Ercan Cakmak

Relationships Between the Phase Transformation Kinetics, Texture Evolution, and Microstructure Development in a 304L Stainless Steel Under Biaxial Loading Conditions: Synchrotron X-ray and Electron Backscatter Diffraction Studies

The relationships between the martensitic phase transformation kinetics, texture evolution, and the microstructure development in the parent austenite phase were studied for a 304L stainless steel that exhibits the transformation-induced plasticity effect under biaxial loading conditions at ambient temperature. The applied loading paths included: pure torsion, simultaneous biaxial torsion/tension, simultaneous biaxial torsion/compression, and stepwise loading of tension followed by torsion (i.e., first loading by uniaxial tension and then by pure torsion in sequence). Synchrotron X-ray and electron backscatter diffraction techniques were used to measure the evolution of the phase fractions, textures, and microstructures as a function of the applied strains. The influence of loading character and path on the changes in martensitic phase transformation kinetics is discussed in the context of (1) texture-transformation relationship and the preferred transformation of grains belonging to certain texture components over the others, (2) effects of axial strains on shear band evolutions, and (3) volume changes associated with martensitic transformation.

Mechanical characterization of an additively manufactured Inconel 718 theta-shaped specimen

Two sets of “theta”-shaped specimens were additively manufactured with Inconel 718 powders using an electron beam melting technique with two distinct scan strategies. Light optical microscopy, mechanical testing coupled with a digital image correlation (DIC) technique, finite element modeling, and neutron diffraction with in situ loading characterizations were conducted. The cross-members of the specimens were the focus. Light optical micrographs revealed that different microstructures were formed with different scan strategies. Ex situ mechanical testing revealed each build to be stable under load until ductility was observed on the cross-members before failure. The elastic moduli were determined by forming a correlation between the elastic tensile stresses determined from FEM, and the elastic strains obtained from DIC. The lattice strains were mapped with neutron diffraction during in situ elastic loading; and a good correlation between the average axial lattice strains on the cross-member and those determined from the DIC analysis was found. The spatially resolved stresses in the elastic deformation regime are derived from the lattice strains and increased with applied load, showing a consistent distribution along the cross-member.

Feasibility Study of Making Metallic Hybrid Materials Using Additive Manufacturing

A metallic hybrid structure, consisting of an Inconel-718 matrix and a Co-Cr internal structure, was successfully manufactured using laser direct energy deposition process. Characterizations were performed using energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), neutron-computed tomography (nCT), and electron back-scatter diffraction (EBSD) to verify the interfaces between Co-rich and Ni-rich phases. nCT revealed the internal structures to be continuous without cracking or significant intermixing due to inter-diffusion of Co and Ni (i.e., dissolved boundaries between the two structures). Minor porosity was detected. EBSD confirmed a good bond at the granular level. No precipitate phases were detected with XRD. EDS revealed dilution/intermixing between the Co and Ni interfaces presumably due to melt-pool overlay between the matrix and the internal structures.

Improving the thermoelectric performance and thermal stability of Ca 3 Co 4 O 9 + δ ceramics by sintering in oxygen atmosphere

AbstractWe report on the influence of sintering gas atmosphere on the thermoelectric (TE) performance of Ca3Co4O9 + δ ceramics made from precursor powders synthesized using chemical sol–gel route. Two sets of polycrystalline pellets were sintered in air and oxygen atmosphere, respectively. Sintering gas atmosphere and sintering temperature were found to have great influence on the Ca3Co4O9 + δ crystal orientation and formation of the Co3O4 secondary phase. All samples, regardless of sintering atmosphere, had similar Seebeck coefficients S that increased with measurement temperature. By contrast, the electrical resistivity ρ changed dramatically as the sintering temperature and atmosphere changed. For samples sintered in air, when the sintering temperature is above 1193 K, ρ increased as the sintering temperature increased. For samples sintering in oxygen, ρ decreased monotonically as the sintering temperature increased up to 1253 K. Benefited from the reduced ρ, the electrical power factor of the sample sintered in the oxygen at 1253 K is 0.539 mW m−1 K−2, and 40% greater than that from the sample sintered at 1193 K in air. Dimensionless figure-of-merit ZT of the sample sintered at 1253 K in oxygen is 0.28 at 1073 K, which is 20% greater than that of samples sintered in air at 1193 K. Overall, the highest TE performance was obtained for Ca3Co4O9 + δceramics sintered in oxygen at 1253 K, which is significantly higher than the Ca3Co4O9 + δ decomposition temperature of 1199 K in air.

Rationalization of solidification mechanism of Nd–Fe–B magnets during laser directed-energy deposition

Near-net fabrication techniques are highly beneficial to minimize rare earth metal usage to fabricate dense fully functional magnets. In this study, feasibility of using the directed-energy deposition technique for fabrication of magnets is evaluated. The results show that despite the ability to fabricate highly reactive materials in the laser deposition process, the magnetic coercivity and remanence of the hard magnets is significantly reduced. X-ray powder diffraction in conjunction with electron microscopy showed that the material experienced a primary Nd2Fe17Bx solidification. Consequently, the absence of the hard magnetic phase resulted in deterioration of the build properties.