gassing and welding, and then hot isostatic pressed under high pressure and temperature. After removing the capsule, the target part can be obtained through simple machining and heat treatment. Therefore, near-net shape hot isostatically pressing (HIP) process has become a prevalent route for the manufacture of key parts with geometrical complexity and difficult-to-machine materials in advanced industrial applications. [6][7][8][9] Typically, capsules are made of carbon steels, and the powder feedstock is difficultto-process materials such as titanium, [10] titanium-aluminum, [11] and superalloys, etc. [12] Close contact between capsules and powders at high pressure and temperature unavoidably lead to a strong elemental diffusion, which affects the surface microstructure and deteriorates the surface performance of parts. [13,14] The main contamination elements were carbon and iron stemmed from carbon steel capsules. Carbides are generated when metal elements in feedstock meet the diffusional carbon. [15][16][17] The precipitation of film-like M 23 C 6 carbides at grain boundaries was deleterious to the stress-rupture limitation of alloys, and the larger blocky MC-type carbides lead to a lower fatigue life. [18][19][20][21] Excessive iron entering into nickelbased superalloys cause composition transformation from Ni matrix to an iron-based alloy, thereby significantly reducing the high-temperature mechanical properties of the alloy. [22,23] Corrosion resistance was impaired at the diffusional interface between AISI 4130 carbon steel and a nickel superalloy due to microstructural variation. [24] Bassini et al. reported that a diffusional layer with a thickness of ≈500 µm between Astrology powder and a low carbon steel capsule was needed to be removed to meet operation conditions. [25] It was also showed that the thickness of the diffusion zone was closely related to the HIP process parameters and more dependent on temperature. [15] Based on the above-mentioned results, elemental migration was likely to induce microstructural variation and new precipitate formation, which was not easy to be predicted under different experimental conditions. Therefore, understanding the diffusional depth and formation mechanism of the interface can provide valuable guidance for the surface reduction after HIP.