Study On The Corrosion Behaviors Of In-situ TiB₂/7050 Aluminum Matrix Composite

With the rapid development of the aviation field,material properties of the critical aero-components are demanding increasingly,which can hardly be satisfied by the traditional aluminum alloys.In-situ Ti B₂/7050 aluminum matrix composite,prepared by melt control based on the conventional 7050high-strength aluminum alloy,has the advantages of light weight,high strength,high stiffness and high fatigue resistance,possible to be used in the critical aero-components.When aircrafts serving in the corrosive environments,aero-components are usually corroded.Considering the flight safety and cost,researches on the corrosion behaviors of in-situ Ti B₂/7050 aluminum matrix composites are necessary to be carried out.In the present thesis,microstructural characteristics in the three directions of in-situ Ti B₂/7050 aluminum matrix composites,including grain orientation,grain morphology and the distribution of Ti B₂ particles,were analyzed to study it’s stress corrosion and exfoliation corrosion behaviors.Influences of microstructures on the corrosion resistance of in-situ Ti B₂/7050 aluminum matrix composite have been revealed.Microstructural observations showed that sizes of most Ti B₂ particles in in-situ Ti B₂/7050 aluminum matrix composites stay in the range of50-500 nm.Along the ED direction the Ti B₂ particles distribute in the form of particle bands and the fiber textures with<100>and<111>can be obviously observed.The aspect ratios of grains in the three directions decrease in the order of ED-ND plane>ED-TD plane>TD-ND plane,while the densities of high angle grain boundaries and the area fractions of Ti B₂ particles decrease in the order of TD-ND plane>ED-ND plane>ED-TD plane.Results of slow strain rate tensile tests in 3.5wt.%Na Cl solution showed that the tensile strength,elongation and rupture time of the composites decrease mainly during in the necking stage.The stress corrosion cracking process can be divided into two stages.First,the initial cracks originate from the sample surface and propagate to the interiors perpendicularly to the loading direction,which causes the cleavage fracture.Second,when reaching the Ti B₂-particle bands,the cracks deflect and tend to propagate along the bands.Stress corrosion cracking can be greatly affected by Ti B₂ particles from the following three aspects:（I）locally agglomerated Ti B₂ particles can induce recrystallization,which provides much more paths for the stress corrosion cracking;（II）dislocations can be pinned by the Ti B₂ particles,which accelerates the hydrogen-induced cracking;（III）there is a large potential difference between the Ti B₂ particles and the Al matrix,which can accelerate stress corrosion cracking.In addition,the loading direction and the aging treatment process have significant impacts on the susceptibility to the stress corrosion cracking of composites.Because of the highest grain boundary density and the Ti B₂particle area fraction,specimen in the ED direction shows the highest stress corrosion susceptibility.Compared with T6,T74 treatment can greatly improve the stress corrosion cracking resistance,however,with about 20%reduction of the tensile strength.The exfoliation corrosion mechanisms of in-situ Ti B₂/7050 composite are as follows.First,the corrosive solution permeates into the sample to a certain depth from the weaknesses on the surface.Second,corrosion products are generated along the texture direction.Third,when the corrosion products accumulate to a certain degree,the surface metal will exfoliate due to the’Wedging Effect’.The permeation of the corrosive solution and the exfoliation corrosion cracking make up the whole process.These two procedures take place alternatively so that the material surface falls off layer by layer,and eventually leading to failure.During the exfoliation corrosion process,the recrystallization induced by Ti B₂ particles will increase the density of the grain boundaries.Ti B₂ particles make it easier to form corrosion paths at the grain boundaries.In the composite,T74and T76 treatments can greatly improve the exfoliation corrosion resistance,among which T74 performs better.Aging treatments mainly affect the permeation depth of the corrosive solution rather than the thickness of the exfoliation layer.