It is of great significance for structural design and engineering evaluation to obtain the elastic–plastic parameters of materials. The inverse estimation of elastic–plastic parameters of materials based on nanoindentation technology has been applied in many pieces of research, but it has proved to be difficult to determine the elastic–plastic properties of materials by only using a single indentation curve. A new optimal inversion strategy based on a spherical indentation curve was proposed to obtain the elastoplastic parameters (the Young’s modulus E, yield strength σ_y, and hardening exponent n) of materials in this study. A high-precision finite element model of indentation with a spherical indenter (radius R = 20 µm) was established, and the relationship between the three parameters and indentation response was analyzed using the design of experiment (DOE) method. The well-posed problem of inverse estimation under different maximum indentation depths (h_max1 = 0.06 R, h_max2 = 0.1 R, h_max3 = 0.2 R, h_max4 = 0.3 R) was explored based on numerical simulations. The results show that the unique solution with high accuracy can be obtained under different maximum press-in depths (the minimum error was within 0.2% and the maximum error was up to 1.5%). Next, the load-depth curves of Q355 were obtained by a cyclic loading nanoindentation experiment, and the elastic–plastic parameters of Q355 were determined by the proposed inverse-estimation strategy based on the average indentation load-depth curve. The results showed that the optimized load-depth curve was in good agreement with the experimental curve, and the optimized stress–strain curve was slightly different from the tensile test, and the obtained parameters were basically consistent with the existing research.

In this study, a ~20 μm (Ni,Pt)Al coating was applied to an IC21 substrate via electroplating followed by aluminizing. A thermal exposure test at 1100 °C in air was used to investigate the microstructure evolution and nanoindentation behavior of samples with and without coating. The experimental results show that the (Ni,Pt)Al coating caused a phase change from β-(Ni,Pt)Al to Ni₃Al, and the Topologically Close-Packed (TCP) phase at the interdiffusion zone grew larger after thermal exposure for 200 h at 1100 °C, while a layer of γ’ phase was formed on the surface of the IC21 substrate. The nanoindentation test indicated that the elastic modulus was reduced in both annealed samples (with and without coating), and after annealing the sample with coating had higher elastic modulus than the sample without coating. This result reveals that the coating effectively prevented the precipitation of refractory metal elements and the coarsening of the microstructure in the substrate, and thus shows that the coating had advantages not only in terms of improving the high-temperature oxidation properties of the substrate, but also played a significant role in improving the mechanical properties of the substrate.