The Study On Alloying And Semi-solid Processing Of Zr-based Bulk Metallic Glass

Bulk metallic glasses （BMG） with excellent mechanical and physical propertiesopen a new research field for structural and functional materials. However thecatastrophic brittle fracture at room temperature has restricted the extensiveapplication of BMG. Extensive effort have been devoted to the finding of new methodfor improving the ductility in BMG. In this paper, based on the main developmentissues in BMG, Zr-based BMG which have not precious metals and toxic elements Beare selected to study the influences of alloying and semi-solid processing on BMG, thefollows are the main conclusions:The effects of the preparation conditions on thermal stability and mechanicalproperties of Zr-based BMG were investigated. By changing the casting conditions,the effects of thermal rate treatment, casting holding time and casting temperature onthermal stability and mechanical properties of BMG were studied under the samecooling rate, thus the optimal preparation conditions were determined. The resultsshow that thermal rate treatment can increase GFA and thermal stability of the BMG,for Zr₅₅Al₁₀Ni₅Cu₃₀BMG, supercooled liquid region△Txand parameter γ increasefrom73K to89K, from0.413to0.417, respectively, as casting voltage increase from7kV to10kV. Under the same heating temperature and cooling rates, the transientnucleation rate and the crystallization trend of liquid melt increase with castingholding time increase. And the random arrangement of atoms change to the orderarrangement, the disordered close-packed structure, GFA and thermal stabilitydecrease. There is a critical value of casting holding time in BMG under the samecooling rate and casting temperature, beyond this critical value, the samples begin tocrystallize. GFA and compression fracture strength increase, compressive plasticitydecrease with casting temperature increase. There is an optimum casting temperatureto the specific composition of BMG. Compression fracture strength and compressiveplasticity increase under the optimum casting temperature. Mechanical properties tosome extents are controlled by casting temperature. The influence of cooling rate ofthe different component samples on GFA and mechanical properties were investigatedunder the same casting conditions. The results show that there are different influencesof different composition samples on GFA and mechanical properties under the samecooling rate, and large-diameter BMG, compared with the same chemical compositionof the small-diameter BMG show more excellent compression plasticity and compression fracture strength.Selected Zr₅₅Al₁₀Ni₅Cu₃₀BMG as the base alloy to study effects and mechanismof adding Fe, Mn, Nb elements on thermal stability and mechanical properties. Theresults show that addition element content of forming amorphous alloy has a certainlimit addition content. When addition content reaches the limit value, it is likely toreact between addition element and original group elements and form crystal phaseunder the same cooling conditions. Adding Fe makes the GFA of BMG increase atfirst and then decrease, thermal stability decrease. Fe addition content is1%, canimprove GFA, reduced glass transition temperature Trgis0.59, parameter γ is0.414.Excessive Fe addition content （4%Fe content） results in crystallization of amorphousmatrix, forming amorphous matrix composite. Fe addition content is2%, the samplehave an excellent mechanical properties, that is, BMG’s compression fracture strengthand plastic strain are1868MPa and2.62%, respectively. Mn addition content is4%,can form BMG which the critical dimension is Ф3mm, thermal stability and GFAdecrease, supercooled liquid region△Txis61K, reduced glass transition temperatureTrgis0.58, parameter γ is0.40, compression fracture strength is1817MPa, has noductility. Adding the appropriate amount of Nb element can improve thermal stabilityand GFA. When Nb addition content is8%, BMG has the best thermal stability andthe highest GFA, supercooled liquid region△Txreaches to86K, parameter γ reachesto0.416. Also Nb addition content is8%, the sample has an excellent mechanicalproperties, that is, BMG’s compression fracture strength and plastic strain are1877MPa and1.92%, respectively.By changing casting voltage（heating power）, thereby changing heatingtemperature, making master alloy temperature enter to solid-liquid two phase region,to study the effects of semi-solid temperature and holding time on precipitatedintermetallic phases appearance, distribution, volume fraction and the relationshipwith mechanical properties of BMG’s composite. Also the enhancement ductilitymechanism was discussed. It show GFA of （Zr0.55Al0.10Ni0.05Cu0.30）96Fe4BMGcomposites increase and precipitated intermetallic second phase volume fractiondecrease with semi-solid temperature increase. As-cast in-situ spherical intermetallicphase/BMG composite which has room temperature compress plasticity wassuccessfully prepared at casting voltage7kV. The BMG composites not only has highcompression fracture strength （reaches2203MPa）, but also has significant roomtemperature ductility （plastic strain reaches5.85%）. Semi-solid temperature has a great influence on BMG composite precipitated intermetallic second phasemorphology. At casting voltage7kV, BMG composite precipitated intermetallicsecond phase morphology is spherical, most of them are10μm-20μm diameter,volume fraction is45.3%, interface between the amorphous matrix, find no otherphases appear. TEM studies have shown that spherical intermetallic phase and theglass substrate has a better interface binding force, effectively solve the wettingproblem, low-energy interface has a strong and stable binding force between sphericalintermetallic phase and the glass substrate. After deformed, the sample surface showsa large number of waved multiple shear bands which pitch is less than1μm, is lessthan the spherical size of the intermetallic second phase. Under the appropriate alloycomposition, semi-solid temperature can be adjusted to obtain as-cast in-situ sphericalintermetallic phase/BMG composite, with the corresponding single-phase BMGcomparison, can effectively improve the plasticity of the material while maintaininghigh compression fracture strength. GFA of （Zr0.55Al0.10Ni0.05Cu0.30）94Fe6BMGcomposites decrease and precipitated intermetallic second phase volume increase withsemi-solid holding time increase. There is a closely relation between mechanicalproperties and intermetallic second phase volume fraction of(Zr_0.55Al_0.10Ni_0.05Cu_0.30)₉₄Fe₆BMG composites. Precipitated intermetallic secondphase volume increase, compression fracture strength and plastic strain decease withsemi-solid holding time increase.