Cyclic Partial Phase Transformations In Low Alloyed Steels: Modeling and Experiments

Mechanical properties of low alloyed steels are directly determined by their microstructures. Thanks to versatility in their microstructures, the mechanical properties of low alloyed steels are much more adjustable than other materials. In the industry, one of the most effective ways to obtain the microstructure for the targeted mechanical properties is via the transformation of the iron lattice from a face centered cubic (FCC) to body centered cubic (BCC). During the lattice transformation, there is also redistribution of carbon or other alloying elements between these two iron lattices. In order to precisely tune the mechanical properties of low alloyed steels, it is required to deeply understand the mechanism of the FCC to BCC transformation. This thesis aims to shed new light on local redistribution of alloying elements at the moving interface during the FCC to BCC lattice transformation ( including both the austenite to ferrite and austenite to bainite transformations in low alloyed steels), and thus improve the mechanism of these two typical phase transformations in steels. For this purpose, novel experimental approaches (the cyclic partial phase transformation approach and the interrupted cooling approach) are designed to reveal the redistribution character of alloying elements and its effect on the phase transformation kinetics. The new experimental results allow a clear discrimination between the correctness of the existing models for both the austenite to ferrite and bainite transformations. New models are also proposed to successfully predict the special features observed in the newly designed experiments. This thesis was granted Chinese Government Award for Outstanding Self-financed Students Abroad.Aerospace Structures and MaterialsAerospace Engineerin