レーザービーム粉末床溶融結合法により作製した Ti–6Al–4Nb–4Zr のクリープ挙動

Powder bed fusion using a laser beam (PBF–LB) was performed for Ti–6Al–4Nb–4Zr (mass％) developed by our group to improve the oxidation resistance at temperatures greater than 600℃ by adding Nb and Zr to near–α alloys. Microstructure evolution of the PBF–LB samples by heat treatment was investigated, especially for heat treatment duration in the α + β phase, cooling rate, and heat treatment in the β phase. The equiaxed α phase formed during heat treatment along the melting–pool boundaries. The high volume fraction of the α phase and high Nb contents in the β phase was obtained by slow cooling (furnace cooling) compared with fast cooling (air cooling). The α/β lamellar structure formed in the melting pool boundaries with 100 µm in size and no equiaxed α phase formed along the boundaries by heat treatment in the β phase regime. Creep life at 600℃ and 137 MPa was similar for the air–cooled and furnace–cooled samples, but the slightly slower deformation was obtained in the furnace–cooled sample. Creep life of the sample heat treated in the β phase region drastically increased due to the absence of the equiaxed α phase. Dominant deformation mechanism of creep was grain boundary sliding. The small equiaxed α phase accelerated grain boundary sliding.