Fe-18Ni-5Si 合金の引張破壊におよぼすミクロ組織の影響

抄録

Structural changes and the tensile fractures have been studied in an aged Fe-18%Ni-5%Si alloy by means of electron microscopy and tensile tests. It has been concluded that the precipitates in the specimen aged at 350 and 400°C for 6 hr are identified as Fe₃Si or (Fe, Ni)₃Si having DO₃ structure, and on aging at 450 and 500°C for 6 hr as Ni₅Si₂ or (Fe, Ni)₅Si₂. In the latter case preferential precipitation occurred at the prior austenite grain boundaries and those of lath martensite besides general precipitation.
The fracture mode of tensile specimens depended on the prior austenite grain size, it changed from dimple fracture to intergranular fracture with increasing grain size. The effective intergranular surface energies in the low- and high-temperature-aged condition were estimated at 7∼8×10⁴ and 2∼4×10⁵ erg/cm² respectively.
It would appear that one of the causes of the embrittlement of this alloy aged at lower temperature is the result of piling-up dislocations at the grain boundaries due to the difficulty of cross slip, on the other hand the decrease in toughness of this alloy aged at higher temperature is due to the precipitation at grain boundaries.