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.

The effects of the γ″-Ni₃Nb phase on fatigue behavior of nickel-based 718 superalloys with standard heat treatment, hot isostatic pressing + solution treatment + aging, and hot isostatic pressing + direct aging were investigated by scanning electron microscope, transmission electron microscopy, and fatigue experiments. The standard heat treatment, hot isostatic pressing + solution treatment + aging, and hot isostatic pressing + direct aging resulted in the formation of more and smaller γ″ phases in the matrix in the nickel-based 718 superalloys. However, the grain boundaries of the hot isostatic pressing + direct aging sample showed many relatively coarse disk-like γ″ phases with major axes of ~80 nm and minor axes of ~40 nm. The hot isostatic pressing + direct aging sample with a stress amplitude of 380 MPa showed the longest high cycle fatigue life of 5.16 × 10⁵ cycles. Laves phases and carbide inclusions were observed in the crack initiation zone, and the cracks propagated along the acicular δ phases in the nickel-based 718 superalloys. The precipitation of fine γ″ phases in the matrix and relatively coarse γ″ phases in the grain boundaries of the hot isostatic pressing + direct aging sample can hinder the movement of dislocation.

Mechanical properties and corrosion resistance of La₂O₃/A356 composites with different contents of La₂O₃ were investigated by optical microscopy, X-ray diffractometry, scanning electron microscopy, electrochemical tests, and immersion corrosion tests. The results show that the addition of La₂O₃ refined the α-Al phase of the A356 matrix, and the long stripe-like Si phase and β-Al₅FeSi phase were transformed into short rod-like forms. The La₂O₃/A356 composites with 1.0 wt.% La₂O₃ exhibited the most optimal mechanical properties and corrosion resistance. The yield strength, ultimate tensile strength, and elongation of La₂O₃/A356 composites with 1.0 wt.% La₂O₃ were higher than those of the matrix. The results of electrochemical experiments and the immersion corrosion test show that the corrosion potential of La₂O₃/A356 composites with 1.0 wt.% La₂O₃ was 72 mV higher than that of the matrix, the corrosion current density was 84.8% lower than that of the matrix, and the impedance Z was improved by 59.1% compared to the matrix. The addition of La₂O₃ improved the mechanical properties of the A356 matrix by refining the grains, inhibiting the nucleation of eutectic Si, and promoting the twinning growth mechanism. Moreover, the effect of La₂O₃ on the micro-galvanic corrosion behavior of A356 was discussed.