Recrystallization control of dilute molybdenum-titanium alloys by multi-step internal nitriding

The control of microstructure and recrystallization temperature in the internally nitrided dilute Mo-Ti alloys was studied through a novel three-step nitriding process at 900 to 1600 °C in N₂ gas, in order to develop highly ductile Mo alloys having a heavily deformed microstructure near the specimen surface. Primary internal nitriding process below the recrystallization temperature of the present alloys at around 1000 °C brings uniform dispersion of superfine Ti-nitride precipitates in the Mo matrix where the deformation microstructure remains. These superfine precipitates decompose and disappear after heating at 1200 °C in a vacuum. However, after secondary nitriding at 1500 °C in a nitrogen atmosphere, those particles grew up to be stick-shaped particles with a length of 50 to 120 nm without the recrystallization of Mo matrix within about 130 μm from the specimen surface. The depth of the region where the recrystalization of Mo matrix was suppressed was greatly dependent upon the secondary nitriding temperature. The third nitriding was conducted at 1500 or 1600 °C. For the specimen nitrided at 1600 °C, the recrystallization temperature in a vacuum was elevated above 1600 °C.