Microstructural Insights into Negative Strain Rate Sensitivity in Additively Manufactured IN718 at Elevated Temperatures

When manufacturers build components for extreme environments, like jet engines or turbines, the materials they choose need to perform flawlessly. That’s why IN718, a nickel-based superalloy, is an excellent candidate because it is strong, handles high temperatures well, and resists deformation under pressure. But with the rise of additive manufacturing (AM)—especially selective laser melting (SLM)—how this alloy behaves is becoming less predictable. AM has revolutionized how parts are made, allowing for intricate designs and reduced waste, but it also creates unique microstructures that can change how materials perform. One surprising behavior in additively manufactured IN718 is something called negative strain rate sensitivity (NSRS). Essentially, instead of getting stronger under faster strain rates, the material weakens—an unusual and potentially dangerous trait, especially for components that face sudden impacts or dynamic forces. This behavior is even more pronounced at higher temperatures, which is concerning because IN718 is typically used in hot, high-stress environments. NSRS seems to stem from the material’s microstructure—things like dislocations, grain boundaries, and solute atoms interacting in unexpected ways. While this phenomenon has been observed before, understanding the exact causes has been tricky, leaving engineers unsure how to prevent it. Recognizing the need for answers, a research team from Northwestern Polytechnical University, including Kai-Yang Zhu, Shi Dai, Shao-Hua Zou, Associate Professor Ya-Jun Yu, and Professor Zi-Chen Deng, took on the challenge. Their work, published in European Journal of Mechanics – A/Solids, investigated the root causes of NSRS in AM IN718 and their experiments showed how features like dislocation movement and solute atom clustering contribute to NSRS, particularly at elevated temperatures. Using crystal plasticity finite element modeling (CPFEM), they recreated these microstructural interactions in a virtual environment, allowing them to predict how the material would behave under different conditions. The insights they gained not only explain why NSRS happens but also offer strategies to mitigate it, such as refining alloy composition or adjusting heat treatment processes.

To start, the authors created samples of IN718 using SLM, a process that builds components layer by layer with a laser. Afterward, the samples went through heat treatments to reduce the internal stresses caused by manufacturing and to refine the microstructure, ensuring the material would behave as it would in real-world applications. They then tested the samples at different strain rates (from slow to fast) and temperatures ranging from 600°C to 800°C, conditions similar to those IN718 faces in service. The results were clear: at 600°C, the material displayed NSRS—it became weaker at higher strain rates. However, by 700°C, this behavior began to disappear, and by 800°C, the material behaved as expected, becoming stronger with faster strain rates. This temperature-dependent shift suggested something significant was happening at the microscopic level. To figure out what, the researchers used powerful imaging tools like electron backscatter diffraction and scanning electron microscopy. These tools revealed that at lower temperatures, carbon atoms in the alloy were interacting with dislocations—tiny defects in the material’s crystal structure. The carbon effectively “pinned” these dislocations, making it harder for the material to deform and causing the strange NSRS behavior. As the temperature increased, the dislocations gained enough energy to break free from this pinning effect, allowing the material to deform more predictably and even strengthening it at higher strain rates. Moreover, the research team used CPFEM advanced computer simulations to recreate what they saw in the lab. These models confirmed their findings and even suggested ways to fix the problem, such as tweaking the alloy’s carbon content or refining its grain structure. This research not only explains why NSRS happens but also provides real-world solutions for making AM IN718 more reliable and safer for high-stress applications.

In conclusion, Northwestern Polytechnical University scientists successfully addressed the NSRS peculiar phenomenon in IN718, where the material weakens under faster strain rates at elevated temperatures. For industries like aerospace and energy, where components face high stresses and temperatures, understanding and fixing this issue is essential. Indeed, the researchers didn’t just identify the problem; but they even explained the microstructural causes and offered important real solutions to improve performance. We believe what makes this study stand out is how it connects what we see on a large scale—like how the material responds to stress—with the microscopic interactions happening inside. They found that NSRS is heavily influenced by something called dynamic strain aging (DSA). At certain temperatures, atoms like carbon cluster around dislocations, essentially “pinning” them in place. This makes it harder for the material to deform smoothly, leading to weaker performance at faster strain rates. However, at higher temperatures, these dislocations can break free, reversing the NSRS and allowing the material to behave more predictably. This insight bridges the gap between mechanical behavior and microstructure, giving engineers a clearer understanding of how these factors interact. Moreover, the study also highlights a challenge unique to additive manufacturing. While AM allows for creating complex shapes and reducing waste, it produces microstructures that are different from traditional manufacturing methods. These unique features can lead to unexpected behaviors, like NSRS. This research shows why it’s essential to develop tailored post-processing treatments and alloy adjustments to address these quirks and ensure consistent performance. It is interesting that the authors proposed actionable solutions and they demonstrated how to stabilize the material’s microstructure and improve its strain rate sensitivity by refining the alloy’s carbon content or adjusting heat treatment protocols. These excellent strategies could make AM IN718 more reliable for demanding applications like jet engines or gas turbines. Furthermore, what is exciting is that the findings aren’t limited to IN718. The methods used—combining hands-on experiments with advanced modeling—can be applied to other alloys and AM materials, paving the way for better-performing components across industries. This study doesn’t just explain a problem; it offers a practical roadmap for building safer, stronger materials in the future.