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Recrystallization as a tool to characterize and control the properties of cold sprayed materials: case study on 316L steel

This study demonstrates that controlled post-spray recrystallization in cold-sprayed 316L stainless steel serves as a powerful tool for characterizing deposition-induced deformation heterogeneity and tailoring microstructural and mechanical properties, driven by intense localized strain and high dislocation densities at particle boundaries.

Original authors: Petr Haušild, Jaroslav Čech, Petr Králíček, Tomáš Heller, Jan Cizek

Published 2026-10-05

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CC BY 4.0

Original authors: Petr Haušild, Jaroslav Čech, Petr Králíček, Tomáš Heller, Jan Cizek

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where metal parts are not forged in the heat of a furnace or cast from molten liquid, but are instead built by firing tiny solid particles at a surface with such incredible speed that they weld together upon impact. This is the realm of cold spray, a technology that has revolutionized how we repair and manufacture metal components. Unlike traditional methods that melt metal, this process keeps the material solid throughout, avoiding the oxidation and structural changes that often weaken metals when they are heated. The secret to this solid-state bonding lies in a violent, localized phenomenon called adiabatic shear instability. When a particle hits a surface at supersonic speeds, the friction and pressure generate intense heat in a tiny, confined zone, causing the metal to soften and flow like a jet, stripping away surface contaminants and allowing fresh metal to fuse. However, this process leaves the resulting material in a chaotic state, riddled with microscopic defects and uneven stress, making it difficult to predict how strong or durable the final product will be.

In a recent study, researchers set out to decode this hidden chaos in a specific type of stainless steel known as 316L. They wanted to understand exactly how the violent impact of the spray process deforms the metal and, more importantly, how they could use heat treatment to tame that deformation and reveal the material's true potential. By firing 316L steel powder at high speeds and then carefully heating the resulting coating, the team discovered that the metal's internal structure holds a detailed map of the impact it endured. They found that the extreme pressure and heat generated during the spray create a massive buildup of defects at the boundaries where particles meet, while the centers of the particles remain relatively untouched. This uneven distribution of stress acts like a stored battery of energy, waiting to be released.

The researchers treated the sprayed steel samples by heating them to temperatures between 900 and 1100 Kelvin. As the samples warmed up, a process called recrystallization began, where the damaged, chaotic metal grains reorganized themselves into fresh, orderly structures. The study revealed that this reorganization did not happen all at once. Instead, it started at the particle boundaries where the damage was worst, gradually spreading inward as the temperature and time increased. After heating at 1100 Kelvin for one hour, the entire coating transformed into a completely new microstructure. What was particularly striking was the pattern of the new grains. They did not form a uniform size; instead, they followed a specific statistical pattern where most grains were small, but a significant number grew much larger. This "heavy-tailed" distribution served as a fingerprint of the original impact, proving that the strain during the spray was not spread evenly but was intensely concentrated in specific zones.

To understand the mechanics behind this, the team calculated the conditions inside the metal at the moment of impact. They estimated that the friction and deformation could raise the local temperature to over 1000 Kelvin, creating a driving force for change that is far more intense than in standard metalworking. However, the rapid cooling that follows the impact prevents the metal from fully reorganizing during the spray itself. Instead, the energy remains trapped in the form of a staggering density of defects, estimated to be as high as 10 to the power of 17 per square meter. This immense stored energy is what fuels the recrystallization when the material is later heated. The researchers also measured the hardness of the material at different stages. They found that while the initial sprayed metal was extremely hard due to its damaged state, heating it to 900 Kelvin changed little. It was only as the recrystallization progressed at higher temperatures that the material softened, yet it retained a strength comparable to high-quality, conventionally processed steel, thanks to the fine grain size achieved through this controlled healing process.

The study concludes that recrystallization is not just a way to fix the material, but a powerful tool for understanding it. By observing how the grains grow and how the hardness changes after heating, scientists can now work backward to determine exactly how much strain and heat the metal experienced during the spraying process. This insight allows for better control over the manufacturing process, ensuring that the final product is not just a coating, but a precisely engineered component with predictable and reliable strength. The work demonstrates that even in the most violent, high-speed manufacturing methods, the history of the material is written in its microstructure, waiting to be read by those who know how to look.

Technical Summary: Recrystallization as a Tool to Characterize and Control the Properties of Cold Sprayed 316L Steel

Problem Statement
Cold spray (CS) deposition relies on adiabatic shear instability (ASI) to achieve solid-state bonding, a process characterized by extreme strain rates ( $10^6–10^9 s^{-1}$ ) and highly localized deformation. While ASI is understood to drive bonding through thermal softening and jetting, the resulting microstructure is notoriously heterogeneous. The relationship between the spatial distribution of deformation-induced defects, the subsequent recrystallization behavior, and the final mechanical properties remains complex. Specifically, the mechanisms governing whether dynamic recrystallization occurs during impact versus static recrystallization during post-spray annealing, and how the extreme strain gradients influence grain size distributions, are not fully resolved. This study addresses the need to link microstructural heterogeneity in CS deposits to their deformation history and mechanical performance.

Methodology
The study utilized 316L austenitic stainless steel as a model material, selected for its chemical stability and intermediate stacking fault energy (~30 mJ/m²), which promotes grain refinement without martensitic transformation.

Sample Preparation: 316L powder (15–45 µm) was deposited using a high-pressure Kinetiks 8000-X system with nitrogen gas at 3.5 MPa and 1000 °C. Deposits were detached from aluminum substrates to avoid contamination.
Thermal Treatment: Samples were annealed in air at 900 K, 1000 K, and 1100 K for varying durations (1 h and 4 h), followed by oil quenching.
Characterization:
- EBSD: Used to analyze grain structure, crystallographic orientation, and Kernel Average Misorientation (KAM) to map geometrically necessary dislocations. The Grain Orientation Spread (GOS) parameter was employed to distinguish recrystallized (strain-free) grains from deformed ones.
- Nanoindentation: Continuous Multi-Cycle (CMC) and constant strain rate indentation were performed to measure hardness and Young's modulus as a function of penetration depth, accounting for the indentation size effect.
- Mechanical Testing: Compression tests were conducted on fully recrystallized samples to determine yield strength.
Theoretical Modeling: Thermomechanical estimates were calculated using the Taylor-Quinney equation for adiabatic heating and the Zener-Hollomon parameter to assess the likelihood of dynamic recrystallization. Grain size distributions were fitted to log-normal models and analyzed using Sellars-type constitutive equations.

Key Results

Microstructural Heterogeneity: In the as-sprayed condition, severe plastic deformation and defect accumulation were highly localized at particle interfaces, consistent with ASI. Particle interiors retained subgrains from the feedstock but remained less deformed.
Recrystallization Kinetics:
- Annealing at 900 K resulted in negligible microstructural change.
- At 1000 K, recrystallization initiated preferentially at particle boundaries (high dislocation density regions) and progressed inward. After 1 h at 1000 K, only ~5% of grains were recrystallized; this increased to 30% after 4 h.
- Complete recrystallization was achieved after 1 h at 1100 K.
Grain Size Distribution: The fully recrystallized microstructure exhibited a heavy-tailed log-normal grain size distribution with a mean size of ~3.0 µm and a geometric standard deviation ( $\sigma_d$ ) of 0.55. This distribution reflects the extreme spatial partitioning of strain: ultra-fine grains formed at particle boundaries due to rapid nucleation and impingement, while larger grains formed in particle cores.
Mechanical Properties:
- Hardness remained largely unchanged after annealing at 900 K but decreased progressively as recrystallization advanced at higher temperatures.
- The fully recrystallized sample (1100 K/1 h) exhibited a yield strength of 388 MPa, closely matching the Hall-Petch prediction of 373 MPa based on the mean grain size. This strength is comparable to wrought 316L in 1/8 Hard temper and approximately double that of annealed bulk 316L.
Thermomechanical Estimates: Calculations suggest local temperatures at particle interfaces reached 1000–1150 K with dislocation densities up to $1.15 \times 10^{17} m^{-2}$ . However, the extremely high Zener-Hollomon parameters ( $Z \approx 10^{24}–10^{27} s^{-1}$ ) indicate that dynamic recrystallization was likely inhibited during the impact event. Instead, the stored energy was retained and released via static recrystallization during post-spray annealing.

Significance and Claims
The paper posits that recrystallization serves as a powerful diagnostic tool for characterizing the deformation history of cold-sprayed materials. By analyzing the kinetics and grain size distribution of post-annealed samples, researchers can infer the severity and localization of strain introduced during deposition. The study demonstrates that the heavy-tailed log-normal grain size distribution is a direct signature of the heterogeneous strain partitioning caused by adiabatic shear instability.

Furthermore, the authors claim that controlled post-spray recrystallization is a viable strategy for tailoring mechanical properties. While the process involves softening relative to the as-sprayed state, the resulting ultrafine-grained microstructure achieves yield strengths significantly higher than conventionally annealed bulk steel, driven primarily by grain boundary strengthening (Hall-Petch effect). The work establishes a quantitative link between localized strain accumulation, recrystallization kinetics, and mechanical property evolution in cold-sprayed 316L stainless steel.

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