Self-organization in immiscible alloy systems under irradiation and severe plastic deformation

Driven Alloys;Immiscible Alloys;Severe Plastic Deformation;Irradiation;Self-Organization

Beach, John Arias

关键词: Driven Alloys;    Immiscible Alloys;    Severe Plastic Deformation;    Irradiation;    Self-Organization;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/101110/BEACH-DISSERTATION-2018.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

There exist many practical examples of extreme environments where materials are commonly forced to operate, ranging from nuclear reactors to alloys being processed through severe plastic deformation. In each of these cases, there exists external dynamical forcing which keeps the material in non-equilibrium conditions. In the case of structural materials in a nuclear reactor environment, there are high fluxes of neutrons which can cause the alloys to become microstructurally unstable and cause their properties to change over time. In addition, nuclear reactors are kept at high temperatures in order to generate power, which can cause other deleterious effects on a material such as creep and grain coarsening. For severe plastic deformation processing of alloys, such as high energy milling, a large amount of energy is imparted in the form of stress, which causes dislocation motion and grain refinement. Temperatures can also vary depending on the process, generally ranging from room temperature to > 0.75 of the melting point. Because each of the environments listed contains an external dynamical force which is driving the materials into non-equilibrium, they are commonly referred to as driven materials.Due to the number of external factors and the numerous practical situations involving driven materials, one may ask if there is a theoretical framework on in which it is possible to not only describe what is occurring but to also predict how the material’s microstructure will evolve over time. After gaining the understanding to be able to predict the material response to such extreme environments, it would be especially beneficial to find a class of materials which would remain stable over time. In other words, rather than determining when an alloy would fail under service, it would be best if it were possible to design an alloy which would simply never fail over its expected lifetime.To achieve this goal, research has been conducted in material science to better understand the fundamentals of driven materials. One of the main areas of focus revolves around the idea of self-organization in nanostructured alloys. A material which self-organizes will reach a steady state microstructure and will not change as long as the environmental parameters do not change. This means that a material which self-organizes would have the same microstructure indefinitely, even in an environment as extreme as a nuclear reactor, and would not undergo the negative effects such as coarsening or phase changes.The property of self-organization has been readily identified in moderately immiscible and highly immiscible alloy systems such as Cu-Ag or Cu-W, respectively. Furthermore, this behavior is experienced in multiple driven systems; both irradiation and severe plastic deformation (SPD) have been shown to share very similar characteristics, even though the underlying mechanisms are different. While existing models such as the “Driven System Model” and “Effective Temperature Model” can explain existing experimental results for alloys systems with low to moderate heat of mixing, there is currently a lack of understanding in how highly immiscible alloy systems can self-organize, especially at low temperatures (T < .25 Tm).In this dissertation, several model experiments were performed to elucidate a mechanism for both irradiation and shear mixing in alloy systems with high heats of mixing. Irradiation and shear deformation experiments were performed at low temperatures (liquid nitrogen/dry ice) to understand the importance of the chemical interactions in these immiscible alloy systems. It was found that the alloys self-organized into nanoprecipitates when under irradiation or severe plastic deformation and that their microstructure was completely independent of the starting conditions.