Impact toughness, crack initiation and propagation mechanism of Ti6422 alloy with multi-level lamellar microstructure

Abstract

The influence of different solution and aging conditions on the microstructure, impact toughness, and crack initiation and propagation mechanisms of the novel α + β titanium alloy Ti6422 was systematically investigated. By adjusting the furnace cooling time after solution treatment and the aging temperature, Ti6422 alloy samples were developed with a multi-level lamellar microstructure, including microscale α colonies and αp lamellae, as well as nanoscale αs phases. Extending the furnace cooling time after solution treatment at 920°C for 1 h from 240 to 540 min, followed by aging at 600°C for 6 h, increased the αp lamella content, reduced the αs phase content, expanded the α colonies and αp lamellae size, and improved the impact toughness from 22.7 to 53.8 J/cm2. Additionally, under the same solution treatment, raising the aging temperature from 500 to 700°C resulted in a decrease in the αs phase content and a growth in the thickness of the αp lamella and αs phase. The impact toughness increased significantly with these changes. Samples with high αp lamellae content or large αs phase size exhibited high crack initiation and propagation energies. Impact deformation caused severe kinking of the αp lamellae in crack initiation and propagation areas, leading to a uniform and high-density kernel average misorientation (KAM) distribution, enhancing plastic deformation coordination and uniformity. Moreover, the multidirectional arrangement of coarser α colonies and αp lamellae continuously deflect the crack propagation direction, inhibiting crack propagation.

1. Introduction

Titanium alloys, known for their low density, high specific strength, excellent corrosion resistance, and low-temperature resistance, are essential materials in aerospace, marine engineering, and biomedical fields [1–3]. Impact toughness serves as a crucial performance metric for evaluating material serviceability under dynamic impact loads. Strength and toughness are two vital properties of metallic materials, but they often display a trade-off, with higher-strength metals typically demonstrating lower ductility and toughness [4–6]. For example, the TC4 alloy with equiaxed microstructures exhibited a yield strength of 813 MPa and an impact toughness of 50 J/cm2. Controlled heat treatment increased the impact toughness to 63 J/cm2 but reduced the yield strength to 743 MPa [7]. The TC21 alloy, when solution-treated in the dual-phase region, achieved a yield strength of 1248 MPa and an impact toughness of 12.3 J/cm2. Transitioning to solution treatment in the single-phase region doubled toughness while reducing the strength by over 300 MPa [8]. In practical applications of titanium alloys, the demand for strength and toughness varies depending on the service conditions. In high-load static structures such as aeroengine compressor disks, high strength is often prioritized to ensure adequate structure stability, but some toughness may be sacrificed. In impact-resistant dynamic structures such as landing gear connectors, the focus is on increasing toughness to enhance damage resistance, which can also result in a partial loss of strength. Therefore, achieving a reasonable balance between impact toughness and strength remains a significant challenge in the development and engineering application of titanium alloy structural materials [9].

The crack propagation path under impact loads is a primary factor affecting the impact toughness of alloys [10]. The lamellar structure of titanium alloys, with high-density α/β phase interfaces, can deflect cracks and prolong the propagation path, thereby enhancing impact toughness [11]. For example, Wu et al. [12] compared the strength and toughness of Ti5321 alloy with bimodal and lamellar structures. The lamellar structure exhibited a superior strength–toughness match (yield strength (σy) = 915 MPa, impact toughness (akv) = 37.5 J/cm2), whereas the bimodal structure had the lowest impact toughness (11.3 J/cm2). Zhang et al. [13] found that increasing the thickness of the lamellar α phase and the content of equiaxed α phase in bimodal TC21G alloy facilitated the formation of the zigzag crack propagation path, which is conducive to the increase of impact toughness. In addition, energy dissipation during crack propagation in titanium alloys is primarily used to overcome plastic deformation at the crack tip [14]. Thus, increasing the plasticity of titanium alloys helps enlarge the plastic zone at the crack tip, improving coordinated deformation ability and blunting cracks [15]. Research by Huang et al. [16] and Zheng et al. [17] indicated that increased dislocation mobility and deformation twinning can relieve stress concentration at the crack tip, thereby