Abstract
We present analytical and numerical results for the dominant mechanisms of pattern selection in two growth regimes which are crucial in elastically influenced solid-solid transformations like the bainitic one. The first growth regime comprises the very early regime, when in a nucleation scenario the size of the nucleus is so small that the bulk crystal structure is typically not yet fully developed and the phase is elastically softened. Here we see a dominant effect of curvature effects in analogy to the theory on growth of lenticular melt inclusions. The second growth regime is of specific interest to bainitic steels. During the bainitic reaction subunits form, grow up to a point where the thermodynamic driving force is kinetically overcome by a deformation-induced growth barrier, stop growth and then nucleate new subunits. Thus, the regime prior to the new subunit nucleation corresponds to the limiting case of vanishing growth velocity. For both, analytical and numerical approach, we use sharp interface descriptions of the problem, for the numerical approach we invoke a representation of the problem in terms of boundary integral equations.
Highlights
Bainitic press hardening is a key production technology for advanced high strength steels, in the automotive industry [1]
We present analytical and numerical results for the dominant mechanisms of pattern selection in two growth regimes which are crucial in elastically influenced solid-solid transformations like the bainitic one
We introduce the basics of the boundary integral method, which is used for the investigations on the terminal subunit growth regime
Summary
Bainitic press hardening is a key production technology for advanced high strength steels, in the automotive industry [1]. It is of specific interest to the industry to take this important technology to the level in terms of process optimisation This technique, using a combination of interrupted cooling and quasi-isothermal holding in the bainitic parameter regime, offers highly desirable strength-failure-strain combinations [2]. An intermediate scenario, where ferrite grows with a partial supersaturation of carbon, with the remaining carbon partitioning into austenite or forming carbides, is described e.g., in [9,10,11] All of these hypothesises effectively consider the competition between carbon escape and interface migration at high supersaturations, the transformation might exhibit a higher velocity than is expected by equilibrium partitioning of carbon. We focus on a regime where the diffusional transport of carbon from the supersaturated austenitic phase at local equilibrium is the rate-limiting mechanism.
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