Effect of post heat-treatment on the microstructure and high-temperature oxidation behavior of precipitation hardened IN738LC superalloy fabricated by selective laser melting

doi:10.1016/j.jmst.2020.11.013

Abstract

Abstract:

The present study investigated the effect of as-built and post heat-treated microstructures of IN738LC alloy fabricated via selective laser melting process on high temperature oxidation behavior. The as-built microstructure showed fine cell and columnar structure due to high cooling rate. Ti element segregation was observed in inter-cell/inter-columnar area. After post heat-treatment, the initially-observed cell structure disappeared, instead bimodal Ni₃(Al, Ti) particles formed. High temperature (1273 K and 1373 K) oxidation test results showed parabolic oxidation curves regardless of temperature and initial microstructure. The as-built IN738LC fabricated via the selective laser melting process displayed oxidation resistance similar to or slightly better than that of IN738LC fabricated via wrought or cast process. Heat-treated SLM IN738LC, although had similar oxidation weight-gain values to those of the SLM as-built material at 1273 K, showed relatively better oxidation resistance at 1373 K. Bimodal Ni₃(Al, Ti) precipitate formed in the post heat treatment changed the local chemical composition, thereby led to changes in alumina former/chromia former location and fraction on the alloy surface. It was concluded that in heat-treated IN738LC increased alumina former fraction was found, and this resulted in excellent oxidation resistance and relatively low weight-gain.

Key words: IN738LC, Selective laser melting, High temperature oxidation, Post heat-treatment, Oxidation mechanism

Kyu-Sik Kim, Sangsun Yang, Myeong-Se Kim, Kee-Ahn Lee. Effect of post heat-treatment on the microstructure and high-temperature oxidation behavior of precipitation hardened IN738LC superalloy fabricated by selective laser melting[J]. J. Mater. Sci. Technol., 2021, 76: 95-103.

Figures/Tables 12

Table 1 Standard chemical compositions of IN738LC alloy and SLM IN738LC used in the present study.

	Ni	Cr	Co	Al	W	Ti	Mo	Ta	Nb	C
Standard IN738LC	Bal.	15.5-16.5	8.0-9.0	3.2-3.7	2.4-2.8	3.0-3.5	1.5-2.0	1.5-2.0	<0.91	<0.11
SLM IN738LC	Bal.	16.5	8.54	2.74	2.39	3.35	1.60	1.75	0.87	0.11

Fig. 1. (a) Laser scanning strategy used in the current study (chessboard scanning with adjacent chessboard block scanned in 90° rotated direction.) and (b) macrograph of selectively manufactured IN738LC.

Fig. 2. Micrographs of defects of selectively manufactured IN738LC alloys under different conditions; (a) as-built, and (b) after solid-solution + aged (blue arrows: pores, red arrows: shrinkage pores or cracks).

Fig. 3. Microstructures of SLM IN738LC observed by EBSD and FE-SEM; (a) grain boundary map, (b) low magnification image, and (c) high magnification image of the as-built sample. (d) grain boundary map, (e) low magnification image, and (f) high magnification image of the heat-treated sample. Black and white lines in (a) and (d) represent high angle grain boundary (over 15°) and low angle grain boundary (2°-15°), respectively.

Fig. 4. XRD analysis of the as-built IN738LC and heat-treated IN738LC fabricated via selective laser melting.

Fig. 5. Variation of TGA weight gain with oxidation time at different temperatures for (a) the as-built IN738LC and (b) compared with heat-treated IN738LC.

Fig. 6. X-ray diffraction analysis of (a) as-built SLM IN738LC and (b) heat-treated SLM IN738LC after isothermal oxidation test at 1273 K and 1373 K.

Fig. 7. SEM micrographs of surface morphologies after oxidation tests at different temperatures for the (a) as-built oxidized at 1273 K, (b) as-built oxidized at 1373 K, (c) heat-treated SLM IN738LC oxidized at 1273 K, and (d) heat-treated SLM IN738LC oxidized at 1373 K.

Fig. 8. Weight gain per unit area versus oxidation time for IN738LC oxidized at different temperatures; (a) 1273 K and (b) 1373 K. Square of weight grain per unit area versus oxidation time at (c) 1273 K and (d) 1373 K.

Fig. 9. Temperature dependence of oxidation rate coefficients of the as-built SLM IN738LC.

Fig. 10. Cross-sectional views and element mappings after isothermal oxidation tests for the (a) as-built IN738LC oxidized at 1273 K, (b) as-built IN738LC oxidized at 1373 K, (c) heat-treated IN738LC oxidized at 1273 K, and (d) heat-treated IN738LC oxidized at 1373 K.

Fig. 11. HR-EPMA mapping of the distribution of elements before oxidation test for the (a) as-built SLM IN738LC and (b) heat-treated SLM IN738LC.

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