New Mg-Based Bulk Metallic Glasses: Casting Preparation And Mechanical Performance Research

Due to the high glass forming ability （GFA） and excellent mechanical properties,especially the low density and high specific strength features, Mg-based bulk metallicglasses （BMGs） are considered as new light weight structural materials with greatapplication prospect that have captured public’s growing attention and concern. In thepresent thesis, new Mg-based BMGs of Mg-Li-Cu-Ni-Y, Mg-Cu-Ti-Y and Mg-Zn-Cu-Gdwith2mm diameter have been successfully prepared by copper mold injection-castingmethod. The phase structures, thermodynamic properties, mechanical properties andfracture surface morphologies of the as-cast samples are systematically investigated byX-ray diffractometry （XRD） and differential scanning calorimetry （DSC）, mechanicaltesting machine and scanning electron microscopy （SEM）, respectively. The primary resultsare listed as follows:（1） The XRD results show that the new as-cast Mg65LixCu20-xNi5Y10（x=0,2,4）,Mg60Cu₃₀-xTi_xY₁₀（x=3,5,8） and Mg₆₅-xZnxCu₂₅Gd₁₀（x=0,3,5） alloy samples possesscompletely amorphous structures, while the as-cast Mg₆₅Li₆Cu₁₄Ni₅Y₁₀, Mg60Cu20Ti10Y10and Mg₅₇Zn₈Cu₂₅Gd₁₀alloy samples consist of both amorphous structures and crystallinestructures. Further analysis indicates that the crystalline structures are mainly composed ofMg2Cu phase. Therefore, it is evident that the key factor in obtaining fully amorphousstructure of the above new Mg-based BMGs is to inhibit the forming of the Mg2Cu phaseduring the fast cooling process of the alloy melts.（2） The DSC measurements indicate that the new as-cast Mg-Li-Cu-Ni-Y, Mg-Cu-Ti-Yand Mg-Zn-Cu-Gd BMGs have good GFA and high thermal stability. However, with theincreasing of the element Li, Ti and Zn content, their GFA and thermal stability graduallyreduced. Thereinto, the increasing content of Li and Ti will decrease both the Tgvalue andTxvalue of the Mg-Li-Cu-Ni-Y and Mg-Cu-Ti-Y BMGs respectively, since the Txvalue hasa greater reduction than that of Tgvalue resulting in the decreasing of their ΔTxvalue.Therefore, it implies that the GFA of both the Mg-Li-Cu-Ni-Y and Mg-Cu-Ti-Y BMGsdecreased correspondingly. This may be associated with the addition of alloying elements Liand Ti having positive heat of mixing with their respective constituent elements （Li-Mg:0kJ/mol; Li-Y:+8kJ/mol; Li-Ni:+1kJ/mol; Ti-Mg:+16kJ/mol; Ti-Y:+15kJ/mol）.With increasing of Zn content in the Mg-Zn-Cu-Gd BMGs, the Tgvalue increases and Txvaluedecreases leading to a decrease of the ΔTxvalue, hence, their GFA decreases.（3） The mechanical perfomance researches demonstrate that the new as-castMg-Li-Cu-Ni-Y, Mg-Cu-Ti-Y and Mg-Zn-Cu-Gd BMGs samples have high compressivefracture strength （σf）, big elastic strain （εe）, high Vickers hardness （Hv） and low elasticmodulus （E）. Thereinto, the σf, εe, Hv and E values of the as-cast above BMGs aredetermined to be729MPa,2.28%,253and36GPa for Mg₆₅Li₂Cu₁₈Ni₅Y₁₀sample;879MPa,3.04%,347and34GPa for Mg₆₀Cu₂₇Ti₃Y₁₀sample and754MPa,3.07%,286and32GPa for Mg₆₀Zn₅Cu₂₅Gd₁₀sample. By contrast, it is easy to see that the element Ti cannotably improve the mechanical properties of Mg-based BMGs.（4） Through the observation of the fracture surface morphologies of the compressivesamples, the fracture behavior of most of the new as-cast Mg-Li-Cu-Ni-Y, Mg-Cu-Ti-Y andMg-Zn-Cu-Gd BMGs samples is proved to be typical brittle fracture manner, which isdominated by a shear fracture mode. Thus, it goes without saying that improving theplasticity remains one of the emphases on the research of Mg-based BMGs.