Elsevier

Applied Surface Science

Volume 542, 15 March 2021, 148592
Applied Surface Science

Full Length Article
Crystallographic orientation relationships and interfacial structures between reinforcement and matrix phases in an in situ (Ti, Nb)B/Ti2AlNb composite

https://doi.org/10.1016/j.apsusc.2020.148592Get rights and content

Highlights

  • (Ti, Nb)B/Ti2AlNb composite was in situ synthesized by spark plasma sintering.

  • Orientation relationships between (Ti, Nb)B and Ti2AlNb matrix were determined.

  • Atomic structures and bonding natures of (Ti, Nb)B/matrix interfaces were revealed.

  • (Ti, Nb)B promoted the preferential precipitation of brittle α2-Ti3Al particles.

  • Inhibiting (Ti, Nb)B/α2 interface by a two-step heat treatment improved the ductility.

Abstract

High-performance (Ti, Nb)B/Ti2AlNb composites are promising high-temperature structural materials for the new generation of aerospace engines. The interfaces between the (Ti, Nb)B reinforcement and matrix phases (i.e. O-Ti2AlNb, α2-Ti3Al and B2 phases) affect the mechanical properties of the composites largely. In this study, the properties of the interfaces were systematically revealed after fabricating a (Ti, Nb)B/Ti2AlNb composite by low-energy ball-milling and spark plasma sintering (SPS). A high-resolution transmission electron microscopy (HRTEM) was adopted to evaluate the crystallographic orientation relationships (ORs) and interfacial structures. (Ti, Nb)B maintains three coherent interfaces with the matrix, and the preferred ORs between them can be expressed as [1 1 2¯ 0]α2//[0 1 0](Ti, Nb)B and (1¯ 1 0 0)α2//(1 0 0)(Ti, Nb)B; [1 1¯ 0]O//[0 1 0](Ti, Nb)B and (1 1 0)O//(1 0 0)(Ti, Nb)B; [1 1¯ 1]B2//[0 1 0](Ti, Nb)B and (1¯ 1 2)B2//(1 0 0)(Ti, Nb)B. The results of first-principles calculations indicated that the (1¯ 1 0 0)α2/(1 0 0)(Ti, Nb)B interface possesses a higher bonding strength compared with the (1 1 0)O/(1 0 0)(Ti, Nb)B and (1¯ 1 2)B2/(1 0 0)(Ti, Nb)B interfaces, and the (Ti, Nb)B reinforcement and the matrix are bonded through strong ionic bonds and weak covalent bonds. Moreover, based on Bramfitt’s lattice mismatch theory, it was found that the (Ti, Nb)B reinforcement acts as an effective substrate in promoting the heterogeneous nucleation and preferred precipitation of the α2 phase. Suppressing the brittle α2 precipitate around the reinforcement improves the ductility of the composite obviously.

Introduction

Despite having advantages in the specific strength and specific stiffness, Ti2AlNb alloys are still difficult to replace the position of nickel-based superalloys such as the Inconel 718 alloy, since the strength and creep properties of Ti2AlNb alloys drop sharply once the ambient temperature is higher than 650 °C [1], [2], [3]. Stiff ceramic reinforcements (e.g. boride and carbide) have been successfully introduced into the soft Ti2AlNb matrix to improve the mechanical properties of Ti2AlNb alloys, which has become a hot spot in the field of high-temperature structural materials [3], [4], [5], [6]. The in situ synthesized (Ti, Nb)B short fibers possess impeccably physical and chemical compatibility with Ti2AlNb alloys and have been regarded as an ideal reinforcement for high-performance Ti2AlNb-based composites [3], [5], [7]. Emura [2], [7] et al. fabricated a 6.5 wt% TiB/Ti2AlNb composite by the hot isostatic pressing of the gas-atomized powders and subsequent hot-rolling. Their results presented that the ultimate tensile strength (UTS) of the composite increased by 48% at 25 °C and 36% at 800 °C, and the fatigue strength at 107 cycles increased by 40% compared with the Ti2AlNb alloy.

Reinforcement/matrix interfaces act as the intermediary to transfer loads from the matrix to the reinforcement, and their bonding strength has a crucial influence on the interfacial load-transfer, strengthening mechanism and fracture mode, determining whether the reinforcement can fully play their strengthening effect and the comprehensive properties of composites. Feng [8] and Liu [9] et al. confirmed that the strength and Young’s modulus of TiB/Ti composites increased with the improvement of the interfacial bonding strength, but the fracture toughness exhibited a contrary tendency. For the interface with an insufficient bonding strength, the crack is prone to initiate in the interfacial zone, which leads to the premature debonding of the interface and poor mechanical properties [10], [11]. Recently, to seek a satisfactory interfacial bonding strength, the in situ synthesis technology is widely employed in which the reinforcements in composites are formed by the chemical reactions between additives and the matrix [5], [12], [13], [14]. Using this technology, Wang [12] et al. synthesized a novel (MgAl2O4 + MgAlB4)/Al composite and Song [13] et al. fabricated a Ti2Al(C, N)/TiAl composite with a core-shell architecture. They all proved that the clean and flat reinforcement/matrix interfaces formed by in situ reactions had sufficient strength to transfer the load from the matrix to the reinforcements, causing that the interfacial debonding was hardly observed on the fracture surface. It has also been verified that the interfaces between the in situ formed reinforcements and the matrix are generally coherent or semi-coherent and free from other interface phases, which guarantees not only a good strengthening effect but also interfacial stability during long-term or high-temperature service [3], [7], [15].

Reinforcement/matrix interfaces can also play a role as the barrier or channel for phase transformations, resulting in the variation of the precipitated behavior in the matrix around the interfaces. It has been well-documented that both the precipitate free zone (PFZ) and the coarse precipitate zone (CPZ) have a tendency to generate in the matrix near the reinforcement/Al interfaces, which significantly deteriorates the mechanical properties of Al-based composites [16], [17], [18], [19]. Except for PFZ and CPZ, the phenomenon that reinforcement/matrix interfaces facilitated the precipitation was also detected by Ma [14] and Strangwood [20] et al. Ma [14] et al. demonstrated that the (Zn1.5Cu0.5)Mg precipitate was prone to precipitate around the TiB2/Al interface in the aged composites, and they attributed this phenomenon to the following two reasons: (a) the pre-existed TiB2/Al interface decreased the energy barrier for nucleation, thereby promoting the heterogeneous nucleation of the precipitate; (b) the highly-dense misfit dislocations on the interface provided a fast diffusion channel for vacancies and solute atoms, thus accelerating the nucleation and growth of the precipitate. A similar phenomenon has also been perceived in (Ti, Nb)B/Ti2AlNb composites that the brittle α2 phase has a preference to precipitate around the (Ti, Nb)B reinforcement [5]. However, it is still indistinct that the mechanism for the α2 preferred precipitation as well as its effect on the mechanical properties.

Generally speaking, the bonding strength of reinforcement/matrix interfaces is difficult to quantify through experimental methods, and similarly, the traditional HRTEM method hardly depicts the atomic configurations and electronic structures of interfacial zones. First-principles calculations based on density functional theory are an effective method to estimate the interfacial properties at the atomic and electronic level. Recently, this method has been successfully applied to unveil the atomic configurations, bonding strength, electronic structures, stability and fracture toughness of ceramic/metal interfaces, such as the MgAl2O4/Al [12], TiC/Al [21] and (Ti, Nb)C/Ni [22] interfaces. Wang [21] et al. employed this method to investigate the properties of the TiC/Al interface and found that the C-terminated (1 1 1)TiC/(1 2 1)Al interface possessed a higher bonding strength than the Ti-terminated (1 1 1)TiC/(1 2 1)Al interface.

Based on the discussion above, it can be concluded that the interfacial properties between the (Ti, Nb)B reinforcement and matrix are the critical factor determining the mechanical properties of (Ti, Nb)B/Ti2AlNb composites, and disclosing the properties of the interfaces is helpful to guide the design and fabrication of the composites. However, to our knowledge, few studies focused on unveiling the natures of the interfaces, for the reason that the research on the (Ti, Nb)B/Ti2AlNb composites is in its initial stage. Besides, the mechanism for the preferred precipitation of α2 is still a mystery. For these reasons, a (Ti, Nb)B ceramic reinforced Ti2AlNb-based composite was fabricated by in situ synthesis technology in this study. Then, the features of the reinforcement/matrix interfaces in the composite including ORs, interfacial structures, atomic configurations, and binding natures were systematically investigated by HRTEM and first-principles calculations. The reason for the α2 preferred precipitation and its effect on the machinal properties of the composite was also evaluated using the classical nucleation theory, Bramfitt’s lattice mismatch theory, heat treatment and tensile test.

Section snippets

Composite synthesis and heat treatment

The (Ti, Nb)B/Ti2AlNb composite was fabricated by low-energy ball-milling and subsequent SPS process [5]. Raw materials included the Ti-22Al-25Nb (all compositions are given in atomic percent in this study) pre-alloyed powders fabricated by the plasma rotating electrode process and mechanically crushed LaB6 powders, and their average particle sizes were 92.0 and 0.92 μm, respectively. The two powders were sealed in stainless-steel milling pots and milled at a low-energy speed of 250 rpm for 8 h

Structure, morphology, composition and elastic properties of boride

The selected area electron diffraction (SAED) pattern shown in Fig. 1(a) reveals that the in situ synthesized boride crystallizes into the B27 structure, which is a primitive orthorhombic crystal structure belonging to the Pnma (62) symmetry group. The boride formed in Ti2AlNb-based composites possesses the same crystal structure as the boride generated in common Ti-based composites [9], [23].

The boride presents a regular polyhedral shape in the transverse section with specific (1 0 0), (1 0 1) and

Conclusion

This research was carried out to provide fundamental insights into the interfacial structures, interfacial binding natures and the crystallographic orientation relationships between the reinforcement and the matrix phases in the (Ti, Nb)B/Ti2AlNb composite. Moreover, it was revealed that the mechanism for the α2 preferred precipitation around the (Ti, Nb)B and its effect on the mechanical property. The following conclusions can be drawn based on the results and discussion.

  • (1)

    The boride in situ

CRediT authorship contribution statement

Ningbo Zhang: Methodology, Software, Investigation, Writing - original draft, Data curation, Validation. Xiuli Han: Conceptualization, Software, Resources. Dongli Sun: Conceptualization, Resources, Supervision. Hao Liu: Methodology, Investigation, Validation. Wei Xue: Data curation. Boyu Ju: Writing - review & editing. Gaohui Wu: Resources, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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