FeCoNiCrMn高熵合金低溫快速撞擊實驗之機械性質與顯微結構研究

Mechanical Property and Microstructure of Cryogenic Temperature and High-stain-rate Compressive Deformation in the FeCoNiCrMn High-entropy Alloy

陳品戎(Pin-Jung Chen)

指導教授 ： 楊哲人

國立臺灣大學/工學院/材料科學與工程學研究所/碩士(2019年)

https://doi.org/10.6342/NTU201901279

摘要

高熵合金作為金屬領域新興材料，近幾年來受到廣泛關注，又以FeCoNiCrMn(或被稱作Cantor alloy)作為最多人研究的系統之一。根據文獻報導，在低溫拉伸實驗中，具有低疊差能之面心立方單相結構FeCoNiCrMn高熵合金會生成大量機械雙晶，雙晶除了能有效阻擋差排移動亦可促使晶粒細化之效用，增加材料之加工硬化率、強度與延展性等機械性能。
本研究藉由霍普金森快速撞擊試驗機，在不同溫度(25℃、-50℃、-100℃、-150℃)和不同應變速率(4000 s-1、9000 s-1)對兩種晶粒尺寸(4.3 μm、45.9 μm)的FeCoNiCrMn高熵合金執行壓縮變形實驗，並透過背向散射電子繞射技術與穿透式電子顯微鏡探討機械性能與顯微結構變化之影響。
實驗結果之應力應變曲線指出，隨著實驗溫度降低或是應變速率提高，材料強度皆會上升，因為提升應變速率和降低溫度有著相同的效果，皆能促使機械雙晶提早形成、提高材料加工硬化率。機械雙晶臨界剪應力亦依循著Hall-Petch理論趨勢，會隨著晶粒縮小而增加，使大晶粒(45 μm)整體強度比小晶粒(4.3 μm)更高。主要透過TEM分析材料內微結構發展，依照變形理論以及TEM結果推論，在變形初期仍以差排為主，隨著差排堆積、糾結使應力快速上升，達到啟動雙晶所需的臨界應力值，單一變體雙晶開始在基地中成核。實驗結果顯示不論在哪個應變速率或溫度，都可以觀察到機械雙晶，且隨著溫度降低雙晶帶的寬度也會增加。除此之外，結構內也觀測到兩組雙晶並存的現象，可分為基地中的Primary twin和Secondary twin、退火雙晶內的機械雙晶以及機械雙晶內的機械雙晶，晚出現的機械雙晶會將原晶粒再次分割，達到晶粒細化的效果也使材料強度增強。本研究亦利用高解析穿透式電子顯微鏡與快速傅立葉轉換圖譜，分析奈米級機械雙晶交互作用情形。

關鍵字

FeCoNiCrMn高熵合金 ； 霍普金森快速撞擊 ； 高應變速率 ； 極低溫 ； 機械雙晶

並列摘要

High entropy alloys are a new type of material in metal field, which have caught a lot of attention in recent years, and FeCoNiCrMn (or named as Cantor alloys) are one of the most studied system. According to published paper, FeCoNiCrMn high entropy alloys with low stacking fault energy and FCC single phase structure generated numerous deformation twins during cryogenic tensile test. Besides blocked dislocation movement, deformation twins also had grain refinement effect, which provided a higher work hardening rate and better mechanical properties.
This experiment took two grain size (4.3μm、45.9μm) FeCoNiCrMn high entropy alloys as material and performed compression test at various temperature (25℃、-50℃、-100C、-150℃) and strain rate (4000 s-1、9000 s-1) by Split-Hopkinson pressure bar, and investigated mechanical properties and microstructure development in material by Electron Back-Scattered Diffraction and Transmission Electron Microscopy.
The results of stress strain curve indicated that stress increased with the decrease of temperature and the increase of the strain rate, because they have same effect in promoting deformation twin development and increasing work hardening rate. The critical resolved shear stress of twinning also followed classic Hall-Petch theory and increased with the decrease of grain size, thus large grain (45 μm) had higher stress than small grain (4.3 μm). Based on deformation theory and experimental results from TEM, it could infer that dislocation slip was main deformation mechanism in the beginning, dislocation accumulation and tangle raised stress quickly and reached critical resolved shear stress of twinning, after then single variant twinning started nucleation in matrix. The results indicated that no matter which temperature or strain rate, deformation twin always appeared, and the thickness of twin band increased with the decrease of temperature. Additionally, there were two variant deformation twins in material which could be classify into three types, first was primary and secondary in matrix, second was deformation twin inside the annealing twin, third was deformation twin inside the deformation twin, the latter one divided the original grain into smaller size which achieved the effect of grain refinement and material strengthening. Finally, this work tried to analyze intersection position between deformation twins through high resolution TEM and fast Fourier transformation.

並列關鍵字

FeCoNiCrMn high entropy alloys ； Split Hopkinson Pressure Bar ； High strain rate ； cryogenic temperature ； Deformation twin

參考文獻

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