Microstructure and properties of modified and conventional 718 alloys

LIU Fang (刘 芳)^{1, 2}, SUN Wen-ru(孙文儒)², DU Jin-hui(杜金辉)³, DONG Jian-xin(董建新)⁴,
GUO Shou-ren(郭守仁)², YANG Hong-cai(杨洪才)¹, HU Zhuang-qi(胡壮麒)²

1. School of Materials and Metallurgy, Northeastern University, Shenyang 110006, China;

2. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China;

3. Central Iron and Steel Research Institute, Beijing 100081, China;

4. University of Science and Technology Beijing, Beijing 100083, China

Received 28 July 2006; accepted 15 September 2006

Abstract: Continuing the effort to redesign IN718 alloy in order to provide microstructural and mechanical stability beyond 650 ℃, IN718 alloy was modified by increasing the Al, P and B contents, and the microstructure and mechanical properties of the modified alloy were compared with those of the conventional alloy by SEM and TEM. The precipitation of the grain boundaries of the two alloys is different. The Cr-rich phase, Laves phase and α-Cr phase are easily observed in the modified alloy. The γ″ and γ′ phases in the modified alloy are precipitated in a “compact form”. The tensile strengths of the modified alloy at room temperature and 680 ℃ are obviously higher than those of the conventional one. The impact energy of the modified alloy is only about half of that of the conventional alloy. Ageing at 680 ℃ up to 1 000 h lowers the tensile properties and impact energy of both the conventional and modified 718 alloys, except increasing the ductility at 680 ℃. It is concluded that the modified alloy is more stable than the conventional one.

Key words: IN718 alloy; microstructure; mechanical properties

1 Introduction

IN718 alloy is mainly strengthened by γ″ phase and a small amount of γ′ phase. The γ″ phase has a higher coherent strain with the γ matrix, giving the alloy high yield strength, good comprehensive properties and broad applications. However, the γ″ phase is relatively unstable and the maximum use temperature is 650 ℃ [1-2]. Many efforts have been made to improve the stability and working temperature of IN718 alloy due to its excellent mechanical and processing properties [3-4]. Previous studies by HORTEN et al [5-8] have shown that the minor elements P and B have a dramatic effect on the elevated temperature creep and stress rupture properties of 718 alloy. Recent work has shown that the P and B modification does not noticeably improve the thermal stability of 718 alloy. A natural development for further improving the elevated temperature properties of 718 alloy will be to combine both the major and minor element approaches[9-13]. In this study, the content of Al, P and B were adjusted in order to improve the use temperature of IN718 alloy, and the microstructure and some mechanical properties of the conventional and modified 718 alloys were compared.

2 Experimental

The compositions of conventional and modified 718 alloys are listed in Table 1. The contents of Al, P and B of the modified 718 alloy were increased to the designed levels, and those of other elements were kept at the same level of IN718 alloy. The alloys were melted by VIM(vacuum induction melting), and homogenized and forged into square bars with 40 mm edge. The bars were given a heat treatment: 960 ℃×1 h, AC; 720 ℃×8 h, furnace cooled at 50 ℃/h to 620 ℃, 620 ℃×8 h, AC. The heat treated bars were then aged at 680 ℃ for 100, 300, 500 and 1 000 h, respectively. The standard tensile samples with a gauge of 5 mm in diameter and 50 mm in length, and standard impact samples with a ‘U’ notch were machined. The tensile properties at room temperature and 680 ℃, and impact toughness at room temperature were measured.

Table 1 Chemical composition of conventional and modified 718 alloys (mass fraction, %)

3 Results and discussion

It is shown in Fig.1 that the tensile strength of the modified 718 alloy is much higher than that of IN718 alloy, while the ductility of the modified alloy is much lower than that of IN718 alloy. With the increase of ageing time, the tensile strength of both the alloys at room temperature and 680 ℃ is decreased. The ductility of both the alloys at room temperature is decreased, while that at 680 ℃ is increased.

It is shown in Fig.2 that the ‘U’ Charpy energy of IN718 alloy is much higher than that of the modified 718 alloy. For both the alloys, the ‘U’ Charpy energy drops rapidly when exposed at 680 ℃ for 500 h, but keeps constant up to 1 000 h. At ageing time of 1 000 h, the impact energy of the modified 718 alloy is only about one third of that of IN718 alloy.

Fig.1 Variation of tensile properties of IN718 and modified 718 alloys with ageing time: (a) Tensiled at room temperature; (b) Tensiled at 680 ℃

Fig.2 Variation of ‘U’ Charpy energies of IN718 and modified 718 alloys with ageing time at 680 ℃

As shown in Fig.3, the precipitates at the grain boundaries of IN718 alloy extend into the grain interior (Fig.3(a)), while those of the modified 718 alloy are all confined at the grain boundaries (Fig.3(b)). After ageing at 680 ℃ for 1 000 h, the grain boundary precipitations of both the alloys are increased markedly.

As shown in Fig.4, a large number of γ″ particles are precipitated in the matrix of IN718 alloy (Fig.4(a)), while the γ″ precipitation is largely reduced and the γ′ precipitation greatly increased in the modified 718 alloy, and the γ′ and γ″ phases are precipitated in a “compact” form in the modified alloy (Fig.4(b)). After ageing at  680 ℃ for 1 000 h, the precipitates in both the IN718 and modified 718 alloys grow markedly (Figs.4(c) and (d)).

The content of Al is increased from 0.45% of IN718 alloy to 1.24% of the modified 718 alloy, and the other main alloying elements are not changed, this will increase the precipitation of γ′ phase and the strength of the modified alloy.

The d-Ni₃Nb phase precipitation consumes large amount of Nb and a γ″?free film is generally formed on the surface of d rods. This film with high ductility is helpful to resist the propagation of grain boundary cracks. As a result, the ductility of IN 718 alloy is increased. The increment of Al addition reduces the d phase precipita- tion and increases the precipitation of the harmful phase such as Laves and α-Cr phases, so the ductility of the modified 718 alloy is reduced greatly. As shown in Fig.5, IN718 alloy exhibits a transgranular fracture mode when tensiled at room temperature, while the modified 718 alloy exhibits an intergranular fracture mode.

Fig.3 Morphologies of grain precipitates of IN718 and modified 718 alloys: (a) IN718 alloy as-heat treated; (b) Modified 718 alloy as-heat treated; (c) IN718 alloy aged at 680 ℃ for 1 000 h; (d) Modified 718 alloy aged at 680 ℃ for 1 000 h

Fig.4 TEM images of g′ and g″ particles of IN718 and modified 718 alloys: (a) IN718 alloy as-heat treated; (b) Modified 718 alloy as-heat treated; (c) IN718 alloy aged at 680 ℃ for 1 000 h; (d) Modified 718 alloy aged at 680 ℃ for 1 000 h

  Fig.5 Fractographs of IN718 and modified 718 alloys tensiled at room temperature: (a) IN718 alloy as-heat treated; (b) Modified 718 alloy as-heat treated

4 Conclusions

1) The tensile strengths at room temperature and 680 ℃ of modified 718 alloy are noticeably higher than those of IN718 alloy, but the tensile ductility and impact toughness are largely lower than those of IN718 alloy.

2) Ageing at 680 ℃ reduces the tensile strength at room temperature and 680 ℃, and the impact toughness at room temperature of both IN718 and modified 718 alloys. Comparatively, ageing at 680 ℃ reduces the tensile ductility of IN718 and modified 718 alloys at room temperature, but increases the tensile ductility of the two alloys at 680 ℃.

3) The increment of Al causes the precipitation of γ″ and γ′ with a ‘compact structure’ in the matrix, reduces the δ precipitation and increases the Laves and α-Cr precipitation in the modified 718 alloy.

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(Edited by CHEN Wei-ping)

Foundation item: Projects(50271072, 50471083) supported by the National Natural Science Foundation of China

Corresponding author: LIU Fang; Tel: +86-24-23971325; E-mail:lpf750413@163.com