The effects of the γ″-Ni₃Nb phase on the mechanical properties of Inconel 718 superalloys, with standard heat treatment, hot isostatic pressing + solution treatment + aging, and hot isostatic pressing + direct aging, were characterized by morphological observation, X-ray diffraction, and tensile experiments. The results of the morphological observation revealed that many fine γ″ precipitates of ~26.49 ± 1.82 nm in mean size were formed in all samples. However, the relatively coarser γ″ precipitates formed in the grain boundaries were only observed in the sample treated with hot isostatic pressing + direct aging. The yield strengths of the hot isostatic pressing + direct aging sample at room temperature and at 650 °C both exhibited the maximum values about 993 ± 5.7 and 811 ± 12.6 MPa, respectively. The γ″ precipitate was considered to be the dominant strengthening phase in the sample according to the lattice misfits (ε) of γ/γ″. The strengthening mechanism of the samples can be explained as the coherency strain strengthening of fine γ″ precipitates. Moreover, due to the coarser γ″ precipitates in the grain boundaries, dislocation-cut ordered particle strengthening also occurred in the sample after hot isostatic pressing + direct aging treatment.

In this work, effect of ultrasonic treatment during solidification on corrosion behavior of Mg-3Al-1Zn and Mg-4Zn alloy was studied by electrochemical, scanning Kelvin probe force microscopy (SKPFM), hydrogen evolution and micro-zone local corrosion simulation tests. Optical microscopy results revealed the second phases were evidently refined as a result of the ultrasonic treatment during solidification. Potentiodynamic polarization measurements showed better corrosion behavior was obtained in ultrasonically treated alloys with refined second phase. Volta potential mapping indicated higher electrochemical nobility of the β -Mg ₁₇ Al ₁₂ and MgZn ₂ phase with respect to the α -Mg matrix. The result of micro-zone local corrosion simulation of Mg-3Al-1Zn alloys showed that the discrete, finer β -Mg ₁₇ Al ₁₂ phase in ultrasonic treated alloy had lower anodic average current density than that of the conventionally cast alloy (about 17.6%).