| Magnesium matrix composite has widespread application prospect in aerospace, national defense and automobile industries. The study of creep is the key need of magnesium matrix composite working in the high temperature. But now in the world, a little of study of the creep about magnesium matrix composite reinforced with short fiber is done. To meet the need of application and study, based on the study of both many researchers and this research group, the study in this paper focuses on the high temperature creep of AZ91D magnesium matrix composite reinforced with aluminum silicate short fiber, provides some experiments and theoretical base for the study of the creep performance and its behavior of magnesium matrix composite reinforced with short fiber working in the high temperature, and provides some theoretical guide in their application in high temperature environment.Tensile creep test is done on the matrix alloy AZ91D and the volume fraction of 20%, 25% and 30% Al2O3 SiO2/AZ91D composites under different temperatures and loads. The results show that, the trends of the creep curves of Al2O3 SiO2/AZ91D composites and AZ91D matrix are different. Creep rate decreases with the increase of temperature and stress, which are the main factors affecting creep rate. With the volume fraction of aluminum silicate short fiber increasing, the creep rate of Al2O3 SiO2/AZ91D composite decreases, indicating that the creep resistance increases. Under the same temperature and close load, the creep rate of Al2O3 SiO2/AZ91D composites is far lower than AZ91D matrix`s, and this shows the creep resistance of the composites is far higher than the one of its matrix.The apparent stress exponent of AZ91D matrix or n is 4.58.3; the apparent stress exponent of Al2O3 SiO2/AZ91D composites or n is 820. And the true stress exponents of Al2O3 SiO2/AZ91D composites and AZ91D matrix are the same, namely, n=3. At the same temperature, the creep threshold stress of Al2O3 SiO2/AZ91D composites is larger than AZ91D matrix`s. The load transfer factor of Al2O3 SiO2/AZ91D composites reduces along with the increase of the temperature and the external stress, and increases along with the increase of volume fraction of the short fiber, and it is the function of the temperature and the external stress. The existence of aluminum silicate short fiber greatly reduces the effective stress of the composites, thus enhancing the ability of creep resistance of the composites. Al2O3 SiO2/AZ91D composites have a high creep apparent activation energy, which should be attributed to the dependence of the threshold stress and transfer factorαof the composites on the temperature. The true creep activation energy of Al2O3 SiO2/AZ91D composites and the one of AZ91D matrix are the same, namely, Qc ?=144.63KJ/mol, and are similar to the diffusion activation energy of Al in the Mg Al alloy. This similar feature between the true creep activation energy and the diffusion activation energy indicates that creep effective stress can be a good description of the creep behavior of the composites.Through the obtained creep parameters, the establishment of creep constitutive equation of different fiber volume fraction AZ91D composites and AZ91D matrix alloy can be achieved. By the creep constitutive equation, the obtained theoretical curves agree well with the experimental curves, indicating that the obtained constitutive equation can explain well the creep constitutive relation between steady creep rate, stress and temperature.By finite element analysis, the impact of the interface thickness, the interface module and the short fiber orientation of Al2O3 SiO2（sf）/AZ91D composite on the maximum fiber axial stress and the steady creep rate is studied. Maximum axial stress of the short fiber is in the fiber center, and the axial stress gradually decreases along the direction of the fiber length. When the external stress is constant, the maximum fiber axial stress increases with decreasing of the thickness of the interface, and the steady creep rate increases with the increasing of thickness of the interface. The maximum fiber axial stress increases with the increasing of the interface modulus, the increasing of the interface module improves the load transfer and the creep resistance. The maximum fiber axial stress decreases with the increasing of the angleθof the fiber orientation, the steady creep rate increases with the increasing of angleθ, indicating that the capacity of the load transfer and the creep resistance of the fiber increases with the decreasing of the angleθ. Finite element simulation results and experimental results can be well matched to better explain the creep behavior.By SEM, TEM, and other modern means, the creep behavior of AZ91D alloy and AZ91D magnesium matrix composites reinforced with aluminum silicate short fiber is studied carefully and completely. The results show that, the threshold stress （breakaway stress） of AZ91D alloy originates from the force of the Al solute atoms air mass towards the moving dislocation. The creep threshold stress of Al2O3 SiO2（sf）/AZ91D composite originates from the drag force on the moving dislocation as the result of the mismatch strain of the short fiber and the matrix as well as from the pinning force of Al atoms air mass on the moving dislocation. The empirical formula of the threshold stress is obtained by experimental data. Threshold stress decreases with increasing temperature, increases with the increase of volume fraction of the short fiber, but not significantly, which is mainly due to the increase of fiber volume fraction caused by precipitation of a greater number ofβMg17Al12 precipitate. Load transfer is the strengthening mechanism of the reinforced aluminum silicate short fiber. The strain inconsistency of the matrix and short fiber produces the drag on the moving dislocations, making the dislocations in the matrix migrate to the interface of the matrix and fiber, the packing density of dislocations at the interface constantly increases, resulting in the stress concentration around fibers, the fibers continuing to accelerate to break into shorter sub fibers, shortening the distance of dislocation motion to the end of the sub fibers, and making dislocation cross the end of the sub fibers; and this is the mechanism of dislocation over the second phase. There is no debonding phenomenon in the good bonding interface of the composite in the creep deformation, and the weak interface of the composite will be severely damaged in the creep deformation, and the holes and cracks of the interface will gradually expand under the stress, resulting in the composite’s changing continuously during the creep deformation, the final formation of macroscopic’s cracking, and leading to the fracture of the composite. The creep mechanism of Al2O3 SiO2（sf）/AZ91D composite and its matrix’s are the same under high temperature and low stress, namely: the creep of Al2O3 SiO2（sf）/AZ91D composite is controlled by the creep of the matrix, and dislocation viscosity slip control plays a leading role, supplemented by grain boundary sliding control. The presence of short fibers hinders the migration of dislocation, reduces the creep rate and improves the creep resistance of the composites.In this paper, through the tensile creep tests on the aluminum silicate short fiber of three fiber volume fraction reinforced AZ91D magnesium matrix composite, the creep behavior is studied and more comprehensive creep parameters are obtained. By the analysis of the experimental data, the relevant creep behavior mechanisms are obtained, which can give some guidance in the theory and practice for magnesium matrix composites reinforced with short fibers. |
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