T.M. Smith

Creep deformation mechanism mapping in nickel base disk superalloys

The creep deformation mechanisms at intermediate temperature in ME3, a modern Ni-based disk superalloy, were investigated using diffraction contrast imaging. Both conventional transmission electron microscopy (TEM) and scanning TEM were utilised. Distinctly different deformation mechanisms become operative during creep at temperatures between 677–815 °C and at stresses ranging from 274 to 724 MPa. Both polycrystalline and single-crystal creep tests were conducted. The single-crystal tests provide new insight into grain orientation effects on creep response and deformation mechanisms. Creep at lower temperatures (≤760°C) resulted in the thermally activated shearing modes such as microtwinning, stacking fault ribbons and isolated superlattice extrinsic stacking faults. In contrast, these faulting modes occurred much less frequently during creep at 815°C under lower applied stresses. Instead, the principal deformation mode was dislocation climb bypass. In addition to the difference in creep behaviour and creep deformation mechanisms as a function of stress and temperature, it was also observed that microstructural evolution occurs during creep at 760°C and above, where the secondary γ′ coarsened and the tertiary γ′ precipitates dissolved. Based on this work, a creep deformation mechanism map is proposed, emphasising the influence of stress and temperature on the underlying creep mechanisms.

HAADF/MAADF Observations and Image Simulations of Dislocation Core Structures in a High Entropy Alloy

High entropy alloys (HEAs) are a new class of multi-component alloys in which the individual elements have similar concentrations. A single-phase solid solution HEA containing 5 elements (Co, Cr, Fe, Mn, and Ni) with equiatomic composition was first discovered by Cantor [1]. Among the surprising characteristics of this fcc HEA are: strong temperature dependence of the yield strength at temperatures around and below room temperature, relatively weak strain-rate dependence over the same temperature range [3]; very large hardening rates [2,3]; and large fracture toughness at room temperature [4]. These features are linked to deformation twinning and dislocation-mediated plasticity, yet presently there is insufficient knowledge of dislocation dissociation, stacking fault energy, or core structures in this alloy. The highly planar deformation involves dislocation arrays on active slip systems (Figure 1a and 1b). This characteristic could imply the presence of short range order, low fault energy, or supplementary displacements in the wake of glide dislocations.

Through-Focal HAADF-STEM Analysis of Dislocation Cores in a High-Entropy Alloy

High-entropy alloys (HEAs) are a new class of multi-component alloys that exhibit surprising characteristics, [1] including very large strain hardening rates, large fracture toughness at room temperature [2], and a strong temperature dependence of yield strength at or below room temperature. These properties are closely linked to nano-twinning and dislocation-mediated plasticity, yet little experimental work has explored dislocation dissociation, stacking fault energy, or core structures in these alloys [3]. In this study, an HEA, containing 5 elements (Cr, Co, Mn, Fe, and Ni) with equiatomic composition was deformed to a 5% plastic strain at room temperature [4]. Post-mortem 3mm disks were electro-polished using a solution consisting of 21% Perchloric acid and 79% Acetic acid and analyzed using a probe-corrected Titan 80-300kV along a [110] zone axis. Highly planar deformation was first observed by Otto et al. [5] and was active for this study as well. This planar deformation, involving dislocation arrays on {111} slip systems, may imply the existence of short-range order, low stacking fault energy (SFE), and/or supplementary displacements in the wake of dislocations.

Quantifying Ordering Phenomena Through High-Resolution Electron Microscopy, Spectroscopy, and Simulation

Advances in aberration corrected scanning transmission electron microscopy (STEM) have allowed researchers to investigate structure-property relationships at the atomic scale [1–4]. In many systems, ordering phenomena at the atomic level can have a dramatic impact on the structural, electronic, and magnetic properties of the material. By combining experiment, simulation, and data processing, these ordering phenomena can be studied in a quantitative way, opening the door for new insights into the structure-property relationships of a wide variety of material systems. Experimentally, high angle annular dark field (HAADF) STEM and energy dispersive X-ray spectroscopy (EDX) are two very powerful techniques for probing compositional variations at the Ångstrom scale. In order to fully understand and quantify these techniques, image simulation and ionization calculations using the quantum excitation of phonons model can be used [5,6]. For the first time, using a double aberration corrected FEI ThemisTM with a Super-XTM XEDS detector compositional mapping of a Ni-based superalloy (commercially available HL-11) was collected at atomic resolution across a stacking fault, as seen in Figure . Individual spectra in the atomic resolution XEDS maps exhibit low signal-to-noise, usually having very few counts per channel. Based on previous characterization of the fault structure determining structural periodicity[4], the data was summed over a repeating unit cell of the fault structure along the [110] projection, resulting in almost an order of magnitude increase in the peak maxima of the XEDS spectra and even larger increase in integrated peak counts. These modified 3D data cubes were then fed into the Bruker Esprit software package and quantified using experimentally determined Cliff-Lorimer k-factors from a solutionized sample of the same material. By preprocessing these data before quantification, the error in the quantification was significantly reduced, allowing for site-specific determination of solute segregation in and around the fault structure in this Ni-based superalloy, leading to the determination of a novel high temperature strengthening mechanism.

Super-X EDS Characterization of Chemical Segregation within a Superlattice Extrinsic Stacking Fault of a Ni- based Superalloy

Superalloys are essential materials for high temperature applications in aerospace and energy production. Improving the temperature capability of disk alloys by a modest 25°C could translate into approximately a 1% increase in aircraft engine efficiency, resulting in significant cost savings as well as benefit environmental impact by reducing carbon emissions. While superalloys are prime candidates for an ICME approach to accelerated alloy development, at present this approach is severely hampered by the lack of quantitative models that connect alloying effects to deformation mechanisms. This linkage is critical for developing quantitative, physics-based deformation models, but has been limited severely by the complexity of the alloys (typically 10 components or more). One aspect, critical to a successful model is accurate determination of the degree of segregation to defects, such as faults, in order to enable the identification of the compositional configuration of such defects.

Fe-25Mn-3Al-3Si TWIP-TRIP Steel Deformed at High Strain-Rates

1. Interdisciplinary Materials Science, Vanderbilt University, Nashville TN 37232, USA 2. Materials Science and Engineering, The Ohio State University, Columbus, OH 43210, USA 3. Microscopy and Microanalytical Sciences, PO Box 7103, Oak Ridge, TN 37831-7103, USA 4. Max-Planck-Institut für Eisenforschung, Max-Planck-Straβe 1, D-40237 Düsseldorf, Germany 5. National Center for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Lab, Berkeley, CA 94720, USA