Effect of Heat Treatment on the Microstructure and Mechanical Properties of the Modified 718 Alloy

doi:10.1007/s40195-018-0790-9

Abstract

Abstract:

M718 alloy with an extra high Mo content of 7.50 wt% which reduced Nb addition and increased Al and Ti additions within the composition specifications of 718 alloy has been designed to increase the service temperature of 718 alloy. And the effect of the heat treatment on the microstructure and mechanical properties of M718 alloy has been investigated in this study. The results showed that Laves phase precipitated on the grain boundaries of M718 alloy instead of δ-Ni₃Nb phase in 718 alloy, and γ′′ and γ′ phases precipitated in the matrix of M718 alloy as that in 718 alloy. Increasing the solution temperature from 960 to 1050 °C noticeably reduced the intergranular precipitation of Laves phase. The precipitation of Laves phase was appropriate at 1020 °C for improving the grain boundary cohesion. Increasing the two-stage aging temperatures markedly increased the sizes of γ′′ and γ′ phases. As a result, the strength of M718 alloy increased.

Key words: Heat treatment, Mo, 718 alloy, Microstructure, Mechanical properties

Da-Wei Han, Lian-Xu Yu, Fang Liu, Bin Zhang, Wen-Ru Sun. Effect of Heat Treatment on the Microstructure and Mechanical Properties of the Modified 718 Alloy[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(11): 1224-1232.

Figures/Tables 13

Table 1 Chemical composition of the tested alloys (wt%)

Alloy	C	Ni	Cr	Al	Ti	Nb	Mo	P	B	Fe
M718	0.04	53.0	19.0	0.70	1.10	4.80	7.50	0.002	0.004	Bal.
718	0.04	53.0	19.0	0.50	1.00	5.30	3.10	0.002	0.004	Bal.

Table 2 Heat treatment preformed on the tested alloys

HT No.	Solution	Aging
1	960 °C?×?1 h, AC	720 °C?×?8 h, cooled at 55 °C/h to 620 °C, 620 °C?×?8 h, AC
2	960 °C?×?1 h, AC	750 °C?×?8 h, cooled at 55 °C/h to 620 °C, 620 °C?×?8 h, AC
3	1020 °C?×?1 h, AC	750 °C?×?8 h, cooled at 55 °C/h to 620 °C, 620 °C?×?8 h, AC
4	1050 °C?×?1 h, AC	750 °C?×?8 h, cooled at 55 °C/h to 620 °C, 620 °C?×?8 h, AC

Fig. 1 OM micrographs of M718 alloy treated by a HT1, b HT3, c HT4, d 718 alloy treated by HT1

Fig. 2 SEM micrographs of M718 alloy treated by HT1 a, HT3 b, HT4 c, 718 alloy treated by HT1 d

Fig. 3 SEM micrograph of the grain boundary precipitates a and corresponding EDS spectrum b of M718 alloy

Fig. 4 Morphology of Laves phase on the grain boundaries in M718 alloy a and corresponding SAED pattern b

Fig. 5 TEM images of γ″ phase and γ′ phase of M718 alloy treated by HT1 a, HT3 b, 718 alloy treated by HT1 c, d SAED pattern taken from the [001] matrix zone axis of M718 alloy

Table 3 Tensile properties of the tested alloys at room temperature and 680 °C

Alloy	Heat No.	Room temperature				680 °C
Alloy	Heat No.	UTS (MPa)	YS (MPa)	EL (%)	RA (%)	UTS (MPa)	YS (MPa)	EL (%)	RA (%)
M718 alloy	HT1	1342	1005	17.0	15.9	1102	836	28.0	24.0
	HT2	1378	1081	19.3	19.1	1149	890	18.3	21.2
	HT3	1413	1121	20.7	30.1	1178	922	25.3	16.6
	HT4	1414	1120	26.0	35.0	1130	895	14.0	23.0
718 alloy	HT1	1479	1201	25.3	43.4	1121	975	19.4	16.1

Fig. 6 Fractographs of the tensile ruptured specimens at room temperature: a M718 alloy treated by HT3, b 718 alloy treated by HT1

Fig. 7 Fractographs of the tensile ruptured specimens at 680 °C of M718 alloy treated by HT3 a, 718 alloy treated by HT1 b, c

Table 4 Stress rupture properties of the tested alloys at 680 °C and 725 MPa

Alloy	HT No.	Life (h)	Elongation (%)
M718 alloy	HT1	22.6	16.8
	HT2	22.8	22.3
	HT3	22.6	21.0
	HT4	23.1	11.4
718 alloy	HT1	4.6	21.6

Fig. 8 Fractographs of the tested alloys stress ruptured at 680 °C and 725 MPa: a M718 alloy treated by HT2, b, c 718 alloy treated by HT1

Fig. 9 Microstructures of cross section of the tested alloys ruptured at 680 °C and 725 MPa: a, c M718 alloy treated by HT1, b, d 718 alloy treated by HT1

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