Influence of multi-step nitriding condition upon mechanical properties of dilute Mo-Ti alloys

Internal nitrided dilute Mo-Ti alloys having a deformation microstructure near the specimen surface were prepared by a novel multi-step internal nitriding at 1423 K to 1873 K in N₂ gas. Thickness of the deformation microstructure can be controlled by varying the nitriding condition. In order to investigate the relationship between the thickness of the deformation microstructure and mechanical properties, static three-point bend tests were performed at temperatures from 77 K to 298 K. Ductile-brittle transition temperature (DBTT) was strongly dependent on the thickness of the deformation microstructure. DBTT of the specimen having the deformation microstructure of 110 μm and 330 μm thickness were 198 K and 170 K, respectively. Yield strangth of these specimens was almost same value at each temperature. The crack initiation area of the specimen having the thin deformation microstructure was an inward recrysallized region, whereas that of the specimen having the thick deformation microstructure was a specimen surface. These results indicate that the difference in the DBTT between the above-mentioned specimens is due to the difference in the fracture strength of the inward region.