細晶鑄造及鋯添加對TM-321超合金中溫機械特性的影響

The Effects of the Fine-grain Process and Zirconium Addition on the Mechanical Properties of the TM-321 Superalloy at Moderate Temperature

10.6841/NTUT.2014.00694

蔡毅龍

TM-321 ； 超合金 ； 細晶 ； 鋯 ； 拉伸 ； 潛變 ； TM-321 ； Superalloy ； Fine-Grain ； Zirconium ； Tensile ； Creep

臺北科技大學工程科技研究所學位論文

2014年

博士

王錫福

繁體中文

細晶鑄造技術的發展係用於提升超合金的中溫強度與潛變性能，且迄今學者對 TM-321超合金細晶鑄造特性之研究闕如，促使本論文進行TM-321超合金細晶製程評估與改良。本研究第一部份控制鑄造參數製備出兩種不同晶粒大小之試桿，接著進行TM-321超合金細晶之拉伸與潛變性能評估。第二部份添加鋯調質細晶TM-321 超合金，以探討不同鋯含量對TM-321超合金機械性能及破壞模式之影響。 本研究所製備細晶試桿的晶粒大小約為90μm，測試結果顯示TM-321超合金細 晶之中溫拉伸與潛變性能均符合EMS-55447規範。此外，由於TM-321超合金較高含量的W、Ta固溶強化元素，較高含量Ta於熱曝露時可抑制γ′相之凝結與成長，並增加曝熱時的穩定性，以及其晶界M6C型碳化物比Mar-M247晶界M23C6型碳化物具有較佳的熱穩定性等因素，使其760℃/724 MPa潛變壽命(101 h)為規範值(23 h)的4.3倍以上，更為傳統鑄造試桿之潛變壽命的6.2倍以上，亦較相同製程之 Mar-M247超合金高出43%。 在鋯對細晶鑄造TM-321超合金之中溫機械特性的影響方面，當鋯含量由0.08 wt%(TZ1)增至0.42 wt%(TZ3)，TM-321超合金的760℃/724 MPa潛變壽命倍增，此歸因於添加鋯產生晶粒細化、γ/γ′顯微組織強化、晶界碳化物細化等綜效。隨鋯含量增加，ii破斷面的裂縫不再只分佈於晶界，逐漸有更多裂縫直接穿過晶粒，使破壞模式由典型的沿晶破壞模式，轉變為穿晶及沿晶混合破壞模式。綜上，本文證實適量添加鋯元素促使TM-321超合金更具細晶鑄造優異的潛變性能，深具將其推廣應用於中溫操作之引擎零組件的潛力。

The fine-grain process (FGP) has been developed to enhance the strength and creep performance of several superalloys at moderate temperature; however, studies on the casting ability of the TM-321 superalloy using the FGP remain scant. Therefore, in this study, the fine-grained TM-321 manufacturing process was assessed and improved. In the first part of the study, the casting parameters were controlled to produce two kind test bars each containing varying grain sizes; the tensile strength and creep properties of fine-grained TM-321 superalloy were subsequently evaluated. In the second part of the study, zirconium (Zr) was added to tailor the TM-321 superalloy, investigating the effects of Zr content on the mechanical properties and fracture modes of the test bars. The grain size of the fine-grained TM-321 superalloy was nearly 90 μm. The results of this study showed that the tensile strength and creep performance of the fine-grained TM-321 test bars all achieved the EMS-55447 specification. In addition, because the TM-321 superalloy contains a higher content of solid-solution strengthening elements W and Ta, and the higher Ta content inhibited the γ′ phase from coagulating and growing at high temperatures during long exposure and increased the thermal stability of the TM-321 superalloy. Moreover, superior to that of the M23C6 carbides of the Mar-M247 superalloy, the thermal stability of the M6C type carbide of the TM-321 superalloy helped improve its mechanical properties. Therefore, the creep life of the fine-grained TM-321 superalloy at 760oC/724 MPa (101 h) is 4.3 times longer than the specification value (23 h), which is also 6.2 times greater than conventionally casting test bars and 1.43 times longer than that of the Mar-M247 superalloy produced using the same manufacturing process. Concerning the influence of Zr content on the mechanical properties of fine-grained TM-321 superalloy at moderate temperature, the results showed that when the Zr content was increased from 0.08 wt% (TZ1) to 0.42 wt% (TZ3), the creep life of TZ3 alloy at 760oC/724 MPa was twice as long as that of the original TM-321 superalloy (TZ1). These results are attributed to the synergic effects of the following strengthening mechanisms: (1) the grain size of the TM-321 superalloy was refined by adding Zr; (2) the Zr addition increased the strength of the γ/γ′ microstructure; and (3) adding Zr refined the grain-boundary (GB) carbides. The fractographic observation of creep and tensile tests demonstrated that crack propagation along the GBs was reduced as the Zr content increased, whereas secondary cracks within the grain interior increased. The fracture modes of the fine-grained TM-321 superalloy were transformed from typical intergranular modes into transgranular and intergranular mixed modes by increasing the Zr content from 0.08 wt% to 0.42 wt%. As mentioned, this study confirmed that adding an appropriate amount of Zr improved the performance of fine-grained TM-321 superalloy and substantially improved its creep life, making the TM-321 superalloy an excellent choice as engine components operating at moderate temperature.