| 其他摘要 | Dynamic plastic deformation (DPD) has proved to be an efficient approach for preparing bulk nanostructured materials. It is necessary to investigate the microstructure and deformation mechanism induced by DPD in order to further develop the DPD technique. This thesis first systematically studied the microstructure and mechanical properties of Cu-Al (Cu-4.5 wt.% Al) samples processed by DPD at room temperature (RT-DPD) and those processed by RT-DPD followed by subsequent annealing. In particular, the microstructural evolution during the nucleation and thickening of shear bands in nano-scale twin/matrix lamellae in RT-DPD Cu-Al were explored in great detail. The thesis then studied the grain orientation dependence of the microstructure in pure Cu processed by DPD at liquid nitrogen temperature (LNT-DPD). The main conclusions are as follows:
1. Microstructural evolution of Cu-Al during RT-DPD
At low strains (0.5), the deformation of Cu-Al during RT-DPD is dominated by deformation twins; whereas at intermediate and large strains ( >0.5), shear bands generated at the highly twinned regions play an important role. The volume fraction of twinned regions in RT-DPD Cu-Al increases with DPD strain at low strains, and then decreases at larger stains as a result of shear banding, revealing a peak value of ~60%. Regions consisting of dislocation structures, within which no deformation twins or shear bands are observed, are seen in all the DPD samples. The microstructural evolution of RT-DPD Cu-Al is typical of low stacking fault energy materials deformed at room temperature and low strain rates.
2. Bulk nanostructured Cu-Al prepared by RT-DPD
A mixed nanostructure was developed in RT-DPD Cu-Al bulk samples deformed to a strain of 1.67. The nanostructure consists of three components: nano-scale deformation twins (twin lamellar thickness, 10 nm; volume fraction, 18%), nano-sized grains in shear bands (transverse grain size, 53 nm; volume fraction, 59%) and nano-scale dislocation structures (dislocation boundary spacing, 88 nm; volume fraction, 23%). Corresponding to the nanostructure, the samples revealed a tensile yield strength (765 MPa) much higher than that of their coarse-grained counterpart (71 MPa). However, the tensile elongation of the samples is quite limited.
3. Microstructural evolution during nucleation and thickening of shear bands in nano-scale twin/matrix lamellae in RT-DPD Cu-Al
The development of shear bands in nano-scale T/M lamellae of a Cu-Al alloy processed by RT-DPD (=1.67) was investigated systematically. Two characteristic stages, nucleation and thickening, were identified based on extensive TEM and high resolution TEM (HRTEM) observations.
In the nucleation stage, most of the shear strains are concentrated in a core region, the thickness of which remains almost constant with increasing shear strain, being 100~200 nm. On the two sides of the core region, two transition layers (TRLs) of deformed T/M lamellar structure with much lower shear strains are present. The nucleation of a shear band is accomplished through the following three steps: (1) Initiation of shear banding (bending, necking and detwinning) of the T/M lamellae. Shear strain inspires the glide of Shockley partial dislocations on successive twin planes, resulting in annihilation of the twin lamellae. (2) Formation of a detwinned dislocation structure (DDS). Dislocation boundaries might be derived from two approaches. One is that for initial shear banding, extended dislocation walls are developed on the interface between the sheared region and non-sheared region, obliquely intersecting the preexisting T/M lamellae. The other is that as the twin boundaries annihilate, the dislocations on twin boundaries tend to accumulate, annihilate and rearrange to form new dislocation boundaries. (3) Transformation of the DDS into a nano-(sub)grained structure (NGS).
As a NGS structure is developed within the core of a shear band, further deformation leads to thickening of the band. The thickening process of shear bands at increasing shear strains is composed of thickening of the core region by transforming the TRLs into the core region with DDS and NGS, analogous to step (2) and (3) of the nucleation process, and outward movement of the TRLs by deforming the adjoining original T/M lamellae. Grain sizes in the well-developed shear bands are obviously larger than the lamellar thickness of original T/M lamellae, and the structure in the out-bound layers of the core is coarser than that in the inner core region. During thickening of a shear band, most of shear strain are anticipated to be concentrated in the vicinity layers of the core/TRL interface boundaries.
4. Mixed-microstructured Cu-Al with a combination of high strength and high ductility by RT-DPD followed by subsequent annealing
During annealing of the RT-DPD Cu-Al samples (=1.67), deformation twins reveals superior thermal stability to that of nano-sized grains in shear bands. After annealing at 300℃ for 20 min, the nano-sized grains fully recrystallized, resulting in grains of tens of micrometers in diameter; whereas a large fraction of deformation twins and dislocation structures are retained. Thus, a mixed microstructure, composed of recrystallized micrometer grains, blocks of deformation twins and blocks of dislocation structures, was obtained. Tensile tests of samples with such a mixed microstructure revealed a combination of high strength (377 MPa) and large uniform elongation (20.8%).
5. Grain orientation dependence of the microstructure of LNT-DPD Cu
The microstructure in LNT-DPD polycrystalline Cu (=0.33) was studied by means of Kikuchi line analysis in SEM-EBSD and TEM. The inhomogeneous occurrence of deformation twins in different grains is caused by a strong grain orientation dependence of the twinning process. Deformation twins tend to occur in grains with orientations near the corner and do not occur in grains near the corner, which can, to large extent, be understood in terms of a Schmid factor analysis. To enhance twinning and the associated structural refinement, the use of samples with an initial [001] fiber texture will be beneficial. |
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