Fe基超合金の時効組織と極低温における機械的性質に及ぼす添加元素の影響

Abstract

A study has been made on effects of C, B, Al, and Mo on cellular precipitation, grain-boundary and intragranular microstructures, and mechanical properties at 4 K of an A286-type iron-base superalloy. Base composition was low C (≤0.005) and Si (≤0.1) Fe-25Ni-15Cr- (2.4 and 3) Ti (mass%), and 0.06C, 0.006B, 1Al, 1-1.5Mo, and 1.4Mo+0.006B were independently added to the alloy in order to explicitly examine the effects of the respective elements.The independent addition of C, B, Mo and Mo+B suppressed the cellular precipitation in the alloy aged at 973 K after solution-treatment at 1373 K for 3.6 ks followed by water quenching. Particularly, C and Mo completely retarded the precipitation for times up to 720 ks. Aluminum, however, showed no retarding effects on the cellular reaction.Microstructural observations and fractographs revealed that there were three metallurgical factors which degraded the low temperature ductility (reduction of area and total elongation in tensile tests) and toughness (Charpy absorbed energy) of the nearly peak-aged alloy: (a) Cellular precipitates, (b) grain-boundary precipitates such as TiC and Fe2B which are responsible for intergranular fracture, and (c) coarse intragranular precipitates which consist mainly of TiC and act as nucreation sites for intragranular voids or cracks. The C addition can only suppress (a), but introduces (b) and (c). The B bearing alloys of which ductility and toughness are higher than those of the C bearing alloys are free from (a) and (c), but not from (b). The Mo addition to low C alloys eliminates these three factors, which results in marked enhancement in ductility and toughness at 4 K.