Study On The Microstructures And Mechanical Properties Of The TiB₂/Al-Zn-Mg-Cu Composites Fabricated By Rapid Solidification Powder Metallurgy

The distribution of the nano-particles in the metal matrix has a significant influence on the mechanical properties of the composites.The nano-particles are difficult to be dispersed uniformly in the matrix through the convential methods of fabricating the particle reinforced metal matrix composites,such as stir casting or mechanical mixing the alloy powders and external particles.Therefore,it is of significance proposing a new effective method to disperse the nano-particles.In the present work,we have proposed to produce the in-situ TiB₂/Al-Zn-Mg-Cu composite castings firstly,then the composite powders with uniformly pre-embedded particles（CPEPs）were obtained through gas atomization,the rapid advancing solid/liquid interface can engulf the TiB₂ particles,and then consolidated the composite powders to fabricate the composites.This thesis focuses on the following scientific issues,（1）the interaction of the solid/liquid surface with particles under rapid solidification condition;（2）the influence of TiB₂particles on the spark plasma sintering mechanism and microstructural evolution during extrusion;（3）the influence of the content of the alloy elements and TiB₂ on the precipitation kinetics and mechanical properties.By comparing the microstructures and mechanical properties of the Al-Zn-Mg-Cu alloys and TiB₂/Al-Zn-Mg-Cu composites fabricated by rapid solidification powder metallurgy,the main conclusions are followed as:The α-Al solids exhibit the equiaxed morphology in the TiB₂/Al-Zn-Mg-Cu composite powders due to the heterogeneous nucleation.However,the α-Al solids exhibit complex structures in the alloy powders.When the alloy powders with size smaller than 3μm,the primary nucleus undercooling is higher than critical nucleus undercooling 392 K and the velocity of the solid/liquid interface is faster than the critical velocity 10 m/s,therefore,the powders exhibit featureless structure.With increasing powder size in the range of the 3～7μm,the cellular structures are found.Most of TiB₂ particles are located within the grains in the composite powders.It is resulted from that the rapid advancing solid/liquid interface velocity of 56～88 mm/s during atomization is higher than the critical velocity 3.2～65mm/s for engulfing TiB₂particles by planar solid/liquid interface.However,there are some larger particles on the grain boundaries due to infinity of the critical velocity when the size of particles is comparable with grain size.It is demonstrated the engulfment of the nanoparticles by solid/liquid interface in the present work coupled with microstructural observations and the critical solid/liquid velocity model for engulfing nanoparticles are developed.The spark plasma sintering stress exponent n of both Al-Zn-Mg-Cu alloy and TiB₂/Al-Zn-Mg-Cu composite powders is in the range of 1～2.The apparent activation energy for sintering Al-Zn-Mg-Cu alloy and TiB₂/Al-Zn-Mg-Cu composite powders are 58.8～62.4 k J/mol,164.5～169.1 k J/mol respectively.Additionally,it is observed that the alloy elements including Zn,Mg and Cu elements are segregated at the primary powder surface and grain boundaries after sintering alloy powders.The alloy elements in the composite powders tend to segregate at the facets of the TiB₂ particles.The introduced TiB₂ particles can accelerate the densification process at the initial sintering stage due to high resistivity of TiB₂ particles,and then impede the densification process at the final sintering stage.The densification of the Al-Zn-Mg-Cu alloy powdes is controlled by grain boundary sliding accommodated with surface diffusion and grain boundary diffusion,while the densification of the TiB₂/Al-Zn-Mg-Cu composite powders is controlled by grain boundary sliding accommodated with volume diffusion.The grains of the TiB₂/Al-Zn-Mg-Cu composites are deformed in the direction perpendicular to the extruded direction at first and then along the extruded direction during extrusion.The<111>and<001>fiber textures are formed in the TiB₂/Al-Zn-Mg-Cu composites after hot extrution due to the preferential activation of S1（111）[-110]and S3（100）[011]slip systems.The TiB₂ particles in the fine-grained composites could also stimulate the initiation of dynamic recrystallization behavior and change the spatial distribution of subgrain boundaries.The grain boundary sliding accommodated with dislocation slip mechanism dominates the deformation mechanism and governs the microstructure evolution.When the Zn content in the TiB₂/Al-Zn-Mg-Cu composites increases from 6.28 wt.%to 10.00 wt.%,the volume fraction and number density of the precipitates increase,the time reaching peak age decreases from 24 h to12 h,the peak age hardness increases from 219.2 HV to 238.5 HV.According the size class model,the accelerared age hardening response results from the enhanced nucleation rate.The yield strength,tensile strength and Young’s modulus of the TiB₂/Al-10Zn-Mg-Cu composites have increased 40 MPa,30 MPa and 9 GPa respectively.The introduced TiB₂particles accelerate the work hardening rate of the composites,the dislocation accumulation parameter k₁ increasese from 3.1×10⁸m^-1of the alloy to 3.6×10⁸ m^-1of the composite.The physical dislocation activation volume decreases from 59b³～74b³ of the alloy to 46b³～65b³of the composite.