Precipitation Behavior of O Phase during Continuous Cooling of Ti-22Al-25Nb Alloy

Dong Li,Weidong Zeng,Xiaobo Liang,Jianwei Xu,Fan Zhang,Xiong Ma

doi:10.3390/met12020291

Dong Li, Weidong Zeng + Show 4 more

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https://doi.org/10.3390/met12020291

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Abstract

The microstructure evolution and formation mechanism of the O phase in a Ti-22Al-25Nb (at.%) orthorhombic alloy resulting from different cooling rates were investigated. The results show that the morphology of the precipitated O phase is significantly affected by the cooling rate. As the cooling rate decreases, the floccular O, composed of many fine acicular O phases, gradually grows into the lamellar O phase. When the alloy is cooled from the B2 phase region, the grain boundary O (OGB) preferentially nucleates at the triple junctions and grain boundaries and forms the flat and the zig-zag OGB according to different cooling rates. The OGB consists of separated, flat OGB parts and unconnected, zig-zag OGB composed of multiple short, separated, flat OGB at a higher cooling rate. The zig-zag OGB presents a connected state due to the sufficient diffusion time at a lower cooling rate. When the alloy is cooled from the (B2 + α2) phase region, the increase of the phase boundary provides favorable conditions for the nucleation of the O phase due to the presence of α2 particles. The precipitated rim O phase appears on the periphery of the α2 particles at lower cooling rates. The analysis indicates that the Widmanstätten intragranular O (OWI) precipitated directly from the B2 phase maintains the plane relation with the parent B2 phase, and the Widmanstätten grain boundary O (OWGB) holds the specific orientation relationship with one of the two adjacent B2 grains. The OGB keeps the specific orientation relationship with one of the B2 grains as much as possible. When it cannot maintain the specific orientation relationship with one of the B2 grains, the OGB maintains a near-orientation relationship with B2 grains on both sides to reduce the nucleation activation energy. Moreover, there can be more than one nucleation site for the O phase on a single B2 grain boundary to form the OGB. The rim O phase formed through a decomposition reaction of α2→α2 (Nb-lean) + O (Nb-rich) is controlled by a diffusional mechanism and maintains a specific orientation relationship, i.e., {001} O//{0001} α2 and <110>O//<112¯0> α2, with the parent α2 particles.

Highlights

With the development of aerospace technology, more stringent requirements are being imposed on the high-temperature service performance of aeroengine materials
There can be more than one nucleation site for the O phase on a single B2 grain boundary to form the OGB
Many studies related to the Ti-22Al-25Nb alloy, such as studies of its phase transformation, microstructure evolution and the relationship between microstructure and mechanical properties, have been conducted by various researchers [7,8,9]

Summary

Introduction

With the development of aerospace technology, more stringent requirements are being imposed on the high-temperature service performance of aeroengine materials. Ti-22Al-25Nb (at.%) alloy, as one of the typical second-generation orthorhombic Ti2 AlNb-based alloys, has a superior balance combination of strength, room temperature ductility and fracture toughness [5,6]. It can be used for long periods of service at 650 ◦ C to 750 ◦ C or for short periods of service at higher temperatures. Zheng et al [12] investigated the effect of three lamellar O of different sizes on tensile properties and found that fine acicular O precipitated during aging treatment improves the tensile strength (1057 MPa) in Ti-22Al-25Nb alloy. Muraleedhran et al [15] investigated the α2 -to-O transformation in Ti-28.5Al-13Nb (at.%)

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