以分子動力模擬探討高熵合金的機械性質與變形機制

Abstract

高熵合金為Yeh於1995年提出的合金系統設計理念。因為擁有優異的機械性質，高熵合金一直是學者們不斷探討的議題。然而，高熵合金微觀結構之變形機制尚未被完全釐清。因此，本研究之目的為透過原子尺度模擬，探討高熵合金材料之元素組成與微觀結構對機械性質與變形機制的影響。本研究以分子動力模擬建立高熵合金的原子模型，高熵合金材料以鈷、鉻、鐵、錳、鎳組成的Cantor Alloy系統為主。研究內容為調配鈷、鉻、鐵、錳、鎳的比例0% ~ 30%，以蒙地卡羅方法隨機均勻分布原子，搭配修正原子鑲嵌勢能（Modified Embedded-Atom Method, MEAM），設計原子尺度模擬的拉伸試驗，釐清元素組成、微觀結構等重要控制變因，如何影響Cantor Alloy系統之機械性質及變形機制。本研究亦找出調配鈷、鉻、鐵、錳、鎳的比例0% ~ 30%，擁有最佳機械性質與變形機制的元素組成。本研究發現改變鈷、鎳、鉻的比例，強度與楊氏係數隨著比例增加而上升，改變錳、鐵的比例，強度與楊氏係數隨著比例減少而下降。本研究分析個別元素之應力分布認為：Cantor Alloy系統之強度與楊氏係數和金屬元素本身的強度與楊氏係數之性質有關。本研究主要分析三種變形機制，包括：滑移誘導塑性變形機制（Slip-Induced Plasticity, SLIP）、孿晶誘導塑性變形機制（Twinning-Induced Plasticity, TWIP）、相變誘導塑性變形機制（Transformation-Induced Plasticity, TRIP），探討三種變形機制與高熵合金延展性之關聯。本研究模擬結果，大部分的差排、疊差（Stacking Fault, SF）、孿晶界（Twin Boundary, TB）都發生在面心立方堆積主要滑移(111)面上，且晶體結構產生越多的差排和SF，擁有越好的延展性，模擬結果符合文獻。本研究模擬結果改變錳的比例30%為TRIP變形機制，以及改變鎳的比例0% ~ 20%，隨著改變鎳的比例下降，微觀結構的六方最密堆積之穩定性增加，六方最密堆積之比例上升，模擬之趨勢皆與文獻之趨勢吻合。本研究模擬結果等莫耳CoCrFeMnNi合金並非擁有最佳強度、楊氏係數、延展性等機械性質的比例，模擬結果符合文獻。本研究發現改變錳、鐵的比例，延展性隨著比例增加而上升。延展性與變形機制有關，SLIP + TWIP + TRIP產生之延展性大於50%，為最好延展性之變形機制。本研究分析並分類三種變形機制之延展性SLIP + TWIP + TRIP、SLIP + TRIP、SLIP + TWIP，歸納出產生最好延展性之變形機制依序為SLIP + TWIP + TRIP > SLIP + TRIP > SLIP + TWIP。

High-entropy alloys were defined by Yeh in 1995. Due to their excellent mechanical properties, high-entropy alloys have gained much attention from academia and industry. However, deformation mechanisms of high-entropy alloys have not been thoroughly clarified. Therefore, the objective of this thesis is to investigate the effect of individual element composition and microstructure on mechanical properties and deformation mechanisms using molecular dynamics simulation. In this study, we established atomistic models using molecular dynamics simulation. The material of high-entropy alloys, composed of cobalt, chromium, iron, manganese, and nickel, is known as Cantor Alloy. We adjusted the proportion of cobalt, chromium, iron, manganese, and nickel in the Cantor Alloy system by 0% to 30% and randomized the atoms evenly with the Monte Carlo method. We used the modified embedded atom method (MEAM) potential energy function for molecular dynamics simulation and simulated high-entropy alloys under uniaxial tensile loading. We clarified the important factors, such as individual element composition and microstructure, and analyzed how these important factors affect the mechanical properties and deformation mechanisms of the Cantor Alloy system. We also found the optimum element composition of Cantor Alloy system for mechanical properties and deformation mechanisms. In this study, we found that change of the proportion of cobalt, nickel, and chromium from 0% to 30%, the strength and Young’s Modulus increase with the proportion and change of the proportion of manganese and iron from 0% to 30%, the strength and Young’s Modulus decrease with the proportion. We analyzed the stress distribution of individual elements and we considered that strength and Young’s Modulus of the Cantor Alloy system are related to the strength and Young’s Modulus of the metal elements. In this study, we mainly analyzed three deformation mechanisms, such as slip-induced plasticity (SLIP), twinning-induced plasticity (TWIP), and transformation-induced plasticity (TRIP). We investigated the relationship between three deformation mechanisms and elongation. The simulation results show that most of dislocations, stacking faults (SF), and twin boundaries (TB) occur on the (111) surface of the face centered cubic crystal structure. Simulation results show that more dislocations and SFs enhance the ductility. In addition, we changed the proportion of manganese by 30% and the deformation mechanism is TRIP. We changed the proportion of nickel from 0% to 20%. As the proportion of nickel decreases, the stability of the hexagonal closest packing of the microstructure increases, and the hexagonal closest packing occurs more. The trend of simulations is consistent with the trend of experimental measurements. We also found that equimolar composition is not the optimum composition of Cantor Alloy system for mechanical properties and deformation mechanisms. In this study, we found that change of the proportion of manganese and iron from 0% to 30%, the ductility increases with the proportion. Ductility is related to the deformation mechanisms. Elongation produced by SLIP + TWIP + TRIP deformation mechanism is greater than 50%, which is the best elongation. We analyzed and classified the elongations of SLIP + TWIP + TRIP, SLIP + TRIP, and SLIP + TWIP deformation mechanisms. We found that elongations produced by SLIP + TWIP + TRIP deformation mechanism are greater than those produced by SLIP + TRIP deformation mechanism. Elongations produced by SLIP + TRIP deformation mechanism are greater than those produced by SLIP + TWIP deformation mechanism.