Microstructure and mechanical properties of Ti₂AlNb diffusion bonding using multi-phase refractory high-entropy alloy interlayer

Highlights

• The Ti₂AlNb was diffusion bonded using multi-phase refractory high entropy alloy as interlayer.

• The high entropy interlayer effectively inhibited the aggregation of interfacial intermetallic compound.

• The welded interface had reliable bonding strength and suitable performance matching.

• The shear strength of joints was higher than that of based metals.

Abstract

This paper conducts diffusion bonding of Ti₂AlNb based alloy, in which a novel refractory high-entropy alloy (Ti₄₀Nb₃₀Hf₁₅Al₁₅; RHEA) was used as the interlayer. The RHEA interlayer is designed to eliminate bond line and restrain the aggregation of orthorhombic (O) phase at bonding interface. The microstructures of joints were investigated by scanning and transmission electron microscopy (SEM and TEM, respectively). Defect-free joints were obtained when bonding was performed in the temperature range of 950–1000 °C at 30 MPa for 2 h. The joint microstructure was mainly composed of a disordered bcc-type solid solution and nanosized basketweave ordered O phase. According to the TEM results, the matrix was rich in Hf, which strengthened the matrix by solid solution strengthening. The tiny O phase retained a specific coherent relationship with the bcc matrix, (001)_O// $\left(01\bar{1}\right)$_bcc and $\left[0\bar{1}1\right]$_O// $\left[\bar{1}11\right]$_bcc, which strengthened the bonding interface by precipitation strengthening. This multiphase coupling interface indicated reliable metallurgical bonding and guaranteed excellent joint performance. The mechanical properties of the joints were evaluated using nanoindentation and shear tests. The microhardness and Young's modulus were distributed evenly without any noticeable fluctuation and ranged from 5.01 to 5.52 GPa and 103.97–117.22 GPa, respectively, illustrating the suitable property matching between the high-entropy interlayer and Ti₂AlNb. The maximum shear strength of the joint was 463 MPa with bonding at 970 °C and 30 MPa for 2 h. The main crack was significantly deflected into the parent materials, rather than propagating along the interface, which further demonstrated that the bonding face had higher strength than the base metals. The precipitation mechanism of the nanoscale O phase was revealed through transmission Kikuchi diffraction–electron backscatter diffraction (TKD-EBSD). The O phase variants formed with equal probability could lead to a basketweave morphology. The successful bonding of Ti₂AlNb using RHEA as the interlayer provides a new interlayer system to bond Ti-based intermetallic compounds.

Introduction

Owing to the addition of niobium (Nb, 17–27 at.%) in Ti–Al alloys, the orthorhombic-phase (O phase) Ti₂AlNb-based alloys with relatively more slip systems display better ductility than Ti–Al alloys [1]. Additionally, with accurate composition design and precise microstructure regulation, Ti₂AlNb-based alloys also exhibit excellent mechanical properties, such as high-temperature strength, creep resistance, and fracture toughness, meeting the requirements for operation under harsh service conditions at high temperatures. Moreover, Ti₂AlNb can be lighter by more than 30% as compared to its superalloy counterparts that serve in the same temperature range [2]. Therefore, Ti₂AlNb was suggested to be a potential candidate for thin-walled complex structural components in aero-engines [3].

A suitable welding method is key to producing such complex components. In view of the complex working conditions and precise parts structure, higher requirements for joint-forming accuracy and mechanical properties naturally arise. First, the welding process should minimize thermal damage to the base metals to retain the properties of the matrix. Second, impurity elements with low melting points should be avoided to improve the high-temperature mechanical properties. Third, the welding deformation should be precisely controlled to ensure the formation of complex parts. Finally, the welding process should overcome the atomic diffusion inertia of high-temperature materials to yield defect-free joints. However, because fusion welding methods, such as electron-beam welding and laser welding, involve melting and rapid solidification, the joints have a severely heterogeneous microstructure and retain high residual stress, resulting in brittle behavior at high temperature [4] and the reheat cracking of joints [5]. In addition, several studies have successfully realized the welding of Ti₂AlNb by brazing [6] using a eutectic filler metal. However, the low-melting-point eutectic filler (Ti,Zr)₂(Cu,Ni) was formed in the joints owing to the introduction of eutectic elements, which would become a weak point in the subsequent high-temperature service of the joints.

Compared with conventional welding methods, solid-state diffusion bonding (DB) can effectively avoid the joint cracking of fusion welding and has the advantages of stable microstructure, minor bonding deformation, and excellent mechanical properties; hence, solid-state DB is currently used to bond Ti₂AlNb-based alloys. Chu [7] investigated the microstructure evolution of Ti₂AlNb at a DB interface in the temperature range of 900–1000 °C and found a clear bond line (BL), characterized by an abundant O–B2 phase interface, within the entire temperature range. The BL formation was related to the different diffusion behaviors of the Ti atoms in the O and B2 phases. Li [8] studied the diffusion behavior of Ti atoms during the DB process of Ti₂AlNb through a molecular dynamics simulation. The results showed that the diffusion coefficient of Ti in the O phase was one order of magnitude lower than that in the B₂ phase. Furthermore, the diffusion activation energy of Ti atoms in the O phase was 79.5% higher than that in the B₂ phase, illustrating that the mass of the O phase in the base metals strongly impeded the diffusion of Ti atoms and pinned the interface. As we know, the BL, as an interfacial defect, severely decreases the performance of joints [9]. To eliminate the BL, increasing the bonding temperature to further promote atomic diffusion is a natural adjustment according to the formation mechanism of the BL. However, this method causes serious thermal damage to the base metal; that is, it causes the dissolution of the O phase and grain growth. In contrast to the increase of the bonding temperature, DB with an interlayer that controls the interface diffusion behavior can eliminate the BL without sacrificing the base metals.

Several attempts have also been made to join Ti₂AlNb alloys with various interlayers, such as Ti–Zr–Ni–Cu + Ti [10], Ti-Ni [11], Ti-Ni-Nb [12], and CP-Ti [13]. However, various intermetallic compounds (IMC) remained distributed along the interface between the interlayer and base metals owing to the strong IMC formation ability of Al with other elements, which leads to potential safety hazards. Therefore, the materials of the interlayer directly affect the phase of the joints and other mechanical properties. The novel multi-phase refractory high-entropy alloys (MP-RHEAs) maintain the superb high-temperature strength exhibited by single-phase bcc RHEAs and further overcome the so-called strength–ductility tradeoff by introducing multi-phase coupling (bcc + hcp, bcc + ordered B₂) [14,15]. Yurchenko [16] investigated the phase transformation and mechanical properties of the RHEA Ti₄₀Nb₃₀Hf₁₅Al₁₅ with different heat treatments. The bcc solid solution (SS) of Ti₄₀Nb₃₀Hf₁₅Al₁₅ was maintained above 900 °C and transformed into a mixture of a bcc matrix and an O phase after annealing at 600 °C; this matrix had an outstanding yield strength (1250 MPa) without excessive plastic sacrifice (16%). Therefore, Ti₄₀Nb₃₀Hf₁₅Al₁₅ was chosen as the interlayer in this study for the following reasons. First, the SS temperature of RHEA is 900 °C, which is the conventional bonding temperature of Ti₂AlNb. Hence, the interlayer could maintain the SS during the bonding process, which tends to weaken the pinning effect and further eliminate BL. Second, the high-entropy effect of the interlayer can inhibit the formation of a brittle IMC at the interface during bonding [17]. Third, the precipitation temperature of RHEA is 600 °C, which meets the service temperature requirement of Ti₂AlNb [18]. Fourth, the element similarity with Ti₂AlNb and excellent strength-toughness matching of RHEA effectively guarantee the reliable connection of joints.

In the present study, Ti₂AlNb-based alloys were diffusion-bonded using an RHEA interlayer. The typical interfacial microstructure, interfacial phases, microstructure evolution, and mechanical properties of the joints were also investigated.