Creep behaviors and mechanisms of Inconel718 and Allvac718plus

Abstract

To investigate creep mechanisms of Inconel718 and Allvac718Plus, creep behaviors of two alloys are studied under the temperature of 700 °C and three applied stresses (560 MPa, 650 MPa and 720 MPa) by using multiple microstructural analysis methods, including scanning electron microscope (SEM), emission scanning electron microscope (FSEM) and transmission electron microscopy (TEM). Result shows that steady-state region is not observed in these two alloys. Creep curves of two alloys are composed of primary region and tertiary region. Tertiary region occupies a dominant position. Ductile dimples are observed on creep fracture surface, indicating creep mechanisms of two alloys are the formation of creep voids. In this study, the formation of creep voids is found to be correlated with three factors which are dislocation multiplication, dislocation motion and dislocation obstacles. Inconel718 has higher dislocation multiplication rate, larger dislocation motion rate, more δ phases and less bands than Allvac718Plus. Therefore, Inconel718 has higher creep voids rate than Allvac718Plus, leading to the result that Inconel718 has shorter creep life than Allvac718Plus.

Introduction

Nickel-based superalloys are an unusual class of alloys which can maintain high corrosion resistance and mechanical strength at high temperature. These materials are widely employed in aerospace, nuclear and gas turbine industries [1], [2]. The unique property of nickel-based superalloys is attributed to strengthening phases. Nickel-based superalloys are predominantly strengthened by γ"-Ni₃Nb and γ′-Ni₃(Al,Ti) precipitates [3], [4]. Inconel 718 (hereinafter referred to as alloy 718) is one of the important nickel-based alloys, which is primarily strengthened by γ"-Ni₃Nb and γ′-Ni₃(Al,Ti) precipitates [5], [6]. Previous studies have found that γ" precipitates are susceptibly transformed to δ phases above 649 °C, leading to thermal instability [7], [8]. Therefore, alloy 718 cannot be used above 649 °C. To obtain an affordable alloy with increased temperature capability, Allvac718Plus (hereinafter referred to as alloy 718Plus) strengthened primarily by γ′ precipitates is developed by ATI Allvac [9]. Compared with alloy 718, the Al + Ti content, Co content, B content and Al/Ti ratio are improved in alloy 718Plus [10], [11]. According to the study [11], creep rupture life and thermal stability increase dramatically as the Al/Ti ratio increases. The study also shows that creep rupture life and thermal stability are improved with the addition of Co. These two properties peak at the 10% Co level. In addition, the improvement of the B content will also improve creep rupture life and creep resistance through strengthening grain boundaries. Based on above researches, alloy 718Plus has excellent ability to maintain thermal stability. Its maximum use temperature is improved to 704 °C [10], [11].

Creep behaviors of two alloys should be investigate to indicate the difference of service performance between alloy 718 and alloy 718Plus. There are lots of publications studying the creep behavior of alloy 718. The study suggests that microcrack initiated at grain boundaries will propagate to become main crack during creep, leading to fracture [12]. It implies that the creep mechanism of alloy 718 is correlated with the formation of cracks. Thus crack growth needs to be investigated under different creep tests. The experiment shows that at low temperatures crack growth is result of a balance of two competing processes, diffusion of point defect and creep deformation process [13]. However, the creep deformation mechanism is changed to dislocation creep at high temperatures [14]. Meanwhile, δ phases are found to be correlated with the formation of cracks [15], [16]. Therefore, it can be speculated that the interaction between dislocations and δ phases might lead to the formation of cracks at high temperatures. In addition, recent studies have found that the twinning mechanism is also a deformation mechanism during creep [17], [18]. The appearance of twins not only promotes creep deformation but also causes the formation of cracks. Although much studies have been done on the investigation of the creep mechanism of alloy718, the formation of cracks still needs to be further explored. More importantly, there is no discussion about the difference of creep mechanisms between alloy 718 and alloy 718Plus.

Bearing in mind above considerations, this paper investigates creep behaviors and creep mechanisms of alloy 718 and alloy 718Plus. Factors which can affect the formation of creep voids are also discussed. As a result, a model about the formation of creep voids is established. The model contains three factors, including dislocation multiplication, dislocation motion and dislocation pile-ups. Based on the model, the reason that alloy 718Plus has better creep resistance than alloy 718 is revealed.