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 ℃ 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 γ',σ,α-Cr,and MC phases after long-term aging.The mass fraction and size of the dispersed spherical y'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 y'gradually decreases.During the long-term aging process,the σ phases are distributed along the grain boundaries in block or strip shape and within grains in needle or strip shape.The distribution of α-Cr phases in the grains is the same as that of σ phases.With the increase in aging time,the number of σ and α-Cr phases is increased and they are gradually coarsened.The σ 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.
Fe-Ni based alloylong-term agingγ'phaseσ phaseα-Cr phase