時效對冷加工及再結晶退火CoCrFeMnNi及Al0.2CoCrFeNi高熵合金之研究

Abstract

本研究針對兩種高熵合金(HEA) CoCrFeMnNi及Al0.2CoCrFeNi，經80%冷滾軋及其後之低溫時效及再結晶退火等對其機械性質及顯微結構的影響進行探討。XRD結果顯示兩種HEAs在滾軋完以及其後的再結晶退火後皆為FCC單相結構。針對CoCrFeMnNi HEA，在80%冷滾軋後直接在450°C/500°C/700°C 進行低溫時效，並測得其硬度隨時效時間變化之曲線。為得知有析出物之後再經短時間退火對機械性質變化的影響，針對冷滾軋後於500°C經析出一段時間後，再於700°C/900°C退火並隨退火時間量測其硬度變化。此外，本研究也比較了未有析出物以及經500°C不同時間的析出後再於900°C/700°C短時間退火之微硬度變化，結果顯示析出強化的效果在900°C短時間退火後便消失，但在700°C者硬度僅略微下降。之後又將700°C短時間退火的試片繼續進行500°C的長時間時效並測量其硬度變化，發現在預先析出過的試片中硬度會較早達到飽和值。在Al0.2CoCrFeNi HEA的部分，在80%冷滾軋後於550°C進行時效，發現其硬度隨時效時間而變化並有兩個硬度最大之峰值。為了解此硬度變化之現象，本研究選擇在550°C下能得到兩個硬度峰值的試片進行TEM的觀察。發現此兩個硬度峰值與在550°C短時間時效析出的GP zones有關，本研究並與同樣熱處理之Al0.2CoCrFeNi HEA小角度X光散射(SAXS)分析研究做比對，推測硬度變化為兩階段析出之GP zones所導致。另外，拉伸試驗的結果也顯示在兩個硬度峰之熱處理條件下具有較大之降伏強度，不過延展性隨著時效時間的增加而下降。而於550°C長時間時效所產生的析出物也由SEM/TEM觀測並做其成分之分析。此外，本研究也針對冷滾軋後先進行再結晶退火之試片進行550°C時效下的硬度變化，發現與冷滾軋再析出者不同，是由於析出物及析出機制不同所導致。而本合金在80%冷滾軋後再於不同在結晶溫度退火後之Hall-Petch關係式及晶粒成長動力學也同時被探討。

In this study, the aging/annealing effects on different thermomechanically treated CoCrFeMnNi and Al0.2CoCrFeNi high-entropy alloys (HEAs) are investigated via Vickers microhardess tests and OM/SEM/TEM observations. XRD results indicate the 80% cold-rolled with and without recrystallization CoCrFeMnNi and Al0.2CoCrFeNi HEAs are single-phase FCC solid solution. For CoCrFeMnNi HEA, the hardness evolutions of 80% cold-rolled and then 450°C/500°C/700°C aged specimens are revealed. In order to find out the aging/annealing effect on precipitated specimens, the hardness evolution of the specimens with and without precipitation short-time annealed at 700°C/900°C and long-time post-aged at 500°C is investigated. Experimental results indicate a rapid softening for 900°C annealing, while a moderate decrease in hardness is found for 700°C annealing. The 700°C short-time annealed and then post-aged at 500°C specimens with 500°C pre-aging reach the hardness maximum faster than those without 500°C pre-aging. For Al0.2CoCrFeNi HEA, the hardness evolution of 80% cold-rolled and then 550°C aged specimens is examined, and two hardness maxima are found and further investigated by TEM observations. Different sizes of GP zones are observed and are suggested to be related to the occurrence of two hardness maxima. The tensile tests of the 80% cold-rolled and then 550°C aged specimens also demonstrate two yield stress maxima, but the ductility decreases as the aging time increases. The hardness evolution of 80% cold-rolled and recrystallized and then aged at 550°C Al0.2CoCrFeNi specimen is also studied. The Hall-Petch relationship and grain growth kinetics of 80% cold-rolled and recrystallized Al0.2CoCrFeNi HEA are also revealed.