| Summary: | 博士 === 國立中山大學 === 材料與光電科學學系研究所 === 106 === In this research, we used the ultrasonic mechanical surface attrition treatment and nanoindentation as research tools, conducting the surface grain refinement and creep properties of high-entropy alloys. The relevant results will be separately described and discussed in the manuscripts.
For the first part, most high entropy alloys (HEAs) are cast to form single phase solid solution. Their hardness and strength at room temperature under the as-cast condition are typically lower than expectation. In the research, the ultrasonic surface mechanical attrition treatment (SMAT) is conducted on the surface of two HEAs, face-centered cubic (FCC) single-phased Fe20Co20Ni20Cr20Mn20 and the face-centered plus body-centered cubic (FCC+BCC) dual-phased Fe18Co18Ni20Cr18Mn18Al9, to upgrade their room temperature surface characteristics. By proper SMAT multiple paths, the grain size can be reduced from ∼50 μm down to ∼0.1-1 μm, the hardness increased from ~2.5-5.0 GPa up to ~5.0-8.5 GPa, and the tensile strength and elongation can be nearly doubled. The gradient refined and strengthened surface layers are demonstrated to appreciably upgrade the HEA performance. The strengthening mechanisms and superposition rules are established and are compared well with the experimental measurements.
Additionally, the creep responses under nanoindentation for the FCC single-phased Fe20Co20Ni20Cr20Mn20 and the FCC+BCC dual-phased Fe18Co18Ni20Cr18Mn18Al9 HEAs are examined on the FCC (111) grains over the temperature regime from 300 to 600oC, under a normalized stress level (σ/E) of 2.5x10-3. The stress exponents for both alloys are found to be about 4, or the strain rate sensitivity is about 0.25, indicating the similar dislocation climb power law creep as the controlling dominant creep mechanism. The extracted activation energy for these two under the “constant normalized stress” is 259±10 and 260±8 kJ/mol, respectively. However, since there is precipitation effect in both alloys over ~400-600oC, the actual activation energy under the “constant structure condition” should be greater than 260 kJ/mol, presumably about 280-300 kJ/mol. The current two HEAs possess relatively large volume, for example, about 250 Å3 at 600oC, larger than those for pure Ni or typical Ni based superalloys (~150 Å3 or less). The current creep response is compared and discussed with that of the Ni based superalloys.
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