Precipitation behavior of the G-phase strengthened 7Ni maraging steels

In this study, G-phase strengthened 7Ni maraging alloys were studied using a combination of thermodynamic prediction based on the TCFE-7 database and advanced experimental techniques, including micro-hardness testing, electron backscatter diffraction (EBSD), in-situ X-ray diffraction (XRD), transmission electron microscopy (TEM), and atom probe tomography (APT). The martensite reversion and phase stability of overcooled austenite were precisely determined for a series of Fe–7Ni–2Si-based alloys, validating the effectiveness of thermodynamic predictions in martensite transformation. Based on these theoretical prediction, aging hardness measurements and microstructural observations further revealed that Ni16X6Si7-G (X = Ti, Nb, Ta) precipitates are effective strengthening phases in 7Ni maraging steel, with the exception of Ni16X6Si7 (X = Mn, Zr) due to their significantly different thermal stabilities. Experimental results showed that the Ni16X6Si7-G (X = Ti, Nb, Ta) precipitates remained stable and densely distributed within the martensitic matrix after aging at 500 °C, resulting in high aging hardness values ranging from 350 to 550 HV. Among the studied alloys, the 1Ti alloy strengthened by the Ni16Ti6Si7-G phase exhibited the finest particle radius (estimated at 1.4 nm) and the highest number density (estimated at 1.9 × 10²⁴/m³). Additionally, it is worth noting that the Ni16Zr6Si7-G phase was believed to form through eutectic reaction with α-Fe during solidification and the Ni16Mn6Si7-G phase was only stable at temperatures below 460 °C and was not detected experimentally. These findings enhance our comprehension of G-phase precipitation and strengthening in 7Ni maraging steel and underscore the potential for utilizing thermodynamic calculations and advanced experimental techniques to guide the design and optimization of high-strength alloys.