Characterisation of 20Cr32Ni1Nb alloys in as-cast and Ex-Service conditions by SEM, TEM and EDX

The microstructure of a 20Cr32Ni1Nb alloy in the as-cast and Ex-Service conditions has been fully characterised using scanning electron microscopy (SEM)/transmission electron microscope (TEM) and EDX. Nb-rich MC and Cr-rich M23C6 carbides were identified in both the as-cast and Ex-Service samples. Minor addition element Mn is present more in M23C6 carbides than in other phases. In addition, a Si-rich intermetallic compound formed on the MC carbides along the grain boundaries in the cast alloy which had been in service around 760 °C for about 4 years. The Si-rich phase was identified as G-phase by the experimental and simulated TEM diffraction patterns. EDX of TEM revealed that its molecular formula is Nb6 (Ni,Fe)16 (Si,Cr)7.