Regulating Precipitate Orientation In Mg-al Alloys By Coupling Twinning And Aging Processes And Its Mechanism

As the lightest metallic structural materials,magnesium(Mg)alloys have a significant application potential due to the strong demands for weight reduction in automotive vehicle and aircraft industries.However,Mg alloys usually show relatively low strength,hindering their industrial applications.Precipitation hardening is one of the most efficient ways to improve strength.The precipitation hardening responses of Mg alloys heavily depend on the morphology and orientation of precipitates.It is commonly accepted that prismatic precipitates in Mg alloys are more effective for precipitation strengthening than the ones formed on the basal planes.Hence,understanding the morphology and orientation relationship(OR)of precipitates and promoting prismatic precipitation is essential for improving the aging hardening responses of Mg alloys.In addition,twin boundary precipitates play a critical role in controlling microstructure and mechanical properties of Mg alloys.Therefore,study of the precipitation behavior of twin boundary precipitates is important to develop the high strength Mg alloys.Mg-Al alloys are most widely used age-hardenable Mg alloys.In this thesis,the hot rolled AZ80 plate was choosen as the starting material.Using transmission electron microscopy(TEM),the crystallography of Mg₁₇Al₁₂ precipitates in Mg-Al alloys was investigated.A novel strategy for regulating precipitate orientation in Mg alloys was proposed and validated through experimental observation.Additional,the microstructure and precipitation crystallography of twin boundary precipitates were systematacially investigated by TEM.The influence of twin boundaries on precipitation was discussed in detail.In direct aged AZ80 alloys,most continuous precipitates are basal plate precipitates that obey the Burgers OR with the Mg matrix.In addition,a few non-basal precipitates were also detected in local regions.Beside the Crawley OR,four new ORs of non-basal precipitates were observed in the Mg matrix.Using the edge-to-edge matching model,these ORs can be explained well.The orientation of basal precipitates in AZ80 alloy is altered by coupling twinning,aging and detwinning processes.This gives the prismatic precipitates with their broad plane parallel to one of the{10(?)0}planes.Tensile and compression tests confirm that such prismatic plates are more effective for dispersion strengthening than basal plates and can significantly enhance the strength of AZ80 alloy.Meanwhile,a crystallography-based algorithm is proposed to predict ORs between the Mg matrix and the prismatic precipitates that result from the TAD process and the predictions are subsequently validated through experimental observation in TEM.Three new ORs between Mg₁₇Al₁₂ precipitates and the Mg matrix are experimentally determined,which agree well with the predictions of the proposed algorithm.After annealing at 250 ^oC,the precipitates are still parallel to{10(?)0}prismatic planes.Moreover,the ORs between the precipitates and the Mg matrix are remained during annealing.Additional,the proposed algorithm was utilized to predict the ORs between the prismatic precipitates and the Mg matrix in other Mg alloys produced via the TAD process.A large number of Mg₁₇Al₁₂ precipitates were formed on{10(?)2}and{10(?)1}twin boundaries in pre-deformed and annealed Mg-Al alloys.The precipitates formed at{10(?)2}twin boundaries have two ORs,i.e.the Burgers OR and Pitsch-Schrader OR(P-S OR).Statistical analysis shows that both ORs were equally observed at the twin boundaries.According to the morphologies of precipitates,the P-S OR precipitates are divided into two types:one is Type I P-S OR precipitates,which obey the P-S OR only with the matrix or twin;another is Type II P-S OR precipitates that have the P-S OR with both the matrix and twin.In the case of the Burgers OR or Type I P-S OR,the habit planes of the precipitates are{011}planes on the coherent side,while they have{112}or other high-index planes on another side.The precipitates exhibit asymmetric morphology associated with the twin boundary.In the case of Type II P-S OR,the precipitates have{011}habit planes on both sides of twin boundary and grow symmetrically into both the matrix and twin.In addition,the variants selection occurred in precipition on twin boundary.For both the Burgers OR and P-S OR,the precipitate variants that have[11(?)]_?or[100]_?parallel to the twin common<2(?)(?)0>axis were formed preferentially.The effect of twin boundary on crystallography of precipitation has been clarified via combined analysis of elastic strain energy and interfacial engery.On the one hand,the elastic strain energy of the P-S OR precipitates formed at twin boundary is lower than that of the Burgers OR preicpitates,resulting in a large number of P-S OR precipitates on{10(?)2}twin boundaries.On the other hand,the Burgers OR preicpitates have lowest elastic strain energy and interfacial engery on the coherent side.Thus,the precipitates with the Burgers OR still formed largely at twin boundaries.The variants selection is also controlled by the elastic strain energy and interfacial engery.It is shown that the preferred variants have the lowest elastic strain energy and interface energy.A statistical result shows that all Mg₁₇Al₁₂ precipitates on{10(?)1}twin boundaries have Potter OR and have a solo variant.The precipitates hold the identical Potter OR with both the matrix and twin and grow symmetrically into both grains.This enables the precipitates to form low energy interfaces with both grains and lowers the activation energy required to form a critical nucleus.