1 State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083 2 Science and Technology on Advanced High Temperature Structural Materials Laboratory, Beijing Institute of Aeronautical Materials, Beijing 100095 3 Beijing Key Laboratory of Special Melting and Reparation of High-end Metal Materials, Beijing 100083
Cite this article:
Yunsong ZHAO,Jian ZHANG,Yushi LUO,Dingzhong TANG,Qiang FENG. EFFECTS OF Hf ON HIGH TEMPERATURE LOW STRESS RUPTURE PROPERTIES OF A SECOND GENERATION Ni-BASED SINGLE CRYSTAL SUPERALLOY DD11. Acta Metall Sin, 2015, 51(10): 1261-1272.
The effect of Hf on the as-cast, heat-treated microstructures and stress rupture properties under 1100 ℃ and 140 MPa was investigated in four second generation Ni-based single crystal superalloys DD11 with various levels of Hf (0~0.80%, mass fraction) additions. The results indicate that increasing Hf addition resulted in decreasing the solidus and liquidus temperatures, while it enhanced the volume fraction of (γ+γ’) eutectic and MC carbide as well as solidification segregation. The number of micropores reduced significantly and the volume fraction of residual (γ+γ’) eutectic and MC carbide increased after heat treatment as Hf content increased. Compared to the Hf-free alloy, the stress rupture life was observed to increase in the alloys with 0.40%Hf, but dropped in the alloy containing 0.80%Hf. Hf addition increased the elemental partitioning ratio of Re, Mo, Cr, resulting in increasing γ/γ’ misfit and decreasing the spacing of γ/γ’ interfacial dislocation networks. The solution strengthing effect was also improved with the enhanced concentration of Re, Mo and Cr in γ phase in Hf-modified alloys. However, when the Hf content was 0.80% in DD11 alloy, the stress rupture properties was decreased obviously due to high volume fraction of residual (γ+γ’) eutectic and MC carbide in heat-treated microstructures.
Fund: Supported by National High Technology Research and Development Program (Nos.2012AA03-A513 and 2012AA03A511), National Basic Research Program of China (No.2010-CB631201) and Science Foundation of Ministry of Education of China (No.625010337)
Fig.1 OM images of as-cast alloys Hf-1 (a), Hf-2 (b), Hf-3 (c) and Hf-4 (d)
Fig.2 BSE-SEM images of the eutectic region of as-cast alloys Hf-3 (a) and Hf-4 (b)
Alloy
Primary dendrite
Volume fraction / %
arm spacing / μm
Eutectic
Carbide
Micropore
Hf-1
329±12
5.5±2.1
0.08±0.05
0.22±0.05
Hf-2
340±11
8.5±2.5
0.15±0.07
0.10±0.03
Hf-3
347±9
9.6±1.7
0.61±0.05
0.08±0.04
Hf-4
337±11
13.3±2.3
1.20±0.07
0.08±0.05
Table 1 Microstructures characterization of as-cast alloys
Fig.3 Solidification segregation coefficients of alloying elements in the as-cast alloys
Fig.4 DSC heating curves of four as-cast alloys (TL—liquidus temperature, TS—solidus temperature)
Fig.5 OM image (a) and interdendrite region BSE-SEM image (b) of alloy Hf-4 after heat treatment at 1320 ℃ for 6 h and then A.C.
Fig.6 Typical microstructures in the dendrite core of alloys Hf-1 (a) and Hf-4 (b) after fully heat treatment
Alloy
Size of γ’ phase / nm
Volume fraction of γ’ phase / %
Channel width of γ phase / nm
Hf-1
380±90
67.3±5.2
72±40
Hf-2
383±62
65.6±6.3
71±50
Hf-3
391±80
65.2±4.3
70±52
Hf-4
395±71
64.3±7.2
71±45
Table 2 Size and volume fraction of γ’ phase, channel width of γ phase in the dendrite cores of alloys after fully heat treatment
Fig.7 OM images of alloys Hf-1 (a), Hf-3 (b) and Hf-4 (c) after fully heat treatment
Fig.8 OM images of un-etched alloys Hf-1 (a), Hf-3 (b) and Hf-4 (c) after fully heat treatment
Alloy
Eutectic
Carbide
Micropore
Hf-1
1.10±0.12
0.05±0.02
0.62±0.12
Hf-2
1.31±0.11
0.12±0.04
0.50±0.09
Hf-3
2.50±0.14
0.31±0.03
0.31±0.08
Hf-4
5.81±0.15
0.95±0.06
0.18±0.09
Table 3 Volume fractions of residual eutectic, carbides and micropores in the interdendrite regions of alloys after fully heat treatment
Fig.9 Elemental partitioning ratio in γ/γ’ phase of alloys after fully heat treatment
Fig.10 γ/γ’ interfacial dislocation networks in alloys Hf-1 (a), Hf-2 (b), Hf-3 (c) and Hf-4 (d) after stress-rupture under 1100 ℃ and 140 MPa
Fig.11 Relationship of average interfacial dislocation spacing and average stress rupture life of alloys under 1100 ℃ and 140 MPa
Fig.12 OM images (a, c) and BSE-SEM images (b, d) of longitudinal sections of stress ruptured fracture of alloys Hf-1(a, b) and Hf-4 (c, d) under 1100 ℃ and 140 MPa
Fig.13 Schematic diagram of MC carbides blocking the diffusion passageways during solution treatment
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