Microstructure And Mechanical Properties Of Al Based Supersaturated Solid Solution Films

As an important surface material,metal films prepared by vapor deposition was found out more and more applications in modern industry.Among them,Al and Al alloy films have attracted more and more attention because of their excellent corrosion resistance and toughness,Hardness improvement is one of the important research directions for this kind of material.High energy and dispersion of vapor deposition particles and the highly non-equilibrium characteristics during film growth create a condition of supersaturated interstitial solid solubility,resulting in grain refinement and significant improvement of mechanical properties,has become an effective way to improve the hardness of Al-based films.Although there have been many current research on Al supersaturated solid solution films,it is still not enough:the material system is homogeneous and mainly concentrated solution in substitutional alloy systems.The microstructure and enhancement mechanisms were not revealed sufficiently.The role of each microstructure factor in strengthening has not been distinguished.Based on these conditions,Al-Ti substitutional supersaturated solid solution alloy film was firstly studied.Al-B interstitial supersaturated solid solution alloy film and Al-TiB₂,Al-TiC composite films with both substitutional and interstitial solid solution characteristics were further studied.The microstructure characteristics and evolution rules of all these supersaturated solid solution films were revealed and their strengthening effects were compared.An experimental simulation method of two-dimensional structure nano-multilayer films was used to reveal the strengthening effect of grain size,grain boundary width and composition three main microstructure factors on nanocrystalline solid solution films.The main conclusions are as follows:1.In the Al-Ti substitutional supersaturated solid solution alloy films,the addition and increase of Ti content resulted in rapid nano crystallization and various types of intermetallic compound formed which gradually made the films into amorphous structures.The hardness of the films increased nearly linearly with the increase of Ti content.The improvement of the film hardness mainly comes from grain refinement and Ti solid solution at low Ti content,while the high Ti content,is more dependent on the increase of intermetallic compounds.2.In Al-B interstitial solid solution films,B atoms are dissolved in the Al lattice and the films formed supersaturated solid solution with a pronounce（111）texture.The film grains were significantly refined with the increase of B content.Compared to other substitutional alloy elements in Al film,the strengthening effect of B is about ten times higher,which is attributed to the significant strengthening effects of interstitial supersaturated solid solution on metal films.Its hardness reached 6.0GPa when containing 1.89at.%B.The strengthening of Al-B films mainly results from the significant lattice distortion and grain refinement caused by interstitial solution of B.3.In the Al-TiB₂ composite films prepared by co-sputtering using Al and TiB₂ targets,Ti and B still exist in the form TiB₂（or Ti B）compounds.These compounds can be dissolved in the Al lattice,but the solubility is less than 2at.%TiB₂.The supersaturated solid solution with both substitutional and interstitial solid solution characteristics resulted in rapid grain refinement and even amorphous structures.The film hardness increased rapidly and reached the highest value of 6.9GPa and decreased slightly due to amorphization.After annealing at 400℃,the grain size remained and the hardness decreased but still remained at a high level.4.The reaction system Al-TiC composite films are still mainly in the form of TiC supersaturated solid solution in the deposition state.The film grains were reduced to nanoscale rapidly and the film gradually turns to amorphous structure.Correspondingly,the hardness of composite films increased rapidly and reached the highest value of 7.0GPa,then slightly decreased due to amorphization.After annealing at 400℃,the film grains grow slightly,the TiC decomposed and Al₃Ti compounds formed.The film hardness decreased correspondingly but remained at a high level.5.Comparing this three kind of films:（1）The strengthening of the substitutional supersaturated solid solution Al-Ti alloy film is supported by the existence and increasing concentration of these intermetallic compounds.Its enhanced efficiency is relatively low;（2）The enhanced efficiency of the interstitial supersaturated solid solution Al-B alloy film is the highest due to the nanocrystallization,severe lattice distortion and preferred orientation.（3）The enhanced efficiency of the TiB₂ and TiC composite films is much higher than Al-Ti alloy film,but slightly lower than Al-B alloy film.6.A two-dimensional multilayered film experimental method was used to realize the isolation and independent change of the main microstructure factors of Al-TiB₂alloy.Their strengthening effects on the film were revealed:Inverse Hall-Petch effect also exists in Al-TiB₂composite films.The critical grain sizes of deviation from the Hall-Petch relationship and the inverse Hall-Petch phenomenon occur at 32 nm and 8nm respectively which were close to that of pure Al predicted by simulations.The change of TiB₂ content and the width of the grain boundary donot obviously changed the hardness of the film.These above results show the new progress of material system development and the revealing of microstructure features in the supersaturated solid solution films.