Microstructure and Growth Mechanism of Overlay Layer Under Combined Action of Boron and Titanium

Literature Overview

The 2017 paper by Wang Yu from the Shenyang Special Equipment Inspection and Research Institute and Gou Jian and Liu Zhengjun from Shenyang University of Technology investigates the microstructural evolution and growth mechanisms in overlay layers containing combined boron (B) and titanium (Ti) additions. Funded by the Liaoning Provincial Doctoral Research Startup Fund (Grant 20131079), this work addresses a critical materials design question for overlay alloys used in pressure vessel and special equipment applications where both wear resistance and bonding integrity are paramount.

Core Technical Content

Thermodynamic Basis for B-Ti Interaction

The combined addition of boron and titanium to iron-based overlay alloys creates a complex thermodynamic landscape with multiple competing phase equilibria:

The key insight from this research is that the combined presence of B and Ti produces synergistic effects that are not simply additive — the formation of TiB₂ (hardness 3000 HV) and Ti₂B phases provides hard phase reinforcement, while the interaction with the iron matrix creates complex carbide-boride networks that influence both hardness and toughness.

Microstructural Evolution

The solidification microstructure of B-Ti-containing overlay alloys exhibits several distinctive features:

1. Primary phase formation — At Ti > 2 wt% and B > 0.5 wt%, primary TiB₂ particles form during initial solidification, acting as heterogeneous nucleation sites for subsequent phases.
2. Eutectic microstructure — The interdendritic regions typically contain complex eutectic structures consisting of martensitic matrix + M₇C₃ carbides + TiB₂ + Fe₂B, creating a multi-phase composite microstructure.
3. Phase distribution — The TiB₂ particles tend to concentrate at interdendritic boundaries and along grain boundaries, creating a network that can either strengthen (at optimal volume fraction) or embrittle (at excessive volume fraction) the microstructure.
4. Growth mechanism — The paper identifies a competitive growth mechanism where TiB₂ particles grow by direct solidification from the liquid, while Fe₂B forms through a peritectic reaction (L + Fe₃C → Fe₂B + Fe), creating a spatially segregated phase distribution.

Mechanical Property Trade-offs

The data clearly demonstrates the fundamental hardness-toughness trade-off, but the B-Ti combination provides a more gradual transition than either element alone, allowing for more precise property tuning.

Growth Mechanism Analysis

The paper provides detailed analysis of the phase growth mechanisms using both thermodynamic modeling and experimental validation:

Nucleation Mechanism

• Heterogeneous nucleation of TiB₂ occurs preferentially at existing oxide inclusions or substrate-derived particles, with a nucleation temperature depression of 100–200°C compared to homogeneous nucleation.
• Epitaxial growth of TiB₂ on Ti-rich dendrite cores creates a coupled growth morphology in which TiB₂ and martensite grow in parallel, producing a lamellar structure.

Growth Kinetics

The growth rate of TiB₂ particles follows a modified Jackson-Hunt model:

• Growth rate increases with undercooling up to a critical value (approximately 50°C below liquidus), beyond which diffusion-limited growth becomes rate-controlling.
• The aspect ratio of TiB₂ particles increases with increasing cooling rate, producing more elongated morphologies at higher solidification rates.
• The interaction between growing TiB₂ particles and advancing dendrite tips creates a complex three-dimensional microstructure that cannot be fully characterized by two-dimensional metallography.

Engineering Practice Implications

Application in Pressure Vessel Overlay

For bimetal pressure vessel fabrication, the B-Ti overlay system offers specific advantages:

1. Controlled hardness gradient — The gradual transition in hardness from substrate to overlay surface reduces stress concentration at the bond line.
2. Improved hydrogen resistance — The TiB₂ phase acts as a hydrogen trap, potentially reducing hydrogen-induced cracking susceptibility in overlay layers exposed to hydrogen service.
3. Corrosion resistance enhancement — Titanium promotes the formation of a protective TiO₂ layer at the overlay surface, complementing the chromium-based passive film.

Process Recommendations

Study Insights and Reflections

This research contributes a nuanced understanding of how B-Ti interactions can be exploited for overlay alloy design. The most significant finding is the identification of a composition window (approximately 2.5–3.5% Ti, 0.6–0.9% B) where the hardness-toughness balance is optimal for industrial applications. This window is narrower than initially expected, emphasizing the need for precise compositional control during overlay welding.

The growth mechanism analysis provides valuable insight into the microstructural evolution that practitioners observe but often struggle to explain. Understanding that TiB₂ forms through heterogeneous nucleation on oxide inclusions has direct implications for process control — substrate cleanliness and flux composition become critical factors that influence overlay microstructure. The competitive growth model explains why overlay layers sometimes exhibit inconsistent properties across the surface — local variations in nucleation site density create spatial heterogeneity in phase distribution.

For pressure vessel applications specifically, the hydrogen trapping behavior of TiB₂ is particularly relevant, as hydrogen-induced cracking remains a major concern for overlay layers in hydrogenation reactors and high-pressure hydrogen service. Further investigation of the long-term hydrogen embrittlement behavior under cyclic loading would strengthen the case for B-Ti overlays in these applications.