Microstructure Evolution of New Superalloy 21Cr-32Fe-41Ni During Long-Term Aging at High Temperature

Fe-Ni based alloys serve in high temperature environment for a long time, so the microstructure stability at high temperature is one of the important indexes of the alloys. The microstructure and properties evolution of a new Fe-Ni based superalloy 21Cr-32Fe-41Ni at 750 degrees C for different aging time were investigated by scanning electron microscope (SEM), transmission electron microscope (TEM), electron probe microanalysis ( EPMA) and chemical phase analysis. The results show that the precipitates in the alloy are mainly gamma', sigma, alpha-Cr, and MC phases after long-term aging. The mass fraction and size of the dispersed spherical gamma' in the matrix are increased rapidly within 500 h aging. With the increase in aging time from 500 h to 2000 h, the increase rate of mass fraction and size of gamma' gradually decreases. During the long-term aging process, the sigma phases are distributed along the grain boundaries in block or strip shape and within grains in needle or strip shape. The distribution of alpha-Cr phases in the grains is the same as that of sigma phases. With the increase in aging time, the number of sigma and alpha-Cr phases is increased and they are gradually coarsened. The sigma phases with discontinuous distribution along the grain are gradually connected, and there is a tendency to develop into a network distribution. With the increase in aging time to 2000 h, the strength of the alloy increases first and then decreases, and reaches the peak after aging for 500 h. While, the hardness keeps increasing.