Internal Structure and Hardness of Surface Oxidized FeCrNiAl Alloys

We have examined the structure and hardness of FeCrNiAl alloys, in which the alumina layer is formed at their surfaces by heat treatment.In FeCrNiAl alloys, the Ni and Zr contents are fixed at 21 mass% and 0.2 mass% respectively, and the Al content is changed to 4.6, 5.3, 6.0 and 6.7 mass% and the sum of Cr and Al contents is changed to 33, 36 and 39 mass%, These samples are heated at 1423 K for 54 ks in air and then examined. The heat treatment produced alumina layers of 3∼4 μm thickness at the surface and root like alumina of 20∼30 μm length stretching out from the alumina layers. The austenite phase appears in the alloys in which the Al content is less than 4.6 mass%, and also it appears in those in which the sum of Cr and Al contents is less than 33 mass% and the Al content is less than 6.0 mass%. The austenite phase is as low as 102 in vickers hardness, so it is harmful to hardening the alloys. NiAl based intermetallic compound particles of about 5 μm diameter (large size) are produced in the alloys, but a region free from the large particles appears extending from the surface to a certain depth. The depth depends on the Cr and Al contents. Even in this region the particles of the NiAl based intermetallic compound of about 0.3 μm diameter are formed. The vickers hardness of the region free from the large size particles is greater than 450. This region is harder than the region with about 5 μm particles.