Metastable austenitic stainless steels combine high formability and high strength, which are generally opposing properties in materials. This property is a consequence of the martensitic phase transformation that takes place during deformation. This transformation is purely mechanically induced although temperature has a very strong influence on the kinetics of the process. As the temperature decreases, the transformation rate increases since martensite becomes more stable relative to austenite. These materials are currently used in industry, for instance in household appliances, although the manufacturing processes for these products are di±cult to optimize. This is due to the absence of material models that can adequately describe the complex mechanical behavior of these steels. Although it is possible to find phenomenological constitutive models for this class of materials in the literature, obtaining parameters for these models requires extensive mechanical testing. The main goal of this study is to develop a constitutive model that incorporates a physically based description of the phase transformation phenomenon. To understand the physics of the mechanically induced transformation, first, mechanical tests have been carried out. The experiments were aimed at determining the effects of stress state and plastic strain on the transformation behavior. The results pronounced the effect of stress over that of plastic strain suggesting that the transformation could be explained by a stress-based model. The crystallography of martensitic transformations has been studied and with a simple model in mesoscale the mechanical driving force concept was exploited. Based on the results of the model, a physical explanation for the transformation was proposed and verified with experiments. A stress based transformation criterion was proposed which was demonstrated to agree very well with experimental results. Furthermore, a continuum level expression for the martensite volume fraction as a function of the applied stress was proposed. Computing the mechanical behavior of the material during transformation requires taking into account the individual behavior of the austenite and martensite phases as well as their mutual interaction. To serve this purpose, a mean-field homogenization model was utilized. Different algorithms found in the literature were tested and two new algorithms were proposed. These were demonstrated to be reliable as well as computationally efficient. A constitutive model was proposed that is based on a combination of the transformation model and the homogenization model. In addition to these, improvements were made to incorporate the effects of evolving volume fractions of the phases as well as the transformation plasticity phenomenon. The results of the algorithm were compared to the experimental results and a good agreement was obtained. The results prove that the stress state and temperature dependent transformation behavior can be described by the stress-driven transformation model accurately, with only a small number of physical parameters.
|Award date||17 Dec 2008|
|Place of Publication||Enschede, The Netherlands|
|Publication status||Published - 17 Dec 2008|