| Abstract:It has been widely understood that applying strain field can impose a greater effect on the habit planes, size, distribution, morphology and density of precipitate phase. In this paper, by means of first-principles calculation and interface thermodynamics, the impact of temperature and Cu activity on interface properties for coherency, semi-coherent and incoherent interface which have been observed on high-resolution electron microscopy were systematically studied. Furthermore, it was thoroughly studied that the coherency and semi-coherent interface energy varies with different strain approach in stress aging. Finally, the comprehensive effect to interface energy of temperature and strain was analyzed and discussed. These results reveal the stress aging mechanism in the view of energetic partly. The main results were shown as follows:(1) The surface stabilities of low index surfaces of Al were investigated by first principles calculations. It was found that the order of Al surface energy was (110)>(112)>(100)>(111).(2) The relaxation results of all calculated interface structures showed that the Cu atom of θ" coherency interface (θ"-CI) is easily stayed in the hollow location which was formed by Al, and the Cu atom of Cu rich interface of θ’coherency interface (8’Cu-CI) and normal9’coherency interface (9’-CI) is easily stayed on the Al-Al bridge location. However, the Cu atom of θ’semi-coherent interface (θ’-SCI) is easily appeared on the top of Al. The relaxation structure of9incoherent interfaces exist as large lattice distortion, the interface atomic migration have taken place in X-Y-Z-three direction.(3) The coherency interface energy always increased with the increasing temperature, the interface energy of θ’Cu-CI was most sensitive to temperature. This kind of interface stability was poorer. At the same temperature, the interface energy of θ"-CI,θ’Cu-CI and9’-Cl in turn increaseed, which suggested the required corresponding energy for the appearing of three kinds of interface increaseed in turn. Moreover, the environment-depend preferred growth of different interface was predicted. And the reason of interface precipitation sequence (from θ"to θ’and θ) in Al-Cu alloys was found by interface energetic.(4) The interfacial bonding strength was assessed by the work of adhesion (Wad) and separation (Wsep). The Wad of θ"-CI was maximum by2.6J/m2. However, The Wad of θ’SCI is minimum by1.52J/m2. When the θ" CI interface fracture occured, The Wsep was maximum by2.12J/m2. However, When the θ’SCI interface fracture occured, the Wsep was minimum by1.71J/m2.(5) In stress (strain) aging, the activity coefficient, γCu, in Al-Cu alloys under strain always increased with the increasing temperature or degree of the imposed strains, but under higher strains or temperature, γcu became less sensitive to temperature.(6) The coherency interface energy always decreased with the increasing of external strain, the interface stability also declined with increasing temperature, which is not conducive to its formation. Applied strain could counteract the effect of temperature and lead to the interface energy decreasing. That resulted in promote θ" and θ’precipitation.(7) At the same strain condition, the interface energy of θ"-CI,θ’ Cu-CI and θ’-CI in turn increased. So, it could be predicted that θ"-CI,θ’ Cu-CI and θ-CI in turn precipitated in strain/stress aging.(8) The impacts of different strain/stress ways to different orientation interface energy were not the same. The calculated results presented that a compressive stress/stain would reduce the interface energy of the vertically oriented slightly more than its parallel counterpart, so that orientation of θ"/θ’would be more favored. If a tensile stress/stain is applied along the same direction, it would reduce the interface energy of the parallel oriented θ"/θ’ slightly more, so as to favor that orientation. This could be regarded as one major mechanism for the stain/stress-orienting effect observed in stress/strain aging. |
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