Experimental and theoretical investigation of precipitate coarsening rate in Z-phase strengthened steels

Two Z-phase strengthened 12% Cr steels were investigated: they are similar in composition, however one steel contains Nb and the other contains Ta. Z-phase precipitates (CrMN, M = Nb or Ta) provide precipitation hardening for creep resistance at 650 °C in these steels. Experimental data based on the transmission electron microscopy in- vestigation of the size evolution of Z-phase precipitates during isothermal ageing at 650 °C show that the Ta-based Z-phase benefits from a five times smaller coarsening constant compared to the Nb-based Z-phase. Theoretical calculations show that this is attributed to the smaller diffusivity of Ta compared to Nb in the steel matrix. Be- sides, comparing the size of the Ta-based Z-phase precipitates in the gauge and head portion of a crept specimen, it is shown that Z-phase coarsens faster under stress.