Investigation of the influence of the sign of the g/g'-lattice misfit on the creep behaviour of single-crystal superalloys with g/g'-microstructure

g/g′-晶格失配是镍基和钴基高温合金的重要结构参数，它决定了析出相强化效率和g/g′-显微组织的热稳定性。在商用镍基高温合金中，使用温度下的失配为负，而在新型钴基高温合金中，失配为正。这重新开启了关于错配标志有利于g/g′-显微组织高温合金力学性能的基本争论。这一问题的实验和理论研究是本提案的主题。本文首先研究了失配符号对g/g′-显微组织单晶高温合金蠕变行为的影响。这里将使用镍基高温合金，首先，因为镍基高温合金中合金元素的g/g'分配行为允许设计具有负和正失配的合金，其次，因为一个巨大的镍基高温合金数据库可用，允许模拟失配和蠕变。利用该模型设计了三种具有负、零和正错配的镍基高温合金，该模型预测了合金成分和温度对错配的依赖。目标失配值对应于涡轮叶片的工作温度，在我们的例子中为1000°C。失配将通过用Ni取代Cr来控制，反之亦然，即其他元素的原子浓度保持不变。这种成分的变化明显改变了错配，但对g′-体积分数和g′-相和g′-相的抗蠕变性能的影响很小，这在我们的前期工作和文献中都有体现。这样的合金设计原则是必要的，以便将成分对错配的影响与其对其他性能（如固溶体强化）的影响分开。失配的符号可以通过两种方式影响蠕变：第一，通过在所有温度下的失配应力符号的反转，第二，通过在高温下漂流方向的改变。为了分离这两种影响，设计的合金的[001]取向单晶将在850°C的较低温度下进行蠕变测试，其中微观结构保持立方体，以及在1000°C的较高温度下进行蠕变测试，其中发生了漂移。实验将伴随蠕变速率的模拟微观结构与不同给定的g'-形态和不匹配的迹象。计划中的研究将阐明哪种失配迹象有利于高温合金的蠕变行为。根据结果，这可能是有针对性地开发具有正失配的高温合金的第一步。

The g/g'-lattice misfit is an important structural parameter of Ni- and Co-base superalloys which determines the efficiency of precipitate strengthening and the thermal stability of the g/g'-microstructure. In commercial Ni-base superalloys the misfit at service temperatures is negative, but in new Co-base superalloys it is positive. This reopens the fundamental debate about the question which sign of the misfit is beneficial for the mechanical properties of superalloys with g/g'-microstructure. The experimental and theoretical investigation of this question is the topic of the present proposal. It starts with investigating the influence of the misfit sign on the creep behaviour of single-crystal superalloys with g/g'-microstructure. Here Ni-base superalloys will be used, first, because the g/g'-partitioning behaviour of the alloying elements in Ni-base superalloys allows to design alloys with negative and positive misfit as well and second, because a huge database is available for Ni-base superalloys, allowing to model misfit and creep. Three comparative nickel-base superalloys with negative, zero and positive misfit will be designed using our model, which predicts the misfit in dependence of alloy composition and temperature. The target misfit values correspond to the service temperature of a turbine blade, in our case 1000°C. The misfit will be controlled by substitution of Cr by Ni and vice versa, i.e. the atomic concentrations of other elements are kept constant. This compositional variation changes the misfit significantly but has only a small influence on the g'-volume fraction and the creep resistance of g- and g'-phase, as shown in our preliminary work and follows from literature. Such a principle of alloy design is necessary in order to separate the effect of composition on the misfit from its effects on other properties, such as solid solution strengthening. The sign of the misfit can influence the creep in two ways: first, via the inversion of the sign of the misfit stresses at all temperatures and second, via the change of the rafting direction at high temperatures. In order to separate both effects, [001] oriented single-crystals of the designed alloys will be tested under creep conditions at a lower temperature of 850°C, where the microstructure remains cuboidal, as well as at a higher temperature of 1000°C, where rafting occurs. The experiments will be accompanied by modelling of creep rate for microstructures with different given g'-morphologies and misfit signs. The planned investigations will clarify which sign of misfit is beneficial for the creep behaviour of superalloys. Depending on the results, it could be a first step to the targeted development of superalloys with positive misfit.