Strengthening Effect Of Each Microstructural Factors In Nanocrystalline Al-Zr Alloy

Nanocrystalline metals show a wide range of application prospects due to the impressive properties,thus their strengthening mechanisms have aroused increasing interest in recent years.Compared with pure nanocrystalline metal,segregation of alloy elements within grain boundary make a more stable structure as well as a more complex strengthening mechanism of the nanocrystalline alloys.The strengthening mechanisms of nanocrystalline alloys not only include fine grain strengthening,but also include solid solution strengthening and grain boundary strengthening.It is feasible to describe the effect of a single factor theoretically,but very difficult experimentally,because they are usually intimately tied with each other and simultaneously change with alloy composition.Despite a great number of studies in this area,the isolation of microstructure factors from each other has not been achieved.Actually this is one of the key issues to hinder in-depth research on the deformation mechanisms of nanocrystalline alloys.Since microstructural factors have significant effects on each other and are difficult to decouple systematically as composition changes,this paper selects Al-Zr alloy system and uses the method of two-demensional nanocrystalline multilayers to isolate the effects of main microstructural factors including grain size,grain interior composition,grain boundary composition,and grain boundary width of nanocrystals.Through the independent change of single factor,the function,proportion,and variation trend of a specific single factor in nanocrytalline alloys were revealed.By comparing with three-demensional nanocrystalline thin films,a bridge was built to connect the mature strengthening theory to nanocrystalline alloys.A strengthening mechanism based on the functional rules of microstructural factors was obtained.The main conclusions in this work are summarized as follows:1.The studies on the Al-Zr alloy films indicate that the films with 0-12.3 at.% Zr content formed highly supersaturated solid solution due to high dispersibility of vapor particles and non-equilibrium growth of film in co-sputtering process.The film grains are refined to nanoscale and the film hardness increased rapidly.A variety of intermetallic compounds are formed with Zr content increasing in the film.The films appear amorphous structure because of mixing of these fine compounds and Al solid solution.The film hardness increase continuously with increasing content of compounds and reaches a high value of 9.8 GPa at 33.3 at.% Zr.2.The effects of main microstructural factors including grain size,grain interior composition,grain boundary composition,and grain boundary width on strengthening are isolated using two-demensional nanocrystalline multilayers.The results indicate that:(1)From the perspective of grain size,similar to the pure nanocrystalline Al,the inverse Hall-Petch phenomenon also exists in nanocrystalline alloys.The critical grain sizes of deviation from Hall-Petch relationship and inverse Hall-Petch phenomenon occurrence are about 40 nm and 10 nm,respectively.These mechanical behaviors of nanocrystalline alloys are similar to those reported in pure metals.(2)The results of intragranular solute content show that when the grain size reduces to 30 nm,the solid solution strengthening still exists in Al-Zr nanocrystalline alloy multilayered films.The strengthening effect matches with the hardness increment calculated through the solid solution strengthening model for coarse-grained materials.(3)The studies on the isolated grain boundary composition and width indicate that the hardness increment caused by grain boundary composition increase is related to the increase of grain boundary hardness.The hardness increment caused by grain boundary width is related to the increase of grain boundary volume fraction.Both these increments are not remarkable and can be calculated by rule of mixture(ROM)through the volume fraction of grain areas and grain boundary areas.3.The variation of each factor with the Zr content is obtained through the characterization of 3D alloy films.Based on the characterization results of microstructural factors in 3D alloy films and the strengthening rules in 2D multilayered films,the variation trend and proportion of a specific single factor with Zr content in nanocrytalline alloys are obtained:(1)Fine grain strengthening is the top contributor on hardness increment of films.The proportion of fine grain strengthening reaches almost 80% of the total hardness increment of films when the Zr content is lower than 1.9 at.%.The proportion decreases with the Zr content increase and is reduced to about 50 % at 11.1 at.% Zr.(2)The hardness increment caused by solid solution strengthening presents a slight decrease with the increase of Zr content.However,the proportion of solid solution strengthening increases because of the decrease of that of fine grain strengthening.Its proportion increases from 10 % at 1.9 at.% Zr to 35 % at 11.1 at.% Zr.(3)The hardness increment caused by grain boundary composition and width increase almost presents linear relationship to Zr content.The changes of their proportions are not obvious,increasing from 10 % at 1.9 at.% Zr to 15% at 11.1 at.% Zr.4.The variation trend of calculated hardness increment of 3D alloy films is coincident with that of measured hardness.They both increase rapidly when the Zr content is lower than 1.9 at.%.In the range of 1.9-8.3 at.% Zr,both of them gradually increase at a lower increase rate.Thereafter,when the Zr content increases to 8.3 at.%,they both present decrease trend.