Characterization of Magnesium and Magnesium Alloys Processed by Equal Channel Angular Extrusion
Krywopusk, Nicholas M
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Magnesium has shown substantial promise as a structural material due to its low density and high specific strength. The lack of primary slip systems and severe tension-compression anisotropy has historically limited traditional forming methods at room-temperature. Thermo-mechanical processing by methods of Severe Plastic Deformation (SPD), such as Equal Channel Angular Extrusion (ECAE), has proven exceptionally effective in microstructural refinement. These successes have included enhanced strength, ductility, and texture modification. Although widely studied, a significant number of gaps in knowledge remain in the processing of magnesium by ECAE, including experimental confirmation of ECAE processing homogeneity, the effect of strain-rate on microstructural evolution, and the relative activation of competing mechanisms for microstructural refinement. Samples of 99.9% pure Mg and AZ31B were processed by ECAE at elevated temperatures for the purposes of microstructural characterization, and to determine the homogeneity of the microstructural refinement within the ECAE processed samples. Characterization was carried out using Electron Backscatter Diffraction (EBSD). The microstructures and textures of both materials were found to be generally uniform throughout the billet. AZ31B was found to have a smaller average grain size and a narrower grain size distribution. Grain boundary misorientation distributions reveal a peak around 30° that is associated with the recrystallization process. In addition, peaks associated with extension twinning were occasionally seen. Both materials exhibit a basal fiber texture expected of the 4BC processing route in ECAE, although AZ31B displayed fewer local variations and a tendency for basal pole splitting. There is some question regarding the extent to which AZ31B exhibits strain-rate hardening, and the conditions under which it occurs. To resolve this question, samples of rolled and ECAE processed AZ31B were mechanically tested under uniaxial compression at multiple quasi-static rates and orientations. The project is intended to dove-tail with the Kolsky bar work being performed by Kannan et al., in order to explore AZ31B strain-rate hardening behavior over a wide range of strain-rates. Both rolled and ECAE processed AZ31B were found to exhibit strain-rate hardening effects. Sigmoidal stress-strain curves were observed in the rolled AZ31B compressed along the ⟨a⟩ axis and both ECAE orientations. In the rolled material, the sigmoidal curves were found to be due to extension twinning. However, no twinning was observed in the ECAE material. The shape of the curve is instead attributed to the concurrent action of stress-induced grain growth and Taylor hardening. Spall is known to be a factor in the high strain-rate failure of magnesium. While a number of studies have investigated spall behavior of magnesium and its alloys, few have explored the micro-mechanics of spall through fractography. Samples of AZ31B processed by the 4BC ECAE route were subjected to spall recovery tests in two different orientations. The resulting spall behavior is studied via fractography. Two potential sites were found for the nucleation of spall: voids at fractured, coarse precipitates and pre-existing voids in the matrix. These two sources appear to produce different spall behaviors. Voids at fractured precipitates cause particle fracture that, in turn, further drives crack growth along the length of the precipitate. In contrast, voids in the matrix undergo ductile growth by nanovoid formation. Although the study is incomplete, these initial results suggest that nanovoids in the extrusion direction oriented samples grow without the assistance of nanoscale precipitates; while in the transverse direction orientation samples nanovoid growth can be linked to nanoscale precipitates. In ECAE processing, the effects of route design and temperature on the final microstructure have been studied extensively. Typically, these experiments were conducted at an optimized and fixed extrusion rate. As such, the systematic effects of strain-rate have received much less attention. Here, billets of pure Mg were ECAE processed at rates of 0.127, 0.381, and 0.762 mm/s for one, two, three, and four passes of the 4BC route. The processed billets were subsequently characterized by EBSD. The area fraction of recrystallized grains was found to increase with higher extrusion rates. Nucleation of recrystallized grains was observed to be largely heterogeneous and orientation dependent. The average recrystallized grain size showed no significant change with pass, but the 0.762 mm/s rate did display a modest reduction. Both the 0.762 and 0.127 mm/s rates were found to possess a greater volume fraction of extension twins than the 0.381 mm/s rate. The larger fraction of twins was hypothesized to be attributed to the necessity for greater strain accommodation at the higher rates, and to the microstructure retaining some orientations favorable for twinning at lower rates. Discerning which processing mechanisms are operative during ECAE processing under different temperature regimes and orientations has proven difficult ex situ. Furthermore, the mechanisms themselves are only partially understood. A neutron transparent ECAE die, including heating and backpressure capability, has been designed and fabricated. Pure magnesium was extruded in two orientations and at three temperatures while using in situ neutron diffraction at the VULCAN diffractometer at the Spallation Neutron Source (SNS). Under all experimental conditions, a large amount of extension twinning was found to occur that produced a basal texture in the longitudinal direction. The extensive twinning was found to mitigate the effects of initial orientation on dynamic crystallization. Samples processed at lower temperatures were found to exhibit more discontinuous dynamic recrystallization-like behavior than those at higher temperatures.