The Effect Of Technics On The Glass Forming Ability And Mechanical Properties Of Cu-based Bulk Metallic Glasses

In recent years,Cu-based metallic glasses especially Cu-Zr-Al system have attracted widely attention since its high strength,high hardness and excellent mechanical properties.Ho--wever,there are also many questions and problems both in the theoretical and applying field of this kind alloys,for example,how to prepare the super-size BMG and reduce the requireme--nts in preparing processes;how to improve their plastic.In this paper,we use magnetic levitation melting-copper mold suction casting equipments series of Cu₄₅Zr₄₂Al₈Ag₅ bulk metallic glasses were prepared by melt overheating treatment and overcooling treatment.And the thermal behaviors,structure and mechanical properties of Cu-based metallic glasses were studied,found some Cu-based amorphous alloys with larger critical size,better mechanical properties and application potentials.The results obtained in this thesis may enrich the theories in Cu-based metallic glasses's fields and promote the application of this system in industry effectively.Cu₄₅Zr₄₂Al₈Ag₅ bulk metallic glasses with 3mm in diameter were prepared by melt overheating treatment,the glass forming ability and thermal stability were studied.The results showed that melt overheating treatment improved the thermal stability of the amorphous alloy and the glass forming ability,three specimens were fully amorphous alloy,there was no crystal precipitation.With the improving of overheating temperature,the thermal stability of Cu-based amorphous alloys and glass forming ability have increased,when the melt superheating is 147K,the amorphous alloys have the best thermal stability(â–³T_χ=75K),at the same time,its glass forming ability is also the largest,its parametersγis 0.4242.The cylindrical samples of Cu-based were prepared by overcooling method,and the glass forming ability and thermal stability were studied.The results showed that,the majority of amorphous alloys were in-situ composites with crystalline precipitates materials.Cu-based metallic glasses showed the better glass forming ability,and the critical size were 12.86-13.76mm which can achieve the centimeter level,the width of supercooled regionâ–³T_χwas in the range of 70-75K,T_rg was in between 0.621-0.627,parameterγwas in the range of 0.4231 -0.4242,the critical cooling rate R_c were reduced obviously,compared to the thermod--ynamic parameters we found that the sample 7 # has the best thermal stability and glass forming ability.The sample's structure was examined by optical microscope,after overheating treatment, The structure of samples was made of amorphous matrix for the distribution of a number of white precipitates,while the overcooling processing of the specimen show three crystal forms of crystal phase which were dentritic crystalspheroidal crystal and equiaxed grains. Measurement results by micro-hardness showed that,the sample's micro-hardness by two preparation methods showed that the hardness decreased gradually from the center to the edge.After superheating treatment the highest value of microhardness was 867.5HV,and after overcooling treatment the highest value of microhardness was 1016.5HV.Compressive property and fracture morphology of Cu₄₅Zr₄₂Al₈Ag₅ amorphous bar with 3 mm in diameter was examined in the different overheating process,the addition of overheating process increased compressive strength of Cu₄₅Zr₄₂Al₈Ag₅ amorphous alloys.With the overhe--ating process increasing,the relative compression ratio increased gradually,the fracture forms were typical brittleness fracture,fracture sufface showed that vein-like pattern.The samples have certain plasticity by overcooling treatment,compressive strength were reduced with the holding time extending,suction casting temperature of 1090K for 2min insulation of the specimen has the greatest compressive strength 2266MPa,the specimen with 4min holding time has the greatest relative compression ratio of 11.7%,the specimen with 4min holding time has best fracture plastic strain of 1.46%.