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Oxidation Behavior and Thermal Stability of Fe- and Cu-Based Bulk Metallic Glasses
|Authors: ||Hsin-Hsin Hsieh|
|Contributors: ||NTOU:Institute of Materials Engineering|
Oxidation Behavior;Thermal Stability;Bulk Metallic Glasses
|Issue Date: ||2011-06-30T07:23:05Z
|Abstract: ||論文探討兩種鐵基(Fe72B22Y6與Fe48Cr15C15Mo14B6Er2)與三種銅基非晶合金(Cu60Hf25Ti15，Cu60Zr30Ti10及Cu60Zr20Hf10Ti10，均為at. %)之氧化行為與熱穩定研究。研究結果顯示，兩種鐵基非晶合金之氧化動力學皆遵守拋物線型定律，唯三元Fe72B22Y6非晶合金在700°C呈現兩段式之氧化動力學行為；根據XRD分析可知，氧化生成物主要為B2O3及少量Fe3O4與FeO，其中，生成B2O3是降低非晶合金氧化速率較相同成份的結晶合金慢的主因。而六元Fe48Cr15C15Mo14B6Er2非晶合金在溫度低於650°C時，其氧化動力學遵守兩段式之拋物線型定律，但當溫度高於675°C時，則轉變成直線定律；由XRD分析可知，鐵基六元非晶合金在650°C以下時，主要生成Cr2O3與少量氧化鐵，當溫度高於675°C時，氧化物主要為氧化鐵以及少量Cr2O3與MoO2，因此，由結果推知，當溫度高於675°C時，生成Fe3O4與MoO2會破壞Cr2O3層的緻密性，進而使合金的氧化速率增快。 此外，三種銅基非晶合金之氧化動力學皆遵守單一式拋物線型定律，且氧化速率隨溫度的上升而加快。由XRD分析得知，Cu60Hf25Ti15非晶合金在低於450°C氧化時，氧化生成物主要為Cu4O3與少量CuO 和HfO2，而當溫度高於475oC時，則主要為CuO 和少量HfO2 與Cu2TiO3；對Cu60Zr30Ti10非晶氧化而言，主要生成CuO/Cu2O與少量cubic-ZrO2 和 ZrTiO4，而Cu60Zr20Hf10Ti10非晶氧化後則主要生成CuO 與少量的cubic-ZrO2與Zr5Ti7O24；其中，相較於純銅氧化生成Cu2O/CuO可知，生成ZrTiO4 與Zr5Ti7O24兩相是造成非晶合金氧化速率加快的主因。 Cu60Hf25Ti15非晶合金之結晶行為主要分為三階段，根據計算得知第一與第二放熱峰之Avrami exponent (n)分別為4與4.5，而第三放熱峰由於放熱反應未於測定溫度內完成，故無法有效評估其結晶行為。根據n值所對應之成長機制可知，第一放熱峰符合固定的成核速率 (constant nucleation rate)特性，而第二放熱峰之結晶行為則為增快的成核速率(increasing nucleation rate)特性。根據XRD與TEM分析結果指出，第一階段之相變化是由非晶結構轉變為部分結晶的Cu3Ti與Hf(Ti)兩相，而第二放熱峰為Cu3Ti的成長與CuHf2的成核，第三結晶峰則可視為Cu3Ti和CuHf2相變為Cu51Hf14及Cu3Ti2。|
The purpose of this dissertation is to study the oxidation behavior and thermal stability of Fe- and Cu-based bulk metallic glasses (BMGs), containing two Fe-based BMGs; Fe72B22Y6 (Fe3) and Fe48Cr15C15Mo14B6Er2 (Fe6), and three Cu-based BMGs; Cu60Hf25Ti15 (CHT), Cu60Zr30Ti10 (CZT) and Cu60Zr20Hf10Ti10 (CZHT) (all in at. %). In general, the oxidation kinetics of the Fe3 BMG followed the parabolic rate law although the two-stage kinetics was noted at 700°C. The scales formed on the Fe3 BMG consisted mainly of boron oxide (B2O3) and minor amounts of iron oxides (Fe3O4/FeO). The formation of B2O3 is responsible for the reduced oxidation rates. The oxidation kinetics of the Fe6 BMG generally followed a two-stage parabolic rate law at T < 650°C, while the single-stage linear rate was observed at higher temperatures (T > 675°C). Based on XRD and energy dispersion spectroscopy (EDS) analyses, a continuous, thin layer of chromium oxide (Cr2O3) dissolved with some iron was formed at T < 650°C, while typical hump iron-oxides intermixed with minor amounts of Cr2O3 and MoO2 were observed at T > 675°C. In addition, a substrate phase transformation from the amorphous structure to Fe-Cr and FeCrMo crystalline phases was detected after the oxidation. Very likely, the formation of the non-protective Fe3O4 and MoO2 breaks the scale integrity and allows the rapid cation/anion transportation, resulting in the fast linear-kinetics behavior of the Fe6 BMG at T > 675°C. In addition, oxidation kinetics of the three Cu-based BMGs also followed the parabolic-rate law with their oxidation rate-constants increased with increasing the temperature. The scale formed on the CHT BMG consisted of mainly Cu4O3 and small amount of CuO and HfO2 at T < 450°C, while mostly CuO and minor HfO2 and Cu2TiO3 were detected at higher temperatures (T > 475°C). The scales formed on the CZT and CZHT BMGs were strongly composition-dependent, consisting of mostly CuO/Cu2O and minor amounts of cubic-ZrO2 and ZrTiO4 for the ternary CZT BMG, and of mainly CuO and minor amounts of cubic-ZrO2 and Zr5Ti7O24 for the quaternary CZHT BMG. It is most likely that the formation of ternary oxides (ZrTiO4 and Zr5Ti7O24) was responsible for the fast oxidation rates for BMGs, as compared to those of pure Cu. Besides, the thermal stability and crystallization behavior of the CHT BMG were further studied by means of continuous heating from 300°C to 700°C. DSC analyses showed that the crystallization process mainly contain three stages. The Avrami exponent (n) for the first peak was 4.0, which indicated a constant nucleation rate during the exothermic reaction, while a value of n = 4.5 obtained for the second peak indicated an increasing nucleation rate during the crystallization. Yet, the n value can not be estimated due to incomplete DSC data present at the maximum operating temperature. XRD and TEM analyses further indicated that the first exothermic peak corresponded to the phase transformation from the fully amorphous to partial crystalline state by forming Cu3Ti and Hf (dissolved with Ti). The second peak represented the grain growth of Cu3Ti and the second-stage nucleation of CuHf2; however, the third stage illustrated the phase transformation from Cu3Ti and CuHf2 to more stable Cu51Hf14 and Cu3Ti2 phases.
|Appears in Collections:||[材料工程研究所] 博碩士論文|
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