Phase transformations during continuous cooling in Inconel 718 alloys manufactured by laser powder bed fusion and suction casting

Highlights

• CCT diagrams of Inconel 718 alloys made by laser powder bed fusion and suction casting are compared.

• The Nb homogeneity can affect the precipitation process of the δ phase during continuous cooling.

• A slow cooling rate of 0.1 K/s can produce the highest hardness in all alloys due to the formation of subgrains.

Abstract

Understanding alloy phase transformations during continuous cooling is important for post-processing design and optimization. In this work, continuous-cooling-transformation (CCT) diagrams of Inconel 718 alloys manufactured by laser powder bed fusion (LPBF) and suction casting are developed under different homogenization conditions. Unlike the available CCT diagrams in the reported studies, no γ″ and γ' precipitates can be observed. NbC and δ are determined to be the precipitates after cooling from the γ matrix. Importantly, homogenization time and manufacturing methods are found to affect the Nb homogeneity in the matrix near NbC particles and thus significantly influence the precipitation process of the δ phase, which has a high content in Nb. In the alloys with high Nb homogeneity, the nucleation process mainly contributes to the precipitation, whereas in the alloys with low Nb homogeneity, the precipitation is primarily associated with the growth process. Subgrains are found to form after cooling at 0.1 K/s and can cause the highest hardness in samples. This work provides a new viewpoint on the study of processing-structure-property relationships during cooling in Inconel 718 and is beneficial to the development of alloy post-processing strategies.

Introduction

Continuous-cooling-transformation (CCT) diagrams of alloys have been used very often to study solid-solid phase transformations. Since the CCT diagrams are conducive to understanding the microstructure-property relationships during processing and thus further provide design solutions for microstructure engineering. Due to the importance of the Inconel 718 in engineering applications with good high-temperature performance [[1], [2], [3]], the evaluation of CCT diagrams becomes critical, especially for additive manufacturing (AM) such as laser powder bed fusion (LPBF). In recent years, the application of Inconel 718 in AM has also proliferated owing to its good weldability as well as the increasing demands for the high-temperature components with complex shapes [[4], [5], [6]]. Therefore, it is essential to understand the phase stability and phase transformations under different heat treatment and cooling conditions.

Inconel 718 is an fcc precipitation-hardenable superalloy strengthened by the γ″ (Ni₃Nb, bct_D0₂₂) plates and the spherical γ' (Ni₃(Al, Ti, Nb), fcc_L1₂) particles [7,8]. Besides, NbC carbides (fcc_B1) usually form along with γ at high temperatures. The δ (Ni₃Nb, D0_a) and Laves_C14 ((Ni, Fe, Cr)₂(Nb, Ti, Mo), hexagonal) are two detrimental intermetallic phases that are often observed in the alloy. The δ phase has a needle shape and usually precipitates along grain boundaries [8,9], and thus potentially can act as a pinning particle for grain size control but degrades strengthening property due to its incoherent phase boundary with the γ matrix [10]. The Laves_C14 phase forms during solidification as a result of Nb segregation at grain boundaries. In order to avoid crack initiation due to segregation, Inconel 718 alloys are often homogenized at elevated temperature as one of the most effective solutions [[11], [12], [13]].

It has been found that the phase transformation behaviors in Inconel 718 strongly depend on the applied heat treatments [[13], [14], [15], [16]]. Therefore, understanding phase transformations during the continuous cooling process is critical for the microstructure engineering. This is particularly important for the AM process, in which the cyclic heating and cooling processes introduce complexity in the microstructure evolution, and thus usually require careful design of both in-situ processing and post-heat treatments [10,14,[17], [18], [19], [20], [21], [22]]. CCT diagrams directly provide the information regarding phase transformations during the continuous cooling processes and are often applied as a tool together with isothermal transformation diagrams for microstructure engineering. Such a diagram can be useful in post-heat treatment but can also be applied to get insight during the cyclic heating and cooling processes when only solid phases are involved.

A few CCT diagrams of Inconel 718 have been previously reported [14,[23], [24], [25]], as summarized in Fig. 1. In order to facilitate discussion, we define three characteristics of the CCT curves, as illustrated in Fig. 1(a), to describe the phase transformation behaviors: (1) starting temperature of the phase transformation; (2) phase formation range, which is defined as the difference between the starting temperature and ending temperature at one certain cooling rate; and (3) critical cooling rate, below which the phase transformation will happen. Garcia et al. [14] used a dilatometer to investigate the effect of homogenizations at 1180 °C for 24, 72, and 90 h on the CCT diagrams of cast Inconel 718 alloys. They found the CCT curves shifted to the slower reaction side, i.e., the right-hand-side of the CCT diagram, with increased homogenization durations (Figs. 1(b)-(f)). The authors [14] also reported that during cooling after 90-h homogenization, new and small MC carbides formed before the δ phase, which was different from the cases of 24 and 72-h homogenizations. This was explained as the Nb segregation along grain boundaries was reduced after long-time homogenization, which increased the Nb supersaturation within the grains and promoted the formation of MC carbides, but limited δ. However, the formation temperature of the δ phase was determined to be quite high (Fig. 1(c)), which is up to about 1130 °C. This temperature is much higher than the reported solvus temperatures of δ from 998.3 to 1027 °C obtained from experiments and CALPHAD (calculation of phase diagrams) calculations [[26], [27], [28], [29]]. In addition, the γ″, γ', and Laves_C14 phases were also found to form after continuous cooling, as can be seen in Figs. 1(d), (e)&(f). γ″ was determined to precipitate prior to γ' during cooling. The critical cooling rates for γ″ were 1– 10 K/s, depending on the homogenization time (Fig. 1(d)). Geng et al. [23] investigated the phase transformation behaviors of γ″ and δ during continuous cooling after homogenization at 1100 °C for 1 h in hot-extruded Inconel 718 alloys. The result deviated significantly from the one by Garcia et al. [14] since they determined the γ″ precipitated under very slow cooling rates of 0.1– 20 K/min (0.0017– 0.33 K/s), while δ formed at cooling rates lower than 5 K/min (0.083 K/s). Slama and Cizeron [24] reported that the δ, γ', and γ″ phases can precipitate respectively after heat treatment at 990 °C for 15 min, as reproduced in Fig. 1(d). The critical precipitation cooling rate of the δ phase was determined to be higher than 100 K/s, which was extremely high compared to the values from other work; while for the γ' and γ″, critical cooling rates were 5 K/s, and 0.2 K/s, respectively. Niang et al. [25] provided a CCT curve for δ measured by differential thermal analysis (DTA) in forged Inconel 718 alloys, the critical cooling rate of δ was evaluated to be about 0.5 K/s (Fig. 1(c)). These results show that the homogenization conditions can affect the phase transformation behaviors during continuous cooling.

Although some CCT diagrams of cast/wrought Inconel 718 have been reported, the results from different work are inconsistent and dependent on the alloy fabrication status. Moreover, it is yet unclear regarding the impact of the manufacturing method on the CCT diagrams, since processing and heat treatment design is often based on the reported diagrams of the cast Inconel 718. However, as indicated by Zhao et al. [13], the as-received and homogenized microstructure of Inconel 718 manufactured by LPBF and suction-cast are significantly different. The as-built microstructure after LPBF shows a strong texture with columnar grain along build direction with a low Nb microsegregation. During homogenization at 1180 °C, the columnar grains in the as-built alloy become equiaxial due to recrystallization and grain refinement occurs as a result of the Zener pinning effect. The Nb homogeneity level in the γ matrix decreases during homogenization. Contrarily, the grain morphology in the as-cast Inconel 718 is equiaxial and the Nb segregation level is high. Moreover, abnormal grain growth can be observed in the suction-cast alloys during homogenization at 1180 °C and the Nb homogeneity level increases. Therefore, the microstructures of LPBF and suction-cast alloys after homogenizations are quite different, indicating the manufacturing methods can also impact the phase transformations of Inconel 718 during continuous cooling processes after homogenization and such effect is worth of more dedicated study.

This work aims at a comprehensive evaluation of the phase transformation behaviors during continuous cooling processes of Inconel 718 alloys under different homogenization conditions and manufacturing methods. Alloys made by both LPBF and suction casting are subject to microstructure analysis and quenching dilatometry. The suction-cast alloys are chosen as a reference for comparison because they have comparative phase transformation behaviors during homogenization processes from AM alloys [13]. Microhardness of alloys after cooling is studied to help understand the microstructure-property correlations under different cooling conditions.