退火後鐵基非晶質薄帶之結構性質分析

Abstract

非晶質金屬由於非晶原子的無序排列，沒有晶體的異相性和晶界，使得磁壁移動與磁化較容易，因而被認定具有優異的軟磁特性，多用在輸配電變壓器之蕊心，是工業上常見的磁性材料。其中，鐵基非晶質薄帶擁有很高的飽和磁通密度、低磁矯頑力和低鐵損值，對節能方面具有重大的經濟效益，因此使用量日益增加。實際投入使用的非晶質薄帶產品會再經由適當的退火熱處理調整性質，但在磁性質提升的同時薄帶出現了轉脆現象，嚴重限制了後續的應用。因此，瞭解退火處理對於非晶質基地結構之影響，以及與之呼應的性質變化，是極其重要的研究課題。在此實驗中針對了兩種不同Si/B 比例的合金薄帶(L-Si alloy (Fe81Si2B16.6C0.4)及 H-Si alloy (Fe80Si8.5B11C0.5))，進行了一系列的退火處理並探討其性質與結構之演變，希能對此一極具發展潛能的材料有更深入的瞭解。本研究可以分為兩個部份，其一為觀察分析退火後的自非晶基地內出現的結晶反應，其二為觀察退火後之性質演變，並根據結構上的變化做出解釋，包含了退火脆化現象及電磁性質的改善等。 經由熱分析及電子顯微鏡觀察，我們得以確認在本研究中所使用的鐵基非晶質薄帶在退火後主要會出現兩種結晶產物，樹狀晶肥粒鐵(α-Fe)及鐵硼結晶相(FexB)。由實驗結果觀察得知，硼含量對於鐵硼結晶相有促進之影響，會導致兩種結晶反應同時發生而抑制樹狀晶肥粒鐵的大小，而高Si/B 比例會有較巨大的樹狀晶肥粒鐵結構，這是因為樹狀晶肥粒鐵的成長與硼的擴散息息相關，硼在肥粒鐵的固溶度極低，結晶過程中會將硼排入非晶基地內。經由穿透式電子顯微鏡繞射分析及背向散射電子繞射分析，在此鐵基非晶薄帶內樹狀晶的成長方向得以確認為〈111〉，與一般認定立方晶系金屬的〈001〉方向不同。此外，薄帶製造過程中對於薄帶的上下表面的殘留應力不一亦對樹狀晶肥粒鐵造成了成長方向差異，殘留應力會誘使樹狀晶肥粒鐵優先朝此表面方向成長，而有較高的結晶度及特定的平行排列分布，但此現象會受到鐵硼相干擾而消弭。 非晶材料的退火脆化問題是實際產業應用的一大限制，本研究利用一簡單之彎曲測試量測退火轉脆溫度，並發現此非晶薄帶呈現兩階段式的轉脆過程。低溫退火即出現脆化現象且具有極為特別的破裂面形貌，是受到退火後非晶基地的結構重整影響，短程有序結構的變化及原子間自由體積的縮減造成了非晶狀態的脆化。高溫退火帶來的結晶組織則嚴重地破壞了薄帶的韌性，雖然晶界帶來的強化效果對於硬度有著顯著的提高，但非晶基地與結晶之不均性造成了薄帶完全脆裂。 低溫退火脆性的破裂形貌與裂紋在非晶基地內的前進機制有直接的關聯，其奈米尺度的局部剪切變形行為與裂紋前進速度的變化在破裂表面留下了大小不一的孔洞及條紋組織。 經過適當的低溫退火處理，此鐵基非晶薄帶之軟磁性質得以有效提升，其依舊是非晶基地內的結構重整現象所造成，由磁滯曲線量測配合電子能量損失能譜、電阻值與居禮溫度的變化分析確認原子間的共用電子鍵結在退火後會增加並提高飽和磁化強度。此外，利用勞倫茲電子顯微技術觀察薄帶之磁疇分布，亦確認了結晶組織在非晶基地內會抑制磁疇壁的移動而使矯頑磁力增高，軟磁特性因此消失。 總括而言，本研究以電子顯微鏡結構分析技術配合各項性質的量測，對於此鐵基非晶質薄帶的退火處理之影響有完整的了解，深入探討樹狀晶組織在非晶基地內之成長過程及非晶基地在原子尺度上之變化與轉脆現象之關聯，依據此一成果得以對此材料提出適當的退火條件來有效提高其性質及應用性。

This work mainly focuses on the investigation of microstructure and properties in annealed amorphous Fe-based amorphous ribbons. A valid structure-properties relationship after annealing treatment is discussed. The research can be divided into two sections: the crystallization behavior in this Fe-Si-B-C amorphous ribbon, and analyses of thermal embrittlement and enhanced magnetic properties after annealing treatment. Two different alloying compositions of amorphous ribbons were applied in this work, L-Si alloy (Fe81Si2B16.6C0.4) and H-Si alloy (Fe80Si8.5B11C0.5). Although crystalline in this Fe-Si-B-C amorphous ribbon is not the desirable structure for industry application, the characteristic crystallization behavior still attracts us many attentions, such as interaction between each crystalline phases and inhomogeneity in individual sides of ribbon. A series of study on crystallization process was carried out with DSC and X-ray diffraction. In this Fe-Si-B-C amorphous ribbon, dendritic crystal (α-Fe) is the main product, and FexB phases appear at higher annealing temperature. The ribbon produced by melt-spinning method is significantly influenced by the manufacturing process and thus has different surface morphology in its individual sides: shiny side for contacting air, and matt side for contacting wheel. Such a discrepancy in morphology also means that there exists different strain energy in individual sides which was confirmed by nano-indentation experiments. The growth direction of dendritic crystal is essential for this investigation and observed with TEM. From x-ray diffraction and electron backscatter diffraction (EBSD) analysis, it indicates that α-Fe crystallites on the shiny side are oriented little preferentially parallel to 〈111〉 direction, while those on the matte side are not. However, treatment at higher temperature drives this discrepancy to decrease. It is therefore to conclude that the strain causes the different preferred texture of α-Fe crystallites in respective sides and the new transformation product FexB at higher annealing temperature inhibits this tendency. A simple bending test was conducted to investigate the ductile-brittle transition behavior in annealed Fe-based amorphous ribbon. The result indicates that the ribbon has two-stage annealed embrittlement, closely related to the crystallization temperature. By assigning structural relaxation and changes in free volume after low-temperature annealing treatment for first-stage embrittlement, a quite special fracture surface morphology corresponding to the crack propagation process was observed by HR-SEM. The second-stage embrittlement is suggested to be caused by transformed dendritic crystalline and exhibits a rather cleavage fracture surface. The enhanced magnetic properties of Fe-based amorphous ribbon after low-temperature annealing treatment also closely relate to structural relaxation. The relationship between structure and changes in magnetic-electrical properties such as hysteresis curve, Curie temperature, electrical resistance, are discussed with the HR-TEM and EELS analysis. The 3d state occupancy of Fe-based amorphous ribbon after each annealed temperature was calculated and showed similar changes with the magnetic properties. This result indicates that heat treatment not only influences the free volume of amorphous matrix (atomic scale) but also causes the changes in electron scale. Further investigation about magnetic domain structure in annealed Fe-based amorphous ribbon was conducted with Lorentz microscopy in TEM. In conclusion, in this research, we used a systematic experimental process to analyze this annealed Fe-Si-B-C amorphous ribbon, and tried to establish the correlation of performed properties and microstructure. A suitable annealing condition for a compromise between embrittlement and enhanced magnetic properties was suggested by the above experimental analysis.