BCC結構中/高熵合金晶粒成長之研究

Abstract

本研究首先從BCC結構的八種含高熔點元素之中/高熵合金(RHEAs) HfMoNbTaTiZr、HfNbTaTiVZr、HfNbTaTiZr、HfNbTaTi、HfNbTaZr、HfNbTiZr、HfTaTiZr及NbTaTiZr透過常溫的軋延測試，發現除已報導具有優良常溫軋延性質的HfNbTaTiZr及HfNbTiZr RHEAs外，HfNbTaTi及HfNbTaZr RHEAs也具有優良的常溫軋延性質。接著探討HfNbTaTiZr、HfNbTaTi、HfNbTaZr及HfNbTiZr RHEAs，經常溫軋延80%與真空封管後，於900℃至1200℃再結晶退火不同時間後的顯微結構與機械性質。由XRD、金相圖、SEM的結果得知，HfNbTaTiZr、HfNbTaTi及HfNbTiZr RHEAs在常溫軋延與再結晶退火後皆為BCC單相結構。由晶粒成長理論分析HfNbTaTi及HfNbTiZr RHEAs再結晶退火後的行為，發現兩者於900℃至1200℃的晶粒成長指數n值分別為0.2669與0.3674，晶粒成長活化能Q值分別為385.9 kJ/mol與190.2 kJ/mol。實驗結果顯示Q值的大小與合金所含的元素、再結晶前的冷加工量、再結晶溫度的範圍與退火時間的長短等因素有關。HfNbTaZr RHEA經常溫軋延80%與真空封管後，於900℃至1000℃退火後會於BCC基地中析出富Ta-Nb之BCC結構析出物與少量富Hf-Zr之HCP結構析出物，於1100℃至1200℃退火則會於BCC基地中析出少量富Hf-Zr之HCP結構析出物。上述四種RHEAs的微硬度試驗結果發現，於80%常溫軋延後，隨著再結晶退火溫度的增加，其硬度值的軟化有逐漸降低的趨勢。此外，本研究也發現HfNbTiZr及HfNbTaTi RHEAs再結晶退火後分別因為晶粒尺寸過大及晶粒尺寸差異不明顯等因素，導致兩者無法由其硬度值來建立Hall–Petch關係式。

Eight BCC-structured refractory high entropy alloys (RHEAs) were tested their rollability at room temperature and found that HfNbTaTiZr, HfNbTaTi, HfNbTaZr and HfNbTiZr RHEAs can be cold-rolled to 80% thickness reduction. The microstructure and XRD tests of these four rollable RHEAs after 80% cold working and annealing were investigated and showed HfNbTaTiZr, HfNbTaTi and HfNbTiZr RHEAs were single-phase BCC solid solution in as-rolled and annealed conditions. The grain sizes of the specimens recrystallized at 900℃ to 1200℃ for various times were measured, and based on the theorem of grain growth, the obtained n value and the activation energy Q were 0.3674 and 190.2 kJ/mol for HfNbTiZr RHEA and 0.2669 and 385.9 kJ/mol for HfNbTaTi RHEA, respectively. Furthermore, the experimental results indicated that the Q values were affected by the alloying elements, the cold working degree, the recrystallization temperature range and the annealing time. The phase decomposition tested by XRD and SEM tests of the HfNbTaZr RHEA annealed at 900℃ to 1200℃ was found to form BCC Ta-Nb-rich and HCP Hf-Zr-rich precipitates(ppts) in the BCC matrix, but only HCP Hf-Zr-rich ppts were formed in 1100℃/1200℃ annealed specimens. The Vickers hardness tests revealed that the hardness softening rate of the cold-rolled specimens recrystallized at 900℃ to 1200℃ decreased as the annealing temperature increased. The hardness evolution of the recrystallized HfNbTiZr and HfNbTaTi RHEAs was also studied and found that the grain sizes were too large to investigate their Hall–Petch relationship.