CrCoNiSi0.15中熵合金於低溫軋延及冷軋延後之退火誘導異常硬化現象與顯微結構研究

Abstract

一些金屬材料，於中、低退火溫度時會發生異常硬化現象(Annealing-induced abnormal hardening)，對材料機械性質具一定影響。然而，此現象之成因目前並無統一說法，因此具高研究價值。 本研究透過改變不同軋延參數，分析及研究CrCoNiSi0.15中熵合金之退火異常硬化現象。將CrCoNiSi0.15中熵合金於低溫(-196 ℃)及室溫下，分別進行50%及70%軋延，並於300 ℃～900 ℃做退火1小時之熱處理，觀察退火溫度對組織的影響。硬度及電子背向散射繞射(EBSD)結果顯示，低溫軋延70%因高密度缺陷導入，具最高硬度，並促使再結晶較早發生與完成，有助於晶粒細化。除外，其於300 ℃～600 ℃之回復階段，發生最顯著的硬化現象，硬度最高峰567.6 ± 19.7 HV落於600 ℃，高於軋延試片的15%。因而，進一步以低溫軋延70%，於600 ℃做5分鐘～1小時之短時間至長時間退火熱處理，結果顯示各持溫時間硬度皆上升。 以穿透式電子顯微鏡(TEM)觀察軋延及退火組織。於軋延組織中，僅低溫軋延70%同時啟動雙晶誘導塑性(TWIP)及相變誘導塑性(TRIP)機制，顯示低溫有助於提升材料強度。而於低溫軋延70%，600 ℃各時間退火過程中，因細晶組織之可動差排源下降及能量釋放，差排經雪崩堆積及分裂機制，形成各形貌的次結構及細晶粒，導致硬度上升。而次結構內部因缺陷重組，被疊差、雙晶、9R結構及HCP相分割為奈米級層狀結構，亦為硬度上升的原因。進一步比較冷軋延及低溫軋延70%，發生硬化之組織，冷軋延次結構內的層狀結構，含大量9R結構，低溫軋延則含大量HCP相，顯示軋延溫度對退火機制具影響力，進而造成不同硬化程度。此外，推測分裂出之細晶粒內部的缺陷，可發展為奈米退火雙晶，並由連續機制形成再結晶晶粒。同時，硬度會隨再結晶開始而下降，9R結構亦可與退火雙晶共存於新晶粒內。

Some metallic materials exhibit “Annealing-induced abnormal hardening” at low or medium annealing temperatures, which affects the mechanical properties. However, the exact mechanisms for this phenomenon remain inconclusive, making it a topic of high research value. This study investigated the annealing-induced abnormal hardening effect with different rolling parameters. CrCoNiSi0.15 medium-entropy alloys were cryo rolled (-196 ℃) and cold rolled to achieve 50% or 70% reduction and annealed for 1 hour at temperatures varying from 300 ℃ to 900 ℃. Then, the effects of annealing temperature on microstructure were observed. Hardness and Electron Backscattered Diffraction (EBSD) analysis showed that 70% cryo-rolled sample displayed the highest hardness due to high-density defects, which promoted earlier recrystallization, resulting in grain refinement. Furthermore, the most significant hardening effect occurred during recovery stage from 300 °C to 600 °C, with the highest hardness of 567.6 ± 19.7 HV observed at 600 ℃, which was 15% higher than 70% cryo-rolled sample. Thus, further annealed 70% cryo-rolled sample at 600 ℃ for durations ranging from 5 minutes to 1 hour. The results revealed that hardness increased for all holding times. Transmission Electron Microscope (TEM) was used to observe rolled and annealed structures. In rolled microstructure, only cryo rolling 70% activated both Twinning-induced Plasticity (TWIP) and Transformation-induced Plasticity (TRIP) effects, demonstrating that low temperature is beneficial for strengthening materials. During annealing at 600 °C for various durations, annealing-induced hardening is related to the reduction of mobile dislocation source and grain boundary relaxation in ultrafine-grained structure. Substructures and fine grains formed through dislocation avalanche and splitting mechanisms, thus enhancing hardness. Additionally, defects rearrangement within substructures also facilitated the formation of hierarchical nanostructure consisting of stacking faults, twins, 9R structure, and HCP phase, contributing to hardening. Comparison of 70% cold-rolled and cryo-rolled samples revealed that hierarchical nanostructure in cold-rolled sample contained a large amount of 9R structure, while those in cryo-rolled sample contained a large amount of HCP phase, highlighting the influence of rolling temperatures on annealing mechanisms and degrees of hardening. Furthermore, defects remained in fine grains may evolve into nano-annealing twins, and fine grains could further develop into recrystallized grains through continuous recrystallization mechanism. Meanwhile, hardness decreased as recrystallization began, and 9R structures could coexist with annealing twins in the new grains.