Preparation,Microstructure And Elevated And Low Temperature Mechanical Behaviors Of B₄Cp/6061Al Composites

Compared with pure aluminum or aluminum alloys,particle-reinforced aluminum matrix composites(PRAMCs)with the higher specific stiffness and specific modulus,lower coefficient of thermal expansion,better elevated temperature performance and creep resistance,and better wear resistance and fatigue resistance have been widely used in many fields,such as aerospace,weapon equipment,automobile,and electronic device.After entering the 21st century,the application range of PRAMCs has been further expanded,therefore,high requirements for specific strength,specific stiffness,and lightweight of PRAMCs are put forward.In order to meet the need for different fields,the core components composed of PRAMCs are usually used in extreme harsh environment,such as elevated temperature,low temperature,and elevated and low temperature alternation.Therefore,mechanical behaviors of PRAMCs at different service temperatures ought to be explored and the coupling relationship of microstructure,mechanical property,and fracture mechanism should be established.In terms of lightweight,high specific modulus,and high specific strength of PRAMCs,and strengthening effect and function characteristics of reinforced particles,6061Al alloy and B4C particle are selected as matrix alloy and reinforcement,respectively.However,mechanical property,strengthening mechanism,and fracture mechanism of B4Cp/6061 Al composites have not been reported,which limits the application of the composites at low temperature.In addition,B4Cp/Al composites have been viewed as desirable neutron absorber materials.Thermal neutrons are absorbed to generate heat,causing an increase in temperature of the composites.Therefore,it is very important to evaluate the mechanical property and failure behavior of B4Cp/6061Al composites at elevated temperatures.As a result,preparation,microstructure and mechanical behavior at elevated and low temperatures of B4Cp/6061Al composites are investigated in this paper.The main research contents of this paper are divided into three parts as follows.(1)Firstly,B4Cp/6061Al composites are fabricated by powder metallurgy.Effect of hot pressing temperature(833 K,853 K,and 873 K)on the microstructure and room temperature mechanical properties of the composites are studied and the fracture behaviors of the composites hot pressed at different temperatures are analyzed.The micropores in the composite hot pressed at 833 K and the reaction products in the composite at 873 K are observed,which results in a decrease in tensile strength of T4 heat treated composites.Then,B4Cp/6061Al composites with various reinforcement contents(0 wt%,5 wt%,10 wt%,and 15 wt%)are prepared via the optimized hot pressing temperature(853 K).Variation in hardness and electrical conductivity as a function of aging time in the composites with different B4Cp contents is discussed.Mechanical properties of peak-aged B4Cp/6061Al composites at room temperature are investigated.The result shows that with increasing the B4Cp content,the yield strength and ultimate tensile strength gradually increase,while the elongation gradually decreases.Furthermore,the yield strength of peak-aged B4Cp/6061Al composites reinforced by different B4Cp contents is quantitatively predicted by direct and indirect strengthening mechanisms.The calculated values of the yield strength of the peak-aged composites are close to the experimental measurements.For the T4 heat treated and peak-aged B4Cp/6061Al composites,the fracture mechanisms at room temperature include the ductile fracture of matrix,B4C particle fracture,and interface decohesion between the B4C particles and matrix.(2)Secondly,tensile property,strengthening-toughening mechanism,and fracture mechanism of peak-aged 5 wt%B4Cp/6061Al composite at cryogenic temperature are studied.The result indicates that as the deformation temperature decreases from 298 K to 77 K,the yield strength,tensile strength,and elongation of the peak-aged 5 wt%B4Cp/6061Al composite increase from 340 MPa to 380 MPa,389 MPa to 487 MPa,and 11.9%to 15.6%,respectively.Compared with the room temperature,the strain-hardening exponent and uniform elongation at 77 K are higher,suggesting that the composite has a better ability to resist deformation at 77 K.The grains with prestored dislocations and recrystallized grains,which are responsible for the increase in tensile strength and ductility at 77 K,are found in the composite.In addition,tensile property and deformation process of notched 5 wt%B4Cp/6061Al composite are also investigated.It is found that decreasing the temperature to 77 K increases the notch sensitivity.The simulation result of stress-strain field based on the two-dimensional actual microstructure shows matrix damage is more likely to occur at the tip of the notch and propagates to the high stress regions near the B4C particles.At 77 K,the fracture mechanisms of smooth and notched composites are similar to that at room temperature,i.e.,ductile fracture of matrix,particle fracture,and interface debonding between the particles and matrix.(3)Finally,microstructure of matrix and B4Cp/Al interface of annealed 10 wt%B4Cp/6061Al composite is characterized.Effect of strain rate and thermal exposure time on the elevated temperature mechanical behavior in the annealed 10 wt%B4Cp/6061 Al composite is investigated.The result shows that with increasing the strain rate,the elevated temperature tensile strength gradually increases,while the elongation gradually decreases.After long time thermal exposure,tensile strength and elongation at elevated temperatures remain unchanged.Furthermore,the tensile strength is further improved by adding nanosized B4C(n-B4C)particles.The microstructure analysis shows that n-B4C particles within the grains and at the grain boundaries are detected.The addition of n-B4C particles can effectively inhibit the dynamic recrystallization process,resulting in the matrix grain refinement,the increase in low angle grain boundary,and the reduction in recrystallization region.As the deformation temperature increases from 373 K to 573 K,the tensile strength increment of the annealed(?+n)-B4Cp/6061Al composite decreases,which is mainly associated with the different strengthening mechanisms caused by n-B4C particles at elevated temperatures.The Orowan strengthening mechanism is dominant at 373 K and dislocation climbing mechanism is predominant at 573 K.For the different deformation temperatures and strain rates,the failure mode of the annealed B4Cp/6061Al composite is mainly ductile fracture of matrix accompanied by a small amount of interface debonding between the particles and matrix at elevated temperatures.