Improvement in recrystallization temperature and mechanical properties of a commercial TZM alloy through microstructure control by multi-step internal nitriding

In order to develop a molybdenum alloy with high recrystallization temperature and excellent mechanical properties, multi-step internal nitriding of a commercial TZM alloy (Titanium-Zirconium-Molybdenum alloy) was studied through optical and transmission electron microscopy, fractographic analysis and three-point bending tests. Two types of nitriding processes, a two-step and another four-step process, were carried out at temperatures between 1423 and 1873 K. Recrystallization was completely suppressed, even at the center of a 1-mm-thick specimen through four-step nitriding, whereas recrystallization proceeded at the specimen center in the case of two-step nitriding. The recrystallization temperature of the TZM alloy in vacuum was raised above 1973 K by four-step nitriding. The ductile-to-brittle transition temperature (DBTT) of the nitrided TZM alloy depended strongly on the thickness of the deformed microstructure. DBTT of the four-step nitriding specimen was below 130 K, which is about 70 K lower than that of the two-step nitriding specimen. The yield stress of the four-step nitriding specimen at 1773 K was about 2.6 times as large as that of the recrystallized specimen.