| 其他摘要 | Magnesium alloys are recognized as the “green structural materials” in the 21st century. The wrought magnesium alloys have more potential in the future applications as compared with the casting alloys, because of their higher strength, better ductility and diversity of mechanical properties. However, at present, there are some problems with the wrought magnesium alloy, such as few alloy-series and the poor formability of Mg, which limited their widely application. In this dissertation, a newly developed high strength wrought magnesium alloy with rare earth elements, i.e. the Mg-6Zn-1Y-Zr (ZW61) strengthened by an icosahedral quasicrystal phase (I-phase), was chosen to be systematically studied in aspects of the evolution of the microstructure and the mechanical properties during thermomechanical processing. The main work and achievements are summarized as follows.
The high temperature flow behaviors and microstructure evolution of the as-cast ZW61 alloy were studied in the temperature range of 250-400ºC under the uniaxial compression. It is shown that, the relationship between the flow stress, strain rate and temperature can be expressed by the power-law equation or the equation corrected by the sinh-function, with the similar linear dependence relations, respectively. The microstructural observation showed that, an inhomogeneous microstructure, i.e. the necklace microstructure, was found in the as-cast ZW61 alloy under most deformation temperatures and strain rates. The explanation for this observation has been given as that the dislocations can not slip smoothly between the grains with random orientations due to the limited slip systems in the as-cast magnesium alloy with a HCP structure. Thus, the deformation has to be accommodated by twinning in the grain interiors, while the dynamic recrystallization (DRX) happens near the grain boundaries due to the high stored energy in those regions. As for the as-extruded ZW61 alloy with an intense initial texture, the activated slip or/and twin systems are significantly different from one orientation to another when samples were compressed in different directions, consequently, the peak flow stress of the ED orientation is the highest and that of the ND one is the lowest. Moreover, the onset of DRX is the earliest in the ND whereas the latest in the ED orientation. Therefore, it is suggested that, besides the deformation temperature, strain rate and strain considered in the conventional metallic alloys, the relative orientation between the stress direction and the initial texture of magnesium alloys should also be an important factor to control the microstructure and mechanical properties during the thermomechanical treatment.
Considering the characteristics of the shear stress of the equal channel angular extrusion (ECAE) with a pure shear direction, the effects of the solution treatment, ECAE temperature, pass numbers and orientation of the billets (route) on the microstructure, texture evolution and mechanical properties of the ZW61 alloy were studied. The results showed that solution treatment before ECAE provided a significant increase in formation efficiency of fine recrystallized grains during ECAE at 350ºC. A homogeneous fully recrystallized microstructure was observed after 8 passes in the as-solutionized alloy, while an inhomogeneous partially recrystallized microstructure was obtained in the as-cast alloy. This is ascribed to the increased homogeneous deformation due to the solution treatment and the increased recrystallization rate due to solution atoms and the fine particles. The ductility of the alloy ECAEed in the as-solutionized state was always significantly better than those of the alloy ECAEed in the as-cast state. However, the strengths of the former were better than those of the latter only after 6 passes, which is attributed to the more strengthening effects of grain refinement and dispersed precipitates after 6 passes in the former.
ECAE processing of the extruded ZW61 alloy with an intense initial basal texture showed that, the yield strength achieved through the single-pass ECAE in the Orient-I (ND of the extruded plate parallels to the ECAE exit direction) was lower than that in the Orient-II (TD of the extruded plate parallels to the ECAE exit direction) when being ECAEed at 200ºC. Similarly, in the materials ECAEed by multi-pass, the yield strengths of the alloy ECAEed through route Bc considerably decreased with the number of passes increasing, and were lower than those of the alloy ECAEed through route A. The texture analysis showed that both of them are attributed to the formation of the texture component with the basal planes inclined at about 45° to the longitudinal direction of the samples when processed by orient-I or route Bc. In addition, both in the single-pass and the multi-pass ECAE, the yield strength of the material ECAEed at 250ºC with a finer grain structure was considerably lower than that ECAEed at 350ºC with a coarser grain structure, which is an abnormal mechanical behavior according to the Hall-Petch relationship. The texture analysis showed that it is attributed to more crystals in the former being oriented to be more favorable for basal slip after being ECAEed at a lower temperature. By taking effects of the ECAE route and the ECAE temperature into account, it is recommended that the ECAE processing via route A and at a moderately higher temperature (about 350ºC) should be carried out in order to obtain better mechanical properties in the ZW61 alloy.
The superplastic deformation behaviors of the ZW61 alloys with mixed fine and coarse grains produced by conventional extrusion and ECAE methods, respectively, have been comparatively studied. It was found that the optimum superplastic condition in the as-extruded alloy was obtained at 450ºC and 3.3×10-4s-1 with the largest elongation to failure of about 450%; while in the as-ECAEed alloy it was at 350ºC and 1.7×10-3s-1 with an elongation of about 800%. The cavities formation were observed to originate from the debonding between the grains boundaries, rather than from the interfaces of the secondary particle and matrix, in both state alloys, however, more and larger cavities were found in the former state alloy. The mechanisms for optimum superplasticity in the as-extruded alloy are supposed to be mainly dominated by dislocation slip/climb in coarse gains and GBS between small grains, in which the initial coarse grains are refined by DRX resulting in a fine and homogeneous microstructure. However, in the as-ECAEed material, it is suggested that GBS mechanism dominates the overall deformation process, which is accommodated with dislocation slip/climb in the coarse grains. |
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