探討鈦和鉬含量及後退火處理對CoCrNi中熵合金薄膜微結構與機械性質的影響

Abstract

本論文研究了鈦、鉬含量以及後退火處理對CoCrNi中熵合金薄膜顯微組織和機械性能的影響。研究分為兩個主要部分：第一部分是探討CoCrNi中熵合金薄膜中不同鉬和鈦含量的影響，而第二部分是研究不同的退火溫度對非晶態Co29.72Cr31.19Ni26.39Mo8.59Ti4.11薄膜的影響。 在研究的第一部分中，使用磁控三靶共濺鍍將摻雜不同含量鉬和鈦的CoCrNi中熵合金薄膜沉積在p型矽基板上。施加到CoCrNi靶材的功率固定為200 W，而 鉬和鈦靶材的相同功率範圍控制從0到90W。隨著鉬和鈦的進一步添加，MoTi70獲得了5.87 GPa的最大降伏強度和11.96 GPa的最高硬度。MoTi80表現出奈米晶和非晶的混合結構，而MoTi90薄膜表現出完全非晶結構。機械性質的提升主要是由於固溶強化、析出強化和晶粒細化強化。 第二部分的重點是對沉積在氧化鋁基板上的Co29.72Cr31.19Ni26.39Mo8.59Ti4.11中熵合金薄膜進行後退火處理。這些薄膜在400至750°C的溫度範圍內進行退火，以分析其微觀結構和機械性能。X光繞射分析證實，隨著退火溫度的升高，結晶度會增加，而TEM分析則證實了析出物的形成。使用奈米壓痕和微柱壓縮測試量測其硬度、降伏強度和斷裂強度。機械性質在650℃退火溫度時達到最大值，硬度為10.79 GPa，降伏強度為6.35 GPa。硬度的增加歸因於奈米晶的形成和生長，以及 HCP Ni3Ti和Cr-Mo σ相的析出硬化。此外，在550、650和750°C退火的薄膜其斷裂應變超過40%，展現了良好的延展性。均勻的塑性流動機制克服了強度-延展性權衡，因為材料可以承受更高的應力並且塑性變形不會出現局部失效。 本研究的結果表明，在CoCrNi基中熵合金薄膜中控制鉬和鈦的含量，並對非晶Co29.72Cr31.19Ni26.39Mo8.59Ti4.11薄膜進行適當的退火處理，可顯著提高機械性能。

This thesis investigates the impact of titanium and molybdenum contents and post-annealing treatment on the microstructure and mechanical properties of CoCrNi medium entropy alloy films (MEAFs). The study is divided into two main parts: the effect of varying Mo and Ti contents in CoCrNi MEAFs, and the subsequent annealing-induced transformations in amorphous Co29.72Cr31.19Ni26.39Mo8.59Ti4.11 films on crystallinity. In the first part of the study, CoCrNi MEAFs doped with different amounts of Mo and Ti were deposited on p-type silicon substrates using magnetron three-target co-sputtering. The power applied to the CoCrNi target was fixed at 200 W, while the Mo and Ti targets were subjected to the same power ranging from 0 to 90 W. For simplicity, the film was denoted as MoTi30 when Mo and Ti targets were subjected to 30 W power, and so on. The structural transformation from FCC to FCC + HCP was observed at MoTi50 due to the precipitation of HCP Ni3Ti. With the further additions of Mo and Ti, the maximum yield strength of 5.87 GPa and hardness of 11.96 GPa were obtained at MoTi70. The films exhibited a mixed nanocrystalline and amorphous structure at MoTi80 and a completely amorphous structure at MoTi90. The improvement in mechanical properties was primarily due to solid solution, precipitation, and grain refinement strengthenings. The second part of the study focused on the post-annealing treatment of amorphous Co29.72Cr31.19Ni26.39Mo8.59Ti4.11 MEAFs deposited on aluminum oxide substrates. The films were annealed at temperatures ranging from 400 to 750°C to examine the effects on their microstructures and mechanical properties. X-ray diffraction analysis confirmed an increase in crystallinity with higher annealing temperatures, while TEM analysis verified the formation of precipitates. The hardness, yield strength, and fracture strength were measured using nanoindentation and micropillar compression tests. The mechanical properties peaked at an annealing temperature of 650°C, with a hardness of 10.79 GPa and a yield strength of 6.35 GPa. The enhancements in hardness were attributed to the nanocrystalline formation and growth, and precipitation hardening from phases such as HCP Ni3Ti and Cr-Mo σ phases. Additionally, the fracture strain for films annealed at 550, 650, and 750°C exceeded 40%, indicating good ductility. The homogeneous plastic flow mechanism overcame the strength-ductility trade-off, as the material can sustain higher stresses and deform plastically without localized failure. This research demonstrates that the controlled addition of Mo and Ti contents to CoCrNi-based MEAFs and applying appropriate annealing treatments to amorphous Co29.72Cr31.19Ni26.39Mo8.59Ti4.11 films, can significantly improve the mechanical properties.