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

镍基高温合金中拓扑密堆积相（TCP）的析出越来越重要，其中σ相是TCP相最重要的代表之一，这是因为新一代的合金被设计成具有更高浓度的难熔元素，以满足高温材料不断增长的要求。由于已知TCP相的显著体积分数对超合金的关键性能（例如蠕变强度、断裂韧性、疲劳和抗氧化性）具有负面影响，因此正在进行研究努力以使这些相不稳定或减缓其沉淀动力学。对于后一个原因，它是至关重要的，有一个更好的理解的因素，影响TCP相在高温合金中的沉淀动力学。本文旨在研究变形高温合金中γ相的存在及其体积分数对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，在超级合金中TCP相的形成通常伴随着TCP颗粒周围γ ′ ′ s的形成。由于γ'相对TCP形成物如Cr、Mo、W等具有低溶解度，预期γ ′ ′ s强烈地减慢TCP颗粒的生长动力学。然而，据我们所知，这一重要的微观结构特征还没有被认为是迄今为止在模拟TCP相沉淀动力学的高温合金。这个问题的研究是复杂的，因为有其他因素也影响TCP相在高温合金中的沉淀动力学（例如，驱动力，元素扩散率，失配，平均晶粒尺寸等）。为了克服这个困难，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，合金设计是由热力学计算指导的。建议研究处于沉淀硬化状态的亚稳合金，其初始γ/γ'显微组织具有相似的晶粒尺寸和相同的γ成分（在给定温度下）。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恒定。在本计画中，将利用原子探针层析术来表征沉淀硬化合金。然后将它们在800至925 °C之间老化不同时间，然后进行几种长度尺度的化学和微观结构研究，以研究σ相沉淀动力学的时间-温度-转变图如何随着γ'体积分数的增加而延迟。总之，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.