Effect of Aging Heat Treatment on the Microstructure and Creep Properties of the Cast Ni-Based Superalloy at Low Temperature

doi:10.1007/s40195-018-0827-0

Abstract

Abstract:

Effect of aging heat treatment on the grain boundary microstructure and creep properties of a cast Ni-based superalloy was investigated. With increasing aging temperature from 750 to 1000 °C, M₂₃C₆ carbides along the grain boundaries evolve from fine distributed block, continuous film into the coarse discrete block. Moreover, the M₂₃C₆ carbides are mainly enveloped within γ′ layers along grain boundaries during 1000 °C aging. Creep rupture lifetime and elongation at 760 °C and 645 MPa are improved with increasing the aging temperature. In particular, the creep rupture lifetime of the specimens aging at 1000 °C is one order of magnitude higher than that of the specimens aging at 750 °C. The enhancement of ductility induced by the γ′ envelopes plays a significant role in the improvement of creep rupture lifetime.

Key words: Superalloys, Creep, Grain boundary, Carbides, Aging heat treatment

Xiang-Wei Li, Li Wang, Xin-Gang Liu, Yao Wang, Jia-Sheng Dong, Lang-Hong Lou. Effect of Aging Heat Treatment on the Microstructure and Creep Properties of the Cast Ni-Based Superalloy at Low Temperature[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(5): 651-658.

Figures/Tables 11

Table 1 Heat treatment conditions of test bars in the present experiment

	Solution and first aging heat treatment	Second aging heat treatment
AHT-1	1235 °C/3 h, AC 1080 °C/4 h, AC	None
AHT-2		750 °C/16 h, AC
AHT-3		850 °C/16 h, AC
AHT-4		1000 °C/16 h, AC

Fig. 1 Morphologies of GB microstructures in the a-c as-cast and d heat-treated samples: a, d SEM images of M23C6 carbides, b STEM images and selected area diffraction pattern of M23C6 carbides, c composition of M23C6 carbides measured by EDS

Fig. 2 Secondary electron images of GB M23C6 during aging heat treatments: a AHT-1, b AHT-2, c AHT-3, d AHT-4

Table 2 Average size of M23C6 carbides in the samples with different heat treatment histories

	AHT-2	AHT-3	AHT-4
Average size (μm)	0.17 ± 0.06	0.36 ± 0.05	0.70 ± 0.11

Fig. 3 Secondary electron images of γ′ after different heat treatments: a AHT-1, b AHT-2, c AHT-3, d AHT-4

Table 3 Average stress-rupture lifetime of specimens aged at different heat treatments

Heat treatments	Average lifetime (h)	Elongation (%)
AHT-1	< 1	1.8 ± 1.3
AHT-2	8 ± 4	2.6 ± 0.2
AHT-3	40 ± 9	3.2 ± 1.0
AHT-4	117 ± 37	4.2 ± 0.9

Fig. 4 Creep curves of the specimens with different heat treatments at 760 °C/645 MPa: a creep strain-lifetime curve, b creep strain rate-strain

Fig. 5 SEM images of fracture surfaces of specimens tested at 760 °C and 645 MPa: a AHT-1, b AHT-2, c AHT-3, d AHT-4

Fig. 6 Images of longitudinal section near the rupture site in the stress-rupture specimens tested at 760 °C and 645 MPa: a AHT-1, b AHT-4

Fig. 7 Relationship among minimum creep rate, creep strain and the time to onset of tertiary creep (tt) for the specimens with different heat treatments

Fig. 8 Relationship between the time to fracture (tf) and the time to onset of tertiary creep (tt) in the specimens with different heat treatments (the dash line is the fitting line of tf/tt = 1.5)

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