Fe-18Ni-2Al合金のミクロ組織と破壊

Abstract

The relation between microstructure and tensile fracture for Fe-18Ni-2Al alloy has been studied. In the alloy aged in the temperature range 480 to 550°C, the precipitates were identified by electron diffraction as NiAl or (Fe, Ni)Al of the B2 type structure.The fracture mode of tensile specimens aged for 3 hr at 480°C, in which intergranular microcracks were initiated before fracture, depended on the prior austenite grain size. In the case of large grain size, the fracture mode was intergranular. On the contrary, specimens with a small grain size was fractured by shearing. The effective intergranular surface energy was estimated as 5.2×105 erg/cm2.Specimens compressed in the quenched condition exhibited sharp and wavy slip bands and passed through the grain boundary by cross slip. In the aged condition the slip bands became obscure and slip occurred mainly on the {110} plane in the 〈111〉 direction even near the grain boundary. This shows that cross slip becomes very difficult and the stress is concentrated at the grain boundary. It is suggested that one of the causes for the brittleness of this alloy is the result of piling-up of dislocations at the grain boundaries due to the difficulty of cross slip and the ease of nucleation and propagation of cracks.