Interfacial Structure And Mechanical Properties Of In-situ Synthesized MgAlB₄ Whisker Reinforced Aluminum Matrix Composites

Aluminum matrix composites(AMCs)have been widely used in aerospace industry,engine manufacturing,precision instruments and other fields due to their characteristics of low density,high strength,high modulus and high thermal stability.Whisker reinforced AMCs have both high strength and good machinability,showing a promising application prospect.The existing whisker reinforced AMCs are mainly prepared using the ex-situ method,which suffers from shortcomings in inhomogeneous dispersion of the whiskers,poor interface bonding,uncontrollable interfacial reaction and causing structure damage of the whiskers.By contrast,the in-situ synthesis of the whiskers in the matrix can not only avoid these problems,but also improve the strength and maintain enough toughness through structural design.Therefore,it is of great significant to develop the in-situ preparation system suitable for aluminum matrix and to realize the in-situ growth of whisker in aluminum matrix in order to prepare high performance AMCs.Herein,in-situ growth of boride whiskers in aluminum matrix was realized using powder metallurgy with aluminum powder,magnesium powder and boron powder(or H₃BO₃)as raw materials.The MgAlB₄ whiskers reinforced AMC and the Mg Al₂O₄particles&MgAlB₄ nano-rods co-reinforced AMC were prepared.The crystal structure of MgAlB₄,the growth mechanism and the intrinsic properties of the whisker,the orientation relationships and interface structure between the reinforcements and the matrix,as well as the mechanical properties and strengthening mechanism of the composite were systematically investigated through experiments and density functional theory(DFT)calculations.The crystallographic features and the formation mechanism of the whiskers were studied by means of component analysis,structural refinement and structural characterization.The results show that the boride whisker is MgAlB₄ with hexagonal lattice,which extends along the direction of[0001]and grows in a layered manner.Magnesium ratio and temperature are key factors to control whisker formation.The addition of magnesium leads to the boride gaining the growth advantage along the[0001]direction,while the high temperature provides the necessary conditions for solid state reaction.The segregation of trace iron on the{101(?)0}surface results in the formation of planar defects that parallel to{101(?)0}planes appearing in the coarse whiskers.The formation of planar defects can be inhibited by decreasing sintering temperature or increasing magnesium content.The stability,electronic structure and mechanical properties of MgAlB₄ and Mg B₂and Al B₂ were compared through DFT calculations.The results show that all of the three compounds have metal conductivity and are stable in thermodynamics and kinetics.Boron atoms bond with each other by covalent bond,and boron atom layers bond with metal atom layers by metallic bond.The strength of the B-B bond varies in the order Al B₂>MgAlB₄>Mg B₂.The strength of Al-B bond in MgAlB₄ is stronger than that in Al B₂,while the Mg-B bond in Mg B₂ is stronger that in MgAlB₄.The mechanical properties of MgAlB₄ are slightly inferior to that of Al B₂,except the tensile strength in[0001]direction.The calculated values of tensile strength and Young's modulus of the MgAlB₄ whiskers are 25.5 GPa and 380 GPa,respectively,making it an ideal reinforcement for AMCs.The properties of the composites,the diversity of orientation relationship and the corresponding interface structure,strengthening and fracture mechanism were studied by testing the mechanical properties of composites,analyzing the microstructure and fracture morphology,characterization of the orientation relationships and interface structures.The results show that the whisker can significantly improve the mechanical properties of the composite,refine the grain,and retain more low angle grain boundaries.The MgAlB₄ nucleates at the close-packed or near close-packed planes of the aluminum matrix and grows into the whiskers.Even if the planes misfit is large,nucleation and growth can still be carried out,which leads to the diversity of orientation relationships.Under the known orientation relationships,MgAlB₄ and Al matrix are combined with the coherent or semi-coherent interface.Strong interfacial bonding leads to whisker fracture prior to the matrix subjected to the applied loading.The improvement of the composite strength is attributed to the combined action of fine grain strengthening,Orowan strengthening and load transfer,during which the load transfer plays a dominant role.Mg Al₂O₄ particle and MgAlB₄ nano-rod co-reinforced AMC was prepared by ball milling-hot pressing-hot extrusion process with boric acid as boron source and oxygen source.The microstructure and mechanical properties of the composites were investigated via microstructure characterization,DFT calculation and mechanical property testing.The results show that the particles and whiskers were distributed in grain and grain boundary after hot extrusion.The reinforcement is chemically bonded to the matrix by forming a coherent interface or coincidence site lattice interface.The ultimate tensile strength of the composite containing 20 wt.%reinforcement is 528 MPa and the specific strength reaches 1.87×10⁵ Nm/kg,increasing by 303%and 286%compared with pure aluminum,respectively,while maintaining an elongation of 6.1%.The in-situ growth of boride whiskers in aluminum matrix has been realized in this work,which facilitates the dispersion of reinforcements and improves interface bonding in whisker reinforced AMCs.A new AMC system based on in-situ synthesis of MgAlB₄whiskers has been preliminarily established,which provides a new route for the preparation of high-performance AMC.