*2.4. Testing*

The hardness was tested by a digital microhardness tester (HVS-1000Z, Shandong Laizhou Huayin Test Instrument Co., LTD., Shandong, China) at a load of 9.8 N with dwell time 30 s. Three different points were tested on each specimen, and the mean value was taken as the hardness of the specimen.

The mechanical properties were tested on a fully automated, closed-loop servo-hydraulic material testing system (MTS810-50 kN), including the ultimate tensile strength (UTS, σb), the 0.2% offset yield strength (YS, σ0.2), and the elongation (δ). The mechanical properties were tested according with the specifications in ASTM standard E8M-1989. In this testing, three tensile specimens were tested for each group experiment, and the results were stated as the mean value. A high-sensitivity extensometer with a gauge distance of 20 mm was installed. The tensile elongation rate for all tests was 2 mm/min.

The S-N curves were tested on the material testing system (MTS810-50 kN) by the stress-control method. All HCF specimens were tested according with the specification of ASTM standard E466-2007. The stress ratio *R* was 0.1, the peak stress *S*max was 160–320 MPa and the loading frequency was 40 Hz. The whole process was completed at room temperature (298 K) and air. The data were monitored and recorded automatically by the computer.
