低熱膨張性41Fe-29Ni-22Co-4Nb-2Cr-1Ti-0.5Al-0.5Si超耐熱合金の時効組織

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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 γ′ (Ni_3(Al, Ti) ; Ll_2) phase in the alloy matrix. This study was carried out to elucidate the relation between the age-hardening and the growth of γ′ 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 γ′ precipitates and the hardness reaches to the maximum in the size of γ′ precipitates 11∿13nm. After reaching to the maximum value the hardness decreases gradually according to the coarsening of γ′ phase. Though the γ′ precipitates in the alloy 929C are relatively stable, they transform gradually to a stable η phase with increasing aging time but never transform to ε phase unlike alloys 909 and 929. (3) The growth kinetics of the γ′ precipitates are explained by Ostwald ripening theory of volume diffusion controlled growth. The apparent activation energy for the growth of γ′ precipitates was estimated to be 259kJ/mol which was nearly equal to those of diffusion of Ti or Fe atoms in γ-iron or nickel. (4) The γ′ phase precipitates in sphere in the early stage of aging and shifts to in cubic with increasing aging time. The γ′ precipitates with the size of 150∿200nm change to a star-like form with eight horns according to the growth of <111>_γ′ direction.
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低熱膨張性41Fe-29Ni-22Co-4Nb-2Cr-1Ti-0.5Al-0.5Si超耐熱合金の時効組織

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