Acta Materialia 50 (2002) 2905–2916 www.actamat-journals.com Oxygen–molybdenum interaction with dislocations in Nb-Mo single crystals at elevated temperatures E. Miura a,* 1 , K. Yoshimi b , S. Hanada b a Department of Materials Processing, Tohoku University, Sendai 980-8577, Japan b Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Received 30 July 2001; received in revised form 12 March 2002; accepted 12 March 2002 Abstract A study using a transmission electron microscope was performed for the deformation microstructures of Nb- 20mol%Mo single crystals containing from 0.03 to 0.65 mol% oxygen and a Nb-40mol%Mo-0.05mol%O single crystal, compressed to approximately 4% plastic strain at 298 K, 973 K and 1473 K. At 298 K, screw dislocations are predomi- nant in all alloys. At 973 K, in Nb-20Mo-0.03O and Nb-40Mo-0.05O, the dislocation microstructure contains the same amount of edge dislocations and small edge dipoles that tend to aggregate into clusters. In Nb-20Mo-0.65O, however, the arrangement consists of long glide loops parallel to the screw direction. At 1473 K, edge dislocations are dominant in all alloys. From the results, the following conclusions are made: (1) Oxygen atoms impede screw dislocation motion at 298 K and 973 K; (2) The influence of oxygen on screw segments is significantly strong at 973 K; (3) At 1473 K, oxygen atoms impede edge dislocation motion. It is inferred that O-Mo atmosphere is formed around dislocations.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Nb-Mo-O single crystals; Transmission electron microscopy (TEM); Dislocation; Mechanical properties; High tempera- ture deformation 1. Introduction It has been known in several studies that Nb- Mo alloys exhibit solid solution strengthening at elevated temperatures because of the size misfit between Nb and Mo [1–3]. In addition, the com- bined effect of Mo and O functions against the * Corresponding author. Tel: +81-95-849-7657; Fax: +81- 95-849-7658. E-mail address: emiura@net.nagasaki-u.ac.jp (E. Miura). 1 Present address: Department of Dental Materials Science, Nagasaki University School of Dentistry, Nagasaki 852-8588, Japan. 1359-6454/02/$22.00  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-6454(02)00115-5 strength of Nb-Mo alloys [1]. It was observed that 1 mol% O-addition strengthens Nb-Mo single crys- tals at temperatures from 298 K up to 1600 K, especially from 500 K to 1473 K, whereas O strengthening in Nb single crystals occurs only at temperatures below 523 K [1]. These results indi- cate that the combined effect of O and Mo changes not only the strength of Nb-Mo alloys but also the effective temperature of O for the solid solution strengthening. We attributed the strengthening observed in Nb-Mo-O alloys to the interstitial-sub- stitutional interaction (the i-s interaction). The slip behavior of Nb-20 mol%Mo single crystal was studied in our previous work [4], and