Creep Behavior of Ti–6Al–4Nb–4Zr Fabricated by Powder Bed Fusion Using a Laser Beam

Abstract

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°C 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°C 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 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.