| 其他摘要 | In order to get a better understanding of the shear deformation model of Equal channel angular pressing (ECAP), the grain refinement mechanism, the interaction between shear deformation and grain boundary (GB), and the competition mechanism between twinning and slipping, we have conducted a series of investigations by using Cu single crystals and bicrystals, Al single crystals, Fe and Cu-3%Si polycrystals. Based on those studies, it is aim to give some instructions on how to design advanced materials through the method of ECAP.
The shear flow lines formed in polycrystalline materials after one-pass ECAP consist of a series of elongated grains, and they make an angle of ~27º with respect to the extrusion direction. The formation mechanism of those shear flow lines can be easily understood from the point of view of materials flow. An equation for the flow line field has been presented to describe the metal flow in ECAP die based on the result of the in-situ modeling experiments. Compressive experiments indicate that the shear flow lines have marked influences on the anisotropic mechanical properties of iron after one-pass ECAP.
The initial crystallographic orientation of Al single crystals has significant influences on the dislocation structure evolution and grain refinement mechanism during ECAP. For crystal I, abundant cell block structures with multi-slip characters were formed, and they should be induced by four symmetric slip systems, while for crystal II, there are two sets of subgrain structures, making an angle of about 70°, which can be attributed to the interactions of the two asymmetric primary slip planes. However, for crystal III, only one set of ribbon structures was parallel to the traces of with the lowest misorienation angle among the three single crystals, which should result from the homogenous slip on the primary slip plane. Based on those results, it is suggested that, in addition to the shear stress along the intersection plane (IP), the shear stress along the direction perpendicular to IP still plays an important role during ECAP.
The GBs of four Cu bicrystals display a very different evolution processes during ECAP, which was induced by the interaction between the shear deformation imposed by ECAP die and the different directions of GBs. The evolution process of GBs in the four bicrystals can be well explained by the view of materials flow. The GB in bicrystals A-0º, C-90º and D-135º can still keep straight after ECAP, while the GB in bicrystal B-45º has been significantly deformed due to the strong interaction between shear bands and GB. The experiments results demonstrate that the shear stress along the direction perpendicular to IP also play an important role when bicrystals subjected to ECAP.
Through specially designing the crystallographic orientation of Cu and Al single crystal, it is successful to acquire abundant deformation twins in Cu and profuse microtwins and stacking faults in Al at low strain rate and room temperature. Those experiments indicate that crystallographic orientation has a marked influence on deformation twinning. Comparing the twinning behaviors of Cu-3%Si alloy with Cu and Al, and analysis based on the fundamental dislocation mechanism, it is found that twinning stress is mainly controlled by three intrinsic factors, such as, SFE, crystallographic orientation and grain size. In order to judge whether twinning can nucleate or not in any given fcc crystal, it is suggested that two main points should be considered. One is the required twinning stress, ，whose value can be evaluated according to its intrinsic factors, such as SFE, crystallographic orientation and grain size. Another one is the applied stress ( ) by the external deformation conditions. Once the external loading mode matches the required twinning stress ( ), deformation twins will naturally nucleate, otherwise, slipping will be the dominating deformation mechanism |
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