Fundamentals of Metastability Engineering for Structural Materials
DescriptionPerhaps the most important challenge for high performance structural materials is the simultaneous achievement of high strength (withstanding large loads) and ductility (undergoing large deformation without fracture). Successes in this area lead to improvements in diverse applications, including transportation systems (energy efficiency/ energy absorption) and biomedical implants. Increasing strength normally implies decreasing ductility; making a material more difficult to plastically deform strengthens it but makes it easier to fracture. While there has been an explosion of interest in making ultra-high strength materials, most of these are brittle. An important strategy to overcome this is to open new pathways for the materials to deform at high stress. Two interesting experimental observations in the past few years suggest a novel path forward. That is, polycrystalline materials, where the grain size is of order 10 nanometers, have been seen to undergo diffusionless phase transformations and/or twinning at extremely high stresses (neither is seen in the corresponding bulk materials). Since these phase transformations change the shape of the material, they can relax stress and impart considerable ductility. This project is particularly current given these experimental observations and because, now, achievable ultra-high strengths mean that materials may be subject to stresses never before possible. This proposal addresses the fundamental mechanisms that control the ability of the material to access these ductilization routes, in order to harness them for the design of future structural metallic alloys.Our approach is multidisciplinary, combining ab initio methods for quantum mechanical accuracy, crystal symmetry, statistical mechanics, and continuum mechanics methods to determine phase stability and transition state methods to predict the competition between different diffusionless transition paths/rates as a function of both stress and temperature. Our own preliminary results demonstrate that the nature of these transformations may be manipulated at high stress. In many metals, the nature of the transformations (martensitic vs. twinning vs. dislocation plasticity) is controlled by the energetics of planar defects. Since our overriding focus is developing alloying strategies for ultra-high strength, transformation-ductilized alloys, we next predict how alloying affects phase transformations. We apply what we learn about the underlying transformation mechanisms and use this understanding to guide material development. Alloying provides the knob that allows us to tune the energetics and barriers that control which mechanisms dominate and thereby manipulate the stress where such mechanisms become important.
|Effective start/end date||1/01/20 → …|