**1. Introduction**

Owing to their extremely high tensile strength, toughness, endurance strength, fatigue strength, corrosion resistance, and oxidation resistance at high temperatures (such as ≥980 ◦C), nickel(Ni)-based superalloys have been widely applied in aviation, aerospace, and military fields [1–3]. Because of the poor casting performance of Ni-based superalloys and the complexity of components, Ni-based superalloys usually need to be welded for their applications. The shortcomings of fusion welding and brazing in the connection of Ni-based superalloys (such as shrinkage stresses induced by rapid precipitation, formation of brittle phases, and the tendency to crack due to local fragility, etc.) limit the industrial application of the process for Ni-based superalloys [4,5]. Paulonis et al. proposed a transient liquid phase (TLP) bonding method [4], where in a thin layer of intermediate alloy with a lower melting temperature is employed as the connecting material. During TLP bonding, a low-melting liquid phase is formed between the parent material and the intermediate layer by heating under vacuum conditions; the liquid phase is then homogeneously diffused and therefore isothermally

solidified, finally forming a joint with a uniform microstructure. Owing to its reasonable welding temperature, low applied pressure requirement, and ability to form a welded joint with excellent mechanical properties, TLP bonding is widely considered as one of the most ideal welding methods for Ni-based superalloys [5–7].

Grain boundaries, as surface imperfections, adversely affect the high-temperature mechanical properties of metals, especially their high-temperature endurance strength. Thus, the high-temperature mechanical properties of single-crystalline alloys are substantially better than those of polycrystalline alloys. Accordingly, single-crystalline alloys are extensively used as the latest generation of Ni-based superalloys. In the TLP bonding of homogenous Ni-based single-crystalline alloys, because the intermediate-layer alloy is a foil-belt amorphous or powdery Ni-based alloy, a region of polycrystalline Ni-based alloy is unavoidably formed in the joint after TLP bonding; the single-crystalline Ni-based alloy (i.e., parent alloy) is usually connected by a polycrystalline Ni-based alloy (i.e., intermediate layer) after TLP bonding. Extensive investigations on the compositions, heat treatments, mechanical properties, and simulations of single-crystalline Ni-based alloys have been reported [8–15]. In addition, some experimental studies that focus on the bonding process, intermediate-layer alloys, or joint microstructures in the welding joints of single-crystalline Ni-based alloys have recently been published [7,16–20]. Thus far, because of limitations in experimental characterization methods (e.g., the magnitude and distribution of stress and strain in the TLP bonded joint cannot be experimentally measured), few studies have been conducted to analyze the influence of the different states of crystallographic orientations between the single-crystalline parent material and the polycrystalline intermediate layer on the mechanical behavior of a TLP bonded joint.

Ni3Al-based alloys belong to Ni-based superalloys, which have excellent mechanical and physical properties [21]. In the present study, the mechanical properties of single-crystalline and polycrystalline Ni3Al were calculated by first-principles calculations based on density functional theory (DFT). In addition, the mechanical behavior of the TLP bonded joint of single-crystalline Ni3Al under a simple tension load was studied via the finite element (FE) method, and the stress concentration caused by the different states of crystallographic orientations between the single-crystalline parent alloy and the polycrystalline intermediate layer was evaluated.
