Congratulations Dr. Krynicki!
Abstract:
Herein the microstructural and mechanical evolution of two distinct materials were characterized thoroughly: Mg-1Zn-0.2Ca and ASTM A516 70 steel. The results generated from these studies are relevant to the advancement of the processing/characterization of lightweight structural metals and to furthering the understanding of high temperature damage mechanisms relevant to the energy sector.
Mg-1Zn-0.2Ca was processed using rolling or conventional extrusion (CE), followed by Equal Channel Angular Extrusion (ECAE), utilizing different routes (which define the rotation of the billet in between extrusion passes), to identify the impact of processing on the microstructure and mechanical properties. Preprocessing via rolling yielded a smaller, more homogeneous initial grain size than that produced by CE. After ECAE via routes 4Bc and 4A, the microstructural differences due to preprocessing were reduced and the mechanical properties showed little variation between rolling and extrusion. The choice of ECAE route (4Bc or 4A) did not impact microstructural refinement. However, the 4Bc route led to enhanced ductility and higher ultimate tensile strengths, as compared to 4A.
Additionally, the impact of grain size on quasi-static and spall strengths of Mg-1Zn-0.2Ca was explored. Samples were processed to generate 3 target grain sizes: ~8, ~26, and ~57 µm. Grain size impacted quasi-static properties, as expected, and led to discontinuous yielding in finer-grained samples. Yet, spall strength did not vary significantly.
In the context of the energy sector, exposure to high temperatures/hydrogen pressures causes the microstructure/mechanical properties of pressure vessel steel to degrade due to high temperature hydrogen attack (HTHA), a significant concern for the petrochemical industry. Most guidelines regarding allowable environmental conditions are based on prior failures in-service. Thus, hydrogen creep data was generated on carbon steel (ASTM A516 70) at various temperatures (357-454°C), applied stresses (66-172 MPa), and hydrogen pressures (1.7-6.9 MPa H2). An unexpected creep plateau (after the tertiary regime) was observed in the low temperature samples. We provide evidence via creep analyses and interrupted tests suggesting that methane pressure, decarburization, hydrogen-defect interactions, and the constrained growth of cavitated material all play significant roles in determining strain rates in the specific creep regimes. We also identify likely mechanisms that control the creep rates in each regime.
