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 ′-晶格失配度是Ni基和Co基高温合金的一个重要结构参数，它决定了沉淀强化的效率和g/g ′-组织的热稳定性。在商用镍基高温合金中，在使用温度下的错配是负的，而在新型钴基高温合金中，错配是正的。这重新开启了关于错配的哪个符号对具有g/g '-微观结构的高温合金的机械性能有利的问题的根本性辩论。对这一问题的实验和理论研究是本提案的主题。首先研究错配符号对具有g/g ′-显微组织的单晶高温合金蠕变行为的影响。这里将使用Ni基超合金，首先，因为Ni基超合金中的合金元素的g/g '分配行为允许设计具有负和正失配的合金，其次，因为Ni基超合金有一个巨大的数据库，允许对失配和蠕变进行建模。三个比较镍基高温合金与负，零和正的错配将设计使用我们的模型，该模型预测的合金成分和温度的依赖性的错配。目标失配值对应于涡轮机叶片的工作温度，在我们的情况下为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恒定。这种组成变化改变了失配显着，但只有一个小的影响，g '-体积分数和蠕变阻力的g-和g'-相，如我们的初步工作，并遵循从文献。合金设计的这种原则是必要的，以便将成分对失配的影响与其对其他性能的影响（如固溶体强化）分开。失配的符号可以通过两种方式影响蠕变：第一，通过在所有温度下失配应力的符号的反转，第二，通过在高温下筏化方向的改变。为了分离这两种效应，将在蠕变条件下在850°C的较低温度下测试所设计合金的[001]取向单晶，其中微观结构保持立方体，以及在1000°C的较高温度下，其中发生筏化。实验将伴随着不同给定的g '-形态和失配符号的微观结构的蠕变速率的建模。计划中的研究将阐明哪种错配迹象对高温合金的蠕变行为有利。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.