Development and Analysis of Low Temperature and High Strain Rate Superplasticity in High-Ratio Extruded AZ31 Mg Alloys

• Main Authors: Hsuan-kai Lin, 林鉉凱
• Other Authors: J. Chih-Ching Huang
• Format: Others
• Language: en_US
• Published: 2005
• Online Access: http://ndltd.ncl.edu.tw/handle/93313166027358214971

博士 === 國立中山大學 === 材料科學研究所 === 93 === There have been numerous efforts in processing metallic alloys into fine-grained materials, so as to exhibit high strain rate superplasticity (HSRSP) and/or low temperature superplasticity (LTSP). The current study is to apply the most simple and feasible one-step extrusion method on the commercial AZ31 magnesium billet to result in low temperature and high strain rate superplasticity (LT&HSRSP). The one-step extrusion was undertaken using a high extrusion ratio at 250-350oC, and the grain size after one-step extrusion became ~1-4 mm. The processed AZ31 plate exhibited high room temperature tensile elongation up to 50%, as well as superior LTSP and/or HSRSP up to 1000%. Meanwhile, the AZ31 alloy was also conducted by equal-channel angular pressing (ECAP). It is demonstrated that an elongation of 461% may be attained at a temperature of 150oC, equivalent to 0.46 Tm where Tm is the absolute melting temperature. This result clearly demonstrates the potential for achieving low temperature superplasticity. A detailed investigation, using x-ray diffraction (XRD), electron back scattering diffraction (EBSD), and transmission electron microscopy / selected area diffraction (TEM/SAD), revealed different textures in the as-extruded and as-ECAP bars. These dominant textures were characteristic of <10 0>//ED in the extruded bars and < 76>//ED in the ECAP condition, where ED is the extrusion direction. The results show that the basal planes tend to lie parallel to the extrusion axis in the extruded bars but there is a rearrangement during ECAP and the basal planes become reasonably aligned with the theoretical shearing plane. As to the extruded plates, the {0002} planes tended to lie on the plane that contains the extrusion axis. At different tensile temperatures, different deformation mechanisms would be dominant. Over the lower loading temperatures within 150-200oC, the true strain rate sensitivity, mt, after extracting the threshold stress is determined to be 0.28, suggesting that power-law dislocation creep but the Qt value is not related to any creep mechanism. It should be partly due to thermal activated dislocation slip mechanism. However, more data need to be tested systematically this part in the future study in order to define the correct deformation mechanism. As to the loading temperatures over 250-300oC, the mt value and the true activation energy for the extruded specimens are calculated to be ~0.4-0.5 and ~90-100 kJ/mol, implying that the major deformation mechanism is grain boundary sliding plus minor solute drag creep, with the rate controlling diffusion step being the magnesium grain boundary diffusion.