Effect of alloying with V and Ti on microstructures and properties in Fe-Ni-Mo-C invar alloys

Mo alloying could improve strength of the conventional Fe-36Ni invar alloy. Unfortunately, increment in tensile strength is low due to rapid growth of the Mo2C carbides during aging for the single Mo-alloyed invar alloy. In this research, several strong carbide-forming alloying elements like V and Ti, together with certain N, were introduced in the Mo-alloyed invar alloy. Our results prove that their addition not only changes precipitated product variety, but also enhances aging temperature of the invar alloy. Precipitation of Ti(C,N) and VC could remain stable nano-sized particles during the whole aging, whereas the Mo2C behaved a different growth mode. The sample aged at 780 degrees C for 5 h presented the maximum tensile strength of 940 MPa, together with a low CTE value of 2.33 x 10(-6) K-1 in the temperature range from -50 to 100 degrees C. Those values were superior to the corresponding data in the single Mo-alloyed one. This unique combination of high strength and low CTE should be attributable to homogeneous precipitation of the nano secondary phase particles. However, preferential growth, especially the Mo2C particles, and aggregation in the precipitated products resulted in an unexpected decrease in strength and increase in the CTE value for the present invar alloy.