Additive manufacturing of nanostructured eutectic NiAl-(Cr,Mo) in-situ composites

轻质NiAl-（Cr，Mo）原位复合材料是航空航天工程和能源转换领域的高温应用材料。通过具有有限冷却速率的常规铸造技术，产生具有不足的机械性能（例如，相对低的强度、不足的断裂韧性和蠕变强度）的相对粗糙的微观结构。然而，电子束辅助增材制造是非常有吸引力的加工纳米共晶NiAl-（Cr，Mo）复合材料。为了成功地加工这种无裂纹的合金并定制其机械性能，必须优化合金成分、工艺参数和相应的微观结构。通过基于新数据库的热力学计算和多标准优化，将选择具有精确共晶成分的合适合金。电子束辅助增材制造同时实现极高的局部温度、冷却速率和温度梯度以及可控的原位热处理。该工艺路线适合于制备具有可调组织的纳米NiAl-（Cr，Mo）原位复合材料。此外，增材制造的共晶合金显示出迄今为止尚未报道的NiAl-（Cr，Mo）的固态不连续沉淀反应。计划在电子束熔化过程中进行复杂的原位热处理，以触发这种不连续的沉淀反应，从而获得有序且排列良好的层状显微组织。这是第一次，这种不连续的沉淀将以一种有益的方式用于产生类似于层状钛铝化物的均匀层状显微组织。晶粒形态、晶体取向和两相之间的晶格失配将根据加工参数和复合材料的组成而变化。高度灵活的扫描策略允许定制的微观结构对各向同性以及各向异性的精细微观结构完全组成的不连续的沉淀相，这有望导致材料性能的显着改善。优化的共晶NiAl-（Cr，Mo）原位复合材料预计将达到比常规铸造材料高50%的强度值和超过20 MPa/m1/2的室温断裂韧性，相当于目前在航空航天工业中使用的金属间化合物TiAl。了解电子束熔炼过程中的工艺策略、相关的温度历史和相应的微观结构之间的相关性是关键。因此，本项目的最终目的是将所有这些知识联合收割机在处理地图链接的工艺参数和显微组织与其机械性能和目前NiAl-（Cr，Mo）作为展示材料的共晶原位复合材料制造的先进的选择性电子束熔化处理技术。

Lightweight NiAl-(Cr,Mo) in-situ composites are interesting candidate materials for high temperature applications in the realm of aerospace engineering and energy conversion. By conventional casting techniques with limited cooling rates relative coarse microstructures are produced with insufficient mechanical properties (e.g. relatively low strength, insufficient fracture toughness and creep strength). However, electron beam assisted additive manufacturing is very attractive to process nanoscale eutectic NiAl-(Cr,Mo) composites. To successfully process such crack free alloys and to tailor the mechanical properties, the alloy composition, the process parameters and the corresponding microstructure have to be optimized.Suitable alloys with exact eutectic compositions will be selected by thermodynamic calculations based on a new database coupled with multi-criterial optimization. Electron beam assisted additive manufacturing simultaneously enables extremely high local temperatures, cooling rates and temperature gradients as well as controllable in-situ heat treatments. This processing route is perfect for preparing nanostructured NiAl-(Cr,Mo) in-situ composites with adjusted microstructures. Additionally, the additively manufactured eutectic alloys show a solid state discontinuous precipitation reaction that has not been reported for NiAl-(Cr,Mo) so far. Sophisticated in-situ heat treatments during electron beam melting are planned to trigger this discontinuous precipitation reaction on purpose to get an ordered and well-aligned lamellar microstructure. For the first time, this discontinuous precipitation will be used in a beneficial way to generate homogeneous lamellar microstructures similar to lamellar Titanium Aluminides. Grain morphology, crystallographic orientation and lattice misfit between both phases will be varied depending on processing parameters and composition of the composite. Highly flexible scan strategies allow tailoring the microstructure towards isotropic as well as anisotropic fine microstructures fully composed of discontinuously precipitated phases which are expected to lead to a significant improvement in material properties. The optimized eutectic NiAl-(Cr,Mo) in-situ composites are expected to reach strength values 50% higher than those of conventionally cast materials and a room temperature fracture toughness of more than 20 MPa∙m1/2, comparable to that of intermetallic TiAl currently used in aerospace industries. It is key to understand the correlation between process strategies during electron beam melting, the associated temperature history and the corresponding microstructure. Therefore, the final aim of this project is to combine all this knowledge in processing maps to link the processing parameters and microstructures with its mechanical properties and present NiAl-(Cr,Mo) as a showcase material for eutectic in-situ composites fabricated by the advanced processing technique of selective electron beam melting.