合金元素对沉淀强化钴基单晶合金组织稳定性影响

Co-based superalloys possess higher incipent melting temperature, excellent resistance to hot corrosion, wear fatigue resistance, thermal fatigue resistance and weldability. New γ′-strengthened Co-Al-W based alloys have therefore attracted extensive interest, which can be developed for new system of superalloys. Prospective applications of the Co-Al-W based alloys are mainly determined by the microstructure stability at elevated temperature. In this project, Co-Al-W-X (X=C, B, Cr, Re, Ta and Hf) quaternary single crystal alloys are investigated as target material. A first-principles study of thermodynamic stability of γ′ precipitates in those quaternary systems is carried out. Then single crystal specimens are made of typical quaternary alloys by Bridgman directional solidification technique. Then the specimens are exposed at elevated temperature for prolonged ageing. By means of OM, SEM, TEM, electron probe, X-ray diffraction and DSC, the effect of alloy agent additions on the microstructure stability, solidus and liquidus temperatures, volume fraction, morphologies, size and component of γ′ precipitates, etc. are systemically investigated. Further, new Co-Al-W based single crystal alloys jointly strengthened by solution, carbide and precipitates will be designed and prepared to study interaction of typical elements on microstructure stability. The laws of variable microstructure stability with elements will be summarized in detail, which provide the fundamentals for improving mechanical properties and designing for the Co-Al-W based superalloy.

钴基合金具有初熔温度度高、抗热腐蚀性能好以及优异的耐磨损疲劳性能、热疲劳性能和焊接性能等优点。因此新型γ′沉淀强化Co-Al-W基合金引起了广泛重视，有望发展成为新的高温合金体系。高温组织稳定性决定了合金应用前景。本项目首先以Co-Al-W-X (X=C, B, Cr, Re, Ta和Hf) 四元单晶合金为研究对象，采用第一原理计算四元单晶合金中γ′相热力学稳定性，制备典型四元合金单晶试样，并进行高温长期时效，然后采用光学显微镜、扫描电镜、透射电镜、电子探针、XRD衍射和DSC等测试手段，系统研究合金元素对合金组织稳定性、固液相线温度、γ′相体积分数、形态、尺寸和成分等的影响。在此基础上，设计制备固溶、碳化物和沉淀相强化兼备的新型Co-Al-W基单晶合金，研究典型合金元素交互作用对高温组织稳定性影响，揭示合金组织稳定性演变规律，提高合金高温力学性能，为新型钴基合金成分设计奠定理论基础。

本项目首先以Co-Al-W-X (X=C, B, Cr, Re, Ta和Hf) 四元单晶合金为研究对象，采用第一原理计算四元单晶合金中γ′ 相热力学稳定性。在此基础上，在Co-Al-W合金体系中分别添加微量元素C、B，固溶强化元素Cr、Re，沉淀强化元素Ta、Ti、Hf、Mo等制备出多元钴基合金，系统研究了合金元素对沉淀强化钴基高温合金组织热稳定性及力学性能的影响。. 添加C元素不仅可以缓慢提升合金的γ′ 相固溶温度、降低合金的错配度，还可以提升合金屈服强度。B元素的添加可降低合金的错配度以及合金的屈服强度和拉伸塑性。Cr元素的添加会降低γ′相固溶温度以及屈服强度，但可提高延伸率。长期时效过程中，随着C元素含量的增加，γ′相900℃粗化速率增加；而B元素的添加则会降低γ′相在900℃粗化速率。. 在多元合金体系中用部分Mo元素取代部分W元素会导致γ′相固溶温度降低，同时密度也有所降低。Mo元素的添加会降低μ相的析出温度，在断裂的过程中，μ相周围的包裹状γ′沉淀相起着重要作用，可以在一定程度上阻止微裂纹的扩展。随着合金体系中Re含量增加，γ′相固溶温度有所提高，γ′相的形态由 球状向立方状转变，合金中的μ相、β相的体积分数均随之增加。2.5Hf合金中的γ′相的体积分数随时效时间的延长而减小，γ基体通道变宽。. 提高合金中Ta、Ti含量，合金的固液相线温度以及初熔温度降低，凝固区间增大，可显著提高γ′相的溶解温度。Ta含量提高，凝固组织的一次枝晶间距减小。热处理后Ta、Ti偏析于γ′相，可增加γ和γ′两相晶格常数；Ta、Ti可提高合金热处理后γ′相的体积分数以及尺寸，可提高γ′相的高温形貌稳定性。提高合金中Ta含量，可提高合金的屈服强度，而提高Ti含量仅能提高合金高温时的屈服强度。提高合金中Ta，Ti含量可提高合金的蠕变性能；并接近第一代镍基单晶合金SRR99。