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The influence of P on tensile properties at room temperature and 650°C as well as the effects of P on rupture properties at 650°C/690 MPa of GH4169 alloy with standard heat treatment (STD) and direct aging heat treatment (DA) were investigated in this paper. Mechanism of the influence of P was also undertaken with microstructure and fractograph analysis. The results show that P content has little effect on tensile properties of alloys. While the stress rupture life improves significantly with an increase of P content due to grain boundary segregation of P increasing grain boundary bonding force. And P has less influence on stress rupture life of GH4169 alloy as STD than that as DA, which may be relevant to the enrichment of P on δ phase interface. Increasing P content from 0.006% to 0.019%, the stress rupture failure mode shows mainly transgranular fracture, decreased fraction of fine microvoids and deep dimple structure arises gradually. Introduction Alloy GH4169 consists mainly of a face centered cubic (fcc) matrix and three major intermetallic phases, namely the metastable γ′′ (DO22 structure) phase and γ' (L12 structure) phase and the equilibrium δ (DOa structure) phase. It has high strength, high ductility, good antioxidation capability and excellent fabrication characteristics below 650°C. Due to these attractive properties, it is the most successful and popular alloy in high-temperature field, widely used in aerospace, power generation and petrochemical industry and other key areas [1]. As domestic economy and national defense develop, there arises a new demand to advance the performance of aircraft engine, especially to improve its reliability and prolong its service life. GH4169 alloys have been extensively used as an important structural material in aero-engines such as disks, blades, and shafts, so it is of great importance to increase stress rupture life of this alloy [2]. Many researchers have indicated that increasing P content properly will improve mechanical properties of alloy GH4169 significantly, especially stress rupture life, contrary to a past view that regarded P as a detrimental element which caused grain boundary embrittlement. But investigators usually focus on GH4169 alloy as STD when they study the effect of P, and little attention has been paid to DA GH4169 alloy. There is still some controversy to the mechanism of P [3-7]. This paper studies the rule of P on tensile and stress rupture properties of GH4169 alloy with different heat treatment. It also discusses the effect mechanism of P, and provides reference to improve the performance of GH4169 alloy. Experimental procedure The ingots were vacuum induction melted (VIM) with each weight of 50 Kg. The chemistry of these blanks is listed in Table 1. P level covers a range of 0.006% to 0.019% in weight percent and is the only apparent variable among compositions. All test alloys were homogenized for 24 hrs at 1160°C and for 72 hrs at 1190°C and rolled to 18mm diameter bars. Specimen blanks for tensile and stress rupture tests were cut from the bars and then given the following different heat treatment. One kind is standard heat treatment: solutionized 1439 2nd International Conference on Electronic & Mechanical Engineering and Information Technology (EMEIT-2012) Published by Atlantis Press, Paris, France. © the authors at 960°C for 1hr, air cooled and then aged for 8 hrs at 720°C, furnace cooled to 620°C, held at 620°C for 8 hrs, air cooled. The other kind is direct aging heat treatment: aged for 8 hrs at 720°C, furnace cooled to 620°C, held at 620°C for 8 hrs, air cooled. These test samples with different heat treatment were then electrolytically etched in 80ml HCl+13ml HF+7ml HNO3 solution at a voltage of 1.6 V for about five seconds. Their microstructure was examined with optical microscopy (OM) and scanning electron microscopy (SEM). Tensile tests were performed at room temperature and 650°C, and stress rupture tests were conducted in air at 650°C and at constant initial applied stress level of 690 MPa. The fracture surface of stress rupture test specimens was also observed by SEM. Table 1. Chemical composition of the investigated GH4169 alloys (wt. %) Alloy No. P C Cr Ni Mo Nb Ti Al B Fe 1 0. 006 0.028 19.03 52.57 3.06 5.39 0.99 0.54 0.0035 Bal. 2 0.0092 0.029 18.94 52.40 3.03 5.39 0.98 0.54 0.0032 Bal. 3 0.0110 0.027 18.95 52.48 3.07 5.41 0.99 0.55 0.0035 Bal. 4 0.0190 0.027 19.01 52.42 3.07 5.39 0.98 0.52 0.0036 Bal. Results The typical optical microstructure of samples before heat treatment is shown in Fig. 1. The alloys contain uniform equiaxed grains about ASTM10 level, and visually no difference exists, precluding possible interference caused by original grain difference. Fig.1 OM micrographs of the as-rolled GH4169 alloys (a) alloy No. 1 (b) alloy No.2 (c) alloy No.3 (d) alloy No.4 Microstructure The microstructure of test alloys with different heat treatment was shown in Figures 2 and 3, representing DA and STD respectively. Owing to merely direct aging between 620°C and 720°C, there is almost no δ phase in grains or at boundaries and the microstructure was basically the same as the rolled (Fig. 2a-d). However, when solutionized at 960°C, a lot of short rod-like δ phases precipitated at grain boundaries, whose quantity and morphology present no apparent relationship with P content (Fig. 3a-d). (a) (b)

Effects of Phosphorus on tensile and stress rupture properties of GH4169 alloy