Deformation And Fracture Behavior Of B₄C/2024Al Composites

This thesis is aimed at preparing B₄C/2024 Al composites by spark plasma sintering.During the process,B4 C was mixed with the 2024 Al that has low density and high strength so that the aluminum matrix composite abtain properties of ligh weight and high strength.However,the intension of strength and stiffness generally leads to a rapid decline of its plasticity,which limits the application of aluminum matrix composite in some related areas.Aluminum matrix composite is a sort of multiphase material which has non-continuous microstructure and nonlinear mechanical properties.In theory,matrix,reinforcement,and interface were able to effect mechanical properties of the composites;however,there exist difficulties in measurement of the interfacial properties: rapid process of fracture of the material,inefficient observation of in-situ,and uncertainty of cracking origin observed by fracture.Thus,finite element method was applied to figure out interfacial properties and some fracture parameters by inversing the correlative experiment data.Besides,the FEM was also significant in discussing the behavior of composite fracture utilizing of presetting the material properties.With regard of 5,10,15% B₄C/2024 Al composites with particle size of 5?m and 17?m,observation of the phase after aging was carried on,the result indicating that B4 C was spaced regularly and had no evident defection.According to density testing,the relative density of B₄C/2024 Al composites was bigger than 99%.By the combination of the SEM and XRD analysis,it was found that micro-size Al2 Cu were diffused in the matrix and there existed no generated Al-B-C interfacial reactant.With the help of TEM,it was observed that a mass of spiculate S' phases were separated from 2024 Al crystals after aging and massive Al7Cu2 Fe phases were existed;additionally,the Al2 Cu on the interface was not totally disappeared after aging.Besides,the mechanical properties of B₄C/2024 Al composites after aging were measured: highest hardness: HB149,highest specific stiffness: 36.56GPa/?g·cm-3?.It is also recorded that the maximum tensile strength of composite 17?m B₄C/2024 Al composites were 478 MPa that was smaller than 5?m B₄C/2024 Al composites,which attended to be 562 MPa.In-situ tensile test and DIC analysis were employed to research the strain field of the composites in the process of tensile deformation.It claimed that three stages existed in the formation and spread of micro-crack.Firstly,initial cracks grew in the matrix;then,the cracks began to spread;Finally,as the growth of new cracks,the cracks converge and cause the fracture of the composites.Furthermore,the effect of particle size to the fracture behavior of the composite B₄C/2024 Al composites was discovered,the 17?m B₄C/2024 Al composite were primarily formed by the opening of the matrix nucleation and interface while the 5?m B₄C/2024 Al composites were fractured due to particle defeats,and corroborative evidence of two fracture models was made by observing the cracks.The thesis established a modeling approach based on practical microstructure,compared finite element method and DIC analysis,and utilized experiment data to get fracture parameter of B₄C/2024 Al composites,fracture strength of B₄C?850MPa?,and interfacial fracture strength?480MPa?.Stress was useful in analyzing the fracture behavior of B₄C/2024 Al composites,the higher the stress triaxiality was,the easier the matrix was to generate microporous aggregate fracture,and the more pervasive the phenomena of the particle fractures was.Lode coefficient and stress concentration factor exert relative less effect on uniaxial tension.High mean stress and stress concentration factor exert relative less effect on matrix crack.The fracture behavior of B₄C/2024 Al composites with different shapes and distributions was also discussed.The fracture behavior of uniform distribution form was dominated by interfacial debonding and particle fracture,which differed from volume fraction and particle shape.Interfacial debonding was the main cause in reticular and agglomerated B₄C/2024 Al composites fracture,and multi-crack propagation was subsistent in reticular B₄C/2024 Al composites.The influence of particle spacing and particle alignment in matrix was studied,and composites with the quadrate B4 C arranged in a row perpendicular to tensile direction and sparse in the horizontal direction,ultimately the excellent mechanical properties of 550 MPa obtained when 5% strain was applied.