**1. Introduction**

Nanograined (NG) metals containing a high volume fraction of grain boundaries have demonstrated much higher mechanical strength than their coarse-grained counterparts [1–3]. Severe plastic deformation (SPD) techniques such as equal channel angular pressing [4,5] and high-pressure torsion [6–8], etc., have been proven effective in grain refinement of metallic materials. However, the same grain boundaries that contribute to the high strength also lead to deterioration of the thermal stability of NG metals [9–12]. The grain boundary energy of nanograins provides a large driving force for grain coarsening. For instance, grain boundary migration takes place at 300 ◦C for nanocrystalline Nb (obtained by high pressure torsion) with an average grain size of 75 nm [13]. Grain growth occurs at temperatures as low as 200 ◦C for nanocrystalline Ni, accompanied by a substantial hardness drop [14]. In electrodeposited nanocrystalline Ni with an average grain size of 10–20 nm, grain coarsening occurs at 80 ◦C [15]. Similarly, grain growth takes place even at ambient temperature in nanocrystalline Cu. The poor thermal stability hinders the application of NG metallic materials at elevated temperatures [16].

Recently, surface mechanical grinding treatment (SMGT) [17], surface mechanical attrition treatment (SMAT) [18–20] and surface mechanical rolling treatment (SMRT) [18] have been applied to introduce gradient microstructures into the surface of metallic materials to improve both strength and ductility. Gradient structures containing an NG top surface layer have been introduced into several types of metals [18,21–24]. It has been reported that these surface modification techniques are more effective than conventional SPD approaches in grain refinement [22]. In contrast to the poor thermal stability of nanograins in most prior studies, it was reported that nanograins smaller than the critical values (70 nm for Cu and 43 nm for Ni) were more stable than larger grains in gradient structured pure Cu and Ni fabricated using SMGT in liquid nitrogen [25]. The surprising observation of enhanced thermal stability of nanograins was attributed to the unique grain boundaries in

**Citation:** Ding, J.; Zhang, Y.; Niu, T.; Shang, Z.; Xue, S.; Yang, B.; Li, J.; Wang, H.; Zhang, X. Thermal Stability of Nanocrystalline Gradient Inconel 718 Alloy. *Crystals* **2021**, *11*, 53. https://doi.org/10.3390/cryst11010053

Received: 12 December 2020 Accepted: 2 January 2021 Published: 11 January 2021

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low energy configurations generated during low-temperature SMGT [25,26]. This evidence implies that the thermal stability of NG alloys may not necessarily be deteriorated after grain refinement.

Inconel 718 (IN718) is a common precipitation-strengthened Ni-based superalloy used for application in high-pressure turbine discs in jet engines [27–35]. However, a majority of the published works focused on wrought IN718 alloys with coarse grains [36,37]. Studies on NG IN718 are limited. Besides, the high-temperature performance of IN718 is primarily determined by the high-density γ" phases formed after annealing [36,38–45]. However, at temperatures above 650 ◦C, the metastable γ" phase transforms to stable δ phase over long-term exposure [36,37,44,46]. The application of IN718 alloy is therefore limited to temperatures below 650 ◦C. In this study, gradient structures containing a severely deformed NG surface layer were introduced into IN718 alloy via the SMGT technique at liquid nitrogen temperature. Studies on the NG IN718 alloy at 700 ◦C for up to 100 h reveal that nanograins have outstanding thermal stability. The underlying nanograin stabilization mechanisms are discussed.
