TiAl基合金の流動層処理による高温耐酸化性の向上

Abstract

TiAl-base alloys are expected to substitute conventional materials for exhaust parts of automobile engines, because they have lower density and higher specific strength at elevated temperatures than heat resistant steels and Ni-base superalloys. The major key to successful application of TiAl alloys is improvement of oxidation resistance at high temperature.A new surface treatment using a fluidized bed with WO3 powder has been developed. The reagent used was a mixture of 40 mass%WO3 powder (100-200 mesh) with 60 mass%Al2O3 powder (80 mesh). The bed was fluidized by an argon gas flow. Specimens were treated in the bed at 1273 K for 7.2 ks. The oxidation tests were carried out at 1173 K and 1223 K for 720 ks in air and in a typical exhaust gas atmosphere. The mass changes were examined, in addition to the characterization of treated and oxidized surface using Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and Auger Electron Spectroscopy (AES).The cyclic oxidation resistance of TiAl-base alloys in air and exhaust gas up to 1223 K was significantly improved. The excellent oxidation resistance obtained is attributable to a continuous and sound Al2O3 surface layer formed during the treatment. This protective layer acts as a barrier against the formation of a complex oxide scale consisting of a TiO2 layer and a porous inner layer of TiO2 and Al2O3. The new surface treatment gives an oxidation resistance superior to that of the Ni-base superalloy Inconel 713C up to 1223 K.