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Nanocluster Strengthened Ferritic Alloys: Characterization & Mechanical Properties

Study on Nanocluster Strengthened Ferritic Alloys using Bright Field STEM Imaging, Compression Creep Testing, and Deformation Mechanisms.

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Nanocluster Strengthened Ferritic Alloys: Characterization & Mechanical Properties

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  1. Characterization and Mechnical Properties of Nanocluster Strengthened Ferritic (NSF) Alloys Peter Sarosi and Michael Mills Materials Science and Engineering - OSU Joachim Schneibel Oak Ridge National Laboratories Support provided by: DOE-Office of Basic Energy Sciences

  2. Bright Field (BF) STEM Imaging Convergent beam Beam rasters across sample specimen Diffracted/ Scattered beam Back focal plane Objective aperture Transmitted beam HAADF detector BF detector Advantages: • BF-STEM images sensitive to sharp crystal curvatures (e.g. dislocations) and mass/thickness • Not sensitive to gradual curvatures (e.g. bend contours) and grain orientation changes • Thicker foils can be analyzed compared with conventional TEM Technique: • Objective aperture selects direct beam which is detected by the BF detector • Diffraction conditions relaxed due to converged beam and angular range of BF detector Deformed Ni Microsample C-TEM BF-STEM BF-STEM Disk superalloy Blade superalloy

  3. Alloy OE-14YWT Compression Creep Testing at 800oC in Air 200 nm • Similar minimum creep rates for testing along longitudinal and transverse directions • Minimal effect of grain shape on creep rate

  4. Constant Strain Rate Compression at 800oC in Air s TEM foil Strain rates: 1x10-5 s-1 1x10-6 s-1 s Alloy OE-14YWT • Small rate sensitivity • Significant strain hardening (e=Asn; n~0.1) • TEM foils extracted in the longitudinal direction for analysis

  5. Initial Characterization of Compression Samples • Straight and pinned dislocations observed in fine grained regions • Dislocation density inhomogeneous – similar to annealed case • No twinning observed as in some nanoscale grain size materials

  6. Initial Characterization of Compression Test • Large grains are nearly defect free! • If present prior to deformation, why so resistant to deformation?

  7. Future work • Transient creep testing as a “Micro-mechanical” probe: Substructure Controlled Mobility Controlled Strain s1 < s2 s1 s2 Time • Form of strain transient following rapid stress change indicative of deformation mechanism. Upon stress increase: • Decreasing creep rate --> Substructure refinement • Increasing creep rate --> Increasing mobile dislocation density

  8. Foil Hole Correlating Dislocations with Clusters e- • Combine HRTEM imaging of dislocations with Energy Filtered TEM to locate clusters (a) 20nm 1 nm

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