Thermal Stability And Crystallization Kinetics Of Zr-based Bulk Amorphous Alloys

Amorphous alloys are metastable on thermodynamics, which will crystallize as be heated. Their original structures and properties will disappear due to crystallization, the disappearance will influence their engineering application. So the study of their thermal stability and crystallization kinetics wil be of great importance. In this thesis, the thermal stability and crystallization kinetics of bulk amorphous alloys Zr60Al15Ni25, Zr65Al10Ni10Cu15 and Zr52.5Al10Ni10Cu15Be12.5 have been systema- tically investigated by X-ray diffraction method (XRD) and differential scanning calorimetry( DSC). The main experimental results and conclusions are listed as follows:(1) Plate-like Zr65Al10Ni10Cu15 and Zr52.5Al10Ni10Cu15Be12.5 bulk amorphous alloys have been prepared by suction casting into a copper mold. The effect of Be addition on thermal stability and crystallization behavior of Zr65Al10Ni10Cu15 bulk amorphous alloy has been investigated by XRD combined with isothermal DSC. XRD results shows that it will crystallize at 683K, with the formation of tetragonal phase Zr2Cu, hexagonal Zr6NiAl2, face-centred cubic Zr2Ni, tetragonal Zr5Al3 and orthorhombic Ni10Zr7. Zr52.5Al10Ni10Cu15Be12.5 bulk metllic glass crystallizes at 748K and leads to the formation of hexagonal ZrBe2, orthorhombic ZrNi phase, tetragonal phase Zr2Al3 and orthorhombic ZrCu, the delay of crystallization of the latter can explain that the addition of Be will enhance the stability of Zr65Al10Ni10Cu15 bulk amorphous alloy. By isothermal DSC analysis, we found that Be addition to the master alloy can increase the incubation period, and the crystallization enthalpy after Be addtion is larger than its master alloy, which indicates that it will release more energy in the crystallization course of the new alloy. By using JMA equation, Avrami exponent n can be got with the value of 1.575~1.953 and 1.712~1.898 for the two alloys, it can be found that they crystallize in the same crystallization mechanisms, namely, controled by diffusion and nucleation rate decreases with time. The activation energy are 431.7kJ/mol for Zr52.5Al10Ni10Cu15Be12.5 bulk amorphous alloy, 23% higher than the master alloy, which also indicates the Be addition to Zr65Al10Ni10Cu15 can improve its thermal stability.(2) Bulk amorphous alloy Zr60Al15Ni25 has been produced by suction casting into a copper mold. Its thermal stability and crystallization behavior have been investigated by XRD and DSC. Phase transformation activation energy Eg, Ex and Ep calculated by Kissiger method are 539.9kJ/mol, 213.1kJ/mol and 261.2kJ/mol, and it shows that Zr60Al15Ni25 bulk amorphous alloy possesses of high thermal stability. In-situ XRD analysis indicates that the difficulty to form ternary phases Zr6NiAl2 and Zr5Ni4Al is the main reason for its high thermal stability. By calculating its degree of crystallinity at different temperatures, we have discovered that it would tend to smaller values at low temperatures, but it would rise rapidly with temperature; crystallization efficiency would decline at higher temperatures because of the less available amorphous phase.(3) The bulk and ribbon samples of Zr65Al10Ni10Cu15 amorphous alloys have been prepared by suction-casting in a copper mold and single roller melt-spinning technique. Their thermal stability has been investigated by DSC. The results indicate that the bulk amorphous alloy has lower Tg and Tx, but a wider supercooled liquid regionΔTx. Using Kissinger and Doyle plots, phase transformation activation energy can be evaluated. It showed that amphous ribbon has larger glass transition and crystallization activation energy, which explains the ribbon possesses higher thermal stability in comparison to the bulk sample.