| 其他摘要 | Mechanical and electrical properties are two of the most important criteria for industrial materials. Amounts of experiments and computer simulations indicated that nanocrystalline (nc) metals with a high density of grain boundaries (GBs) usually exhibit substantial high strength, very limited plastic strain, diminishing work hardening capacity and reduced conductivity, compared with their conventional coarse-grained (CG) counterparts. The above mentioned properties of nc metallic materials had been severely blocked their industrial applications. Twin boundaries (TBs), special type of low-energy boundaries, can serve as barriers to dislocation motions like conventional GBs. While the interaction between TBs and dislocations is basically different from that between GBs and dislocations. Meanwhile, the coherent TBs possess a very low capacity to scatter conduction electrons. Nevertheless, up to now, a clear scenery of the influence of high density of coherent TBs on mechanical and electrical properties of nc materials is still lacking. Therefore, understanding of intrinsic mechanisms for hardening and conducting of nanoscale coherent twin structures becomes more and more crucial.
In this work, high-purity Cu specimens with nanoscale growth twins and nc 316L stainless steel (316L SS) samples were synthesized by means of pulsed electrodeposition (PED) technique and surface mechanical attrition treatment (SMAT), respectively. We systematically investigated the mechanical and electrical properties of these samples. The main results of this work are as follows:
1. Optimizing the microstructures by modifying deposition parameters, two kinds of ultrafine-grained (ufg) Cu samples with nanoscale twins were prepared by means of PED technique: (I) keeping average grain size constant (400~500 nm), while changing twin lamellar spacing from 15 to 90 nm; (II) keeping twin lamellar spacing constant (~60 nm), while changing grain size from 460 nm to 1500 nm.
2. Transmission electron microscope (TEM) observations revealed that GB morphology and energy are related to twin density in nano-twinned Cu (nt-Cu) samples. For the nt-Cu with high twin density, the faceted GBs with low interfacial energy are straight and clear. Whereas the GBs of the nt-Cu with relatively low twin density are quite curved and show large crystallographic width. There are a few {211}/{211} and {11 44}/{223} incoherent TBs in electrodeposited Cu samples, besides a high density of {111}/{111} coherent TBs. A striking feature of these structures is a bending of the {111} planes that run continuously through the incoherent boundaries.
3. Keeping the grain size of Cu specimens roughly constant (400-500 nm), the influence of twin lamellar spacing on work hardening response of nt-Cu was investigated by recourse to uniaxial continuous tensile, loading-unloading and cold rolling tests at room temperature (RT). Net flow stress increment and work hardening rate of the nt-Cu are enhanced substantially with decreasing twin lamellar spacing (from 90 nm to 20 nm), which may be due to that TB can be effective obstacles for the motion of dislocations. Whereas, the work hardening exponent increases slightly (from 0.22 to 0.30) with increasing twin density in nt-Cu, which is comparable to that of the ufg Cu with identical grain size but without twins and much higher than those of nc Cu samples. This suggests that the introduction of high density of TBs is beneficial to elevate the work hardening capacity of nt-Cu. However, the ability to store immobile dislocations can not be promoted evidently in the submicron grains when the twin lamellar spacing reduces from 90 nm to 20 nm. The possible factors as follows: (I) The twin lamellar structure is two-dimension, which limits dislocation interactions in three dimensions and consequently results in the absence of dislocation locks in the lattice and TBs. (II) Dislocation interactions from multi-slip systems can not occur extensively. (III) Reactant Shockley partials, being left at the TBs and leading to TB migration, do not contribute to work hardening significantly.
4. Comparing mechanical properties of two nt-Cu samples with similar twin spacing of ~60 nm but with different grain sizes (450 nm and 1500 nm, respectively), it is observed that the yield strength is independent on grain size, that indicated the high yield strength of nt-Cu originates from the strengthening of high density of TBs. However, both the ductility and the work hardening capacity show an increasing trend with increasing grain size in the nt-Cu. For the nt-Cu sample with a grain size of 1500 nm, the elongation to failure and work hardening exponent are as high as 15% and 0.39, both of which are much higher than those of nt-Cu with a grain size of 460 nm. The elevated ductility and work hardening ability may come from increasing the grain size so that there are a longer shear path of abundant dislocations along the TBs and suppressed dynamic recovery. When nonuniform plastic deformation occurs, the disappearance of TBs and the formation of dislocation cells dominate the dynamic recovery and consequently give rise to the diminishing of work hardening rate in the nt-Cu with a grain size of 1500 nm. A strong dependence of structural evolution on the grain size and strain was investigated systematically by TEM observations in nt-Cu.
5. The effect of twin density on electrical resistivities of nt-Cu samples is systematically investigated through holding grain size roughly identical. The electrical resistivity of the Cu specimen with a twin spacing of 15 nm at RT is 1.75 μΩ cm (the conductivity is about 97% IACS), which is comparable to that of CG pure Cu specimen. A reduction in twin density (with twin lamellar spacings of 35 nm and 90 nm, respectively) results in an increment in electrical resistivity from 1.75 to 2.12 μΩ cm. The increased electrical resistivities of the Cu samples were ascribed dominantly to the intrinsic GB scattering, while the GB defects and GB energy would decrease with increasing twin density. Plastic deformation would induce an apparent increase in the resistivity. The higher twin density, the higher increment of RT resistivity was detected in the Cu specimens subjected to 40% rolling strain. Both the deviated TBs and strained GBs may give rise to an increase in the resistivity. High strength and good conductivity are achieved simultaneously in the nt-Cu.
6. A nc 316L austenitic stainless steel sample with a mean grain size of 40 nm was prepared by means of SMAT. Uniaxial tensile tests at RT showed that nc 316L SS sample exhibits a yield strength as high as 1450 MPa, which is about six times higher than that of CG sample. This ultrahigh strength of the nc sample, which still follows the Hall-Petch relation extrapolated from CG structures, is attributed to the effective blockage of lattice dislocation motions by nanometer-sized grains. |
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