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Research Papers: Gas Turbines: Manufacturing, Materials, and Metallurgy

# Computational Design of Corrosion-Resistant Fe–Cr–Ni–Al Nanocoatings for Power Generation

[+] Author and Article Information
K. S. Chan, W. Liang, N. S. Cheruvu

Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238

D. W. Gandy

Electric Power Research Institute, Charlotte, NC 28262

J. Eng. Gas Turbines Power 132(5), 052101 (Mar 04, 2010) (9 pages) doi:10.1115/1.3204651 History: Received March 25, 2009; Revised April 03, 2009; Published March 04, 2010; Online March 04, 2010

## Abstract

A computational approach has been undertaken to design and assess potential Fe–Cr–Ni–Al systems to produce stable nanostructured corrosion-resistant coatings that form a protective, continuous scale of alumina or chromia at elevated temperatures. The phase diagram computation was modeled using the THERMO-CALC ® software and database (Thermo-Calc® Software, 2007, THERMO-CALC for Windows Version 4, Thermo-Calc Software AB, Stockholm, Sweden; Thermo-Calc® Software, 2007, TCFE5, Version 5, Thermo-Calc Software AB, Stockholm, Sweden) to generate pseudoternary Fe–Cr–Ni–Al phase diagrams to help identify compositional ranges without the undesirable brittle phases. The computational modeling of the grain growth process, sintering of voids and interface toughness determination by indentation, assessed microstructural stability, and durability of the nanocoatings fabricated by a magnetron-sputtering process. Interdiffusion of Al, Cr, and Ni was performed using the DICTRA ® diffusion code (Thermo-Calc Software® , DICTRA , Version 24, 2007, Version 25, 2008, Thermo-Calc Software AB, Stockholm, Sweden) to maximize the long-term stability of the nanocoatings. The computational results identified a new series of Fe–Cr–Ni–Al coatings that maintain long-term stability and a fine-grained microstructure at elevated temperatures. The formation of brittle $σ$-phase in Fe–Cr–Ni–Al alloys is suppressed for Al contents in excess of $4 wt %$. The grain growth modeling indicated that the columnar-grained structure with a high percentage of low-angle grain boundaries is resistant to grain growth. Sintering modeling indicated that the initial relative density of as-processed magnetron-sputtered coatings could achieve full density after a short thermal exposure or heat-treatment. The interface toughness computation indicated that the Fe–Cr–Ni–Al nanocoatings exhibit high interface toughness in the range of $52–366J/m2$. The interdiffusion modeling using the DICTRA software package indicated that inward diffusion could result in substantial to moderate Al and Cr losses from the nanocoating to the substrate during long-term thermal exposures.

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## Figures

Figure 1

Computational results of Al contents required to suppress σ-phase formation in Fe–Cr–Ni–Al at a temperature in the range of 327–1027°C

Figure 2

Computed pseudoternary phase diagrams for Fe–Cr–Ni–Al alloys at 727°C: (a) 1 wt % Al, (b) 3 wt % Al, (c) 6 wt % Al, and (d) 10 wt % Al

Figure 3

Computed pseudoternary phase diagrams for Fe–Cr–Ni–Al alloys with 10 wt % Al for various temperatures: (a) 427°C, (b) 527°C, (c) 627°C, and (d) 827°C

Figure 4

Predicted grain growth kinetics using the activated energy values for equiaxed and columnar grain structures compared with experimental data for Fe–18Cr–8Ni–10Al nanocoatings with a columnar grain structure at 750°C

Figure 5

Grain diameter distribution for Fe–18Cr–8Ni–10Al nanocoating in the as-coated condition

Figure 6

Colored–coded orientation map for as-coated Fe–18Cr–8Ni–10Al nanocoating on 304 stainless steel substrate

Figure 7

Distribution of grain boundary disorientation determined by EBSD for as-coated Fe–18Cr–8Ni–10Al nanocoating on 304 stainless steel substrate

Figure 8

Theoretical relative density based on the sintering model compared with the experimental data of Fe–18Cr–8Ni–10Al coating on Fe–18Cr–8Ni substrate—the initial relative density of the as-processed coating must be greater than 98% in order to achieve the full density in less than 45 h of exposure at 750°C

Figure 9

Rockwell C indentation on a Fe–20Cr–8Ni–10Al nanocoating on Fe–18Cr–8Ni substrate resulted in a circular indent of radius rp and a cracked/debonded zone of radius rd—the ratio of rd/rp was utilized to deduce the elastic strain energy release rate or interface toughness during steady-state interface debonding using the procedure developed by Drory and Hutchinson (31)

Figure 10

Diffusion couple used in the interdiffusion computation using DICTRA

Figure 11

Computed concentration profiles for Fe–18Cr–8Ni–10Al coating on Fe–18Cr–8Ni substrate after various times of exposure at 750°C: (a) Al distribution, (b) Cr distribution, and (c) Ni distribution

Figure 12

Computed concentration profiles for Fe–25Cr–40Ni–10Al coating on Fe–18Cr–8Ni (304 SS) substrate after 825 h at 750°C: (a) Al concentration, (b) Cr concentration, and (c) Ni concentration

Figure 13

XRD patterns of Fe–18Cr–8Ni (304SS) and Fe–18Cr–8Ni–10Al (304SS+10Al) show the presence of σ-phase without Al addition in 304 SS and the absence of σ-phase in Fe–18Cr–8Ni–10Al with 10 wt % Al addition—these specimens have been subjected to 990 one-hour thermal cycles at a peak temperature of 750°C

Figure 14

Calculated concentration profiles compared against experimental data for Fe–18Cr–8Ni–10Al coating on Fe–18Cr–8Ni substrate after 825 h at 750°C: (a) Al content, (b) Cr content, and (c) Ni content

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