Control On Microstructure Of Mg-Sn-based Alloys And Research On The Precipitates Crystallography In Several Mg Alloys

Guided by crystallography of phase transformation, a novel Mg-Sn-based alloywith high Mn content, i.e., Mg-7.5Sn-2.2Mn (wt.%) alloy (TM72), has been designed.According to a comprehensive survey, two crystallographic criteria are proposed topredict precipitates that are likely to grow along [0001]_αof Mg matrix. β-Mn issupposed to be a potential candidate. The existence of rod-like β-Mn precipitates along[0001]_αis confirmed by transmission electron microscopy (TEM) investigation on theaged alloy.Another novel Mg-Sn-based alloy has been designed by micro-alloying Ag inMg-Sn-Zn-Mn alloy. The resulted alloy is Mg-7Sn-3Zn-1Ag-0.2Mn (TZQM7310). Theage-hardening effect of TZQM7310is much higher than that of TZM730. As toTZQM7310alloy aged at200C, Mg₅₁Zn₂₀forms at the early aging stage andtransforms to MgZn₂at the aging peak. In the overaging period, metastable Mg₅₄Ag₁₇phase is identified. The element Ag changes the precipitation sequence of the alloy andresults a more uniform dispersion density of the precipitates.Crystallography of the precipitates in aged TM72, TZM730, TZQM7310andMg-6Zn-5Al (ZA65) alloys has been quantitatively studied by TEM. Fourteen neworientation relationships (ORs) of different precipitates in Mg alloy have been identified,including7,1,1,2and3ORs for precipitates of Mg2Sn, MgZn₂, β-Mn, Mg₅₄Ag₁₇andÏ„-Mg₃₂(Al, Zn)₄₉, respectively. The ORs of the latter three precipitates are firstinvestigated. Approach based on analysis of g vectors and singularity of interfacialstructures has been employed to interpret interfacial facets of the precipitatescorresponding to9of the14new ORs. The interpretation of the facets of OR10andOR11type Mg2Sn precipitates lead to general solutions of OR and lattice parameters ofcubic phase under the condition of their major facets normal to g//g//g. Moreover, amore precise method has been developed to measure the angular deviation in the OR,and it was used to identify near OR1type Mg2Sn precipitates.A new concept of secondary constrained coincidence site lattice (secondary CCSL)and the associated model have been proposed. Its applications provided consistentinterpretation of the morphology and the ORs of the Type Ï„ precipitates and the OR5 type Mg2Sn precipitates.