Phase Transformation and Mechanical Properties of G Phase (Mn6Ni16Si7) in Mn-Ni-Si Model Alloys after 1,000°C Annealing.

Mn6Ni16Si7 intermetallic compounds, referred to as the G phase, and their precursors are recognized as potential secondary phases precipitated in neutron-irradiated steels and are the main cause of embrittlement in low-alloyed reactor pressure vessel steels under long-term operation. To obtain the mechanical properties and thermal stability of the G phase, two Mn-Ni-Si model alloys (composed of 21 mol%Mn-58 mol%Ni-21 mol%Si and 30 mol%Mn-58 mol%Ni-12 mol%Si) were annealed at 1,000°C. The existence of the G phase in both annealed ternary model alloys was confirmed by XRD and SEM/EDS. Meanwhile, the process of different Mn-Ni-Si ternary regions to the G phase transformation under 1,000°C annealing has also been discussed. Young's modulus and nano-hardness of the G phase were measured using the nanoindentation technique. The results showed similar values for the G phase in two alloys with Young's modulus of approximately 220 GPa, which is similar to Young's modulus of iron. This fact suggests the necessity of reconsideration of the hardening model contributed by G phase precipitates in reactor pressure vessels in the future. [ABSTRACT FROM AUTHOR]