Micro-/Meso-Structure Control of Multi-Hostmetal Alloys by Massive Nitrogen Supersaturation.

The low-temperature plasma nitriding was utilized to describe the microscopic solid-phase separation in the austenitic stainless-steel type AISI316, induced by the nitrogen supersaturation. This nitrogen supersaturated layer with the thickness of 60 µm had a two-phase nanostructure where the nitrogen-poor and nitrogen-rich clusters separated from each other. Due to this microscopic solid-phase separation, iron and nickel atoms decomposed themselves from chromium atoms and nitrogen solutes in this nitrogen supersaturated AISI316 layer. These microscopic cluster separation and chemical decomposition among the constituent elements in AISI316 were induced in the multi-dimensional scale by the plastic straining along the slip lines in the (111)-orientation from the surface to the depth of matrix. The nitrogen solute diffused through the cluster boundaries into the depth. With the aid of masking technique, this nitrogen supersaturation and nanostructuring was controlled to take place only in the unmasked AISI316 matrix. The nanostructures with two separated clusters were mesoscopically embedded into AISI316 matrix after the masking micro-textures. This microscopic and mesoscopic structure control was available in surface treatment of multi-host metals such as superalloys and high entropy alloys.

Effect of Alloying Elements and Low Temperature Plasma Nitriding on Corrosion Resistance of Stainless Steel.

Although nitriding treatment usually improves the hardness and wear resistance of stainless steel, it also reduces its corrosion resistance. The effects of different nitriding temperatures and time and main alloying elements in stainless steel on the properties of the martensitic precipitation hardening of stainless steel were studied by first-principles calculations and experiments in this study. The results showed that the corrosion resistance of the martensitic stainless steel 0Cr17Ni4Cu4Nb was much lower than that of 1Cr15Ni2Mo2Cu before and after nitriding. According to the density functional theory calculation results, the molybdenum-containing stainless steel had higher stability and corrosion resistance and a lower Fermi level, electron conduction concentration and electrochemical activity than the niobium-containing stainless steel before and after nitriding. In addition, at the same temperature, the surface hardness of the 1Cr15Ni2Mo2Cu steel increased linearly with the prolongation of nitriding time, but its corrosion resistance decreased. Under the same nitriding time (24 h), the nitriding temperature increased from 300 to 450 °C, and the surface hardness and nitriding layer depth of the nitriding steel increased gradually, while the corrosion resistance decreased gradually. These results were attributed to the Cr-poor phenomenon caused by the formation of CrN. The 1Cr15Ni2Mo2Cu martensitic stainless steel obtained a high surface hardness after nitriding at 300 °C for 24 h, and the corrosion resistance did not decrease.