1 Department of Materials Physics and Chemistry, School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China 2 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article:
Hongwei ZHANG,Xuezhi QIN,Xiaowu LI,Lanzhang ZHOU. Incipient Melting Behavior and Its Influences on the Mechanical Properties of a Directionally Solidified Ni-Based Superalloy with High Boron Content. Acta Metall Sin, 2017, 53(6): 684-694.
A new directionally solidified Ni-based superalloy is developed for industrial gas turbine applications, which has high strength and excellent hot corrosion resistance at high temperatures. The high strength of the alloy is primarily derived from precipitation hardening by ordered L12γ′ phase. To achieve a uniform distribution of precipitated γ′ particles for optimized mechanical properties, the suitable heat treatments are used. However, the heat treatment temperature in Ni-based superalloys is limited by the problem of incipient melting. Incipient melting microstructrue evolution during heat treatment has been hardly reported. Therefore, the behaviors of incipient melting and its effect on mechanical properties in the new directionally solidified superalloy DZ444 with high boron have been investigated in this work. The results show that some interdendritic regions of the as-cast DZ444 sample exhibit many of γ′/γ eutectic, MC carbides and multi-phase eutectic-like constituent which are composed of boride, Ni5Hf and η phases. During solution treatments, incipient melting does not occur in boride or Ni5Hf phase with low melting point firstly, but appears in γ matrix around multi-phase eutectic-like constituent which is affected significantly by borides. Compared to DZ444 alloy with the normal boron content, incipient melting occurs at the lower temperature in the range between 1160 ℃ and 1170 ℃. Incipient melting can occur significantly with the increase of the solid solution temperature or time. Incipient melting consists of typical γ dentrites and a lot of tiny precipitation particles after the water quenching (WQ) method following solution treatment. However, incipient melting forms multi-phase eutectic-like constituent, γ matrix and γ′/γ eutectic successively during air cooling (AC) following solution treatment, and the morphology of multi-phase eutectic-like constituent is similar to that of as-cast alloy. Firstly, a so-called incipiently melted circle (IMC) forms around multi-phase eutectic-like constituent; with the increase of the solid solution temperature or time, IMC extends inwards which makes γ matrix and multi-phase eutectic-like constituent in this circle melt successively. Finally, a incipiently melted pool forms gradually. Incipient melting is limited to the IMC below 1200 ℃ and the area of incipient melting changes with temperature or time correspondingly. However, incipiently melted region (IMR) expands outwards continuously which makes γ matrix outside the incipiently melted circle melt when the temperature is higher than 1210 ℃. Especially, IMR swallows up plenty of γ matrix, and many matrix islands, regions unmelted, exist in IMR above 1250 ℃ which destroys the continuity of the matrix significantly. The incipient melting has a minor effect on the tensile properties, nevertheless, decreases the creep-rupture properties remarkably. The degradation of mechanical properties mainly results from the increasing of the incipient melting area fraction and size.
Fund: Supported by National Natural Science Foundation of China (No.51001101) and National High Technology Research and Development Program of China (No.2012AA03A501)
Fig.1 SEM (a~d) and TEM (e, f) images of as-cast microstructures of DZ444 alloy with high boron (specimens of Figs.1a and b were etched by electrolyte A which removes the γ’ precipitates in the matrix , specimens of Figs.1c and d were etched by electrolyte B which removes the γ matrix in the alloy) (a) dendrite morphology (b) γ/γ’ eutectic (c) borides (d) multi-phase (generally including γ, η, boride and Ni5Hf phases) eutectic-like constituent (e) TEM image of boride (Inset shows SAED pattern of boride) (f) TEM image of η and Ni5Hf phases (Insets show SAED patterns of Ni5Hf and η)
Fig.2 BSE (backscattered electron) image (a) and EPMA elemental mapping results of multiphase eutectic-like constituent for Cr (b), Mo (c), W (d), B (e), C (f), Al (g), Co (h), Ni (i), Hf (j), Ti (k) and Ta (l)
Phase
Al
Ti
Cr
Co
Ni
Mo
Hf
Ta
W
MC(1)
0.6
47.9
2.0
1.2
6.5
7.0
3.3
0.2
31.3
MC(2)
0.6
12.4
4.2
2.9
13.4
2.5
48.4
4.4
11.1
η
3.1
8.9
4.2
7.5
63.3
0.6
8.9
1.5
1.9
Boride(1)
0.2
2.2
42.1
3.4
10.2
15.9
0
0
26.1
Boride(2)
0.3
2.7
23.8
2.9
11.6
18.6
1.0
0
39.2
Ni5Hf
0.7
2.6
3.7
8.0
56.3
0.8
18.9
5.9
3.1
Table 1 Compositions of various phases around γ/γ′ eutectics in the as-cast alloy (mass fraction / %)
Fig.3 SEM images of IMR in the DZ444 alloy with high boron solid solution treated at 1170 ℃ for 5 min (a), 10 min (b) and 30 min (c) and then cooled by WQ (Specimens were etched with electrolyte A, IMR—incipiently melted region, IMC—incipiently melted circle, WQ—water quenching)
Fig.4 SEM images of IMR in the DZ444 alloy with high boron solid solution treated at different temperatures for different times and then cooled by WQ (Specimens were etched with electrolyte A) (a) 1180 ℃, 5 min (b) 1190 ℃, 10 min (c) 1200 ℃, 10 min (d) 1200 ℃, 30 min
Fig.5 SEM images of IMR in the DZ444 alloy with high boron solid solution treated at different temperatures for different times and then cooled by WQ (Specimens were etched with the electrolyte B) (a) 1180 ℃, 5 min (b) 1190 ℃, 10 min (c) 1200 ℃, 10 min (d) 1200 ℃, 30 min
Fig.6 SEM images of IMR in the DZ444 alloy with high boron solid solution treated at different temperatures for different times and then cooled by AC (Specimens were etched with electrolyte B, AC—air cooling) (a) 1180 ℃, 5 min (b) 1190 ℃, 10 min (c) 1200 ℃, 10 min (d) 1200 ℃, 30 min
Fig.7 Low (a~c) and high (d~f) magnified SEM images of IMR in the specimens solution treated at 1210 ℃(a, d), 1230 ℃ (b, e) and 1250 ℃ (c, f) for 2 h and then followed by AC (Specimens were etched with electrolyte B)
Fig.8 SEM images of IMR in the specimens solution treated at 1210 ℃ (a), 1230 ℃ (b) and 1250 ℃ (c) for 2 h and then followed by AC (Specimens were etched with electrolyte A)
Solution temperature / ℃
Area fraction / %
Size / μm
1210
5.3
49.8
1230
7.1
52.0
1250
18.6
88.5
Table 2 Volume fraction and size changes of IMR with different temperatures
Fig.9 Morphology of IMR after solution treatment at 1210 ℃, 2 h, AC and then high temperature ageing at 1080 ℃, 4 h, AC (Specimens were etched with the electrolyte B)
Heat treatment
Tensile property
Stress-rupture property
σb / MPa
σ0.2 / MPa
δ / %
φ / %
t / h
δ / %
HT1
707
457
29.8
40.3
55
30.6
HT2
690
460
27.3
35.0
44
27.8
HT3
665
447
24.5
32.3
22
13.5
Table 3 Tensile properties at 900 ℃ and stress-rupture properties at 930 ℃, 275 MPa of DZ444 alloy with high B content under different heat treatments
Fig.10 SEM images of cracks mainly initiated in IMR of tensile ruptured specimens at 900 ℃ (HT1) (a) and after stress-rupture test (HT3) (b) (Specimens were etched with electrolyte A)
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