Microcrack Nucleation and Propagation Investigation ofInconel 740H Alloy Under In SituHigh Temperature Tensile Test

doi:10.11900/0412.1961.2017.00218

Acta Metall Sin

2017, Vol. 53

Issue (12): 1627-1635 DOI: 10.11900/0412.1961.2017.00218

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Microcrack Nucleation and Propagation Investigation ofInconel 740H Alloy Under In SituHigh Temperature Tensile Test

Jin WANG¹, Yuefei ZHANG¹(

), Jinyao MA¹, Jixue LI², Ze ZHANG²

1 Institute of Microstructure and Property of Advanced Materials, Beijing University of Technology, Beijing 100124, China
2 Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310058, China

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Jin WANG, Yuefei ZHANG, Jinyao MA, Jixue LI, Ze ZHANG. Microcrack Nucleation and Propagation Investigation ofInconel 740H Alloy Under In SituHigh Temperature Tensile Test. Acta Metall Sin, 2017, 53(12): 1627-1635.

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Abstract

As a result of increasing energy demands and accelerated environmental problems, there is an urgent need to improve the thermal efficiency of ultra supercritical (USC) power plants. To achieve this goal, advanced ultra-supercritical (A-USC) technologies with the main steam temperature of 700~750 ℃ and pressure of 35 MPa have been developed quickly in recent years. One of the most promising candidate Ni-based superalloys for the main steam pipe of 700 ℃ ultra-supercritical coal-fired power plants is Inconel 740H, which is a modified version of Inconel 740 developed by Special Metals Corp (SMC). Compared with IN740, the Ti/Al ratio in IN740H is lowered in order to stabilise the microstructure at long ageing times. In addition, the Nb content is lowered to improve the weldability. In this work, the microstructure evolutions, the nucleation and propagation mechanisms of microcracks in the nickel base superalloy Inconel 740H at 750 ℃ high temperature were studied by the self-developed in situ high temperature tensile stage inside a SEM. The results showed that under the uniaxial tensile stress at 22 ℃ room temperature and 750 ℃ high temperature conditions, the grain boundaries of Inconel 740H alloy are always the most primary crack sources. The strength of grain boundaries is higher than that of grains under the room temperature, and the microcracks will be nucleated at the grains as well, but the relative strength of grain boundaries will be weaken under the high temperature, which makes the microcracks tend to nucleate at grain boundaries. The experimental results also showed that the influence of high temperature on the mechanical properties is very significant, the high temperature reducing the activate energy of slip and weakening the strength of the grain boundaries, so that more slip systems activated and the grain boundaries occurring bending and sliding deformation, so further enhance the ability of plastic deformation of alloy. However, the reduction of relative strength of alloy grain boundaries leads to microcracks nucleation and propagation more easily from grain boundaries and lower the yield strength and tensile strength of alloy.

Key words: Inconel 740H in situ SEM high temperature tensile microcrack nucleation and propagation

Received: 07 June 2017

ZTFLH:

TG132.3

Fund: Supported by National Key Scientific Instrument and Equipment Development Project (No.11327901)

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	Jin WANG
	Yuefei ZHANG
	Jinyao MA
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	Ze ZHANG

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https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00218 OR https://www.ams.org.cn/EN/Y2017/V53/I12/1627

Fig.1 Dimensions of an in situ tensile specimen(unit: mm)

Fig.2 In situ high temperature tensile stage (a) and schematic of the cross section of the main working parts (b)

Fig.3 OM image of Inconel 740H alloy after standard heat treatment (a), histogram of grain size (within the observation gauge) (b), SEM images of grain boundaries and twin boundaries (c, d) and SEM images of (NbTi)C and Cr₂₃C₆ (e, f) (T—twin, TB—twin boundary)

Fig.4 Force-displacement curves of Inconel 740H alloy at 22 and 750 ℃

Table 1 Mechanical properties of Inconel 740H alloy in situ tensile specimens at 22 and 750 ℃

Fig.5 Evolutions of microcrack nucleation of Inconel 740H alloy at 22 ℃ under 0 N (a), 599 N (b), 628 N (c) and 634 N (d) (SB—slipping band)

Fig.6 Evolutions of microcrack propagation of Inconel 740H alloy at 22 ℃ under 642 N (a), 507 N (b), 459 N (c) and rupture (d) (r₁~r₄ show the orientations, G_a and G_b show the grains, DB—deformation band)

Fig.7 Evolutions of microcrack nucleation of Inconel 740H alloy at 750 ℃ under 0 N (a), 354 N (b), 612 N (c) and 611 N (d) (Fig.7b is the enlarged view of rectangle area in Fig.7a which shows the location of crack a, and inset in Fig.7b shows the enlarged view of crack a)

Fig.8 Evolutions of microcrack propagation of Inconel 740H alloy at 750 ℃ under 600 N (a), 504 N (b), 472 N (c) and rupture (d)

Fig.9 Schematics of microcrack nucleation and propagation in Inconel 740H alloy at 22 ℃ (a~c) and 750 ℃ (d~f) under uniaxial tensile stress (σ—tension stress, τ—shear stress) (a, d) microcrack nucleation (b, e) microcrack propagation (c, f) crack connection

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