Elastic consideration of the precipitation in model alloys of maraging steels: theory and experimental validation

Abstract

The unique combination of high strength and high fracture toughness in maraging steels is attributed to the precipitation of mainly two phases, Ni 3 Mo and Ni 3 Ti, in the soft bcc martensitic matrix. Experimentally, it is difficult to distinguish between these phases; moreover, the effect of each precipitating phase on the properties of the steel is ambiguous. In the present work, we tackle these questions by analyzing the elastic fields and elastic energy associated with each precipitating phase and their role in the determination of the shape and relative arrangement. Using a semi-analytic approach based on the solution of the equations of elasticity by Fourier transform, the elastic fields and elastic energy associated with each precipitate were calculated. The calculations show that the minimum self-energy of both precipitates is accomplished when the smallest crystallographic misfit lies parallel to the longest geometric axis. The combination of elastic energy and surface energy provides an explanation to the observed morphologies of the precipitates, each type having an elongated shape but with a different aspect ratio. According to the calculations, the minimum self-elastic energy associated with Ni 3 Ti coherent precipitate is 2.8-fold larger than the energy of Ni 3 Mo. This implies for a larger strengthening effect of the former, as was confirmed experimentally in model alloys, one without Mo and the other without Ti. The elastic model also predicts that elongated shape and the anisotropic misfit strains favor several vertical arrangements of both precipitates, as was also verified experimentally by transmission electron microscopy of aged model alloys.

Addressing the issue of precipitates in maraging steels – Unambiguous answer

“…Since Fe-Mo precipitates are not coherent with the matrix, contrary to Ni 3 Mo and Ni 3 Ti, its formation barrier is much higher. It was calculated in [32] that the elastic energy in the matrix surrounding a Ni 3 Mo precipitate is two orders of magnitude lower than in the vicinity of the Ni 3 Ti precipitate, implying that the formation barrier of Ni 3 Mo is low. Thus, although unstable, Ni 3 Mo precipitate forms at the early stages of aging along with the stable Ni 3 Ti phase.…”

Kinetics of Martensite Reversion to Austenite during Overaging in a Maraging 350 Steel

“…(Received on September 26, 2018; accepted on January 16,2019) The present work has studied the kinetics of partial reversion of martensite into austenite, which occurred during overaging between 520 and 600°C in a maraging 350 steel. The microstructural modifications were followed using optical microscopy, scanning electron microscopy, X-ray diffraction and ferritoscopy.…”

“…and would be used in the formation of the austenite reversion and the molybdenum used in the formation of the Fe 2 Mo. 1,6,11,13,14) Studies performed by Moshka and coauthors 15) and by Shmulevitsh and coauthors 16) showed that the Ni 3 Mo precipitate is thermodynamically more unstable than the Ni 3 Ti precipitate. In this way, it is more probable that the dissolution of the Ni 3 Mo precipitate may give origin to the reverted austenite and to the Fe-Mo precipitates simultaneously.…”

Investigation on the Fatigue Behaviors of Maraging Steel at Different Stress Ratios

“…Maraging steel is a typical high-strength steel, which has been taken as significant material options for the production of rocket engines, aircraft, cryogenic missiles, deep manned cabin, etc. [1][2][3][4][5][6] . In addition to the tensile properties, the fatigue properties of maraging steel are also the significant factors influencing its industry application.…”