Materials Science Forum Vol. 1175

Abstract: AlCoCrFeNi high entropy alloys (HEA) have superior strength and corrosion resistance at both room and high temperatures and are expected to application in elevated temperature environments. However, it is not clear the relationship between the harmonic structure and the mechanical properties of these HEAs at elevated temperatures. The harmonic structure is composed of dispersed coarse grains and fine grains that are networked around them. In this study, the harmonic structure AlCoCrFeNi HEA was fabricated by mechanical milling (MM) / spark plasma sintering (SPS) process and the microstructure and elevated temperature mechanical properties of AlCoCrFeNi HEA are investigated in detail. AlCoCrFeNi mixed powders with average particle sizes of 14.6 and 82.4 μm were treated with MM. The MM powders were consolidated by SPS at 1173 to 1373 K. Mechanical properties were evaluated by compression tests at room temperature to 1073 K. Microstructural observation was performed using a scanning electron microscope, electron back scattered diffraction and energy dispersive X-ray spectrometer. The conventional SPS compacts have modulated structure with BCC and B2 phase and grain boundary precipitates with FCC phase. While the MM-SPS compacts have a similar structure of the conventional compacts at dispersed region and an equiaxed nanograins including a σ phase at network region. MM compacts with harmonic microstructure demonstrate high compression strength compared to conventional compacts at room temperature to 673 K. However, conventional microstructure compacts have higher strength than harmonic structure above 873 K. These results suggest that the harmonic structure has unique deformation behavior at elevated temperatures.

Abstract: Selective laser melting (SLM) can produce Ni-based superalloys with a unique hierarchical structure consisting of micrometer-scale crystallographic lamellar microstructure and nanometer-scale cellular structure under optimized process parameters. This work investigated the effects of input energy density on the morphology of the cells and its influence on the tensile properties of Ni-based superalloy prepared by SLM. We found that the cell spacing decreases with decreasing input energy density. Further investigation of the cells clarified that the boundary of cells is a low angle grain boundary with dislocation cell wall and segregation of certain elements such as Nb and Ti. Moreover, it was demonstrated that the boundary of cells performs as a significant barrier to the griding dislocation. Thus, the cell boundary leads to strong strengthening through the Hall-Petch law.