Properties Optimization And Microstructure Investigation Of The Mg-Sn-Mn And Mg-Cu-Mn Based Alloys

Characterization and interpretation on the detailed microstructure of Mg alloys arestrategically important for us to understand the principles of phase transformations andthe relationship between mechanical properties and microstructure, which are essentialfor alloy design. Mg-Sn and Mg-Cu based alloys are two key alloy systems for theirgreat promising properties of heat-resistance and damping respectively. However, themechanical properties of these alloys are still lower. In the present work, micro-alloyingmethod has been employed to improve the properties of both Mg-Sn-Mn andMg-Cu-Mn alloys. Some basic scientific questions, e.g., the effect of the micro-alloyingelements on the precipitates microstructure of the age-hardening Mg-Sn-Mn alloy, thedefects configurations in the micro-alloyed Mg-Cu-Mn damping alloy, and thecrystallography of the important precipitates in the alloys have been investigateddetailedly and systematically. Analytical electron microscopy (AEM) is the mosteffective way for resolving these problems.Different micro-alloying elements or combined elements, i.e., Ag, Ag+Zn, Cu orCu+Al have been added to the Mg-Sn-Mn alloys to improve its ageing hardeningresponse. Two kinds of Mg-Sn alloys, Mg-1.5Sn-0.5Mn-0.5Ag-0.5Zn and Mg-1.3Sn-0.5Mn-0.8Cu-2.0Al (at.%) with significantly enhanced age-hardening response andheat-resistant properties have been developed. In all the modified alloys, the precipitatesdensity has been increased greatly and the precipitates size has also been refinedsignificantly at the peak-aged state. The stable heat-resistant property is attributed to thestabilized precipitates size by the micro-alloying elements during the over-aged period.Addition of Zn in the Mg-Sn-Mn-Zn-Ag alloy promotes the formation of’-Mg54Ag17(’) phase, which has a larger size along the c axis of α-Mg matrix and thus theycan hinder the dislocation slip more effectively. Cu or Cu+Al additions to theMg-Sn-Mn alloy lead to the formation of the Mg2Cu or Al0.93Cu1.07Mgintermetalics with high melting points, which are retained at the grain boundaryduring the solution-treatment, and thus they are beneficial for the creep-resistantproperty. In all the Mg-Sn alloys, the growth of the dominant Mg2Snprecipitates exhibit a clear anisotropic behavior. Segregation of Ag or Cu in the Mg2Sn precipitates provides us an important clue on their promotion role in thenucleation of these precipitates.Detailed characterization on the precipitation crystallography has beencarried out and several new crystallographic features of various precipitates basbeen observed. Nine new orientation relationship (OR) between precipitates andα-Mg has been observed including:1from Mg2Sn,1from α-Mn,5form’ and2from Mg25.04Ag7.96. Each of the reproducible interfaces of these precipitates isnormal to a g in reciprocal space.’ precipitates of OR1and OR2aretwin-related with a twin plane of {3-10}’. High resolution transmissionelectron microscopy (HRTEM) images on the periodic dislocationconfigurations in the interfaces and the periodic steps in the irrational facetshave been obtained for the dominant OR1Mg2Sn and OR1’ precipitates. Theconstrained coincidence site lattice (CCSL) models have been used to interpretthe precipitates morphology. Dislocation and step configurations calculated bythe secondary CCSL model and O-lattice model shows excellent consistencewith the experimental results.A proper addition of Zn and Y has improved the damping property of Mg-Cu-Mnalloy significantly. The dominant dislocations in the alloy are arranged in parallel rowswith regular spacing, indicating they are easy to move, which provides the experimentalevidence of the dislocation mechanism of internal friction. A certain number of twinsobserved in the alloy also provide evidence on the inference that the deviation of ‘G-L’plot from a straight line is resulted from twin. In addition to the14H and18Rlong-period stacking ordered structures (LPSO), clear diffraction pattern from the24Rstructure has been firstly observed.6H,7H and8H are suggested as the buildingblock of18R,14H and24R structures respectively. The LPSO structures alwayscontain stacking defects of different building blocks. A novel6H-8H modulatedperiodic structure has also been discovered.