Study On The Rmodynamics And Mechanical Behaviors Of Ti-Cu-Ni-Zr Alloys

Bulk metallic glasses (BMGs) are expected to be the engineering material intwenty-first Century, which must meet the design requirements of green engineeringmaterials. In addition to have excellent mechanical properties with low cost, BMGsshould be environment friendly and recyclable. Since the first discovery of metallicglasses, most of them cannot meet the design requirements as "green" materials.Therefore, it is a big challenge for the design and development of "green" bulkamorphous alloys. It is therefore very important to study the glass forming ability(GFA) and mechanical properties of new class of BMGs.Among all reported BMGs, Ti-based or Ti-Cu-based BMGs have beenattracting an increasing attention owing to their small density and high strength.However, the Ti-based BMGs with the high GFA generally contain toxic elements(Be) or noble metal elements (Pd), which are not able to meet the designrequirement of "green" materials. In this paper, the Ti-Cu-Ni-Zr and Hf addedTi-Cu-Ni-Zr alloy compositions are designed by using the "proportional mixing ofbinary eutectics" method. Based on the relationship between the GFA and Gibbsfree energy difference (ΔG) of the undercooled melt and crystal, the specific heat ofthe crystal and supercooled liquid region was measured by DSC (DifferentialScanning Calorimeter) and DTA (Differential Thermal Analyzer). The ΔG valuesfor two classes of BMGs were calculated, and the relationship between the GFA andΔG was also analyzed.Based on the Dubey’s expression of ΔCp, the hyperbolic equation of ΔCpwasestablished; in addition, the expressions of ΔG, ΔH, ΔS were also established. Byusing the experimental data of the Ti-Cu-Ni-Zr and Hf added Ti-Cu-Ni-Zr BMGs,the fitting degree was analyzed between the ΔCp, ΔG, ΔS and ΔH and theexperimental values. Moreover, by using the experimental data of other amorphousalloys, the maximum error formulas of hyperbolic and Dubey’ expressions of ΔCpwere provided, and the applied condition of Dubey’s expression of ΔCpwas alsodiscussed.Based on the hole theory of the liquid state, the hyperbolic equation of ΔCpwas established, which suggests the expression has the base of theory. Fordemonstrating the fitting degree between the caculated values by the hyperbolicequation of ΔCpand expermental values of ΔCp, the ΔG, ΔH and ΔS values werecalculated by using the experimental data of fifteen BMGs. By using theexperimental data of the Ti35.37Cu45.11Ni8.88Zr10.64and Ti37.65Cu43.25Ni9.6Zr9.5, the relationship between the ΔG values calculated by by using the hyperbolic equationof ΔCpand GFA was discussed. Moreover, the relationship beweenn the fittingdegree of the caculated ΔCpvalues and the heat vibration of disorder atoms was alsodiscussed in amorphous alloys.The Ti-Cu-Ni-Zr(Hf) BMGs were compressed at room temperature, and theyield strength, fracture strength, and plastic strain were obtained. By observed thefracture morphology of Ti-Cu-Ni-Zr samples, the phenomenon of melt flow wasfound in the area of stress concentration. The increasing temperature of shear bandwas calculated, when the fracture or the serrated flow of samples occurs. Therelationship between the releasing elastic energy and the melting amorphous phaseswas revealed. In addition, the morphology of fracture was observed by PSEM(Phenom Scanning Electron Microscopy) for the Hf added Ti-Cu-Ni-Zr samples,andthe height of "bumps" was calculated at the boundary of vein pattern. Therelationship between the plasticity and "bumps" was analyzed. Based on theserrated flow in the stress-strain curve, a dynamic model of formation for a singleshear band was established and the formula of multiple shear bands for singlesample was improved. In addition, the relationship between the serrated flow for asingle shear band and multiple shear bands was also discussed, which explained themechanical behavior of Ti-Cu-Ni-Zr and Hf added Ti-Cu-Ni-Zr BMGs compressedat room temperature.For the application of engineering materials at low temperatures, theTi-Cu-Ni-Zr BMGs were compressed at low temperature. The yield strength,fracture strength and plastic strain were obtained at low temperature. Through thecontinuous different loading rates, the stresses were obtained under the differentstrain rates. The relationship between the stress overshoots and the change of strainrate was discovered; the Newton and non-Newton viscous behaviors were found atlow temperature. In addition, the morphology of fracture was observed by SEM(Scanning Electron Microscopy) for the sample compressed at low temperature, andthe forming cause of "fishbone" pattern was analyzed on the fracture.