分子動力學於單元素至高熵合金結構與性質之模擬

Molecular dynamics (MD) simulation is a powerful tool in materials research. It has been widely used to provide an atomic description of the crystallization and glass forming processes during rapid solidification of alloys. Quantum mechanics based many-body Tight-Binding (TB) potential model has been utilized in our MD simulation to simulate structural and thermal properties of several metals and alloy systems. Structure evolution was simulated for alloys consisting of two to eight equal-molar elements Ni, Al, Cu, Co, Ti, V, Zn, Zr as being molten, rapidly-solidified (at 2 x 1013 K/s), and annealed, respectively. We found that as the number of elements n < 4 the melt-quenched alloys tend to form amorphous structure; however when n is five and more, the alloys show a liquid-like solidified structure. We propose to estimate the glass-forming-ability (GFA) criterion of alloys by simulation of reduced glass transition temperature (Trg). Trg of CuxZr100-x (x= 46, 50, 62) correspond well to experimental values in literature. We calculated GFA of TixCo100-x (x= 60 ~ 84) alloys and found potential glass-forming compositions, which were experimentally verified to be bulk metallic glasses. We succeeded for the first time in literature the prediction of BMG compositions before experimental trial-and-errors. We tried to bring closer the mechanical melting to thermodynamic melting by incorporating into the TB-potential an extra lattice vibration energy in MD calculations. Calculated melting point of single elements was 0.5 to 9.8 % varied from the experimental ones instead of 20 to 30 % in conventional simulation. This method works well for elements with a low ratio between Debye temperature and melting temperature.

關鍵字

分子動力學；高熵合金；非晶質合金

並列摘要

分子動力學模擬對材料研究而言為一有力的工具，且廣泛地應用於快速凝固合金之結晶化及玻璃形成程序上之研究。本研究利用以量子力學為基礎所簡化之緊束勢能模型來描述原子之運動行為，進行一連串之分子動力學相關之研究。本研究包含三大主題，首先，以二元至八元等莫耳比例之鎳、鋁、銅、鈷、鈦、釩、鋅、鋯合金為題，探討這些合金經過熔融、固化(以 2 x 1013 K/s 之冷卻速度冷卻)與退火等程序，其結構及性質之變化。發現其結構變化與其元素多寡有絕對關係。當元素量為四元或更少時，合金固化後傾向成為非晶質結構；當合金組成元素為五元及以上時，固化之合金傾向形成類液體結構。第二主題，以CuxZr100-x (x= 46, 50, 62) 及 TixCo100-x (x= 60, 70, 77 and 84)之合金系統，計算其約化玻璃轉化溫度(Trg= Tg/Tm，Tg為玻璃轉化溫度、Tm為熔點)，以探討其非晶形成能力。結果發現Cu-Zr系之Trg 計算值與文獻報導值吻合。計算亦得到Ti-Co系中具有高Trg之成分，並據以實驗驗證相吻合。本研究因而首先披露以MD模擬，定量預測高玻璃形成能力合金成分的方法。於開發新式塊狀非晶時，這將有助於以模擬方法先預測具有潛力的合金成份，可降低實驗嘗試錯誤的次數。最後，為了縮短熔點計算值與實驗值之巨大差異，我們利用了調整勢能參數之方式，將無因次晶格振動能加進傳統緊束勢能參數中，計算單元素之熔點。結果發現可將原先計算之熔點（實為機械式熔融）誤差20～30％降為0.5～9.8％。我們成功地發展出使機械熔點逼近熱力學熔點的MD法，並發現特別適用於Debye溫度與熔點比值小於0.3之單元素。

並列關鍵字

Molecular Dynamics ； High Entropy Alloys ； Amorphous Alloys

參考文獻

[2] W. Klement, R.H. Wilens, P. Duwez, “Non-crystalline Structure in Solidified Gold–Silicon Alloys”, Nature 187 (1960) 869.

[3] W.L. Johnson, “Thermodynamic and kinetic aspects of the crystal to glass transformation in metallic materials”, Prog. Mater. Sci. 30 (1995) 81.

[4] A.L. Greer, “Metallic Glasses”, Science 267 (1995) 1947.

[5] H.A. Daveis, Amorphous Metallic alloys (Butterworth-Heinemann, 1993).

[7] H.W. Kui, A.L. Greer, and D. Turbull, “Formation of bulk metallic glass by fluxing”, Appl. Phys. Lett. 45 (1984) 615.

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分子動力學於單元素至高熵合金結構與性質之模擬

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