Study On The Tensile Deformation And Fracture Behavior Of Spray Deposition 7075Al/SiC_p Composites Sheet At Elevated Temperature

Discontinuously reinforced aluminum matrix composites exhibit unusual combinations of mechanical, physical properties such as high stiffness and strength, high elastic modulus, good wear resistance, and good dimensional stability while retaining relatively low density, are attractive for many structural applications, especially in the aerospace and automobile industries. However, a significant disadvantage of these kinds of material was their low room temperature ductility and poor toughness for the existence of a large amount of reinforcements. In the present study, the elevated temperature tensile deformation and fracture behavior of spray deposition 7075Al/SiC_p composites sheet was studied by uniaxial tensile tests on WDE-200 tensile machine at temperatures range from 300 to 450℃and at strain rates between 0.001 and 0.1s-1. The deformation microstructures and tensile fracture surfaces were examined by Leitz MM-6 metallographic and JSM-6700F scanning electron microscopy (SEM). The effect of heat treatment on high temperature deformation and fracture behavior was also studied after solution and aging heat treatment.1. The flow stress of spray deposition 7075Al/SiC_p composites sheet are decreased with increasing deformation temperature and decreasing strain rate. Deformation initially, the flow stress shows obvious strain hardening trend with the increased of deformation degree, and then due to the offset of softening process, the increase gradually is slow and eventually reach a steady-state flow, until a fracture is occurred;2. The flow stress of spray deposition 7075Al/SiC_p composites are analyzed after elevated temperature tensile tests, the results show that the maximum strain rate sensitivity exponent is only 0.22 which is much lower than that of typical superplastic materials. The total elongation increased with increasing temperature but decreased with increasing strain rate, the maximum elongation of 71.07% can be obtained at the strain rate of 0.001s-1and at temperature of 450℃in the present testing. The elevated temperature deformation behavior of 7075Al/SiC_p composites sheet can be described by a Power ? Arrheniusmodel and Z parameter, a flow stress constitutive equation is obtained as follow:ε= 4.28×10¹⁴σ^5.06 exp( - 332×10³/RT) 3. SiC particles and the microstructures of matrix alloy are examined after elevated temperature tensile, the results show that SiC particle distribute in the composites are more homogeneous at lower strain rate and higher deformation temperature, and also the broken degree. The dimension of matrix grains become refining and cavities in the surface are increased with the strain rate increasing, but decreased with the temperature increasing. The grains are stretched along the tensile direction.4. The flow stress behavior of spray deposition 7075Al/SiC_p composites sheet which conducted solution and ageing heat treating are similar to hot rolling state, however, the strain hardening trend become slower and fracture trend is accelerated. The tensile and yield strengths are increased resulted from solution, deposition and refining strengthening. The total elongations are decreased which can still reach 10% above, strain rate sensitivity exponents m are decreased and hot activation energy Q are increased gradually which indicated the workability of the composites become worse, but the combination properties are improved. After solution heat treating, there are no coarse secondly grains found in the composites, but amount of ellipse precipitated phase and fineness secondly grains can be easy found after aging heat treating.5. The elevated temperature fracture mechanics are studied through examined the fracture surface: SiC particles fracture, extracting from the matrix, interfacial debonding and matrix failure are the main reasons resulted the finally tensile fracture.By compared the elevated temperature tensile fracture of 7075Al/SiC_p composites sheet after or before heat treating, the results indicate when composites is being hot rolling state, the dimples are deeper and secondly precipitated phase can not be examined, the debonding is more obviously. While after heat treating, the dimples become shallow and amount of precipitated phase can be found. Large and regions of clustered particles are found to be the locations prone to damage, which are also the reason caused materials failure.