Isothermal Crystallization Kinetics And Behavior Of NiTiZrAlCuSi Bulk Metallic Glass

In this dissertation, the Bulk Metallic Glass(BMGs) rod of 3mm in diameter with the nominal component Ni₄₂Ti₂0Zr_21.5Al₈Cu₅Si_3.5 was prepared by copper mould casting methods. The isothermal crystallization kinetics of this alloy was investigated by differential scanning calorimetry (DSC), the isothermal crystallization behavior was studied by means of X-ray diffraction (XRD) and transmission electron microscopy (TEM).The isothermal DSC data shows that the relationship between incubation timeÏ„, crystallization peak time tp, the ending crystallization time te and annealing time Ta can be fitted by a two order exponential function.The z(x) function was used to verify the applicability of the JMA model; the results revealed that JMA equation can be used to describe the isothermal crystallization process of Ni₄₂Ti₂0Zr_21.5Al₈Cu₅Si_3.5 metallic glass. The kinetics of isothermal crystallization was analyzed by means of JMA model. Average value of the Avrami exponent shows that the kinetics process of two annealing temperature regions, i.e., 818⁸28K and 833⁸58K, was governed by different mechanisms. The local Avrami exponent demonstrated that the first-stage of crystallization was a one-dimensional, interface-controlled process. During the middle stage of crystallization, the local Avrami exponent at low annealing temperature region have a value greater than 2.5 means that an increasing nucleation rate and growth process. The local Avrami exponents at high annealing temperature region close to 4.0 means that a constant nucleation and growth rate process. At the finally stage, Avrami exponent at low temperature region close to 3.0 means a site saturation nucleation and growth process. Avrami exponent at high temperature region lose to 2.0 means an interface-controlled nucleation and growth process. The apparent activation energy is 492.37kJ/mol at low temperature region and 473.74kJ/mol at high temperature region, which implies that this alloy is more stable than the other alloys. At high temperature region, the local activation energy of initial stage has a higher value, then local activation energy Ec(x) become stable at main stage and decreases rapidly at the finally stage. With annealing temperature region increasing, Ec(x) decreases gradually.The crystallization product was investigated by XRD, the crystallization phase of the alloy is Ti(Cu,Al)₂ (Hexagonal) and Ni(TiZr) (Monoclinic) after annealing at 858K for 20 min. For sample annealing 60 min, the crystallization phase of the alloy is (TiZr) (Cu,Al)₂ and Ni(TiZr). The crystallization product annealing at supercooled liquid region was also analyzed. Main crystallization phase after annealing at 828K for 4.5 to 6.5 min is NiTi (Monoclinic) and Ti(Cu,Al)₂ (Hexagonal). The crystallization product after annealing at 828K for 8.5 min is identified to be Ni(TiZr) and Ti(Cu,Al)₂. The statistical average crystal size D presents a three order polynomial relationship with isothermal annealing time, which demonstrates that the isothermal process annealing before 8.5 min is mainly a nucleation stage. However, the process annealing after 8.5min is dominated by growth of the nuclei.The quenched sphere-like crystal in as-cast alloy was also studied. The crystal was indexed as NiTi₂ (Face-centered Cubic). During annealing at 828K for 4.5min, the crystal decomposes and forms a Hexagonal Al₂Zr phase. Some sphere-like crystals still exist in matrix after annealing for 6.5min, it disappears after annealing for 8.5min and Al₂Zr phase grows rapidly.