Aged Microstructures of a Low Thermal Expansion 41Fe-29Ni-22Co-4Nb-2Cr-1Ti-0.5Al-0.5Si Superalloy

摘要

41Fe-29Ni-22Co-4Nb-2Cr-1Ti-0.5Al-0.5Si alloy (alloy 929C) is the latest low thermal expansion superalloy with a good resistance to SAGBO (Stress Accelerated Grain Boundary Oxidation) embrittlement at elevated temperatures. This alloy is strengthen by the precipitation of #gamma#' (Ni_3 (Al,Ti); L1_2) phase in the alloy matrix. This study was carried out to elucidate the relation between the age-hardening and the growth of #gamma#' precipitates in alloy 929C by micro Vickers hardness test and transmission electron microscopy.The results obtained in this study are as follows:(1) Comparing with the similar superalloys like alloys 909 and 929, the age-hardening rate in alloy 929C is slow; its peak hardness is high, and the overaging is suppressed.(2) Age-hardening proceeds with growing #gamma#' precipitates and the hardness reaches to the maximum in the size of #gamma#' precipitates 11 approx 13 nm. After reaching to the maximum value the hardness decreases gradually according to the coarsening of #gamma#' phase. Though the #gamma#' precipitates in the alloy 929C are relatively stable, they transform gradually to a stable #eta# phase with increasing aging time but never transform to #epsilon# phase unlike alloys 909 and 929.(3) The growth kinetics of the #gamma#' precipitates are explained by Ostwald ripening theory of volume diffusion controlled growth. The apparent activation energy for the growth of #gamma#' precipitates was estimated to be 259 kJ/mol which was nearly equal to those of diffusion of Ti or Fe atoms in #gamma#-iron or nickel.(4) The #gamma#' phase precipitates in sphere in the early stage of aging and shifts to in cubic with increasing aging time. The #gamma#' precipitates with the size of 150 approx 200nm change to a star-like form with eight horns according to the growth of <111>_(#gamma#') direction.