| Al/Al-Cu graded materials were fabricated by powder metallurgy technology at 600℃under the pressure of 25 MPa for 1 h. The microstructure, composition and mechanical properties of the prepared specimens were studied. The results showed that Al/Al-Cu graded materials with dense structure and compositional continuous change were obtained. The precipitated phase CuAl2 distributed in the grain boundaries of the matrix. It could be seen that there is a continuous graded increase of hardness from pure Al to Al-4 wt% Cu layer. Vickers hardness of the Al-4 wt% Cu layer reaches to 80 HV. And the flexural strength reaches to 166.40 MPa which is one time higher than that of pure Al. This is attributed to the dispersion strengthening effect of CuAl2 phase. With the increase of Cu content, the fracture mode changed from tough fracture to the tendency of brittle fracture.Heat treatment was carried out for the graded material. It could be concluded from the results of Vickers hardness that the optimized heat treatment effect was solid solution treated at 503℃for 3 h, water quenched and artificial aged at 150℃for 15 h.The microstructure, composition and mechanical properties under heat treatment were investigated. The result showed that the distribution of second phase obviously changed after solid solution treatment and artificial aging treatment. During solution treatment, almost all the CuAl2 dissolved into the matrix and formed supersaturated solid solution at the room temperature. After solid solution treatment, the as-formed supersaturated solid solution will precipitate during aging processing. There are some flake-like and needle-like phases namedθ" phase andθ' phase which are precipitated during artificial aging process. And the majority of the precipitates phases areθ" phase. The strengthening mechanism will change with the changes of composition in the following sequence:CuAl2 second phase strengthening→Solid solution strengthening→Dispersion strengthening (θ" andθ' phase). Hence, the strength and hardness will increase. At F state, the distribution of Cu element has a steep increase from pure Al layer to Al-4 wt% Cu layer. However, the distribution of Cu element becomes continuous gradually after T6 heat treatment, which is attributed to the diffusion of Cu element during heat treatment. It also could be seen the fracture surface changes from tough fracture to the tendency of brittle fracture after heat treatment. Under the control of constant displacement rate, the creep crack growth of the graded material can be divided into slow expansion stage and steady expansion stage. There is no obvious relationship between creep crack growth rate (da/dt) and stress intensity factor (K), but the relationship between da/dt and net section stress (σnet) is good with the corresponding formula da/dt=1.24σnet1.73. From the path analysis of the crack propagation, at the initial stage of creep crack growth, cavity grew at the matrix grain boundary at first and aggregated together in front of the pre-crack and then micro-cracks formed. The main crack grew along one of the micro-cracks. At the steady stage of creep crack growth, a lot of crack deflection and branching could be found. Because the strength of Al-4 wt% Cu layer is higher than pure Al layer and some hard phases exist, the creep crack propagation resistance will increase, and the crack growth rate will therefore decrease. The fracture surface of creep crack growth was tough inter-granular fracture. |
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