熱機處理對六元富鈦中熵合金之微結構與機械性質影響之研究;The effect of thermo-mechanical treatment on the microstructure and mechanical properties of hexamerous titanium-rich medium-entropy alloy

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Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/90214

Title:	熱機處理對六元富鈦中熵合金之微結構與機械性質影響之研究;The effect of thermo-mechanical treatment on the microstructure and mechanical properties of hexamerous titanium-rich medium-entropy alloy
Authors:	蔡翰林;Tsai, Han-Lin
Contributors:	機械工程學系
Keywords:	中熵合金;輕量化;熱機處理;介金屬化合物;析出強化;Medium-entropy alloys;lightweight;thermomechanical treatment;intermetallic compounds;precipitation strengthening
Date:	2022-09-23
Issue Date:	2022-10-04 12:16:53 (UTC+8)
Publisher:	國立中央大學
Abstract:	中熵合金因其具有優異的材料性質以及靈活的設計彈性使其在合金開發上具有重要的影響力。本次實驗基於先前研究開發之Ti65(AlCrNbV)32Ni3中熵合金並採用不同的熱機處理製程：(一) 熱軋50 %後再冷軋70 %、(二) 熱軋50 %後再冷軋80 %以及(三)冷軋85 %等三種加工方式，再搭配不同時間900℃的熱處理，使材料內部的微結構發生改變。藉由分析其拉伸機械性質與微結構的變化探討合金的強化機制以及不同熱機處理參數對富鈦中熵合金之影響。根據XRD結果顯示，合金經熱機處理後內部有析出物的生成，分析繞射峰比對為Ti2Ni與TiNi之介金屬化合物。在拉伸測試方面，一開始隨著退火熱處理時間增加合金降伏強度急遽下降，並在退火到一定時間後合金機械性質開始呈現穩定。其中，經熱軋後的材料其降伏強度可維持在1200 MPa，而延性則在20%左右；而無經過熱軋後的材料其降伏強度則維持在1100 MPa，而延性則可達25%以上。另外可以發現隨著冷加工量的提高，較高的加工量使合金再結晶溫度降低，亦使熱處理時機械性質穩定的時間越早發生。造成合金機械性質穩定的原因推斷為析出硬化與晶界強化兩種機制的拮抗，當熱處理時間增加，再結晶晶粒成長使得晶界密度降低進而造成強度下降，但熱處理時間的增加亦會使析出物成長提升合金的強度，兩種強化機制的作用使得合金在特定時間內可維持一定機械性質。而當熱處理時間超過臨界值進入過時效階段，析出物無法有效強化合金，使得合金機械性質開始下降。本次研究中，Ti65(AlCrNbV)32Ni3經熱軋50%冷軋70%後再900℃熱處理30分鐘其降伏強度與延展性可達1203MPa及20%。對比過去快速退火之研究數據(降伏強度1250MPa、延展性16%)，其強度略低50MPa左右但延展性大幅提升了約25%。;Medium-entropy alloys (MEAs) possess huge influence in alloy exploration due to their outstanding material properties and alloy design flexibility. This study is based on the previously Ti65(AlCrNbV)32Ni3 MEAs and conduct three different thermomechanical treatment processing: (1) hot rolling 50% and then cold rolling 70%, (2) hot rolling 50% and then cold rolling 80%, and (3) cold rolling 85%, respectively. And then heat treatment at 900 ℃ was used with different times to change the internal microstructure of the MEAs. The strengthening mechanism and the effect of the thermomechanical treatments on the Ti-rich MEAs were discussed by analyzing the results of the microstructure and tensile mechanical properties of the MEAs. According to the XRD results, the alloy has formed precipitates after thermomechanical treatment. The precipitates were identified as Ti2Ni and TiNi intermetallic compounds. In tensile testing, the yielding strength reduce significantly with the increase of annealing time at the beginning, and the mechanical properties of the MEAs began to stabilize after annealing to a certain time. Among them, the yield strength of MEAs with the hot rolling can be maintained at 1200 MPa, while the ductility is around 20%. The yield strength without hot rolling is maintained at 1100 MPa, and the ductility can reach more than 25%. The reason for the stable mechanical properties is inferred to be the confrontation between precipitation hardening and grain boundary strengthening. When the heat treatment time increases, the growth of recrystallized grains reduces the grain boundary and thus reduce the strength, but the increase of heat treatment time will also improve precipitate. The growth of the precipitiates increases the strength of the alloy, and the effect of the two strengthening mechanisms enables the MEAs to maintain certain mechanical properties. Finally, when the heat treatment time exceeds to critical, the precipitates cannot effectively strengthen the MEAs, and the mechanical properties of the MEAs begin to decline. In addition, with the increasing of cold working, the higher working amount will reduce the recrystallization temperature making the mechanical property stabilization earlier. In this study, Ti65(AlCrNbV)32Ni3 with hot rolling 50% then cold rolling 80% and then subjected to 30 min with 900℃ annealing, the yield strength and ductility of Ti65(AlCrNbV)32Ni3 can reach to 1203 MPa and 20%, respectively. Compared with the previous study of rapid annealing (the yield strength of 1250 MPa, the ductility of 16%), despite the strength is slightly reduce about 50 MPa, the ductility is greatly improved more than 25%.
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