Microstructure and Performance of FeAlCuCrNiNbx High Entropy Alloy Overlay Welded Layers

Literature Overview

This paper by Su Yunhai, Liang Xuewei, Deng Yue, and Liu Yunqi from Shenyang University of Technology, published in Welding Journal (2020, Vol. 41, No. 4, pp. 38–43), investigates the influence of niobium (Nb) content on the microstructure, microhardness, wear resistance, and electrochemical corrosion resistance of FeAlCuCrNiNbx high entropy alloy (HEA) overlay layers deposited on carbon steel substrates via GMAW. The study is supported by the National Key R&D Program of China (2017YFB1103603) and the Liaoning Provincial Natural Science Joint Fund (20180510030). The research addresses a critical gap in surface engineering: how alloying element content in HEA overlay systems affects tribological and corrosion performance, which is directly relevant to applications in pipelines, valves, and pump components exposed to abrasive and corrosive service conditions.

Core Technical Findings

Phase Composition and Microstructure

The overlay layers exhibit a BCC solid solution matrix dominated by the Fe-Cr phase, with minor amounts of MC-type eutectic carbides. The microstructure is characterized by a typical dendritic morphology, consisting of gray dendrite arms (DR) and white interdendritic regions (ID). This dendritic structure is consistent with the rapid solidification conditions inherent to arc welding processes, where the cooling rate typically ranges from 10 to 1000 K/s depending on heat input and thermal mass of the substrate.

The presence of MC carbides (likely NbC or Cr7C3) is significant from a wear resistance perspective. These carbides act as hard second-phase particles that impede dislocation motion and provide wear resistance through micro-ploughing and micro-cutting mechanisms. However, the volume fraction and distribution of these carbides are critical; excessive carbide precipitation at grain boundaries can compromise toughness and increase susceptibility to cracking.

Effect of Nb Content on Microhardness and Wear Resistance

The peak microhardness of 602 HV at x = 0.8 is attributed to the synergistic effect of solid solution strengthening (from the BCC matrix) and precipitation hardening (from Nb-rich MC carbides). Beyond x = 0.8, the marginal increase in Nb content leads to a slight decrease in hardness, possibly due to changes in carbide morphology or the onset of phase instability in the BCC matrix.

Electrochemical Corrosion Performance

All Nb-containing HEA overlay layers exhibit lower self-corrosion current density (icorr) compared to 304 stainless steel, indicating superior corrosion resistance. The optimal corrosion resistance is achieved at x = 1.0, where the polarization curves show the lowest icorr and the slowest corrosion rate. This improvement is attributed to the formation of a more stable and uniform passive film on the surface, enriched with Cr and Nb oxides. The Nb element promotes the formation of thermodynamically stable Nb2O5 in the passive film, which enhances passivity even in aggressive chloride environments.

Interpretation and Engineering Implications

The results demonstrate a clear trade-off between wear resistance (optimized at x = 0.8) and corrosion resistance (optimized at x = 1.0). In engineering practice, this trade-off must be resolved based on the dominant failure mode of the component. For example, in pipeline applications where erosion-corrosion is a concern, a composition near x = 0.8 to 1.0 would provide a balanced performance envelope.

From a welding process perspective, GMAW deposition of HEA layers presents challenges related to dilution, porosity, and cracking. The dilution rate between the HEA overlay and the carbon steel substrate can significantly alter the final composition of the deposited layer. Engineers should account for dilution in design calculations, potentially requiring multi-pass deposition or the use of a transition layer to achieve the desired composition in the final overlay.

The dendritic microstructure observed in the overlay layers suggests that post-weld heat treatment (PWHT) could be beneficial to refine the microstructure and reduce residual stresses. However, the effectiveness of PWHT on HEA systems is still an area of active research, as the high entropy stabilizes the BCC phase and may resist phase transformations at conventional austenitization temperatures.

Key Questions and Reflections

A critical question raised by this study is whether the observed performance can be maintained under cyclic loading and thermal cycling conditions, which are common in pipeline and valve applications. The dendritic microstructure, while providing good hardness, may be susceptible to crack initiation at dendrite junctions under fatigue loading. Additionally, the long-term stability of MC carbides at elevated temperatures (> 500°C) should be evaluated, as coarsening of carbides can reduce wear resistance over time.

Another consideration is the scalability of this process. GMAW is a well-established industrial process, but achieving consistent composition control in HEA overlay welding requires precise wire composition and stable arc parameters. In mass production settings, process monitoring and feedback control systems would be necessary to maintain overlay quality.

Summary

This study provides valuable data on the Nb content optimization for FeAlCuCrNiNbx HEA overlay layers, demonstrating that x = 0.8 yields optimal wear resistance (602 HV, 0.30 g wear loss) while x = 1.0 provides superior corrosion resistance compared to 304 stainless steel. The BCC matrix with MC carbide reinforcement offers a promising approach for surface hardening of carbon steel components in abrasive and corrosive service environments. Engineers applying these findings should carefully consider the dilution effects, post-weld treatment requirements, and the wear-corrosion trade-off when selecting the optimal composition for specific service conditions.