Effect of γ’ volume fraction on the precipitation kinetics of the σ phase in wrought Ni-base superalloys

在镍基高温合金中，以σ相为代表的拓扑紧密堆积（TCP）相的析出具有越来越重要的意义，因为新一代的合金被设计成含有更高浓度的耐火元素，以满足日益提高的高温材料要求。由于已知TCP相的显著体积分数会对高温合金的关键性能（如蠕变强度、断裂韧性、疲劳和抗氧化性）产生负面影响，因此正在进行研究工作，以使这些相不稳定或减缓其析出动力学。由于后者的原因，更好地了解影响高温合金中TCP相析出动力学的因素至关重要。本文旨在研究形变高温合金中γ′相及其体积分数对σ相析出动力学的影响。研究这种行为的动机是基于一个简单的观察，即高温合金中TCP相的形成通常伴随着TCP颗粒周围γ′包络层的形成。由于γ′相对Cr、Mo、W等TCP形成物具有较低的溶解度，因此γ′包膜有望强烈地减缓TCP颗粒的生长动力学。然而，据我们所知，到目前为止，在高温合金中tcp相析出动力学的模拟中还没有考虑到这一重要的微观结构特征。这个问题的研究是复杂的，因为还有其他因素也会影响高温合金中TCP相的析出动力学（例如驱动力、元素扩散系数、失配、平均晶粒尺寸等）。为了克服这一困难，本项目的独创性在于研究具有不同γ′体积分数的模型合金，其中这些因素保持恒定。为了达到这一目标，合金设计以热力学计算为指导。提出研究具有相似晶粒尺寸和相同γ成分（在给定温度下）的初始γ/γ′组织的沉淀硬化态亚稳合金。后者将确保TCP相形成的驱动力、TCP相与γ基体之间的失配以及扩散系数保持不变。在本项目中，原子探针层析成像将用于表征沉淀硬化合金。然后在800 - 925°C之间进行不同时间的时效，然后在几个长度尺度上进行化学和微观结构研究，以研究σ相析出动力学的时间-温度转变图如何随着γ′体积分数的增加而延迟。总的来说，系统地了解相稳定性和相变动力学之间的关系将为开发具有优化性能的新一代变形高温合金提供基础。

The precipitation of topologically close-packed (TCP) phases, among which the σ phase is one of its most important representatives in nickel-base superalloys, is a matter of increasing significance because new generations of alloys are designed with higher concentrations of refractory element to meet the ever-increasing requirements of high-temperature materials. As significant volume fractions of TCP phases are known to have negative effects on key properties of superalloys (e.g. creep strength, fracture toughness, fatigue and oxidation resistances), research efforts are being made to either destabilize these phases or decelerate their precipitation kinetics. For the latter reason, it is vital to have a better understanding of the factors that affect the precipitation kinetics of TCP phases in superalloys. The present proposal aims to investigate how the presence of the γʹ phase and its volume fraction influence the precipitation kinetics of the σ phase in wrought superalloys. The motivation to study this behavior is based on a simple observation, i.e., the formation of TCP phases in superalloys is typically accompanied with the formation of γ’ envelops around TCP particles. Since the γ’ phase has a low solubility for TCP formers such as Cr, Mo, W, etc., the γ’ envelops are expected to strongly decelerate the growth kinetics of TCP particles. However, to the best of our knowledge, this important microstructural feature has not been considered so far in simulations of TCP-phase precipitation kinetics in superalloys. The investigation of this issue is complex because there are other factors that also affect the precipitation kinetics of TCP phases in superalloys (e.g. driving force, elemental diffusivities, misfit, mean grain size, etc.). To overcome this difficulty, the originality of the present project consists in investigating model alloys with different γ’ volume fractions in which these factors are kept constant. To reach this goal, alloy design is guided by thermodynamic calculations. It is proposed to investigate metastable alloys in the precipitation hardened state with an initial γ/γ’ microstructure that have similar grain sizes and the same γ composition (at a given temperature). The latter will ensure that the driving force for TCP-phase formation, the misfit between TCP phases and γ matrix, and the diffusivities remain constant. In the present project, atom probe tomography will be employed to characterize the precipitation hardened alloys. These will be then aged between 800 and 925 °C for various times followed by chemical and microstructural investigations at several length scales to investigate how time-temperature-transformation diagrams for the σ-phase precipitation kinetics are delayed with increasing γ’ volume fraction. Overall, the systematic understanding of relationships between phase stability and transformation kinetics will provide the basis for developing new generations of wrought superalloys with optimized properties.