| In this paper,the 2024 Sc Al alloy was prepared by the traditional casting method,2024 Sc Al matrix composites reinforced by the 3 wt.% short carbon fiber with length of 2mm,3mm,4mm and 5mm respectively were prepared by liquid stirring casting method.The carbon fibers were treated by chemical copper plating process.The microstructures of the composites and tensile fracture morphology were observed by Scanning Electron Microscopy(SEM).The phases in the materials were analyzed by Energy Dispersion Spectrum(EDS).The density and densification degree of the materials were measured by the Archimedes principle.Conventional mechanical properties and micro-yield properties of materials were tested by the tensile test and the loading-unloading test at room temperature.The two-stage aging was used to treat the materials after solution treatment.The thermal expansion coefficient of the materials were measured by a thermal expansion meter.The results show:(1)It was observed that the microstructure of the as-cast 2024 Sc aluminum alloy,which was mainly composed of the gray α-Al matrix and white continuous second phase due to composition segregation during un-equilibrium solidification.The carbon fiber in the composites was distributed evenly,and a slight interfacial reaction was occurred in the interface.After homogenization,the continuous white second phase in these materials became discontinuous.Parts of Zr,Sc atoms in these materials was diffused to form solid solution and the rest as the form of compounds.After extrusion,the grains were elongated,and the second phase and carbon fibers were distributed along the extrusion direction.(2)It was found that both of the theoretical density and the experimental density of the composites were less than 2024 Sc aluminum alloy,and the density of as-cast materials was less than the density after extrusion.With increasing the length of carbon fiber,the experimental density of the as-cast composites become bigger gradually and then smaller.The density difference among the composites decreased after extrusion.After extrusion,the porosity of the composites decreased by 71.3%,59.4%,80.2% and 80% respectively.Moreover,with the increasing of the carbonfiber length,the densification degree of the composites increased,leading to the improvement of mechanical properties of the composites.(3)The two-stage aging treatment(130℃/3h+200℃/(5h,10 h,15h))was used to treat the materials after the solution treatment(495℃/2h).The conventional mechanical properties of the treated materials was tested.It was found that the tensile strength,yield strength and the elastic modulus of materials increased gradually and decreased finally with increasing the length of carbon fiber from 2 mm to 5 mm.When the second stage aging time was 10 h,it was peak aging time.Then the highest tensile strength,yield strength and elastic modulus of the composites were 472 MPa,411MPa and 87 GPa respectively.However,the second stage peak aging time reduced to 5h for the composite with 5mm carbon fiber length,while the tensile strength and yield strength of the composite were 494 MPa and 419 MPa respectively.When the carbon fiber was added into the 2024 Sc Al alloy,the tensile strength,yield strength and elongation of the composite decreased.However,the elastic modulus of composites obviously increased compared with the 2024 Sc Al alloy,and the peak value was 90 GPa.It was observed that the carbon fibers in composites were pulled out(early failure)or pull off through the fracture morphology.(4)It was found that the work hardening rate in the micro-yield stage was much higher than that in the yield stage.After solution treatment(495℃/ 2h),the micro-yield strength of the materials reached to maximum value at peak aging state(130℃/ 3h +200℃/10h).With increasing the carbon fiber length,the micro-yield strength of composites increased firstly and then decreased.The micro-yield strength of the composite reached the peak value(304MPa)when the carbon fiber length was 4mm.The micro-yield strength of composites was lower than that of 2024 Sc Al alloy.(5)It was found that the relative elongation of the materials changed linearly with temperature.The addition of carbon fiber decreased the thermal expansion coefficient of the materials.The thermal expansion coefficient of the composites decreased at first and then increased with increasing the carbon fiber length.When the carbon fiber length was 4mm,the thermal expansion coefficient of the composite was the smallest,and this value was 24.2×10-6 /℃.In a word,the composites with 4mm carbon fiber length had the bestcomprehensive mechanical properties... |
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