| 摘要: | 因其優異的材料性能和靈活的設計條件，多主元素合金（Multi-principal Element Alloys, MPEAs）突破傳統的合金設計框架並成為當今材料發展的主流之一。而又因其混合熵值的不同，又可在進一步分成高熵合金（High-entropy alloys, HEAs）與中熵合金（Mediun-entropy alloys, MEAs）。而由於科技發展帶動之技術指標的提高與能源效率的追求，開發同時具高強度且優異延展性的輕量材料是目前結構材料學界的主要研究方向之一，而熱機處理為一普遍且有效提升合金機械性能之方法。本次研究將以本實驗室開發之數種改質後輕量富鈦中熵合金（Ti65Ni3 MEA、Ti65Zr7 MEA與Ti65Bx series MEAs）為基礎，透過熱機處理與搭配快速退火製程，以期開發出具有低密度（~ 5 g/cm3）、高強度（YS ≥ 1200 MPa）與高延展性（EL% ≥ 10%）之中熵合金。
在Ti65Ni3部分，該合金先進行三種不同的滾軋製程，分別為（1）冷軋70%（CR70）、（2）冷軋85%（CR85）與（3）熱軋50%然後冷軋70 %（HR50CR70）。接著再進行快速退火（升溫速率15℃/秒）熱處理至743℃、770℃、812℃和881℃。透過X光繞射分析（X-ray diffraction analysis, XRD）可以證明Ti65Ni3在經熱機處理後仍可維持單一BCC固溶相。而透過電子背向散射繞射（Electron Back Scatter Diffraction, EBSD）的觀察可以看到合金的再結晶率隨著退火溫度的升高而增加。而在拉伸測試方面，隨著退火溫度的升高，Ti65Ni3的降服強度降低，延展性亦隨之增加。比較三種滾軋製程可以看出，較高的冷加工量會使合金再結晶的行為提前；而熱軋製程則可細化合金晶粒，使合金在冷軋時能更為有效的積累應變能，進而也使得再結晶溫度降低。其中，經CR85和HR50CR70製程的Ti65Ni3合金再經快速退火至770℃後皆擁有優異的機械性質表現，其降伏強度分別可達1250和1232 MPa，且同時具有相似的延展性（EL% ~ 16%）。
在Ti65Zr7部分，該合金則進行兩種不同的滾軋製程，分別為（1）熱軋50%然後冷軋70 %（HR50CR70）與（2）熱軋50%然後冷軋80 %（HR50CR80）。再進行兩種不同升溫速率（15℃/秒和25℃/秒）的快速退火分別至30、40、50、60秒。一樣透過XRD觀察，經熱機處理後的Ti65Zr7依舊可維持單一BCC結構。而透過EBSD觀察可以看出，在相同時間下，較高的退火升溫速率將使合金具有更高的再結晶率；較高的冷加工量則可使再結晶的合金晶粒尺吋降低。而透過拉伸測試得知，採用HR50CR70製程的Ti65Zr7再經快速退火（15℃/秒）至30 到50秒後展現優異的機械性能。其降伏強度可達1200MPa且同時具有15%以上之延性。此外，經過適當的熱機處理製程並透過分析加工硬化率，可以發現Ti65Zr7合金具有異構組織強化的機制。
在Ti65Bx部分，Ti65B0、Ti65B0.1和Ti65B0.2 合金統一先熱軋50%然後再冷軋80%（HR50CR80），之後再進行快速退火（15℃/秒）至700、800和900℃。由光學顯微鏡（Optical microscope, OM）可以得知隨著硼含量的添加，合金的晶粒尺吋也隨之下降。而經熱機處理後，XRD的分析結果則有析出物的特徵峰出現。因此利用掃描式電子顯微鏡（Scanning Electron Microscope, SEM）與穿透式電子顯微鏡（Transmission electron microscope, TEM）對析出物進行微觀分析，可發現析出物是生成在晶界上且其成份被確定為TiB。而在相同退火溫度下，合金的強度與硬度隨著硼含量的上升而增加。而在OM的觀察下可看見隨著硼含量的上升合金的再結晶比例則越低，推斷是因為生長於晶界上的析出物阻礙差排移動，致使合金的再結晶溫度升高。而在再結晶後，各合金的延性均提升至超過20%。其中，經HR50CR80製程的Ti65B0.2合金再經800℃退火後，展現優異的綜合機械性質（YS. 1275 MPa，EL. 10%）。
另外亦與金屬工業研究發展中心（Metal Industries Research & Development Centre, MIRDC）合作，透過感應凝殼熔煉成功製備工業尺吋級的Ti65Zr7合金。並再與台鋼航太科技股份有限公司（S-Tech Corp.）攜手完成大件Ti65Zr7合金之熱鍛製程。在鑄態方面，隨著與澆口距離的增加，合金的晶粒尺吋會降低，而硬度則隨之增加。而在經熱機處理後其機械性質與實驗室製備的樣品相近，展現優異的工業應用潛力。
整體而言，本研究的目的是透過熱機械處理來優化輕量富鈦中熵合金的材料性能，並分析不同微量元素的添加、熱軋的有無、冷軋加工量與退火加熱速率等參數對合金微結構與機械性質之影響。本次成功開發具優異機械性質的輕量高性能富鈦中熵合金，對比國際上其它研究亦展現相當優異的機械性質。同時相關的研究成果與經驗可作為藍圖，為未來開發相關輕量中/高熵合金提供參考與路徑。;Due to the excellent material properties and flexible design conditions, the multi-principal elements alloys has breakthrough the traditional alloy design concept. According to the difference in mixing entropy, it can be further divided into high-entropy alloys (HEAs) and medium-entropy alloys (MEAs). Because of the improvement of technical standards and the pursuit of energy efficiency, the development of lightweight materials with high strength and ductility is one of the main issue in the structural materials community. The thermo-mechanical treatment is a widely and effectively processing to enhance the mechanical properties of the metallic materials. In this study, several lightweight Ti-rich MEAs (Ti65Ni3, Ti65Zr7, Ti65Bx series) were conducted with thermo-mechanical treatment to optimize the microstructure and mechanical properties. The purpose of this study is to strengthen the MEAs with low density (~ 5 g/cm3), high strength (YS ≥ 1200 MPa) and high ductility (EL% ≥ 10%).
In Ti65Ni3 MEA section, the alloy was treated by three types of thermo-mechanical treatment: (1) Cold-rolled 70% (CR70), (2) Cold-rolled 85% (CR85), (3) Hot-rolled 50% then cold-rolled 70% (HR50CR70). Then these three different processed alloys were rapidly annealed at temperature of 743°C, 770°C, 812°C, and 881°C, respectively with a heating rate of 15°C /sec. Through the XRD analysis, it can be confirmed that the MEAs can maintain as a single solid solution structure after thermo-mechanical treatment. Meanwhile, the recrystallization ratio increases with annealing temperature via the EBSD observation. The results of tensile testing revealed that with increasing annealing temperature, the yield strength decreases and the ductility increases, respectively. Compare different process of thermo-mechanical treatment, it can be noticed that high cold-rolled amount can make the recrystallization behavior earlier, and the hot rolled process can make efficient in strain energy accumulation to reduce the recrystallization temperature. Among of all, the specimen conducted with CR85 and HR50CR70 process exhibit outstanding yield strength of 1250 MPa and 1232 MPa with annealing at 770°C, respectively. In addition, both the MEAs present similar tensile ductility of 16%.
In Ti65Zr7 MEA section, the MEA was subjected to two types of rolling process, hot-rolled 50% then cold-rolled 70% (HR50CR70) and hot-rolled 50% then cold-rolled 80% (HR50CR80), and then conducted in different annealing heating rate (15°C/sec and 25°C/sec) with different time. By XRD observation, the MEA can maintain as BCC structure after thermo-mechanical treatment. Through EBSD observation, it can be seen that under same annealing time, the recrystallized ratio of 25°C/sec is larger than 15°C/sec, and grain size of HR50CR80 sample is smaller than HR50CR70 sample. The tensile testing results show the alloy with HR50CR70 process then with annealing for 30 to 50 seconds present outstanding mechanical properties which yield strength can be higher than 1200 MPa with more than 15% ductility. In addition, conducted with proper thermo-mechanical treatment, it can be noticed that Ti65Zr7 MEA possess the characteristic of the hetero-structure strengthening by observing the work hardening rate.
In Ti65Bx MEA section, the Ti65B0, Ti65B0.1 and Ti65B0.2 MEA were conducted with hot-rolled 50% then cold-rolled 80% (HR50CR80) and then subjected the 15°C/sec annealing to 700, 800 and 900°C. It can be noticed that adding boron can reduce the grain size of the alloy by OM images. After thermo-mechanical treatment, the XRD results showed the characteristic peak of the precipitate. Through SEM and TEM, the precipitate can be observed on the grain boundary and is identified as TiB intermetallic compound. The strength and hardness increase with increasing boron content under same heat treatment conditions. In terms of microstructure, the recrystallization behavior of the alloy tends to be tough by boron doping, resulting in an increase of recrystallization temperature. After recrystallization, the elongation of Ti65B0, Ti65B0.1, and Ti65B0.2 exceeded 20%. Among of all, the Ti65B0.2 MEA processed HR50CR80 then annealed at 800°C presents the optima synergy of yield strength and ductility (YS. 1275 MPa, EL. 10%).
In industry-dimension production of Ti65Zr7 MEA, the ingots are successfully fabricated by induction skull melting. The microstructure and the mechanical properties of the as-cast and thermo-mechanical treatment sample are investigated. It can be noticed that when the distance to the sprue increase, the grain size will be decrease while the hardness increase. And the mechanical properties of industry-dimension ingot are similar with the laboratory samples that exhibit huge potential for industry application.
Overall, the purpose of this study is to optimize the material properties of the lightweight Ti-rich MEAs through thermo-mechanical treatment. The effects of minor element addition, hot-rolled process, cold-rolled amount and different annealed heating rate on microstructure and mechanical properties are fully discussed. Through these experimental results, it can provide a blueprint for subsequent lightweight Ti-rich MEAs development. Undoubtedly, the MEAs with excellent material properties and flexible alloy design will have wide development and application in structural material community in the future. |
