Low Ni alumina-forming austenitic (AFA) heat-resistant steel is an advanced high-temperature stainless steel with reduced cost, good machinability, high-temperature creep strength, and high-temperature corrosion resistance. Using the First-principles approach, this study examined the effect of Mn content on austenite stability and mechanical properties at the atomic level. Adding Mn to low Ni-AFA steel increases the unit cell volume with an accompanying increase in the absolute value of formation energy; the austenite formed more easily. The austenitic matrix binding energy decreases and remains negative, indicating austenite stability. As the Mn content increases from 3.2 to 12.8 wt%, the system's bulk modulus (B) rises significantly, and the shear modulus (G) falls. In addition, the system's strength and hardness decrease, and the Poisson ratio of the austenite matrix increases with improved elasticity; the system has excellent plasticity with an increase in the B/G. For the Fe22–Cr5–Ni3–Al2 system, with the increase of Mn content, the electron density distribution between the atoms is relatively uniform, and the electrons around the Mn atoms are slightly sparse, which will slightly reduce the structural stability of the matrix. The experiment demonstrated the matrix maintains the austenitic structure when adding 3.2–12.8 wt% Mn elements to low Ni-AFA steel. At an Mn content of 8 wt%, the overall mechanical properties of the high-Mn AFA steel are optimal, with a tensile strength of 581.64 MPa, a hardness of 186.17 HV, and an elongation of 39%.

Aluminum foam-filled tubes are complicated structures created by filling one or more thin-walled metal tubes with varying shapes in cross-sections with aluminum foam. We optimized the structure of aluminum foam-filled tubes using software simulation, compression, and three-point bending experiments. Filling aluminum foam not only improves the axial compressive performance and bending strength of the thin-walled metal tube but also eliminates the disadvantage of the low strength of the aluminum foam. The aluminum foam-filled single tube exhibited significant improvements in load-bearing and energy absorption, with a one-time increase in load-bearing and five times increase in energy absorption compared with an empty tube. The aluminum foam-filled single tube with a smaller diameter-to-thickness ratio has a higher load-bearing capacity. In contrast, the length-to-diameter ratio has a lower impact on load-bearing capacity. Once the filling length reaches the effective filling length, the structure can still support higher loads and effectively reduce its overall weight. The compression and bending properties in the double-tube structure filled with foam aluminum improved significantly compared with the empty tube and single-tube structure filled with aluminum foam. The total compressive energy absorption capacity in the double-tube structure filled with aluminum foam is 2.01 times that of the empty tube and 1.81 times that in the single-tube-filled structure. When the wall thickness of the filled stainless steel tube is 1.0 mm, the total bending energy absorption and the specific absorption energy of the aluminum foam-filled double tube structure are 1.5 and 2.1 times that of the corresponding aluminum foam-filled single tube, respectively.