Nickel-based alloys, owing to their excellent high-temperature strength, creep strength, and acceptable corrosion resistance, have become the preferred material for hot-end components such as aero-engines and high-temperature gas turbines. As-cast nickel alloy impellers, during complex solidification, typically experience such problems as coarse microstructure, alloying element segregation, uneven distribution of strengthening phase, porosity, and concentration of residual stresses, which considerably reduce their service durability and life. The employment of scientific heat treatment regimens to control the as-cast microstructure has become therefore a principal direction to increase the quality of cast impeller castings.
Introduction
With progression in aerospace propulsion to better thrust-to-weight ratio and greater turbine inlet temperature, more severe requirements are placed on the high-temperature capability, microstructure stability, and damage tolerance of impeller materials. Nickel-based alloys, due to their γ/γ’ two-phase strengthening mechanism, possess the potential to develop satisfactory mechanical properties at temperatures beyond 1000°C and are being widely used in principal parts such as high-pressure compressors and medium-high pressure turbines of aero-engines.
However, it must be mentioned that although integral casting of impellers with complicated geometries is possible through casting processes, they necessarily introduce microstructural flaws such as coarse grains, element segregation, eutectic brittle phases, etc. These “native defects” are liable to induce initiation of fatigue cracks and stress concentration under high-temperature and high-stress conditions, leading to premature failure under service conditions. Therefore, from the perspective of engineering practice, it is deemed necessary to regulate the as-cast microstructure via heat treatment regimes—the sole effective means of enhancing the uniformity and distribution of strengthening phases of impeller castings, yet one of key assurances of their long-term stability in operation under multi-level thermal and mechanical loads.
Characteristics of As-Cast Microstructure and Analysis of Performance Bottlenecks
As-cast nickel-based alloy impellers often exhibit considerable microstructure inhomogeneity, and it has a profound influence on their subsequent heat treatment response, service stability, and processing performance. Especially for aerospace or energy applications working at high-temperature and high-stress conditions, the as-cast defects straight restrict the deployment of the alloy’s potential performance directly. The following starts from typical as-cast microstructure defects and systematically elaborates their bottleneck mechanisms at the performance level:
Coarse Dendrites with Strong Orientation
In actual cast practices, nickel-base alloy impellers are prone to directional solidification or equiaxed solidification technologies. However, due to the geometric complexity and low mold thermal conductivity, the cooling rate is typically low, easily causing coarse columnar dendrite microstructures. The dendrites are strongly oriented, with reduced grain boundaries but high-density distribution, leading to greater local mechanical property anisotropy. The stress concentration susceptibility is further enhanced, especially under thermal cycling or high-cycle fatigue loading conditions, when grain boundaries become favorable paths for the development of cracks. In addition, strengthening phase distribution within dendrites is typically nonequaxial, hence limiting the overall load-carrying capacity of the alloy.
Element Segregation and Brittle Phase Enrichment
High-content alloying elements like Nb, Ti, and Mo exhibit significant interdendritic segregation in the terminal solidification stage. That is, Nb- and Mo-rich regions tend to form Laves phases, while the segregation of Ti is beneficial for forming continuous eutectic γ/γ’ structures. Such low-melting-point or hard and brittle phases are mostly located at grain boundaries or between dendrites and significantly degrade microstructure continuity. During service, they are easily used as sources for crack initiation, accelerating the process of material fracture due to thermal fatigue and stress cycling. Meanwhile, the presence of Laves phases also decreases the homogeneity of element diffusion during heat treatment, affecting the secondary precipitation behavior of γ’ strengthening phases.
Coarsened Carbides and Uneven γ’ Phase Precipitation
Primary MC-type coarse carbides are often encountered in as-cast microstructures. Though they possess a certain pinning effect, their irregular and rough morphology readily leads to stress concentration during dislocation motion. Simultaneously, size, distribution, and volume fraction of γ’ strengthening phases in the as-cast state also tend to agglomerate or coarsen, especially in regions with slow cooling rates, whose strengthening effect is significantly weaker than that of fine γ’ particles with uniform and dispersed state. In addition, such cluster γ’ phases are not effective in engagement with sliding dislocations, significantly affecting the strengthening potential of the alloy.
Significant Residual Stress
As-cast impellers, due to cooling rate fluctuations and complex geometric configurations, are prone to significant residual stress during late solidification. Especially in regions of substantial wall thickness variations and rim-hub transition zones, thermal stress and structural stress intersect, forming multiaxial stress concentration regions. These regions are prone to becoming initiation sites of fatigue cracks during service and also affect the stability of subsequent heat treatment, even leading to deformation or propagation of microcracks during the process.
Briefly speaking, the adverse characteristics of as-cast microstructures not only decrease the mechanical properties of impellers but also affect the uniformity and strengthening efficiency of subsequent heat treatment. Therefore, establishing a scientific and reasonable heat treatment regime has become a significant method for improving their microstructure and service performance.
Construction of Heat Treatment Regimes and Mechanisms of Microstructure Control
As structural components under high-temperature operation, the fundamental guarantee for nickel-based alloy impeller performance is in a satisfactory microstructure, and heat treatment regimes are the fundamental technical means to guarantee microstructure optimization. Aimed at coarse dendrites, element segregation, brittle phase enrichment, and irregular distribution of strengthening phases in as-cast microstructures, there is a need to reconstruct and regulate the microstructure through scientific and systematic heat treatment channels, thereby significantly improving the general mechanical properties and service stability of the material. Currently, the heat treatment processes adopted for nickel-based alloy impellers are primarily comprised of three links as follows: solution heat treatment, single/double-stage aging treatment, and combination treatment of multi-stage processes. All these kinds of heat treatment processes play a crucial role in microstructure evolution.
Solution Heat Treatment: A Prerequisite for Microstructure Homogenization
Solution heat treatment is normally conducted in the high-temperature range of 1080–1200°C. Its chief duty is to effectively dissolve unfavorable phases precipitated in the as-cast state (such as Laves phases, eutectic γ/γ’ phases, and some coarse MC-type carbides), and at the same time activate atomic diffusion mechanisms through high temperature to facilitate the redistribution of the segregated elements such as Nb, Ti, and Mo in the matrix to achieve chemical composition homogenization. During the process, the chemical potential gradient in the dendrites diminishes progressively, and the microstructure flattens and becomes continuous to establish the foundation for ordered precipitation of the next strengthening phases.
However, temperature and solution heat treatment duration must be precisely managed. Very high temperature or long holding time can easily cause abnormal grain growth, leading to excessive dissolution of grain boundary strengthening phases and increased susceptibility to grain boundary sliding, thereby compromising the high-temperature creep resistance and low-cycle fatigue life of the material. Accordingly, for the design of heat treatment processes, thermal expansion curve analysis and differential thermal analysis (DTA) must be combined to determine the optimum solution window in order to facilitate removal of undesirable phases as well as controllability of microstructure.
Aging Treatment: Precise Control of Strengthening Phase Precipitation
After solution treatment, precipitation morphology and size distribution control of γ’ strengthening phases is the key target of aging treatment. Single-stage aging is usually performed at 870–900°C for 4–16 hours, promoting uniform precipitation and dispersed distribution of nanoscale γ’ phases in the γ matrix and fully exercising the precipitation strengthening mechanism. The particle size of γ’ phases precipitated during this stage is normally regulated at 20–50 nm, which can effectively hinder dislocation movement and improve the yield strength and high-temperature deformation resistance of the material.
On the other hand, a two-step aging treatment such as “870°C × 8 h + 760°C × 16 h” is able to further refine particle size distribution and γ’ phase nucleation density while maintaining strengthening efficiency. By taking advantage of the larger γ’ cores obtained in high-temperature primary stage, a lower secondary temperature can be maintained for fine γ’ secondary precipitation, thereby achieving the multi-scale synergistic strengthening of particles and improving creep resistance and rupture life of the material. Double aging also can effectively suppress over-coarsening of γ’ particles and achieve microstructure stability and resistance to fatigue damage at long-term service conditions.
Multi-Stage Treatment and Stress Relief: In-Depth Guarantee of Structural Integrity
As working conditions of impellers become more serious, conventional heat treatment processes can hardly meet requirements for high consistency of microstructure control and elimination of defects. Thus, usage of multi-stage treatment technology in new heat treatment processes is developing into one of the key break-through trends. Realization of multi-level control on grain growth, precipitation strengthening stages, and interface stress evolution at different stages can be achieved through implementation of stepwise heating, multi-stage holding, and slow cooling technologies. At the same time, the addition of hot isostatic pressing (HIP) to heat treatment not only has the ability to effectively close microvoids and microcracks formed during as-casting but also relieve residual stress and improve material density and overall structural integrity, significantly improving fatigue life and service reliability. The HIP process reconstructs and heals defects by means of high pressure and high temperature action, which is among the main technical means to extend the service life of cast nickel-based alloy impeller structures.
In theory, heat treatment regimes not only belong to a repair method of as-cast microstructures but also to a necessary route to construct functional microstructures. By adjusting the solution process heat treatment window, aging regime parameters, and adding composite treatment bondings such as HIP, the synergistic optimization among γ’ precipitation behavior and microstructure continuity can be achieved, ultimately developing a high-strength, high-stability, and high-fatigue-life service-oriented microstructure with a strong guarantee for nickel-based alloy impellers’ safe operation under complicated working conditions with high temperatures.
Experimental Results and Microstructure-Property Responses
With a specific type of high-strength nickel-based alloy (IN738LC) impeller casting as an example for exploration, comparative experiments were conducted. The microstructure characteristics and variations in mechanical properties subsequent to various heat treatments are as follows:
| Treatment Regime | Average γ’ Particle Size | Microhardness (HV) | Tensile Strength (MPa) | Elongation (%) |
| As-cast (untreated) | 120 nm | 295 | 820 | 4.2 |
| Solution + single-stage aging | 45 nm | 435 | 1120 | 5.8 |
| Solution + double-stage aging | 35 nm | 462 | 1165 | 6.5 |
From the above data, it can be seen that after rational solution treatment, primary interdendritic Laves phases and MC carbides in the castings are significantly reduced, and the strengthening phases are more dispersed. Especially after double aging treatment, the size of the γ’ phases becomes more evenly distributed and finer, and microhardness and tensile strength significantly increase, with minimal improvement in plasticity, reflecting good microstructure control effect.
Analysis of Influencing Factors of Heat Treatment Regime Parameters
It is our belief that the optimal design of a heat treatment process should consider in-depth synergistic effects of the following parameters:
Coupling effect of solution temperature and holding time
Moderate temperature increase can promote homogenization of microstructure, but too high temperature (>1130°C) will cause abnormal grain growth. As shown in the experiment, 1100°C × 30 min is a relatively good condition, where the grain size is refined to grade 6.4 and the peak hardness is after strengthening phase precipitation (164.5 HV), with strength and plasticity in equilibrium state.
Cooling rate control
Moderate rapid cooling will homogenize the microstructure and prevent re-precipitation of low-melting-point phases but can cause thermal stress cracks when too rapid, especially in complex structures or thin walls areas.
Optimized design of alloying elements
Ti and Al promote the precipitation of γ’ phases, and Nb promotes the formation of MC-type carbides—a balance must be struck between strengthening effect and microstructure stability to prevent brittle fracture caused by the aggregation of secondary phases.
Control of initial as-cast quality
Casting defects (e.g., shrinkage cavities, inclusions) will undermine the consistency of heat treatment, and subsequent microstructure control failure will result. Therefore, high-quality casting processes (e.g., VIM+VAR, direction solidification) are needed for ensuring the effectiveness of heat treatment.
Conclusion
In summary, heat treatment regimes are a determining factor in the microstructure and service performance of nickel-based alloy impeller castings. Through scientific control of solution temperature, precipitation strengthening behavior during ageing regimes can be improved effectively, leading to refinement of microstructure and uniform distribution of strengthening phase, and thereby enhancing their service stability under high-temperature and high-load conditions greatly. With the advent of simulation technology, multi-stage process, smart control means, nickel-based alloy heat treatment will be designed towards higher precision, higher efficiency, and stronger adaptability, and thereby form a solid foundation for the localization and high reliability of the next-generation aero-engine key components.
