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

抄録

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.

 

Mater. Trans. 64 (2023) 1175-1182に掲載．AbstractおよびFigs. 5-9のキャプションを修正．

Creep tests were performed at 600℃ and 137 MPa for the PBF-LB, and the creep curves were plotted together with the forged samples performed in the previous study. The creep life strongly depends on melting pool size or grain size [16]; that is, the shortest creep life was obtained in the forged sample with bi-modal (10 µm), then it increased as grain size increased as B-HT (100 µm), D-HT (300 µm). The longest one was obtained in the forged sample with a lamellar structure (550 µm). Creep behavior in primary and steady-state regimes of samples heat treated at 970℃ followed by air cooling or furnace cooling were similar in that of B-HIP due to similar microstructure. The creep strain was smaller in the furnace-cooled sample with a large volume fraction of the α phase and high Nb contents in β phase, but the effect of the lamellar spacing was small. Heat treatment at 1100℃ in β phase effectively improved creep life.