| 其他摘要 | Both grain boundary and twin boundary are interfaces that can effectively block dislocation motions. It has been found that when the grain/twin size decreases below 100 nm, the nanograins and nano-twins can strengthen materials more efficiently. However, the well-known methods of severe plastic deformation can only prepare bulk ultrafine-grained materials with grain sizes in the range of submicrometer, and the methods of electro-deposition or magnetron sputtering can only fabricate nano-grained/nano-twinned foil samples with very thin thickness. The proper method to fabricate real flaw-free and contamination-free bulk nanostructured (NS) samples has not been developed yet, which directly limits the industrial utilization of bulk NS materials. Recently, through the method of dynamic plastic deformation (DPD), high-density nano-twins and nanograins are successfully induced into bulk brass sample, which paves the way for the systematic and thorough researches on the microstructures and mechanical properties of bulk NS materials.
In this work, bulk NS brass samples were fabricated through dynamic plastic deformation at liquid nitrogen temperature (LNT-DPD). Firstly, the microstructural evolution and mechanical properties of LNT-DPD brass samples were investigated systematically. Secondly, the influences of deformation conditions (strain path change, strain rate and temperature) on the microstructures and mechanical properties were studied. Finally, the microstructures and mechanical properties of annealed LNT-DPD brass samples were investigated. The main results are as follows:
1. The microstructural evolution of LNT-DPD brass can be classified into three stages:
(i) Stage I (0-0.2). The microstructure is characterized by planar dislocations.
(ii) Stage II （0.2-0.8). The major deformation mode is deformation twinning. Both the twin layer thickness and twin boundary spacing decrease with the increasing strain. The value is nearly equal to the value when strain > 1.2, which corresponds to the saturated state of twin density.
(iii) Stage III (0.8-1.6). The major deformation mode is shear banding. Shear bands swallow twin lamellae and widen with the increasing strain, and meanwhile the structure inside shear bands develops from elongated grains into equiaxed grains. The ultimate microstructure is composed of nano-scale twin bundles and nanograins. The average twin boundary spacing is in the range of 10-20 nm and the average nanograin size is in the range of 40-70 nm.
2. Uniaxial tensile tests were conducted on the LNT-DPD brass samples with different strains. With the increasing strain, the yield strength increased significantly from 80 MPa to 720 MPa, while the uniform elongation decreased from 50 % to 2-3 %. The twin strengthening in bulk sample follows the modified Hall-Petch relationship, and plays a dominant role in the total strengthening. The occurrence of shear bands weakens the twin strengthening and exerts bad influences on the ductility, leading to a catastrophic decrease of uniform elongation.
3. A series of investigations were conducted on the influences of strain path change, strain rate and temperature on the microstructures and mechanical properties of deformed brass. The results are as follows:
(i) The strain path change leads to a transient stage during which strength decreases and strain hardening rate increases when brass samples are reloaded. This transient stage diminishes gradually with the subsequent increasing strain, and has little influences on the mechanical properties of brass samples with large deformation strain. The strain path change can effectively activate more twin systems and accelerate the formation of twin intersections.
(ii) The influences of strain rate and temperature can be integrated and evaluated by Zener-Hollomon parameter (Z). It is shown that the twin layer thickness, twin boundary spacing and nanograin width decrease with the increasing LnZ, and the yield strength increases with the increasing LnZ. Compared to other samples, LNT-DPD sample has smaller twin size (11 nm, 14 nm), smaller grain size (38 nm) and higher yield strength (720 MPa), which can be attributed to the larger Z (LnZ = 90) due to higher strain rate and lower temperature. High strain rate and low temperature can refine the structural size efficiently and enhance the yield strength correspondingly.
4. Isothermal annealing was conducted on two kinds of LNT-DPD samples, LNT-DPD_0.8 and LNT-DPD_1.6 sample, prepared at strain = 0.8 and 1.6 respectively. The microstructures and mechanical properties after annealing were investigated. The results are as follows:
(i) The thermal stability of LNT-DPD_0.8 sample is better than that of LNT-DPD_1.6 sample. This is because a large number of nanograins elevate the total energy level of LNT-DPD_1.6 sample, leading to larger nucleation driving force and lower static recrystallization (SRX) temperature.
(ii) The SRX happens primarily in the twinned regions in LNT-DPD_0.8 sample, while happens preferentially in the nano-grained regions in LNT-DPD_1.6 sample. The SRX leads to the decrease of volume fractions of twin lamellae but little change of twin boundary spacing.
(iii) With the increasing annealing time, the strength decreases and ductility increases for both kinds of LNT-DPD samples. The strength-ductility trends of annealed LNT-DPD_0.8 samples and as-deformed LNT-DPD samples are overlapped, which can be attributed to their similar microstructural components. Under the same annealing conditions, LNT-DPD_1.6 samples have better strength-ductility combination compared with LNT-DPD_0.8 samples. With the same uniform elongation, the yield strength of annealed LNT-DPD_1.6 samples is higher than that of annealed LNT-DPD_0.8 samples by 100200 MPa, which is attributed to thinner twin lamellas and smaller SRX grains. |
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