具有γ/γ'-Co3(Al,W)共格组织的新型Co-Al-W基合金高温低拉伸塑性行为和机理研究

The Co-Al-W based alloy with excellent strength at high temperatures is one of the most important issues of the Co-based alloys.Intermetallic compound γ'-Co3(Al,W), a phase precipitated from the γ-Co matrix, is the guarantee for the outstanding strength of the Co-Al-W based alloys with a coherent γ/γ' microstructure at high temperatures through the coherent strengthening and flow stress anomalies mechanisms. We have found that the Co-Al-W based alloys with a coherent γ/γ' -Co3(Al,W) microstructure show the tensile elongation larger than 13% at room temperature due to dimple failure mode, and their yield strength at 600oC-800oC is about double of the traditional Co-based alloys. It is interesting that, however, the tensile elongation at 600oC-800oC decreases considerably from 13% at room temperature to 3% caused by intergranular fracture. The grain boundary character distribution (GBCD), segregation of elements (including impurity and alloying elements) on the grain boundaries and brittleness of the γ'-Co3(Al,W) phase at high temperatures are believed to be the keys that result in the anomalous low tensile ductility and intergranular failure of the γ/γ'-Co3(Al,W) coherent microstructure. In this application, the polycrystalline and single crystal γ/γ'-Co3(Al,W) coherent microstructure alloys and γ'-Co3(Al,W) alloys with minor addtions of the rare earth elements Y and Ce will be designed and propared. The tensile mechanical properties and faiulre modes of the γ/γ'-Co3(Al,W) coherent microstructure alloys at room- and high- temperatures dependent on the boundary character distribution (GBCD), distribution of the rare earth elements and the behavior of the γ'-Co3(Al,W) phase will be investigated. We will also analyze the failure mechanisms of the γ/γ'-Co3(Al,W) coherent microstructure at room and high temperatures, reveal effect of the grain boundary character distribution (GBCD), the distribution of the rare earth elements and the behavior of the γ'-Co3(Al,W) phase on the anomalous low tensile ductility of the coherent γ/γ'-Co3(Al,W) microstructure at high-temperatures, and confirm the dominant factor and mechanism. Finally, the strengthening and toughening principle for the composition and microstructure design of the γ/γ'-Co3(Al,W) coherent microstructure will be proposed.

新型高温高强Co-Al-W基合金是Co合金的重要发展方向，γ-Co中的金属间化合物析出相γ'-Co3(Al,W)的共格强化和反常屈服效应是Co-Al-W基合金高温高强度的组织保证。申请者发现室温拉伸时γ/γ'共格组织的延伸率大于13%，发生穿晶韧窝型断裂；600oC-800oC时屈服强度达到传统Co基合金的1倍，但延伸率急剧下降到3%以下，发生了沿晶断裂。晶界结构（晶界特征分布）、元素（杂质和合金元素）在晶界偏聚以及γ'相高温脆性是导致γ/γ'共格组织高温低拉伸塑性的关键因素。本项目研究微量稀土元素Y、Ce掺杂前后对多晶和单晶γ/γ共格组织合金和γ'单相合金的晶界结构（多晶体）、微观组织、室温至高温力学性能的影响规律，明确γ'和γ/γ'共格组织高温变形断裂形式和机理，揭示晶界结构、微量稀土元素和γ'相性能等因素引起γ/γ'共格组织高温低拉伸塑性的机制，提出γ/γ'共格组织的强韧化方法。

多晶体金属间化合物及其合金在室温和高温下往往塑性不足，沿晶断裂是其脆性的主要原因。杂质元素在晶界的偏聚和更多的大角晶界降低了晶界强度，造成低应力沿晶脆性断列。本课题针对具有γ-CoSS/γ'-Co3(Al, W)共格组织的新型Co-Al-W基合金的室温和高温力学性能开展工作，明确高温反常塑性的机理和影响因素，提出基于微合金化的γ/γ’共格组织的强韧化方法。. 以Co-9Al-9W成分为基础，用稀土元素Y(0.01~0.2at%)、Ce(0.01~0.2at%)和活性元素Mg(0.1~1at%)微合金化，应用电弧熔炼和螺旋选晶法制备了具有γ/γ'组织的多晶和单晶合金以及γ'-Co3(Al, W)单相单晶，研究微合金化对晶界杂质分布、晶界特征和显微组织影响规律，应用原位拉伸方法研究室温和高温下Co-9Al-9W基合拉伸力学性能和变形断裂方式。取得主要结果：.1）微合金化不影响Co-Al-W基合金的铸态和时效态组织。时效态合金具有γ-CoSS/γ'-Co3(Al, W)共格组织结构。微合金元素偏聚于晶界和γ'相中，适量添加（0.05at%）排除晶界上杂质元素O，提高小角晶界和对称晶界比例；.2）微合金化均提高γ-CoSS/γ'-Co3(Al, W)组织的室温延伸率（大于20%）。Ce添加的多晶体合金700℃下还保持6%以上塑性，700℃以上发生沿晶解理，塑性大幅下降；而单晶体合金室温~900℃塑性良好，发生韧窝型断裂；.3）{1 1 1}<1 1 0>位错起源于γ通道随后切割进入邻近的γ'-Co3(Al, W)中。小角度晶界是滑移线穿过晶界的必要条件：当两个晶粒间几何排列因子M>0.8，施密特因子S>0.3时，滑移线穿过小角度晶界；大角度晶界M值往往较小，开动相邻晶粒中施密特因子较大的滑移系；.4）室温~900℃范围，单晶体合金屈服强度高于多晶体强度，两者在700~800℃发生反常屈服。多晶体700℃反常屈服是位错密度明显提高，更多位错被γ'相钉扎；而单晶体800℃反常屈服是形成反相畴界，其中有高密度的[110]/2位错。.5）首次制备了γ'-Co3(Al, W)单相单晶合金，从室温到900℃塑性良好，在600℃延伸率甚至超过45%，发生微孔聚集型韧性断裂；反常屈服发生在600~700℃。由于没有界面，屈服强度远低于两相合金。