated with the silicide phases. This study focuses on characterization of the microstructure of the Nb-27Ti-5Si-10Cr (at%) system using SEM and TEM analysis.