| Because of its excellent mechanical properties,thermal properties and nuclear properties,beryllium and beryllium aluminum alloys are the key materials to develop aerospace industry and the monopoly materials related to national security.Beryllium-aluminum alloys have specific modulus and additional properties that outperforms traditional aerospace alloys,making it an excellent candidate in aerospace applications to reduce weight and dramatically increase the performance of stiffness-driven components,which can be used as an alternative to beryllium.Pure A1 was selected as the matrix of powder metallurgy beryllium aluminum alloys in the prior work,which limited the performance of beryllium aluminum alloys.In this paper,the composition of beryllium aluminum alloys are designed from the point of view of alloy strengthening,wettability and interfacial reaction controlling.Beryllium aluminum alloys with excellent mechanical properties are prepared by powder metallurgy technology.The microstructure,interface characteristics and aging behavior of beryllium aluminum alloys are analyzed by X-ray diffraction,scanning electron microscopy,high resolution transmission electron microscopy and differential scanning calorimetry.The main contents and conclusions are as follows:?1?A new approach to predict the yield strength of beryllium aluminum alloy is developed.The particle cracking and particle debonding have been incorporated into the model based on the Weibull statistics and the Eshelby's equivalent inclusion approach.The results show that the yield strength of beryllium aluminum alloy are effected by the strength of aluminum alloy matrix,the grain size of beryllium phase and the interfacial bonding strength.The effect of alloying elements on the interfacial bonding strength between beryllium phase and aluminum phase was discussed from the point of view of thermodynamics.The wettability between liquid aluminum and solid beryllium,liquid aluminum and solid beryllium oxide determine the bonding strength between beryllium phase and aluminum phase.The wettability between liquid aluminum and solid beryllium,liquid aluminum and solid beryllium oxide can be improved by adding Mg and Si to aluminum alloy matrix,thus the interfacial bonding strength can be improved.Beryllium aluminum alloys with excellent properties can be obtained by selecting Al-Mg-Si alloy as the matrix of beryllium aluminum alloy.?2?The composition uniformity of the mixed powder increases with the increase of milling time,but the oxygen content increases continuously.The beryllium aluminum mixed powder with uniform composition can be obtained by ball milling for 6h and the oxygen content is less than 1.5wt.%.The cold isostatic pressing behavior of beryllium aluminum mixed powder accords with the powder pressing double logarithmic equation.The density of beryllium aluminum compact increases almost linearly with the increase of cold isostatic pressing pressure.When the cold isostatic pressing pressure is 270MPa,the density of beryllium aluminum compacts can reach to 87%of theoretical density.With the rising of HIP sintering temperature,the aluminum phase in beryllium aluminum alloy will agglomerate,while and the aluminum phase in beryllium aluminum alloy will be melted and exudated with the rising of vacuum hot pressing sintering temperature and spark plasma sintering temperature.Beryllium aluminum alloy with high interfacial bonding strength,fine microstructure and excellent mechanical properties can be prepared by hot isostatic pressing sintering,hot pressing sintering and spark plasma sintering when the sintering process parameters are optimized.?3?The effects of different beryllium particle size,different aluminum matrix and different sintering techniques on the microstructure and mechanical properties of beryllium aluminum alloy are studied.The elongation is basically unchanged when the grain size of beryllium phase is reduced from 40?m to 10?m but the yield strength and tensile strength increased from 195MPa to 265MPa,310MPa to 383 MPa,respectively under the condition of the same beryllium content.The elastic modulus of Be-38wt.%A1 and Be-38wt.%Al-Mg-Si is basically the same,but the yield strength and tensile strength of Be-38wt.%Al-Mg-Si are higher than those of Be-38wt.%Al.The densification sintering temperature of hot isostatic pressing sintering was lower than that of vacuum hot pressing sintering.The elastic modulus,yield strength and tensile strength of hot isostatic sintering beryllium aluminum alloy were slightly improved.Hot isostatic pressed beryllium aluminum alloy has the highest mechanical properties.The yield strength and tensile strength are 265MPa and 383 MPa respectively,which are close to the yield strength and tensile strength of rolled powder metallurgy beryllium aluminum alloy with the same beryllium content prepared by the United States.?4?The microstructure of beryllium aluminum alloy and the characteristic of interface were characterized by scanning electron microscopy and high resolution transmission electron microscopy.The results show that the microstructure of hot isostatic Be-38wt.%Al-Mg-Si is mainly composed of Al phase and Be phase.Mg and Si are mainly distributed in Al matrix.There was no reaction product at the interface in Be/Al-Mg-Si composite,only with a small amount of nanometer BeO particles distributed along the interface,which tended to form semi-coherent interface with Be.It is impossible to form a complete three-dimensional coherent interface or semi-coherent interface between beryllium and aluminum because of their different crystal structure.In order to minimize the interfacial energy,In order to minimize the interface energy,Be phase and Al phase tilt at a certain angle and maintain a specific crystal orientation relationship:[011]Al//???Be,???Al?0.8°???Be?5?The aging behavior of Be/Al-Mg-Si composite fabricated by powder metallurgy technique was investigated by Micro Vickers hardness measurements,high resolution transmission electron microscopy and differential scanning calorimetry experiments.It is found that Be/Al-Mg-Si composite exhibited an accelerated age hardening phenomenon as high density dislocations are incorporated by Be particles.The aging precipitation sequence of Be/Al-Mg-Si composite is similar to Al-Mg-Si alloy,but the formation of GP zone is suppressed which can be attributed to the decrease of vacancy concentration caused by Be/AI interfaces and the segregation of Mg and Si on Be/Al interfaces.The temperature of ?",?' and? precipitates' formation in Be/Al-Mg-Si composite are lower than that in Al-Mg-Si alloy.The ?" needles which are responsible for the maximum age-hardening effect in Al-Mg-Si alloys precipitate independently or precipitate along dislocations in the A1 matrix.?6?The yield strength and tensile strength of hot isostatic pressed Be-38wt.%Al-Mg-Si increase significantly after aging treatment.When the aging temperature is 170? and the aging time is 8 h,the yield strength and tensile strength of Be-38wt.%Al-Mg-Si are 340MPa and 416 MPa respectively,which are higher than the maximum yield strength and tensile strength of powder metallurgy beryllium aluminum alloy with the same beryllium content prepared by the United States. |
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