PSI - Issue 7

L.L. Liu et al. / Procedia Structural Integrity 7 (2017) 174–181 L. L. Liu et al. / Structural Integrity Procedia 00 (2017) 000–000

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on carbide itself, which become origins of fatigue cracks during the loading process [15] . The specimens used in this paper were taken from an actual GH4169 compressor disc of a civil aero-engine. The composition of nickel base GH4169 superalloy used in this study is listed in Table1. The master alloy was produced by vacuum induction melting, electroslag remelting (ESR) and vacuum arc remelting (VAR) technologies. Then, the compressor disc was heat treated by two stages, namely solution and subsequent double aging. The compressor disc was solution heat treated at a temperature of 980 °C and held for 100 min, followed by an air cool to 720 °C. Afterwards, the compressor disc was aging at 720 °C for 520 min, cooling to 620 °C at a rate of 50 °C/h and then held for 480 min, followed by an air cool.

Table1 Composition of GH4169 (wt%).

Nb

Mo

Fe

Cr

Al

Ti

Si

C

Mn

S

B

P

Ni

5.11

3.02

19.46

18.93

0.53

1.03

0.08

0.035

0.03

0.003

0.003

0.006

Bal.

2.2 Experimental Procedure

LCF test of smooth specimens were used to investigate the relationship between the grain size ( d ) and lifetime ( N f ). Geometry of the smooth specimen used in the LCF experiment is illustrated in Fig.1. The configuration of round specimen with 100 mm in length and 6 mm in diameter. The LCF test was carried out at 600 °C using stress controlled with triangular wave loading mode on a Shimadzu-EHF-EM100 servo-hydraulic testing machine. The frequency of the fatigue testing was 0.2 Hz and the stress ratio was kept being 0.1 with the maximum strain of 1%, 0.9%, 0.8%, 0.7% respectively. The specimens which cut from each fatigue test specimen 1cm beneath the fracture surface were used for microstructure observation. Each sample was prepared by standard grinding and polishing procedures, then, etched in a solution (100 ml CH 3 CH 2 OH, 100 ml HCl, 5 g CuCl 2 ) for 90 s. The Image-Pro Plus (IPP) software was used to estimate the grain size based on optical microscope pictures. By SEM on the fracture surfaces, the possible microscopic mechanism of the grain size-dependent LCF behaviour of GH4169 superalloy was observed.

Fig.1 Geometry of the smooth specimen used in the LCF experiment (unit: mm).

3. Result and discussion

3.1 Microstructure and LCF properties

The LCF lifetime under the same test condition has a great dispersion. The microstructures of GH4169 alloy with different fatigue lives are compared in Fig. 2. As shown in picture, there are a large number of short rod-like δ phases at the grain boundaries, and the grain sizes of different samples are different obviously. The average grain sizes which were defined as an equivalent diameter and the area fraction of δ phase of the smooth specimens were measured by IPP software with a result that the average grain size of each specimen is from 4.99um to 10.04um and the area fraction of δ phase is from 7.83% to 18.89%. The specimens ’ experimental lifetime and grain size divided by average lifetime and the average grain size under the same load level respectively, and the relationship between the experimental lifetime and the mean grain size after dimensionless treatment is shown in Fig.3 (a). The same